NOAA Sees Potential for "Hyperactive" Hurricane Season; Record High Sea Surface Temperatures Among Contributing Factors

The National Oceanic and Atmospheric Administration (NOAA) last week (27 May 2010) released its 2010 Atlantic Hurricane Season Outlook, citing record high sea surface temperatures (SSTs) among the factors contributing to what may be an "exceptionally active (or hyperactive) season."  The high SSTs result from a combination of natural climate variability with a very likely contribution from global warming driven by increasing atmospheric concentrations of greenhouse gases. 

In its 2010 Atlantic Hurricane Season Outlook, NOAA employs a measure called the Accumulated Cyclone Energy (ACE) index, to assess the upcoming hurricane season (1 June - 31 November 2010) and to compare it to earlier seasons. The ACE index reflects the number, duration and intensity of tropical storms. According to the outlook,  there is a 70% chance that the ACE for the 2010 hurricane season will range between 155% and 270% of the median value for the 1950-2000 seasons. NOAA says that a value above 117% of the median would be  an "above-normal" season, and one above 175% of the median is an "exceptionally active (or hyperactive)" season.

Based on the ACE range for 2010, NOAA says there is a 70% chance that the Atlantic hurricane season would see 14-23 named storms, 8-14 hurricanes, and 3-7 major hurricanes. "Therefore, this season could see activity comparable to a number of extremely active seasons since 1995," NOAA says.  "If the 2010 activity reaches the upper end of our predicted ranges, it will be one of the most active seasons on record."

In a press release (NOAA Expects Busy Atlantic Hurricane Season, 27 May 2010), NOAA cites three key variables that suggest a particularly active season:

1. "Upper atmospheric winds conducive for storms. Wind shear, which can tear apart storms, will be weaker since El Niño in the eastern Pacific has dissipated. Strong wind shear helped suppress storm development during the 2009 hurricane season.
2. Warm Atlantic Ocean water. Sea surface temperatures are expected to remain above average where storms often develop and move across the Atlantic. Record warm temperatures – up to four degrees Fahrenheit above average – are now present in this region.
3. High activity era continues. Since 1995, the tropical multi-decadal signal has brought favorable ocean and atmospheric conditions in sync, leading to more active hurricane seasons. Eight of the last 15 seasons rank in the top ten for the most named storms with 2005 in first place with 28 named storms."

Sea Surface Temperatures

NOAA data show that global sea surface temperatures (SSTs) were the highest on record for April, with a temperature anomaly of 0.5706°C above the 20th century average.   The figure below  shows that one area where the temperature anomaly is particularly large is the tropical Atlantic Ocean, especially in what is called the hurricane "Main Development Region" (MDR) off the African coast. 

Above: Sea Surface Temperature Anomalies.  Source: National Oceanic and Atmospheric Administration, National Climatic Data Center.

Atmospheric disturbances called African waves form in this area and Jeff Masters of the Weather Underground explains that "[t]hese 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.) "

In Record Atlantic SSTs continue in the hurricane Main Development Region (15 May 2010), Masters reports:

"SSTs in the Main Development Region (10°N to 20°N and 20°W to 85°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 high April SST anomaly does not bode well for the coming hurricane season. The three past seasons with record warm April SST anomalies all had abnormally high numbers of intense hurricanes."

In its 2010 Atlantic Hurricane Season Outlook, NOAA says that "[r]ecord warm SSTs are now present in the MDR, with departures exceeding +1.5oC nearly everywhere east of the Caribbean Islands."  The agency expects above-average temperatures to persist in the MDR during the peak of the hurricane season, from August through October, adding that "[s]everal climate models are predicting either near-record or record SSTs in the MDR during August-October."

NOAA explains:

"A set of factors likely combined to produce the record warmth now in the Atlantic. Based on the observations, the likely cause of the extreme Atlantic warming is a pronounced weakening of the northeasterly trade winds that led to a sharp increase in Atlantic SSTs during February and March. This increase occurred in combination with the typical warming associated with El Niño. It is also superimposed upon the background warming associated with the warm Atlantic phase of the multi-decadal signal that has been in place since 1995, and with longer term trends."

Connections Between Rising Greenhouse Gases, Rising Sea Surface Temperatures and More Energetic Hurricanes.

Though the hurricane outlook does not specify what the "longer term trends" are and how they contribute to the potentially "hyperactive" hurricane season, NOAA and others have elsewhere made it clear that SST's are rising in the MDR, that rising greenhouse gases very likely have contributed to the rising SSTs, and that the higher SSTs are correlated with more energetic hurricane seasons. 

Last year (2009), NOAA Administratrator Jane Lubchenco and the Director of the White House Office of Science and Technology Policy, John Holdren, submitted to Congress the report, Global Climate Change Impacts in the United States.  Here is what the report says about the connection between rising SSTs and more active hurricane seasons:

"Tropical storms and hurricanes develop and gain strength over warm ocean waters. As oceans warm, they provide a source of energy for hurricane growth. During the past 30 years, annual sea surface temperatures in the main Atlantic hurricane development region increased nearly 2°F. This warming coincided with an increase in the destructive energy (as defined by the Power Dissipation Index, a combination of intensity, duration, and frequency) of Atlantic tropical storms and hurricanes. The strongest hurricanes (Category 4 and 5) have, in particular, increased in intensity."

The graph below shows the strong correlation between hurricane power and sea surface temperature in the Atlantic and the overall increase in both during the past 30 years...

Evidence of increasing hurricane strength in the Atlantic and other oceans with linkages to rising sea surface temperatures is also supported by satellite records dating back to 1981. An increase in the maximum wind speeds of the strongest hurricanes has been documented and linked to increasing sea surface temperatures.

While there is a correlation between SSTs in hurricane formation regions and the intensity of Atlantic hurricane seasons, the exact nature of the relationship is not entirely understood.  We do not fully understand how those elevated SSTs have interacted with other variables to influence the frequency, intensity and other characteristics of hurricanes.  

And what about the role of rising greenhouse gases in driving up sea surface temperatures?  According to  Weather and Climate Extremes in a Changing Climate. Regions of Focus: North America, Hawaii, Caribbean, and U.S. Pacific Islands (2008), a report from the USGCRP:

"It is very likely that the human-induced increase in greenhouse gases has contributed to the increase in sea surface temperatures in the hurricane formation regions. Over the past 50 years there has been a strong statistical connection between tropical Atlantic sea surface temperatures and Atlantic hurricane activity as measured by the Power Dissipation Index (which combines storm intensity, duration, and frequency). This evidence suggests a human contribution to recent hurricane activity. However, a confident assessment of human influence on hurricanes will require further studies using models and observations, with emphasis on distinguishing natural from human-induced changes in hurricane activity through their influence on factors such as historical sea surface temperatures, wind shear, and atmospheric vertical stability."

As for the future, Global Climate Change Impacts in the United States says:

"Projections are that sea surface temperatures in the main Atlantic hurricane development region will increase at even faster rates during the second half of this century under higher emissions scenarios.

 ... As ocean temperatures continue to increase in the future, it is likely that hurricane rainfall and wind speeds will increase in response to human-caused warming.

Analyses of model simulations suggest that for each 1.8°F increase in tropical sea surface temperatures, core rainfall rates will increase by 6 to 18 percent and the surface wind speeds of the strongest hurricanes will increase by about 1 to 8 percent." 

For a short backgrounder on trends in the intensity of tropical storms in the Atlantic, and on the relationship between SSTs and storm intensities, see Tropical Cyclone Intensity [PDF], a two-page excerpt from Climate Change Indicators in the U.S, a report from the U.S. Environmental Protection Agency (dated 27 April 2010)

Online Resources:

NOAA:

• NOAA Expects Busy Atlantic Hurricane Season.  Press release (27 May 2010) from NOAA.
• 2010 Atlantic Hurricane Season Outlook.  Report (27 May 2010) from NOAA.

WWF Climate Change blog:

• Climate Change Puts Trillions of Dollars in Assets at Risk Along U.S. Coasts
• Conference Focuses on Preparing the Gulf Coast for Climate Change Impacts
• Coastal Cities of Southern Virginia Among the Most Vulnerable in U.S. to Climate Change by mid-Century

U.S. Global Change Research Program:

• Global Climate Change Impacts in the United States.  2009.
• Weather and Climate Extremes in a Changing Climate.  2008.

The hurricane season of 2010 arrives.  Posted at the WunderBlog (1 June 2010) by Jeff Masters.

Articles, statements and other materials on the connections between climate change and hurricanes:

• Global Warming and Hurricanes: An Overview of Current Research Results.  From Thomas R. Knutson, Geophysical Fluid Dynamics Laboratory/NOAA.  Last updated 5 May 2010.

• Tropical Cyclones and Climate Change. (To access the paper, click on the title and then run the executable file).   By Thomas R. Knutson et al.  Published in Nature Geoscience, 21 February 2010.

• Modeled Impact of Anthropogenic Warming on the Frequency of Intense Atlantic Hurricanes. By Morris A. Bender et al.  Science, 22 January 2010: Vol. 327. no. 5964, pp. 454 - 458.

• Statement on Tropical Cyclones and Climate Change.  Authored by participants in the WMO International Workshop on Tropical Cyclones, IWTC-6, San Jose, Costa Rica, November 2006.

• The Hurricane-Climate Connection [PDF].  By Kerry Emanuel.  In Bulletin of the American Meteorological Society (BAMS), May 2008.

Above: Sea Surface Temperatures at the Start of 2010 Hurricane Season.  Source: NASA Earth Observatory.