Pacific ENSO Update

2nd Quarter, 2006 Vol. 12 No. 2


Replace Old TEXT!!!!!!!!!!!!!!!!!!!!!! in rainfall marked the first three months of 2005. At most recording locations, February’s rainfall was very low; as low as one inch in several locations in Chuuk State and the Republic of Palau. A special abbreviated supplement to the ENSO Newsletter was issued to address islander’s concerns that the very dry February would lead to a more sustained drought. Forecasters at PEAC felt that February’s very low rainfall was an extreme short-term event that would abate in subsequent months. Throughout Micronesia, one month of very low rainfall is sufficient to cause substantial draw-down of stream flow on high islands (such as Pohnpei), problematic losses to rain catchments and other sources of surface water, stress to local crops, and wildfires on islands with large tracts of grasslands (such as Guam). In March and April 2005, rains returned in abundance to many islands, alleviating concerns of a prolonged drought.

According to the U.S. Climate Prediction Center (CPC), the climate of the tropical Pacific is now in a transition to ENSO Neutral after having entered a weak El Niño that began in the second half of 2004. The islands of Micronesia are typically drier than normal in the first few months during the year that follows an El Niño. With this in mind, PEAC’s outlook was for moderately drier than normal conditions throughout most of Micronesia for the first several months of 2005. Despite February’s very low rainfall, most islands have averaged near normal rainfall during the first quarter of 2005. February’s very low rainfall is probably not related to changes in the status of ENSO, but rather to the influence of the Madden Janian Oscillation (MJO). The MJO, also referred to as the 30-60 day oscillation, is the main intra-annual (less than a year) fluctuation explaining weather variations in the tropics. The manifestation of the MJO signal at most islands of Micronesia is pronounced alternations of periods of wet weather with periods of hot dry weather.

For the first quarter of 2004, most of the islands of Micronesia and American Samoa had near normal to above normal rainfall (Fig. 1a, 1b). Only the northernmost islands, such as the CNMI and Kwajalein, experienced persistent dryness that was considerably less than normal for this period.

Near normal rainfall is anticipated throughout much of Micronesia and American Samoa for the next 3 to 6 months; however, large month-to-month variability is expected.

The general consensus among international computer climate forecasts is for a continued slide away from weak El Niño conditions back toward ENSO Neutral conditions during mid-2005. For more information, see the CPC EL NIŅO/SOUTHERN OSCILLATION (ENSO) DIAGNOSTIC DISCUSSION for Feb..