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.
Highlights
Precipitation in the summer monsoon season accounts for more than 80% of India’s annual rainfall and is crucial for the region’s agriculture and economy [1]
This study helps to bring out plausible reasons through circulations, dynamic and thermodynamic characteristics involved in monsoon depression days in association with the Intraseasonal Oscillation (ISO)
Same thing have been applied for the all the parameters: precipitable water, relative humidity and sea level pressure data to identify the synoptic condition during monsoon depression days over the Bay of Bengal associated with the intraseasonal oscillations
Summary
Precipitation in the summer monsoon season accounts for more than 80% of India’s annual rainfall and is crucial for the region’s agriculture and economy [1]. Two such possible mechanisms are interannual and intraseasonal variations of large-scale summer circulation in the WTP—SCS region induced by 1) the interannual variation of the sea surface temperature (SST) in the eastern tropical Pacific and 2) the northward migration of the 30 - 60-day monsoon trough/ridge from the equator to around 20 ̊N. The present study emphasis the role of monsoon depression in association with ISO for the study period, 1990-2014
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