Wednesday, April 20, 2011

Atlantic Hurricane Activity and the Southern Oscillation Index

(published in commodityweather)

We know that El Nino years tend to correlate with decreased tropical activity in the Atlantic Basin, usually resulting fewer named storms.  This does not mean that there won’t be any significant hurricanes during El Nino years, but the overall activity will be suppressed by cooler Sea Surface Temperatures (SST) across the central Atlantic, fewer storms generating off of the West African coast, and increased shearing.  So during La Nina years, the probability of more active seasons tends to increase.  Recently, the Southern Oscillation Index has weakened, but is still strongly in positive phase (+SOI is a La Nina indicator) and the central and northern Atlantic has a positive SST profile.  Most dynamical and statistical seasonal models are showing that the La Nina has peaked, but the transition to a neutral Nino state may still take some time.  This means that La Nina conditions can still be present when we enter the 2011 hurricane season, and that may guide how the season starts.
In the chart above, the blue bars depict the number of classified hurricanes (greater than Category 1) each year between 1966 and 2010, and the red line represents the average April through September SOI value.  Note that the general pattern of higher groups of years with positive SOI values typically corresponds with more active tropical seasons over the last four decades.  While there is still uncertainty surrounding the behavior of La Nina during the hurricane season, as we approach the start to the 2011 hurricane season, indicators are pointing to another active year.

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