El Niño began in the summer of 2009, and became strong by the end of the year. During the first few months of 2010 El Niño conditions matured, and as of April 2010 most indicators are showing signs of conditions returning to neutral. During an El Niño, dry conditions are typically experienced in the western Pacific, beginning first in Australia and Indonesia during July, and working their way into Micronesia and Hawai’i in the months that follow. When El Niño conditions became moderate to strong in the fall of 2009, very dry conditions were expected across Micronesia and Hawai’i during the first few months of 2010. A team of PEAC scientists from the University of Guam coordinated with the Guam WFO to make several outreach visits within Micronesia during the fall of 2009 to discuss the anticipated effects of El Niño. The UOG team visited the Republic of the Marshall Islands (RMI), Yap, Palau, Kosrae, and Saipan, while a WFO representative visited Chuuk, Pohnpei, Rota, Saipan and Guam. At the time of the first visits in October, it was thought that the biggest threats from El Niño would be an enhanced risk of typhoon (hurricane) activity through December 2010 in Hawai’i and from the RMI westward to Guam. This increased threat of typhoon occurrence was to be followed by potentially severe dry conditions throughout this region in the first few months of 2010. Guam, the CNMI, and Yap were threatened by Super Typhoon Nida (~160 kt) in November. Hawai’i was threatened by Hurricane Neki in late October. During the second half of 2009 and January 2010, rainfall amounts were above normal in American Samoa, and near normal at most locations across Micronesia. Hawai’i, however, did experience persistent dry conditions during this time period. At the end of January 2010, it seemed that Micronesia had been spared the worst-case scenario of late-season typhoons and the onset of severe drought as discussed in outreach visits.
Unusual conditions developed in February 2010, as almost everywhere in Micronesia became extremely dry. Water resources were particularly stressed in Chuuk State and the RMI. Even in typically wet Kosrae, water managers began to worry. Monthly rainfall totals of one inch or less were common across the region. In the words of one RMI forecaster, the month was “…windy, dusty, salty, brown, and very dry!” A renewed fear of prolonged and severe dry conditions arose as tropical cyclone activity pushed eastward into French Polynesia, drying out the northern islands. In response, WSO Guam began a weekly ENSO advisory, and PEAC issued a special bulletin.
As abruptly as the rains had stopped, rainfall returned to portions of Micronesia during the second half of March. Two large and intense tropical cyclones formed in the South Pacific: Tomas and Ului. Tomas hit parts of Fiji, while Ului moved westward across the Coral Sea and made landfall on the northeast coast of Australia. These two cyclones both occurred in normal locations for this time of year, and seemed to mark the end of the El Niño-related eastward displacement of tropical cyclone activity. In association with the formation of Tomas and Ului, a northern hemisphere TC (Omais) developed south of Guam and moved between Yap Island and Ulithi. Omais brought rain to many locations, and preceded a well-established trade wind trough across the low latitudes of Micronesia. Rainfall was abundant at many locations during March, with over 20 inches falling on portions of Pohnpei Island, Kosrae, and Kapingamarangi. Despite these developments, several locations at higher latitudes (e.g., northern RMI, Guam, and the CNMI) have remained relatively dry. We are cautiously optimistic that the trade wind trough will continue to bring adequate rainfall to islands south of 7° N, and begin its seasonal northward shift, returning normal rainfall to most islands, by May or June.
During the 1st Quarter of 2010, the total rainfall at most of the U.S.-Affiliated Pacific Islands (USAPI) was below normal (see Figure_1). February 2010 was particularly dry, with only Pago Pago on American Samoa recording higher than normal rainfall totals. Several nearby tropical cyclones and the northwest monsoon provided American Samoa with this abundant rainfall. During March, American Samoa became abruptly dry, while rainfall returned to portions of the island groups in Micronesia.
Sea-level variation in the USAPI is sensitive to the ENSO cycle, with lower (higher) sea level typically observed during El Niño (La Niña) events. The monthly mean sea level in March 2010 remained steady, and in some cases recorded a slight rise. Monthly maxima for March were higher than February at most stations, due to strong trade winds returning to the western and central tropical Pacific. Sea-level forecasts call for continuing slight rise in the coming seasons, as the current El Niño event weakens and prevailing conditions shift back to ENSO-neutral.
The following comments from the EL_NIÑO/SOUTHERN_OSCILLATION_(ENSO)_DIAGNOSTIC_DISCUSSION were posted on the U.S. Climate Prediction Center web site on April 8, 2010:
ENSO Alert System Status: El Niño Advisory
Synopsis: El Niño is expected to continue through the Northern Hemisphere spring 2010 and transition to ENSO-neutral conditions by Northern Hemisphere summer 2010. El Niño weakened to moderate strength during March 2010, with sea surface temperature (SST) anomalies decreasing slightly, but still exceeding +1°C across much of the central and eastern equatorial Pacific Ocean at the end of the month. Subsurface heat content anomalies (average temperatures in the upper 300m of the ocean) decreased during March in response to the eastward expansion of below-average temperature anomalies at depth (100-200m) into the east-central Pacific. Anomalous tropical convection remained consistent with El Niño, with enhanced convection over the central and eastern Pacific and suppressed convection over Indonesia. The equatorial low-level easterly trade winds strengthened near the Date Line, while upper-level easterly wind anomalies became confined to the eastern Pacific. Collectively, these oceanic and atmospheric anomalies reflect an ongoing, but weakening El Niño.
Nearly all models predict decreasing SST anomalies in the Niño-3.4 region through 2010, with the model spread increasing at longer lead times (Fig. 6). The majority of models predict the 3-month Niño-3.4 SST anomaly will drop below +0.5°C by May-June-July 2010, indicating a transition to ENSO-neutral conditions that will likely persist through Northern Hemisphere summer. Over the last couple months, an increasing number of models, including the latest runs from the NCEP Climate Forecast System (CFS), are predicting below-average temperatures in the Niño-3.4 region by Northern Hemisphere fall, with some forecasts meeting thresholds for La Niña. However, it should be noted that model skill is at a minimum during this time of year, and also that the majority of models continue to indicate the persistence of ENSO-neutral conditions through 2010.
Expected El Niño impacts during April-June 2010 include drier-than-average conditions over Indonesia and enhanced convection over the central and eastern equatorial Pacific Ocean. For the contiguous United States, potential El Niño impacts include above-average precipitation for the southeastern states, while above-average temperatures are most likely for the Pacific Northwest.