May 23rd, 2012

Spring Conditions Suggest Tempered Atlantic Hurricane Season, Part I

Posted at 1:00 AM ET

On the heels of two very active years, the 2012 April hurricane season forecasts expect that activity will be around 20 percent below the long term average (1950-2011). This briefing discusses how current atmospheric and oceanic patterns will likely reduce storm formation and how these same patterns are expected to contribute to the hurricane steering flow through the heart of the season.

  • The Main Development Region (MDR), birthplace of over 80 percent of major hurricanes in the Atlantic basin, currently has sea surface temperatures cooler than any period since 2002, and they are forecast to remain cooler than recent years through the 2012 hurricane season.
  • Sea surface temperature (SST) anomalies in the North Atlantic are consistent with the likely continuation of the positive phase of the North Atlantic Oscillation (NAO) that was seen in much of the winter and spring and were largely responsible for the very warm U.S. winter. The positive NAO drives unusually strong trade winds across the tropical Atlantic, increasing wind shear and helping to maintain or strengthen the cool SST anomalies in the tropical North Atlantic.
  • SSTs in the eastern Pacific are forecast to approach El Niño levels during the peak of hurricane season, and have already risen to higher levels in the far eastern portion of the basin. Recent research (1) shows an especially strong relationship between warming in this eastern region and decreased tropical development in the MDR due to increased wind shear and large scale sinking motion that inhibits thunderstorm activity.

The three limiting factors for development described above are the driving forces for the end of April forecasts calling for an average of 11 named storms, five hurricanes and two major hurricanes. The forecasts are shown in the table below. These seasonal forecasts have historically achieved modest accuracy in forecasting the upcoming season by April. The forecast accuracy increases with the June forecasts and then increases again by the August forecasts.

72table

The ACE index is a wind energy index, defined as the sum of the squares of the maximum sustained surface wind speed (knots) measured every six hours for all named storms while they are at least tropical storm strength. NOAA and some other hurricane season forecasters use the ACE index in order to categorize North Atlantic hurricane seasons as being above normal, near normal or below normal.

Despite 19 named storms forming in each of the past two seasons, a record was set last year for the most consecutive seasons without a major hurricane making landfall in the United States. The six consecutive seasons, stretching back to the occurrence of Hurricane Wilma in 2005, was the longest period on record through at least the 1850s.

Interestingly, the increased SSTs in the Atlantic that caused the increase in activity beginning in 1995 actually have been shown by several recent studies to decrease the proportion of hurricanes to hit the United States by up to 40 percent. (2)  The above normal SSTs have had this impact by causing changes in the location of storm formation and in the atmospheric steering currents that prevail through the season.

Notes:

1. Kossin, James P., Suzana J. Camargo, Matthew Sitkowski, 2010: Climate Modulation of North Atlantic Hurricane Tracks. J. Climate, 23, 3057-3076.

2. C. Wang, H. Liu, S., K. Lee, and R. Atlas, 2011: Impact of the Atlantic warm pool on United States landfalling hurricanes.
Geophys. Res. Lett., 38, L19702.

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