Impact of vertical structure of the atmosphere on the variability in summer monsoon rainfall over Gangetic West Bengal, India

The influence of temperature (T), potential temperature (circle minus), vertical velocity (omega), specific humidity (q), geopotential height (hgt), zonal wind (ZW), total cloud cover (TCC), momentum flux (mf), latent heat flux (lhf), tropopause height, and kinetic energy of wind (KE) at various pressure levels from 1000 to 10 hPa on the variability in summer monsoon rainfall (SMR) over Gangetic West Bengal (GWB) (21.5 degrees N-24.5 degrees N; 86 degrees E-89 degrees E), India is explored in this research with NCEP reanalysis data set during the period from 1986 to 2016. The said period has been divided into three categories: excess rainfall years (ERY), normal rainfall years (NRY), and deficit rainfall years (DRY) based on the percentage rainfall anomaly over GWB. The principal component analysis (PCA) has been implemented with the evaluation of eigenvectors and correlation coefficients to extract the significant parameters from the original data set. The result shows slight decrease in rainfall and its anomaly over GWB during the summer monsoon (SM) season (JJAS) from 1986 to 2016, which is statistically insignificant. However, the rainfall is observed to be maximum for all the categories in the month of July with decreasing trend until September, whereas the rainfall is minimum in the month of June. The study shows slight cooling in tropospheric temperature than the climatology during ERY compared with NRY and DRY. It is observed that the availability of tropospheric moisture is more during ERY and is much less during DRY. The descending motion during DRY is found to lead subsidence and thus increase in surface temperature. The weak upper level wind during DRY leads to the reduction in moisture advection. The geopotential height is observed to descend during DRY making the level colder, whereas it goes up for NRY and ERY generating warmth in the upper level. The cold anomaly at upper level pulls down the tropopause height during DRY, whereas the warm anomaly during ERY and NRY pushes it up. The total cloud cover anomaly is observed to be positive during ERY and NRY with higher value for ERY and negative anomaly during DRY.