Tuesday, October 02, 2007

The North Atlantic Oscillation

For the past five years I've spent several weeks each winter with the Humpback Whales of the Silver Bank in the Dominican Republic. I'm planning to go again this year. Naturalist Tom Conlin of Aquatic Adventures leads expeditions that give us the opportunity to observe these magnificent animals and their fascinating behaviour.

Being moored 80 miles offshore on the Silver Bank, I've taken a keen interest in the weather. In the winter of 2005 (February/March) in particular, I noticed a seemingly relentless series of "cold fronts" which brought high winds, rain and cooler weather from the north. Typically the wind on the Silver Bank (which is north of the Dominican Republic and south of the Turks and Caicos) blows from the east at 10-15 knots and brings pleasant warm temperatures in the 80's farenheight. The cold fronts would bring higher winds (20-30 knots), cooler temperatures and rain, often in a series of very localized squalls.

The North Atlantic Oscillation seems to be a major factor in the winter weather for a vast region including the Silver Bank: see the maps on this NAO page at Columbia University. It appears that when the NAO index is negative, the easternly winds reaching the Silver Bank will be weaker. The page also says the the Eastern U.S. experiences more cold air outbreaks. The Silver Bank cold fronts seemed to originate from cold systems in the U.S. which then moved south (my interpretation from looking at the satellite weather maps available on the boats).

So the question is: was the NAO index actually particularly negative in the winter of 2005? And of course, what are is prospect for the upcoming winter of 2008?

And the answer is ...

Apparently the NAO index was dropping rapidly in the winter of 2005, it was in transition from a fairly high index to a fairly low one. Here's a graphic I found at NOAA.

NOAA graphic

The next step would be to find actual weather data for the region as opposed to relying on my subjective recollections.

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