Meteorologist Dr. Jeff Masters, in a WunderBlog repost.
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 Ni±a event are beginning to show up.
A borderline moderate/strong La Ni±a 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 Ni±a 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 Ni±a 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 Ni±a event. An animation of SSTs since late November shows this developing warm tongue nicely. Springtime is the most common time for a La Ni±a event to end; since 1950, half of all La Ni±as ended in March, April, or May. The weakness displayed by the current La Ni±a event has prompted NOAA’s Climate Predictions Center to give a 50% chance that La Ni±a will be gone by June. If La Ni±a 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 Ni±a event. Image credit: NOAA Climate Prediction Center.
What does this mean for hurricane season?
As many of you know, the phase of the El Ni±o/La Ni±a is critical for determining how active the Atlantic hurricane season is. La Ni±a or neutral conditions promote very active Atlantic hurricane seasons, while El Ni±os sharply reduce Atlantic hurricane activity, by increasing wind shear. Will the probable demise of La Ni±a this spring allow an El Ni±o 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 Ni±a events during February (26% of all years.) In half of those years, La Ni±a 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 Ni±o. So historically, the odds do not favor a transition to El Ni±o by hurricane season. The latest set of computer model forecasts of El Ni±o/La Ni±a (Figure 2) also reflect this. Only two of the sixteen models predict El Ni±o conditions by hurricane season.
Figure 2. Predictions made in January 2011 of the evolution of El Ni±o/La Ni±a over the coming year shows that only two of the sixteen models predict El Ni±o conditions by hurricane season, while four predict La Ni±a and ten predict neutral conditions. Image credit: Columbia University IRI.
— Dr. Jeff Masters.