Initiation and Intensification of Tropical Depressions over the Southern Indian Ocean: Influence of the MJO

Abstract

sing ERA-Interim global atmospheric reanalysis, an original tracking approach is developed to follow tropical low pressure systems from the early tropical depression (TD) stage up to possible intensification into developed tropical cyclones (TCs). The different TC stages are identified using the IBTrACS dataset. This approach detects many more TD initiations compared to IBTrACS alone and thus gives a more comprehensive dataset to study the cyclogenesis by considering separately TD initiations and the probability of intensification.In the south Indian Ocean (SIO), the MJO modulation of the number of TCs is primarily due to the modulation of the number of TD initiations and secondarily to the probability of their intensification. The TD initiations are more probable at 55°, 75°, and 95°E and can be primarily attributed to the development of an unstable cyclonic meridional shear of the zonal wind at low levels. The reinforcement of this shear results from (i) a heat low, related to a precipitation anomaly, which triggers westerly winds equatorward of the initiation region and (ii) an easterly wind strengthening south of the initiation regions due either to a reinforcement of the subtropical high (for western and central SIO) or to a large-scale depression over the western Maritime Continent (for eastern SIO). Over the western and central SIO, the concomitance of precipitation and subtropical high anomalies at the origin of the shear reinforcement could be partly stochastic, giving a weaker relation with MJO and ENSO. Over the eastern SIO, the large-scale MJO (and ENSO) perturbation pattern alone can reinforce the shear, giving a larger modulation of the number of TD initiations.