Friday, October 30, 2009

North Atlantic Hurricane Season slowest since 1997 -- Figure

Global and Northern Hemisphere Tropical Cyclone Activity remains near 30-year historical lows -- three years in a row now of considerably below-average activity globally. -- Figure

Consequence of the transition from La Nina to El Nino during the past year

Oct 29: The North Atlantic hurricane season has not produced a storm in over 3-weeks and, if no more develop, the season overall would rank as the slowest since the El Nino year of 1997. Hurricanes Bill and Fred accounted for over 82% of the Accumulated Cyclone Energy [ACE**] -- a metric that combines intensity, duration, and frequency of hurricanes and tropical storms during a year. The remaining storms were weak, rather short-lived and unremarkable. Indeed, the Accumulated Cyclone Energy [ACE] of 44 ranks among the slowest during the past half-century. Elsewhere, the Northern Hemisphere and Global ACE when calculated either with 12- or 24-month running sums, remains just above historical 30-year lows. Indeed, the global ACE sunk to record low levels during the early summer prior to the typhoon activity in the Western Pacific and the hurricane activity in the Eastern Pacific. While it may seem like the world has experienced considerable tropical cyclone activity lately, 2009 as a whole is still well behind normal or climatology. The previous Southern Hemisphere cyclone season including the Southern Indian and Pacific Oceans along with the Australian region produced historically low levels of ACE (from Oct 2008 - Apr 2009). So a global sum during the past 12 or 24 months will simply show the depressed tropical cyclone activity experienced.

This is a natural consequence of the rather unusual flip from strong La Nina to El Nino conditions during the past calendar year, which did not happen at all during the period of 1976-2006 as indicated by the MEI-ENSO INDEX (LINK). It is expected by NOAA and others that the current-El Nino is locked in for the rest of winter 2009-2010 and may indeed strengthen. This would suggest enhanced typhoon activity in the Western Pacific throughout the rest of the fall and winter which will necessarily increase the NH ACE. The Southern Hemisphere TC season may begin at any time now, but most activity is experienced between January and March.

**Note: The Accumulated Cyclone Energy metric combines frequency, duration, and the intensity of tropical cyclones into one value that can be calculated from historical storm records as well as current operational center (i.e. NHC) advisories. The ACE is simply the wind speed squared (times 10^4 kts^2) for each 6-hour storm location and intensity estimate -- added up for an entire season or whatever period you wish to define. CLIMO based upon 1979-2008 climatology.

Click on the above link for charts and more information.


I hope I'm not setting us up for a big fall by posting this with a month to go in the season, but I thought it was interesting that this is the slowest hurricane season since 1997.

So much for global warming?

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