Interannual variability in Monsoon Low Level Jet and Indian summermonsoon rainfall (ISMR)

The strong cross-equatorial Low Level Jet (LLJ) exists over the Indian Ocean and the South Asia is an important component in modulating the Indian Summer Monsoon Rainfall (ISMR). The interannual variation of the Monsoon LLJ (MLLJ) is examined using the interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim), Indian Monsoon Data Assimilation and Analysis (IMDAA), and Modern-Era Retrospective Analysis for Research and Application (MERRA) reanalyses data at different pressure levels during the 40 years period (1979-2018) and its implications on ISMR is studied using 0.25°× 0.25° gridded rainfall data for same period from IMD (Indian Meteorological Department). In June, July, August and September, the analysis of MLLJ strength at different level shows that the maximum wind speed in MLLJ core region is fluctuated between the levels 850hpa to 950hpa.The maximum MLLJ strength is calculated by taking maximum of wind speed over the core region (0°N- 20°N, 40°E- 80°E) of MLLJ and for the pressure levels (1000hpa to 700hpa) for each year and the MLLJ height is defined as the vertical pressure level at which the MLLJ strength is maximum. The trend analysis of maximum MLLJ strength and MLLJ height is carried out using reanalyses data and results suggest that there is an increasing trend in maximum wind speed during ISM months for ERA and MERRAdata while decreasing trend is seen with IMDAA. Also, the MLLJ height is showing negative trend for July and September months in MERRA data. The spatial correlation analysis of MLLJ strength and height with ISMR over India is computed to analyze the impact of inter annual variations of MLLJ in ISMR.It is noted thatthe MLLJ strength for June and September months has strong positive correlation with the rainfall of these months comparedto the months of July andAugust. Results show that MLLJ (strength and height) plays an important role in modulating rainfall particularly during the onset andwithdrawal phases of Indian summer monsoon.