Pacific ENSO Update

1st Quarter, 2005 Vol. 11 No. 1


According to the U.S. Climate Prediction Center (CPC), the climate of the tropical Pacific entered El Niño in the second half of 2004. The CPC’s definition of El Niño requires three consecutive months with SSTs at least 0.5 degrees Celsius warmer than normal in a region of the central equatorial Pacific designated as “Niño 3.4.” This threshold was reached in September 2004, and has continued since then. The atmospheric response to El Niño (persistent negative values of the Southern Oscillation Index, for example) has been weak. Widespread dryness associated with El Niño, which typically begins at many locations in Micronesia late in the year, was not extreme.

For the 2004 calendar year, most of the islands of Micronesia and American Samoa had near normal to above normal rainfall (Fig. 1a, 1b). Due in part to the near passage of several tropical cyclones, the island of Guam had its second wettest year in its rainfall time series, which extends back to the 1950’s. At 21 selected locations throughout the region, 13 had above normal annual rainfall (Fig 1a). Annual rainfall totals exceeded 120% of normal at Ulithi, Guam, Saipan, Tinian, Rota, and Kapingamarangi. Annual rainfall totals were below 80% of normal at American Samoa, Peleliu, and Pingelap. During the final months of 2004, a wide expanse of drier than normal weather spread across the tropical western Pacific in a “horseshoe”- or “boomerang”-shaped area extending from Fiji westward to the Solomon Islands and Indonesia, and across most of Micronesia in the North Pacific. Drier than normal weather is anticipated throughout much of Micronesia for the next 3 to 6 months; however, a major El Niño-related Micronesia-wide drought is not expected.

The general consensus among international computer climate forecasts is for the weak El Niño conditions to weaken for the next three months, and gradually subside back toward ENSO neutral conditions during mid-2005. Historically, climate forecasts made during this time of the year show little skill, so it is difficult to predict conditions beyond three months with any reliability; most models to show a return to ENSO Neutral conditions. The following comments from the EL NIŅO/SOUTHERN OSCILLATION (ENSO) DIAGNOSTIC DISCUSSION were posted on the U.S. Climate Prediction Center web site on February 10, 2004:

Synopsis: A transition from weak warm-episode (El Niño) conditions to ENSO-neutral conditions is expected during the next three months.

Sea surface temperature (SST) anomalies decreased in the equatorial Pacific everywhere east of the date line during January 2005, resulting in decreases in all of the Niño indices with the exception of Niño 4. However, positive sea surface temperature (SST) anomalies greater than +1°C (~1.8°F) persisted in portions of the central and western equatorial Pacific. By early February 2005, positive equatorial SST anomalies greater than +0.5°C (~0.9°F) were found from 140°E eastward to 155°W. The pattern of anomalous warmth in the equatorial Pacific in recent months and the most recent 5-month running mean value of the Southern Oscillation Index (-0.5) indicate that a weak warm (mid-Pacific El Niño) episode is in progress. However, through December there was a lack of persistent enhanced convection over the anomalously warm waters of the central equatorial Pacific, which limited El Niño-related impacts….

Based on the recent evolution of oceanic and atmospheric conditions and on a majority of the statistical and coupled model forecasts, it seems most likely that weak warm episode (El Niño) conditions will gradually weaken during the next three months and that ENSO-neutral conditions will prevail during the last half of 2005.”