November 20, 2010

From La Nina to What?

Filed under: Oscillations — Erin @ 9:03 pm

We are currently in a La Nina episode of moderate strength. La Nina is the stage of the El Nino-Southern Oscillation in which the equatorial Pacific is cooler than average:

La Nina

This episode has likely played a large part in the 2010 Atlantic hurricane season’s intensity and length. It probably also affected large-scale weather patterns during the intense drought over the American South (which still continues, despite rain, because have a significant rain deficit to make up.) La Nina is expected to continue through the winter. La Nina winter conditions for the Southeast are, in my opinion, wretched: Warmer and drier than normal. If we get a snow event, it’ll be in spite of the ENSO pattern. However, by no means should snow be ruled out; winter precipitation is determined by short-term synoptic-scale weather systems more than anything else. Indeed, our first freeze this year happened earlier than last year, which was an El Nino winter (El Nino winters for the Southeast are cooler and wetter than usual).

This La Nina event is following a pattern and timeline of strengthening most similar to the 1998/1999 La Nina event, as the official list of ENSO statuses indicates. That year was not known as a “wintry” winter for the South. There was a devastating ice storm in some areas, but the storm’s description makes it quite clear that this was an aberration that effectively defied the La Nina conditions that had been controlling the winter previously:

Much of the fall and early winter season of late 1998 had been very mild for southerners. But on Tuesday December 22nd, a strong surge of shallow Arctic air drove southward and spelled a quick end to the unseasonable warmth and helped set the stage for one of winter’s nasty tricks. A moist southwest flow above the cold air brought several upper level disturbances. This combination of cold air and moisture held together for several days and brought periods of freezing rain and sleet to the three state region, beginning Tuesday night and not ending until Christmas morning. Up to 2 inches of ice accumulated on power lines and much of the area experienced long power outages, nearly seven days in some cases. Hardest hit was an area from northeast Louisiana to north central Mississippi where tree and power line damage was moderate to severe. The prior warmth had left most trees with a little more foliage than usual and greater potential for ice accumulation.

(Please note that I am NOT forecasting an ice storm for the Southeast this winter! It is not possible to forecast an event like this far in advance.)

I have a growing suspicion that the most profound impact, or influence perhaps, of this year’s La Nina will actually be felt next year. There is a link between intense tornado seasons and a La Nina the previous winter. Some notorious examples are 2008 (the Super Tuesday tornado outbreak, among others), 1999 (the record F5 tornado in Oklahoma), 1974 (the unprecedented and historically unique Super Outbreak, which had either 6 or 7 F5 tornadoes), and 1971 (the most recent F5 tornado in Mississippi). The mechanics of this phenomenon are not well understood yet, and the statistical link is not as highly correlated as it might be. In fact, there may even be a lesser link between El Nino winters and an active tornado season. But there is a growing consensus in meteorology that La Nina winters tend to precede active springtime tornado seasons.

The long-range ENSO computer models are suggesting that this La Nina episode may go away by next year, leaving us in a slightly negative neutral state:

ENSO models

If this plays out, we should consider another long-term implication, the effect of the assumed demise of La Nina on next year’s hurricane season. From this far ahead in time, we have no skill at forecasting any other factor except the Atlantic multi-decadal oscillation that entered a warm phase in 1995, so the status of ENSO would play a large part in long-range hurricane season forecasts. From the current warm phase of the AMO, hurricane seasons in which a La Nina episode from the previous winter met its demise include 2008, 2001, 2000, and 1996. From the previous warm AMO phase, such years include 1968, 1963, 1957, and 1951. However, I do not think that these years from the 50s and 60s are that applicable to this year, because this year’s La Nina is already stronger than any of those were. A couple of them were quite shallow, in fact. And three of the years saw the development of an El Nino following the La Nina, an occurrence that right now seems unlikely.

Given the strength of our current La Nina already, I actually would not be that surprised if the computer models shifted to predict the continuation of La Nina through next winter. Some of them do, in fact. In any case, whether we shift to neutral by next summer or remain in a (weakened but reintensifying) La Nina, it seems that next year’s hurricane season is likely to be active just as this year’s was. 2011 has the potential of being quite a headline-generating weather year!

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