The author, Phil Klotzbach, produces Colorado State University’s seasonal Atlantic hurricane outlooks.

Temperatures at the surface of the tropical Atlantic Ocean have become remarkably colder than normal. The cold water could have profound impacts on this year’s hurricane season, since warm water is the fuel source for tropical storms.

Prospects for a second straight busy hurricane season have diminished markedly. Coastal residents on edge after the horrible 2017 season may be relieved by this news but should remain vigilant. It can take just one storm to make even a quiet season memorable.

Currently, sea surface temperatures averaged over the tropical Atlantic (shown in the black box in the image below) are the coldest that they have been in the middle of June since at least the early 1980s.

Colder water means less fuel for storms, and it also tends to be associated with higher pressure and a more stable atmosphere — both of which suppress strong thunderstorms that are the building blocks of hurricanes.

How did the tropical Atlantic get this cold and end up so different from last year?

It happened fast. In March of this year, sea surface temperatures across the tropical Atlantic were near average and remarkably similar to the pattern one year ago, ahead of the devastating 2017 Atlantic hurricane season. The average sea surface temperatures over the tropical Atlantic in March 2017 and March 2018 were within 0.02 degrees (0.01 degrees Celsius) of each other, as apparent in the animation below.

But from April 1 through early June last year, the atmospheric pressure pattern across the Atlantic Ocean developed very differently — essentially opposite to this year’s.

In 2017, the subtropical Atlantic was characterized by much lower than normal pressures. The counterclockwise circulation around the low pressure zone drove winds from west in the tropical Atlantic, and that weakened the easterly trade winds.

Weaker trade winds meant less mixing of the ocean water, less evaporation and less upwelling of cold water, which collectively led to very warm tropical Atlantic Ocean temperatures. At this time last year, tropical Atlantic sea surface temperatures were the third-warmest since 1982.

But the situation reversed this year. Stronger-than-normal high pressure in the subtropical Atlantic has driven stronger trade winds that have intensified ocean mixing, evaporation and upwelling, and the tropical Atlantic has turned quite cold.

The sea surface in the tropical Atlantic is the coldest on record (since 1982) and about 3 degrees (1.7 degrees Celsius) cooler than last year at this time. While 3 degrees may not seem like much, that’s a huge year-to-year difference for the tropical Atlantic historically.

Although a colder tropical Atlantic is typically associated with reduced Atlantic hurricane activity, there is still a window for current trends to reverse and for it to warm. That said, time is running out for the sea surface temperature pattern to change significantly. Hurricane season is already underway and moves toward its peak August into October.

Of course, even if the hurricane season ends up relatively quiet overall, it’s worth reiterating that coastal residents should not let their guard down. Quieter seasons sometimes still bring terrible storms. A classic case of this was 1992, when the “A” storm did not form until Aug. 17. That storm happened to be Hurricane Andrew, which devastated South Florida.