Pacific ENSO Update

4th Quarter, 2007 Vol. 13 No. 4


According to the U.S. Climate Prediction Center (CPC), the present oceanic and atmospheric anomalies are consistent with the development of La Niña conditions in the tropical Pacific. Many of the islands of Micronesia are typically slightly wetter than normal during a La Niña year, unless it is the year directly following El Niño, in which case most islands are 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 a result of the westward shift of tropical cyclone development during La Niña. The typhoon threat is reduced at most islands during La Niña, especially for islands east of the longitude of Guam (145° E).

During the 3rd Quarter of 2007, most of the islands of Micronesia had rainfall totals that were below normal. Many locations (especially those further to the east) had 3-month rainfall totals that were less than 80% of normal. Saipan (Capitol Hill), Palikir (Pohnpei Island), and Wotje (Northern RMI) received less than 70% of the normal rainfall expected during these three months, which are in the heart of the rainy season. Only two locations (American Samoa and Kapingamarangi, FSM) had 3rd Quarter 2007 rainfall totals in excess of 120% of normal (see Figures 1a, 1b). The highest 3rd Quarter rainfall total recorded in Micronesia was the 52.16 inches at the Palau International Airport, followed closely by the 51.78 inches at Luweech on Yap Island. The lowest recorded 3rd Quarter 2007 rainfall total was the 17.73 inches of rain at Utirik in the northern RMI, followed closely by the 18.47 inches recorded at Wotje (also in the northern RMI). During the 3rd Quarter of 2007, individual monthly rainfall totals exceeding 20 inches occurred only at some locations in the Republic of Palau, some locations on Yap Island, at Kapingamarangi, and in American Samoa. Individual monthly rainfall totals of less than 10 inches occurred at Guam, the CNMI, Chuuk State, Pohnpei State, the central and northern RMI, and even at Kosrae where the 9.28 inches of rain during August was about 60% of the normal for that month.

Near normal rainfall is anticipated throughout much of Micronesia during the next 3 to 6 months. Tropical cyclone activity, which has been very quiet throughout the entire western North Pacific during 2007, should return to near normal in Micronesia for the remainder of 2007. (See each island summary for the meaning of a “normal” tropical cyclone threat.)

Considerable sea level was recorded across all USAPI locations in the July-August-September season, with some stations (such as Malakal, Palau and Marianas, Guam) recording more than 8 inches of rise in a month. While many stations recorded rise in the previous three quarters, the rise during the 3rd Quarter of 2007 is particularly great. As mentioned, the sea-level variability in the Pacific Islands is sensitive to ENSO-events, with low sea-level typically recorded during El Niño and high sea-level during La Niña events. Consistent with the shift toward La Niña conditions, the sea level has been rising in the vicinity of the USAPI since January. With the continued development (and possible strengthening) of La Nina conditions over the next several months, we may expect to observe several inches of sea-level rise in all of these stations during the upcoming seasons.

The following comments from the EL NIÑO/SOUTHERN OSCILLATION (ENSO) DIAGNOSTIC DISCUSSION were posted on the U.S. Climate Prediction Center web site on October 11, 2007:

Synopsis: La Niña will likely continue into early 2008.

La Niña conditions strengthened during September 2007, as negative SST anomalies along the equator expanded westward and now extend from 170ºE to the South American coast… The magnitude of the negative SST anomalies increased in all of the Niño regions... And the upper-ocean heat content in the central and east-central equatorial Pacific remained below average during September. Consistent with these conditions, the low-level easterly winds and upper-level westerly winds remained stronger than average across the central equatorial Pacific, convection remained suppressed throughout the central and eastern equatorial Pacific, and enhanced convection again covered parts of Indonesia and the far western Pacific. Collectively, these oceanic and atmospheric conditions reflect a strengthening La Niña.”

“The recent SST forecasts (dynamical and statistical models) for the Niño 3.4 region indicate a weak-to-moderate La Niña continuing into early 2008. Current atmospheric and oceanic conditions and recent trends indicate that La Niña will continue and may strengthen during the next 3 months.”

“Expected La Niña impacts during October – December include a continuation of above-average precipitation over Indonesia and below-average precipitation over the central equatorial Pacific…”