Relationship between the position and intensity of Low-Level Jet stream and Indian summer monsoon rainfall

A strong equatorial Low-Level Jet (LLJ) exists over the Indian Ocean plays a crucial role in modulating the Indian Summer Monsoon Rainfall (ISMR). The variation in strength and height of Monsoon LLJ (MLLJ) can provide an important lead in predicting the variations in ISMR. The variability of MLLJ is examined using the fifth generation ECMWF (European Centre for Medium-Range Weather Forecasts) reanalysis ERA-5, National Centers for Environmental Prediction-Climate Forecast System Reanalysis (NCEP-CFSR) / Climate Forecast System, version 2 (CFSv2) and the Modern-Era Retrospective Analysis for Research and Application, version-2 (MERRA-2) during 40 years period (1979–2018). The results suggest that there is an increasing trend in MLLJ strength during July and September, but a decreasing trend during August. The MLLJ level showed an upward trend (increase in height from the surface level) in all the reanalysis data sets. The ISM rainfall over Western Ghats region and central India showed strong positive (and negative) correlation with the MLLJ strength (and height) except during August. It is noted that the influence of the MLLJ on rainfall over the Indian subcontinent is stronger during the onset and withdrawal phases of monsoon. The increasing trend in MLLJ height indicated the possibility of increase in rainfall over many regions in India especially Western Ghat and Central India. The relationship between MLLJ strength and level with ISMR is strongest during the month of September. Our analysis also showed that lag correlation between MLLJ and ISMR is relatively weaker when compared to zero lag correlations.