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Pacific ENSO Update

4th Quarter, 2011 Vol. 17 No. 4


After relaxing during the middle of 2010, La Niña was back in full force by October 2011, with weather patterns across the Pacific similar to those that had dominated during the strong La Niña event of 2010. During the first half of 2011, La Niña conditions faded to ENSO-neutral. Rainfall was near-normal or above-normal at most locations (see Figures 1a and 1b). Abundant rainfall finally returned to Kapingamarangi Atoll near the equator, where it had been very dry for the last 5 months of 2010 and the first two months of 2011. During the 3rd Quarter months of July, August and September, the weather patterns across Micronesia seemed to be returning to those more typical of the summer: monsoonal southwest winds pushed eastward into Guam and the CNMI, and a few monsoon depressions formed in the region eastward from Palau through the longitudes of Guam and Chuuk.

During September, the basin became quite active for tropical cyclones, with a total of nine numbered tropical cyclones active in the basin at some time during the month. The typhoons were still displaced to the north and west, however, with the Philippines and the Ryukyu Islands bearing the brunt of their inclement weather. During October, tropical cyclone activity abruptly entered a quiet phase throughout the basin as La Niña once again became established. The east wind anomalies of La Niña ended the monsoon flow across Micronesia, and stifled all further tropical cyclone development. The odd collapse of tropical cyclone activity seen for much of the past decade was back, with anomalous easterly surface winds found along the equator from the International Date Line to the Philippine Islands. This is an unusual pattern because in the boreal fall, westerly surface winds typically begin their push further to the east at low latitudes in Micronesia. In the South Pacific, several of the island groups experienced very dry conditions. Tuvalu and the islets of Tokelau Atoll were extremely dry, and the U.S. Coast Guard was involved in a mission to deliver water to Tokelau. American Samoa was also very dry, and wild fires were a problem there in September.

Hawaii has also reentered into very dry conditions. This is due to above normal temperatures and below normal rainfall throughout the 3rd Quarter of 2011 (please see the Hawaii Local Summary for more information). Drought conditions still persist and have even worsened on the Big Island and on Maui in the past few months. Rainy conditions in early November have been welcomed across the state. Additionally, relief may be in sight as the NOAA Climate Prediction center indicates that probabilities favor above-normal precipitation during early 2012 as a part of the forecast mature phase of La Niña.

The following comments from the EL NIÑO/SOUTHERN OSCILLATION (ENSO) DIAGNOSTIC DISCUSSION were posted on the U.S. Climate Prediction Center web site on October 06, 2011:

“ENSO Alert System Status: La Niña Advisory

Synopsis: La Niña conditions are expected to gradually strengthen and continue through the Northern Hemisphere winter 2011-12.

During September 2011, La Niña conditions strengthened as indicated by increasingly negative sea surface temperature (SST) anomalies across the eastern half of the equatorial Pacific Ocean. The weekly Nino indices continued their cooling trend and all are currently at or below –0.5°C. Consistent with this cooling, oceanic heat content (average temperature anomalies in the upper 300m of the ocean) remained below-average in response to a shallower thermocline across the eastern Pacific Ocean. Also, convection continued to be suppressed near the Date Line, and became more enhanced near Papua New Guinea. In addition, anomalous low-level easterly and upper-level westerly winds persisted over the central tropical Pacific. Collectively, these oceanic and atmospheric patterns reflect the continuation of La Niña conditions.

Currently, La Niña is not as strong as it was in September 2010. Roughly one-half of the models predict La Niña to strengthen during the Northern Hemisphere fall and winter. Of these models, the majority predict a weak La Niña (3-month average in the Nino-3.4 region less than -0.9°C). In addition, a weaker second La Niña winter has occurred in three of the five multi-year La Niñas in the historical SST record since 1950. However, the NCEP Climate Forecast System (CFS.v1) predicts a moderate-strength La Niña this winter (between –1.0°C to –1.4°C) and CFS.v2 predicts a strong La Niña (less than –1.5°C), which rivals last year’s peak strength. For CFS forecasts made at this time of year, the average error for December-February is roughly ±0.5°C, so there is uncertainty as to whether this amplitude will be achieved. Thus, at this time, a weak or moderate strength La Niña is most likely during the Northern Hemisphere winter.”

Pacific ENSO Applications Climate (PEAC) Center
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Page Last Modified: December 10 2011 01:44:31 GMT


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