Pacific ENSO Update

3rd Quarter, 2008 Vol. 14 No. 3


During the first half of 2008, the climate of the tropical Pacific Ocean evolved from La Niña to ENSO-neutral conditions. While this transition was completed during the month of June, the atmosphere, however, still continues to exhibit some behavior more typical of La Niña: enhanced trade winds across Micronesia and a westward displacement of tropical cyclone activity. Few climatic extremes, wet or dry, have been noted.

The total rainfall during the first half of 2008 was within the range of 80% to 120% of normal at most locations within the U.S.-Affiliated Pacific Islands (USAPI) (see Figures 1a, 1b). Six-month rainfall totals were less than 80% of normal at some locations in the northern RMI and Yap State. The lowest 2008 1st half rainfall totals of 19.42 inches, 21.66 inches and 21.95 inches occurred at Saipan, Andersen AFB (Guam) and Wotje, respectively. Six-month rainfall totals in excess of 120% of normal were seen at in American Samoa and at several locations on Pohnpei Island and its outer atolls. The 122.09 inches recorded at the Kosrae airport was the highest recorded total rainfall in the USAPI during the first half of 2008, followed by the 116.97 inches of rain at Palikir on Pohnpei Island. The highest departure from normal occurred at Nukuoro, with 115.57 inches of rain (148% of normal) for the first half of 2008.

Most of the Hawaiian Islands have been extremely dry for the first half of 2008 (see Figures 1a, 1b), with drought conditions reported across the state. Windward areas of the Big Island have been placed under moderate drought. Degraded pasture conditions are severely affecting livestock, and brush fire danger is also very high. Drought conditions across Maui range from moderate to severe, and sugar planting in central Maui has been suspended due to lack of water. In west Molokai, users of irrigation water from the Kualapu‘u Reservoir remain under a mandatory 20% reduction on water use. On east Oahu, extreme drought conditions have developed. Irrigation water levels continue to drop in the Waimanalo Reservoir, where officials have placed a 30% mandatory restriction on irrigation water use. The latest long-lead Hawaiian Outlook issued by the NOAA Climate Prediction Center continued to project below-normal rainfall in the Hawaiian Islands through December 2008, which is deep into the islands’ normal wet season.

No typhoons directly affected any island in the western North Pacific basin during the first half of 2008. Several of the year’s typhoons, however, evolved through their tropical disturbance and depression stages in portions of Micronesia, contributing to rainfall totals at some locations from Chuuk westward to Palau. Extreme daily rainfall amounts of 3 to 4 inches occurred at some locations, but these values are not considered that unusual. Excessive rain rates of 6 inches or more in 24 hours are typically associated with tropical cyclones, which have been notably absent in Micronesia since 2007. (See the Tropical Cyclone section for the latest typhoon outlook for 2008, and refer to each island’s summary for the definition of “normal” tropical cyclone threat).

Sea level variation in the USAPI is sensitive to the ENSO cycle, 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 slightly elevated at all USAPI stations for another 1 to 3 months, although sea-level at all locations should begin slowly receding toward normal levels. This trend is consistent with the recent transition from La Niña to ENSO-neutral conditions.

The following comments from the EL NIÑO/SOUTHERN OSCILLATION (ENSO) DIAGNOSTIC DISCUSSION were posted on the U.S. Climate Prediction Center web site on on July 10, 2008:

Synopsis: ENSO-neutral conditions are expected to continue into Northern Hemisphere Fall 2008.

“A transition from La Niña to ENSO-neutral conditions occurred during June 2008, as sea surface temperatures (SSTs) returned to near-average across the central and east-central equatorial Pacific Ocean...

Similar to past transitions, La Niña continues to linger in the atmospheric circulation, but with diminishing strength. Enhanced low-level easterly winds and upper-level westerly winds remain across the central equatorial Pacific, while convection continues to be suppressed in the central equatorial Pacific and slightly enhanced over the far western Pacific. Collectively, these atmospheric and oceanic anomalies are consistent with a return from La Niña to ENSO-neutral conditions.

Most forecast models indicate ENSO-neutral conditions will continue through the Northern Hemisphere Winter 2008-09. Despite this model consensus, the possible development of El Niño or La Niña cannot be ruled out due to uncertainty in model forecasts and because ENSO events often form during the second half of the year. Based on current atmospheric and oceanic conditions, recent trends, and model forecasts, ENSO-neutral conditions are expected to continue into Northern Hemisphere Fall 2008.”