Baobab isotope records and rainfall forcing in southwest Madagascar over the last 700 years

Sea Surface Temperature (SST, Ifaty) Oxygen isotopes of coral 1660-1994 334 Zinke et al., 2022 Southern Annual Mode Index (SAM) Mid-latitude to polar domain proxy records 1000-2007 1007 Abram et al., 2014 Pacific Decadal Oscillation (PDO) Tree rings 993-1996 1003 MacDonald & Case 2005 HadISST1.1 SST dataset (Index referring to IOD) Calculated anomalous SST gradient between the western equatorial Indian Ocean and the south- eastern equatorial Indian Ocean. Based on coral isotopes and Ca/Mg ratios. 1981-2010 29 Saji & Yamagata 2003 266 267 The identification of monotonic trends within the record was conducted using a non- 268 parametric Mann-Kendall trend test (Nasri & Modares, 2009; Pohlert, 2018) combined with a 269 Least Squares Regression to evaluate the rate of change in rainfall per year in the regional 270 records. In addition, gridded datasets downloaded from GPCC monthly total precipitation 271 from around Betioky at 2.5° x 2.5° resolution were compared to the composite record. For 272 correlations, significance at a level of 0.05 were accepted. .CC-BY 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted August 20, 2025. ; https://doi.org/10.1101/2025.08.15.670475doi: bioRxiv preprint 273 3. RESULTS 274 The calibrated radiocarbon dates for the four trees suggest that they grew over the last 700 years 275 (1302 – 2013 CE) (Table SI1). The most parsimonious age models, that reconcile the AMS dates 276 and stable carbon isotope records, are shown in Fig. 2. All the trees show linear growth over time 277 except for GTR, which demonstrated a hiatus from 1500 CE to 1700 CE. This is not uncommon 278 in baobabs (Patrut et al., 2017). The age model assigned most of the AMS ages within 1-sigma 279 error of 68%. 280 281 Fig. 2: Age-models for (A) DFL, (B) DFS, (C) GTR with the hiatus indicated in dashed line, and 282 (D) TSP, based on 52 radiocarbon dates. The horizontal lines are the 1-sigma calibration 283 intervals for the radiocarbon dates. The bold line represents the age model that best intercepts the 284 1-sigma calibration range for the radiocarbon dates 285 The corrected δ13C time series from the four trees range between -26.2‰ (1400 CE) and -24.5‰ 286 (1650 CE) with a mean of -25.3‰, and a variation of about 1.7‰ (Fig. 3). The trend analysis on 287 the composite δ13C records using the Mann-Kendall test shows that there is a marginal 288 decreasing trend of the isotope data over time (S=-5.07, p<0.01) with a difference of about - 289 0.7‰ between 1300 CE and 2013 CE. A least square linear regression is significant (F= 80.6, p 290 = 0.001) but with an R2 of 0.10. The change point analysis revealed a number of major shifts in 291 the mean values of the composite isotope data becoming either more positive or negative in the 292 time series (significant at 95%). These occur at approximately 1400 CE, 1480 CE, 1500 CE, 293 1630 CE, 1660 CE, 1820 CE, and 980 CE (Fig. 4). 294 295 Fig. 3: Corrected δ13C time series of four baobabs from southwest Madagascar with inverted y- 296 axis indicating drier (less negative isotope value) and wetter conditions (more negative isotope 297 value). 298 .CC-BY 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted August 20, 2025. ; https://doi.org/10.1101/2025.08.15.670475doi: bioRxiv preprint 299 Fig. 4: The composite record from four trees (black) with wetter periods (blue) and drier periods 300 (yellow). Error bars represent standard errors. 301 The isotope biweight mean composite and the GPCC monthly total precipitation both shows 302 similar trends over time. A linear regression of precipitation data from the GPCC monthly total 303 precipitation from 1900–present revealed a decreasing but not significant trend over time (β = - 304 0.67, p = 0.17), with year explaining only ~1.7% of the variance (R² = 0.017). Conversely, the 305 isotope data showed a small but significant increasing trend (β = +0.00112, p = 0.015) reflecting 306 decreasing rainfall, though the model explains less than 1% of variance (R² = 0.0099). These 307 results suggest subtle but differing temporal behaviours in the two proxies potentially associated 308 with the associated ages. Both records show a slight decrease in rainfall as reflected by the 309 precipitation value and the more positive isotope value around 1950 and then from 1970-1990 310 while more wetter periods are recorded around 1960 and 2000 (Fig. 5). 311 312 Fig. 5: Comparison of the baobab δ13C records with existing model datasets: Black indicates the 313 baobab δ13C composite records from 1900-2015 with linear regression indicated in grey. Blue 314 and dark blue shows GPCC total precipitation at 2.5° x 2.5° resolution from around Betioky 315 between 1900-2013 along with the associated linear model. 316 The correlation analysis of potential rainfall drivers in Southwest Madagascar has been 317 calculated (Table 3). The results diverse correlation values and significance (Table 3). There is a 318 negative correlation between SST and the isotope data an overall significant positive correlation 319 between SAM and the isotope records with recorded weak correlation since the 16th century. 320 Related to the Pacific Decadal Oscillation (PDO) reconstruction, no significant correlation has 321 been recorded during the entire period but during the LIA negative PDO anomalies are 322 associated with decreases in rainfall particularly between 1600 – 1750 CE (r=-0.30, p<0.001), 323 whereas in the record before and after this period, they are associated with increases in rainfall 324 (r=0.18, p=0.004 and r=0.18, p=0.001 respectively) (Fig. 7C). 325 .CC-BY 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted August 20, 2025. ; https://doi.org/10.1101/2025.08.15.670475doi: bioRxiv preprint 326 Table 3: Correlation between baobab isotope data and various environmental indices. 327 Pearson correlation coefficients (r) and associated p-values are shown. Sample size (n) indicates 328 the number of data points compared for each dataset. Dataset Compared with Baobab Isotope Data Time Period (AD) Sample Size (n) Pearson Correlation (r) p-value Sea Surface Temperature (SST, Ifaty) 1660–1994 334 –0.22* < 0.001* Southern Annual Mode Index (SAM) 1000–2007 1007 0.16* 0.05 HadISST1.1 SST dataset (IOD Index) 1981–2010 29 –0.06 > 0.05 329 Note: * indicates significance at α = 0.05.* 330 4. DISCUSSION 331 4.1. Rainfall record of southwest Madagascar for the last 700 years 332 The similarities between the δ13C records and the GPCC precipitation data meets the theoretical 333 expectation, and reaffirms the results obtained for baobab isotope controls on the African 334 mainland (Woodborne et al., 2015; 2016; Fig. 5). The baobab δ13C record can thus be interpreted 335 as a proxy for local effective rainfall in southwest Madagascar, reflecting decadal to centennial 336 variability in the last 700 years. Accordingly, lower δ13C values indicate wetter periods while 337 more positive isotope ratios correlate to drier conditions. 338 The chronology of the four baobabs ranges from 1300 CE until 2013 with a hiatus in the GTR 339 core from 1500 CE to 1700 CE. Punctuated growth models have been noted in other baobabs 340 (Patrut et al., 2017) and may arise because of attenuated growth rates over time, or possibly due 341 to lobate development of the stem resulting in the reallocation of resources to another part of the 342 tree. The GTR baobab was collected near the River Mangoky in southwest Madagascar where a 343 sediment core was taken from nearby Lake Tsizavatsy. The age-depth model of the Tsizavatsy 344 core shows a hiatus from 1400 to 1900, suggested to be associated with a regional drought event 345 (Razanatsoa et al., 2021). Although the other trees show positive isotope excursions (dry events) 346 during this period, there are oscillations of wetter conditions covered by the present records (Fig. .CC-BY 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted August 20, 2025. ; https://doi.org/10.1101/2025.08.15.670475doi: bioRxiv preprint 347 3). Notwithstanding the gap, the record from GTR prior to and after the hiatus was combined 348 with the composite record of all trees to provide a full rainfall proxy record for the last 700 years 349 for southwest Madagascar covering the Little Ice Age (LIA; Lechleitner et al., 2017; Putnam & 350 Broecker, 2017) and the Anthropocene. 351 Synchronicity in the baobab δ13C records from southwest Madagascar demonstrate regional 352 variability with a succession of wet and dry cycles on the scale of decades to centuries (Fig. 4). 353 The rainfall in the region was suggested to be variable with drought events recorded in 354 Southwest Madagascar prior 1000 CE based on stalagmite records (Faina et al., 2021) with 355 drying trends being recorded on the isotope tree records since 1300 CE. Both records, baobab 356 δ13C and stalagmite δ¹⁸O show similar trends and patterns demonstrating high agreement in the 357 variability of rainfall over time despite a weak negative Pearson correlation between the two 358 variables (See Table 3). This non correlation could be explained by the uncertainties associated 359 with the two age models. Before 1700 CE, both records show different trends with discrepancies 360 around mid-17th century where the tree record showed drier conditions while the stalagmites is 361 showing wetter conditions. Post 1700 CE similarities between the stalagmite 𝛿18O and 𝛿13C tree 362 records are recorded with wet condition around 1700-1800 CE, dry condition 1800-1900 CE and 363 a wetter condition again between 1900-2000 CE. Very recently (post-1950 CE), both records 364 reflected a severe drought (Fig. 6). 365 Fig. 6: Comparison between baobab δ13C records from tree rings in southwest Madagascar 366 (black) with stalagmite δ18O records from Asafora cave from the southwest coast and just 367 southeast of the Velondriake Marine Reserve (green, from Faina et al., 2021). 368 The composite baobab record shows increasing δ13C values indicating marginal drying at the 369 locations of the trees over the past 700 years, corresponding with previous findings suggesting 370 increasing aridity since 1000 CE (e.g. Burney, 1993; Burns et al., 2016; Virah-Sawmy et al., 371 2016, Razanatsoa et al., 2021). Pollen records of the last millennia show a synchronous 372 desiccation from various regions in Madagascar including the southwest (Burney, 1993; Burns et 373 al., 2016; Virah-Sawmy et al., 2016) which agrees with reduced length of wet periods in the tree 374 records compared to regional records. .CC-BY 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted August 20, 2025. ; https://doi.org/10.1101/2025.08.15.670475doi: bioRxiv preprint 375 4.2. Synoptic drivers of rainfall in southwest Madagascar 376 When compared with other baobab δ13C records from South Africa (Woodborne et al., 2015; 377 2016), the rainfall patterns for southwest Madagascar and the South African summer rainfall 378 zone are in phase for most of the last 700 years. Wet (dry) periods in the South African Pafuri 379 and Mapungubwe records (22 °S) corresponded to similar wet (dry) periods from southwest 380 Madagascar. This suggests that southwest Madagascar rainfall responds to similar drivers of the 381 summer rainfall zone in southern Africa, including Agulhas Current sea-surface temperature 382 variations regulated by the East-West displacement of the TTTs (Woodborne et al., 2016) in 383 addition to the regulatory effect of the island’s mountains (Donque, 1975; Jury & Huang, 2004). 384 However, further investigation of the various drivers of rainfall is required to provide more 385 understanding of local and regional temporal changes. 386 4.2.1. ITCZ and SAM modulated rainfall during Early Little Ice Age between 1370 387 and 1500 388 The composite baobab record from southwest Madagascar shows a wet period between 389 approximately 1370 – 1500 CE (Fig. 4) which coincides with the second phase 1495 – 1833 CE 390 of a wet-neutral-wet cycle recorded in northwest Madagascar (Scroxton et al., 2017). The 391 movement of the ITCZ has a significant impact on rainfall variability in Madagascar (Haug et 392 al., 2001; Liu et al., 2003; Verschuren et al., 2000; Schneider et al., 2014). A southward shift was 393 recorded at the beginning of the Little Ice Age around 1300 CE (Chiang & Bitz, 2005; Broccoli 394 et al., 2006; Lechleitner et al., 2017; Putnam & Broecker, 2017) and this may have resulted in 395 increased rainfall for southwestern Madagascar. Wetter conditions are also evident in several 396 East African sediment records during this period, including Lake Chilwa (Crossley et al., 1984), 397 Lake Malawi (Johnson et al., 2001), Lake Massoko (Barker et al., 2000), Lake Tanganyika (Alin 398 & Cohen 2003), Lake Victoria (Stager et al., 2005), Lake Naivasha (Verschuren et al., 2000; 399 Tyson et al., 2001; Tierney et al., 2013) suggesting a common driver of rainfall, most likely the 400 southwards movement of the ITCZ. 401 The wetter conditions from 1370 – 1500 CE in southwestern Madagascar occur when the 402 Southern Annular Mode (SAM) was at its most extreme negative phase in the fifteenth century 403 (Abram et al., 2014; Fig. 7A). Negative SAM indices imply an expansion of the westerly .CC-BY 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted August 20, 2025. ; https://doi.org/10.1101/2025.08.15.670475doi: bioRxiv preprint 404 circumpolar vortex, which has significant impacts on temperature and precipitation over all four 405 Southern Hemisphere continents (Gillett et al., 2006), and particularly over Africa south of 25°S, 406 where an increase in precipitation is associated with the northward migration of the westerlies 407 during the austral winter. The response is most pronounced along the east coast, where it is 408 associated with anomalous easterly winds advecting more moisture off the SWIO (Gillett et al., 409 2006). No significant response to SAM has been reported for Madagascar on the basis of 410 instrumental data (Gillett et al., 2006) although heavy rainfall events have been associated to 411 atmospheric circulation displays a Southern Annular Mode-like pattern throughout the 412 hemisphere (Randriamahefasoa & Reason, 2017). The relationship between SAM and rainfall in 413 the baobab record presented here is not consistent with an overall significant positive correlation 414 (r=0.16, p<0.001). However, since the 16th century the relationship is weak and non-significant 415 suggesting that the southward contraction of the westerly winds during the positive SAM reduces 416 their influence on the region. When the westerlies migrate southward during a positive SAM 417 phase, they cease to be a driver of rainfall, and decadal to centennial rainfall variability responds 418 to other forcing. The lack of consistent correlation throughout the records also supports recent 419 findings related to the lack of a forced response in SAM variability prior to the 20th century 420 (King et al 2023). 421 Our composite record suggests that rainfall responds to subtropical forcing during extreme 422 negative SAM phases as the westerly winds migrate northwards bringing wetter condition to the 423 subtropics including southern Madagascar (Fig. 7A), but otherwise it responds to tropical forcing 424 determined by the position of the ITCZ during the austral summers. Evidence of this mechanism 425 operating during glacial periods is derived from marine records and model analysis of the late 426 Glacial maximum over southern Africa (Anderson et al., 2009; Sigman et al., 2010; Miller et al., 427 2019b; Engelbrecht et al., 2019; Hahn et al., 2021). The mechanism may explain the rainfall 428 maximum during the 14th and 15th centuries, but further research is needed to elucidate the 429 influence of the SAM on southern African climate during the Little Ice Age. 430 Fig. 7: Comparison of the baobab δ13C records (black) with (A) the Southern Annual Mode 431 (SAM) index (orange) (Abram et al. 2014), (B) the Pacific Decadal Oscillation (PDO) index 432 (blue) (MacDonald & Case 2005), (C) Sea Surface Temperature (SST) from Ifaty, southwest 433 Madagascar (grey) (Zinke et al. 2022), and (D) HadISST1.1 SST dataset that refers to Indian 434 Ocean Dipole variability (red, Saji & Yamagata 2003) .CC-BY 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted August 20, 2025. ; https://doi.org/10.1101/2025.08.15.670475doi: bioRxiv preprint 435 4.2.2. Regional and localised rainfall drivers during the LIA 1600-1750 436 Wet conditions during the 14th and 15th centuries are followed by extremely dry conditions 1600 437 – 1750 CE in southwest Madagascar. This period is characterised by a positive phase of the 438 SAM that commences at 1500 CE (Abram et al., 2014) implying reduced effect of the westerlies. 439 It is also a period of reduced sunspot activity known as the Maunder Minimum (1645 – 1715) 440 (Mann et al., 2009; Scroxton et al., 2017) with decreased global temperatures at the maximum of 441 the Little Ice Age (LIA; Tyson et al., 2001). It has been speculated that there was a southward 442 migration of the ITCZ during the maximum of the LIA (Jury & Huang, 2004; Russel et al., 2007; 443 Tadross et al., 2008; Voarintsoa et al., 2017). Our results show drier condition during this period 444 starting around 1600 CE and has also been experienced in northwest of Madagascar around 1700 445 CE (Scroxton et al., 2017). Dry periods were also experienced across the African continents 446 including East Africa (Russell & Johnson, 2007; Tierney et al., 2013), and the southern African 447 summer rainfall area (Huffman, 2004; PAGES 2k Consortium, 2013; Macron et al., 2014; 448 Chevalier & Chase, 2015; Huffman & Woodborne, 2016; Woodborne et al., 2016). Pollen 449 evidence from the Lake Longiza in southwest Madagascar suggested an increase in grass and 450 decrease in trees and shrubs such as Arecaceae, Pandanus, and Acacia potentially associated 451 with the drying in the region (Matsumoto & Burney, 1994; Razanatsoa et al., 2022). Lake 452 Tsizavatsy, from the same region, showed a hiatus in its sediment deposition between 1400 – 453 1900 CE (Razanatsoa et al., 2021) suggesting the drying of the lake during this period, which is 454 consistent with the tree records. The evidence of dry conditions from records across southern 455 Africa does not support southward migration of the ITCZ during this period and if the migration 456 occurred, there might have been other drivers that inhibited its effect leading to a decrease in 457 rainfall during this period. 458 The Pacific Decadal Oscillation (PDO) is linked to ENSO in relation to drought patterns across 459 Africa through teleconnections that influence the longitudinal position of climate systems. The 460 (PDO) reconstruction shows that during the LIA negative PDO anomalies are associated with 461 decreases in rainfall particularly between 1600 – 1750 CE (r=-0.30, p<0.001), whereas in the 462 record before and after this period, they are associated with increases in rainfall (r=0.18, p=0.004 463 and r=0.18, p=0.001 respectively) (Fig. 7C). Despite the change in the sign of the correlation, 464 this evidence suggests that climate forcing is dominated by tropical forcing. (Thompson et al., .CC-BY 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted August 20, 2025. ; https://doi.org/10.1101/2025.08.15.670475doi: bioRxiv preprint 465 2003; MacDonald & Case 2005; Hoell et al., 2017). Changes in the TTTs position which tend to 466 propagate eastward, from southern Africa to the Mozambique Channel and southern Madagascar 467 is known to have a strong influence on intra-seasonal and even interannual rainfall variability in 468 the region (Macron et al., 2014). This has particularly been suggested to be dictated by the 469 migration of the ITCZ (Chase & Meadows, 2007; Sachs et al., 2009) and to increase during La 470 Nina conditions (Manhique et al., 2011; Ratna et al., 2012; Macron et al., 2014). Moreover, its 471 persistence was suggested to be maintained by variation of SST anomalies over the Agulhas 472 Current (Manhique et al., 2011; Vigaud et al., 2007). Comparison of the baobab δ13C records 473 from 1660 – 1994 CE with the 300-year Agulhas Current Sea surface temperature (SST) record 474 from Ifaty in southwest Madagascar (Zinke et al., 2014) shows a significant negative correlation 475 (r=-0.22, p<0.001). Positive (negative) SST corresponds with higher (lower) rainfall in the 476 baobab record (Fig. 7B). The coolest oceanic temperatures in the coral record, with anomalies of 477 -0.3 – -0.5 °C between 1675 – 1760 CE, correspond to the driest period in southwest 478 Madagascar. Similar patterns of rainfall were noted in the summer rainfall area of the adjacent 479 African mainland (Woodborne et al., 2015). Variation in SST in the western Indian Ocean are 480 determinant in the IOD with a suggested negative relationship with eastern African rainfall 481 responses (Hoell et al., 2017; Taylor et al., 2021). 482 4.2.3. Mixed effect of changes in ITCZ position, human land-use and climate change 483 from 1750 – 2013 CE 484 Around 1750 CE until early 1800 CE, at the end of the LIA, there is a relatively wet period 485 recorded in the baobab δ13C data. A relatively dry period after 1860 CE is similar to conditions 486 experienced over the summer rainfall zone in southern Africa. These periods coincided with 487 more extreme ENSO warm phases (Nash, 2017) with the warmest period in the Agulhas SST 488 record between 1880 CE and 1900 CE and a northward migration of the ITCZ (Zinke et al., 489 2014; Railsback et al., 2018). 490 The comparison of the composite baobab δ13C record with the Dipole Mode Index (DMI) that 491 were used as an index of the IOD (1870 – 2013) shows very low to no correlation (Table 3, Fig. 492 7D) but with a noticeable positive but not significant correlation since 1980 CE (r= 0.27, p= 493 0.08). The effect that the IOD has on equatorial climate forcing is similar to the ENSO or PDO .CC-BY 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted August 20, 2025. ; https://doi.org/10.1101/2025.08.15.670475doi: bioRxiv preprint 494 effects as it is driven by an equatorial SST differential across the Indian Ocean while ENSO is 495 driven by a gradient in the Pacific Ocean. The effect in the subtropical region of southwestern 496 Madagascar appears to be dominated by the Pacific Ocean influences on global climate 497 (ENSO/PDO) which is influential in the latitudinal position of the TTTs system. 498 Around 1950 CE, conditions in southwest Madagascar are as wet as at any time in the record. 499 This corresponds to a positive SST anomaly, positive phase of IOD and a negative PDO phase. 500 Despite suggested changes in the impact of ENSO cycles on the SST in the region of the SWIO 501 since 1970 (Zinke et al., 2014), our results show typical PDO/rainfall phasing with more 502 negative PDO corresponding to high rainfall while the inverse is not always true. Records 503 suggest that the IOD intensified following the onset of global warming during the 20th century 504 along with forced response of SAM (Abram et al., 2008; Namakura et al., 2009; Watanabe et al., 505 2019; King et al 2023), with increased evidence of human induced climate change (IPCC, 2021), 506 evidence of increased river runoff and shifts in human land-use through slash and burn 507 agriculture were recorded in coral records from eastern Madagascar (Grove et al., 2013). Pollen 508 records from the region suggest a decrease in the tree component including Arecaceae coinciding 509 with an increase in Poaceae and pioneer taxa such as Asteraceae mostly likely associated with 510 tree cutting associated with agriculture expansion (Razanatsoa et al., 2022). These suggest that 511 changes in rainfall in the region led to changes in land-use. There is a return to drier conditions 512 around 1980 called “belt of iron” also recorded in historical records peaking at the beginning of 513 the 1990s (Von Heland & Folke, 2014). This was followed by a trend towards wet conditions in 514 the past 20 years similar the instrumental record (Tadross et al., 2008). 515 4.3. Implications for future climate change risk and adaptation 516 The last 300 years of the baobab composite record show the interacting global, regional and local 517 drivers that have influenced rainfall variability in southwest Madagascar. The dominant effect of 518 the position of the ITCZ during the austral summer is evident, and this is troubling in the context 519 of forecast climate change. The position and zone of the ITCZ is predicted to narrow with a 520 northward shift over eastern Africa and the Indian Ocean and a southward shift in the eastern 521 Pacific and Atlantic oceans by 2100, which would severely reduce rainfall in the region 522 (Mamalakis et al., 2021). In terms of climate risk and adaptation, southwest Madagascar will 523 likely experience a drier climate with more frequent and prolonged droughts as already predicted .CC-BY 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted August 20, 2025. ; https://doi.org/10.1101/2025.08.15.670475doi: bioRxiv preprint 524 in the IPCC 6th assessment report (IPCC, 2021). Climatic factors are an important driver of 525 economic, environmental and societal decisions (Boutin & Smit, 2016) and would be crucial for 526 the future of these dry areas. Indeed, the population are dependent on rainfall as a source of 527 water and for agriculture due to the lack of infrastructure and the limited permanent water ponds 528 (Hänke et al., 2017; Carriere et al., 2018). The effect of drought and lack of rainfall has already 529 led the Mikea forager communities to diversify their livelihoods with seasonal agriculture to 530 ensure food security (Razanatsoa et al., 2021). Some adaptations that have been established 531 elsewhere and could be conducted in the region include the introduction of new drought resistant 532 crops, (e.g. Thomas et al., 2007; Yaro et al., 2014). Multiple species livestock herding with cattle 533 and goat pastoralism (Kaufmann & Tsirahamba, 2006; Hänke & Barkmann, 2017) and livelihood 534 diversification including work that is not farming (e.g. crafts and services for local markets) were 535 suggested to be a major adaptive strategy under drying conditions in a short and long term and 536 buffer livelihoods in the face of environmental change (e.g. Kuiper et al., 2007; Tambo & 537 Abdoulaye, 2013). Further understanding of the effect of future climate on these populations and 538 their surrounding environment is critical in planning strategies of adaptation in terms of 539 livelihoods but also water provision also in the coming years. 540 5. CONCLUSION 541 Baobab δ13C data from southwest Madagascar are a proxy for changing rainfall over the last 700 542 years. The inferred wettest period was between 1370 – 1500 CE while the driest period occurred 543 between 1600 – 1750 CE. High centennial variability was recorded with a decreasing rainfall 544 trend and reducing duration of wet periods over time. The comparison of the records with 545 existing records of rainfall drivers at local, regional, and global scales shows that the baobab 546 rainfall proxy record is not dominated by the influence of any single forcing over the entire 547 record. The westerlies may play a role during extreme negative phases of the SAM, while 548 latitudinal shifts of the ITCZ are the dominant low frequency driver of rainfall. At a more local 549 level, the role of SST seems to dominate variability possibly through longitudinal influences on 550 the position of the TTTs system which is also influenced by the PDO/ENSO system. Localised 551 climate forcing in relation to the Southwest Madagascar Coastal Current (SMACC) within the 552 greater Agulhas Current system has been suggested (Ramanantsoa et al., 2018). What emerges .CC-BY 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted August 20, 2025. ; https://doi.org/10.1101/2025.08.15.670475doi: bioRxiv preprint 553 from comparisons with other rainfall proxy records on the island of Madagascar, and from the 554 adjacent African mainland, is that the temporal trends are not consistent, probably reflecting the 555 contrasting dominance of different drivers in different regions. A rainfall dipole exists between 556 southern Africa and Madagascar (Jury et al., 2015; 2016; Woodborne et al., 2016; Barimalala et 557 al., 2018) with increases in precipitation over southern Africa extending from Mozambique to 558 Angola coincident with a decrease in rainfall most of Madagascar (Barimalala et al., 2018) but 559 not in the southwest region. The mountains that extend from the north to the south of 560 Madagascar (>1500 m elevation) reduce the direct transport of moisture from the Indian Ocean 561 toward southern Africa (Barimalala et al., 2018) and southwest Madagascar. The Agulhas 562 current SST forcing of Madagascar rainfall is opposite to that in southern African where negative 563 SST anomalies were associated with wetter conditions over the southern African interior 564 (Woodborne et al., 2015). These contradictions suggest that the variation in rainfall is not a 565 simple intensification or weakening of the existing climate patterns, but rather a response in the 566 synoptic systems to tropical (ENSO/PDO), extratropical (SAM), and localised (SST) forcing. 567 Why some forcing appears to dominate at certain periods and not at others is unclear, and the 568 evidence presented here suggests that synergistic effects might be explored in global climate 569 models. 570 The potential effect of climate change and land-use change were also recorded at the near present 571 period, as well as possible effects of SAM if there is a northward migration of westerlies similar 572 to what happened around 1300 CE. The data generated here provide the opportunity to unravel 573 the relative importance and interaction between global, regional, and local drivers across the 574 southern and eastern African region. These findings are crucial in the simulations of rainfall 575 projections to help evaluate the impacts and trends of migration of the westerlies and 576 anthropogenic induced climate change on the African continents that are not fully understood. 577 For southwest Madagascar with an expected drier climate and increasing occurrence of severe 578 drought conditions predicted for the near future, understanding the risks, and establishing 579 adaptation strategies particularly in terms of livelihood could avoid disastrous famine. .CC-BY 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is The copyright holder for this preprintthis version posted August 20, 2025. ; https://doi.org/10.1101/2025.08.15.670475doi: bioRxiv preprint 1. ACKNOWLEDGEMENT We would like to thank the Ministry of Environment and Sustainable Development of Madagascar and Madagascar National Park for providing the permission for the field campaign, sampling and exportation. We acknowledge ESSA-Forêts Mention Foresterie et Environnement de l’Ecole Supérieure des Sciences Agronomiques, Université d’Antananarivo – MADAGASCAR for collaborating with the obtention of the research permit. We also acknowledge all the field assistants that have participated in retrieving the cores and Tsilavo Razafimanantsoa for his input on the map. 1. DATA REFERENCES Zinke, J., B. Loveday, C. Reason, W.-C. Dullo, and D. Kroon. (2014). Madagascar corals track sea surface temperature variability in the Agulhas Current core region over the past 334 years. Scientific Reports, 4, 4393. doi: 10.1038/srep04393. NOAA's National Centers for Environmental Information (NCEI) Abram, N. J., Mulvaney, R., Vimeux, F., Phipps, S. J., Turner, J., & England, M. H. (2014). 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FUNDING This project has been funded as part of the Faculty PhD fellowship (University of Cape Town, R.E.) 2015-2018 and the Applied Centre for Climate and Earth Systems Science (ACCESS NRF UID 98018, R.E.) project, the UCT University Research Committee accredited (URC) and COVID supplemental support from the University of Cape Town [URC, 2019-2020] and the NRF/SASSCAL (Southern African Science Service Centre for Climate Change and Adaptive Land Management, grant number 118589), the NRF/African Origins Platform (grant number 117666), and NRF Competitive Programme for Rated Researchers (Grant Number 118538). Supplementary Information SI1: Radiocarbon dates of each tree replicate from the four trees collected in southwest Madagascar .CC-BY 4.0 International licensemade available under a (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. 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