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Pacific ENSO Update

4th Quarter, 2010 Vol. 16 No. 4


CURRENT CONDITIONS

According to the U.S. Climate Prediction Center (CPC), La Niña conditions continued through October in the tropical Pacific. Oceanic and atmospheric indices of ENSO are now indicative that a moderate to strong La Niña is underway. La Niña conditions are anticipated to last at least through the next northern spring. Weather patterns during the past 6 months were clearly that of La Niña: strong and persistent, easterly, low-level wind flow across Micronesia, and monsoon and tropical cyclone activity pushed to the northwest of normal with elevated sea level. Tropical cyclone activity in the western North Pacific is at historical lows, with only 15 numbered tropical cyclones through mid-October. A sharp reduction of tropical cyclone activity is typical during a year that follows an El Niño, but this year's reduction is unprecedented, and continues a multi-year run of quiescence across the entire tropical Pacific.

Rainfall has been near normal across most of Micronesia (see Figures 1a and 1b). The lack of the monsoon trough and westward displacement of tropical cyclones have resulted in an extended period of “tranquil” weather (i.e., few notable extremes of heavy rain). The current La Niña event has seen a strong westward extension of the equatorial tongue of cold water. Because of this, atolls near the equator (e.g., Kapingamarangi, Nauru, and western Kiribati ) have become very dry. As part of Pohnpei State, the island of Kapingamarangi has received drought advisories from the Weather Forecast Office (WFO) Guam and its home Weather Service Office (WSO) on Pohnpei Island. Equatorial dryness has recently edged northward to Nukuoro (~4°N) and Kosrae (~6°N), but overall, these locations have received ample rainfall. The Hawaiian Islands remain very dry, with extreme drought conditions on large portions of the Big Island, eastern Oahu, Kauai, western Molokai, Lanai, and Maui.

Near to above normal rainfall is anticipated throughout much of Micronesia during the next three to six months, with the exception of Kapingamarangi, where dry conditions will continue for the next few months. American Samoa is entering its rainy season, and abundant rains are expected there. Most locations in Hawaii will soon enter their winter rainy season, and things look to continue on the dry side. When seasonal rains return to Micronesia next spring (e.g., April to June at Pohnpei Island), there is the possibility of above normal rainfall. When climate indices show a strong La Niña at the beginning of the year, and then steadily fall toward ENSO-neutral or El Niño conditions, Micronesia tends to experience very wet conditions during the first 6 months of the year. During such years, the trade wind trough (the band of heavy rain showers where the NE trades and the SE trades converge) is active and persistent across Micronesia. During January and February, Abundant rain showers are confined first to locations between 3°N to 6°N. Later in the northern spring (April and May), the trade wind trough progresses northward to bring abundant rainfall to locations up to 8°N, which includes Pohnpei Island, Chuuk Lagoon and the Mortlock Islands, and Palau. Islands north of 10°N have their greatest chance for above normal rainfall in the first half of the year primarily when the climate state is transitioning from La Niña or ENSO-neutral to El Niño.

The following comments from the 07 October 2010 EL NIÑO/SOUTHERN OSCILLATION (ENSO) DIAGNOSTIC DISCUSSION, were posted on the U.S. Climate Prediction Center web site:

“ENSO Alert System Status: La Niña Advisory

Synopsis: La Niña is expected to last at least into the Northern Hemisphere spring 2011.

Consistent with nearly all of the forecast models, La Niña is expected to last at least into the Northern Hemisphere spring 2011. Just over half of the models, as well as the dynamical and statistical averages, predict La Niña to become a strong episode (defined by a 3-month average Niño-3.4 index of –1.5°C or colder) by the November-January season before beginning to weaken. Even though the rate of anomalous cooling temporarily abated during September, this model outcome is favored due to the historical tendency for La Niña to strengthen as winter approaches...

Likely La Niña impacts during October-December 2010 include suppressed convection over the central tropical Pacific Ocean, and enhanced convection over Indonesia. The transition into the Northern Hemisphere fall means that La Niña will begin to exert an increasing influence on the weather and climate of the United States. Also, La Niña can contribute to increased Atlantic hurricane activity by decreasing the vertical wind shear over the Caribbean Sea and tropical Atlantic Ocean . Conversely, La Niña is associated with suppressed hurricane activity across the central and eastern tropical North Pacific.”



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Page Last Modified: January 15 2011 01:16:37 GMT

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