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Southern Indian Ocean hurricanes

May 6, 2019

At this time of year, tropical storms and hurricanes are not high in the consciousness of those of us in the Northern Hemisphere as the hurricane season has not yet begun. However, in the Southern Hemisphere the hurricane seasons are just drawing to a close, and for the southwestern Indian Ocean this has been a very active and very deadly season that has attracted some press coverage.

Let’s take a look at how seasons generally progress in the southwestern Indian Ocean, which for purposes of tropical cyclone climatology extends westward from 90° E to the east coast of Africa between the equator and 40° S. The season runs from Nov. 15 through April 30 of the subsequent year, the Southern Hemisphere’s summer and early autumn. Since the 1980 season (Nov. 15, 1979, to April 30, 1980) the number of tropical cyclones in this region has ranged from nine to 22. The average number of systems was just a bit more than 15 per season.

As of the official end of the 2019 season, there have been 15 systems; so from the standpoint of the number of systems, the season has been average. However, all 15 of these systems were at least tropical storms, and that is a record high. Eleven of the tropical storms were designated cyclones (the equivalent of hurricanes in the North Atlantic and eastern Pacific basins) and 10 of those were intense tropical cyclones (roughly equivalent to Category 3 hurricanes) with both of those statistics record highs. Perhaps most important, nearly 1,100 people lost their lives as a result of tropical cyclones in this region for this season, which is the second most ever recorded, and the amount of damage has reached more than $1 billion, which is the highest total ever. Both of these statistics are not final yet, as one of the most devastating storms (in fact, the strongest storm of the season) has occurred in the final week of the season.

As in the Northern Hemisphere, tropical cyclones tend to track from east to west in the tropical latitudes of the Southern Hemisphere, and — depending on the steering currents — may turn to higher latitudes at some point. But sometimes there are some oddities in the tracks of storms depending on how the prevailing atmospheric flow is set up around the system. For the southwestern Indian Ocean region, the coastal areas prone to impacts from tropical cyclones include the Mascarene Islands (Mauritius and Reunion), Madagascar and the coast of southeastern Africa, especially Mozambique. Some of the smaller islands are also vulnerable, including portions of the Seychelles, but the islands that are farther north and closer to the equator are less likely to be impacted since the Coriolis effect is minimized at the equator and thus rotating weather systems cannot be sustained in that region. Other areas of impact include more inland locations in Africa where heavy rains can produce flooding. We will take a quick look at the two storms that did the most damage this season and caused the greatest number of casualties.

Figure 1: Track data for Intense Tropical Cyclone Idai from Meteo France, which tracks systems in the southwestern Indian Ocean from the Regional Specialized Meteorological Center at Reunion Island.

Intense Tropical Cyclone Idai was an unconventional system in several respects. This system initially developed as a tropical depression on March 4, 2019, in the Mozambique Channel, then moved west-northwest across the central coast of Mozambique and tracked north-northwest, moving inland to southern Malawi over the course of a couple of days. During this time, it produced large amounts of rain leading to serious flooding and caused several fatalities and significant damage in Mozambique and Malawi. Even though it was over land through this period, it maintained its status as a tropical depression.

The system then turned southwest and headed back to the coast, eventually moving back over the Mozambique Channel on March 9. It was only at this time that the system began to become stronger, and it strengthened rapidly into an intense tropical cyclone by March 11, at which point it was located near northern Madagascar and producing significant impacts there.

Figure 2: Visible satellite image of Idai from NASA in the Mozambique Channel.

Idai then began a track toward the west-southwest and crossed the Mozambique coast near the city of Beira, where the greatest amount of damage and the highest number of fatalities occurred. It continued west, moving inland, and large amounts of rain and serious flooding occurred over much of southern Mozambique and Zimbabwe. Figure 1 shows the track of Idai; Figure 2 is a visible satellite image showing the system over the Mozambique Channel when it was close to its peak intensity. Figure 3 is an enhanced infrared image showing the system on its way toward its landfall near Beira.

Figure 3: Infrared satellite image of Idai in the Mozambique Channel on March 13, 2019, generated by the Naval Research Laboratory at Monterey, Calif.

The factors that made this system unusual include its persistence over land for several days after its first landfall as a tropical depression, and its re-emergence and subsequent intensification into an intense tropical cyclone. This allowed it to make its second, more devastating landfall — though the first landfall did produce very serious flooding over a large area. The death toll for this one system was over 1,000, and this number could still be a low estimate due to significant outbreaks of disease in the Beira area resulting from destruction of infrastructure and lack of resources in the wake of the storm. The death toll from this storm is the highest for a storm in this region since a cyclone that impacted Mauritius in 1892.

Figure 4: Satellite image of Intense Tropical Cyclone Kenneth nearing the northern coast of Mozambique on March 25, 2019, from the Joint Typhoon Warning Center.

Mozambique was not finished with tropical cyclones for this season, though, as another intense tropical cyclone made landfall in the northern portion of the country in the waning days of the season in late April. Intense Tropical Storm Kenneth was a somewhat more conventional system in that it developed over open waters northeast of Madagascar, then tracked west-southwest while strengthening. This system actually became stronger than Idai, reaching the equivalent of a Category 4 hurricane before making landfall in northern Mozambique near the city of Pemba. At landfall, Kenneth was the most intense tropical cyclone ever observed in Mozambique, but it weakened rapidly after crossing the coast. Its death toll was not nearly as high as Idai, but at least 45 people lost their lives, including some on the island of Comoros. The damage was significant, but the estimates are still incomplete as this newsletter is being written. Figure 4 is a satellite image of Kenneth prior to landfall in northern Mozambique.

The impacts of these two systems are likely to last for several years.

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