The busy hurricane season about to 'go Greek'

14th September 2020 Last updated at 12:52

There is only one name left on the pre-determined Atlantic hurricane season naming list before it "goes Greek".

With the formation of Tropical Storm Teddy and Tropical Storm Vicky on Monday, there have now been 20 named storms. With one more name left on this year's list, Wilfred, the letters of the Greek alphabet will be used to name further storms.

This would be only the second time this has occurred.

It has been an extremely busy season. By mid-September, statistics from the United States' National Hurricane Center show that we would normally have had seven named systems. This year, however, has been far from normal with 13 more than average.

One of the most remarkable features of the season has been how early each of the named storms formed in relation to previous years. Teddy and Vicky for example, were the earliest 'T' and 'V' named storms on record.

None of this, though, has surprised meteorologists as conditions were forecast to come together for a busy season.

Anomaly map showing that sea surface temperature for most of the North Atlantic is higher than average
NCEP/NOAA
The sea surface temperature for most of the North Atlantic is currently higher than average

Seasonal forecasts from the National Oceanographic Atmospheric Administration (NOAA) and Colorado State University in the US both predicted 2020 would see around double the 1981-2010 average number of named storms. There are only 21 names on each year's dedicated World Meteorological Organization list so, if the forecasts come to fruition, the Greek alphabet will be used to name the rest.

Gerry Bell, NOAA's lead hurricane season forecaster, stated that his organisation's forecast of as many as 25 named storms was a first for them.

The Greek alphabet has only been used once before, in the record-breaking 2005 hurricane season when there were 27 named storms. It ended with Tropical Storm Zeta at the end of December.

It was also the season when Hurricane Katrina, a Category 5 hurricane, caused extensive damage in Louisiana and Mississippi - one of the costliest natural disasters in US history.

Why has this season been so busy?

Like baking a cake, there are a number of ingredients we need to produce a hurricane or tropical storm. These include a sea surface temperature greater than 26C and instability in the atmosphere around western Africa. There also needs to be little wind shear - changes in wind speed and direction throughout the atmosphere that can affect the development of storms.

And that is what we have seen so far this year. Sea surface temperatures in the Atlantic have consistently been around 1-2C above normal over the summer, with meteorologists at Colorado State University suggesting sea surface temperature anomalies are ranked as the fourth warmest on record.

Through the rest of September and into October, the sea surface temperature will remain high enough to supercharge tropical storm formation.

The other main ingredient for a tropical storm - wind shear - has also been extremely low throughout July. The seasonal forecasts issued suggest there is a strong connection between July wind shear and August-October averaged wind shear, so we can therefore also expect this to enhance storm activity in the Atlantic.

Anomaly map showing that sea surface temperature for much of the eastern Pacific is lower than average
EARTHNULLSCHOOL
The sea surface temperature for much of the eastern Pacific is currently lower than average

The final factor contributing to the forecast of an active season is a natural climatic pattern called ENSO - El Nino Southern Oscillation. This describes the state of sea surface temperatures and wind patterns across the Pacific Ocean, which have climatic implications across the globe.

When this oscillation is in a neutral or negative phase - known as La Nina - hurricane activity tends to be increased. NOAA announced in mid-September that waters in the eastern Pacific had cooled enough for La Nina to form .

Is climate change a contributing factor?

Linking tropical cyclones to climate change is complicated. Climatologists are actively researching this area and studies so far suggest that we could see more intense major storms in a warming world.

However, as there are so many factors involved we cannot say whether man-made climate change has an effect on individual seasons such as this.