Published: 2007-03-12 10:38:05
Updated: 2007-03-12 10:38:05
Posted March 12, 2007
By Mike Moss
A modest El Nino episode, involving warmer than normal sea surface temperatures in the central and eastern equatorial Pacific, faded rather rapidly from mid January into mid February, and after a brief period of near-neutral (sometimes called "La Nada" conditions) it appears the pattern is making a fairly quick transition toward a La Nina configuration which is almost the opposite of El Nino in that sea surface temperatures are cooler than normal. The implications for our weather are tough to pin down because La Nina often doesn't show a real strong influencing signal over our area during the spring and summer months, with just a slight tendency toward increased rainfall. In the winter, it often leaves the southeastern U.S. somewhat drier and warmer than normal, but is also known for some degree of instability in the large scale pattern through the winter months, with sudden development of deep troughs and bursts of very cold weather here. These episodes usually aren't all that frequent, but they do mean that just because a La Nina may tilt us toward an overall warm/dry winter, a few significant episodes of wintry weather can't be ruled out.
All of that, of course, is more relavant by now to next winter, assuming La Nina is still in place. While there are no guarantees, it is a reasonable assumption, as La Ninas that develop heading into Spring often maintain themselves into the the following spring. In terms of other weather effects to watch for, there have been some correlations between La Nina and enhanced tornado activity in the lower and middle Mississippi Valley region and possibly into North Carolina as well. Perhaps of equal or greater direct interest to us, while El Nino has a tendency to suppress tropical cyclone development in the Atlantic basin, La Nina tends to be associated with elevated numbers of tropical storms and hurricanes. It isn't a perfect correlation of course, because other factors come into play as well, but it will be interesting to see what kind of numbers we end up with in the upcoming season if La Nina holds into the Fall.
The image above is a recent satellite estimate of sea surface temperature departures from normal. The most relevant part of the map is the narrow band along the equator extending west from South America that is a rich shade of blue, indicating temperatures that have fallen to around one to two degrees celsius below normal.
You can also see an animation showing how the anomalies have changed through recent months at
and a similar animation showing a vertical cross-section of temperatures along the equator, where you can actually see cold subsurface water making its way eastward and toward the surface over the past couple of months, at
A recent NOAA press release about the Nino to Nina evolution is at