Pacific ENSO Update

3rd Quarter, 2005 Vol. 11 No. 3


Tranquil weather conditions prevailed throughout Micronesia and the USAPI during the first half of 2005, and continued into the months of July and August. Through the first half of 2005, most islands have had near normal rainfall. Other than very dry conditions throughout most of Micronesia in February, and a spate of South Pacific hurricanes near American Samoa (also in February), there were no other noteworthy monthly anomalies, and very few noteworthy short-term extreme events. Nearly 11 inches of rain in 24 hours at the WSO Pohnpei on April 30 stands out as one of the most extreme rain events in Micronesia so far this year. Strong trade winds prevailed during the first few months of 2005, and high surf was unusually persistent at many locations. The number of tropical cyclones in the western North Pacific basin was lower than average, and tropical cyclone formation was north and west of normal, keeping most of the tropical storms and typhoons away from Micronesia.

According to the U.S. Climate Prediction Center (CPC), the climate of the tropical Pacific is now in ENSO Neutral. The weak El Niņo that began in the second half of 2004 ended in early 2005. The islands of Micronesia are typically drier than normal in the first few months during the year that follows an El Niņo, and for the whole year after El Niņo, the month-to-month variability of rainfall is not as great as it is during El Niņo (primarily a result of the westward shift of tropical cyclone development during the year that follows an El Niņo event).

For the first half of 2005, most of the islands of Micronesia had near normal rainfall (Fig. 1a, 1b). Only the northernmost islands, such as Guam, the CNMI, and Kwajalein, experienced persistent dryness that was considerably less than what is normally expected. January through June rainfall totals below 80% were experienced on Peleliu, Saipan, Tinian, Rota, Kwajalein and Utirik. January through June rainfall totals in excess of 120% of normal were experienced at WSO Chuuk and at Polowat. American Samoa was wetter than normal during the first six months of 2005. The highest official rainfall total recorded in Micronesia for the first 6 months of 2005 was the 107.47 inches at the Nautilus Hotel on Kosrae. The UOG/CSP experimental rain gage on top of Nahna Laud (Pohnpei Island's highest mountain) recorded 182.07 inches of rain during the first six months of 2005. The lowest recorded six month rainfall total during the first half of 2005 was the 14.05 inches of rain at Capitol Hill, Saipan. Near normal rainfall is anticipated throughout much of Micronesia and American Samoa for the next three to six months. Tropical cyclone activity, which has been very quiet in Micronesia so far in 2005, should return to near normal in the latter half of 2005 (see each island summary for the meaning of a "normal" tropical cyclone threat).

The general consensus among international computer climate forecasts is for a continuation of ENSO Neutral conditions for the next 3 to 6 months. For more information, see the CPC EL NIÑO/SOUTHERN OSCILLATION (ENSO) DIAGNOSTIC DISCUSSION for August on p. 12.