A preliminary study of climatological associations and characteristics of tropical cyclones in the SW Indian Ocean

Summary

Tropical cyclones (TC) in the data-sparse SW Indian Ocean region are studied through climatological and statistical associations and case study structure. Differences between summers with more and less TC are identified with a view to the prediction of seasonal frequencies. In summers with more TC, upper easterlies and lower westerlies over the equatorial zone north of Madagascar form a Walker cell anomaly in conjunction with the east phase of the stratospheric quasi-biennial oscillation (QBO), while sea surface temperatures (SST) are above normal in the preceding spring (>28°C). In the sub-tropics, easterly trade winds strengthen while mid-latitude westerlies shift polewards and SST are below normal (<23°C). OLR departures in “more TC” summers are <−15 Wm⁻² over region frequented by tropical cyclones.

Two tropical cyclone events are selected for analysis which rank highest in terms of rainfall on Mauritius. Danielle formed near 13°S, 65°E and tracked southwest across Mauritius on 19 January 1964. A radiosonde time-height section is analysed for departures from climatology and thermodynamic structure. The profile of equivalent potential temperature is rather uniform near the center of the TC, decreasing from 350°K near the surface. Dry stable air is present in the 600hPa layer around the perimeter. TC Hyacinthe was quasistationary to the east of Madagascar causing rainfall in excess of 500 cm on Reunion Island from 15–27 January 1980. OLR anomaly plots and satellite imagery indicate that Hyacinthe was spawned in association with an eastward moving convective wave and reached maximum intensity (−92 Wm⁻²) and radius (>1000 km) from 21 to 26 January 1980.