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Pacific ENSO Update

1st Quarter, 2011 Vol. 17 No. 1


El Niño conditions matured during the first few months of 2010, and by the end of June 2010, the Pacific Basin entered a La Niña. The climate state of the Pacific Basin progressed to moderate to strong La Niña conditions from August through the end of the year. December’s standardized Tahiti-minus-Darwin sea-level pressure (The NOAA version of the Southern Oscillation Index -- SOI) value of +3.2 is the highest monthly value of the SOI in the CPC archive, which goes back to 1951. The weather patterns across Micronesia were generally those expected during moderate to stronger La Niña events. Examples include very strong and persistent easterly surface winds and a westward displacement of tropical cyclone activity. The strength of the low-level easterly winds in the central equatorial Pacific was the highest in the CPC archived record. The CPC archive of low-level winds in the equatorial Pacific begins in 1979, and contains information from ships, buoys and weather satellites. In response to these strong easterly low-level wind anomalies, the sea level across Micronesia rose during the latter half of 2010, from near normal at the beginning of the year, to several inches above normal by the end of the year. Additional alterations to the weather patterns were observed that are typical during years that follow El Niño (regardless of whether the climate state enters La Niña or returns to ENSO-neutral). These include a substantial reduction of tropical cyclones in the western North Pacific basin, a near elimination of the normal monsoon trough in Micronesia, increased rainfall over Indonesia, reduced rainfall over the equatorial portion of the central and western Pacific to about 150 °E, and a reduction of annual rainfall at most sites (see Figure 1). A deep collapse (the biggest ever observed) in the number of tropical cyclones in the North Pacific Basin during 2010 was far beyond what is normally seen during similar years (see the discussion on tropical cyclone activity).

In American Samoa, the 2010 annual rainfall was near normal. The start of the 2010-2011 rainy season in American Samoa was generally tranquil and a bit drier than average. A tropical cyclone (TC08P – Wilma) hit American Samoa on January 23rd, 2011, and the island experienced wind gusts up to 70mph. During 2011, it is anticipated that the most likely scenario would be for the current La Niña conditions to slowly fade back to ENSO-neutral by June or July. In this scenario, Guam and the CNMI would probably have near normal rainfall for the foreseeable future.  

In the state of Hawaii, recent near-normal rainfall has eased drought conditions in most areas. With the beginning of the wet season in October 2010, pastures and general vegetation conditions have greatly improved. A record daily maximum rainfall total of 5.41 inches fell at the Honolulu International Airport on Sunday, December 19th, breaking the old record of 5.28 inches set in 1955. This one day event doubled December’s monthly rainfall total to 11.73 inches, set the monthly percent of normal precipitation to a whopping 412%, and pushed the annual total much closer to normal.

The following comments from the 06 January 2011 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 continue well into the Northern Hemisphere spring 2011.

A moderate-to-strong La Niña continued during December 2010 as reflected by well below-average sea surface temperatures (SSTs) across the equatorial Pacific Ocean. All of the Niño indices were –1.5°C at the end of December, except for the easternmost Niño-1+2 region. The subsurface oceanic heat content (average temperatures in the upper 300m of the ocean, continued to reflect a large reservoir of below-average temperatures at depth in the central and eastern equatorial Pacific. Convection remained enhanced over Indonesia and suppressed over the western and central equatorial Pacific. Also, enhanced low-level easterly trade winds and anomalous upper-level westerly winds continued over the equatorial Pacific. Collectively, these oceanic and atmospheric anomalies reflect the ongoing La Niña.

The current ENSO model forecasts have not changed significantly compared to last month. La Niña is currently near its peak and is expected to persist into the Northern Hemisphere spring 2011 at a lesser intensity. Thereafter, there remains considerable uncertainty as to whether La Niña will last into the Northern Hemisphere summer (as suggested by the NCEP CFS 10 February 2011…

The next ENSO Diagnostics Discussion is scheduled for 10 February 2011. To receive an e-mail notification when the monthly ENSO Diagnostic Discussions are released, please send an e-mail message to:”

Pacific ENSO Applications Climate (PEAC) Center
c/o NOAA NWS - Weather Forecast Office Honolulu
2525 Correa Road, suite 250
Honolulu, HI 96822
(808) 956-2324

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Page Last Modified: February 17 2011 06:12:46 GMT


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