Pacific ENSO Update

1st Quarter, 2008 Vol. 14 No. 1


Moderate La Niña conditions are present in the Pacific Basin, and are expected to persist through the northern hemisphere’s spring months. During the calendar year 2007, the climate of the tropical Pacific steadily evolved from ENSO-neutral conditions toward full-blown La Niña conditions by year’s end. Oceanic cooling observed along the equator in the central and eastern Pacific increased in magnitude and extent during the course of the year. Climatic affects of La Niña were noted for much of 2007, and included well-known La Niña-related anomalies such as below normal tropical cyclone activity across most of Micronesia, a weak monsoon, and abnormally strong and widespread easterly surface winds in the low latitudes. The weather was generally tranquil at most locations, with few extremes of rainfall and damaging wind events. Rising sea levels (an effect of La Niña) coupled with high surf caused some problems with inundation, particularly on some of the atolls of Chuuk and Pohnpei states.

The 2007 annual rainfall at most of the U.S.-Affiliated Pacific Islands (USAPI) was somewhere in the range of 80% to 120% of normal at most locations (see Figures 1a, 1b). Abundant rainfall in the final three months of the year pushed many islands that were dry in the first half of 2007 into the near-normal range for 2007 totals. Annual rainfall totals were less than 75% of normal at only two locations: at some of the atolls of the northern RMI, and at Woleai in the southern part of Yap State. The lowest 2007 annual rainfall totals of 47.71 inches and 49.95 inches occurred at Utirik and Wotje (northern RMI), respectively. Annual rainfall totals for 2007 in excess of 115% of normal were seen across locations in American Samoa, Yap Island, and some of the atolls of Pohnpei State. The annual total of 221.98 inches of rain at Aasufou, American Samoa was the highest recorded value in the USAPI for 2007, followed by 199.41 inches of rain at the Kosrae airport and 193.76 inches of rainfall at the WSO Kolonia, Pohnpei Island.

While no typhoons directly affected any island in the western North Pacific basin during 2007, several of the year’s typhoons, evolved through their tropical disturbance and depression stages in portions of Micronesia, contributing rainfall to some locations from Pohnpei westward to Yap. Beginning in December 2007, the trough axis of the Australian northwest monsoon set up across the Indonesian sea eastward to Fiji and Tonga. Two Madden-Julian Oscillation (MJO)-related enhancements of the Australian Northwest Monsoon have occurred (one in early December and the other ongoing in mid-January). These MJO episodes result in an extension of the monsoon trough and its associated northwesterly monsoonal surface winds across the Coral Sea from Australia’s Cape York Peninsula toward Fiji and Tonga. Tropical cyclone activity associated with the monsoon trough has been focused primarily in the seas north of Australia and in the Coral Sea out to Fiji. American Samoa has so far been spared any problematic tropical cyclone activity.

Sea-level variation in the USAPI is sensitive to ENSO-cycle, with low sea-level observed during El Niño and high sea-level during La Niña years. All locations across the USAPI have continued to record a rise in sea level during the 4th quarter of 2007, consistent with the continuing La Niña conditions. Several locations recorded substantial sea level deviations (see Table 2) in the 4th quarter, although compared to November 2007, a slight downward trend was observed in December 2007.

The following comments from the EL NIÑO/SOUTHERN OSCILLATION (ENSO) DIAGNOSTIC DISCUSSION were posted on the U.S. Climate Prediction Center web site on January 10, 2008:

Synopsis: La Niña is expected to continue into Northern Hemisphere spring 2008.

“La Niña remained at moderate strength during December 2007, with below-average sea surface temperatures (SSTs) extending from 160ºE to the South American coast... Consistent with these oceanic conditions, stronger-than-average low-level easterly winds and upper-level westerly winds continued across the central equatorial Pacific, convection remained suppressed throughout the central equatorial Pacific, and slightly enhanced convection covered the far western Pacific. Collectively, these oceanic and atmospheric conditions reflect a mature La Niña.

Over half of the recent SST forecasts (dynamical and statistical models) predict a moderate strength La Niña to continue through February-April, followed by weaker La Niña conditions. Current atmospheric and oceanic conditions and recent trends are consistent with a likely continuation of La Niña into the Northern Hemisphere spring 2008... Expected impacts during JFM include a continuation of below-average precipitation over the central and eastern equatorial Pacific. Recent MJO activity has contributed to short-term fluctuations in low-level winds and convection over the equatorial Pacific, which has acted to modify some of the typical La Niña impacts on a sub-seasonal timescale.”