Variability of upper-ocean characteristics and tropical cyclones in the South West Indian Ocean

Track and intensity are key aspects of tropical cyclone behavior. Intensity may be impacted by the upper-ocean heat content relevant for TC intensification (known as T-dy) and barrier layer thickness (BLT). Here the variability of T-dy and BLT in the South West Indian Ocean and their relationships with tropical cyclones are investigated. It is shown that rapid cyclone intensification is influenced by large T-dy values, thick barrier layers and the presence of anticyclonic eddies. For TC generation in the South West Indian Ocean, the parameter T-dy was found to be important. Large BLT values overlay with large T-dy values during summer. Both fields are modulated by the westward propagation of Rossby waves, which are often associated with ENSO. For example, the 1997-1998 El Nino shows a strong signal in T-dy, SST, and BLT over the South West Indian Ocean. After this event, an increasing trend in T-dy occurred over most of the basin which may be associated with changes in atmospheric circulation. Increasing SST, Power Dissipation Index and frequency of Category 5 tropical cyclones also occurred from 1980 to 2010. To further examine the links between tropical cyclones, T-dy, and BLT, the ocean response to Category 5 Tropical Cyclone Bansi that developed near Madagascar during January 2015 was analyzed. Its unusual track was found to be linked with the strengthening of the monsoonal north westerlies while its rapid intensification from Category 2 to Category 4 was linked to a high-T-dy region, associated with a warm core eddy and large BLT.