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Pacific ENSO Update

4th Quarter, 2009 Vol. 15 No. 4


According to the U.S. Climate Prediction Center (CPC), weak El Niño conditions continued through September in the tropical Pacific. While oceanic and atmospheric indices of El Niño remain weak, they are anticipated to elevate to moderate levels in the next few months. In contrast to the past two years of tranquil weather throughout most of the USAPI, the shift of the climate to El Niño has brought more extreme weather conditions, including typhoon passages, heavy rain events, and high surf to many of the northern hemisphere islands.

During the 3rd Quarter of 2009, there was a mixed-bag of rainfall anomalies across the USAPI, with some islands very wet (e.g., American Samoa, Guam WSO, and Kapingamarangi, with 3-month rainfall in excess of 125% of normal), and some islands were quite dry (e.g., Woleai, Polowat, Wotje, and Utirik, with 3- month rainfall below 80% of normal) (see Figure_1). Hawaii was also drier than normal at most stations. Hurricane Felicia brought much needed rainfall in August, but not enough to relieve many leeward regions of the continuing drought situation.

The monsoon of the western North Pacific has been active, with several episodes of westerly winds within Micronesia that extended all the way to the RMI. Some tropical cyclones passed through the region, with two serious island strikes: Ngulu (an atoll in Yap State), and Alamagan (a remote high island in the CNMI). Tropical cyclone activity in the central Pacific was also enhanced, which is typical during an El Niño year.

Near normal rainfall is anticipated throughout much of Micronesia during the next three months, thereafter declining to below normal conditions for the first several months of 2010. Typically it becomes drier than normal throughout Micronesia and Hawaii in the winter and spring months (e.g., January to June 2010) that follow an El Niño year (e.g., 2009). Tropical cyclone activity is expected to be enhanced in Micronesia through January of 2010. Additionally, tropical cyclone activity will likely be enhanced in American Samoa beginning in late November and extending through late April of 2010.

Sea-level variation in the USAPI is sensitive to the ENSOcycle, with low sea level observed during El Niño years and high sea level during La Niña years. Sea levels have been above normal since early 2007. Current forecasts indicate that sea levels will remain slightly elevated at all USAPI stations for another 1 to 3 months, although sea level at all locations should begin receding considerably toward normal. No further rise is expected.

The following comments from the EL_NIÑO/SOUTHERN_OSCILLATION_(ENSO)_DIAGNOSTIC_DISCUSSION were posted on the U.S. Climate Prediction Center web site on October 8, 2009:

ENSO Alert System Status: El Niño Advisory
Synopsis: El Niño is expected to strengthen and last through the Northern Hemisphere winter 2009-2010.

“A weak El Niño continued during September 2009, as seasurface temperature (SST) anomalies remained nearly unchanged across much of the equatorial Pacific Ocean. Since the transition to El Niño conditions during June, the weekly values of the Niño-3.4 index have remained between +0.7C and +0.9C. Subsurface oceanic heat content (average temperatures in the upper 300m of the ocean) anomalies continued to reflect a deep layer of anomalous warmth between the ocean surface and the thermocline, particularly in the central and east-central Pacific.”

“The pattern of tropical convection also remained consistent with El Niño, with enhanced convection over the west-central Pacific and suppressed convection over Indonesia. In addition, two westerly wind bursts were observed over the western equatorial Pacific, the first occurring early in the month and the second occurring near the end of the month. These oceanic and atmospheric anomalies reflect an ongoing weak El Niño.”

“A majority of the model forecasts for the Niño-3.4 SST index suggest that El Niño will reach at least moderate strength during the Northern Hemisphere fall (3-month Niño-3.4 SST index of +1.0C or greater). Many model forecasts even suggest a strong El Niño (3-month Niño-3.4 SST index in excess of +1.5C) during the fall and winter, but in recent months some models, including the NCEP CFS, have overpredicted the degree of warming observed so far in the 3.4 region. Based on the model forecasts, the seasonality of El Niño, and the continuation of westerly wind bursts, El Niño is expected to strengthen and most likely peak at moderate strength.”

“Expected El Niño impacts during October-December 2009 include enhanced precipitation over the central tropical Pacific Ocean and a continuation of drier-than-average conditions over Indonesia.”

Pacific ENSO Applications Climate (PEAC) Center
c/o NOAA NWS - Weather Forecast Office Honolulu
2525 Correa Road, suite 250
Honolulu, HI 96822
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Page Last Modified: June 01 2010 21:01:46 GMT


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