It's official: El Nino is back. The warming taking place in the equatorial Pacific Ocean, which brought an end to La Nina earlier this year, has reached the degree that it now qualifies as an El Nino. The Climate Prediction Center issued their first advisory on the new El Nino on July 9.
The image at left shows the sea surface temperature (SST) anomalies across the Pacific basin, as of July 12 (click to open a full-size version in a new window). Most of the equatorial Pacific between Indonesia and the west coast of South America is painted in the reds indicating SSTs that are warmer than normal by 0.5-1.5 C. The SST anomalies near the South American coast are quite strong; the yellow shades indicate waters that are on the order of 2-3 C warmer than normal.
The SST anomalies, when averaged over a set of geographic regions within the equatorial Pacific, yield the standard temperature anomaly indices that are used to gauge the oceanic component of ENSO (El Nino-Southern Oscillation). Typically, the rule of thumb is that when these indices reach +0.5 C (or more), we are in El Nino. If the anomalies are -0.5 C (or less), we are in La Nina. In between is ENSO-neutral. Most of the SST indices are now in the range of +0.6 C to +0.9 C, so the situation now officially qualifies as El Nino.
This nascent El Nino may have some legs to it. Subsurface waters (down to about 1000 feet deep) across the equatorial Pacific are also warmer than normal, which tells us that there actually is a mass of anomalously warm water extending to some depth, and not just a thin skin of unusually warm waters at the surface. In addition, the forecast plumes from IRI also suggest this El Nino could be more than a passing fad. Most of the forecasts agree that the El Nino will continue into the fall season, and it could grow stronger. It isn't out of the question that the El Nino could grow to moderate strength.
The ramifications of El Nino on the weather in the U.S. are many. However, El Nino's impacts during summer are rather muted and it won't be until the fall and winter that the weather patterns over the U.S. begin to show a more definitive El Nino influence. What can be seen during the summer is the impact that El Nino has on the degree of tropical activity in the Atlantic basin. Typically, El Nino generates stronger winds than usual at high levels in the atmosphere. These stronger winds in turn lead to greater wind shear, and wind shear is the enemy of many a tropical cyclone. The development of El Nino was anticipated to a certain degree in many of the seasonal outlooks for hurricanes and tropical storms in the Atlantic, and is the primary reason why most of the outlooks call for near or below normal activity.
In our next entry, we'll take a look at some recently published research documenting a variant of El Nino, known colloquially as "modoki El Nino." In this form, the warm anomalies in the equatorial Pacific are displaced to the central Pacific as opposed to the eastern Pacific, which could affect the degree of suppression of tropical activity in the Atlantic.
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