Record Atlantic SSTs continue; very active hurricane season foreseen by CSU and TSR

Sea Surface Temperatures (SSTs) in the Atlantic's Main Development Region for hurricanes had their warmest March on record, 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 (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 85°W) were an eye-opening 1.26°C above average during March. This easily beats the previous record of 1.06°C set in 1969. SSTs in the Main Development Region are already warmer than they were during late June of last year, which is pretty remarkable, considering that March is one of the coldest months of the year for SSTs in the North Atlantic. Last month's anomaly of 1.26°C tied with June 2005 as the greatest monthly anomaly ever recorded in the Atlantic MDR.


Figure 1. The departure of sea surface temperature (SST) from average for April 12, 2010. Image credit: NOAA/NESDIS.

What is responsible for the high SSTs?
As I explained in detail in last month's post on record February SSTs in the Atlantic, 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. This heating is superimposed on the very warm global SSTs we've been seeing over the past few decades due to global warming. Global and Northern Hemisphere SSTs were the 2nd warmest on record this past December, January, and February. We are also in the warm phase of a decades-long natural oscillation in Atlantic ocean temperatures called the Atlantic Multi-decadal Oscillation (AMO). This warm phase began in 1995, and has been partially responsible for the high levels of hurricane activity we've seen since 1995.

What does this imply for the coming hurricane season?
The high March SST anomaly does not bode well for the coming hurricane season. The three past seasons with record warm March SST anomalies all had abnormally high numbers of intense hurricanes. Past hurricane seasons that had high March SST anomalies include 1969 (1.06°C anomaly), 2005 (0.93°C anomaly), and 1958 (0.93°C anomaly). These three years had 5, 7, and 5 intense hurricanes, respectively. Just two intense hurricanes occur in an average year. The total averaged activity for the three seasons was 15 named storms, 11 hurricanes, and 6 intense hurricanes (an average hurricane season has 10, 6, and 2.) Both 1958 and 2005 saw neutral El Niño conditions, while 1969 had a weak El Niño. So, even if this year's El Niño lingers on into hurricane season, it may not protect us from a hyper-active hurricane season--the weak El Niño year of 1969 had 18 named storms, 12 hurricanes, and 5 intense hurricanes.

April forecast for the 2010 Atlantic hurricane season issued by Colorado State University
A well above-average Atlantic hurricane season is on tap for 2010, according to the latest seasonal forecast issued last week by Dr. Phil Klotzbach and Dr. Bill Gray of Colorado State University (CSU). The Klotzbach/Gray team is calling for 15 named storms, 8 hurricanes, and 4 intense hurricanes. An average season has 10 named storms, 6 hurricanes, and 2 intense hurricanes. The forecast calls for 30% above-average chance of a major hurricane hitting the U.S., both along the East Coast (45% chance, 31% chance is average) and the Gulf Coast (44% chance, 30% chance is average). The Caribbean is also forecast to have an above-average risk of a major hurricane (58%, 42% chance is average.)

The forecasters cited two main reasons for their forecast of an active season:

1) Sea surface temperature (SST) anomalies in the tropical Atlantic are at their warmest levels on record in the Main Development Region for hurricanes in the tropical Atlantic. Warmer-than-normal waters provide more heat energy for developing hurricanes. In addition, an anomalously warm tropical Atlantic is typically associated with lower sea level pressure values and weaker-than-normal trade winds, indicating a more unstable atmosphere with decreased levels of vertical wind shear, favoring hurricane development. Part of the reason for the substantial warming is because a weaker than average Bermuda-Azores High drove weak trade winds over the winter and early spring. These weaker winds acted to reduce evaporative cooling of the ocean. Weaker winds also decreased the mixing of cool waters to the surface from below.

2) Hurricane activity in the Atlantic is lowest during El Niño years and highest during La Niña or neutral years. The CSU team expects the current weak to moderate El Niño conditions to transition to neutral and perhaps weak La Niña conditions by this year's hurricane season. April and May are typically the months when the atmosphere will swing between El Niño and La Niña, which makes any seasonal forecasts of hurricane activity during April low-skill. The current computer models used to predict El Niño (Figure 2) mostly favor neutral conditions for the coming hurricane season. The models used include statistical models, which observe how previous El Niño events have evolved, and sophisticated computer-intensive dynamical models (similar to the GFS model we use to make weather forecasts). The reliability of all of these models is poor, but the CSU team believes the ECMWF model (yellow-orange squares) is the best one. The ECMWF model only goes out to JJA (June-July-August) in this plot, and is forecasting neutral conditions.


Figure 2. Computer model forecasts of El Niño/La Niña made in March. The forecasts that go above the red line at +0.5°C denote El Niño conditions; -0.5°C to +0.5°C denote neutral conditions, and below -0.5°C denote La Niña conditions. Three computer models predict El Niño conditions and three predict La Niña for the upcoming hurricane season (ASO, August-September-October). However, most of the models predict neutral conditions. Image credit: Columbia University's IRI.

Analogue years
The CSU team picked five previous years when atmospheric and oceanic conditions were similar in April to what we are seeing this year. Those five years were 2005, the most active Atlantic season in history; 1998, which had Category 5 Hurricane Mitch in the Western Caribbean; 1969, which featured Category 5 Hurricane Camille, the strongest hurricane ever to hit the U.S.; 1966, which had Category 4 Hurricane Inez that killed 1,000 people in the Caribbean and Mexico; and 1958, which had five major hurricanes. The mean activity for these five years was 16 named storms, 10 hurricanes, and 5 intense hurricanes.

How accurate are the April forecasts?
While the formulas used by CSU do well in making hindcasts--correctly modeling the behavior of past hurricane seasons--their April hurricane season forecasts have no skill in predicting the future. This year's April forecast uses the same formula as was used in 2008 and 2009. This scheme successfully predicted an active hurricane season in 2008, but failed to properly predict the relatively quiet 2009 hurricane season. A different formula was used prior to 2008, and the April forecasts using that formula showed no skill over a simple forecast using climatology. CSU maintains an Excel spreadsheet of their forecast errors (expressed as a mathematical correlation coefficient, where positive means a skilled forecast, and negative means they did worse than climatology) for their their April forecasts. For now, these April forecasts should simply be viewed as an interesting research effort that has the potential to make skillful forecasts. The next CSU forecast, due on June 2, is the one worth paying attention to. Their early June forecasts have shown considerable skill over the years.

2010 Atlantic hurricane season forecast from Tropical Storm Risk, Inc.
The British private forecasting firm Tropical Storm Risk, Inc. (TSR), issued their 2010 Atlantic hurricane season forecast last week, and they are also calling for a very active year: 16.3 named storms, 8.5 hurricanes, and 4.0 intense hurricanes. TSR predicts a 74% chance of an above-average hurricane season, 20% chance of a near-normal season, and only a 6% chance of a below normal season. They give a 77% chance that 2010 will rank in the top third of most active hurricane seasons on record.

Like the CSU April forecasts, the TSR April forecasts have little skill. I like how TSR puts their skill level right next to the forecast numbers: 12% skill above chance at forecasting the number of named storms, 7% skill for hurricanes, and 6% skill for intense hurricanes. That's not much better than flipping a coin.

TSR projects that 5.1 named storms will hit the U.S., with 2.3 of these being hurricanes. The averages from the 1950 - 2009 climatology are 3.2 named storms and 1.5 hurricanes. Their skill in making these April forecasts for U.S. landfalls is 9 - 13% above chance. In the Lesser Antilles Islands of the Caribbean, TSR projects 1.6 named storms, 0.7 of these being hurricanes. Climatology is 1.1 named storms and 0.5 hurricanes.

TSR cites two main factors for their forecast of an active season: they predict slower than normal trade winds from July - September over the Main Development Region (MDR) for hurricanes over the Atlantic (the region between 10° - 20° N from Central America to Africa, including all of the Caribbean). Trade winds are forecast to be 0.81 meters per second (about 2 mph) slower than average in this region, which would create greater spin for developing storms, and allow the oceans to heat up due to reduced evaporational cooling and reduced mixing of cooler sub-surface waters to the surface. TSR forecasts that SSTs will be 0.42°C above average in the MDR during hurricane season.


Figure 3. Accuracy of long-range forecasts of Atlantic hurricane season activity performed by Phil Klotzbach and Bill Gray of Colorado State University (colored squares) and TSR (colored lines). The CSU team's April forecast skill is not plotted, but is less than zero. The skill is measured by the Mean Square Skill Score (MSSS), which looks at the error and squares it, then compares the percent improvement the forecast has over a climatological forecast of 10 named storms, 6 hurricanes, and 2 intense hurricanes. TS=Tropical Storms, H=Hurricanes, IH=Intense Hurricanes, ACE=Accumulated Cyclone Energy, NTC=Net Tropical Cyclone Activity. Image credit: TSR.

I'll have a new post by Thursday.
Jeff Masters