Tuesday, June 1, 2010

Hurricane Watch: Everything You Want to Know About Current Conditions and the 2010 Outlook

With the start of the season we'll check in with Dr. Jeff Masters and his Wunderblog:
The hurricane season of 2010 is upon us. With unprecedented sea surface temperatures in the Atlantic, El Niño gone and possibly transitioning to La Niña, a massive oil slick in the Gulf of Mexico, a million earthquake refugees in Haiti at the mercy of a hurricane strike, and an ever-increasing number of people living on our coasts, the arrival of this year's hurricane season comes with an unusually ominous tone.

NOAA is forecasting a very active and possibly hyperactive season, and Dr. Bill Gray has said he expects "a hell of a year." However, our ability to forecast hurricane activity months in advance is limited, and we don't yet know how the large scale weather patterns like the Bermuda High will set up during the peak part of hurricane season. In particular, I very much doubt that we are in for a repeat of the unprecedented violence of the Hurricane Season of 2005, with its 28 named storms, 15 hurricanes, and 7 intense hurricanes.

While sea surface temperatures are currently warmer this year than in 2005, that year featured some very unusual atmospheric circulation patterns, with a very strong ridge of high pressure over the eastern U.S., record drought in the Amazon, and very low surface pressures over the Atlantic. A repeat of 2005's weather patterns is unlikely, though I am expecting we will get at least four major hurricanes this year. An average year sees just two major hurricanes.

Figure 1. Tracks of all June tropical storms and hurricanes in the Gulf of Mexico, 1995 - 2009. Allison was a subtropical storm (coded blue). Image credit: NOAA Coastal Services Center.

The latest long-range computer model guidance suggests there's no reason to suspect that the first two weeks of this year's hurricane season will bring any unusual activity. Climatologically, June is typically the quietest month of the Atlantic hurricane season. On average, we see only one named storm every two years in June. Only one major hurricane has made landfall in June--Category 4 Hurricane Audrey of 1957, which struck the Texas/Louisiana border area on June 27 of that year, killing 550. The highest number of named storms for the month is three, which occurred in 1936 and 1968. In the fifteen years since the current active hurricane period began in 1995, there have been eleven June named storms (if we include 2008's Tropical Storm Arthur, which really formed on May 31). Five tropical storms have formed in the first half of June in that 14-year period, giving a historical 36% chance of a first-half-of-June named storm. Five June storms in the past 14 years have passed close enough to the Deepwater Horizon oil spill location to have caused significant transport had there been an oil slick on the surface.

Sea Surface Temperatures
Sea Surface Temperatures (SSTs) are at record high levels over the tropical Atlantic between Africa and Central America this year (Figure 2). As I discussed in my May 15 post, the area between 10°N and 20°N, between the coast of Africa and Central America (20°W - 80°W), 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.) SSTs in the Main Development Region (10°N to 20°N and 20°W to 80°W) were an eye-opening 1.46°C above average during April. This is the third straight record warm month, and the warmest anomaly measured for any month--by a remarkable 0.2°C. The previous record warmest anomalies for the Atlantic MDR were set in June 2005 and March 2010, at 1.26°C. The Arctic Oscillation (AO) and its close cousin, the North Atlantic Oscillation (NAO), are largely to blame for the record SSTs. 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. If the difference in sea-level pressure between Iceland and the Azores is small (negative NAO), this creates a weak Azores-Bermuda High, which reduces the trade winds circulating around the High. During December - February, we had the most negative AO/NAO since records began in 1950, and this caused trade winds between Africa and the Lesser Antilles Islands in the hurricane Main Development Region to slow to 1 - 2 m/s (2.2 - 4.5 mph) below average. 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 heated up significantly, relative to normal, over the winter and Spring.

However, over the past two weeks, the AO/NAO has trended close to average, and trade winds over the tropical Atlantic have increased to near normal speeds as the Bermuda-Azores High has strengthened. SST anomalies have been falling in recent weeks, and will continue to fall in the coming two weeks, based on the latest forecast from the GFS model. While I expect that record SSTs will continue into mid-June, current trends suggest that by July, SST anomalies will be close to what they were in 2005. SST anomalies in the MDR could fall below the record 2005 levels by the peak part of hurricane season, August - October. Even so, SSTs in the Caribbean this year will be plenty warm to cause an abnormal number of major hurricanes. These warm SSTs may also cause extensive damage to the coral reefs, which suffered huge die-offs from the record SSTs of 2005.

Typically, June storms only form over the Gulf of Mexico, Western Caribbean, and Gulf Stream waters just offshore Florida, where water temperatures are warmest. SSTs are 28 - 30°C in these regions, which is about 0.5 - 1.5°C above average for this time of year. June storms typically form when a cold front moves off the U.S. coast and stalls out, with the old frontal boundary serving as a focal point for development of a tropical disturbance. African tropical waves, which serve as the instigators of about 85% of all major hurricanes, are usually too far south in June to trigger tropical storm formation. Every so often, a tropical wave coming off the coast of Africa moves far enough north to act as a seed for a June tropical storm. This was the case for Arthur of 2008 (which also had major help from the spinning remnants of the Eastern Pacific's Tropical Storm Alma). Another way to get Atlantic June storms is for a disturbed weather area in the Eastern Pacific Intertropical Convergence Zone (ITCZ) to push north into the Western Caribbean and spawn a storm there. This was the case for Tropical Storm Alberto of 2006 (which may have also had help from an African wave). SSTs are too cold in June to allow storms to develop between the coast of Africa and the Lesser Antilles Islands--there has only been once such development in the historical record--Ana of 1979.

Figure 2. Sea Surface Temperature (SST) departure from average for May 31, 2010. SSTs averaged more that 1°C above average over the entire tropical Atlantic and Gulf of Mexico. Note the large region of below average SSTs along the Equatorial Pacific off the coast of South America, signaling the possible start of an La Niña episode. Image credit: NOAA/NESDIS.

Wind shear
Wind shear is usually defined as the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude). In most circumstances, wind shear above 20 knots will act to inhibit tropical storm formation. Wind shear below 12 knots is very conducive for tropical storm formation. High wind shear acts to tear a storm apart. The jet stream's band of strong high-altitude winds is the main source of wind shear in June over the Atlantic hurricane breeding grounds, since the jet is very active and located quite far south this time of year.

The jet stream over the past few weeks has been locked into a pattern where a southern branch (the subtropical jet stream) brings high wind shear over the Caribbean, and a northern branch (the polar jet stream) brings high wind shear offshore of New England. This leaves a "hole" of low shear between the two branches off the coast of North Carolina, which is where Invest 90L formed.