Pacific ENSO Update

4th Quarter, 2004 Vol. 10 No. 4


During September and October, a wide expanse of drier than normal weather spread across the tropical western Pacific in a “horseshoe”- or “boomerang”-shaped area extending from Fiji westward to the Solomon Islands and Indonesia, and across most of Micronesia in the North Pacific. This large-scale pattern of dry weather is typical of El Niño, but may have other causes such as the Madden/Julian Oscillation (MJO), or other rainfall processes not related to El Niño. By September 2004, the sea surface temperatures in the central equatorial Pacific had warmed to the threshold of El Niño, according to the U.S. Climate Prediction Center (CPC). The oceanic definition of El Niño used by the CPC is three consecutive months with the SST at least 0.5 degrees Celsius warmer than normal in a region of the central equatorial Pacific near the International Date Line designated as “Niño 3.4.” The following two impacts normally associated with an El Niño event have occurred:

  • a change in the distribution of tropical cyclones in the western North Pacific (several typhoons formed east of normal) and
  • a record number of typhoons affected Japan. (During El Niño years, typhoons tend to recurve, and thereby lessen the risk of a strike on the coast of China and increase the risk of a strike in the main islands of Japan.)

However, the following two impacts normally associated with an El Niño have not developed at this time.

  • a substantial erosion of the cold tongue of SST along the equator in the eastern equatorial Pacific and
  • reduced hurricane activity in the Atlantic, Caribbean, and Gulf of Mexico during the 2004 hurricane season. (The Atlantic, Caribbean, and Gulf of Mexico had a relatively active season, especially in the Florida area.)

During recent months there has been extreme month-to-month variations of rainfall and typhoon formations. The rainfall at Guam from June through October (Fig. 1) illustrates the huge magnitude of the fluctuations between periods of excessive rain and dry breaks. The time between periods of excessive rainfall and between the dry breaks has been on the order of 40 days, suggesting that a phenomenon know as the Madden Julian Oscillation (MJO) was occurring. The MJO, also referred to as the 30-60 day oscillation, turns out to be the main intra-annual (less than a year) fluctuation that explains weather variations in the tropics. In general, MJO activity begins in the Indian Ocean and moves slowly eastward through the tropical Pacific Ocean. The active phase of the MJO is associated with increased rainfall, westerly winds extending eastward to Pohnpei or beyond, and increased chances for typhoon formation in Micronesia. The MJO affects the intensity and break periods of the Asian and Australian monsoons and interacts with El Niño. Wet spells in the Australian monsoon occur about 40 days apart. Wet spells and dry spells in Micronesia often occur about 40 days apart.

Most of the islands of Micronesia had wetter than normal rainfall totals during each of the months from January through August 2004. The pattern of a wet June, a dry July, a wet August, a dry September, and a dry October (Fig. 1) occurred across much of Micronesia. Persistent dryness became established across most of Micronesia during September and October. The persistent dryness was most extreme at Palau where the 3-month rainfall total for August, September and October of 19.54 inches was only 48% of normal. American Samoa had been wet, but entered a prolonged period of drier than normal weather beginning in April 2004. Guam experienced two months (June and August) with over 30 inches of rain! The dry weather of September and October may be a signal of the weak El Niño conditions in the Pacific basin. Drier than normal weather could persist throughout much of Micronesia for the next 6 months; however, a major Micronesia-wide drought is not expected.

The general consensus among international computer climate forecasts is for weak El Niño conditions to persist for the next three to six months, and then gradually subside back toward El Niño neutral conditions during mid-2005. The following comments from the EL NIÑO/SOUTHERN OSCILLATION (ENSO) DIAGNOSTIC DISCUSSION were posted on the U.S. Climate Prediction Center web site on November 10, 2004:

Synopsis: Warm-episode conditions are expected to continue into early 2005.

Positive sea surface temperature (SST) anomalies greater than +0.5°C persisted across most of the equatorial Pacific during October 2004). By early November, positive equatorial SST anomalies greater than +1°C were found from 160°E eastward to 150°W and locally in the area around 120°W. The increase and eastward expansion of the area of anomalous warmth in the central and east-central equatorial Pacific during July-October indicates the early stages of a warm ( El Niño) episode.

“Since late 2003 [Madden-Julian] MJO activity has resulted in week-to-week and month-to-month variability in many atmospheric and oceanic indices. … Based on the recent evolution of oceanic and atmospheric conditions and on a majority of the statistical and coupled model forecasts, it seems most likely that warm episode (El Niño) conditions will persist through early 2005.”