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Great Depression! Tropical Cyclone Energy at 30-year lows
Great Depression ! Global tropical cyclone energy remains near 30-year lows !
Figure: 12-month running sums of Accumulated Cyclone Energy for the entire
globe during the past 31-years -- 1979 to October 29, 2009 (current). Average is 769.
With the Northern Hemisphere Tropical Cyclone (TC) seasons winding down in the Eastern Pacific and North Atlantic, it is as good as time as any to
take account of what the Earth has offered during the past 12-months in terms of TC Accumulated Cyclone Energy (ACE). Since the last Southern Hemsiphere
season was quite uneventful with well-below normal ACE, and the continued Northern Hemisphere inactivity, the sum of the two = global ACE reached
record low values in the early summer of 2009 and has slightly recovered only just recently.
Figure: 24-month running sums of Accumulated Cyclone Energy.
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. 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.
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.
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.
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