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Pacific ENSO Update

1st Quarter, 2012 Vol. 18 No. 1


The state of the climate during 2011 began as strong La Niña. The La Niña waned over the course of the first few months of 2011, and conditions evolved to ENSO-neutral by the summer. The weather patterns across Micronesia during the first half of 2011 were generally those expected for La Niña; for example, very strong and persistent easterly surface winds and a westward displacement of tropical cyclone activity. In response to the strong easterly low-level wind anomalies, the sea level across Micronesia remained higher than normal during all of 2011. During the summer, while the climate system had evolved to ENSO-neutral, the monsoon advanced into parts of Micronesia. For several weeks, spread across July, August and September, Guam and the CNMI experienced light-to-moderate southwesterly wind accompanied by occasional heavy monsoonal rain showers and some impressive lightning displays in monsoon convection. Several tropical disturbances evolved into tropical depressions shortly after passing Guam and the CNMI, and contributed directly to heavy rain events in those islands and also in Yap State and in the Republic of Palau. In October of 2011, the climate system once again shifted back toward La Niña, ending the monsoon early and effectively halting late season (Oct, Nov, Dec) tropical cyclone activity in the basin and especially in the region. The year’s final tropical cyclone, TC 27W (Washi), passed over Palau in mid-December as a tropical depression, and later passed over the central Philippines as a tropical storm, with deadly flooding there. No significant damage was reported from Palau. La Niña strengthened through November and December 2011, and a weak-to-moderate La Niña continues at the time of this writing. The start of the 2011-2012 rainy season in American Samoa was generally tranquil and a bit drier than average. Since the passage of Cyclone Wilma in January 2011, no tropical cyclones have affected American Samoa through January 2012.

The State of Hawaii overall in 2011 received a sufficient amount of rain with some exceptions. There seemed to be a line drawn over Maui where to the east (Big Island) received below-normal rainfall, while to the west (Oahu, Kauai, Molokai, and Lanai) collected above-normal rainfall during the first 6 months of 2011. Specifically in the month of May an upper level low abided over top of most of the islands, which cause several thunderstorms. This tripled the normal amounts of rainfall in Lihue (on Kauai) and Honolulu (on Oahu) for the month. The rainfall continued into June, causing the Punaluu Stream rain gauge (on the windward side of Oahu) to report an amazing 8.53 inches in three hours with a total of 10.13 inches in 24 hours on the 5th. The normal rainfall amount for June on Oahu is only 0.18 inches. However, in September and October as the climate shifted back into La Niña status, all islands saw well below-normal rainfall and warmer-than normal temperatures. These conditions negatively impacted any strides made to eradicate drought on the islands (especially on the Big Island and Maui) at the beginning of the year. It should be noted that drought that has been present in some capacity since June 2008 in the State. More rainfall is needed to promote a full recovery from this drought.

Although, persistent trade winds in November and December helped bring abundant amounts of rainfall for the eastern areas on most of the Hawaiian Islands, the leeward sides received very little rain. This led to the end of the year rainfall totals (depicted on page 9) of the Big Island and Maui near-normal rainfall totals, 90% and 83% respectively. Kauai and Oahu had well above-normal annual rainfall totals with 153% and 172% for the year of 2011 (see Figure 1).

The following comments from the 05 January 2012 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 spring 2012.”

“During December 2011, below-average sea surface temperatures (SST) associated with La Niña continued across the eastern and central equatorial Pacific Ocean. The weekly SST index in the Niño-3.4 region remained near –1.0°C throughout the month, indicating a weak to moderate La Niña. The oceanic heat content (average temperature in the upper 300m of the ocean) anomalies strengthened across the eastern Pacific, reflecting a large area of below-average temperatures in the subsurface. In the atmosphere, anomalous low-level easterly and upper-level westerly winds strengthened over the central and west-central Pacific. Convection remained suppressed in the western and central Pacific and enhanced over northern Australia and parts of Indonesia and the Philippine Islands. Consistent with these conditions, the Southern Oscillation Index (SOI) also strengthened. This evolution is consistent with past events, in which the atmospheric components of La Niña become strongest and most well-defined during the Northern Hemisphere winter. Collectively, the ongoing oceanic and atmospheric patterns reflect the continuation of a weak to moderate La Niña.”

Pacific ENSO Applications Climate (PEAC) Center
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Honolulu, HI 96822
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Page Last Modified: March 22 2012 05:05:15 GMT


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