So far the hurricane season for the Atlantic has been pretty quiet for 2009. Ryan Maue from Florida State University explains why. In related news, Al Gore has dropped the [hurricane frequency] related slide in his traveling PowerPoint show. – Anthony
Great Depression! Tropical Cyclone Energy at 30-year lows
Both Northern Hemisphere and South Hemisphere AND therefore 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.
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.
Figure: 24-month running sums of Accumulated Cyclone Energy.
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 (El Nino years are the converse, with must less activity, as forecast by Gray and NOAA for 2009). 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. During the 2008-2009 TC season, the Southern Hemisphere ACE was 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.
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. Many papers have suggested that these changes are already occurring especially in the strongest of hurricanes due to warming sea-surface temperatures, but the methodology and data issues with each of these papers perhaps overshadows the conclusions. The notion that the overall global hurricane energy or ACE has collapsed does not contradict the recent climate change / TC linkage literature 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 (or noise) 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.
Ryan Maue’s Seasonal Tropical Cyclone Activity Update
Current Tropical Cyclones = 0
September 21: As far as I can tell using the best-tracks, the last day in September without Accumulated Cyclone Energy (ACE, Bell et al. 2000) being registered was September 24, 2003. During the past 20-years, *15* September days have not had an active tropical cyclone with *5* of those days occurring at the end of September 1999.
Through September 21, comparing 2009 to the previous 20 Septembers for NH ACE, the current total from the operational advisories of about 71 is one standard deviation below the 20-year mean (mean=117,sigma=36). These links are for two plots that show the yearly Northern Hemisphere ACE for September and the average ACE per day during the month.
Since daily ACE represents a 4-times daily sum of wind speed squared, an “average” September 21st could see one of the following (among other combos):
One TC at 125 knots
Two TCs at 90 knots
Three TCs at 70 knots or
Six TCs at 50 knots
Current TCs = 0
September 15: Global Hurricane Frequency [storms with maximum intensity greater than 64 knots] has dramatically collapsed during the past 2-3 years. When measured using 12 or 24 month running sums, the number of tropical cyclones at hurricane intensity is clearly at a 30-year low. HOWEVER, the number of tropical cyclones with intensity greater than 34-knots has remained at the 30-year average (83 storms per year). More on the distinction in an upcoming paper currently submitted for publication.
Global Tropical Cyclone ACE valid September 22, 2009 00Z
Thru Oct 31
Thru Sep 30
|S Hemisphere||107||229||Out of||Season||—|
Sorted monthly data: Text File
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