La Nina conditions have officially ended, as the Climate Prediction Center recently released the final advisory on our now-departed La Nina. Still, the atmosphere - especially over North America - remains in a very lethargic, La Nina-like configuration.
The image above left is a global plot of sea surface temperature (SST) anomalies. The area of interest is the equatorial Pacific, where there is now virtually no blue painted on our map from Indonesia to the South American coast. SSTs in the tropical Pacific are no longer cooler than normal, and are actually a little warmer than normal. Most of the equatorial Pacific surface waters are averaging about 0.3 degrees Celsius warmer than normal of late.
The warm anomalies also extend to some depth below the surface. The plot at left shows the depth of the temperature anomalies in the equatorial Pacific as of May 13. Longitude is on the horizontal axis, with the left edge of the plot representing Indonesia, and the right edge South America. Depth is on the vertical axis, with the surface at the top and greater depths toward the bottom. While the anomalies at the surface of the Pacific may not be terribly impressive, there is a substantial reservoir of warmer than normal water just below the surface, which extends across almost the entire basin. Much of this water is a solid 2 degrees Celsius warmer than normal. Some of this warm subsurface water is making its way to the surface in the eastern Pacific.
Despite the presence of weak warm anomalies at the surface, and the large pool of warmer water lurking below the surface, conditions are currently ENSO-neutral. However, given the warm subsurface water (which can often be a predictor of surface temperatures in a few months), it is likely that additional warming will continue heading into the summer months. At left is the latest ENSO forecast plume from the International Research Institute for Climate and Society, which displays a collection of ocean temperature forecasts from various dynamical and statistical models. Most of the forecasts show increasing SST anomalies, though most keep the values of the anomalies within the ENSO-neutral category (from -0.5 Celsius to +0.5 Celsius). Some of the models do predict the development of El Nino conditions later this year.
In the near term, the consensus is for the ENSO-neutral conditions to continue into the summer months. By the Fall, there is a chance that continued warming could bring about El Nino conditions. IRI pegs this chance at a little less than 50/50. This could have implications for the upcoming Atlantic hurricane season, which begins officially on June 1. Should El Nino conditions manifest earlier, or be stronger than the current thinking suggests, it could quash tropical activity in the Atlantic in the latter part of the season, similar to what was seen in 2006 when no storms were observed after October 2 due to the onset of El Nino. With this in mind, many of the recent updated tropical season outlooks have been revising their forecast numbers downward, to account for the possible appearance of El Nino later in the season. The consensus is still for a season with activity levels above the long-term average, but not as great as the recent (1995-present) average.
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