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Pacific ENSO Update

3rd Quarter, 2009 Vol. 15 No. 3


During the first half of 2009, the climate of the tropical Pacific transitioned from ENSO-neutral conditions to El Niño. On July 9th, the U.S. Climate Prediction Center’s ENSO Alert System Status was elevated from the level of “El Niño Watch” to “El Niño Advisory”. Manifestations of the shift in climate state to El Niño consisted of tropical Pacific oceanic warming at the surface and at depth, a fall of sea level across most of Micronesia, and a weakening of the strong easterly wind flow that has\ dominated the region for the past two years. While the ocean has shown the strongest indications of El Niño with its extensive warming, the atmosphere has not yet shown many signs typical of El Niño; tropical cyclone distribution remains shifted to the west of normal, trade winds have persisted in many regions, and there have been few episodes of strong westerly surface winds at low latitudes that are typical during the development of El Niño. Sea levels, which have been very high throughout much of the region for the past few years, have begun to fall in response to El Niño, but still remain elevated above normal at most locations.

The total rainfall during the first half of 2009 was somewhat dry throughout the region, with a few exceptions (see Figures 1a and 1b). In the west, at Palau and Yap, the first half of 2009 was wetter than normal. With the ongoing and persistent westward shift of tropical cyclone activity, these areas still catch heavy rainfall from disturbances and tropical depressions that head toward the Philippines and Taiwan. Nukuoro and Kapingamarangi were also wet during the first half of 2009 due to their southerly latitude. The 101.47 inches recorded at the Kosrae airport was the highest recorded rainfall total in the USAPI during the first half of 2009. The most problematic rainfall anomaly during the first half of 2009 was the persistent, very dry conditions in some of the northern atolls of the Republic of the Marshall Islands, where some locations have been critically dry for many months. Six-month rainfall totals were about half of normal at Kwajalein, Utirik, Wotje, and other atolls of the northern RMI. Six-month rainfall totals below 20 inches were noted in parts of the CNMI, Guam and in the northern RMI.

The Hawaiian Islands were generally very dry during the first half of 2009. Leeward areas, especially on Maui and the Big Island, did not receive much rainfall and remain under drought conditions. Pasture conditions have also been deteriorating on portions of all the islands and impacts to livestock operations have been increasing. With the development of El Niño, Hawaii may receive below normal rainfall during the winter months.

No typhoons directly affected any island in the western North Pacific basin during the first half of 2009. Some of the early stages in the development of the year’s first few named tropical cyclones evolved through their tropical disturbance and depression stages within the territorial waters of Palau and Yap States.

Sea level variation in the USAPI is sensitive to the ENSOcycle, with low sea level observed during El Niño events and high sea level observed during La Niña events. Sea levels have been higher than average in the Pacific since early 2007. Consistent with the recent transition from ENSO-neutral to El Niño, sea levels will remain slightly elevated at all USAPI and Hawaiian stations for the next three months. However, no further rise is expected. As El Niño conditions develop further, sea levels across the USAPI and Hawaii are expected to fall to normal or below normal levels over the next 3-6 months, depending on the strength of the event.

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


“With the transition to El Niño conditions, the trade winds should continue to weaken, tropical cyclone activity and rainfall distribution should return to near normal, and sea levels should slowly fall toward normal levels. Most Micronesian locations have, in fact, seen sea levels fall over the past month or two. As El Niño conditions further develop, the region will see more westerly monsoon flow and a more typical tropical cyclone distribution in Micronesia. American Samoa is in its dry season, which should be relatively normal and cyclone-free. Generally ample but somewhat below normal rainfall has returned to most locations in Micronesia. The Republic of the Marshall Islands, however, has had well-below normal rainfall, and the National Weather Service and PEAC have recommended continued water conservation for the Marshall Islands for the next few weeks. Other locations will have near normal rainfall, but will likely see high month-to-month variability with occasional episodes of strong westerly winds and heavy rainfall. Increased tropical cyclone activity is expected to affect Micronesia by September, and dry conditions could evolve by the end of the year.”

Pacific ENSO Applications Climate (PEAC) Center
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Honolulu, HI 96822
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Page Last Modified: June 01 2010 22:10:36 GMT


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