Pacific ENSO Update

2nd Quarter, 2007 Vol. 13 No. 2


El Niño conditions persist in the Pacific and are expected to continue for the next 3 to 6 months. El Niño is anticipated to weaken during this time, as the Southern Oscillation Index and NIÑO 3.4 sea surface temperatures are thought to have reached their peak in December.

For all of 2006, the rainfall throughout Micronesia was near normal (± 20%) at most locations (see charts, Fig. 1a, 1b), with few extreme events of rainfall and no destructive typhoons experienced at any island. The anticipated eastward shift of tropical cyclone activity in the fall of 2006 did not materialize. Instead, several typhoons in the formed to the west of Guam and had enormous impacts to the Philippines . This was more akin to the typical behavior of typhoons during La Niña events than during El Niño.

Based on the anticipated slow demise of El Niño during the first half of 2007, tropical cyclone activity is expected to be below normal throughout Micronesia during this time period. In American Samoa , however, the risk of a tropical cyclone is enhanced, with two or three named cyclones expected to form near the islands during January - April. American Samoa is expected to be very wet through April of 2007 , with a high risk of at least gale-force winds related to the close passage of a tropical cyclone or the penetration of a very active northwest monsoon into the region.

Below normal rainfall is anticipated throughout most of Micronesia and Hawaii for the next six months. Island residents are urged to participate in 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, a more serious effort may be required to ensure adequate water supplies for all islanders. Most islands are expected to receive 60% to 70% of the rainfall normally expected during the first six months of 2007. However, month-to-month variations in rainfall patterns (which are unpredictable) may cause any island to experience less than half of normal rainfall during any one or two of the months from January through June of 2007. By comparison, many locations received considerably less than half of normal rainfall for ALL of the months during the first half of 1998! The risk of wildfires will be high beginning in late January through June on islands that typically experience them at this time of year. Residents are urged to clear away brush and loose combustible materials located near their homes.

The sea-level variation in the western North Pacific islands has been identified to be sensitive to ENSO-cycle, with low sea-levels during El Niño and high sea-levels during La Niña events. Consistent with the on-going weak El Niño event, the sea-level has recorded some drop. Because the onset of the current El Niño occurred relatively late in the season (September 2006), considerable sea level drop is yet to be seen, but will most likely be visible in the months to come.

Many Micronesian islands typically enter their dry season at the start of the calendar year. For islands north of 10º N, the normal dry season may persist through June or July. For islands that are along the ITCZ rainfall maximum (such as Pohnpei, Kosrae, and the southern Marshalls, the normal dry season is typically experienced as one or two moderately dry (10-inch) months occurring anytime within January - March. El Niño-related dry conditions manifest throughout Micronesia as below-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 , with a tendency for dry conditions to be experienced only after a strong El Niño and wet conditions experienced during some weak and moderate El Niño events.

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 January 11, 2007:

Synopsis: El Niño conditions are likely to continue through March-May 2007.

“Equatorial Pacific SST anomalies greater than +1ºC were observed in most of the equatorial Pacific between 170ºE and the South American coast. The latest SST departures in the Niño regions are around 1.0ºC, except near 0.5ºC for Niño 1+2. The increase in SST anomalies during the last half of 2006 was accompanied by weaker-than-average low-level equatorial easterly winds across most of the equatorial Pacific and negative values of the Southern Oscillation Index (SOI). Collectively, these oceanic and atmospheric anomalies indicated the development of El Niño in the tropical Pacific.”

“Most of the statistical and coupled models... indicate that SST anomalies are near their peak and that decreasing anomalies are likely during February-May 2007. Recent observed trends in the upper ocean tend to support those forecasts. …”

“Global effects that can be expected during January-March include drier-than-average conditions over portions of Malaysia , Indonesia , northern and eastern Australia , some of the U.S.-affiliated islands in the tropical North Pacific…”