Record warmth in Atlantic Main Development Region for hurricanes

Sea Surface Temperatures (SSTs) in the Atlantic's Main Development Region for hurricanes were at their highest February level on record last month, according to an analysis of historical SST data from the UK Hadley Center. SST data goes back to 1850, though there is much missing data before 1910 and during WWI and WWII. The region between 10°N and 20°N, between the coast of Africa and Central America, is called the Main Development Region (MDR) because virtually all African waves originate in this region. These African waves account for 85% of all Atlantic major hurricanes and 60% of all named storms. When SSTs in the MDR are much above average during hurricane season, a very active season typically results (if there is no El Niño event present.)


Figure 1. The departure of sea surface temperature (SST) from average for March 7, 2010, as derived from the AMSR and AVHRR satellite data. Image credit: NOAA.

SSTs in the Main Development Region (10°N to 20°N and 20°W to 85°W) were an eye-opening 1.02°C above average during February. This easily beats the previous record of 0.83°C set in 1998. SSTs in the Main Development Region are already warmer than they were during June of last year, which is pretty remarkable, considering February is usually the coldest month of the year for SSTs in the North Atlantic. The 1.02°C anomaly is the 6th highest monthly SST anomaly for the MDR on record. The only other months with higher anomalies all occurred during 2005 (April, May, June, July, and September 2005 had anomalies of 1.06°C - 1.23°C).

What is responsible for the high SSTs?
Don't blame El Niño for the high Atlantic SSTs. El Niño is a warming of the Pacific waters near the Equator, and has no direct impact on Atlantic SSTs. Instead, blame the Arctic Oscillation (AO) or its close cousin, the North Atlantic Oscillation (NAO). The AO and NAO are climate patterns in the North Atlantic Ocean related to fluctuations in the difference of sea-level pressure between the Icelandic Low and the Azores-Bermuda High. They are some of the oldest known climate oscillations; seafaring Scandinavians described the pattern several centuries ago. Through east-west oscillation motions of the Icelandic Low and the Azores-Bermuda High, the AO/NAO controls the strength and direction of westerly winds and storm tracks across the North Atlantic. A large difference in the pressure between Iceland and the Azores (positive NAO) leads to increased westerly winds and mild and wet winters in Europe. Positive NAO conditions also cause the Icelandic Low to draw a stronger south-westerly flow of air over eastern North America, preventing Arctic air from plunging southward. In contrast, if the difference in sea-level pressure between Iceland and the Azores is small (negative NAO), westerly winds are suppressed, allowing Arctic air to spill southwards into eastern North America more readily. The winter of 2009 - 2010 has seen the most negative AO and NAO patterns since record keeping began in 1950, which caused a very cold winter in Florida and surrounding states. A negative AO/NAO implies a very weak Azores-Bermuda High, which reduces the trade winds circulating around the High. During December - February, trade winds between Africa and the Lesser Antilles Islands in the hurricane Main Development Region were 1 - 2 m/s (2.2 - 4.5 mph) below average (Figure 2). Slower trade winds mean less mixing of the surface waters with cooler waters down deep, plus less evaporational cooling of the surface water. As a result, the ocean has heated up significantly, relative to normal, over the winter. This heating is superimposed on the very warm global SSTs we've been seeing over the past decade, leading to the current record warmth. Global and Northern Hemisphere SSTs were the 2nd warmest on record in both December and January.


Figure 2. Sea level pressure averaged for the period December 2009 - February 2010 (left) and the sea level pressure averaged for the period December - February from the long-term mean (1968 - 1998). This winter, the Azores-Bermuda High was about 3 - 4 mb weaker than in a typical winter, due to strongly negative AO/NAO conditions. Image credit: NOAA/ESRL.


Figure 3. Departure of surface wind speed from average for December 2009 - February 2010. Winds were about 1 - 2 m/s (2.2 - 4.5 mph) lower than average over the Atlantic hurricane Main Development Region (MDR). Image credit: NOAA/ESRL.

What does this imply for the coming hurricane season?
According to Dr. Phil Klotzbach of the University of Colorado, February temperatures in the MDR are not strongly correlated with active hurricane seasons. The mathematical correlation between hurricane season Accumulated Cyclone Energy (ACE) and February SSTs is only 0.26, which is considered weak. Past hurricane seasons that had high February SST anomalies include 1998 (0.83°C anomaly), 2007 (0.71°C anomaly), and 1958 (0.68°C anomaly). These three years averaged 13 named storms, 8 hurricanes, and 3 intense hurricanes, which is considerably higher than the average of 10, 6, and 2. The big question is, how long will the strong negative AO/NAO conditions keep the Azores-Bermuda High weak? Well, the AO has risen to near-neutral values over the past week, and the latest 2-week forecast from the GFS model show that the AO and NAO will not be as strongly negative during March. This should allow the Azores-Bermuda High to strengthen some this month and increase the trade winds over the MDR. However, I still expect we'll set a record for warmest-ever March SSTs in the Main Development Region. Longer term, the crystal ball is very fuzzy, as our ability to predict the weather months in advance is poor. The long-range NOAA CFS model is predicting SSTs in the Atlantic MDR will be about 0.70°C above average during the peak months of hurricane season, making it one of the top five warmest years on record--but not as warm as the unbelievable Hurricane Season of 2005, which averaged 0.95°C above normal during August - October. The other big question is, when will El Niño fade? El Niño is currently holding steady at moderate intensity, and I expect that will continue through at least mid-April. It is possible El Niño will linger long enough into the year that it will create increased wind shear that will suppress this year's hurricane season.

Brazilian disturbance
An area of disturbed weather off the coast of Brazil, near 24S 36W, has changed little over the past two days. This disturbance still has a slight potential to develop into subtropical or tropical depression by Wednesday, according to the latest runs of the ECMWF, GFS, and NOGAPS models. Satellite imagery shows little organization to the cloud pattern, and only limited heavy thunderstorm activity. Wind shear over the region is about 20 knots, which is rather high, and should keep any development slow. Sea surface temperatures are about 27°C, about 1°C above average, which is warm enough to support a tropical storm. The system is small, limiting its potential to become a tropical cyclone. I don't think it will become a subtropical depression.

I'll have a new post on Wednesday.
Jeff Masters