Posted by: JeffMasters

A significant shift is occurring in the Equatorial waters of the Eastern Pacific off the coast of South America, where the tell-tale signs of the end to the current La Niņa event are beginning to show up. A borderline moderate/strong La Niņa event has been underway since last summer, with sea surface temperatures (SSTs) about 1.5°C below average over a wide stretch of the Equatorial Pacific. These cool SSTs have altered the course of the jet stream and have had major impacts on the global atmosphere. The La Niņa has been partially responsible for some of the extreme flooding events in recent months, such as the floods in Australia, Sri Lanka, and Colombia. La Niņa is also largely to blame for the expanding drought over the southern states of the U.S. But in the last few weeks, SSTs in the Equatorial Pacific have undergone a modest warm-up, and these temperatures are now about 1.2°C below average. A region of above-average warmth has appeared immediately adjacent to the coast of South America--often a harbinger of the end to a La Niņa event. An animation of SSTs since late November shows this developing warm tongue nicely. Springtime is the most common time for a La Niņa event to end; since 1950, half of all La Niņas ended in March, April, or May. The weakness displayed by the current La Niņa event has prompted NOAA's Climate Predictions Center to give a 50% chance that La Niņa will be gone by June. If La Niņa does rapidly wane, this should help reduce the chances for a continuation of the period of high-impact floods and droughts that have the affected the world in recent months.


Figure 1. A comparison of the the departure of sea surface temperature (SST) from average between this week and two months ago shows that a tongue of warmer-than-average waters has appeared off the coast of South America, possibly signaling the beginning of the end of the current La Niņa event. Image credit: NOAA Climate Prediction Center.

What does this mean for hurricane season?
As many of you know, the phase of the El Niņo/La Niņa is critical for determining how active the Atlantic hurricane season is. La Niņa or neutral conditions promote very active Atlantic hurricane seasons, while El Niņos sharply reduce Atlantic hurricane activity, by increasing wind shear. Will the probable demise of La Niņa this spring allow an El Niņo to take its place by this fall? Well, don't get your hopes up. Since 1950, NOAA's Climate Prediction Center records that there have been sixteen La Niņa events during February (26% of all years.) In half of those years, La Niņa was still active during the August - September - October peak of Atlantic hurricane season, six (38%) transitioned to neutral conditions, and only two (12%) made it all the way to El Niņo. So historically, the odds do not favor a transition to El Niņo by hurricane season. The latest set of computer model forecasts of El Niņo/La Niņa (Figure 2) also reflect this. Only two of the sixteen models predict El Niņo conditions by hurricane season.


Figure 2. Predictions made in January 2011 of the evolution of El Niņo/La Niņa over the coming year shows that only two of the sixteen models predict El Niņo conditions by hurricane season, while four predict La Niņa and ten predict neutral conditions. Image credit: Columbia University IRI.