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.
As previously reported here and here at Climate Audit, and chronicled at my Florida State Global Hurricane Update page, both Northern Hemisphere and overall Global hurricane activity has continued to sink to levels not seen since the 1970s. Even more astounding, when the Southern Hemisphere hurricane data is analyzed to create a global value, we see that Global Hurricane Energy has sunk to 30-year lows, at the least. Since hurricane intensity and detection data is problematic as one goes back in time, when reporting and observing practices were different than today, it is possible that we underestimated global hurricane energy during the 1970s. See notes at bottom to avoid terminology discombobulation.
Using a well-accepted metric called the Accumulated Cyclone Energy index or ACE for short (Bell and Chelliah 2006), which has been used by Klotzbach (2006) and Emanuel (2005) (PDI is analogous to ACE), and most recently by myself in Maue (2009), simple analysis shows that 24-month running sums of global ACE or hurricane energy have plummeted to levels not seen in 30 years. Why use 24-month running sums instead of simply yearly values? Since a primary driver of the Earth’s climate from year to year is the El Nino Southern Oscillation (ENSO) acts on time scales on the order of 2-7 years, and the fact that the bulk of the Southern Hemisphere hurricane season occurs from October – March, a reasonable interpretation of global hurricane activity requires a better metric than simply calendar year totals. The 24-month running sums is analogous to the idea of “what have you done for me lately”.
During the past 6 months, extending back to October of 2008 when the Southern Hemisphere tropical season was gearing up, global ACE had crashed due to two consecutive years of well-below average Northern Hemisphere hurricane activity. To avoid confusion, I am not specifically addressing the North Atlantic, which was above normal in 2008 (in terms of ACE), but the hemisphere (and or globe) as a whole. The North Atlantic only represents a 1/10 to 1/8 of global hurricane energy output on average but deservedly so demands disproportionate media attention due to the devastating societal impacts of recent major hurricane landfalls.
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.
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.
GreenJibe 
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