Analysis of low level moisture transport over South Asia

Abstract

This study investigates long-term variation of low-level vertically integrated moisture flux (VIMF) for 37-year from 1979 to2015 and its contributions to the total rainfall over South Asia (SA), which includes also the Bay of Bengal and the eastern part of Arabian Sea. For this purpose wind, specific humidity, precipitation and evaporation data are collected from ERA-Interim reanalysis data generated by European Centre for Medium- Range Weather Forecasts (ECMWF) having horizontal resolution of approximately 80 km. The region is further sub divided into six areas i.e. South-West (SW), Bay of Bengal (BoB), South-East (SE), North-West (NW), Bangladesh and surrounding (BAN) and North-East (NE) for understanding details of spatial and temporal characteristics of lowlevel moisture flux and its effect on precipitation. SW, BoB and SE domains are considered as Southern part and NW, BAN and NE domains are considered as Northern part. The averaged VIMF is found to be 113.71 kg m-1 s-1 for the total study area. The annual variation of VIMF remains almost constant for 37 years over SA with average increasing rate of only 0.14% per year. Southern parts of the study area (SW, BoB and SE) acquire 67% of the total flux, whereas, 33% of the total moisture flux is observed over the Northern parts (NW, BAN and NE), which are mainly the land area. According to season, about 41% of the total VIMF is found to be advected in the monsoon season, whereas, only 17% of the total VIMF is found in the winter season. The monthly value of VIMF (191.74 kg m-1 s-1) is highest in July over SA which is 33% more than the minimum value in March. For six individual domains, highest amount of VIMF (27% of the total VIMF) is found over the BoB whereas only 6% of the total VIMF is found over the NE domain. The SW, SE, NW and BAN regions receive 21%, 19%, 13% and 14% of the total flux, respectively. Annual variation of VIMF for individual domain is also found constant except in the NE domain where VIMF is increased significantly with coefficient of determinant (R²) value of 0.54. However, the NE region has the lowest amount of VIMF (35.01 kg m-1 s-1). Seasonal variation in the domains indicates that the Southern parts of the study area have two times higher VIMF than the Northern parts during the the monsoon season. The lowest amount of VIMF approximately 29.73 kg m-1 s-1 is observed in January for the Northern region and for the Southern region the lowest value of approximately 76.78 kg m-1 s-1 is found in March.