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Pacific ENSO Update

3rd Quarter, 2012 Vol. 18 No. 3


During the first half of 2012, La Niña ended and the climate state of the tropical Pacific became ENSO-neutral. With the central and eastern equatorial Pacific sea-surface and sub-surface waters undergoing a gradual warming, El Niño is expected to develop in the second half of 2012. The atmosphere however, has yet to exhibit strong signals of impending El Niño, and in fact, the unusual tranquil weather similar to what has been observed over the past decade has persisted. The monsoon has been weak or absent, rainfall throughout most of Micronesia has been near normal (see figure 1), and there have been few notable extremes of climatic elements, except for persistently high sea levels. At nearly all recording locations, the rainfall during the first half of 2012 was much less than during the first half of 2011 when a similar shift of La Niña to ENSO-neutral occurred. Abundant rainfall typically occurs in years that begin as La Niña and transition to ENSO-neutral or to El Niño sometime later in the year. The recent high rainfall at Kapingamarangi (located close to the Equator) is one climatic item that is typical of the early stages of El Niño. Another typical harbinger of El Niño observed in the first half of 2012 was a spate of tropical cyclone developments in May and June; but this did not persist, and the tropics once again became unusually quiet during July. Because of the recent decadal trend toward reduced numbers of tropical cyclones in the western North Pacific basin, the City University of Hong Kong Laboratory for Atmospheric Research declined to issue further outlooks for tropical cyclone activity there until the cause of the recent trend can be established and incorporated into the forecasts (see the tropical cyclone discussion on page 3 for more information).

During July 2012, two consecutive hurricanes (Daniel and Emilia) formed in the eastern Pacific Ocean and progressed westward toward the central Pacific. However, neither system had a significant impact on the Hawaiian Islands, as they encountered unfavorable environmental sea surface temperature and wind shear conditions in the central Pacific. The Lihue, Honolulu, Kahului, and Hilo airport observing sites look to finish the month of July with well below-normal rainfall.

The following comments from the EL NIÑO/SOUTHERN OSCILLATION (ENSO) DIAGNOSTIC DISCUSSION were posted on the U.S. Climate Prediction Center/NCEP and the International Research Institute (IRI) for Climate and Society web site on July 5th, 2012:

ENSO Alert System Status: El Niño Watch

“Synopsis: Chances increase for El Niño beginning in July-September 2012.

During June 2012, ENSO-neutral continued as reflected in both the oceanic and atmospheric anomalies. However, positive equatorial Pacific sea surface temperature (SST) anomalies have grown, exceeding +0.5°C across the eastern Pacific Ocean by the end of June. SST anomalies increase moving from the westernmost Niño 4 region to the Niño 1+2 region adjacent to South America, which remained near +1.5°C during the month. The oceanic heat content anomalies (average temperature in the upper 300m of the ocean) increased during June, as above-average sub-surface temperatures became more entrenched in the equatorial Pacific. This warming was consistent with a weakening of the low-level trade winds across the east-central equatorial Pacific, along with a weakening of the persistent pattern of enhanced convection near Papua New Guinea. The observations are consistent with ENSO-neutral, but reflect a likely progression towards El Niño.

There continues to be a substantial disparity between the statistical and dynamical model SST forecasts for the Niño-3.4 region. The dynamical models, including the NCEP Climate Forecast System, largely favor the development of El Niño by July-September 2012, while the majority of statistical models predict ENSO-neutral through the rest of 2012. The forecaster consensus largely favors the dynamical model outcome because those models tend to exhibit greater skill emerging from the Northern Hemisphere “spring barrier”, and also due to the strengthening of observed signals indicating an evolution towards El Niño.”

Pacific ENSO Applications Climate (PEAC) Center
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Honolulu, HI 96822
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Page Last Modified: December 12 2012 23:18:31 GMT


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