Pacific ENSO Update

2nd Quarter, 2007 Vol. 13 No. 2

Guam and CNMI text Guam/CNMI:  While rainfall on Guam during the 3rd quarter of 2006 was greater than normal at most reporting locations, extreme month-to-month variation in Guam’s rainfall was observed. Very wet conditions persisted for all of July through the first half of August, followed by very dry conditions from mid-August to the last week of September (less than 4 inches of rain at the WSO). Wet conditions returned again in late September and lasted through late October. During the mid-August to late-September dry spell, it became so dry that wildfires were observed in the southern grasslands. This rainfall pattern is possibly the result of the Madden Julian Oscillation (MJO). The MJO (also referred to as the 30-60 day or 40-50 day oscillation) appears to be the main intra-annual fluctuation that can be used to explain weather variations in the tropics. While the MJO affects the entire tropical troposphere, it’s effects are most evident in the Indian and western Pacific Oceans. The MJO involves variations in wind, sea surface temperature (SST), cloudiness, and rainfall. The manifestation of the MJO signal in Micronesia is to produce several weeks of wet weather broken by a week or two of hot dry weather. Click here to see a chart of Guam's 3rd quarter rainfall as it relates to the MJO.

Rainfall totals at stations in the CNMI during the 3rd quarter of 2006 were generally dry, especially during August and September. Rainfall for September, typically the wettest month of the year, was less than half on Tinian and Rota. July marked the only month during which any of the recording locations in the CNMI received more than 100% of normal rainfall.

Guam and CNMI Rainfall Summary 3rd Quarter 2006

Station   Jul. Aug. Sep. 3rd Qtr
Guam International Airport WSO Rainfall (inches) 20.56 12.07 8.96 41.59
% of Normal 195% 88% 66% 110%
Anderson Air Force Base Rainfall (inches) 16.38 11.33 8.75 36.46
% of Normal 150% 84% 66% 97%
Dedado (Ypapao) Rainfall (inches) 20.78 12.45 10.00 43.23
% of Normal 178%* 85%* 66%* 104%*
Saipan International Airport Rainfall (inches) 12.78 9.57 8.02 30.37
% of Normal 158% 77% 59% 89%
Capital Hill Rainfall (inches) 15.09 10.45 8.00 33.54
% of Normal 168% 84% 59% 96%
Tinian Airport Rainfall (inches) 11.09 7.46 6.40 24.95
% of Normal 123% 60% 47% 71%
Rota Airport Rainfall (inches) 10.75 9.82 6.44 27.01
% of Normal 103% 74% 48% 73%
* % of normal for Dededo is with respect to WSO Finigayan

Climate Outlook:

With the onset of El Niño in the 2nd half of 2006, the rainfall on Guam and in the CNMI is likely to be below normal through the first half of 2007. El Niño conditions provide an opportunity for tropical cyclones to form further east than normal (e.g., in the Marshall Islands) and thereby increase the threat of a damaging typhoon on Guam and in the CNMI. During non-El Niño years, the odds of typhoon force winds on Guam or on any individual island in the CNMI is about 1 in 10; however, during El Niño years that chance rises to about 1 in 3. Dangerous surf from a typhoon does not require that the typhoon pass close to any location, so it is almost certain that at least one episode of dangerous typhoon-generated waves will affect the islands. Every year several lives are lost due to hazardous surf and the rip currents produced by them.

Rainfall is anticipated to be below normal for Guam and the CNMI during the upcoming dry season. The dry season months are expected to be drier than normal, and the dry season may be lengthened on both ends (meaning an early start beginning in November 2006 and an extension into June or July 2007). Island residents are urged to begin voluntary water conservation measures now, and to undertake low-cost repairs and maintenance to water infrastructure. If extreme dry conditions develop in the next 3 to 6 months, a more serious effort may be required to ensure adequate water supplies for all islanders. The risk of wildfires will be high beginning in late January and continuing through June. Residents are urged to clear away brush and loose combustible materials located near their homes. During the record drought of the dry season of 1998, nearly 12% of Guam’s land area was scorched by wildfires.

Predicted rainfall for Guam and the CNMI from November 2006 through October 2007 is as follows:

Inclusive Period

% of Long-Term Average Rainfall
Nov - Dec 2006 (End of Rainy Season)
Jan - Mar 2007 (Next Dry Season)
Apr - Jun 2007 (Next Dry Season)
Jul - Sep 2007 (Next Rainy Season)

*A typhoon passage in these months could result in an extreme 24-hour rain event

source: UOG-WERI