Pacific ENSO Update

2nd Quarter, 2007 Vol. 13 No. 2


El Niño conditions in the ocean abruptly ended at the beginning of February 2007, and oceanic and atmospheric circulation patterns are now in an ENSO-neutral state. Some of the islands in the RMI, Chuuk State , and Guam have experienced very dry conditions for some or all of the past three months. Three-month rainfall totals were less than half of normal at some locations in the northern RMI (see figures, Fig. 1a, 1b). Other island groups have had adequate to abundant rainfall. Three-month rainfall totals exceeded 125% of normal at some locations in Chuuk State, Pohnpei State, American Samoa, and (surprisingly) Saipan. The 51.67 inches of rainfall at American Samoa during the first three months of 2007 was the highest recorded value in the USAPI, followed closely by the just-over 50 inches recorded in the same time period at Nukuoro in Pohnpei State. The year’s first typhoon (Kong-Rey) formed southwest of Pohnpei near the end of March, bringing substantial rainfall to locations in Pohnpei State, Chuuk State, and many islands of the CNMI. It tracked too far north and east of Guam and Rota, however, to provide much relief to ongoing dryness there.

Based on the anticipated demise of El Niño, below normal rainfall was forecast throughout most of Micronesia and Hawaii for the first quarter of 2007, while American Samoa was forecast to be wet. Widespread dryness did not occur, but more localized extreme dryness was experienced especially in the northern RMI, where it was so dry that emergency measures were needed to ensure adequate water supplies. Recently, some much-needed rains have occurred to alleviate extreme dryness in the northern RMI. Persistent very dry weather on Guam has exacerbated the wildfire situation, with many hundreds of acres scorched in the southern grasslands. American Samoa was overall very wet in the first quarter of 2007. Only four tropical cyclones (Zita, Arthur, TC 11P, and Becky,) formed in the South Pacific during the 1st quarter of 2007, and contrary to expectations, none of them affected American Samoa .

Many of the islands of Micronesia have now passed through the heart of their dry seasons, and the slow northward march of heavy convective rainfall has already begun. It is likely that the worst of persistent dryness is over for many of the islands of the RMI that have been extremely dry. On some islands (especially those north of 10º N), it is still anticipated that May and June will be drier than normal, but in general, a near-normal distribution of rainfall is anticipated for the next three months. Rainfall forecasts issued by NOAA’s Climate Prediction Center for Hawaii give equal chances of below average, near average, or above average rainfall through early summer. Sea-level variation in the USAPI is sensitive to ENSO-cycle, with low sea-level observed during El Niño and high sea-level during La Niña years. Consistent with the transition from ENSO-neutral to La Niña conditions, observations reveal that the sea level has already started to record a rise this year as most locations across the USAPI have recorded moderate positive deviations.

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 April 05, 2007:

Synopsis: A transition from ENSO-neutral to La Niña conditions is possible within the next 3 months.

“The pattern of anomalous sea surface temperatures (SSTs) during March 2007 was consistent with ENSO-neutral conditions in the tropical Pacific, with average to slightly below-average SSTs extending from the date line to the west coast of South America . …”

“The upper-ocean heat content remains below-average across the central and east-central equatorial Pacific, with temperatures at thermocline depth generally 3°-5°C below average. … Stronger than-average low-level easterly winds persisted throughout the month of March over the central equatorial Pacific, and convection was enhanced over the western equatorial Pacific and Indonesia and suppressed near the date line. Collectively, these atmospheric and oceanic conditions are consistent with a trend towards a Pacific cold (La Niña) episode.

“Most of the statistical and coupled model forecasts, including those from the NCEP Climate Forecast System (CFS), indicate additional anomalous surface cooling during the next several months. Some forecast models, especially the CFS, indicate a transition to La Niña during May-July 2007. This forecast is consistent with the observed trends in atmospheric and oceanic conditions. However, the spread of the most recent statistical and coupled model forecasts (ENSO-neutral to La Niña) indicates considerable uncertainty as to when La Niña might develop and how strong it might be.”