Ask Eric: Why The Quiet Tropics, And Will They Pick Up?

Why has the hurricane season been less active so far and do you expect things to pick up before the season is out?

- Sean

A question many are asking this year Sean! On one hand, it's been fairly odd because water temperatures are plenty warm, there hasn't been widespread wind shear, and it's not an El Nino year (which tend to be quiet due to increased shear across the Atlantic). So if you look at those factors, you'd expect it to be quite active. And that's why the forecasts were all for a very active season.

I'd stress here (and we've said the same on air a few times) is that this isn't necessarily an inactive season, it's just not an intense one. As of September 30th we've had 10 named storms, which is a pretty busy season on paper. But we've only had 2 hurricanes, and both of them were quite weak (Humberto and Ingrid). One metric we look at to judge tropical activity is ACE (Accumulate Cyclone Energy)...and by looking at that it really tells the story. In the Northern Hemisphere, year-to-date ACE through this writing is at 188. The average year-to-date reading for the northern hemisphere is 408. So we are WELL below average, in historic territory. The Atlantic also has not seen a 'Major' Category 3 or higher hurricane, which hasn't happened since 1994 and has only happened 4 times in the satellite era. Even more striking, the Eastern Pacific basin has not seen a Major, either!

There are a couple of reasons being tossed out there as to why it's been quiet, at least for the Atlantic Basin. Atlantic high pressure has been oriented in a way as to keep a flow of dry and often dusty air off or Spain/Portugal and Africa. This dry and dusty layer of air holds down instability, which is what typically helps these storms get going in the open waters of the Atlantic. The so-called "Saharan Air Layer" has been persistent for most of the season.

There's also been a fair amount of wind shear in the western Atlantic, in places like the Bahamas and off the East Coast of the U.S.

As to why there hasn't been much activity in the Caribbean and Gulf, I'm not so sure. It's a complex work of atmospheric art that creates these beasts, and I'm sure many studies will be undertaken to try and figure out why globally things have shut down.

And one final theory is that the temperature gradient across the Atlantic is much smaller this year, due to well above-average sea surface temperatures (SSTs) across the North Atlantic. The purpose of hurricanes is to transport massive amounts of heat from the equatorial regions toward the poles, and perhaps this smaller gradient has lead to a smaller requirement for heat transport.

As to your question of 'will the season pick up?' - it doesn't really look like it. There may be some action in the Gulf of Mexico over the next 10 days, but there aren't many other signs that the Atlantic will wake up in the weeks to come. By that time, we're getting into late October, and wind shear becomes even more of a factor. That being said, Sandy was born in late October, and look how that story ended. So we can't let our guard down, even when times seem quiet. All it takes is one.