Pacific ENSO Update

1st Quarter, 2006 Vol. 12 No. 1


On February 2nd, NOAA's Climate Prediction Center announced the official return of La Niña. According to the press release, "Oceanic sea surface temperatures have met the operational definition of La Niña for the November through January period. La Niña is the periodic cooling of ocean waters in the east-central equatorial Pacific, which can impact the typical alignment of weather patterns around the globe. Noaa predicts this event will likely remain into9 late spring and possibly into late summer."

Typically, the islands of Micronesia are slightly wetter than normal during a La Niña year, unless it is the year directly following El Niño. The month-to-month variability of rainfall is less during La Niña than during El Niño; this is primarily a result of the westward shift of tropical cyclone development during thisLa Niña. The typhoon threat is reduced at most islands during La Niña, as it was during the latter half of 2005 when climatic conditions trended toward La Niña.

During 2005, most of the USAPI had near normal annual rainfall with annual totals falling between 80% and 120 % of the long-term average (Fig. 1a, 1b). Annual rainfall totals for 2005 exceeded 120% of normal only at a few locations including American Samoa, Polowat and Kapingamarangi. Annual rainfall totals for 2005 were less than 80% of normal only at some locations in the northern RMI, including Kwajalein, Wotje and Utirik. The highest official annual rainfall total recorded in Micronesia during 2005 was the 217.87 inches at Palikir, Pohnpei Island followed next by the 200.21 inches recorded at the airport on Kosrae. Rainfall on the summit of Nahna Laud (the highest mountain in the interior of Pohnpei Island) was once again over 300 inches (according to a rain gage placed there as part of the University of Guam/Conservation Society of Pohnpei experimental rain gage network). The lowest recorded annual rainfall total during 2005 was the approximately 41 inches of rain at Wotje in the northern RMI. Abundant rainfall is anticipated throughout much of Micronesia and American Samoa for the next three to six months. Tropical cyclone activity, which was very quiet in Micronesia during 2005, should be slightly reduced for most of 2006, returning to near normal during September through December of 2006 (see each island summary for the meaning of a “normal” tropical cyclone threat).

Collectively, the present oceanic and atmospheric anomalies are consistent with the development of weak La Niña conditions in the tropical Pacific. The most recent EL NIÑO/SOUTHERN OSCILLATION (ENSO) DIAGNOSTIC DISCUSSION stated that “current conditions (stronger-than-average easterly winds over the central equatorial Pacific) and recent cooling trends in observed oceanic conditions support the continuation of La Nina conditions in the tropical Pacific during the next 3-6 months.”.