One of the factors used in predicting the number of tropical storms and hurricanes we expect in the Atlantic Ocean is the temperature of the ocean, since tropical activity generally only starts when the sea surface is at least 80 F. That is the temperature needed to help tropical waves develop the vertical circulation needed to grow into a full-blown TS or hurricane. Over the last year, the Atlantic Ocean surface has been setting records for abnormally high temperatures, and this is leading to concern about how these temperatures will affect the start of the season and the number of storms we can expect. Last year, we had 20 named storms even though we were in an El Nino, which usually keeps tropical waves from developing. This year, with a La Nina likely to be in place by mid-summer, there will be less atmospheric resistance to the development of storms, and so the number is likely to be higher than last year. The early forecasts set the number of likely named storms in the mid-20s, although that will be tweaked later this spring and early summer once we get a better handle on how quickly the El Nino to La Nina transition is occurring.
Scientists are not sure why the Atlantic Ocean has been so warm, but are looking at a number of factors, including the impacts of reduced sulfur in marine fuels, the impact of the Hunga Tonga eruption, and the continuing warming trend due to fossil fuels causing warming of the planet. You can read more in the New York Times here.
