Pacific ENSO Update

2nd Quarter, 2007 Vol. 13 No. 2


Weak El Niño conditions have developed in the Pacific and are predicted to become weak to moderate through the Northern Hemisphere Spring 2007. Conditions reflecting the onset of El Niño include warmer than normal SST in the central and eastern equatorial Pacific, and a shift of the large-scale atmospheric pressure patterns that have caused the Southern Oscillation to become negative. During years that follow a strong El Niño (e.g., 1983 and 1998), almost all of Micronesia, much of Hawaii, and American Samoa experience a period of prolonged dryness. In Micronesia, dry conditions associated with El Niño may begin in November of the El Niño year and extend until June or July of the year following El Niño. In Hawaii, dry conditions related to El Niño occur in the same months as they do in Micronesia, but while this is the normal dry season in Micronesia, it is the normal winter rainy season for much of Hawaii. Years following weak or moderate El Nino events may be very dry, but most often they are not quite as dry for years following strong events.

Most islands of Micronesia typically enter their dry season at the start of the calendar year; on islands north of 10º N, the normal dry season may persist until June or July. El Niño-related dry conditions manifest as less than normal rainfall during the typical dry season months and as an extension of the length of the dry season in both directions (e.g., earlier than normal dry season onset, and a later than normal end to the dry season.) The effects of ENSO on the rainfall in American Samoa are somewhat less defined than in Micronesia. In American Samoa, dry conditions tend to be experienced only after a strong El Niño and wet conditions experienced during weak La Niña events.

Rainfall during October 2006 was slightly below normal in many locations throughout Chuuk State, the Republic of Palau, and at many locations throughout Pohnpei State (see 3rd Quarter Rainfall, Fig. 1a, 1b). October rainfall was above normal at many locations in the RMI, especially at Kwajalein where October’s total of 19.91 inches was much above the normal 11.91 inches. By early November, Guam and the CNMI became very dry.

During the 3rd Quarter (July, August and September) of 2006 rainfall totals were less than 80% of normal only at locations in the CNMI and at the WSO Majuro. Rainfall totals in excess of 120% of normal occurred in several locations including American Samoa, portions of the Republic of Palau, portions of Chuuk State, and some of the outer Islands of Pohnpei State (Nukuoro and Kapingamarangi). The greatest departure from normal occurred at Kapingamarangi where the 3-month total of 43.30 inches was almost twice normal (193%) (Fig. 1b). Tinian Airport received 24.95 inches (71% of its normal) and was also the lowest rainfall total in the region.

Below normal rainfall is anticipated throughout much of Micronesia and Hawaii for the next six months. Island residents are urged to begin voluntary water conservation measures, and to undertake low-cost repairs and maintenance of the means of obtaining drinking water. If extreme dry conditions develop in the next 3 to 6 months, then a more serious effort may be required to ensure adequate water supplies for all islanders.

El Niño conditions may cause two or three tropical cyclones to form in the region east of Pohnpei and into the Marshall Islands from now through January 2007. Tropical cyclones that form this far to the east increase the odds that some of the islands of Micronesia will experience a tropical storm or typhoon. Particularly at risk are Guam and the CNMI where the odds of a damaging typhoon rise to 1 in 3 for the months of November through January of an El Niño year (the risk in non-El Niño years during these months is approximately 1 in 10). Though the risk of a typhoon is normally very low in the Marshall Islands, El Niño increases their chances as well. Notable El Niño-related tropical cyclones in the Marshall Islands include Typhoon Zelda (1991) and Typhoon Paka (1997). In American Samoa the risk of a damaging hurricane is increased during moderate El Niño events. Since the current event is expected to remain weak to moderate, the risk of a hurricane in American Samoa is expected to be greater than normal.

The following comments from the CPC EL NIÑO/SOUTHERN OSCILLATION (ENSO) DIAGNOSTIC DISCUSSION were posted on the U.S. Climate Prediction Center web site on November 9, 2006:

Synopsis: El Niño conditions are likely to continue into early 2007.

“ … [current] oceanic and atmospheric anomalies are consistent with the early stages of El Niño in the tropical Pacific... Global effects that can be expected during November-March include drier-than-average conditions over most of Malaysia, Indonesia, [and] some of the U.S.-affiliated islands in the tropical North Pacific, …”