El Ni�o gone; La Ni�a on the way?

The El Ni�o event of 2006-2007 is over. Ocean temperatures in the Equatorial Eastern Pacific have cooled rapidly over the past four weeks, resulting in near-normal water temperatures and an end to the El Ni�o event that began in September 2006. By definition, an El Ni�o episode occurs when Sea Surface Temperatures (SSTs) are at least +0.5�C above normal in the region 120�W-170�W and 5�S-5�N (called the Ni�o 3.4 region). SSTs in this region reached +0.5�C above normal in September, and fell below +0.5�C above normal in late January. A time series of the departure of SST from normal (Figure 1) shows the rapid cooling over the past four weeks to near-normal values in this El Ni�o 3.4 region (black box on the plots).


Figure 1. Departure of Sea Surface Temperatures from normal for the past four weeks. The black box marks the region 120�W-170�W and 5�S-5�N (called the Ni�o 3.4 region). Note the rapid cooling to below-normal values in portions of this box. Image credit: NOAA Climate Prediction Center.

What does this mean for Atlantic hurricane season?
The demise of El Ni�o is bad news for those living along the hurricane-prone areas of the Atlantic coast. El Ni�o conditions tend to suppress Atlantic hurricane activity, as occurred in the 2006 hurricane season. It is extremely unlikely we will see a return to El Ni�o conditions this fall. A decay of El Ni�o this time of year is very rarely followed by a resurgence later in the year, and only one of the 20 or so computer models used to forecast El Ni�o is forecasting this to happen this year. It is much more likely that we will see a full-fledged La Ni�a episode develop. Indeed, La Ni�a may be already be well on its way--NOAA chief Conrad Lautenbacher remarked in a press release today, "we're seeing a shift to the La Nina, it's clearly in the data". He was refering to a large pool of cooler than normal waters that has developed in recent weeks in the sub-surface waters of the Equatorial Eastern Pacific. This is a prime situation for a La Ni�a to develop, and several of the long-range computer models are predicting La Ni�a conditions for the coming hurricane season (Figure 2). These models are not very reliable, however, and it is equally probable that we will see El Ni�o-neutral conditions--the absence of either a La Ni�a or El Ni�o--for the coming hurricane season. La Ni�a conditions usually cause Atlantic hurricane seasons that are much more active than average, so El Ni�o-neutral conditions would probably be more welcome than a La Ni�a. Remember, though, that the worst hurricane season on record--the infamous Hurricane Season of 2005--occurred with El Ni�o-neutral conditions. I am expecting a much more active hurricane season than the mild season of 2006 as a result of this month's demise of El Ni�o.


Figure 2. Computer model forecasts of the departure of SST from normal in the region 120�W-170�W and 5�S-5�N (called the Ni�o 3.4 region). Temperatures +0.5�C above normal in this region indicate an El Ni�o episode; temperatures -0.5�C below normal indicate an La Ni�a. Three of the 14 models plotted predict La Ni�a conditions during the upcoming hurricane season (ASO, August-September-October), one model predicts El Ni�o conditions, and the other ten predict El Ni�o-neutral conditions. Image credit: International Research Institute ofr Climate and Society.

I'd like to welcome our new featured blogger, Mike Theiss! Mike is a professional weather photographer and storm chaser, and will be sharing his awesome storm photos with us for the coming tornado season. He also documents all landfalling U.S. hurricanes (check out his amazing Katrina videos), so tune in this hurricane season to his blog!

Jeff Masters