Tropical cyclogenesis conditions in the Southwestern Indian Ocean

Mature tropical cyclones around the world are associated with similar atmospheric and oceanic processes. The necessary conditions for these storms to occur (warm SST, atmospheric instability, high humidity in the low to mid-troposphere, low wind shear, enough Coriolis force) are identical over the Tropics. However, the environment within which initial disturbances evolve into balanced warm-core cyclonic circulations are fairly different in the various basins. This is partly due to the influence of large-scale climatic cycles (e.g., ENSO, AMO, IOD, ... ) and synoptic-scale propagating modes (e.g., tropical waves, Madden-Julian Oscillation, ... ). While many studies have examined the specific situations of the Atlantic and the western North Pacific, storm formation is much less known in the southern hemisphere.

Concerning the South-Western Indian Ocean (SWIO), Bessafi and Wheeler (2006) have shown a large and statistically significant modulation by MJO and convectively coupled equatorial Rossby waves, and a small yet significant modulation by Kelvin waves. Others (e.g., Ho et al. 2006, Kuleshov et al. 2008, etc.) have studied the impact of ENSO on the formation area and tracks of tropical cyclones in the SWIO.

The present study concerns the analysis of cyclogenetic evolution of named storms (tropical storms and cyclones) in the SWIO during 13 seasons from 1999-2000 to 2011-2012, from ECMWF ERA-Interim reanalyses and Meteosat-7 images, in connection with ENSO, MJO and tropical waves.

First, an objective method is used to detect tropical storms (also identified in the IBTrACS database) in the ERA-Interim reanalyses. The overall characteristics of cyclonic perturbations (warm/cold cores, Hart 2003, baroclinicity and upper-level forcing, McTaggart-Cowan et al. 2008 and 2013) that evolve into tropical storms and cyclones in the SWIO will be discussed.

Then, an objective determination of relevant predictors and their associated weight, followed by the determination of their combination in a linear or an exponential form to derive an environmental cyclogenesis index adapted to the peak of the SWIO cyclonic season (December - March) will be presented.

A spectral analysis in space and time of the different dynamical and thermodynamical environmental parameters will then be detailed. This analysis shows slow (period > 10 days) and fast (period < 10 days) eastward and westward tropical waves. Finally, the relationship between these waves and developing and non-developing storm cases will be discussed.