Look Out for More Hurricanes: La Nina Is Here

We weather nerds have been expecting this for some time. But now it's official: According to the National Oceanic and Atmospheric Administration, La Nina conditions have officially set in across the tropical Pacific ocean. Why should you care? Because among many other things, La Nina -- the twin sister of El Niño -- generally means long and particularly active Atlantic hurricane seasons.

During El Nino, the pool of deep warm water located in the western Pacific moves eastward all the way to the coast of the Americas. As a result, that extra warmth triggers dramatic thunderstorm and hurricane activity over the ocean. Particularly for the Eastern Pacific region, one often sees enhanced hurricane activity during an El Nino year.

Like El Nino, La Nina too has a dramatic influence on global weather patterns, but in the other direction. As you can see in the figure below, rather than warm anomalies -- which we saw during the relatively quiet 2006 Atlantic hurricane season -- we're now witnessing cool anomalies in the Pacific regions that are generally used by scientists to determine whether the ocean is in an El Nino, La Nina, or neutral state: Furthermore, when scientists examine the forecasting models that they use to determine how conditions should trend in the future, most show a continuation -- or intensification -- of La Nina over the next several months.

Now let's turn to the implications for hurricanes. As one scientific paper puts it:

During cold events in the eastern Pacific, the odds are significantly higher that the U.S. will experience greater impacts because of a larger number of tropical cyclones and higher intensities for each storm. Over the 73 years covered by this study, the total numbers of tropical storms and hurricane landfalls were 58 during El Nino years versus 82 during La Nina years. The average Saffir/Simpson category of landfalling tropical cyclones (counting tropical storms as zero) is 0.93 during El Nino years and 1.33 during La Nina years. This translates to a modest, but significant (at the 94% level), difference of about 6 m/s in wind speed (from 30.6 to 36.3 m/s). Because damage increases with at least the square of wind speed (Pielke and Landsea 1998), the greater intensity translates to a substantial increase in damage. The average damage per storm in El Nino years is $800 million vs. $1,600 million in La Nina years.

Note that these figures are from a paper published well before the dramatically damaging 2004 and 2005 hurricane seasons or the relatively quiet 2006 one (which, again, was an El Nino). But my sense is that scientists generally agree that such findings remain quite accurate.

One reason that El Nino tends to be much easier on the U.S. for hurricanes is that the intense Pacific thunderstorm activity that it triggers in turn affects wind patterns over the Atlantic. In particular, El Nino tends to create a situation in which developing hurricanes are more likely to be inhibited or even torn apart by so-called vertical wind shear. Once again, with La Nina it's the converse: There tends to be a lessening of vertical wind shear over the Atlantic. Meanwhile, more hurricanes develop from so-called African easterly waves, and these storms tend to be stronger and to threaten the U.S. and Caribbean, rather than re-curving harmlessly out to sea. As a result, NOAA explains: "The chances for the continental U.S. and the Caribbean Islands to experience a hurricane increase substantially during La Nina, and decrease during El Nino."

So in short, it looks like the 2006 and 2007 Atlantic hurricane seasons may be mirror opposites. We've already seen two Category 5 Atlantic hurricanes this year, and with La Nina in place, it's a safe bet that we have a long way yet to go before we're totally in the clear.