Mean dynamical characteristics of the asian summer monsoon with a global analysis - forecast system

Abstract

The dynamical characteristics of Asian summer monsoon are examined with a global spectral model. In addition to the seasonal circulation features, the large scale budgets of kinetic energy, vorticity and angular momentum are examined making use of mean analysis and forecast fields (upto day 5) for summer season comprising June, July and August, (JJA) 1994. Apart from elucidating the systematic errors over the monsoon region, the study expounded the influence of these errors on associated dynamics. The significant errors in the low level flow (850 hPa) evince (i) weakening of south easterly trades and cross equatorial flow into the Northern Hemisphere and (ii) weakening of westerly flow over the North Indian Ocean. Similarly, in the upper level flow (200 hPa) they indicate (i) weakening of Tibetan anti-cyclone and (ii) reduction of return flow into the Southern Hemisphere. The balance requirements in the mean analysis as well as forecast fields are fairly in agreement with the observed over the summer monsoon. The monsoon domain is discerned as the source region of kinetic energy and vorticity. Both are produced in the monsoon region and transported horizontally across. The model forecasts fail to retain the analyzed atmospheric variability in terms of mean as well as transient circulations which is revealed by kinetic energy budget. This is further corroborated by vorticity and angular momentum budgets. It is noticed that the forecasts in all ranges produce feeble monsoon circulation which weakens considerably with increase in the forecast period.