Indian summer monsoon cyclogenesis and its variability

Abstract

MONSOON disturbances forming over the north Bay of Bengal and moving towards the WNW across northern India are the main rain producers of the Indian summer monsoon. There are some (monsoon) seasons when these disturbances fail to develop in adequate number and some that do form move towards the northwest or north—these are periods of drought. Understanding how monsoon disturbances develop and why they sometimes do not is an interesting scientific and an important economic problem. Studies so far on monsoon cyclogenesis^1,2 have shown that the monsoon atmosphere is not baroclinically unstable. Shukla¹ obtained barotropically unstable upper tropospheric modes and emphasised the role of the conditional instability of the second kind (CISK) but failed to get a preferred scale of maximum growth for lower tropospheric disturbances. R.N.K. et al.² found slowly growing barotropically unstable modes in the lower troposphere corresponding to monsoon disturbances. Mak³ studied the baroclinic instability of monsoon zonal flow with a superposed meridional component, but the meridional winds required for instability were large compared to observed winds. Lorenz⁴ studied the barotropic instability of Rossby waves and found that shorter waves can be barotropically unstable. V.S. et al.⁵ showed that the superimposition of a long stationary wave, of the scale of wave number two, on monsoon zonal flow yielded fast growing upper tropospheric modes and lower tropospheric modes of scale 2,500 km, a doubling time of 5 days and slow westwards movement. These correspond well to the observed monsoon disturbances. We report here a combined barotropic–baroclinic stability analysis of monsoon zonal flow with a superimposed long stationary wave (global monsoon) using a two-level quasi-geostrophic model. We show that the phase of the global monsoon wave is a factor in deciding whether a particular year is good for monsoons or for drought.