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.
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.
During the summer and fall of 2007, as the Atlantic hurricane season failed to live up to the hyperbolic prognostications of the seasonal hurricane forecasters, I noticed that the rest of the Northern Hemisphere hurricane basins, which include the Western/Central/Eastern Pacific and Northern Indian Oceans, was on pace to produce the lowest Accumulated Cyclone Energy or ACE since 1977. ACE is the convolution or combination of a storm's intensity and longevity. Put simply, a long-lived very powerful Category 3 hurricane may have more than 100 times the ACE of a weaker tropical storm that lasts for less than a day. Over a season or calendar year, all individual storm ACE is added up to produce the overall seasonal or yearly ACE. Detailed tables of previous monthly and yearly ACE are on my Florida State website.
Previous Basin Activity: Hurricane ACE
|* Southern Hemisphere peak TC activity occurs between October and April. Thus, 2008 values represent the period October 2007 - April 2008.
The table does not include the Northern Indian Ocean, which can be deduced as the portion of the Northern Hemisphere total not included in the three major basins. Nevertheless, 2007 saw the lowest ACE since 1977. 2008 continued the dramatic downturn in hurricane energy or ACE.
The following stacked bar chart demonstrates the highly variable, from year-to-year behavior of Northern Hemisphere (NH) ACE. The smaller inset line graph plots the raw data and trend (or lack thereof). Thus, during the past 60 years, with the data at hand, Northern Hemisphere ACE undergoes significant interannual variability but exhibits no significant statistical trend.
So what to expect in 2009? Well, the last Northern Hemisphere storm was Typhoon Dolphin in middle December of 2008, and no ACE has been recorded so far. The Southern Hemisphere is below normal by just about any definition of storm activity (unless you have access to the Elias sports bureau statistic creativity department), and the season is quickly running out. With La Nina-like conditions in the Pacific, a persistence forecast of below average global cyclone activity seems like a very good bet. Now if only the Dow Jones index didn't correlate so well with the Global ACE lately...
Hurricane is the term for Tropical Cyclone specific to the North Atlantic, Gulf of Mexico, Caribbean Sea, and the Pacific Ocean from Hawaii eastward to the Mexican coast. Other names around the world include Typhoon, Cyclone, and Willy-Willy (Oz) but hurricane is used generically to avoid confusion.
Accumulated Cyclone Energy or ACE:
is easily calculated from best-track hurricane datasets, with the one-minute maximum sustained wind squared and summed during the tropical lifecycle of a tropical storm or hurricane.