Tropical cyclones – massive storms that can span over 1,000km in diameter – get their energy from the ocean heat. There’s widespread agreement that ocean surface temperatures of at least 26⁰C are required for tropical cyclones to form.
As the oceans warm, the locations of where tropical cyclones form and intensify (reach their highest windspeeds) have been shifting over the past 30 years. Their area of occurrence has been moving towards the poles at a rate of about 50 km to 60 km each decade. The warm temperatures needed for tropical cyclones to form are now being found in what used to be cooler ocean regions.
Before the 1980s, southern hemisphere tropical cyclones formed and moved over regions between 5° and 20° south of the equator. This tropical region was where ocean waters were warmest.
We conducted research to find out whether this poleward shift was also present at a smaller spatial scale such as in the South West Indian Ocean, from the East African coastline to 100° east of this.
The countries most affected by tropical cyclones in this ocean are the islands of Madagascar, Mauritius and La Reunion. Also affected is Mozambique, which has a coastline, and Zimbabwe, which is landlocked.
In future South Africa, which has an extensive eastern coastline and borders on Mozambique as well as Zimbabwe, could also be at risk if the storms begin to occur further south.
Most countries in southern Africa don’t have the resources to adapt to major disasters. This makes it all the more important to try to determine when and where tropical cyclones are more likely to make landfall.
Our research found that it’s possible for storms to affect regions that are considered outside the usual zones of storm activity. Climate variables such as wind fields and sea surface temperature all influence the location and path of storms.
The research focused on storms that made their landfall south of the tropic of Capricorn, which is 23.5° south of the equator. This is south of the usual tropical cyclone formation zones.
We used data from the National Ocean and Atmospheric Association. This data tracks the location, maximum windspeed and pressure of these storms every six hours over their lifetime. Over the study period (1960-2015) 29 storms had occurred south of the Tropic of Capricorn. The data allowed us to see the path each one took before it made landfall.
We also looked at sea surface temperatures to capture any impacts warmer oceans were having on the storms.
Our study has confirmed that the Southwest Indian Ocean does experience tropical cyclones outside of the tropics. We were unable to find a statistically significant trend of cyclones moving further south within this discrete dataset. But what the data showed was that there was a cyclical tendency. Some decades (1970-1979 and 1990-2010) experienced storms further south in the South West Indian Ocean than others (1960-1969 and 1980-1989). These abnormally southward occurring storms also increased in intensity over time. Their greater windspeeds made them more dangerous. This phenomenon has also been observed in global trends.
We also looked at the relationship between different track types (the direction the storm moved in) and variability in the climate, such as the El Niño Southern Oscillation. This variability is defined by changes in normal temperatures, winds and pressure. We found that when El Niño occurred, there was a greater tendency for storms to recurve – that is, change direction by taking a semi-circular route instead of a straight or diagonal line.
We found that recurving tracks also accounted for the majority of the landfalls south of the Tropic of Capricorn. Notable storms are tropical cyclones Gafilo in 2004 and Ernest in 2005, which hit Madagascar’s southernmost regions.
The large-scale wind fields and the area of ocean warmer than 26°C were shown to influence the storms’ path.
Our research suggests that countries around the South West Indian Ocean need to prepare for the possibility of more tropical cyclones reaching their shores. Being unprepared puts them at greater risk. As the storm locations increase in variability, it becomes more difficult to adapt.
Countries such as Japan and the United States continue to improve their short-term forecasts in order to warn people and help them evacuate in time. Southern hemisphere countries need to do the same and start creating plans of action for the future – even if they don’t need them immediately.