Time-Space Characteristics of Seasonal and Interannual Variations of Atmospheric Water Balance over South Asia

Abstract

The atmospheric water balance over different domains within the South Asian monsoon region has been studied using moisture convergence (C) computed from JRA-25, ERA-40 and NCEP/NCAR reanalysis datasets, GPCP precipitation data (P) and evaporation data (E) as a residual of these two parameters. The seasonal climatology of P, C, and E for the selected regions shows generally large contribution of E to P. The inter-annual characteristics of P, C and E over selected key domains within the South Asian monsoon region have also been examined for both the early (June and July: JJ) and late summer (August and September: AS) monsoon periods from 1979 to 2000. The spatial and temporal characteristics of the hydrological cycle and the contribution of E and C to P are discussed in detail. One important aspect on the seasonal timescale is that from the dry regions in the northwest to the central and the wettest northeast regions, the monthly variations of E or C are large during the monsoon months specific to those regions. However, the interannual variability of P over each domain is not necessarily influenced by the same criteria like C or E, which influences the mean seasonal precipitation. It is also evident that the structure of variability for early (JJ) and late (AS) summer precipitation is different over the South Asian monsoon region. Over northwest India E is dominant on the seasonal timescale, but C contributes higher to interannual variability of P. On the other hand, over central India C is dominant during early summer (JJ) on the seasonal timescale, but E contributes higher to P variability on the interannual timescale, and during late summer (AS) E is dominant on the seasonal timescale, but C contributes higher to P variability on the interannual timescale. Over northeast India, C is dominant on the seasonal timescale, but E contributes higher to interannual variability of P. The importance of land-atmosphere interaction over each domain is discussed. The regionality in the mechanism of precipitation generation and its contribution to the India summer monsoon precipitation variability are also discussed in detail. The role of evaporation variability of precipitation is stronger over the Bay of Bengal sector and the role of convergence on the interannual variability of precipitation is stronger over the Arabian Sea sector.