Seasonal forecasts for the 2023 Atlantic hurricane season were below normal as the forecast for the El Niño Southern Oscillation (ENSO) shifted towards an El Niño. Then the forecasts shifted to average or slightly above average as the season began depending on average. So what changed?

Over the last few years we have had La Niña conditions. During La Niña equatorial ocean water in the Pacific are cooler than normal. The result tends to be a more active Atlantic hurricane season due to a reduction in vertical wind shear (see 2020). Hurricanes can strengthen quickly when there is no vertical wind shear and all other conditions are favorable.

So during an El Niño when the equatorial Pacific sea surface temperature (SST) is above normal, we would expect to see less hurricanes in the Atlantic due to stronger than average vertical wind shear. So why are we expecting more Atlantic hurricanes in 2023 instead of less?

The reason is because the sea surface temperature in the Atlantic is hotter than normal over a large area. Hurricanes need warm ocean waters to form, and the hotter the water, the stronger they can get. So the El Niño and warm SST are working against each other and almost balancing out.

Since the Atlantic is a large basin, strong vertical wind shear doesn’t always cover the entire basin. Warmer SST over a larger area means that hurricanes can form over a larger area. And this is how we get towards a slightly above average season in the Atlantic.

Although ENSO plays a large role in how the Atlantic hurricane season will play out, it is not the only factor. It is also important to remember that regardless of whether it is an active or below-average season, it only takes one hurricane to cause catastrophic damage. Always be prepared.