Pacific ENSO Update

2nd Quarter, 2008 Vol. 14 No. 2


According to the U.S. Climate Prediction Center (CPC), the present oceanic and atmospheric anomalies are consistent with the continuation of moderate La Niña conditions in the tropical Pacific. Typically, many of the islands of Micronesia are slightly wetter than normal during a La Niña year; however, if the La Niña event follows directly after an El Niño year, most islands tend to be drier than normal (particularly during the first half of the year). The month-to-month variability of rainfall is not as great during La Niña as it is during El Niño, primarily as a result of the westward shift of tropical cyclone development during La Niña. The typhoon threat during La Niña is reduced at most islands, especially for islands east of 145° E longitude (Guam and eastward).

During the 1st Quarter of 2008, most of the islands of Micronesia reported near normal rainfall totals (see Figures 1a, 1b). Several heavy daily rainfall events helped push Guam and the CNMI above their normal rainfall, although conditions there have become dry in recent weeks. All Yap locations were drier than normal, but no problems with water supply were reported. Most of Chuuk, Pohnpei, and Kosrae states were near normal or slightly wetter than normal. Although the rainfall throughout the Marshall Islands was generally near normal for the 1st Quarter of 2008, persistent dryness commenced in March and extended into the first two weeks of April at Majuro and atolls to the north (including Kwajalein, Wotje, and Utirik). After several years of mostly above normal rainfall, Kapingamarangi became progressively drier in the 1st Quarter, and the 2.74 inches of rain received in March 2008 marked only 20% of normal. Kapingamarangi often becomes dry during La Niña episodes as anomalous easterly winds drive the equatorial cold tongue of water to its longitude. American Samoa became hot and dry during the 1st Quarter of 2008, which typically occurs during La Niña as the South Pacific Convergence Zone (SPCZ) and the Australian northwest monsoon retreat to the west.

Near normal rainfall is anticipated throughout much of Micronesia during the next 3 to 6 months. Tropical cyclone activity, which was very quiet in Micronesia (and throughout the entire western North Pacific) during 2007, should be near normal for the remainder of the 2008 calendar year. (See the Tropical Cyclone section for the latest typhoon outlook for 2008, and refer to each island’s summary for the definition of “normal” tropical cyclone threat).

Sea level variation in the USAPI is sensitive to the ENSO-cycle, with low sea level observed during El Niño years and high sea level during La Niña years. Sea levels began rising in early 2007, and have been above normal for many months. Current forecasts indicate that sea levels will remain elevated at all USAPI stations for another 1 to 3 months (see Table 2). This trend of elevated sea level is consistent with the on-going La Niña conditions; however, as La Niña conditions weaken, the sea-level is expected to begin receding toward normal levels. Collectively, the present oceanic and atmospheric anomalies are consistent with the continuation of La Niña conditions in the equatorial Pacific. There have recently been some early signs of weakening of the current La Niña, but most climatic trends and model guidance support the continuation of La Niña through the Northern Hemisphere’s spring months of 2008.

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

Synopsis: La Niña is expected to continue for the next 3 months.

“La Niña declined to moderate-strength during March 2008 as negative sea surface temperature (SST) anomalies weakened across the central and east-central equatorial Pacific... In the central Pacific, the subsurface temperature anomalies also lessened (averaging - 1°C to - 4°C at thermocline depth), and became increasingly confined to the surface region. This evolution led to a significant weakening of the negative ocean heat content anomalies (average temperatures in the upper 300m of the ocean). Despite this oceanic trend, the atmospheric conditions continue to strongly reflect La Niña. Enhanced low-level easterly winds and upper-level westerly winds persisted across the central equatorial Pacific, convection remained suppressed throughout the central equatorial Pacific, and enhanced convection covered the far western Pacific. Collectively, these atmospheric and oceanic conditions indicate an ongoing, but weaker, La Niña.

The recent dynamical and statistical SST forecasts indicate La Niña will become weak and persist through May-June-July 2008. Thereafter, nearly one-half [of the models] indicate La Niña could continue well into the second half of the year. Based on current atmospheric and oceanic conditions and recent trends, La Niña is expected to continue for the next 3 months. ”