A study on the distinct features of the Asian summer monsoon during the years of extreme monsoon activity over India

Abstract

In this paper, interseasonal characteristics of the Asian summer monsoon in the years of 1987 and 1988 are studied as 1987 is characterized by a large deficiency of monsoon rainfall (drought) and that of 1988 by a large excess monsoon rainfall (flood) over India. In order to compare the similarities and differences seen in the large scale dynamics and energetics of the Asian summer monsoon during the years of extreme monsoon activity, uninitialized analyses (12 Z) of the European Centre for Medium Range Weather Forecasts (ECMWF), U.K. are utilized in this study for the summer monsoon seasons of 1987 and 1988. It is found that the excess rainfall season (1988) is characterized by much stronger tropical easterly jet (TEJ) associated with the upper tropospheric easterlies and the East African low level jet (Somali Jet) associated with lower tropospheric westerlies. Such a feature mainly determines the strength of the reverse Hadley circulation which normally covers the South Asian continent during the northern summer. Further, the energetics of the TEJ show that the monsoon of 1988 has comparatively stronger zones of kinetic energy flux divergence (convergence) at its entrance (exit) regions. These zones of kinetic energy flux divergence are largely maintained by the adiabatic processes over the strong kinetic energy flux divergence zones over the Bay of Bengal and east central Arabian Sea as compared to that of 1987. Apart from this, both the zonal and meridional components of the ageostrophic flows are found to be stronger during 1988 monsoon season. Analysis of the vertically integrated thermodynamical features of the monsoon indicate that the monsoon of 1988 was characterized by an excess import of heat and moisture into the monsoon atmosphere as compared to that of 1987. Further, from the quantitative estimation of certain significant heat and moisture budget parameters during the contrasting monsoon seasons of 1987 and 1988, it becomes evident that considerable differences exist in the quantities of adiabatic production of heat energy, diabatic heating and the moisture source/sink.