Inter-annual and intra-seasonal variation of some characteristics of monsoon disturbances formed over the Bay

The characteristics of monsoon disturbances during drought and flood years for the period 1971-96 are studied to find out their inter-annual variations. Variations of some of the characteristics of monsoon disturbances formed over Bay during 1979-88, with respect to different monsoon conditions such as strong, weak and break monsoons, are also studied. The results show that monsoon disturbance days are higher during flood years than during drought years. Drought years are associated with higher chances of low pressure areas to intensity into depressions, less westward movement, more horizontal extent, intense pressure departure from normal in comparison with flood years. However, more monsoon disturbances tilt significantly during flood years. The rainfall associated with these disturbances is highly variable and does not depend on the density, horizontal and vertical extent of the individual system. More number of lows intensify into depressions during strong monsoon conditions compared to those of weak monsoon conditions. Lows and depressions during strong monsoons have more westward movement and longer life period. Generally, very few lows form during break monsoon and none of them intensify into depression. Hence, the presence of mid-tropospheric heating during strong and weak monsoons is essential for the formation of depression. Synoptic systems which abate break monsoon condition and re-establish normal monsoon are also discussed.

A Genesis Index for Monsoon Disturbances

Synoptic-scale monsoon disturbances produce the majority of continental rainfall in the monsoon regions of South Asia and Australia, yet there is little understanding of the conditions that foster development of these low pressure systems. Here a genesis index is used to associate monsoon disturbance genesis in a global domain with monthly mean, climatological environmental variables. This monsoon disturbance genesis index (MDGI) is based on four objectively selected variables: total column water vapor, low-level absolute vorticity, an approximate measure of convective available potential energy, and midtropospheric relative humidity. A Poisson regression is used to estimate the index coefficients. Unlike existing tropical cyclone genesis indices, the MDGI is defined over both land and ocean, consistent with the fact that monsoon disturbance genesis can occur over land. The index coefficients change little from their global values when estimated separately for the Asian–Australian monsoon region or the Indian monsoon region, suggesting that the conditions favorable for monsoon disturbance genesis, and perhaps the dynamics of genesis itself, are common across multiple monsoon regions. Vertical wind shear is found to be a useful predictor in some regional subdomains; although previous studies suggested that baroclinicity may foster monsoon disturbance genesis, here genesis frequency is shown to be reduced in regions of strong climatological vertical shear. The coefficients of the MDGI suggest that monsoon disturbance genesis is fostered by humid, convectively unstable environments that are rich in vorticity. Similarities with indices used to describe the distribution of tropical cyclone genesis are discussed.