Understanding the Role of Monsoon Depressions on Intraseasonal Oscillations over Indian Sub-Continent

Abstract

Summer monsoon and its synoptic, meso-scale systems provide 80% of total annual rainfall over India. Monsoon depressions are weak, low-pressure circulation within the monsoon trough that forms in the head Bay of Bengal and moves northwestward and westward across the Indian sub-continent during summer monsoon season. Another convective system from this planetary scale circulation is the Intraseasonal Oscillation (ISO) also occurs in the daily mode. It is very important to study the impact of monsoon depressions in synoptic scale on ISOs through its strength, frequency and duration. In the recent decades, frequency of monsoon depressions and sea surface temperature over head Bay is decreasing while the intensity is increasing over Bay of Bengal. The study brings out the relationship between the ISOs and monsoon depressions over Bay of Bengal through their structure and movement for the period 1990-2014. Composites of monsoon and no monsoon depression days are calculated and found that rainfall is mainly occupied over Kerala and Interior Peninsula within the range of 10 -50 cm during monsoon depression days. Relative vorticity brings out that the shallower layer of convergence mainly from 1000 -850 hPa level and a deeper layer of weak divergence above it is mainly associated with the depression. Next, thermodynamic structure of monsoon depression and its intensity is directly proportional to the increasing of CAPE. Wavelet spectrum also indicates the intraseasonal oscillations are very active during monsoon depression days. Finally this study helps to bring out plausible reasons through circulations, dynamic and thermodynamic characteristics involved in monsoon depression days in association with the ISOs.

Influence of cloud microphysical parameterization on the characteristics of monsoon depressions over the Indian region

“…(2014) argued the reason for north‐westward propagation (based of 100 MD analysis) could be due to adiabatic nonlinear advection of potential vorticity maximum (at 500 hPa). The previous study has also observed the warming in the equatorial Indian Ocean due to the presence of anomalous sea surface temperature, which plays an essential role in the genesis of MD (Suneetha et al ., 2018). Some other studies (e.g., Rao, 1971; Krishnamurti et al ., 1976; Sikka, 1977) have found that dynamic instability through wind shear (vertical and horizontal) associated with large scale flow could be the significant reason for MD growth; however, Ditchek et al .…”