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.
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 [
Large meridional shear of the eastward component of winds and high cyclonic vorticity at low levels over the monsoon trough region favor growth of these instabilities. Recent studies [
Daily averaged NCEP/NCAR global wind, precipitable water, relative humidity, omega, sea level pressure, monsoon depression tracks data from IMD (2.5˚ lat × 2.5˚ long) during 21 year period (1990-2014) have been used in the present study. The IMD daily gridded rainfall datasets are collected at 0.5 lat/long resolution [
We have calculated the monsoon depression days (178) and no monsoon depression days (2872) from 1990-2014. With rainfall we have plotted the spatial and temporal maps of the rainfall to identify the highest and lowest amount of rainfall during monsoon and no monsoon depression days. 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. Wavelet analysis is used to find out the periodicity, number and intensity of intraseasonal oscillations by using interactive wavelet plot available through the website http://paos.colorado.edu/research/wavelets/. Monsoon depression day’s tracks have been drawn to examine the movement of the track in excess and deficient monsoon rainfall years. The monsoon depression days are less compare no monsoon depression days.
Monsoon depressions are the primary rain bearing systems during summer monsoon season over India.
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The significant phenomenon noticed associated with the depression is the shallower layer of convergence below the 850 hPa level and a deeper layer of weak divergence above it. This result is a very characteristic feature of the monsoon depression with low level of non divergence approximately around 850 hPa level. The series of low level circulation vortices are mainly observed along the east coast of India with peak intensity over Head Bay of Bengal during monsoon
depression days (
Relative humidity indicates that the saturation is mainly observed over 5˚N to 16˚N (~80%) during monsoon depression days (
monsoon depression during monsoon depression days. High relative vorticity helps to maintain low surface pressure. Positive values of relative vorticity are associated with regions of low pressure. As air parcels move through the region of maximum vorticity, their spin slows resulting in divergence. Divergence at the 500 mb level removes mass from the air column causing a drop in surface barometric pressure. In turn, lower pressure at the surface results in convergence promoting rising air parcels in the column. These rising air parcels contribute to the formation of precipitation so prevalent with monsoon depressions. During no monsoon depression days most of the places are negative relative vorticity over India compare to monsoon depression days.
The cyclonic circulation is mainly prevails over head bay of Bengal and the intensity of south esterlies are more (16.5 m/s) during monsoon depression days while weak westerlies prevails (12 m/s) during no monsoon depression days (
Monsoon depression days from 1990-2014, the SLP it is observed that two closed isobars of pressure 1998 and 1000 hPa centered at N.E coast and head Bay of Bengal from the IMD criteria it is where there are two closed isobars with difference of 2 hPa there is a depression formation in that region. Over the central B.O.B and south peninsular India the isobars are extending with the SLP of 1002 - 1006 hpa (
The ω-distribution in the lower troposphere shows a centre of maximum upward motion around the 900 - 850 mb level. As the depression intensifies, the upward motion is always present on the west side of the depression centre. The presence of upward motion at lower levels is mainly due to frictional forces and other baroclinic and dynamical forces (
The composites of monsoon depression with available data are drawn for the study period during summer monsoon season.
Wavelet spectrum indicates the intra-seasonal oscillations are very active during monsoon depression days. More number of oscillations are seen in the June month there are four high frequency oscillations (10 - 20-day). In the month of July, august the frequency and intensity of oscillations are less. In September there is high intensity of oscillations 10 - 20 day is extended in to 30 to 60 day oscillation (
Monsoon depressions and Intraseasonal oscillations both are internally coupled ocean atmosphere interacting systems provides significant contribution of summer monsoon rainfall over India. Monsoon depression (178) and no monsoon depression (2080) days are calculated from the available life period of monsoon depressions over Bay of Bengal. It is pertinent that the frequency of monsoon depressions is decreasing over Bay of Bengal. In addition, the SSTs over Head Bay of Bengal also decrease with no significant trend. Anomalous SST provides that equatorial Indian Ocean warming play a significant role for generating the cyclogenesis of monsoon depressions. The spatial distribution of summer monsoon rainfall is mainly observed over southern peninsular India during monsoon depression days. The monsoon depression associated extreme rainfall events > 12.5 cm are mainly occupied over Jharkhand and some parts of west coast of India. Dynamic and Thermodynamic variables insights that probable reasons for providing the moisture transport from the Indian ocean to head Bay of Bengal. Intraseasonal oscillation like 10 - 20 is very active during monsoon depression days while 30 - 60 day oscillations are in no monsoon depression days. Later the study extended for active and deficient monsoon years through circulations features at 850 and 200 hPa level. The monsoon depressions frequency is more in active rather than deficient years.
The authors are thankful to the NCEP/NCAR team and India Meteorological Department for providing the datasets. The second author is grateful to the Department of Science & Technology, Government of India, New Delhi for their financial support through DST-INSPIRE fellowship.
Suneetha, P., Lakshmi, K.N., Sai, R.R., Peddada, L., Zedek, D.M. and Kumar, O.B. (2018) Understanding the Role of Monsoon Depressions on Intraseasonal Oscillations over Indian Sub-Continent. International Journal of Geosciences, 9, 236-253. https://doi.org/10.4236/ijg.2018.94015