Atlantic hurricane season preview: what are SSTs doing?

Atlantic hurricane season is still a long way off, but we can start looking at Sea Surface Temperatures (SSTs) in the Atlantic to get some idea of the severity of the coming season. A plot of the current SST anomalies (that is, the departure of the temperature from average) shows that the Caribbean and Atlantic waters stretching to the coast of Africa remain much warmer than normal, as they have been since 2004 (Figure 1). How does this warmth compare to the record-breaking SSTs observed during the disastrous Hurricane Season of 2005? Figure 2 show the difference in SST between 2007 and 2005 for February, and we can see that SST were about 0.5 �C warmer in February 2005 vs. February 2007 in the region we care about--the hurricane Main Development Region (MDR) between 10 �N and 20 �N extending from Africa to the Central American coast.


Figure 1. The departure of Sea Surface Temperature from average for March 7, 2007. Image credit: NOAA/NESDIS.


Figure 2. The difference in Sea Surface Temperature (SST) between February 2007 and February 2005. Cool colors (blues and purples) are shown where the SSTs were warmer during 2005. Warm colors (yellows and oranges) are shown where it was warmer in 2007. Note the presence of an El Nino event in 2007 but not 2005 caused warmer SSTs in the Equatorial Eastern Pacific. SSTs in the Hurricane Main Development Region (red box) were about 0.5 �C warmer in February 2005 vs. February 2007. Image credit: NOAA Earth System Research Lab.

The SST forecast
What can we expect SSTs to do in the coming months? NOAA's SST forecast (Figure 3) for the peak months of hurricane season (August, September, and October) projects a continuation of the above-normal SSTs at about 0.5 �C above normal. This is a lot of extra energy to fuel intense hurricanes, but not nearly as extreme as the 1-2 �C above normal SSTs observed in 2005. Long range forecasts of SST are not very reliable, but this forecast appears to be a reasonable one. It would take a major reduction in the trade winds over the coming months to allow SSTs to climb to levels seen in 2005 (slower trade winds reduce the amount of evaporative cooling, resulting in increased SSTs). While it is impossible to predict what the trade winds might do over the next few months, a sustained weakening of the trade winds for many months is an event that is unlikely. The best guess right now is that SSTs will be above normal this hurricane season, but nothing like observed in 2005. Based on this expectation, plus the demise of El Nino, and the fact we are in an active hurricane period that began in 1995, I am expecting a hurricane season perhaps 50% above average in number of storms and intense storms--but not a repeat of 2005.


Figure 3. The departure of Sea Surface Temperature (SST) from a 1990-2003 average as forecast by NOAA's Climate Forecast System (CFS) model. Note that this model is forecasting a moderate La Nina event in the Eastern Pacific during the 2007 hurricane season, and SSTs about 0.5 �C above normal in the Main Development Region (MDR) for Atlantic hurricanes. The MDR is the region between 10 �N and 20 �N extending from Africa to the Central American coast, and includes all of the Caribbean Sea.

The steering pattern forecast
The next key question is--what will the steering pattern be for 2007? Will there be a trough of low pressure over the Eastern U.S. that recurves storms out to sea, as happened in 2006? Or, will a ridge of high pressure set up, steering hurricanes into the Caribbean, Florida, and U.S. Gulf coast, as happened in 2004 and 2005? I won't have a speculation on that until late May.

My next blog will be Friday, when I plan to review a just-released book about the most intense tornado of all time--the May 3, 1999 Oklahoma City twister. It's a great read.

Jeff Masters