La Niña weaker; may be gone by summer

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 Niña event are beginning to show up. A borderline moderate/strong La Niña 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 Niña 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 Niña 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 Niña event. An animation of SSTs since late November shows this developing warm tongue nicely. Springtime is the most common time for a La Niña event to end; since 1950, half of all La Niñas ended in March, April, or May. The weakness displayed by the current La Niña event has prompted NOAA's Climate Predictions Center to give a 50% chance that La Niña will be gone by June. If La Niña 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 Niña event. Image credit: NOAA Climate Prediction Center.

What does this mean for hurricane season?
As many of you know, the phase of the El Niño/La Niña is critical for determining how active the Atlantic hurricane season is. La Niña or neutral conditions promote very active Atlantic hurricane seasons, while El Niños sharply reduce Atlantic hurricane activity, by increasing wind shear. Will the probable demise of La Niña this spring allow an El Niño 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 Niña events during February (26% of all years.) In half of those years, La Niña 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 Niño. So historically, the odds do not favor a transition to El Niño by hurricane season. The latest set of computer model forecasts of El Niño/La Niña (Figure 2) also reflect this. Only two of the sixteen models predict El Niño conditions by hurricane season.


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

I'll have a new post by Thursday.

Jeff Masters