Weakening of lower tropospheric temperature gradient between Indian landmass and neighbouring oceans and its impact on Indian monsoon

The study shows that in the scenario of global warming temperature gradient (TG) between Indian landmass and Arabian Sea/Bay of Bengal is significantly decreasing in the lower troposphere with maxima around 850 hPa. TG during pre-monsoon (March to May) is reducing at a significant rate of 0.036°/year (Arabian Sea) and 0.030°/year (Bay of Bengal). The above alarming results are based on sixty years (1948–2007) of daily temperature and wind data extracted from CDAS-NCEP/NCAR reanalysis datasets. TG based on ERA-40 data also indicates a decreasing trend of 0.0229°/year and 0.0397°/year for Arabian Sea and Bay of Bengal respectively. As TG is not governed by any type of significant oscillation, there is a possibility of TG tending to zero. It is further observed that the rate of warming over the oceans is more than that over the land which has resulted into the weakening of TG. Pre-monsoon TG has significant correlations with All India Seasonal Monsoon Rainfall (AISMR) kinetic energy of waves 1 and 2 at 850 hPa kinetic energy, and stream function at 850 hPa over Indian landmass during monsoon season. Except AISMR, the decreasing trends observed in all the above parameters are significant. All India rainfall for July and August together shows a significant decreasing trend of 0.995mm/year. Reducing number of depressions and cyclonic storms and increasing number of break days during monsoon over India are the reflections of the weakening of TG.