Pacific ENSO Update
Special Bulletin March 1, 2005
SPECIAL ISSUE BULLETIN
Due to the abrupt onset of very dry conditions throughout much of Micronesia during February 2005, and other unusual weather conditions in other areas of the tropical Pacific (such as the occurrence of three hurricanes near Samoa), this special bulletin is being issued with updated information to the last full issue of Pacific ENSO Update (1st Quarter, 2005). This bulletin will be followed by the next full issue of Pacific ENSO Update for the 2nd Quarter, 2005, to be issued in May.
While there has been a slight cooling of sea surface temperatures (SST) in the eastern and central Pacific since the last issue of Pacific ENSO Update, the weather patterns throughout Micronesia and the South Pacific shifted dramatically during February. Whereas rainfall was abundant throughout the region during January 2005, widespread dry conditions became established across much of Micronesia during February 2005. The abrupt onset of dry weather during February 2005 was associated with a major eastward shift of the South Pacific portion of the Southern Hemisphere summer monsoon system along with its accompanying tropical cyclones. Three major hurricanes formed east of the International Date Line in the South Pacific: Nancy, Olaf, and Percy. These hurricanes affected American Samoa , several islands of the Cook Islands and French Polynesia . The official monthly Southern Oscillation Index (SOI) for January was +0.3 (the first time the monthly value has been positive since July 2004). However, the SOI fell substantially during February. On the 26 th of February the 30-day SOI was sharply negative (near -2.5). The very low SOI reflected higher than normal pressure in northern Australia (+1.8 hPa at Darwin ), and lower than normal pressure in portions of French Polynesia (-3.5 hPa at Tahiti ).
According to the U.S. Climate Prediction Center (CPC), the climate of the tropical Pacific entered El Niño in the second half of 2004. Until now, the atmospheric response to El Niño has been weak. Although the climate anomalies of February 2005 (e.g., extreme dryness in much of Micronesia and intense hurricanes east of the International Date Line) are typically associated with El Niño, it is not yet clear whether these very unusual weather patterns are being caused by El Niño, or by some other short-term forcing of the climate system, perhaps in response to warmer than normal SSTs in the Southern Hemisphere. In any case, the outlook from the CPC is still for the weak El Niño conditions to gradually subside back toward El Niño neutral conditions during mid-2005. In this scenario, one would expect drier than normal weather throughout much of Micronesia for the next 2 to 3 months; however, the widespread extreme dryness of February 2005 is unlikely to be repeated , and many islands in the eastern parts of Micronesia such as Kosrae and Pohnpei should see a return to near normal rainfall by April. At other islands such as Kwajalein , Yap , Chuuk and Palau , the return of near normal rainfall may be delayed until May. For the CNMI, normal rainfall may be delayed until June.
LOCAL VARIABILITY SUMMARIES
AMERICAN SAMOA : During the first two months of 2005, a highly unusual number of tropical cyclones passed through the Samoa region with some of the islands greatly affected. Hurricane Olaf passed near Manu’a with serious effects there. January rainfall for Pago Pago was 13.25 inches or 95% of that normally expected, and February rainfall was 10.55 inches (83%). Rainfall and cyclone activity predictions in the 1 st Quarter 2005 Pacific ENSO Update are still valid.
Chuuk: January rainfall for Weno Island was 20.93 inches or 196% of normal. The rainfall total fell to only 1.91 inches (31%) during February. At Lukunoch the rainfall total for January and February was 16.86 inches (158%) and 0.42 inches (7%) respectively. To the west at Polowat, January was very wet with a total of 23.92 inches (299%), while February was dry with 4.24 inches (68%). The March through May rainfall prediction is reduced from 80% of normal in the 1 st Quarter Newsletter to 70% of normal.
Pohnpei: At Kolonia, the January rainfall was 14.33 inches (110%). During February, the rainfall total fell to only 3.16 inches (29%). As a result of the low rainfall in Pohnpei, stream flow decreased markedly, and popular crops such as Yams and Sakau were stressed. Burning of the roadside vegetation was common. March through May rainfall prediction is reduced from 85% of normal in the 1 st Quarter Newsletter to 70% of normal, and recovering as indicated in the Newsletter thereafter. Rainfall predictions for Kapingamarangi remain unchanged.
Kosrae: Rainfall at the Kosrae Airport during January was 26.91 inches (187%), and during February it was 8.47 inches (52%). Rainfall at Kosrae was not interrupted as much during February as at other islands. Rainfall predictions for Kosrae that appear in the 1 st Quarter Newsletter are still valid.
Yap : Rainfall was abundant throughout Yap State during January, especially at the atolls. The total was 6.94 inches (95%) near the Yap airport, 12.68 inches (204%) at Ulithi, and 17.75 inches at Woleai (166%). During February it became very dry throughout Yap State . The total rainfall was 2.04 inches (34%) near the Yap airport, 1.92 inches (38%) at Ulithi, and 2.16 inches (29%) at Woleai. March through May rainfall prediction is reduced from 70% of normal in the 1 st Quarter Newsletter to 60% of normal, and recovering thereafter as indicated in the Newsletter.