Tropical depressions, cyclogenesis and the MJO over the Southern Indian Ocean

The origin of the relation between the MJO and the cyclogenesis above the south Indian Ocean during austral summer (October to March) is analyzed. The emphasis is put on the physical process at the origin of the initiation of tropical depressions (TD) and on the probability for a TD to reach the Tropical Storm strength (sustained wind > 33 kt). An original TD tracking approach, based on ERA-Interim geopotential fields, is developed. Cyclogenetic TDs are identified by a comparison with the IBTrACS dataset. About 1/3 of the TD are cyclogenetic and this proportion is only slightly modulated by the MJO. The MJO modulation of the number of cyclones is thus mostly due to the modulation of the number of TD initiations.

The TD are initiated mostly over three regions centered at 10°S and at 55°E, 75°E and 95°E. The TD initiation is due to the development of an unstable (inversion of the PV meridional gradient) cyclonic meridional shear of the zonal wind at low-levels. This shear results first from a heat low, due to a convective perturbation, which triggers westerly winds north of the TD initiation region. The shear is reinforced by large-scale geostrophic easterly wind south of the initiation regions due either to a reinforcement of the subtropical high (for the western IO) or to a large-scale depression above the Maritime Continent (for the eastern IO). This large-scale depression is related more directly to the MJO than the subtropical high and the MJO thus modulates primarily the TD initiation in the eastern IO (a factor of 5 between active and inactive MJO phases).