The Atlantic hurricane season officially began on June 1 and continues until the end of November. This year, we have seen an unusually quiet start to the season with no named systems yet. In a “normal” season we would typically have two named systems (tropical storm or hurricane) by the beginning of August. So now the questions become, “Why has it been so quiet?” and, “Can we expect the tropics to remain quiet this season?”
The answer to the former question has to do with a couple of different weather features. Firstly, over the last several months, waters in the equatorial eastern Pacific have warmed significantly, rising above normal when compared to climatology. This is part of an ongoing and natural temperature oscillation known as El Niño, which has developed much as predicted in the “Hurricane Season Outlook” Hot Topic posted in May.
When an El Niño develops, it has an effect on the jet stream winds across the Caribbean Sea. The jet stream tends to shift southward into the Caribbean, producing stronger westerly winds aloft over the Caribbean and nearby Atlantic waters. Typically, winds aloft over the Caribbean are light and easterly at this time of year. When there are westerly winds aloft, they can act counter to the easterly trade winds closer to the surface and any squalls that try to develop will get “sheared” apart by these opposing winds.
The second root cause of the quiet tropical season has to do with a semi-permanent high pressure area located from the central Atlantic to near the Azores. This high has been stronger than normal this summer and as a result, the trade winds have been stronger than normal south of this high. But the inhibiting factor of the shear created by the westerly winds aloft is exacerbated when surface easterly winds are stronger than normal. The two opposing wind directions shear squall activity apart.
Stronger trade winds also result in water temperatures being cooler than normal, and of course, tropical systems rely on warm waters for their fuel. Stronger winds cool the water in two ways – stronger winds tend to stir the water more vigorously, which results in more “upwelling” (when cooler water beneath the surface rises to the surface).
Additionally, stronger winds result in an increased evaporation rate from the sea surface and evaporation is a cooling process (think of when you climb out of the water and you get that chill. The cooling effect you’re feeling comes from water evaporating from your skin).
So will the tropics remain quiet? Well, that can be answered mainly by looking at El Niño. Indications at this time are that the weak El Niño in place now is likely to strengthen further over the next two to three months. This would likely result in a further increase in westerly winds across the Caribbean, making it difficult for tropical systems to form or survive in this area. There are no indications at this time of the Azores high weakening in the near future, either.
Both factors would likely bring about a continued “quieter than normal” season.
It must be noted that we are still coming into the statistical height of the tropical season. Systems become more likely to form outside of the Caribbean, especially into the Atlantic, as we get into the Cape Verde season, as well as in the Gulf of Mexico and waters east of the U.S. East Coast.
Hot Topic: 2009 Atlantic Hurricane Season Outlook
Hot Topic: Storm Outlook: Eastern Pacific