| classicman |
03-16-2009 06:34 PM |
Global and Northern Hemisphere Tropical Cyclone Activity [still] lowest in 30-years
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Tropical cyclone (TC) activity worldwide has completely and utterly collapsed during the past 2 to 3 years with TC energy levels sinking to levels not seen since the late 1970s. This should not be a surprise to scientists since the natural variability in climate dominates any detectable or perceived global warming impact when it comes to measuring yearly integrated tropical cyclone activity. With the continuation (persistence) of colder Pacific tropical sea-surface temperatures associated with the effects of La Nina, the upcoming 2009 Atlantic hurricane season should be above average, as we saw in 2008. Nevertheless, since the Atlantic only makes up 10-15% of overall global TC activity each year (climatological average during the past 30 years), continued Northern Hemispheric and global TC inactivity as a whole likely will continue.
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http://www.coaps.fsu.edu/~maue/tropi...unning_ace.jpg
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Why the record low ACE?
During the past 2 years +, the Earth's climate has cooled under the effects of a dramatic La Nina episode. The Pacific Ocean basin typically sees much weaker hurricanes that indeed have shorter lifecycles and therefore -- less ACE . Conversely, due to well-researched upper-atmospheric flow (e.g. vertical shear) configurations favorable to Atlantic hurricane development and intensification, La Nina falls tend to favor very active seasons in the Atlantic (word of warning for 2009). This offsetting relationship, high in the Atlantic and low in the Pacific, is a topic of discussion in my GRL paper, which will be a separate topic in a future posting. Thus, the Western North Pacific (typhoons) tropical activity was well below normal in 2007 and 2008 (see table). Same for the Eastern North Pacific. The Southern Hemisphere, which includes the southern Indian Ocean from the coast of Mozambique across Madagascar to the coast of Australia, into the South Pacific and Coral Sea, saw below normal activity as well in 2008. Through March 12, 2009, the Southern Hemisphere ACE is about half of what's expected in a normal year, with a multitude of very weak, short-lived hurricanes. All of these numbers tell a very simple story: just as there are active periods of hurricane activity around the globe, there are inactive periods, and we are currently experiencing one of the most impressive inactive periods, now for almost 3 years.
http://www.coaps.fsu.edu/~maue/tropi..._1950_2008.jpg
Bottom Line
Under global warming scenarios, hurricane intensity is expected to increase (on the order of a few percent), but MANY questions remain as to how much, where, and when. This science is very far from settled. Indeed, Al Gore has dropped the related slide in his PowerPoint (btw, is he addicted to the Teleprompter as well?) Many papers have suggested that these changes are already occurring especially in the strongest of hurricanes, e.g. this and that and here, due to warming sea-surface temperatures (the methodology and data issues with each of these papers has been discussed here at CA, and will be even more in the coming months). The notion that the overall global hurricane energy or ACE has collapsed does not contradict the above papers but provides an additional, perhaps less publicized piece of the puzzle. Indeed, the very strong interannual variability of global hurricane ACE (energy) highly correlated to ENSO, suggests that the role of tropical cyclones in climate is modulated very strongly by the big movers and shakers in large-scale, global climate. The perceptible (and perhaps measurable) impact of global warming on hurricanes in today's climate is arguably a pittance compared to the reorganization and modulation of hurricane formation locations and preferred tracks/intensification corridors dominated by ENSO (and other natural climate factors). Moreover, our understanding of the complicated role of hurricanes with and role in climate is nebulous to be charitable. We must increase our understanding of the current climate's hurricane activity.
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A very interesting read. Some of which is above me. I highly recommend it still. There are some really smart people here and I'd love your/their take on it.
(Sorry the photos are so large If a mod could reduce that'd be great)
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