Occurrence of monsoon depressions in the Southwest Indian Ocean: Synoptic descriptions and stratosphere to troposphere exchange investigations

Abstract

This study focuses on Monsoon Depressions (MD) in the Southwest Indian Ocean (SWIO) basin. A MD is a subcategory of tropical depressions. Some aspects of MD dynamical characteristics differ from those of the classical tropical depressions. A MD displays a broader horizontal extension (>1000 km), and its structure is marked by deep convection clusters that are generally poorly organized. These convective clusters often develop as a wide convection belt in the eastern semicircle of the system. The MD nucleus is characterized by an area of weak winds about 200 km wide and by a hazy central cloud system, surrounded by a larger belt of strong winds. In the SWIO basin, between 2000 and 2008, 5 systems, out of the 100 tropical cyclones that were followed by RSMC La Réunion, comply with these criteria and then have been identified as MD. Using potential vorticity from European Centre for Medium‐Range Weather Forecasts analyses as stratospheric tracer, stratospheric signatures have been identified in the troposphere for the five events, involving different dynamical mechanisms: tropopause fold between an anticyclone and MD, tropopause fold between MD and a jet front system, and cut‐off low. The dynamical structure of MD, especially the external belt of strong winds, seems to favor tropopause disturbances and leads to stratosphere–troposphere exchange.