The orographic effects of Reunion Island on tropical cyclone tracks

Orography is already known to have significant effects on cyclone tracks. Previous studies have shown the track deflection of tropical cyclones by Taiwan among other mountain ranges. Two behaviors of cyclones passing over Taiwan have been identified: a continuous track and a discontinuous track. The track deflections by the islands of the South-West Indian Ocean such as La Reunion and Madagascar still need to be objectively documented and defined. It is interesting to note that Taiwan has an intermediate orography between a meso-alpha mountain range such as Madagascar and a meso-gamma high conical-shaped one such as La Reunion.

This study focuses on the deflection of the cyclone tracks near Reunion Island. First, an objective climatological study is performed in the South-West Indian Ocean (SWIO) basin about the change of direction and speed of cyclones. Near the Reunion Island and when these changes of direction and speed are significantly different from the mean of the SWIO basin, we can define a radius of influence of Reunion Island that depends of cyclone intensity. The map of change of direction and speed in the influenced area of the Reunion Island allows us define a limit of 250km for significant change.

The control parameters that explain track deflections in the case of Taiwan are also applied to the case of a smaller island, such as Reunion Island, showing different results. In order to understand the dynamical reasons that explain the deflection, several numerical idealized experiments are performed and shown, with and without orography of Reunion Island. Different parameters are worth exploring, such as the distance between the island and the centre of the tropical cyclone. The effect of extending various thresholds wind (and not only the radius of maximum wind) is also considered.