SYMMETRICAL INSTABILITY OF MONSOON FLOWS

Using a zonally symmetric multi-level moist linear model, we have exam ined the possibility of symmetric instability in the monsoon region. S tability analyses with a zonally symmetric model using monthly ECMWF ( Jan-Dec) zonal basic flows revealed bath unstable as well as neutral m odes. In the absence of cumulus heating, the linear stability of the m onsoon Bow changes dramatically with the emergence of many unstable mo des in the month of May and lasting through August; whereas with the i nclusion of cumulus heating unstable modes appear early in April with substantially enhanced growth rates. This onset of instability of the May basic state may signal the abrupt transition of the south Asian me ridional monsoon circulation. The abrupt nature of the monsoon transit ions was also clearly seen in the ECMWF 5-day mean meridional circulat ion in the South Asian monsoon region. The most unstable modes have do ubling time of about 1 to 2 days. The amplitude structure of these uns table modes were mainly confined to the equatorial regions. The growth rates and the amplitude structure of the most unstable modes agree re asonably with the corresponding growth rate and structure of the merid ional monsoon cell during the monsoon transition.