El Nino Returns
Posted July 13, 2009
After a period in which Pacific ocean temperatures have held for a while in a neutral or "La Nada" pattern, in the wake of a modest La Nina last year, the equatorial warming that we know as El Nino has been developing over the past month or two, and in June crossed some defining thresholds that appear to signal a new El Nino episode that is expected to strengthen and probably last at least into the coming winter. So far, it is rather weak, and model projections currently indicate a likely weak to moderate episode ahead.
The attached pair of maps from NOAA shows sea surface temperature and departure from normal over the Pacific, and you can make out a strip of temperatures along the equator that has reached an anomaly of +1 degree Celsius or so over the central and eastern ocean areas. The second image is a graph of these departures from normal averaged over four sections of ocean that specialists use to analyze the intensity of El Nino. In particular, the one labeled Nino 3.4 indicates El Nino conditions when the departure is more than one half degree in the positive direction (and vice versa for La Nina), a criteria that was surpassed during June. Finally, the last graph shows a measure of the average upper ocean heat content along the equatorial Pacific (not just surface temperatures, but including subsurface temperatures). When this variable runs above normal for a while, it often precedes a period of El Nino conditions, and there is a rather strong upward trend that has become apparent in this measure of heat in recent months.
What does all of this mean for us? Probably not a lot in the short term, as weak to moderate El Nino events have very modest climate signals in North Carolina, and much of the U.S., during the summer and early fall. The impacts increase late fall into winter, especially for more intense episodes, and can include cooler than normal temperatures and slightly enhanced precipitation for North Carolina, with the main precipitation effect over southeastern parts of the state (and a larger impact toward Florida).
Another well-publicized El Nino influence is its tendency to result in increased vertical wind shear across the Caribbean, and a resulting suppression of Atlantic hurricane frequency and intensity. It will be interesting to see if the next round of seasonal hurricane forecasts (issued in early August) cut back a little from the near to slightly above normal numbers that have been predicted so far...