Post by Dr. Ryan N. Maue

Update: Andrew Revkin writes a nice piece at his DotEarth NY Times blog that “very politely” repudiates McKibben. Remember, the NY Times editorial board completely agrees with McKibben on the Tar Sands pipeline issue.

Update: ThinkProgess spins a narrative that says Irene is worse from global warming. (Disconnected, hand-wavy narrative)

Bill McKibben authors a bizarre piece in the Daily Beast where he not only blames the strength of Hurricane Irene on global warming but connects the storm to President Obama’s expected approval of the Keystone Pipeline transport of Canadian Tar Sands to terminals in the United States. While the second part of his thesis is political in nature, the first part is quite easy to fact check, and comes up woefully short. McKibben has no expertise in tropical cyclone science, and relies on the expert quotes of Weather Underground blogger Dr. Jeff Masters who has provided a laudable public service with his Irene coverage.

McKibben begins: “Irene’s got a middle name, and it’s Global Warming.”

I doubt there is a tropical cyclone scientist that would go on record and make such a foolish statement, but who knows.

Normally, says Jeff Masters of Weather Underground, it’s “difficult for a major Category 3 or stronger hurricane crossing north of North Carolina to maintain that intensity, because wind shear rapidly increases and ocean temperatures plunge below the 26°C (79°F) level that can support a hurricane.” The high-altitude wind shear may help knock the storm down a little this year, but the ocean temperatures won’t. They’re bizarrely high—only last year did we ever record hotter water. Sea surface temperatures 1° to 3°F warmer than average extend along the East Coast from North Carolina to New York. Waters of at least 26°C extend all the way to southern New Jersey, which will make it easier for Irene to maintain its strength much farther to the north than a hurricane usually can,” says Masters. “These warm ocean temperatures will also make Irene a much wetter hurricane than is typical, since much more water vapor can evaporate into the air from record-warm ocean surfaces. The latest precipitation forecast from NOAA’s Hydrological prediction center shows that Irene could dump over eight inches of rain over coastal New England.”

Masters is alluding to the process known as “extratropical transition” in which a fully tropical hurricane becomes enmeshed with the midlatitude westerlies and evolves into a more typical extratropical cyclone. The “tropical phase” hurricane encounters upper-level winds that are very strong which causes significant vertical shear. This shear “tilts” the hurricane inner-core — a situation that is not optimal for the maintainence of deep convection around the entire eye. Also as Irene reaches the Virginia border, it will encounter cooler SSTs, almost 10 degrees C cooler than its present location in the Bahamas. The combination of dry continental air entrainment and cooler SSTs will immediately decrease the inner-core convection and help to “poof” out the storm.

Here’s a model depiction of the rapid structure change expected with Irene: snapshots from the simulated GOES-12 brightness temperatures from the NCEP-NAM 12 km model.

Update 08/28/2011: Here’s the verifying satellite image, about 8-hour behind the third simulated panel…model did pretty good.

After 72-hours, Irene will look like a Nor’easter snowstorm on satellite with very cold cloud-tops on the NW flank or comma-head of the storm. Considerable rain will occur before Irene makes landfall, as well as during its trip up the eastern seaboard. However, the symmetric appearance of a major hurricane with an eye will be long gone.

Masters is quoted that “bizarrely high” SSTs along Irene’s path will cause Irene to be a much wetter and apparently longer-lasting hurricane that normal. This assertion is true if “all else is equal”. However, before attributing the “anomalous SST” to global warming, one must control all other variables in this complex situation. That requires considerable sensitivity research with state-of-the art numerical weather prediction (and climate) models. With very poor in-situ historical observations of the global oceans, it is still a quite daunting task to attribute SST anomalies in the meandering and variable Gulf Stream to global warming. This hand-waving theory may not hold up when a rigorous scientific hypothesis is tested, yet McKibben does not provide a citation or reference aside from Masters’ quotations, which are not peer-reviewed in the slightest.

I plot up daily the current SSTs as well as the anomalies for each August 25 from 1979 to 2011 for the North Tropical Atlantic here. The path Irene is expected to take does not go over “bizarrely high” SSTs by any stretch of the imagination. The 26-degree C isotherm is just about at its average location for the past 30-years.

If Irene occurred in September, the SSTs would be warmer than August, which does not imply that global warming aided the storm’s development. Thus, one must look at the variability (variance) of local and regional SSTs as well as the actual SSTs to gauge an accurate understanding of tropical cyclone intensity change. With the current track very similar to Floyd 1999, one should expect similar impacts in terms of precipitation and wind “if all else is equal”. However, nature rarely operates in text book manner especially in the field of meteorology.

While some tropical cyclone scientists are probably sympathetic with McKibben’s political goals, I will keep my eyes peeled for one that will go on record agreeing with McKibben’s stretched scientific logic. In his mind, Bush caused Katrina and Obama caused Irene. Hopefully McKibben and the media will let this crisis go to waste.

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