Subsurface mixing and ventilation of oxygen minimum zone waters in the southern Bay of Bengal during the summer monsoon

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N. Vinayachandran, Jenson George, Amit Sarkar, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3790094/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract During the summer monsoon, the local wind forcings around Sri Lanka causes the formation of a cold dome called the Sri Lanka Dome (SLD), which upwells subsurface waters. To the east of SLD, the summer monsoon current (SMC) flows into the Bay of Bengal (BoB), transporting high-salinity water from the Arabian Sea. We show that the SMC and the upwelled waters of the SLD are ventilated episodically during summer monsoon in the southern BoB, leading to a net exchange of low oxygen subsurface waters with saturated mixed layers. We observed presence of hypoxic boundary < 63 µmol kg − 1 very close to the surface. Within the SLD, it shoaled between 35 to 40 m, with oxygen values reaching as low as 6 µmol kg − 1 at the bottom of the thermocline. Negative fluxes showing the ingassing rates ranged between − 0.33 and − 9.43 µmol m − 2 sec − 1 within the SLD and SMC. We propose that the episodic ventilation seen during this investigation may lead to disequilibrium between mixed layer and below thereby contributing to mid-depth oxygen enrichment. This study possibly illustrates a pathway through which the oxygen minimum zone in BoB may be gaining oxygen, thereby preventing from becoming denitrifying. Earth and environmental sciences/Biogeochemistry Earth and environmental sciences/Climate sciences Earth and environmental sciences/Environmental sciences Earth and environmental sciences/Ocean sciences Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction The Bay of Bengal (BoB) is a semienclosed basin in the North Indian Ocean characterized by strong surface layer stratification 1,2 . The strongest stratification covaries with the onset of the summer monsoon (May to September) in the northern BoB, where heavy rainfall and river influx result in a low-salinity surface layer 3, 4, 5 . In contrast to the northern BoB, the southern BoB receives less rainfall; therefore, the surface salinity is greater 6, 7, 8 . The existence of a strong perennial OMZ (oxygen minimum zone) with moderate depth oxygen depletion has been reported 9, 10 . Despite this, no denitrification has been observed, which is generally characterized by the presence of secondary nitrite maxima typically associated with Winkler oxygen (O 2 ) close to ~ 4 µmol kg − 1 in the Arabian Sea 9, 11, 12, 13, 14, 15 . Within the OMZ core, O 2 concentrations can range between < 63 and < 4 µmol kg − 1, and such low O 2 levels are particularly harmful to marine 14 . The low oxygen content at mid-depth in the Bay of Bengal can be attributed to primary production in surface waters along with the rapid sinking of organic matter and moderate bacterial respiration rates 12, 16, 17, 18 . Recent observations highlighted very low levels of O 2 in the BoB and OMZ, and a further decrease in O2 may lead to an anammox process 19 . These authors reported low but significant nitrogen loss in the Bay of Bengal. However, this hypothesis was recently contested by who reported episodic O 2 injection into the BoB OMZ due to the presence of eddies 20 . These authors showed that even though the western boundary upwelling system in the BoB is weak anticyclonic eddies that form in the east supply O 2 -rich waters to the OMZ as it moves toward the western BoB 20 . Based on the mean lifetime of the anticyclonic eddies, ventilation rates at 100–300 m were estimated to be 0.06 ± 0.019 µmol kg − 1 day − 1 , which is 3 to 4 times greater than the bacterial respiration rate (0.019 µmol kg − 1 day − 1 ) 20 . In addition, an exchange of waters with O 2 values between 5 and 10 µmol kg − 1 with the core of the BoB OMZ is recently reported 21 . These authors highlighted that such processes prevent the BoB OMZ from becoming denitrifying. More recently, it has been shown that Persian Gulf Water brings O 2 -rich waters to the BoB and possibly plays an important role in keeping O 2 levels higher below which ecological functioning would be significantly affected 22, 23 . Thus, new mechanisms have been identified recently that keep BoB O 2 levels just above the denitrification threshold; these mechanisms are not well understood and are not well represented in biogeochemical models 24 . During the summer monsoon, due to local wind forcings, the formation of a cold dome, the Sri Lankan dome (SLD), occurs in the southern BoB. The upward Ekman pumping induced by this cyclonic curl brings cooler water to the near-surface layers 25 . The upwelling accompanying the SLD influences the local environment by modulating water column properties by cooling the sea surface temperature and enhancing biological production and air-sea interactions 26, 27 . Further to the east of the SLD, the intrusion of the summer monsoon current (SMC) occurs during the same time, and the eastward flow in the North Indian Ocean during the summer monsoon is called the summer monsoon current 3, 8 . The direction of the SMC is eastward to the south of India, and the SMC turns around Sri Lanka and enters the BoB, carrying high-salinity water from the Arabian Sea along its path 4, 28, 29 . When lighter water from lower-salinity BoB water is encountered, the Arabian Sea water subducts beneath the latter 6 . High-salinity (35–35.6 psu) water intruded below the mixed layer to a maximum depth of approximately 200 m 25, 30 . The nutrient-rich water carried by the SMC promotes an increase in surface chlorophyll throughout its path 26, 31 . The biogeochemical manifestations of the upwelled waters associated with the SLD and its interaction with the SMC in the vicinity of the southern BoB are poorly understood. Taken together, these findings underscore that understanding the O 2 biogeochemical cycles in the BoB is particularly important, especially during the summer monsoon, as upwelling around the Sri Lankan Dome (SLD) coincides with the flow of Arabian Sea high-salinity water mass (ASHSW) in this region during which both are characterized by low O 2 content 4, 5 . The data presented here were collected during the Bay of Bengal Boundary Layer Experiment (BoBBLE) field program in the southern BoB 5 (Fig. 1 ) from 24 June to 24 July 2016, coinciding with the summer monsoon. The present study investigated the oxygen distribution associated with the upwelled waters of the SLD and SMC and discussed the impact of episodic ventilation of subsurface waters observed during the curise. We hypothesize that the episodic ventilation observed during this investigation may have led to O 2 disequilibrium/anomalies within the surface mixed layers and contributed to mid-depth oxygen enrichment. Our study illustrates a possible mechanism by which the BoB oxygen minimum zone may gain oxygen, which may counteract OMZ expansion, as predicted, and possibly maintain the oxygen levels of the southern BoB above the denitrifying threshold. Results Thermohaline structure and O 2 within SLD At the beginning of our observations at TSW and Z1 (28 to 30 June; Fig. 2 ), the surface mixed − 29. The water column distribution of O 2 up to a depth of 1050 m is illustrated in Fig. 3 , and the temporal variability is shown in Fig. 4 . The surface water was less saline, which created an oxygenated freshwater layer on the top. It is apparent that this layer remained well oxygenated with concentrations > 190 µmol kg − 1 . Within the SLD, oxygen concentrations as low as < = 50 µmol kg − 1 were observed at a shallow depth of 50 m and remained close to 30 m on the 30th June, after which a freshening event was recorded on the 2nd June. Below a depth of 100 m, O 2 concentrations varied between 6 and 10 µmol kg − 1 within the SLD. This value is just above the denitrifying threshold and remains constant until a depth of 200 m. Below this depth, it showed a minor increase and ranged between 16 µmol kg − 1 and 24 µmol kg − 1 until a depth of 500 m. The observed nitrite values are presented in Fig. 5 . Variability in O 2 within the SMC While TSW and Z1 were within the SLD in the western part of the study area, Z2 to Z3 were located within the core of the SMC (Fig. 1 ). Our hydrographic observations at Z2, Z3, and the TSE suggest the presence of high-salinity water in the Arabian Sea (Fig. 3 ). The oxygen concentrations within the top 100 m at Z2 were marginally greater than those observed within SLD, and a gradual increase was noted within Z3 and TSE. Within Z2, the hypoxic boundary (~ 63 µmol kg − 1 ) was located much deeper (108 m) than what was observed within SLD (~ 40–45 m) and thereafter moved close to 158 m (Fig. 3 ) within Z3; after this, there was strong depletion in O 2 concentrations, attaining a minimum of 13 µmol kg − 1 at a depth of 220 m (Fig. 3 ). However, at the TSE, upward propagation was noted, and the hypoxic boundary shoaled between 65 and 88 m (Fig. 3 ). BL erosion and ventilation of O 2 at the Time Series Station (TSE) The formation of BL and associated erosion were pronounced at this location (TSE; Fig. 2 ). The temporal variability of the thermohaline structure shows two freshening events (4 to 5th July and 10 to 15 July 2016) (Fig. 2 and Supplementary Fig. S2). During these events, the MLD was confined to the base of the low-salinity BoB surface waters, and the formation of the BL took place. Briefly, a three-layer structure was observed initially at TSE. At first, there was the presence of a fresh surface mixed layer with a thickness of 10 to 20 m, below which there was the evolution of the BL which had the same temperature but higher salinity, after this there was intrusion of high salinity core layer associated with the SMC. The barrier layer formed rapidly after the first frenshing event. The sea surface salinity on 4 July decreased by 0.3 in 2 h, decreasing from 34.3 psu at 0530 UTC to 33.9 psu by 0730 UTC. The mixed layer depth (MLD) decreased from 70 to 18 m, leading to the formation of a barrier layer that was approximately 50 m thick and had a temperature of 29°C. The upper layer warmed by approximately 0.3°C relative to the barrier layer below, and a diurnal cycle occurred. The second barrier layer formed gradually. Consequently, this new barrier layer formed at a thickness of approximately 40 m . Oxygen anomalies and fluxes in the surface layer and during deep mixing events in the southern Bay of Bengal The average O 2 anomalies and fluxes associated with the mixed layer are presented in Table 1 . The anomalies were strongest within the SLD, where the presence of hypoxic waters was observed at much shallower depths. The O 2 anomalies ranged between − 3.2 and − 76.3 µmol kg − 1 during this investigation. The derived mixed layer ventilation times for complete equilibration of upwelled waters were on the order of 3.7 days within the SLD and 7.2 within the SMC based on the observed gas transfer velocity and MLD. In general, negative fluxes representing the ingassing rates ranged between − 0.33 and − 9.43 µmol m − 2 sec − 1 within the SLD and SMC. The highest ingassing rate of 9.43 µmol m − 2 sec − 1 was associated with station Z1 (SLD), which had BoB OMZ waters very close to the surface. Table 1 Estimates show O 2 anomalies Δ [O 2 ] = ([O 2 ] m - [O 2 ] sat ]) and expected fluxes of O 2 under shallow mixed layer condition and deep mixing events observed at Z1 and TSE. Erosion of barrier layer at TSE resulted in mixing of low O 2 SMC waters during the BoBBLE campaign. Note strong under saturation was observed below surface layer at all stations. Positive [Δ O 2 ] indicates that the ocean is supersaturated and will tend to outgas to the atmosphere and negative means under saturation and ocean will gain O 2 . Stn ID Lat [°N] Long [°E] MLD (m) Average Δ [O 2 ] (within MLD in µmol kg − 1 ) Outgassing/ Ingassing rates within MLD in (µmol m − 2 sec − 1 ) TSW 7.999 85.3013 30 − 6.7 − 0.69 Z1 7.999 86.0183 22 − 76.3 − 9.43 Z2 8.013 87.0071 18 − 26.9 − 2.79 Z3 8.007 88.0018 24 − 3.2 − 0.33 TSE (During BL erosion) 8.007 89.0006 60 − 6.9 − 0.71 TSE (Without BL) 8.007 89.0006 18 + 10.9 + 1.13 Discussion The atmospheric conditions observed during the BoBBLE campaign are described elsewhere 5 . Briefly, there was influence of intraseasonal variability in June 2016. The sampling locations in the southern BoB were under the influence of a convectively active phase of the boreal summer intraseasonal oscillation (BSISO), which propagated northward and was replaced by a convectively suppressed phase of the BSISO during July 2016 5 . Toward the end of the campaign, conditions returned to the convectively active phase, with the incursion of the next cycle of the BSISO. Hence, the main BoBBLE deployment sampled the transition between the end of one active BSISO event, the subsequent suppressed phase and the initiation of the active phase in the following BSISO event. Therefore, our observations captured in detail the warming of the ocean mixed layer and preconditioning of the atmosphere to convection 5 . During BoBBLE upwelled waters from the SLD and SMC were sampled to determine the influence of upwelling waters on the oxygen distribution in the southern BoB. While stations TSW and Z1 were within the SLD in the western part, Z2 to Z3 were located within the core of the SMC and TSE on the outer edge of the SMC. We show that the presence of SMCs alongside the upwelling waters of the SLD was episodically ventilated during the summer monsoon in the southern BoB. The water masses within TSW and Z1 had signatures of upwelling, with negative sea level anomalies observed within the SLD (Supplementary Fig. 1). At TSW and Z1, the hypoxic boundary (< 63 µmol kg − 1 ) shoaled just below the mixed layer (~ 35–40 m). The impingement of the oxygenated mixed layer during deepening within the SLD is shown in Fig. 6 . An increase of ~ 38 µmol kg − 1 was observed at 50 m after the first mixing event, with the hypoxic boundary shifting from 28 m to 50 m, thereby increasing the concentration of O 2 at the top of the thermocline. Based on the calculations the relative increase in oxygen concentration was found to be ~ 0.34 µmol kg − 1 for one mixing event (assuming a mixing depth 50 m) within the SLD. This result is comparable to the eddy-induced oxygen enrichment observed within the northern BoB 32, 20 . Within the SLD no secondary nitrite maxima were observed in our dataset, suggesting the absence of denitrification (Fig. 5 ). This result is consistent with the O 2 concentrations observed within the OMZ (Fig. 3 ). In general, the water columns at TSW and Z1 were less oxygenated than those at stations Z2, Z3 and TSE (Fig. 3 ),, which were under the influence of SMC. During the investigation period, the SMC was fully developed with near-surface speeds of 0.5 –1 ms − 1 29 (Fig. 1 ). The most common feature here was the presence of a high-salinity core (Fig. 2 & Supplementary Fig. 2). The subsurface high-salinity core is the expression of ASHSW transported by the subsurface branch of the SMC with a salinity greater than 35 psu. Our observations from the southern BoB show strong shoaling of hypoxic waters very close to the surface. For example, it was close to 40 m within the SLD and varied between 65 and 80 m within the SMC. This presumably interchanged with the surface mixed layer during deep mixing events (Fig. 6 ). As the spread of high-salinity water along the SMC weakens vertical stratification, which in turn causes BL erosion and subsequent ventilation 30 ; therefore, the exchange of low-O 2 subsurface waters with O 2− rich surface waters is expected. Moreover, we did not observe any secondary nitrite maxima at any of the stations investigated, which possibly supports the hypothesis that periodic oxygen enrichment in subsurface waters keeps the bay above the denitrifying threshold. The mixed layer deepening and erosion of the barrier has been described in detail in George et al. (2019) 30 . After the first freshening event, the mixed layer deepened on 6 July in response to a salinization event that occurred between 6 and 9 July (Supplementary Fig. 2). During this event, the surface salinity increased from 33.84 to 34.35 psu. This led to mixed layer deepening and erosion of the barrier layer (Fig. 2 and Supplementary Figs. 2 and 3). In contrast, when a barrier layer (BL) was present, the surface mixed layer was shallow and less saline. These processes occurred episodically during our investigation, where O 2 -depleted subsurface waters mixed with the surface layer, initiating a possible mixing process. The air-sea exchange anomalies are presented in Table 1 . In general, the thickness of the BL layer ranged between 40 and 50 m and was formed due to the advection of low-salinity (33.35–33.8 psu) water at the TSE. Due to this BL, the surface mixed layer remained confined to the upper 10 to 20 m, whereas the isothermal layer penetrated until 60 m (see Fig. 2 b). The salinity budget presented in George et al. (2019) 30 showed that during BL erosion, advection brought high-salinity surface waters (~ 34.5 psu) with weaker stratification to the time series location and replaced the three-layer structure with a deep mixed layer (~ 60–70 m). The resulting weakened stratification at the time series location led to the mixing of low O 2 subsurface waters. The changes in O 2 concentrations with depth after BL erosion are shown in Fig. 6 . Once the barrier layer was eroded, the O 2 concentration at 60 m increased to ~ 135 µmol kg − 1 from earlier observations of ~ 63 µmol kg − 1 (Fig. 3 ), suggesting O 2 intrusion from the upper layers. The hypoxic boundary shifted from 60 m to 125 m after BL erosion. Further we also calculated how much O 2 can be supplied to the low-oxygen waters below due to BL erosion, as observed in the TSE. Our calculations suggest that the relative increase in oxygen content due to BL erosion considering a 4-day mixing event within an SMC for a 1° × 1° grid can reach ~ 100 µmol kg − 1 . While the concentration of O 2 in surface waters is an end product of complex interactions of O 2 produced via primary production and lost via respiration, ventilation through atmospheric exchange, and advection by currents, a decrease in O 2 in subsurface waters is generally due to poor ventilation and associated heterotrophic processes 33, 34 . The deepening of the mixed layer between 2 and 4 July is consistent with our hypothesis, which suggests mixing of the low-O 2 subsurface waters with the oxygenated surface mixed layer. Large-scale analysis of oxygen data from Arabian Sea OMZ waters revealed that isopycnal resupply of O 2 was the dominant process only between 300 and 500 m, while at 200 to 250 m, O2 was the most likely diapycnal process due to eddies 35 . Therefore, the exchange between the SLD and SMC at greater depths cannot be ignored; however, we focused here on mixed layer deepening, which was restricted to ~ 70 m, and calculated the relative increase in oxygen concentration that can occur if the parcel of water is redistributed over a given area as detailed in method section. Our observations at TSE capture an important mechanism wherein low O 2 ASHSW is observed to be occasionally ventilated. The depth profile of salinity vs. O 2 also suggested the occasional presence of ASHSW waters at depths of 40 to 50 m, with O 2 concentrations between 80 and 100 µmol kg − 1 (Fig. 7 ). A potential temperature and salinity diagram also suggested the presence of Persian Gulf Water (PGW) and Red Sea Water (RSW), which were recently detected in the Bay of Bengal 36, 37 . The detailed physical processes involved and their implications for subsurface oxygen distribution are further discussed in Sheehan et al., 2020 37 . Several authors have reported that SLDs form during summer monsoons, and SMC flow occurs until September in the southern BoB 4, 38 . BoBBLE observations were carried out between 24th June and 23rd July 2016. Although we captured two freshing events and two BL erosion events within the SMC between 24th June and 25th July, more such events are expected to occur until the SMC remains active, coinciding with the active summer monsoon phase. Furthermore, the relative increase in the oxygen content because of mixing events needs detailed investigation. We believe that episodic exposure to low-O 2 waters may result in quick equilibration with the saturated surface mixed layer and represent a potential pathway of O 2 enrichment in this region. As the expansion of O 2 minimum zones is already predicted by climate models 39, 40 , understanding the complex interplay that maintains this delicate balance between O 2 supply and its subsequent demand in the BoB becomes important, as this may be crucial for deciding on future deoxygenation scenarios in this region. Methods Physical setting during BoBBLE field expedition The sampling stations were along 8◦ N, extending from 85.3◦ E (hereafter referred to as TSW) to 89◦ E (hereafter referred to as TSE), with three additional stations in between, referred to as Z1, Z2, and Z3. The transects from TSW to Z3 were sampled between 24 July to 3 July, and the TSE was sampled on the 4th of July. Thereafter, time series observations were carried out at the TSE (8° N and 89° E) between the 4th and 15th of July 2016. The TSW station was located within the SLD (Fig. 1 ). Sea level anomalies (SLAs) from the region highlight the evolution of the SLD and are represented by negative SLAs (Supplementary Fig. 1). Z1 is on the outer edge of the SMC to the west, and station TSE is east of the SMC. Stations Z2 and Z3 were sampled within the core of the SMC (Fig. 1 ). The transect runs across the productive regions of the SLD and SMC and is further detailed in 31 Thushara and et al., 2019. Biogeochemical analysis A factory-calibrated SeaBird Electronics (SBE) 9/11 + Conductivity-Temperature-Depth profiler (CTD) was used to measure the vertical profiles and collected water samples at all the points marked in Fig. 1 . Nominally, the casts were collected to a depth of 1000 m. A total of 138 CTD casts were taken, including from a time series station (TSE) 5, 30 . In addition, hydrographic samples were collected at discrete depths to measure various biogeochemical parameters and to compute air-sea O 2 fluxes 41 . For this purpose, the CTD was attached to a rosette frame with 12 Niskin sampler bottles (10 L each) and sampled at fixed depths. Water samples for chemical measurements were collected from the Niskin bottles using Tygon tubing connected to the spout. Analysis of dissolved O 2 (DO) at a precision of ± 0.03 µM was based on the Carpenter (1965) modification of the traditional Winkler titration. These O 2 data sets were used to calibrate the CTD O 2 with the Winkler DO and CTD O 2 data, which yielded a coefficient of determination of 0.99 (n = 306, p < 0.01). The mixed layer depth (MLD) is calculated as the depth at which the density is equal to the sea surface density plus an increase in density equivalent to 0.8°C 30 . The isothermal layer is defined as the depth where the temperature is 0.8°C less than the SST, and the barrier layer is the layer between the base of the isothermal layer and the base of the mixed layer (BL). Apparent oxygen utilization (AOU) was calculated using the calibrated CTD data and the solubility equations of 42, 43 Garcia and Gordon (1992) and coefficients of Benson and Krause (1984). Calculation of the relative increase in dissolved O 2 that can occur within the BoB OMZ and ASHSW due to mixing An attempt is made to understand the magnitude of O 2, which can be supplied to the BoB OMZ due to observed deep mixing events within the SLD and BL erosion within the SMC. The ventilation rate of the mixed layer/equibriation time can be calculated by dividing the mixed layer thickness by the gas transfer velocity, as further detailed in 41 Roy et al., 2021. For this purpose, we considered the water mass within the SLD as a parcel of water with a width (W) of 100 km that contains the BoB surface waters and OMZ waters below. Therefore, this mass of water can be written as M parcel (1) = W× H × U × Δt × ρ, ( 1 ) where W (denotes the approximate width of the parcel within the SLD assumed here to be 100 km), H is the vertical extent of the deep mixing (in this case, 50 m as observed), U is the lateral speed of the parcel taken as ~ 0.5 m s − 1 15 within the SLD, Δt is the approximate time line of mixing, taken here as 1 day, and ρ is the density of the parcel of water (taken as 1022 kg m − 3 ). This gives M parcel (1) as = 2.20 × 10 14 kg. The additional oxygen added from the surface mixed layer to the OMZ due to the mixing event can be written as the product of the excess oxygen observed (38 µmol kg − 1 ) multiplied by M parcel (1), which equates to 8.36 × 10 15 µmol. However, the actual increase will depend on how much this parcel of water is redistributed. For a grid area between 7°N and 8°N and between 85°E and 86°E and a mixing depth of 50 m with a ρ of 1022 kg m − 3, Mparcel (2) equals 6.29 × 10 14 kg. Our calculations suggest that the relative increase in the oxygen concentration will reach ~ 0.34 µmol for one mixing event until 50 m within the SLD. Similarly, we calculated how much O 2 can be supplied to the low-oxygen waters below due to BL erosion within the SMC. For this purpose, we used an excess O 2 of 72 µmol kg − 1 , W (as 100 km), H (as 70 m), U (as 0.5 ms − 1 ), Δt (as 4 days), and ρ (1022 kg m − 3 ). This gives M parcel (3) as = 1.23 × 1015 kg. However, the additional oxygen added due to BL erosion to OMZ waters below can be written as the product of the excess oxygen observed (72 µmol kg − 1 ) multiplied by M parcel (1), which equates to 88.56 × 10 15 µmol. As above, the increase in oxygen concentrations will depend on the volume over which it is redistributed; if redistributed over a 7°N to 8°N and 87°E to 88°E and mixing depth of 70 m with a ρ of 1022 kg m − 3, Mparcel (4) equals 8.81 × 10 14 kg. Therefore, the actual increase in the oxygen content due to BL erosion considering a 4-day event within an SMC within a 1° × 1° grid was found to be ~ 100 µmol. Fluxes of O 2 due to upwelling/deep mixing events and erosion of the barrier layer (BL) during the BoBBLE campaign The net air-sea flux of O 2 is directly proportional to the partial pressure difference across the air-sea interface or its corresponding concentration anomaly. Therefore, Δ O 2 = [O 2 ]m – [O 2 ]s, where [O 2 ]m represents the measured concentration of O 2 in µmol kg − 1 and [O 2 ]s represents the saturated concentration of O 2 ; these values can be deduced as a product of the solubility coefficient (S A ) and partial pressure of O 2 in the atmosphere (p A ), where the units of the solubility coefficient are in mmol m − 3 atm − 1 . The solubility coefficients are calculated based on the in situ temperature and salinity at a given depth. Therefore, areal fluxes (µmol m − 2 sec − 1 ) across the water–atmosphere boundary can be calculated according to the expression: F = Kw Δ O 2 ( 1 ) where ‘Kw’ is the gas transfer velocity (cm hr − 1 ) and Δ O 2 is the difference between the measured O 2 and its solubility. The gas transfer velocity ‘kw’ (cm hr − 1 ) was calculated according to the equation given by Wanninkhof (1992), which depends on the wind speed measured at a height of 10 m (u 10) and Schmidt number (Sc) and is shown in Eq. 2 below. Kw = 0.31.U 2 . (Sc/600) −0.5 ( 2 ) Sc is the kinematic viscosity of water divided by the diffusion coefficient of O 2 . The coefficients for calculating the Schmidt number (below Eq. 3), which is a function of temperature and salinity (35 psu) for O 2, were taken from 44 Sarmiento and Gruber (2006), and the equation for gas transfer velocity from 45 Wanninkhof (1992) was used to estimate the fluxes during barrier layer erosion (deep mixing events). Sc = A - BT- CT 2 - DT 3 ( 3 ) where the in situ temperature (T) is in °C. The surface wind speed during the first half of the BoBBLE observation period was on the order of 8–12 ms − 1 . These wind speeds are typical for the southern BoB during the summer monsoon 5 . A positive value of flux in ( 1 ) means outgassing O 2 , whereas a negative flux denotes ingassing. Based on the wind speed, the estimated gas transfer velocity (K W ) ranged between 37.42 and 44.53 cm h − 1 for oxygen. Declarations Author Contribution RR, AS, PNV, JG, CLP: Conceptualization, Methodology, Formal analysis and investigation, original draft preparation, RR & PNV: Conceptualization, review and editing, Funding acquisition and supervision, PNV: Review and editing. Data availability The datasets used during the current study is available from the corresponding author on reasonable request. References Shetye, S. R., A. D. Gouveia, D. Shankar, S. S. C. Shenoi, P. N. Vinayachandran, D. Sundar, G. S. 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Naqvi, S.W.A., Naik, H., Pratihary, A., D'souza, W., Narvekar, P.V., Jayakumar, D.A., Devol, A.H., Yoshinari, T. and Saino, T., 2006. Coastal versus open-ocean denitrification in the Arabian Sea. Biogeosciences, 3 (4), pp.621–633. Naqvi, S.W.A., Moffett, J.W., Gauns, M.U., Narvekar, P.V., Pratihary, A.K., Naik, H., Shenoy, D.M., Jayakumar, D.A., Goepfert, T.J., Patra, P.K. and Al-Azri, A., 2010. The Arabian Sea as a high-nutrient, low-chlorophyll region during the late Southwest Monsoon. Biogeosciences, 7 (7), pp.2091–2100. Ittekkot, V., Nair, R.R., Honjo, S., Ramaswamy, V., Bartsch, M., Manganini, S. and Desai, B.N., 1991. Enhanced particle fluxes in Bay of Bengal induced by injection of fresh water. Nature, 351 (6325), pp.385–387. Prasanna Kumar, S., Nuncio, M., Narvekar, J., Kumar, A., Sardesai, D.S., De Souza, S.N., Gauns, M., Ramaiah, N. and Madhupratap, M., 2004. Are eddies nature's trigger to enhance biological productivity in the Bay of Bengal?. Geophysical Research Letters, 31 (7). Narvekar, J. and Kumar, S.P., 2006. Seasonal variability of the mixed layer in the central Bay of Bengal and associated changes in nutrients and chlorophyll. Deep Sea Research Part I: Oceanographic Research Papers, 53 (5), pp.820–835. Bristow, L.A., Callbeck, C.M., Larsen, M., Altabet, M.A., Dekaezemacker, J., Forth, M., Gauns, M., Glud, R.N., Kuypers, M.M., Lavik, G. and Milucka, J., 2017. N 2 production rates limited by nitrite availability in the Bay of Bengal O2 minimum zone. Nature Geoscience, 10 (1), pp.24–29. Sarma, V.V.S.S. and Udaya Bhaskar, T.V.S., 2018. Ventilation of O 2 to O 2 minimum zone due to anticyclonic eddies in the Bay of Bengal. Journal of Geophysical Research: Biogeosciences, 123 (7), pp.2145–2153. Johnson, K.S., Riser, S.C. and Ravichandran, M., 2019. O 2 variability controls denitrification in the bay of Bengal O 2 minimum zone. Geophysical Research Letters, 46 (2), pp.804–811. Jain, Vineet, D. Shankar, P. N. Vinayachandran, A. Mukherjee, and P. Amol. "Role of ocean dynamics in the evolution of mixed-layer temperature in the Bay of Bengal during the summer monsoon." Ocean Modelling 168 (2021): 101895. Sheehan, P.M., Webber, B.G., Sanchez-Franks, A., Matthews, A.J., Heywood, K.J. and Vinayachandran, P.N., 2020. Injection of O2ated Persian Gulf Water into the southern Bay of Bengal. Geophysical Research Letters , 47 (14), p.e2020GL087773. McCreary Jr, J.P., Yu, Z., Hood, R.R., Vinaychandran, P.N., Furue, R., Ishida, A. and Richards, K.J., 2013. Dynamics of the Indian-Ocean O2 minimum zones. Progress in Oceanography, 112 , pp.15–37. Vinayachandran, P. N., and Toshio Yamagata. "Monsoon response of the sea around Sri Lanka: generation of thermal domesand anticyclonic vortices." Journal of Physical Oceanography 28, no. 10 (1998): 1946–1960. Vinayachandran, P. N., P. Chauhan, M. Mohan, and S. Nayak. "Biological response of the sea around Sri Lanka to summer monsoon." Geophysical Research Letters 31, no. 1 (2004). Vos, A.D., Pattiaratchi, C.B. and Wijeratne, E.M.S., 2014. Surface circulation and upwelling patterns around Sri Lanka. Biogeosciences, 11 (20), pp.5909–5930. Murty, V.S.N., Sarma, Y.V.B., Rao, D.P. and Murty, C.S., 1992. Water characteristics, mixing and circulation in the Bay of Bengal during southwest monsoon. Journal of Marine Research, 50 (2), pp.207–228. Webber, B.G., Matthews, A.J., Vinayachandran, P.N., Neema, C.P., Sanchez-Franks, A., Vijith, V., Amol, P. and Baranowski, D.B., 2018. The dynamics of the Southwest Monsoon current in 2016 from high-resolution in situ observations and models. Journal of Physical Oceanography, 48 (10), pp.2259–2282. George, Jenson V., P. N. Vinayachandran, V. Vijith, V. Thushara, Anoop A. Nayak, Shrikant M. Pargaonkar, P. Amol, K. Vijaykumar, and Adrian J. Matthews. "Mechanisms of barrier layer formation and erosion from in situ observations in the Bay of Bengal." Journal of Physical Oceanography 49, no. 5 (2019): 1183–1200. Thushara, V., Vinayachandran, P.N.M., Matthews, A.J., Webber, B.G. and Queste, B.Y., 2019. Vertical distribution of chlorophyll in dynamically distinct regions of the southern Bay of Bengal. Biogeosciences, 16 (7), pp.1447–1468. Sarma, V.V.S.S., Jagadeesan, L., Dalabehera, H.B., Rao, D.N., Kumar, G.S., Durgadevi, D.S., Yadav, K., Behera, S. and Priya, M.M.R., 2018. Role of eddies on intensity of O2 minimum zone in the Bay of Bengal. Continental Shelf Research, 168 , pp.48–53. Wyrtki, Klaus. "The oxygen minima in relation to ocean circulation." In Deep Sea research and oceanographic abstracts , vol. 9, no. 1–2, pp. 11–23. Elsevier, 1962. Loescher, C.R., Bange, H.W., Schmitz, R.A., Callbeck, C.M., Engel, A., Hauss, H., Kanzow, T., Kiko, R., Lavik, G., Loginova, A. and Melzner, F., 2016. Water column biogeochemistry of oxygen minimum zones in the eastern tropical North Atlantic and eastern tropical South Pacific oceans. Biogeosciences, 13 (12), pp.3585–3606. Banse, K., Naqvi, S.W.A., Narvekar, P.V., Postel, J.R. and Jayakumar, D.A., 2014. Oxygen minimum zone of the open Arabian Sea: variability of oxygen and nitrite from daily to decadal timescales. Biogeosciences, 11 (8), pp.2237–2261. Kumar, S. Prasanna, and T. G. Prasad. "Formation and spreading of Arabian Sea high-salinity water mass." Journal of Geophysical Research: Oceans 104, no. C1 (1999): 1455–1464. Sheehan, Peter MF, Benjamin GM Webber, Alejandra Sanchez-Franks, Adrian J. Matthews, Karen J. Heywood, and P. N. Vinayachandran. "Injection of oxygenated Persian Gulf Water into the southern Bay of Bengal." Geophysical Research Letters 47, no. 14 (2020): e2020GL087773. Burns, Jessica M., Bulusu Subrahmanyam, and V. S. N. Murty. "On the dynamics of the S ri L anka D ome in the B ay of B engal." Journal of Geophysical Research: Oceans 122, no. 9 (2017): 7737–7750. Stramma, L., Prince, E.D., Schmidtko, S., Luo, J., Hoolihan, J.P., Visbeck, M., Wallace, D.W., Brandt, P. and Körtzinger, A., 2012. Expansion of oxygen minimum zones may reduce available habitat for tropical pelagic fishes. Nature Climate Change, 2 (1), pp.33–37. Bopp, L., Le Quéré, C., Heimann, M., Manning, A.C. and Monfray, P., 2002. Climate-induced oceanic O2 fluxes: Implications for the contemporary carbon budget. Global Biogeochemical Cycles, 16 (2), pp.6 – 1. Roy, R., Vinayachandran, P.N., Sarkar, A., George, J., Parida, C., Lotliker, A., Prakash, S. and Choudhury, S.B., 2021. Southern Bay of Bengal: A possible hotspot for CO 2 emission during the summer monsoon. Progress in Oceanography , 197 , p.102638. Garcia, H.E. and Gordon, L.I., 1992. O2 solubility in seawater: Better fitting equations. Limnology and oceanography, 37 (6), pp.1307–1312. Benson, B.B. and Krause Jr, D., 1984. The concentration and isotopic fractionation of O2 dissolved in freshwater and seawater in equilibrium with the atmosphere 1. Limnology and oceanography, 29 (3), pp.620–632. Wanninkhof, R., 1992. Relationship between wind speed and gas exchange over the ocean. Journal of Geophysical Research: Oceans, 97 (C5), pp.7373–7382. Sarmiento, J.L. and Gruber, N., 2006. Ocean biogeochemical dynamics . Princeton University Press. Additional Declarations No competing interests reported. Supplementary Files SF1.docx Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-3790094","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":264545525,"identity":"3f21958f-7150-4d04-ab5a-c3fae9de37cd","order_by":0,"name":"Rajdeep Roy","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABCUlEQVRIiWNgGAWjYHCCBAkIzXzgQMIPGwYGCeK1sCU++NiTRpQWmBoeY8MZbIcJazFvP/DwxoeKO/n8Eglm0jw85xP7ZzcffMBQYxONS4vMmYRkyxlnnlnOnJGQJs1jcTtxxp1jyQYMx9JyG3A6CqiSt+2wgcGNhGNAW24nNtzIMZNgbDiMWwv/gzTpv/9AWhLbpHnYziXOJ6hFAmgLUAFQSzIz0PsHEjcQ1vIg2bLn2DMDyZ5njMBATjbeeCMt2SABn1/4cxJv/Ki5Y8DPnv8BGJV2svNuJB988KHGBqcWYHQkAIkDDAwCCWCuI1hlAk7lIMB+AKKF/wCYa49X8SgYBaNgFIxIAAA7P2SohNlxwwAAAABJRU5ErkJggg==","orcid":"","institution":"National Remote Sensing Centre","correspondingAuthor":true,"prefix":"","firstName":"Rajdeep","middleName":"","lastName":"Roy","suffix":""},{"id":264545526,"identity":"0b7ae400-96e5-4ba2-a705-6d5056f40187","order_by":1,"name":"P. N. Vinayachandran","email":"","orcid":"","institution":"Indian Institute of Science","correspondingAuthor":false,"prefix":"","firstName":"P.","middleName":"N.","lastName":"Vinayachandran","suffix":""},{"id":264545527,"identity":"6a17ea76-b966-4890-81ef-4a2625b1541a","order_by":2,"name":"Jenson George","email":"","orcid":"","institution":"National Centre for Antarctic and Ocean Research","correspondingAuthor":false,"prefix":"","firstName":"Jenson","middleName":"","lastName":"George","suffix":""},{"id":264545528,"identity":"fa8895cd-0451-4375-8527-0fd34145fbb9","order_by":3,"name":"Amit Sarkar","email":"","orcid":"","institution":"Kuwait Institute of Scientific Research","correspondingAuthor":false,"prefix":"","firstName":"Amit","middleName":"","lastName":"Sarkar","suffix":""},{"id":264545529,"identity":"55556490-d588-4efe-8ebe-e1eb4f31436c","order_by":4,"name":"Chandanlal Parida","email":"","orcid":"","institution":"Indian Institute of Science","correspondingAuthor":false,"prefix":"","firstName":"Chandanlal","middleName":"","lastName":"Parida","suffix":""}],"badges":[],"createdAt":"2023-12-22 05:29:47","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-3790094/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3790094/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":49018678,"identity":"32b2b7d0-48a9-4f9f-998f-8b398b3a497f","added_by":"auto","created_at":"2024-01-01 06:45:40","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":638051,"visible":true,"origin":"","legend":"\u003cp\u003eMap of the study area and location of observations (pink star). Shading shows bottom topography (m) with its scale shown to the right of the panel. Vectors are surface currents (m/s) averaged over the period 27 June -- 02 July 2016, based on \u0026nbsp;\u0026nbsp;OSCAR Ocean Surface Current Analysis Real-time, Lagerloefet al. (2002) data with the scale vector shown at the top of the panel. Pink contours are sea surface salinity averaged over the period 25 June -- 4 July 2016, based on SMAP (Soil Moisture Active Passive, Entekhabi et al. (2010)) data. The location of observation (pink star) are TSW (8◦ N, 85.3◦ E), Z1 (8◦ N, 86.0◦ E), Z2 (8◦ N, 87.0◦ E) Z3 (8◦ N, 88.0◦ E) and TSE (8◦ N, 89.0◦ E). The main branches of the SMC is shown by the red arrow. Stations under the influence of SLD and SMC are marked in yellow.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-3790094/v1/6ba4531e40d64d7c92536a94.png"},{"id":49018890,"identity":"ac26d6bf-3d0d-481e-874c-ff0f76d92071","added_by":"auto","created_at":"2024-01-01 07:01:40","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":683642,"visible":true,"origin":"","legend":"\u003cp\u003eDuring BoBBLE, a section along 8°N (from TSW to TSE, shown in Figure 1) was sampled from 28 June to 4 April and then a time series at TSE (89 °E, 8°N) until 15 July. a) Upper panel shows the vertical structure of the salinity (shading with the scale shown to the right of the panel) observed during the BoBBLE campaign. The 35 psu isohaline is additionally highlighted by contours. The mixed layer and isothermal layer are shown by the purple and cyan lines, respectively. b) Lower panel shows the temporal variability of the thermal structure observed during this investigation. The changes in the mixed layer and isothermal layer are shown by purple and the cyan lines, respectively. \u003cstrong\u003eNote:\u003c/strong\u003eThe 10-day time series at TSE captured two freshening events, one during 4–6 July and the other during 8–9 July. During the first event the mixed layer depth (MLD) decreased from 70 to 18 m, leading to the formation of a barrier layer that was about 50 m thick and had a temperature of 29°C. (Figure reproduced from Roy et al., 2021 with permission from Elsevier).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-3790094/v1/5a338e3f90fb2e3a2364a278.png"},{"id":49018677,"identity":"42638be6-70dc-45bc-91f1-2127c21bb791","added_by":"auto","created_at":"2024-01-01 06:45:40","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":140895,"visible":true,"origin":"","legend":"\u003cp\u003eDepth profiles of a) Oxygen (black); b) salinity (blue); c) temperature (red) from various stations sampled during BoBBLE expedition. Note the depth profiles covers unique biogeochemical signatures of various water mass encountered within the SLD and SMC. Note within SLD (Z1 and Z2) sharp decrease in oxygen concentrations are observed within top 100 m.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-3790094/v1/76c4d30b059907fb4892ea94.png"},{"id":49018683,"identity":"6bc2669b-8e0f-422c-8ca6-1ab636047481","added_by":"auto","created_at":"2024-01-01 06:45:40","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":817300,"visible":true,"origin":"","legend":"\u003cp\u003ea) Shows the depth profile of oxygen along the stations TSW to TSE in (µmol kg\u003csup\u003e-1\u003c/sup\u003e\u003cstrong\u003e)\u003c/strong\u003e. b) Corrosponding plot showing the changes in the AOU (µmol kg\u003csup\u003e-1\u003c/sup\u003e) for the same locations. Refer Fig 2b for stations locations along the transcet.\u0026nbsp; The 63 µmol kg\u003csup\u003e-1\u003c/sup\u003e\u003csup\u003e\u003cstrong\u003e \u003c/strong\u003e\u003c/sup\u003econtour\u003csup\u003e\u003cstrong\u003e \u003c/strong\u003e\u003c/sup\u003eis shown in black demarcating the hypoxic boundary Mixed layer variability is shown in pink and distribution of the isothermal layer is shown in cyan (same as figure 2). Note that within the SLD, the hypoxic boundary is observerd very close to 45 m thereby coinciding with the MLD and isothermal layer. Further, AOU below the thermocline shows a steep decrease from to 200 to 100 µmol kg\u003csup\u003e-1 \u003c/sup\u003eat all stations.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-3790094/v1/1678bc2a01e7357a187c1c0f.png"},{"id":49018764,"identity":"d5a203dd-4541-47f1-8690-f472d31e874e","added_by":"auto","created_at":"2024-01-01 06:53:40","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":79030,"visible":true,"origin":"","legend":"\u003cp\u003eShows the depth profiles of nitrite (µmol kg\u003csup\u003e-1\u003c/sup\u003e) within a) SLD ( TSE and Z1); b) SMC (Z2 and Z3); c) at TSE (inside the high salinity core)\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-3790094/v1/e96e8b6da61a2c19ed643610.png"},{"id":49018684,"identity":"c7f8fb93-59d9-4fb7-b8ad-ccf20cb7ed0d","added_by":"auto","created_at":"2024-01-01 06:45:40","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":201762,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of water column depth profiles of O\u003csub\u003e2\u003c/sub\u003e. a) within the SLD after the increase in MLD from 30 to 45 m following the deep mixing event; b) observed O\u003csub\u003e2\u003c/sub\u003e concentrations before (blue) and after (red) the BL erosion,\u0026nbsp; at TSE. The dashed vertical line represents the 63 µmol kg\u003csup\u003e-1\u003c/sup\u003e hypoxic boundary. The mixing events capture penetration of relatively oxygenated mixed layer to deeper depths.\u003c/p\u003e","description":"","filename":"6.png","url":"https://assets-eu.researchsquare.com/files/rs-3790094/v1/3a22ca25c2ded5aa6e21bf08.png"},{"id":49018681,"identity":"f45f95c8-be8b-4b01-bd9b-cc047ac23ad7","added_by":"auto","created_at":"2024-01-01 06:45:40","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":367029,"visible":true,"origin":"","legend":"\u003cp\u003eTop panel shows the T – S characteristics and associated O\u003csub\u003e2 \u003c/sub\u003edistribution\u003cstrong\u003e \u003c/strong\u003eobserved during BoBBLE campaign. Varying degree of O\u003csub\u003e2\u003c/sub\u003e concentrations are observed associated with these water masses. a) Presence of ASHSW, Persian Gulf Water (PGW) and Red Sea Water (RSW) are demarcated by distinict T – S features. b) Bottom panel shows the depth distribution of the water masses. Note the presence of BoB OMZ waters are visible very close to the mixed layer (30 - 40 m within SLD). Further, few data suggest already ventilated Arabian Sea high salinity waters very close to 20 to 40 m,\u0026nbsp; presumably equilibrated after barrier layer erosion.\u003c/p\u003e","description":"","filename":"7.png","url":"https://assets-eu.researchsquare.com/files/rs-3790094/v1/f3f37738bea54782c3d3fd53.png"},{"id":56149033,"identity":"ce262b31-4c54-4003-aa5c-b34943abb4a1","added_by":"auto","created_at":"2024-05-09 06:40:47","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":3312986,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3790094/v1/4b417c92-7c63-4008-92b0-9033723aaa74.pdf"},{"id":49018679,"identity":"392bafde-aaa4-4cbc-8829-96548aab2c6e","added_by":"auto","created_at":"2024-01-01 06:45:40","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":1754957,"visible":true,"origin":"","legend":"","description":"","filename":"SF1.docx","url":"https://assets-eu.researchsquare.com/files/rs-3790094/v1/83be3c72c9ee2367c02d320b.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Subsurface mixing and ventilation of oxygen minimum zone waters in the southern Bay of Bengal during the summer monsoon","fulltext":[{"header":"Introduction","content":"\u003cp\u003eThe Bay of Bengal (BoB) is a semienclosed basin in the North Indian Ocean characterized by strong surface layer stratification \u003csup\u003e\u003cb\u003e1,2\u003c/b\u003e\u003c/sup\u003e. The strongest stratification covaries with the onset of the summer monsoon (May to September) in the northern BoB, where heavy rainfall and river influx result in a low-salinity surface layer \u003csup\u003e\u003cb\u003e3, 4, 5\u003c/b\u003e\u003c/sup\u003e. In contrast to the northern BoB, the southern BoB receives less rainfall; therefore, the surface salinity is greater \u003csup\u003e\u003cb\u003e6, 7, 8\u003c/b\u003e\u003c/sup\u003e. The existence of a strong perennial OMZ (oxygen minimum zone) with moderate depth oxygen depletion has been reported \u003csup\u003e\u003cb\u003e9, 10\u003c/b\u003e\u003c/sup\u003e. Despite this, no denitrification has been observed, which is generally characterized by the presence of secondary nitrite maxima typically associated with Winkler oxygen (O\u003csub\u003e2\u003c/sub\u003e) close to ~\u0026thinsp;4 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e in the Arabian Sea \u003csup\u003e\u003cb\u003e9, 11, 12, 13, 14, 15\u003c/b\u003e\u003c/sup\u003e. Within the OMZ core, O\u003csub\u003e2\u003c/sub\u003e concentrations can range between \u0026lt;\u0026thinsp;63 and \u0026lt;\u0026thinsp;4 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1,\u003c/sup\u003e and such low O\u003csub\u003e2\u003c/sub\u003e levels are particularly harmful to marine \u003csup\u003e\u003cb\u003e14\u003c/b\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe low oxygen content at mid-depth in the Bay of Bengal can be attributed to primary production in surface waters along with the rapid sinking of organic matter and moderate bacterial respiration rates \u003csup\u003e\u003cb\u003e12, 16, 17, 18\u003c/b\u003e\u003c/sup\u003e. Recent observations highlighted very low levels of O\u003csub\u003e2\u003c/sub\u003e in the BoB and OMZ, and a further decrease in O2 may lead to an anammox process \u003csup\u003e\u003cb\u003e19\u003c/b\u003e\u003c/sup\u003e. These authors reported low but significant nitrogen loss in the Bay of Bengal. However, this hypothesis was recently contested by who reported episodic O\u003csub\u003e2\u003c/sub\u003e injection into the BoB OMZ due to the presence of eddies \u003csup\u003e\u003cb\u003e20\u003c/b\u003e\u003c/sup\u003e. These authors showed that even though the western boundary upwelling system in the BoB is weak anticyclonic eddies that form in the east supply O\u003csub\u003e2\u003c/sub\u003e-rich waters to the OMZ as it moves toward the western BoB \u003csup\u003e\u003cb\u003e20\u003c/b\u003e\u003c/sup\u003e. Based on the mean lifetime of the anticyclonic eddies, ventilation rates at 100\u0026ndash;300 m were estimated to be 0.06\u0026thinsp;\u0026plusmn;\u0026thinsp;0.019 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e day\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, which is 3 to 4 times greater than the bacterial respiration rate (0.019 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e day\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) \u003csup\u003e\u003cb\u003e20\u003c/b\u003e\u003c/sup\u003e. In addition, an exchange of waters with O\u003csub\u003e2\u003c/sub\u003e values between 5 and 10 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with the core of the BoB OMZ is recently reported \u003csup\u003e\u003cb\u003e21\u003c/b\u003e\u003c/sup\u003e. These authors highlighted that such processes prevent the BoB OMZ from becoming denitrifying. More recently, it has been shown that Persian Gulf Water brings O\u003csub\u003e2\u003c/sub\u003e-rich waters to the BoB and possibly plays an important role in keeping O\u003csub\u003e2\u003c/sub\u003e levels higher below which ecological functioning would be significantly affected \u003csup\u003e\u003cb\u003e22, 23\u003c/b\u003e\u003c/sup\u003e. Thus, new mechanisms have been identified recently that keep BoB O\u003csub\u003e2\u003c/sub\u003e levels just above the denitrification threshold; these mechanisms are not well understood and are not well represented in biogeochemical models \u003csup\u003e\u003cb\u003e24\u003c/b\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eDuring the summer monsoon, due to local wind forcings, the formation of a cold dome, the Sri Lankan dome (SLD), occurs in the southern BoB. The upward Ekman pumping induced by this cyclonic curl brings cooler water to the near-surface layers \u003csup\u003e\u003cb\u003e25\u003c/b\u003e\u003c/sup\u003e. The upwelling accompanying the SLD influences the local environment by modulating water column properties by cooling the sea surface temperature and enhancing biological production and air-sea interactions \u003csup\u003e\u003cb\u003e26, 27\u003c/b\u003e\u003c/sup\u003e. Further to the east of the SLD, the intrusion of the summer monsoon current (SMC) occurs during the same time, and the eastward flow in the North Indian Ocean during the summer monsoon is called the summer monsoon current \u003csup\u003e\u003cb\u003e3, 8\u003c/b\u003e\u003c/sup\u003e. The direction of the SMC is eastward to the south of India, and the SMC turns around Sri Lanka and enters the BoB, carrying high-salinity water from the Arabian Sea along its path \u003csup\u003e\u003cb\u003e4, 28, 29\u003c/b\u003e\u003c/sup\u003e. When lighter water from lower-salinity BoB water is encountered, the Arabian Sea water subducts beneath the latter \u003csup\u003e\u003cb\u003e6\u003c/b\u003e\u003c/sup\u003e. High-salinity (35\u0026ndash;35.6 psu) water intruded below the mixed layer to a maximum depth of approximately 200 m \u003csup\u003e\u003cb\u003e25, 30\u003c/b\u003e\u003c/sup\u003e. The nutrient-rich water carried by the SMC promotes an increase in surface chlorophyll throughout its path \u003csup\u003e\u003cb\u003e26, 31\u003c/b\u003e\u003c/sup\u003e. The biogeochemical manifestations of the upwelled waters associated with the SLD and its interaction with the SMC in the vicinity of the southern BoB are poorly understood. Taken together, these findings underscore that understanding the O\u003csub\u003e2\u003c/sub\u003e biogeochemical cycles in the BoB is particularly important, especially during the summer monsoon, as upwelling around the Sri Lankan Dome (SLD) coincides with the flow of Arabian Sea high-salinity water mass (ASHSW) in this region during which both are characterized by low O\u003csub\u003e2\u003c/sub\u003e content \u003csup\u003e\u003cb\u003e4, 5\u003c/b\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe data presented here were collected during the Bay of Bengal Boundary Layer Experiment (BoBBLE) field program in the southern BoB \u003csup\u003e\u003cb\u003e5\u003c/b\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) from 24 June to 24 July 2016, coinciding with the summer monsoon. The present study investigated the oxygen distribution associated with the upwelled waters of the SLD and SMC and discussed the impact of episodic ventilation of subsurface waters observed during the curise. We hypothesize that the episodic ventilation observed during this investigation may have led to O\u003csub\u003e2\u003c/sub\u003e disequilibrium/anomalies within the surface mixed layers and contributed to mid-depth oxygen enrichment. Our study illustrates a possible mechanism by which the BoB oxygen minimum zone may gain oxygen, which may counteract OMZ expansion, as predicted, and possibly maintain the oxygen levels of the southern BoB above the denitrifying threshold.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e \u003cb\u003eThermohaline structure and O\u003c/b\u003e \u003csub\u003e \u003cb\u003e2\u003c/b\u003e \u003c/sub\u003e \u003cb\u003ewithin SLD\u003c/b\u003e\u003c/p\u003e \u003cp\u003eAt the beginning of our observations at TSW and Z1 (28 to 30 June; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e), the surface mixed \u0026minus;\u0026thinsp;29. The water column distribution of O\u003csub\u003e2\u003c/sub\u003e up to a depth of 1050 m is illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e, and the temporal variability is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The surface water was less saline, which created an oxygenated freshwater layer on the top. It is apparent that this layer remained well oxygenated with concentrations\u0026thinsp;\u0026gt;\u0026thinsp;190 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Within the SLD, oxygen concentrations as low as \u0026lt;\u0026thinsp;=\u0026thinsp;50 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e were observed at a shallow depth of 50 m and remained close to 30 m on the 30th June, after which a freshening event was recorded on the 2nd June. Below a depth of 100 m, O\u003csub\u003e2\u003c/sub\u003e concentrations varied between 6 and 10 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e within the SLD. This value is just above the denitrifying threshold and remains constant until a depth of 200 m. Below this depth, it showed a minor increase and ranged between 16 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and 24 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e until a depth of 500 m. The observed nitrite values are presented in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e.\u003c/p\u003e\u003cp\u003e \u003cb\u003eVariability in O\u003c/b\u003e\u003csub\u003e \u003cb\u003e2\u003c/b\u003e \u003c/sub\u003e \u003cb\u003ewithin the SMC\u003c/b\u003e\u003c/p\u003e \u003cp\u003eWhile TSW and Z1 were within the SLD in the western part of the study area, Z2 to Z3 were located within the core of the SMC (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Our hydrographic observations at Z2, Z3, and the TSE suggest the presence of high-salinity water in the Arabian Sea (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The oxygen concentrations within the top 100 m at Z2 were marginally greater than those observed within SLD, and a gradual increase was noted within Z3 and TSE. Within Z2, the hypoxic boundary (~\u0026thinsp;63 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) was located much deeper (108 m) than what was observed within SLD (~\u0026thinsp;40\u0026ndash;45 m) and thereafter moved close to 158 m (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e) within Z3; after this, there was strong depletion in O\u003csub\u003e2\u003c/sub\u003e concentrations, attaining a minimum of 13 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at a depth of 220 m (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). However, at the TSE, upward propagation was noted, and the hypoxic boundary shoaled between 65 and 88 m (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cb\u003eBL erosion and ventilation of O\u003c/b\u003e \u003csub\u003e \u003cb\u003e2\u003c/b\u003e \u003c/sub\u003e \u003cb\u003eat the Time Series Station (TSE)\u003c/b\u003e\u003c/p\u003e \u003cp\u003eThe formation of BL and associated erosion were pronounced at this location (TSE; Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). The temporal variability of the thermohaline structure shows two freshening events (4 to 5th July and 10 to 15 July 2016) (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Supplementary Fig. S2). During these events, the MLD was confined to the base of the low-salinity BoB surface waters, and the formation of the BL took place. Briefly, a three-layer structure was observed initially at TSE. At first, there was the presence of a fresh surface mixed layer with a thickness of 10 to 20 m, below which there was the evolution of the BL which had the same temperature but higher salinity, after this there was intrusion of high salinity core layer associated with the SMC. The barrier layer formed rapidly after the first frenshing event. The sea surface salinity on 4 July decreased by 0.3 in 2 h, decreasing from 34.3 psu at 0530 UTC to 33.9 psu by 0730 UTC. The mixed layer depth (MLD) decreased from 70 to 18 m, leading to the formation of a barrier layer that was approximately 50 m thick and had a temperature of 29\u0026deg;C. The upper layer warmed by approximately 0.3\u0026deg;C relative to the barrier layer below, and a diurnal cycle occurred. The second barrier layer formed gradually. Consequently, this new barrier layer formed at a thickness of approximately 40 m .\u003c/p\u003e \u003cp\u003e \u003cb\u003eOxygen anomalies and fluxes in the surface layer and during deep mixing events in the southern Bay of Bengal\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe average O\u003csub\u003e2\u003c/sub\u003e anomalies and fluxes associated with the mixed layer are presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. The anomalies were strongest within the SLD, where the presence of hypoxic waters was observed at much shallower depths. The O\u003csub\u003e2\u003c/sub\u003e anomalies ranged between \u0026minus;\u0026thinsp;3.2 and \u0026minus;\u0026thinsp;76.3 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e during this investigation. The derived mixed layer ventilation times for complete equilibration of upwelled waters were on the order of 3.7 days within the SLD and 7.2 within the SMC based on the observed gas transfer velocity and MLD. In general, negative fluxes representing the ingassing rates ranged between \u0026minus;\u0026thinsp;0.33 and \u0026minus;\u0026thinsp;9.43 \u0026micro;mol m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e sec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e within the SLD and SMC. The highest ingassing rate of 9.43 \u0026micro;mol m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e sec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was associated with station Z1 (SLD), which had BoB OMZ waters very close to the surface.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eEstimates show O\u003csub\u003e2\u003c/sub\u003e anomalies Δ [O\u003csub\u003e2\u003c/sub\u003e] = ([O\u003csub\u003e2\u003c/sub\u003e]\u003csub\u003em\u003c/sub\u003e - [O\u003csub\u003e2\u003c/sub\u003e]\u003csub\u003esat\u003c/sub\u003e]) and expected fluxes of O\u003csub\u003e2\u003c/sub\u003e under shallow mixed layer condition and deep mixing events observed at Z1 and TSE. Erosion of barrier layer at TSE resulted in mixing of low O\u003csub\u003e2\u003c/sub\u003e SMC waters during the BoBBLE campaign. Note strong under saturation was observed below surface layer at all stations. Positive [Δ O\u003csub\u003e2\u003c/sub\u003e] indicates that the ocean is supersaturated and will tend to outgas to the atmosphere and negative means under saturation and ocean will gain O\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"6\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eStn ID\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eLat [\u0026deg;N]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eLong [\u0026deg;E]\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eMLD\u003c/p\u003e \u003cp\u003e(m)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eAverage Δ [O\u003csub\u003e2\u003c/sub\u003e] (within MLD in \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e )\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eOutgassing/ Ingassing rates within MLD in (\u0026micro;mol m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e sec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTSW\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e7.999\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e85.3013\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e30\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026minus;\u0026thinsp;6.7\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u0026minus;\u0026thinsp;0.69\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZ1\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e7.999\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e86.0183\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e22\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026minus;\u0026thinsp;76.3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u0026minus;\u0026thinsp;9.43\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZ2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e8.013\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e87.0071\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026minus;\u0026thinsp;26.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u0026minus;\u0026thinsp;2.79\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eZ3\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e8.007\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e88.0018\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026minus;\u0026thinsp;3.2\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u0026minus;\u0026thinsp;0.33\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTSE (During BL erosion)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e8.007\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e89.0006\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e60\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e\u0026minus;\u0026thinsp;6.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e\u0026minus;\u0026thinsp;0.71\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eTSE (Without BL)\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e8.007\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e89.0006\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e18\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e+\u0026thinsp;10.9\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e+\u0026thinsp;1.13\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eThe atmospheric conditions observed during the BoBBLE campaign are described elsewhere\u003csup\u003e\u003cb\u003e5\u003c/b\u003e\u003c/sup\u003e. Briefly, there was influence of intraseasonal variability in June 2016. The sampling locations in the southern BoB were under the influence of a convectively active phase of the boreal summer intraseasonal oscillation (BSISO), which propagated northward and was replaced by a convectively suppressed phase of the BSISO during July 2016 \u003csup\u003e\u003cb\u003e5\u003c/b\u003e\u003c/sup\u003e. Toward the end of the campaign, conditions returned to the convectively active phase, with the incursion of the next cycle of the BSISO. Hence, the main BoBBLE deployment sampled the transition between the end of one active BSISO event, the subsequent suppressed phase and the initiation of the active phase in the following BSISO event. Therefore, our observations captured in detail the warming of the ocean mixed layer and preconditioning of the atmosphere to convection\u003csup\u003e\u003cb\u003e5\u003c/b\u003e\u003c/sup\u003e .\u003c/p\u003e \u003cp\u003eDuring BoBBLE upwelled waters from the SLD and SMC were sampled to determine the influence of upwelling waters on the oxygen distribution in the southern BoB. While stations TSW and Z1 were within the SLD in the western part, Z2 to Z3 were located within the core of the SMC and TSE on the outer edge of the SMC. We show that the presence of SMCs alongside the upwelling waters of the SLD was episodically ventilated during the summer monsoon in the southern BoB. The water masses within TSW and Z1 had signatures of upwelling, with negative sea level anomalies observed within the SLD (Supplementary Fig.\u0026nbsp;1). At TSW and Z1, the hypoxic boundary (\u0026lt;\u0026thinsp;63 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) shoaled just below the mixed layer (~\u0026thinsp;35\u0026ndash;40 m). The impingement of the oxygenated mixed layer during deepening within the SLD is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. An increase of ~\u0026thinsp;38 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e was observed at 50 m after the first mixing event, with the hypoxic boundary shifting from 28 m to 50 m, thereby increasing the concentration of O\u003csub\u003e2\u003c/sub\u003e at the top of the thermocline. Based on the calculations the relative increase in oxygen concentration was found to be ~\u0026thinsp;0.34 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for one mixing event (assuming a mixing depth 50 m) within the SLD. This result is comparable to the eddy-induced oxygen enrichment observed within the northern BoB \u003csup\u003e\u003cb\u003e32, 20\u003c/b\u003e\u003c/sup\u003e. Within the SLD no secondary nitrite maxima were observed in our dataset, suggesting the absence of denitrification (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). This result is consistent with the O\u003csub\u003e2\u003c/sub\u003e concentrations observed within the OMZ (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn general, the water columns at TSW and Z1 were less oxygenated than those at stations Z2, Z3 and TSE (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e),, which were under the influence of SMC. During the investigation period, the SMC was fully developed with near-surface speeds of 0.5\u003csup\u003e\u0026ndash;1\u003c/sup\u003e ms\u003csup\u003e\u0026minus;\u0026thinsp;1 \u003cb\u003e29\u003c/b\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The most common feature here was the presence of a high-salinity core (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e \u0026amp; Supplementary Fig.\u0026nbsp;2). The subsurface high-salinity core is the expression of ASHSW transported by the subsurface branch of the SMC with a salinity greater than 35 psu. Our observations from the southern BoB show strong shoaling of hypoxic waters very close to the surface. For example, it was close to 40 m within the SLD and varied between 65 and 80 m within the SMC. This presumably interchanged with the surface mixed layer during deep mixing events (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). As the spread of high-salinity water along the SMC weakens vertical stratification, which in turn causes BL erosion and subsequent ventilation \u003csup\u003e\u003cb\u003e30\u003c/b\u003e\u003c/sup\u003e; therefore, the exchange of low-O\u003csub\u003e2\u003c/sub\u003e subsurface waters with O\u003csub\u003e2\u0026minus;\u003c/sub\u003erich surface waters is expected. Moreover, we did not observe any secondary nitrite maxima at any of the stations investigated, which possibly supports the hypothesis that periodic oxygen enrichment in subsurface waters keeps the bay above the denitrifying threshold.\u003c/p\u003e \u003cp\u003eThe mixed layer deepening and erosion of the barrier has been described in detail in George et al. (2019) \u003csup\u003e\u003cb\u003e30\u003c/b\u003e\u003c/sup\u003e. After the first freshening event, the mixed layer deepened on 6 July in response to a salinization event that occurred between 6 and 9 July (Supplementary Fig.\u0026nbsp;2). During this event, the surface salinity increased from 33.84 to 34.35 psu. This led to mixed layer deepening and erosion of the barrier layer (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Supplementary Figs.\u0026nbsp;2 and 3). In contrast, when a barrier layer (BL) was present, the surface mixed layer was shallow and less saline. These processes occurred episodically during our investigation, where O\u003csub\u003e2\u003c/sub\u003e-depleted subsurface waters mixed with the surface layer, initiating a possible mixing process. The air-sea exchange anomalies are presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. In general, the thickness of the BL layer ranged between 40 and 50 m and was formed due to the advection of low-salinity (33.35\u0026ndash;33.8 psu) water at the TSE. Due to this BL, the surface mixed layer remained confined to the upper 10 to 20 m, whereas the isothermal layer penetrated until 60 m (see Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb). The salinity budget presented in George et al. (2019) \u003csup\u003e\u003cb\u003e30\u003c/b\u003e\u003c/sup\u003e showed that during BL erosion, advection brought high-salinity surface waters (~\u0026thinsp;34.5 psu) with weaker stratification to the time series location and replaced the three-layer structure with a deep mixed layer (~\u0026thinsp;60\u0026ndash;70 m). The resulting weakened stratification at the time series location led to the mixing of low O\u003csub\u003e2\u003c/sub\u003e subsurface waters. The changes in O\u003csub\u003e2\u003c/sub\u003e concentrations with depth after BL erosion are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e. Once the barrier layer was eroded, the O\u003csub\u003e2\u003c/sub\u003e concentration at 60 m increased to ~\u0026thinsp;135 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e from earlier observations of ~\u0026thinsp;63 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e), suggesting O\u003csub\u003e2\u003c/sub\u003e intrusion from the upper layers. The hypoxic boundary shifted from 60 m to 125 m after BL erosion. Further we also calculated how much O\u003csub\u003e2\u003c/sub\u003e can be supplied to the low-oxygen waters below due to BL erosion, as observed in the TSE. Our calculations suggest that the relative increase in oxygen content due to BL erosion considering a 4-day mixing event within an SMC for a 1\u0026deg; \u0026times; 1\u0026deg; grid can reach\u0026thinsp;~\u0026thinsp;100 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. While the concentration of O\u003csub\u003e2\u003c/sub\u003e in surface waters is an end product of complex interactions of O\u003csub\u003e2\u003c/sub\u003e produced via primary production and lost via respiration, ventilation through atmospheric exchange, and advection by currents, a decrease in O\u003csub\u003e2\u003c/sub\u003e in subsurface waters is generally due to poor ventilation and associated heterotrophic processes \u003csup\u003e\u003cb\u003e33, 34\u003c/b\u003e\u003c/sup\u003e. The deepening of the mixed layer between 2 and 4 July is consistent with our hypothesis, which suggests mixing of the low-O\u003csub\u003e2\u003c/sub\u003e subsurface waters with the oxygenated surface mixed layer.\u003c/p\u003e \u003cp\u003eLarge-scale analysis of oxygen data from Arabian Sea OMZ waters revealed that isopycnal resupply of O\u003csub\u003e2\u003c/sub\u003e was the dominant process only between 300 and 500 m, while at 200 to 250 m, O2 was the most likely diapycnal process due to eddies \u003csup\u003e\u003cb\u003e35\u003c/b\u003e\u003c/sup\u003e. Therefore, the exchange between the SLD and SMC at greater depths cannot be ignored; however, we focused here on mixed layer deepening, which was restricted to ~\u0026thinsp;70 m, and calculated the relative increase in oxygen concentration that can occur if the parcel of water is redistributed over a given area as detailed in method section. Our observations at TSE capture an important mechanism wherein low O\u003csub\u003e2\u003c/sub\u003e ASHSW is observed to be occasionally ventilated. The depth profile of salinity vs. O\u003csub\u003e2\u003c/sub\u003e also suggested the occasional presence of ASHSW waters at depths of 40 to 50 m, with O\u003csub\u003e2\u003c/sub\u003e concentrations between 80 and 100 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). A potential temperature and salinity diagram also suggested the presence of Persian Gulf Water (PGW) and Red Sea Water (RSW), which were recently detected in the Bay of Bengal \u003csup\u003e\u003cb\u003e36, 37\u003c/b\u003e\u003c/sup\u003e. The detailed physical processes involved and their implications for subsurface oxygen distribution are further discussed in Sheehan et al., 2020 \u003csup\u003e\u003cb\u003e37\u003c/b\u003e\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eSeveral authors have reported that SLDs form during summer monsoons, and SMC flow occurs until September in the southern BoB \u003csup\u003e\u003cb\u003e4, 38\u003c/b\u003e\u003c/sup\u003e. BoBBLE observations were carried out between 24th June and 23rd July 2016. Although we captured two freshing events and two BL erosion events within the SMC between 24th June and 25th July, more such events are expected to occur until the SMC remains active, coinciding with the active summer monsoon phase. Furthermore, the relative increase in the oxygen content because of mixing events needs detailed investigation. We believe that episodic exposure to low-O\u003csub\u003e2\u003c/sub\u003e waters may result in quick equilibration with the saturated surface mixed layer and represent a potential pathway of O\u003csub\u003e2\u003c/sub\u003e enrichment in this region. As the expansion of O\u003csub\u003e2\u003c/sub\u003e minimum zones is already predicted by climate models \u003csup\u003e\u003cb\u003e39, 40\u003c/b\u003e\u003c/sup\u003e, understanding the complex interplay that maintains this delicate balance between O\u003csub\u003e2\u003c/sub\u003e supply and its subsequent demand in the BoB becomes important, as this may be crucial for deciding on future deoxygenation scenarios in this region.\u003c/p\u003e "},{"header":"Methods","content":"\u003cp\u003e \u003cb\u003ePhysical setting during BoBBLE field expedition\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThe sampling stations were along 8◦ N, extending from 85.3◦ E (hereafter referred to as TSW) to 89◦ E (hereafter referred to as TSE), with three additional stations in between, referred to as Z1, Z2, and Z3. The transects from TSW to Z3 were sampled between 24 July to 3 July, and the TSE was sampled on the 4th of July. Thereafter, time series observations were carried out at the TSE (8\u0026deg; N and 89\u0026deg; E) between the 4th and 15th of July 2016. The TSW station was located within the SLD (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). Sea level anomalies (SLAs) from the region highlight the evolution of the SLD and are represented by negative SLAs (Supplementary Fig.\u0026nbsp;1). Z1 is on the outer edge of the SMC to the west, and station TSE is east of the SMC. Stations Z2 and Z3 were sampled within the core of the SMC (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e). The transect runs across the productive regions of the SLD and SMC and is further detailed in \u003csup\u003e\u003cb\u003e31\u003c/b\u003e\u003c/sup\u003e Thushara and et al., 2019.\u003c/p\u003e \u003cp\u003e \u003cb\u003eBiogeochemical analysis\u003c/b\u003e \u003c/p\u003e \u003cp\u003eA factory-calibrated SeaBird Electronics (SBE) 9/11\u0026thinsp;+\u0026thinsp;Conductivity-Temperature-Depth profiler (CTD) was used to measure the vertical profiles and collected water samples at all the points marked in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. Nominally, the casts were collected to a depth of 1000 m. A total of 138 CTD casts were taken, including from a time series station (TSE) \u003csup\u003e\u003cb\u003e5, 30\u003c/b\u003e\u003c/sup\u003e. In addition, hydrographic samples were collected at discrete depths to measure various biogeochemical parameters and to compute air-sea O\u003csub\u003e2\u003c/sub\u003e fluxes\u003csup\u003e\u003cb\u003e41\u003c/b\u003e\u003c/sup\u003e. For this purpose, the CTD was attached to a rosette frame with 12 Niskin sampler bottles (10 L each) and sampled at fixed depths. Water samples for chemical measurements were collected from the Niskin bottles using Tygon tubing connected to the spout. Analysis of dissolved O\u003csub\u003e2\u003c/sub\u003e (DO) at a precision of \u0026plusmn;\u0026thinsp;0.03 \u0026micro;M was based on the Carpenter (1965) modification of the traditional Winkler titration. These O\u003csub\u003e2\u003c/sub\u003e data sets were used to calibrate the CTD O\u003csub\u003e2\u003c/sub\u003e with the Winkler DO and CTD O\u003csub\u003e2\u003c/sub\u003e data, which yielded a coefficient of determination of 0.99 (n\u0026thinsp;=\u0026thinsp;306, p\u0026thinsp;\u0026lt;\u0026thinsp;0.01). The mixed layer depth (MLD) is calculated as the depth at which the density is equal to the sea surface density plus an increase in density equivalent to 0.8\u0026deg;C \u003csup\u003e30\u003c/sup\u003e. The isothermal layer is defined as the depth where the temperature is 0.8\u0026deg;C less than the SST, and the barrier layer is the layer between the base of the isothermal layer and the base of the mixed layer (BL). Apparent oxygen utilization (AOU) was calculated using the calibrated CTD data and the solubility equations of \u003csup\u003e\u003cb\u003e42, 43\u003c/b\u003e\u003c/sup\u003e Garcia and Gordon (1992) and coefficients of Benson and Krause (1984).\u003c/p\u003e \u003cp\u003e \u003cb\u003eCalculation of the relative increase in dissolved\u003c/b\u003e O\u003csub\u003e2\u003c/sub\u003e \u003cb\u003ethat can occur within the BoB OMZ and ASHSW due to mixing\u003c/b\u003e\u003c/p\u003e \u003cp\u003eAn attempt is made to understand the magnitude of O\u003csub\u003e2,\u003c/sub\u003e which can be supplied to the BoB OMZ due to observed deep mixing events within the SLD and BL erosion within the SMC. The ventilation rate of the mixed layer/equibriation time can be calculated by dividing the mixed layer thickness by the gas transfer velocity, as further detailed in \u003csup\u003e\u003cb\u003e41\u003c/b\u003e\u003c/sup\u003e Roy et al., 2021. For this purpose, we considered the water mass within the SLD as a parcel of water with a width (W) of 100 km that contains the BoB surface waters and OMZ waters below. Therefore, this mass of water can be written as\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003eM\u003csub\u003eparcel (1)\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;W\u0026times; H \u0026times; U\u0026thinsp;\u0026times;\u0026thinsp;Δt\u0026thinsp;\u0026times;\u0026thinsp;ρ, (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e)\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere W (denotes the approximate width of the parcel within the SLD assumed here to be 100 km), H is the vertical extent of the deep mixing (in this case, 50 m as observed), U is the lateral speed of the parcel taken as ~\u0026thinsp;0.5 m s\u003csup\u003e\u0026minus;\u0026thinsp;1 \u003cb\u003e15\u003c/b\u003e\u003c/sup\u003e within the SLD, Δt is the approximate time line of mixing, taken here as 1 day, and ρ is the density of the parcel of water (taken as 1022 kg m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e). This gives M\u003csub\u003eparcel (1)\u003c/sub\u003e as =\u0026thinsp;2.20 \u0026times; 10\u003csup\u003e14\u003c/sup\u003e kg. The additional oxygen added from the surface mixed layer to the OMZ due to the mixing event can be written as the product of the excess oxygen observed (38 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) multiplied by M\u003csub\u003eparcel (1),\u003c/sub\u003e which equates to 8.36 \u0026times; 10\u003csup\u003e15\u003c/sup\u003e \u0026micro;mol. However, the actual increase will depend on how much this parcel of water is redistributed. For a grid area between 7\u0026deg;N and 8\u0026deg;N and between 85\u0026deg;E and 86\u0026deg;E and a mixing depth of 50 m with a ρ of 1022 kg m\u003csup\u003e\u0026minus;\u0026thinsp;3,\u003c/sup\u003e Mparcel \u003csub\u003e(2)\u003c/sub\u003e equals 6.29 \u0026times; 10\u003csup\u003e14\u003c/sup\u003e kg. Our calculations suggest that the relative increase in the oxygen concentration will reach\u0026thinsp;~\u0026thinsp;0.34 \u0026micro;mol for one mixing event until 50 m within the SLD.\u003c/p\u003e \u003cp\u003eSimilarly, we calculated how much O\u003csub\u003e2\u003c/sub\u003e can be supplied to the low-oxygen waters below due to BL erosion within the SMC. For this purpose, we used an excess O\u003csub\u003e2\u003c/sub\u003e of 72 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, W (as 100 km), H (as 70 m), U (as 0.5 ms\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e), Δt (as 4 days), and ρ (1022 kg m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e). This gives M\u003csub\u003eparcel (3)\u003c/sub\u003e as =\u0026thinsp;1.23 \u0026times; 1015 kg. However, the additional oxygen added due to BL erosion to OMZ waters below can be written as the product of the excess oxygen observed (72 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) multiplied by M\u003csub\u003eparcel (1),\u003c/sub\u003e which equates to 88.56 \u0026times; 10\u003csup\u003e15\u003c/sup\u003e \u0026micro;mol. As above, the increase in oxygen concentrations will depend on the volume over which it is redistributed; if redistributed over a 7\u0026deg;N to 8\u0026deg;N and 87\u0026deg;E to 88\u0026deg;E and mixing depth of 70 m with a ρ of 1022 kg m\u003csup\u003e\u0026minus;\u0026thinsp;3,\u003c/sup\u003e Mparcel\u003csub\u003e(4)\u003c/sub\u003e equals 8.81 \u0026times; 10\u003csup\u003e14\u003c/sup\u003e kg. Therefore, the actual increase in the oxygen content due to BL erosion considering a 4-day event within an SMC within a 1\u0026deg; \u0026times; 1\u0026deg; grid was found to be ~\u0026thinsp;100 \u0026micro;mol.\u003c/p\u003e \u003cp\u003e \u003cb\u003eFluxes of O\u003c/b\u003e \u003csub\u003e \u003cb\u003e2\u003c/b\u003e \u003c/sub\u003e \u003cb\u003edue to upwelling/deep mixing events and erosion of the barrier layer (BL) during the BoBBLE campaign\u003c/b\u003e\u003c/p\u003e \u003cp\u003eThe net air-sea flux of O\u003csub\u003e2\u003c/sub\u003e is directly proportional to the partial pressure difference across the air-sea interface or its corresponding concentration anomaly. Therefore, Δ O\u003csub\u003e2\u003c/sub\u003e = [O\u003csub\u003e2\u003c/sub\u003e]m \u0026ndash; [O\u003csub\u003e2\u003c/sub\u003e]s, where [O\u003csub\u003e2\u003c/sub\u003e]m represents the measured concentration of O\u003csub\u003e2\u003c/sub\u003e in \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and [O\u003csub\u003e2\u003c/sub\u003e]s represents the saturated concentration of O\u003csub\u003e2\u003c/sub\u003e; these values can be deduced as a product of the solubility coefficient (S\u003csub\u003eA\u003c/sub\u003e) and partial pressure of O\u003csub\u003e2\u003c/sub\u003e in the atmosphere (p\u003csup\u003eA\u003c/sup\u003e), where the units of the solubility coefficient are in mmol m\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e atm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe solubility coefficients are calculated based on the in situ temperature and salinity at a given depth. Therefore, areal fluxes (\u0026micro;mol m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e sec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) across the water\u0026ndash;atmosphere boundary can be calculated according to the expression:\u003c/p\u003e \u003cp\u003eF\u0026thinsp;=\u0026thinsp;Kw Δ O\u003csub\u003e2\u003c/sub\u003e (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e)\u003c/p\u003e \u003cp\u003ewhere \u0026lsquo;Kw\u0026rsquo; is the gas transfer velocity (cm hr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and Δ O\u003csub\u003e2\u003c/sub\u003e is the difference between the measured O\u003csub\u003e2\u003c/sub\u003e and its solubility. The gas transfer velocity \u0026lsquo;kw\u0026rsquo; (cm hr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) was calculated according to the equation given by Wanninkhof (1992), which depends on the wind speed measured at a height of 10 m (u\u003csub\u003e10)\u003c/sub\u003e and Schmidt number (Sc) and is shown in Eq.\u0026nbsp;2 below.\u003c/p\u003e \u003cp\u003eKw\u0026thinsp;=\u0026thinsp;0.31.U\u003csup\u003e2\u003c/sup\u003e. (Sc/600)\u003csup\u003e\u0026minus;0.5\u003c/sup\u003e (\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e)\u003c/p\u003e \u003cp\u003eSc is the kinematic viscosity of water divided by the diffusion coefficient of O\u003csub\u003e2\u003c/sub\u003e. The coefficients for calculating the Schmidt number (below Eq.\u0026nbsp;3), which is a function of temperature and salinity (35 psu) for O\u003csub\u003e2,\u003c/sub\u003e were taken from \u003csup\u003e44\u003c/sup\u003e Sarmiento and Gruber (2006), and the equation for gas transfer velocity from \u003csup\u003e45\u003c/sup\u003e Wanninkhof (1992) was used to estimate the fluxes during barrier layer erosion (deep mixing events).\u003c/p\u003e \u003cp\u003eSc\u0026thinsp;=\u0026thinsp;A - BT- CT\u003csup\u003e2\u003c/sup\u003e - DT\u003csup\u003e3\u003c/sup\u003e (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e)\u003c/p\u003e \u003cp\u003ewhere the in situ temperature (T) is in \u0026deg;C. The surface wind speed during the first half of the BoBBLE observation period was on the order of 8\u0026ndash;12 ms\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. These wind speeds are typical for the southern BoB during the summer monsoon \u003csup\u003e\u003cb\u003e5\u003c/b\u003e\u003c/sup\u003e. A positive value of flux in (\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e) means outgassing O\u003csub\u003e2\u003c/sub\u003e, whereas a negative flux denotes ingassing. Based on the wind speed, the estimated gas transfer velocity (K\u003csub\u003eW\u003c/sub\u003e) ranged between 37.42 and 44.53 cm h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e for oxygen.\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eRR, AS, PNV, JG, CLP: Conceptualization, Methodology, Formal analysis and investigation, original draft preparation, RR \u0026amp; PNV: Conceptualization, review and editing, Funding acquisition and supervision, PNV: Review and editing.\u003c/p\u003e\u003ch2\u003eData availability\u003c/h2\u003e \u003cp\u003eThe datasets used during the current study is available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eShetye, S. R., A. D. Gouveia, D. Shankar, S. S. C. Shenoi, P. N. Vinayachandran, D. Sundar, G. S. Michael, and G. 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Princeton University Press.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-3790094/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3790094/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eDuring the summer monsoon, the local wind forcings around Sri Lanka causes the formation of a cold dome called the Sri Lanka Dome (SLD), which upwells subsurface waters. To the east of SLD, the summer monsoon current (SMC) flows into the Bay of Bengal (BoB), transporting high-salinity water from the Arabian Sea. We show that the SMC and the upwelled waters of the SLD are ventilated episodically during summer monsoon in the southern BoB, leading to a net exchange of low oxygen subsurface waters with saturated mixed layers. We observed presence of hypoxic boundary\u0026thinsp;\u0026lt;\u0026thinsp;63 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e very close to the surface. Within the SLD, it shoaled between 35 to 40 m, with oxygen values reaching as low as 6 \u0026micro;mol kg\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e at the bottom of the thermocline. Negative fluxes showing the ingassing rates ranged between \u0026minus;\u0026thinsp;0.33 and \u0026minus;\u0026thinsp;9.43 \u0026micro;mol m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e sec\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e within the SLD and SMC. We propose that the episodic ventilation seen during this investigation may lead to disequilibrium between mixed layer and below thereby contributing to mid-depth oxygen enrichment. This study possibly illustrates a pathway through which the oxygen minimum zone in BoB may be gaining oxygen, thereby preventing from becoming denitrifying.\u003c/p\u003e","manuscriptTitle":"Subsurface mixing and ventilation of oxygen minimum zone waters in the southern Bay of Bengal during the summer monsoon","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-01-01 06:45:36","doi":"10.21203/rs.3.rs-3790094/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"4ddc3667-5f8f-4001-990e-192326906960","owner":[],"postedDate":"January 1st, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[{"id":27879153,"name":"Earth and environmental sciences/Biogeochemistry"},{"id":27879154,"name":"Earth and environmental sciences/Climate sciences"},{"id":27879155,"name":"Earth and environmental sciences/Environmental sciences"},{"id":27879156,"name":"Earth and environmental sciences/Ocean sciences"}],"tags":[],"updatedAt":"2024-05-09T06:32:26+00:00","versionOfRecord":[],"versionCreatedAt":"2024-01-01 06:45:36","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-3790094","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3790094","identity":"rs-3790094","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}