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Pacific ENSO Update

3rd Quarter, 2011 Vol. 17 No. 3


At the beginning of 2011, the climate of the Pacific basin was dominated by the typical local and regional anomalies associated with La Niña. These included: A strongly positive SOI, cool equatorial SST, strong easterly surface winds, elevated sea level in Micronesia, and very dry conditions at some low-latitude western Pacific atolls (e.g., Kapingamarangi and the atolls of western Kiribati). Through the first half of 2011, the La Niña waned, and many of its associated anomalies subsided or reversed. The SOI fell, the SST warmed, the easterly surface winds weakened somewhat, and the localized island droughts came to an end. With the shift of La Niña to ENSO-neutral during the first half of 2011, rainfall was abundant throughout the USAPI. Above normal 6-month rainfall values dominated, with magnitudes equal to or exceeding 100% of average at locations in every island group except American Samoa and Kosrae (see Figures 1a and 1b). Rainfall totals were equal to or less than 85% of average only at Aasufou, Kapingamarangi, and two of four stations on Kosrae. The highest observed 6-month rainfall total was the 103.03 inches (156%) recorded at the Weather Service Office (WSO) Palau, with the 100.69 inches (103%) recorded at Palikir in 2nd place. The lowest 6-month rainfall was the 26.24 inches recorded at Saipan, which ironically was 135% of the average value during this time of the dry season in the CNMI.

Very dry conditions were experienced at Kapingamarangi during the 7-month period August 2010 through February 2011, and likely caused harm to water supplies, vegetation and crops. Drought quenching rainfall returned to Kapingamarangi in March 2011 when 8.46 inches (61%) of rainfall was observed, and recently a whopping 17.91 inches (247%) was recorded there in June 2011. The warming of the SST in the equatorial western Pacific along with the penetration of westerly surface winds to near the longitude (155°E) of Kapingamarangi accompanied the dramatic increase of rainfall there.

Forecasts from computer models available to PEAC largely anticipated the widespread abundant rainfall during the first half of 2011. Abundant rainfall typically occurs in years that begin as La Niña and transition to ENSO-neutral or to El Niño sometime later in the year. Most of the PEAC outlooks reflected this.

Most of the State of Hawaii has been receiving sufficient amounts of rain during the first half of 2011. Currently, no drought conditions are reported on Lanai, Oahu, or Kauai. Drought symptoms are improving on Maui as well. On the Big Island, conditions have been slowly improving but areas of severe and moderate drought still persist. Oahu experienced a particularly wet June, resulting in a 6-month total at 316% of normal.

The following comments from the ENSO DIAGNOSTIC DISCUSSION were posted on the U.S. Climate Prediction Center web site on July 7, 2011:

“ENSO Alert System Status: Not Active (NA)

Synopsis: ENSO-neutral conditions are expected to continue into the Northern Hemisphere fall 2011.

During June 2011, ENSO-neutral conditions continued as reflected by the overall pattern of small sea surface temperature (SST) anomalies across the equatorial Pacific Ocean. All of the latest weekly Niño index values were near average, ranging between 0.0°C and 0.4°C. The subsurface oceanic heat content anomaly (average temperature anomalies in the upper 300m of the ocean) remained elevated, but weakened slightly throughout the month, in accordance with the declining strength of above-average temperatures at depth. While weak, the atmospheric circulation anomalies remained consistent with certain aspects of La Niña. In particular, convection continued to be enhanced over eastern Indonesia and suppressed over the central equatorial Pacific, mainly south of the equator. Also, anomalous low-level easterly and upper-level westerly winds persisted over the central Pacific. Collectively, these tropical Pacific anomalies indicate ENSO-neutral conditions, but the atmospheric circulation continues to be characteristic of La Niña.

Forecasts from a majority of the ENSO models, indicate ENSO-neutral will continue into the Northern Hemisphere fall 2011. However, over the last couple of weeks, forecasts created by the NCEP Climate Forecast System (CFS) have begun to indicate the re-emergence of La Niña during Northern Hemisphere fall 2011. Combined with the recent weakening of the positive subsurface ocean anomalies and the lingering La Niña state of the atmosphere, the possibility of a return to La Niña during the Northern Hemisphere fall 2011 has increased over the past month. However, ENSO-neutral remains most likely into the Northern Hemisphere fall 2011, with most models and all multi-model forecasts predicting ENSO-neutral to continue through early 2012.”

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: August 05 2011 22:59:03 GMT


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