TITLE:
Understanding the Role of Monsoon Depressions on Intraseasonal Oscillations over Indian Sub-Continent
AUTHORS:
P. Suneetha, K. Naga Lakshmi
KEYWORDS:
Depressions, ISOs, CAPE, Relative Vorticity
JOURNAL NAME:
International Journal of Geosciences,
Vol.9 No.4,
April
30,
2018
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.