Comparative study on monsoon depressions and tropical depressions over the Bay of Bengal from the year 2000 to 2018

Abstract

Tropical depression (TD) is a low-pressure system over the tropical ocean organized by deep convections with a surface wind speed of less than 34 kts, which can turn into a tropical cyclone (TC) under the favorable dynamics and thermodynamics environmental conditions. Pre-monsoon (March-may) and post-monsoon (October-December) are conducive to initiate TCs through tropical depressions over the Bay of Bengal (BoB). On the other hand, the cyclonic disturbances that formed in the BoB during the monsoon season (June-September) are known as monsoon depression (MD) which normally may not turn into a TC due to strong wind shear. The monsoon and tropical depressions developed over the BoB during the study period from 2000 to 2018 are analyzed using the six hourly reanalysis data from the European Center for Medium-Range Weather Forecasts (ECMWF) with the horizontal resolution of 0.75°×0.75°. The MDs usually originate near the northmost of the BoB and generally move in a direction between the west and north-northwest without recurving, which are different from TDs. Both types of depressions are initiated due to the influence of monsoon trough which is located above 20N during the development of MD and below 20N in the case of TD. The sea surface temperatures of 29C and low vertical wind shear (<6 ms^(-1)) are favorable for the formation of both depressions. The relative humidity is quite high (≥80%) around the MDs, whereas, non-uniform moisture coexists in the TD environment. Mean sea level pressure (MSLP) during MDs is ≤994 hPa, whereas, most of the cases it is more than 1000 hPa for TDs, which indicates the occurrence of intense depressions in monsoon season. Considering the distribution of meridional wind around the depressions, almost all depressions show symmetric structure during TDs, whereas, both symmetric and asymmetric structure are found for MDs. Also, the average vorticity for MDs is found to be ~19 x 10-5 s-1 which is greater than that in TDs (~12 x 10-5 s-1), indicating strong rotation. The height of the depressions in MDs extended 800-500 hPa, but it is up to 600 hPa during TDs. The average equivalent potential temperature for MDs is approximately 83C whereas for TDs it is 75C, which represents high instability at low-level. The anomalies of different parameters are also analyzed in this study for both TDs and MDs.