May 24th, 2012

Spring Conditions Suggest Tempered Atlantic Hurricane Season, Part II

Posted at 1:00 AM ET

The Atlantic Warm Pool (AWP) is defined as an area of the ocean with SSTs greater than 28.5°C (83°F). When this area is very large the average position where storms form in the MDR shifts to the east (1), making it more likely that the storm will recurve harmlessly north through the central Atlantic. This shift in genesis position occurs due to the more rapid development of tropical waves moving off of Africa into the Atlantic as these disturbances encounter more favorable atmospheric and oceanic conditions. Once a storm forms it is steered by deeper level flow in the atmosphere, making it more susceptible to being pulled north by passing mid-latitude troughs. A quicker, more easterly forming storm also experiences longer exposure to an internal storm tendency to move north that is called the Beta Effect. The opposite effect occurs during years with cooler SSTs in the tropical Atlantic, with storms taking longer to form, shifting the genesis location westward. This allows the storms to be steered by the shallow trade winds and flow further west, threatening landfall in the western Atlantic/Caribbean Sea.

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Storm genesis locations (denoted by black circles) for the top ten largest Atlantic Warm Pool years (upper) and smallest (lower) for the period 1979-2009 along with average Sea Surface Temperature (°C) for the August - October period. Note the significant increase in development in the eastern Atlantic in the large AWP years shown in the upper image. Images taken from Wang 2012.

Observational and modeling studies have both shown that a large AWP or the related Atlantic Multi-Decadal Oscillation (AMO) causes a general pattern of higher pressure/ridging to occur over the southeastern United States and lower pressure/troughing over the northeastern US out into the northwest Atlantic. This atmospheric pattern, which is also consistent with the negative phase of the North Atlantic Oscillation (NAO), means that the subtropical ridge (Bermuda High) is weaker and is shifted northeast. Storms that form in the MDR with this pattern round the southern side of the Bermuda High, then head north into this area of troughing, and proceed northeast away from the East Coast of the United States. A strong Bermuda High and more westward track are associated with a smaller AWP as shown in the lower image below. 

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Average Sea Level Pressure for the top ten largest Atlantic Warm Pool years (upper) and smallest (lower) for the period 1979-2009. The Main Development Region is shown by the black box with the characteristic steering pattern for storms that develop in the MDR shown in the dashed line. Images taken from Wang 2012.

Even with a forecast of reduced overall activity this upcoming season, there is still a very good chance that some tropical activity will make landfall in the United States. In an average year (based on the last 100 years) there is a 97 percent chance that a tropical storm or stronger will make U.S. landfall (2), an 84 percent chance that a hurricane will make landfall and a 52 percent chance that a major hurricane will make landfall. As of the middle of May, the following large scale pattern effects that will help determine where storms track are expected for the 2012 hurricane season.

  • The movement in the average position for formation of storms to the west in the MDR in response to cooler ocean temperatures and higher shear will favor a more westward track toward the western Caribbean/Gulf of Mexico or Florida.
  • Continuation of the positive North Atlantic Oscillation seen over the past six months into the summer and fall would strengthen and shift the Bermuda High to the southwest, consistent with the pattern associated with a small Atlantic Warm Pool. This would steer storms westward, favoring U.S. landfalls in the Southeast or the Gulf of Mexico.
  • Warming in the far eastern Pacific has been shown to increase development in the western Caribbean and Gulf of Mexico, as the increased regional moisture and associated instability favor increased deep thunderstorm activity. In addition, there is no significant increase in shear in these regions associated with El Niño. This could further favor a Gulf or Florida landfall this season.

With the expected below normal levels in overall hurricane activity it is unlikely that there will be multiple major hurricane landfalls in 2012. However, it is certainly possible, based on forecasted steering/genesis location indicators, that the season could finally end the drought of major U.S. hurricane landfalls, particularly in the Southeast or Gulf of Mexico.

Guy Carpenter will provide another update in early July. If you have any questions, please contact your GC catastrophe modeling representative.

Notes:

1. Colbert, Angela J., Brian J. Soden, 2012: Climatological Variations in North Atlantic Tropical Cyclone Tracks. J. Climate, 25, 657-673.

 

2. Klotzbach, Phil J., William M. Gray, 2012:  Extended Range Forecast of Atlantic Seasonal Hurricane Activity and Landfall Strike Probability for 2012. The Tropical Meteorological Project,  Colorado State University, accessed May 10, 2012, url: http://tropical.atmos.colostate.edu/forecasts/2012/apr2012/apr2012.pdf

 

3. C. Wang, 2012: Climate Factors Influencing Hurricane Landfall. NOAA/CPO/MAPP Webinar, accessed May 10, 2012, url: http://www.climate.noaa.gov/cpo_pa/mapp/webcasts/presentations/2012/4-10/Wang.pdf.

 

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