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Pacific ENSO Update

2nd Quarter, 2009 Vol. 15 No. 2


According to the U.S. Climate Prediction Center (CPC), the present oceanic and atmospheric anomalies indicate an ongoing transition from La Niña to ENSO-neutral. Precipitation and surface wind anomalies in the tropical Pacific have reflected La Niña for the past several months, characterized by a westward retraction of deep tropical convection toward Indonesia, suppressed precipitation centered on the Date Line, and enhanced easterly winds across most of Micronesia. The weather across the USAPI has remained tranquil, with no extremes of high wind and few extremes of heavy rain. For the past two months, persistent dry conditions have threatened the water resources of some atolls in the RMI. The most troublesome aspect of recent La Niña conditions has been elevated sea level. During the 1st Quarter of 2009, most of the islands of Micronesia, Hawaii and American Samoa had rainfall totals that were near normal (see Figures 1a, 1b). Unusually persistent trade winds continued to dominate the weather across Micronesia. There were no early season tropical cyclones in Micronesia, and tropical cyclone activity in the South Pacific was mostly confined to Australian waters eastward to Fiji.

Although the present state of the Pacific climate appears to be transitioning from La Niña to ENSO-neutral, the wind and rainfall patterns affecting the USAPI are still those typical of La Niña. These anomalies, which include enhanced trade winds, above normal sea level and a suppression of tropical cyclone activity, could continue through June. The very strange collection of climate anomalies that existed in the western Pacific for most of 2008 (persistent strong easterly wind, nearly complete suppression of tropical cyclone activity eastward of Guam’s longitude, and high stands of the sea throughout Micronesia) are not likely to recur in such strong measure during the coming months of 2009.

Near normal rainfall is anticipated throughout much of Micronesia during the next 3 to 6 months. Tropical Cyclone activity, which has been very quiet in Micronesia and throughout much of the western North Pacific for the past two years, should rebound somewhat during 2009, but will still be relatively quiet everywhere eastward of the Marianas (see each island’s summary for the meaning of a “normal” versus a “suppressed” tropical cyclone threat).

Sea level variation in the USAPI is sensitive to the ENSO cycle, with low sea level observed during El Niño events and high sea level observed during La Niña events. Sea levels have been higher than average in the Pacific since early 2007. Current forecasts indicate that sea level across the USAPI will remain elevated 2-6 inches above normal for the next 3 months, while sea level at Hawaiian stations is expected to remain near normal.

The following comments were taken from WFO Guam’s MONTHLY PACIFIC ENSO DISCUSSION FOR MICRONESIA AND AMERICAN SAMOA, issued in April 2009:

“While sea surface temperatures (SSTs) remained cooler than normal in the eastern and east-central equatorial Pacific, they continued to exhibit warming throughout the month. Positive subsurface heat content anomalies in the upper 300 meters of the equatorial western and central Pacific Ocean pushed eastward, while negative subsurface anomalies weakened in the eastern Pacific. During most of April, the atmosphere continued to exhibit La Niña characteristics, with below normal convection near the International Date Line and enhanced convection across Indonesia. Equatorial low-level easterly and upper-level westerly winds also continued across the Pacific during March and into April. CPC noted that ‘Collectively, these oceanic and atmospheric anomalies are consistent with a weakening La Niña.’

CPC expects La Niña to give way to ENSO-neutral conditions by May. Most of the latest climate forecast models support this view. This week, equatorial low-level westerly winds penetrated eastward to 145ºE. In addition, the strong persistent trade winds of early April have temporarily weakened. These occurrences support a weakening of the atmospheric affects of La Niña. As La Niña wanes, the trade winds will weaken, and sea levels will slowly decrease for the next couple of months. This should reduce the risk for non-tropical cyclone-related inundation to the high and low islands. With the emergence of equatorial westerly winds, we could see a more typical May-June beginning of tropical cyclone activity in western Micronesia. American Samoa will be moving into its dry season, which should be relatively normal. Rains have returned to most locations in Micronesia south of 9ºN. The northern Republic of the Marshall Islands, northern Yap State and the Marianas will likely have below normal rainfall for the next month or so. Chuuk and Pohnpei States and the southern Marshall Islands should return to near normal rainfall, while Kosrae State and the Republic of Palau will likely have normal to above normal rainfall.”

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 22:44:43 GMT


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