Pacific ENSO Update

1st Quarter, 2009 Vol. 15 No. 1


The year of 2008 both began and ended with La Niña conditions, with ENSO indices remaining neutral for most of the year. But while the prevailing state of the climate was ENSO-neutral, the weather patterns were far from normal. Climatic effects more typical of La Niña were noted for much of 2008, and included well-known La Niña-related anomalies such as below normal tropical cyclone activity across most of Micronesia, a weak monsoon, higher than normal sea-level, and abnormally strong and widespread easterly surface winds in the low latitudes. Oceanic cooling observed along the equator in the central and eastern Pacific increased in magnitude in late 2008, nudging the climate back into La Niña by year’s end.

During the calendar year 2008, the weather throughout the U.S.-Affiliated Pacific Islands (USAPI) was generally tranquil, with no destructive wind events and few extremes of rainfall. However, the most dramatic climate extreme of the year occurred during the week of December 8-15, 2008, when an unusual pattern of gale-force winds located in the subtropics of the western North Pacific near the International Date Line generated oceanic swell that traveled to the south and caused phenomenal surf throughout eastern Micronesia and all the way south to the northern coast of Papua New Guinea (see Press Release). Massive inundation was experienced in many locations, with damage to infrastructure, personal property, and crops. Damages also occurred in the RMI, and a full investigation of the damages in all of the USAPI is now underway. The PEAC Center is committed to a thorough meteorological examination of this event, with a special report to follow.

The 2008 annual rainfall at most of the USAPI recording sites was below normal (see Figures 1a, 1b ). Annual rainfall totals were less than 75% of normal at some of the atolls of the northern RMI, at Woleai in the southern part of Yap State, at Polowat in the western portion of Chuuk State, at Kahului Airport on Maui, and on parts of Guam. Annual rainfall totals with amounts below 70 inches occurred at Woleai, Saipan, Guam and in some of the northern atolls of the RMI. Annual rainfall totals exceeding 200 inches occurred on Pohnpei Island, Nukuoro, and Kosrae. The 223.67 inches recorded at the Kosrae Airport was the USAPI’s highest reported annual rainfall total during 2008. After a drier-than-normal October, the Hawaiian islands settled into a typical rainy season pattern, and most stations ended the year with near-normal rainfall. A series of storms throughout November and December brought much-needed rainfall and helped improve drought conditions to many areas of the state. However, the excessive rainfall in December caused significant flooding across portions of Kauai and Oahu, producing severe damage to homes, infrastructure, and farm lands. One particularly strong thunderstorm produced a rare tornado as it made landfall over the Pakala area of south Kauai on December 13.

No typhoons directly affected any island in the central or western North Pacific basin during 2008 (see the PEAC Tropical Cyclone Summary). In fact, until Typhoon Dolphin tracked past Guam in mid-December, the Guam Weather Forecast Office had issued no tropical cyclone watches or warnings in its area of responsibility. Guam and the CNMI were placed under a tropical storm watch and a tropical storm warning as Dolphin passed by (rather uneventfully).

Sea-level variation in the USAPI is sensitive to the ENSOcycle, with low sea-level observed during El Niño years and high sea-level during La Niña years. Sea-levels have been above normal since early 2007 (see Table 2). Current forecasts indicate that sea-levels will remain elevated at all USAPI stations through the next several months, as La Niña conditions continue to develop.

The following comments were taken from WFO Guam’s MONTHLY PACIFIC ENSO DISCUSSION FOR MICRONESIA AND AMERICAN SAMOA, issued in January 2009:

“The return to La Niña conditions will keep trade winds stronger and more persistent than normal. This will reinforce the higher than normal sea-levels we have experienced in the western Pacific, exposing most of the Micronesian islands, especially the low islands, to periods of coastal flooding and inundation, particularly during full moon and new moon phases and during high surf episodes. This situation could last into early summer. Tropical cyclone activity will be pushed to the west, reducing the risk of hurricanes to American Samoa and likely delaying the Northern Hemisphere typhoon season for Micronesia. Rainfall in American Samoa will likely be below normal due to reduced monsoon activity. The Republic of the Marshall Islands, and Yap State and Chuuk State in the FSM will likely have below normal rainfall for the next few months. The Marianas will experience high month to-month rainfall variability during its dry season. Pohnpei State will have near normal rainfall, while Kosrae State will have normal to above normal rainfall. Rainfall for Palau will also be normal to above normal.”