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GW and the 2006 no-hurricane season

Posted by GliderGuider (A) on January 2, 2007 at 11:22:51

Down below, Road Warrior mentioned the lack of Atlantic hurricanes this last year as a datum that argues against global warming. That got me interested enough to poke around and learn a bit more about what affects hurricane formation. It turns out that the lack of activity last year isn't evidence against GW, it just illustrates how conditions in one part of the world can affect those elsewhere through un-obvious linkages.

Hurricane formation is intimately linked to heat energy in the atmosphere, that powers the thunderstorms that in turn seed the hurricanes. If the atmosphere doesn't warm up enough in the critical Atlantic regions, the hurricanes don't form. What keeps the air from warming up? Dust carried our over the ocean from the Sahara and Sahel by the Saharan Air Layer. If the conditions in the Sahara and Sahel become drier, the desert expands, exposing more sand. In addition, drought conditions mean the sand doesn't get washed out of the air.

So in an odd sort of way, the lack of Atlantic hurricanes may be evidence in support of global warming. Warming of the Atlantic waters may contribute to hurricanes in some years, and an increase in Saharan dust may act to suppress them in other years. It's all about climate variability.

The summertime dust that affects Atlantic tropical storms originates over the southwestern Sahara (18 - 22 N) and the northwestern Sahel (15 - 18 N). The dust that originates in the Southwest Sahara stays relatively constant from year to year. However, the dust from the northwestern Sahel varies significantly from year to year, and understanding this variation may be a key factor in improving our forecasts of seasonal hurricane activity in the Atlantic. The amount of dust that gets transported over the Atlantic depends on a mix of three main factors: the large scale and local scale weather patterns (windy weather transports more dust), how wet the current rainy season is (wet weather will wash out dust before it gets transported over the Atlantic), and how dry and drought-damaged the soil is.

The level of drought experienced in the northwestern Sahel during the previous year is the key factor of the three in determining how much dust gets transported over the Atlantic during hurricane season, according to a January 2004 study published in Geophysical Research Letters published by C. Moulin and I. Chiapello. In 2005, precipitation across the northwestern Sahel averaged near normal, so I'm a bit surprised we saw so much dust over the Atlantic. So far in 2006, precipitation in the northwestern Sahel has been lower than in 2005.

If the research cited above is any indication, we should have at least as much dust over the Atlantic during the 2007 hurricane season as the 2006 hurricane season had, which should act to hamper hurricane formation in the region between Africa and the Lesser Antilles Islands.

• http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=598&tstamp=200612&page=1   (Open in New Window)

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