Pacific ENSO Update

4th Quarter, 2004 Vol. 10 No. 4

Guam and CNMI text Guam/CNMI:  Rainfall on Guam during the heart of its summer rainy season (June through October) was excessive, with enormous month-to-month variation (see Fig. 1). The rainfall for June, July, August, September, and October at the Guam International Airport (GIA) was 38.03 inches (588%), 10.11 inches (96%), 37.32 inches (272%), 10.85 inches (80%), and 9.89 inches (82%) respectively. The total of 106.20 inches was 187% of normal for the 5-month period, and was the highest total rainfall for that period among all recording stations in Micronesia (with the exception of the WERI/CSP rain gage on top of Nahna Laud in Pohnpei — see Fig. 2 a, b). The total rainfall at the GIA of 129.59 inches from January through October of 2004 already far exceeds its mean annual rainfall of approximately 91 inches. The total at the GIA for June 2004 of 38.03 inches was the second highest monthly total ever recorded, second only to the 38.49 inches recorded in August 1997! The rainfall total at the GIA for August 2004 of 37.32 inches was the third highest monthly total ever recorded at the GIA, and only the 5th time that over 30 inches has been recorded there in one month. At Andersen Air Force Base (AAFB), the rainfall for June, July, August, September, and October was 36.39 inches (574%), 8.91 inches (82%), 32.52 inches (242%), 8.78 inches (66%), and 9.57 inches (74%) respectively, amounting to 169% of normal for the 5-month period. The total rainfall at AAFB of 121.61 inches from January through October of 2004 is already 23 inches in excess of the mean annual rainfall there.

The month-to-month variability of rainfall on Guam during the past five months has been extreme: an enormous amount of rain fell during June (breaking many records); it was relatively dry during July; an enormous amount of rain fell during August; September and October were both relatively dry (Fig. 1). Such high month-to-month variation of rainfall could have been a manifestation of the Madden/Julian Oscillation. Persistent dryness during September and October may be partly associated with the development of weak El Niño conditions. If the weak El Niño conditions persist, then the rest of 2004 could be drier than normal for Guam. The upcoming dry season may also see less-than-normal rainfall.

Two typhoons adversely affected Guam through October 2004: Tingting and Chaba. During June 27-28, Typhoon Tingting passed 220 miles northeast of Guam and produced extraordinary amounts of rainfall. 24-hour rainfall totals exceeded 20 inches at many locations. Extensive flooding and mudslides caused much property damage. Westerly gales occurred for several hours on Guam during the passage of Tingting, with a peak gust at the GIA of 58 mph from the west on the night of the June 27. Very high surf was also experienced for several days during and after the passage of this typhoon. There were 7 deaths attributable to the high surf produced by Tingting. On the night of August 21, Typhoon Chaba passed between Rota and Tinian (where damage to property and vegetation was extensive). On Guam, Chaba produced heavy rains with a peak 24-hour total of 9.05 inches at the GIA. Westerly winds exceeded typhoon force in gusts on much of the northern half of the island. The highest recorded gust on Guam during the passage of Chaba was 79 mph at Apra Harbor. The peak gust at the GIA was 66 mph. Fortunately, Guam narrowly escaped the brunt of Typhoon Chaba, and only minor property damage was reported there.

The distribution of rainfall in the CNMI during this year’s rainy season was similar to that on Guam: an extraordinarily wet June, a dry July, an extraordinarily wet August, followed by drier than normal conditions in September and October. During August of 2004, all recording stations on Saipan received over 30 inches of rain. There is no rain-recording station on Saipan that has ever reported more than 30 inches of rain in one month! Typhoon Tingting contributed to the excessive rainfall total during June; Typhoons Chaba and Songda contributed to the excessive rainfall total during August.

The rainfall for June, July, August, September, and October at the Saipan International Airport (SIA) was 18.44 inches (397%), 8.78 inches (108%), 34.15 inches (273%), 8.88 inches (66%), and 7.08 inches (66%) respectively, amounting to 156% of normal for the 5-month period. The 34.15 inches of rainfall at SIA during August is an all-time record, exceeding by far the previous monthly extremes of 24.80 inches recorded there in August 1981 and 24.07 inches in August 1993. The rainfall total of 86.39 inches at SIA from January through October already far exceeds the mean annual rainfall there of 74 inches. The rainfall for the 5-month period June, July, August, September, and October 2004 at Capitol Hill was 19.50 inches (336%), 11.89 inches (132%), 35.75 inches (286%), 5.64 inches (40%), and 5.73 inches (48%) respectively, amounting to 152% of normal for the 5-month period. The 35.75 inches of rainfall at Capitol Hill during August is an all-time record, exceeding by far the previous monthly extremes of 28.59 inches recorded there in August 1993 and 25.06 inches in August 1994.

Rainfall amounts for June, July, August, September, and October at the Tinian Airport were 18.80 inches (324%), 1.71 inches (19%), 37.85 inches (303%), 9.16 inches (68%), and 9.16 inches (76%) respectively, amounting to 145% of normal for the 5-month period. The 37.85 inches of rainfall at the Tinian Airport during August is an all-time record. On the night of 22 August, Typhoon Chaba passed to the south of Tinian. Rainfall during the two-day period of August 22 through August 23 was 9.88 inches. The peak wind gust recorded at the Tinian Aiport during Chaba was 81 mph.

Rainfall amounts for June, July, August, September, and October at the Rota Airport were 25.44 inches (410%), 8.65 inches (83%), 27.98 inches (212%), 14.71 inches (68%), and 8.53 inches respectively, amounting to 152% of normal for the 5-month period. On the night of August 22, the center of Typhoon Chaba passed just to the north of Rota, placing that island in its eye wall and highest wind speeds. Rainfall at the Rota Airport during the two-day period of August 22 through August 23 was 9.60 inches. The Rota Aiport closed at 12:30 PM on August 22, before the arrival of the worst of the typhoon later that night, but an automated observing station at the airport recorded a peak gust of 136 mph at 8:51 pm. Wind gusts as high as 140 mph were estimated from the extensive damage on portions of Rota that were indicative of a low-end CATEGORY 3 typhoon on the Saffir-Simpson Tropical Cyclone Scale — a scale relating tropical cyclone wind speed to potential damage for the tropical Pacific that was adapted by PEAC scientists Chip Guard and Mark Lander from the well-known Saffir-Simpson Hurricane Scale used in the Atlantic.

There is still a threat of a typhoon for Guam and the CNMI through January of 2005. The pattern of warm SST associated with the ongoing weak El Niño should allow for at least one or two more tropical cyclones to develop in the eastern Caroline Islands or in the Marshall Islands that travel westward to threaten Guam and the CNMI.

Predicted rainfall for the Mariana Islands from Nov 2004 through Oct 2005 is as follows:

 
Inclusive Period

% of Long-Term Average
Guam/Rota
Saipan/Tinian
    Nov - Dec 2004 (End of the Rainy Season)
90%
85%
Jan - May 2005 (Next Dry Season)
80% 75%
Jun - Oct 2005 (Next Rainy Season Onset)
95% 90%
source: UOG-WERI