Even with the expected active 2010 North Atlantic hurricane season, which accounts on average for about 1/5 of global annual hurricane output, the rest of the global tropics has been historically quiet. The Western North Pacific this year has seen 8-Typhoons, the fewest in at least 65-years of records. Closer to the US mainland, the Eastern North Pacific off the coast of Mexico has uncorked a grand total of 7 tropical storms of which 3 became hurricanes, the fewest since at least 1970. Global, Northern Hemisphere, and Southern Hemisphere Tropical Cyclone Accumulated Energy (ACE) remain at decades-low levels. With the fantastic dearth of November and December global hurricane activity, it is also observed that the frequency of global hurricanes has continued an inexorable plunge into into a double-dip recession status. With 2010 [possibly but not probably] being the hottest year ever, we will likely see the fewest number of global tropical cyclones observed in at least three-decades…

Post by Ryan Maue: A year ago, I walked into a Tallahassee Borders and snapped an IPhone photo (that’s my thumb) of Al Gore’s new book cover and marveled at the locations of hurricanes in a globally warmed future. The book was released at the tail end of the 2009 Atlantic hurricane season, which uncorked one of the quietest years on record. With Copenhagen, Cancun, and the hottest year ever come and gone, you would think that global climate disruption was spinning up cyclones with reckless abandon. Remember, after Katrina in 2005, scientists published alarming papers linking increases in hurricane activity worldwide to global warming. Fast forward 5-years: the inconvenient truth is that aside from the Atlantic basin, global tropical cyclone or hurricane activity during 2010 has tanked to the lowest levels in decades. So what happened?

Natural variability of the climate, which is coincidentally about as poorly understood and predicted as any anthropogenic effects on it, has returned the global atmosphere-ocean system to a state that favored fewer, less intense, and shorter-duration tropical cyclones (TCs). Each year, 80 to 90 TCs form in the global tropics distributed unequally among the basins of the Pacific, Indian, and Atlantic oceans, in both hemispheres. The dominant mode of global tropical variability is the El Nino Southern Oscillation (ENSO), and it usually determines the genesis location, track, and flavor of the world’s TCs on interannual timescales of 2 to 7 years. The Pacific Decadal Oscillation (PDO) is a longer-time scale mechanism (30-years) that has been linked to a variety of climate phenomena, most notably the Great Climate Shift of 1976. A cold/warm PDO is associated with more La Ninas/El Ninos, as has been observed during the past 60-years. The global historical TC records and ENSO + PDO tell us to a large degree what to expect in a given year with respect to tropical cyclone activity. While 2010 saw a bumper crop of Atlantic storms accurately forecasted by everyone expecting a transition to La Nina (except for that sorta important US landfall #) during the current North Atlantic active period , the Pacific was historically quiet.

Many readers will recognize the figure above as the trace of Accumulated Cyclone Energy (ACE) which is easily calculated from the historical hurricane records. It’s apparent that considerable swings in activity have occurred during the past 40-years, which is still way-too-short of a time series to say anything meaningful about the acknowledged interdecadal climate mechanisms at play here. Regardless, global ACE during 2010 sunk to lowest levels since 1977. The fantastic dearth of November and December activity globally has pushed the tally lower, and I am beginning to wonder when/if a “recovery” will occur. Is this 5-year lull in overall global TC activity going to continue? While prognosticators are heralding an active Southern Hemisphere TC season ahead, the expected continuation of La Nina will not only continue into a bone-chilling Northern Hemisphere winter, but also depress 2011 Pacific TC activity.

Here’s a new figure that may be rather interesting:

Not only has the ACE plunged to decades lows, but now the frequency/number/count of global tropical storms and hurricanes has followed suit. So during the past few years, we have experienced fewer AND less intense AND shorter duration tropical cyclones globally all at the same time — and while the planet is experiencing the warmest year ever. Here is the 12-month running sum image. During the past 12-months, our Blue Marble has spun up 64 named tropical cyclones, the lowest number on record for any consecutive 12-month period since 1970. The previous low was October 1976-September 1977 with 67 tropical storms, during the so-called Great Climate Shift. The Southern Hemisphere would need to produce at least 25 storms during the next four months to pick that statistic out of the tank.

Bottom line: The 2010 *new* WMO consensus (Knutson et al.) on tropical cyclones and climate change is summarized on Roger Pielke Jr’s blog: “…we cannot at this time conclusively identify anthropogenic signals in past tropical cyclone data.” Thus, it is perhaps best for Al Gore to continue his Photoshop endeavors to move merchandise since Mother Nature is not cooperating.

For those interested in the raw numbers and additional information about global TC activity, my continually updated FSU webpage has plenty of fodder.

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