Simulating the influence of sea-surface-temperature (SST) on tropical cyclones over South-West Indian ocean, using the UEMS-WRF regional climate model

Abstract

Tropical cyclones remain a major threat to the lives, property and economy of communities around the South West Indian ocean (SWIO), notably Southern Africa and Madagascar. This study uses the weather research forecast (WRF) model to perform a series of simulations for tropical cyclone Enawo with the aim of investigating the effect of an increase or decrease (by 2$^{\circ}C$) in sea surface temperature (SST) on the intensity of the tropical cyclone (using windspeed, precipitation and pressure as measures of cyclone intensity). The experiment uses the data from European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 re-analysis dataset to validate the results of the WRF model which was ran using boundary conditions data from climate forecast system reanalysis (CFSR). The results indicate that the WRF model performs reasonably well in simulating the track and windspeed of the tropical cyclone, when compared to observational data. In simulating the tropical cyclone, the WRF model shows that an increase in the SST by 2$^{\circ}C$ generally increases the intensity of the tropical cyclone formed. This is evident in the increasing maximum precipitation rate as well as windspeed, and decreasing minimum pressure. An increase in SST also causes the emergence of a second low pressure system. On the other hand, a decrease in the SST by 2$^{\circ}C$ leads to a minute effect in the intensity but generally acts to decrease it. This results in a smoother track path for the tropical cyclone, a decrease in the maximum precipitation rate and windspeed, and an increase in the surface pressure. The results of this study have shown that increasing the global temperature by around 2$^{\circ}C$ - violation of the Paris Accord - would lead to more violent and unpredictable tropical cyclones within the SWIO, and hence more destruction and loss of lives.