Friday, August 21, 2009

Hurricane Bill, the Atlantic hurricane season and Pacific warming

The first Atlantic hurricane of the season, Hurricane Bill, is on its way north towards the east coast of Canada (and creating huge waves in Bermuda and the northeastern US). Forecasters expect Bill to pass the coast of Nova Scotia on Sunday and run towards Newfoundland and Labrador.

The offshore path of the first and only hurricane of the Atlantic season brings to mind an interesting paper published earlier this summer, that warrants more attention than (I think) it received. In this post a few weeks back, before I disappeared for the Ontario leg of the Canadian Summer of ’09 heat wave tour (ah, Rex Murphy, what happened to global cooling?), I briefly mentioned the paper by Kim et al. that looked at the response of Atlantic hurricane activity to different types of El Nino events.

The conventional thinking is that Atlantic hurricane activity is low during El Nino events. Basically, the increase in eastern and central equatorial Pacific Ocean temperatures that happens during El Ninos shifts upper-level atmospheric circulation, which in turn, creates wind shear in Atlantic that disrupts hurricane activity.

Kim et al. went an important a step further. They found that the relationship depends on the nature of the Pacific warming.

The above figure tells the story. When the warming occurs throughout the eastern equatorial Pacific (EPW), the more classic El Nino, there are significantly fewer hurricanes in the in the Caribbean and up the eastern seaboard of North America. When warming is centered in central equatorial Pacific (CPW), as happened in 2002 and 2004, there actually is a significant increase in hurricanes reaching North America. Central Pacific warming events basically cause less vertical wind shear.

The ongoing development of El Nino conditions in the Pacific is the major reason for predictions of a less active than normal hurricane season in the Atlantic (and an active eastern Pacific season). That Pacific surface temperature anomaly is currently centered in the eastern Pacific, which suggests that all other factors being equal, which of course they never are, there is a higher likelihood of hurricane track density depicted in Part A of the figure above.

Now one hurricane does not a season make: you could argue that Bill's track is loosely bucking that prediction. Nonetheless, it will be interesting to follow hurricane development this year, and during the next episode of central Pacific warming, to see whether Kim et al. are correct in asserting that location of all those flapping butterflies in the Pacific predictably determines the development of storms in the Atlantic.

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