CURRENT CONDITIONS

Most Micronesian islands have a distinct rainy season and dry season. The length of the rainy season and dry season is primarily a function of latitude. Islands closer to the equator (e.g., Pohnpei) have a short dry season and a long rainy season; islands further away from the equator (e.g., Guam and Saipan) have longer dry seasons. Most islands of Micronesia typically enter their dry season at the start of the calendar year; on some islands (especially those north of 10º N), the dry season persists until June or July. During years that follow a strong El Niño (e.g., 1983 and 1998), almost all of Micronesia experiences a long and prolonged dry season.

In contrast the rainy season in American Samoa occurs during the dry season in Micronesia, and vice versa. 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 after a strong El Niño and wet conditions experienced during weak La Niña.

2006 started out with climate patterns typical of weak La Niña that later relaxed to those expected for ENSO-Neutral conditions. Most of the statistical and dynamic forecast guidance suggested that most islands of Micronesia and American Samoa would be wetter than normal under this scenario. Throughout the first half of 2006, however, many islands were drier than anticipated by the models and official PEAC forecasts. Overall, the weather during the first half of 2006 was very tranquil with few extremes of very wet or very dry conditions (Fig 1a, Fig 1b).Overall, Micronesian weather during the first half of 2006 was very tranquil with few extremes of very wet or very dry conditions. Much of Hawaii and American Samoa had a very wet 1st quarter followed by drier 2nd quarter conditions. During the first six months of the calendar year 2006 rainfall totals were less than 80% of normal at some locations within Yap State, the northern RMI, Guam and the CNMI. Rainfall totals in excess of 120% of normal occurred in only a few locations in Micronesia-including some of the outer Islands of Pohnpei State (Nukuoro and Kapingamarangi), and the southern RMI (Mili)-and in American Samoa and Hawaii. The highest rainfall total recorded in Micronesia for the first six months of 2006 was at Kosrae airport with 106.54 inches; Nukuoro and Palikir followed next with 100.25 inches and 95.64 inches, respectively. The highest rainfall amount on Kosrae was, ironically, slightly less than normal (98%). The lowest recorded rainfall total in Micronesia during the first half of 2006 was the 14.91 inches at Tinian in the CNMI. Only some rainfall totals in the CNMI, Guam and in the northern RMI were less than 20 inches during the first half of 2006.

At the end of June, the monsoon trough worked its way into the western Caroline Islands bringing heavy rains to Palau and Yap, and contributed to the formation of Tropical Storm 03W and Super Typhoon 04W (Ewiniar). The establishment of the monsoon trough across the western half of the Caroline Islands in late June brought widespread rainfall to almost all island groups in Micronesia except for Guam and the CNMI, where abundant rains did not arrive until early July when the very large Tropical Storm Bilis (05W) passed through the region.

Most statistical and coupled model forecasts indicate ENSO-neutral conditions in the tropical Pacific through the end of 2006, with some of the models trending toward weak El Niño. The spread of the most recent statistical and coupled model forecasts (ENSO-Neutral to weak El Niño) indicates uncertainty in the outlooks for the last half of the year. However, current conditions support those forecasts indicating that ENSO-Neutral conditions will continue for the next several months. Based upon this, near normal rainfall is anticipated throughout much of Micronesia, American Samoa and Hawaii for the next six months. 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 August 10, 2006:

“Synopsis: ENSO-neutral conditions are expected to continue for the next one to three months, with a 50% chance that weak El Niño conditions will develop by the end of 2006.

 Equatorial surface and subsurface temperature anomalies increased during July 2006, with SST anomalies greater than +0.5C observed in most of the equatorial Pacific between 130ºE and 140ºW. As a result, positive SST anomalies were observed in all of the Niño regions . During July, low-level (850-hPa) easterly winds were weaker than average across most of the equatorial Pacific, and the Southern Oscillation Index (SOI) was negative for the third consecutive month. Beginning in February the basin-wide upper ocean heat content increased, and since early April positive anomalies have been observed. Positive upper-ocean heat content anomalies are usually a precursor to warm (El Niño) episodes.

The statistical and coupled model forecasts range from ENSO-neutral to weak warm (El Niño) episode conditions for the remainder of 2006 and into early 2007 . The forecasts are consistent with the recent build up in upper-ocean heat content along the equator , indicating a trend toward warm-episode conditions. In the absence of any strong intraseasonal (Madden-Julian Oscillation – MJO) activity, a continued slow trend toward warm-episode conditions is expected. Therefore, ENSO-neutral conditions are expected to continue for the next one to three months, with a 50% chance that weak El Niño conditions will develop by the end of 2006