Atlantic tropical storms forming at a record pace
The Atlantic region is currently setting a record-breaking pace for the formation of tropical cyclones. As of 18 September 2020, a total of 21 storms had formed – more than any other year on record at this stage in the June to November season. Only once have more storms been recorded in a season, that was 2005 when there were 28. There are 21 names on the Atlantic storm naming list, when these have been allocated letters from the Greek alphabet are used. Therefore the next storm to be named will be Tropical Storm Alpha. However, that only paints part of the picture of what has been happening in the Atlantic and other parts of the northern hemisphere this year.
A combination of several climate factors is driving the active Atlantic storm season this year. A main driver is the development of La Niña conditions in the eastern Pacific, which acts to reduce wind shear over the Atlantic allowing storms to form more readily. High wind shear prevents or slows tropical storm formation. The sea-surface temperatures over large parts of the Atlantic have been higher than average and the west African monsoon has also been strong meaning the easterly waves which cross west Africa and produce Atlantic storms have been potent.
Although there has been a record number of Atlantic storm formations, eight have become hurricanes (winds 74 mph or greater) and just two have become a ‘major’ hurricane (winds 111 mph or greater). This is a lower proportion of hurricanes and major hurricanes than would be expected from a total of 21 tropical storms based on past climate figures. Also, several of this season’s storms were quite short lived, particularly early in the season. Thus, by some measures other than the number of storms, this season has had lower levels of activity than some previous seasons to date.
One feature of the current storm season is the high number of storms reaching landfall over the USA. Tropical Storms Bertha, Cristobal, Fay and Marco and Hurricanes Hanna, Isaias, Laura and Sally have all come ashore over the USA.
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