Author: Julian Heming, Met Office Tropical Prediction Scientist.
Update 21 Sept 2020
Shortly after publishing the blog below two more storms were named. A low pressure system near Portugal was named Subtropical Storm Alpha, which made landfall later on Friday 18th September. Then the evening of the 18th Tropical Storm Beta formed, this storm is expected to come ashore over Texas soon.
There have now been a total of 23 named storms so far this season and this is the first time Greek names have had to be used in 15 years.
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.
Hurricane Laura was the strongest of these bringing wind gusts of over 130 mph and a storm surge of near 15 feet at its peak. Although weaker than Laura as measured by wind speed, Hurricane Sally became slow moving at landfall and dropped as much as 30” (750 mm) of rainfall over some parts of southern USA. In addition, Hurricane Paulette made landfall directly over Bermuda in the Atlantic Ocean. This is fourth hurricane landfall over this small Atlantic territory in the last six years.
Meanwhile another tropical depression has formed in the Gulf of Mexico and is expected to become a hurricane over the next few days.
Elsewhere in the Globe
So have other parts of the globe seen similar levels of activity? In short, the answer is ‘no’. Both the eastern and western Pacific have seen tropical cyclone activity well below average to this point in the season. By one measure, the western Pacific has had only 40% of usual activity. However, some parts of this basin have still been hit hard. Two tropical storms (Hagupit and Jangmi) and three typhoons (Bavi, Maysak and Haishen) have all impacted the Korean Peninsula.
The North Indian Ocean has a split cyclone season and in the first half there were two cyclones. The strongest of these was Amphan which struck the India/Bangladesh border region and to date is the strongest tropical cyclone of the northern hemisphere season.
The Mediterranean Sea occasionally sees storms which have some characteristics of tropical cyclones – sometimes referred to as ‘medicanes’. In 2018 a particularly strong one, known locally as Zorbas, hit southern Greece causing some impacts. A medicane named Ianos, came ashore over the Greek island of Kefalonia overnight 17th to 18th September causing coastal flooding, downing trees and damaging buildings.
Is climate change affecting tropical storm intensity and frequency?
The effects of climate change on tropical cyclone activity remains complex. It is not possible to conclusively state the extent to which climate change may have influenced the frequency or intensity of this year’s North Atlantic Hurricane season. More research is needed to understand the relationships and relative contributions of the various physical processes at play and the way in which climate change is influencing them.
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