Pacific ENSO Update

4th Quarter, 2008 Vol. 14 No. 4

CURRENT CONDITIONS

According to the U.S. Climate Prediction Center (CPC), the present oceanic and atmospheric anomalies are consistent with the continuation of ENSO-neutral conditions in the tropical Pacific. Although the word “neutral” might imply that conditions are “middle-ground” or “near normal”, the tropical Pacific atmospheric circulation patterns during recent months have been quite unusual.

While the commonly used climate indices (Southern Oscillation Index, Oceanic Niño Index, Multivariate ENSO Index, etc.) support the claim of ENSO-neutral conditions, recent circulation anomalies in the Pacific basin have exhibited characteristics of BOTH La Niña and El Niño simultaneously. For example, low-level equatorial westerly winds over the Eastern Pacific and low level equatorial easterlies over the Indian Ocean are consistent with El Niño. The tendency for anomalously suppressed convection over the Maritime Continent region is also consistent with weak El Niño conditions. Conversely, the Central Pacific has been cooler than average, and sea-levels across Micronesia remain higher than average (with values typically seen during La Niña). The persistent brisk trade winds across Micronesia, along with below normal cloudiness and a shift of tropical cyclone activity to the north and west, are also typical of La Niña.

Strong and persistent low-level equatorial easterly wind anomalies over the western North Pacific associated with this pattern have almost completely suppressed the normal hot bed of tropical cyclone activity over the deep tropics (see the PEAC Tropical Cyclone summary). The overall basin-wide tranquil weather has led to extraordinary departures from normal seasonal rainfall across the U.S. Affiliated Pacific Islands (USAPI) (see Figures 1a, 1b). Unusually persistent trade winds have dominated the weather across Micronesia. The monsoon trough was absent, and tropical cyclones stayed out of the region. There were no notable extremes of wind and few extremes of rainfall at any location during the 3rd Quarter of 2008.

Surely, coupled ocean-atmosphere climate dynamics related to the present state of ENSO are responsible for much of the basin’s unusual weather patterns. This year’s strange collection of climate anomalies offers strong motivation to extend our characterizations of ENSO beyond the oversimplification of labeling the ENSO state strictly as El Niño, La Niña, or ENSO-neutral. This is not a one-time occurrence; the weak El Niño of 2006-2007 had many La Niña characteristics in the western North Pacific. Feedback from the Pacific Island communities is encouraged and welcome.

Near normal rainfall is anticipated throughout much of Micronesia during the next 3 to 6 months. Tropical cyclone activity, which has been very quiet in Micronesia and throughout much of the western North Pacific for the past two years, should remain suppressed for the remainder of 2008. (See each island’s summary for the meaning of a “normal” versus a “suppressed” tropical cyclone threat.)

Sea-level variation in the USAPI is sensitive to the ENSOcycle, with low sea-level observed during El Niño years and high sea-level during La Niña years. Sea-levels have been above normal since early 2007 (see Table 2). Current forecasts indicate that sea-levels will remain slightly elevated at all USAPI stations for another 1 to 3 months, although sea-level at all locations should begin slowly receding toward normal.

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 9, 2008:

Synopsis: ENSO-neutral conditions are expected to continue into early 2009

“ENSO-neutral conditions continued during September 2008... Sea-surface temperatures (SSTs) remained slightly below average in the central Pacific, and slightly above-average in the eastern Pacific.”

“Although ENSO-neutral conditions have been in place since June 2008, the atmospheric circulation over the western and central tropical Pacific continues to reflect lingering aspects of La Niña... Overall, the ocean-atmosphere system remains consistent with ENSO-neutral conditions.”

“Most of the dynamical and statistical SST forecasts for the Niño 3.4 region indicate a continuation of ENSO-neutral conditions (-0.5°C to 0.5°C in the Niño-3.4 region) into the first half of 2009. While the model spread continues to include possibilities ranging from El Niño to La Niña, the recent decrease in subsurface and surface temperatures favors a return to La Niña over the development of El Niño. However, based on current atmospheric and oceanic conditions, recent trends, and model forecasts, ENSO-neutral conditions are expected to continue into early 2009.”