Phil Leggiere, writing for Homeland Security Today (May 28, 2010), noted that “all signs point to active hurricane season.” Yesterday, echoing the findings of numerous private forecasters, the National Oceanic and Atmospheric Administration (NOAA) Climate Prediction Center (CPC) issued a 2010 hurricane season outlook calling for an 85 percent chance of an “above normal” season.
The hurricane season officially begins next Tuesday June 1. NOAA estimated a 70% probability that 14-23 named storms, 8-14 hurricanes and 3-7 major hurricanes would hit the Atlantic basin this season. By contrast only eleven named storms three hurricanes, and two major hurricanes struck the Atlantic during the 2009 season.
NOAA calls atmospheric conditions of the past few months “very conducive to increased Atlantic hurricane activity.” This expectation is based on the prediction of three climate factors, according to the outlook : the tropical multi-decadal signal, which has contributed to the high-activity era in the Atlantic basin that began in 1995, exceptionally warm sea surface temperatures in the tropical Atlantic Ocean and Caribbean Sea (called the Main Development Region), and La Niña conditions in the tropical Pacific, with La Niña becoming increasingly likely.
“During 1995-2009,” the outlook says, “ some key aspects of the tropical multi-decadal signal within the MDR have included warmer than average SSTs, reduced vertical wind shear and weaker easterly trade winds, below-average sea-level pressure, and a configuration of the African easterly jet that is more conducive to hurricane development from tropical waves moving off the African coast. Many of these atmospheric features typically become evident during late April and May, as the atmosphere across the tropical Atlantic and Africa begins to transition into its summertime monsoon state.”
Several of these conditions are now present, the report adds, and they are expected to persist through the hurricane season.
Further, the report says, “ The El Niño episode, which contributed to the below normal Atlantic hurricane season last year, has dissipated. Conditions in the equatorial Pacific Ocean are now favorable for the development of La Niña. Also, in the upper atmosphere the pattern of circulation (i.e. streamfunction) anomalies during the last 30 days, and the last 60 days, indicates cyclonic anomalies in the central subtropical Pacific of both hemispheres.”
This pattern, according to the report, suggests that the atmosphere has already transitioned out of its El Niño state observed last winter and early spring. “The conditions expected this year have historically produced some very active Atlantic hurricane seasons,” the outlook concludes. “The 2010 hurricane season could see activity comparable to a number of extremely active seasons since 1995. If the 2010 activity reaches the upper end of our predicted ranges, it will be one of the most active seasons on record.”
Over the past several weeks scientists have expressed concern that a particularly active hurricane season could exacerbate problems of the Deepwater Horizon oil spill. “With this year’s hurricane season likely to be a severe one, with much above average numbers of hurricanes and intense hurricanes, we have the unwholesome prospect of a hurricane churning through the largest accidental oil spill in history,” Dr. Jeffrey Masters, founder of The Weather Underground, wrote on his blog Wednesday. “A hurricane has never passed over a sizable oil spill before, so there are a lot of unknowns about what might happen.”