Symmetric instability of monsoon flows

Abstract

Using a zonally symmetric multi-level moist linear model, we have examined the possibility of symmetric instability in the monsoon region. Stability analyses with a zonally symmetric model using monthly ECMWF (Jan — Dec) zonal basic flows revealed both unstable as well as neutral modes. In the absence of cumulus heating, the linear stability of the monsoon flow changes dramatically with the emergence of many unstable modes in the month of May and lasting through August; whereas with the inclusion 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 meridional monsoon circulation. The abrupt nature of the monsoon transitions was also clearly seen in the ECMWF 5-day mean meridional circulation in the South Asian monsoon region. The most unstable modes have doubling time of about 1—2 days. The amplitude structure of these unstable modes were mainly confined to the equatorial regions. The growth rates and the amplitude structure of the most unstable modes agree reasonably with the corresponding growth rate and structure of the meridional monsoon cell during the monsoon transition.