4th Quarter 1998 - Vol.4 No.4
The very strong 1997 El Niño event peaked around January 1998 and phased out in the May to June time frame of 1998. Around this time, sea surface temperatures (SSTs) returned to near normal values in all equatorial Pacific regions near the west coast of South America. By July, the equatorial SSTs in the central Pacific had become more than 1øC (1.8øF) cooler than normal, indicating that weak La Niña (cold episode) conditions had developed. This rapid cooling suggested that a strong La Niña could develop, but the cooling slowed and has since hovered around the 1øC cooler than normal value. The equatorial cold tongue associated with the La Niña now extends from the west coast of South America westward to 160ø East. Some climate models are predicting strong La Niño conditions by early 1999, while others suggest that the La Niña will remain weak until mid-1999. With stronger than normal easterly low level equatorial winds and cooler than normal sub-surface SSTs (a shallower than normal thermocline) across the eastern and central equatorial Pacific, we expect some strengthening of the La Niña to a moderate event, peaking in January 1999 and phasing out by June. Thus, SSTs point toward a return to near normal rainfall and tropical cyclone activity in late 1998 for most of Micronesia, but drier than normal winter and early spring rainfall conditions are espected to prevail in the Marshall Islands and in near-equatorial regions.
After reaching its lowest point in March 1998, the Southern Oscillation Index (SOI) began to rise and became positive in May. It has hovered slightly above +1 since July. It may rise to near +2.0 by March 1999, then begin to fall to normal values (near zero) by September. This behavior of the SOI supports moderate La Niña conditions through the northern hemisphere spring. Thus, the behavior of the SOI points to a return to near normal rainfall and tropical cyclone activity for most of Micronesia, but less than normal rainfall will prevail during winter and early spring in the Marshall Islands and in near-equatorial regions.
Sea level heights should rise slightly in the western Pacific as water temperatures warm to deeper levels and increase the volume of basin water. Sea levels should be lower than normal in the central and eastern equatorial Pacific where La Niña waters are colder than normal and water volume is reduced.
In the western North Pacific, tropical cyclone activity during 1998 was the lowest on record. While we correctly predicted that most tropical cyclone activity would be north and west of Micronesia , and would be below normal, we did not expect such a low number of tropical storms and typhoons. The first tropical storm did not occur until July 8 (Tropical Storm Nicole) and the first typhoon did not occur until August 3 (Typhoon Otto). And, the only three storms to affect Micronesia were weak at the time -- Tropical Storm Zeb, which moved 37 nm north of Yap on October 10; Tropical Storm Babs, which also moved north of Yap on October 15; and, midget Tropical Storm Alex, which hit Rota in the CNMI on October 11. Tropical cyclones can still affect the western half of Micronesia until about mid-December. Any late season tropical cyclones are expected to intensify west of the Mariana Islands and most likely hit the Philippines.
In the Southwest Pacific, 10 tropical cyclones developed east of the international date line from October 1997 into May 1998 (typical of strong El Niño activity), with eight affecting French Polynesia. In addition, an unexpectedly large number of cyclones (7) also developed west of the date line, although none affected eastern Australia. Few, if any, tropical cyclones of severe tropical storm or hurricane intensity are expected east of the date line until the next El Niño event.
In February and March 1997, the coupled atmosphere-ocean models that PEAC staff has consulted on a regular basis indicated the development of an El Niño warm event. By May, it was clear that an ENSO warm event was developing very quickly, and we alerted our clients through the Pacific ENSO Update. Chip Guard from Water and Energy Research Institute (WERI) at the University of Guam produced rainfall forecasts in terms of percent of normal rainfall for three-monthly seasons and provided those to the Guam and the CNMI governments. Other governments then asked for quantitative forecasts, which Chip produced. PEAC issued its first quantitative rainfall forecasts in the Pacific ENSO Update in October 1997.
In September 1997, Chip Guard and WERI staff began briefings for government officials in the American Flag Pacific Islands on the impending drought. Chip reviewed the rainfall and typhoon forecasts and suggested that governments begin preparing to respond immediately. In Yap, a drought task force was in place before Chip arrived. It had been organized in response to the forecasts issued in the Pacific ENSO Update. Chip reinforced the message already being conveyed by the public information program that the task force had developed. The task forces in the RMI, the FSM, Palau, and Guam developed mitigation plans. They mounted public information campaigns to inform the public about what to expect from the El Niño, to explain measures that could be taken to conserve water and prevent outbreaks of diseases, and to warn of the increased wildfire risk and actions to reduce the risks. In Pohnpei State, a video was produced and aired on the public television station four times a day from early January through May. Hotlines were set up, brochures were developed, public service announcements were made on local radio and television stations, and presentations on El Niño and the drought were done in schools.
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The Water and Environmental Research Institute (WERI) at the University of Guam conducts research in hydrology, meteorology, climatology, water quality, hydrogeology, and other related areas. The Institute offers a BS in Pre-Engineering and an MS in Environmental Sciences. The Institute has a faculty of eight, which includes six professors and two research associates. WERI is one of fifty-three US land grant water institutes, but is unique in that it is not only the water institute for Guam, but is also the water institute for the Federated States of Micronesia (FSM) and the Commonwealth of the Northern Mariana Islands (CNMI). Dr. H. Galt Siegrist is the Director.
As a water institute, WERI conducts annual fresh water-related research and training for the governments of Guam, the FSM, and the CNMI. This may entail ground water modeling, surface water resources management, water quality assessment, erosion studies, hydraulics, hydrologic engineering, and meteorology/climatology. The meteorology and climatology research at WERI is both basic and applied, and includes general tropical meteorology, tropical cyclone motion and structure change, monsoon dynamics, climate variability, climate change, and meteorological satellite techniques and interpretation.
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NOAA's Climate Prediction Center in Washington D.C. routinely monitors the latest forecast results from several ENSO models. The PROGNOSTIC DISCUSSION FOR LONG-LEAD OUTLOOKS outlines the these results.
Hawaii: No Flash Flood Warnings were issued for the state in November. Urban and Small Stream Flood Advisories were issued for portions of windward Oahu (10 Nov), and for portions of the windward districts on the island of Hawaii (17 and 18 Nov) and the island of Maui (17 Nov).
Rainfall amounts were a mixed bag throughout the state. Individuals in some areas may feel like the enhanced rainfall expected during a La Niña episode is here. Others, especially in the leeward areas of the islands of Maui and Hawaii, continue to see below normal rainfall.
Large scale trade winds continued to play a dominant role in the weather pattern over the Hawaiian Islands throughout the month. These trade winds were light during the first week of November. However, an upper level trough to the west of the state produced relatively unstable conditions and enhanced shower activity over the islands. Normally, the month of November marks the start of the cool season with the island chain normally experiencing the passage of several cold fronts or shear lines. Subtropical cyclones, or kona storms, can also develop west of the islands to produce prolonged periods of heavy rain. During November 1998, only one shear line (the remnant of a cold front) was able to reach the island chain. A band of showers ahead of the shear line produced heavy rain for a brief period along the northern sections of windward Oahu. The shear line stalled over the island of Kauai and the Kauai Channel on 11 November before lifting out toward the northeast. Fragments of this shear line and subsequent shear lines embedded within the trade flow hit the islands from the east northeast over the period from 12 November to 20 November. This caused enhanced trade showers, mainly over the windward sections of the islands. Upper level troughs helped enhance the shower activity, especially over the islands of Maui and Hawaii during this period.
For a more complete summary, and the county by county wrap-ups, please see the November Precipication Summary from the National Weather Service Honolulu Weather Forecast Office.
American Samoa: After several months of drought conditions, we are expecting near normal rainfall to return to the Samoa region. As anticipated in the last Pacific ENSO Update, rainfall for July through October of 1998 was well below normal, but highly variable. At Pago Pago, rainfall values for July (1.42 inches) and August (1.59) were a very low 23% of normal. Amounts began to pick up in September and October with 4.30 inches (64%) and 6.31 inches (58%), respectively. July and August rainfall for the Samoa region came exclusively from showers embedded in enhanced trade wind flow or from the occasional passage of weak, decaying cold fronts (shear lines). Our last Pacific ENSO Update stated: "Dry conditions are expected to continue into September." and "Wet weather is expected to return in October." This occurred, and we expect rainfall amounts to reach near normal or slightly above normal values from November through February as the South Pacific Convergence Zone (SPCZ) is anchored over or near the islands. With our prediction of a moderate instead of a strong La Niña event, we don't expect the next dry season in Samoa to be affected too severely. Tropical cyclone activity in the southern hemisphere will move west of the international date line, reducing the threat to the Samoa region.
The rainfall predictions for American Samoa and surrounding islands are:
Inclusive Period % of long-term average Samoa Region Jul-Oct 98 42% Nov 98-Feb 99 110% Mar-Sep 99 90% Oct-Dec 99 100%
- sources: UOG-WERI and PEAC
Guam/CNMI: After a near record drought for Guam and the CNMI, we expect rains to return to near normal values. 1998 could well be the driest year on record for the Mariana Islands. Rainfall began to increase in June on Guam and in July in the CNMI as the result of heavier rain events associated with upper level disturbances in the Tropical Upper Tropospheric Trough. For July and August, rainfall at the Guam International Airport (GIA) was 5.30 inches (50%) and 4.44 inches (32%), and that at Andersen Air force Base (AAFB) was 4.28 inches (40%) and 5.26 inches (39%). July and August rainfall at Saipan International Airport (SIA) was 5.90 inches (74%) and 5.46 inches (47%). At Capitol Hill, amounts were 8.50 inches (94%) and 8.72 inches (70%), respectively.
In September and October, rainfall at GIA increased to 16.44 (122%) and 9.27 (77%), respectively, while that at AAFB increased to 16.69 (125%) and 8.02 inches (62%). At SIA, the respective values for September and October were 4.89 (44%) and 4.68 inches (43%). Capitol Hill was wetter with 8.65 inches (69%) and about 8.50 inches (71%), respectively. Tinian rainfall was similar to that on Saipan, with values of 6-7 inches (depending on location) in August, September, and October. The NASA rain gage network located at the Rota Resort and Country Club on Rota showed August, Sepetember and October amounts of 4.87 inches (36%), 23.84 inches (179%) and about 5.72 inches (44%). The Sabana area was probably somewhat wetter than the northern Golf Resort area, but not significantly so.<.P>
Only one monsoon episode developed across the Mariana Islands during the entire year, lasting a few days in September. Also in September, rainfall associated with disturbances induced by upper level low pressure systems in the Tropical Upper Tropospheric Trough (TUTT) shifted from a position north of the Mariana Islands to near 10ø North latitude. Most of the rain by-passed the Marianas, but Guam and Rota received more of the spotty rainfall than did Saipan and Tinian.
Tropical cyclone activity could affect the Mariana Islands until mid-December 1998. The threat will resume in April and May 1999 and again from September to mid-December 1999.
Our rainfall predictions through December 1999 for Guam/Rota and Saipan/Tinian are:
Inclusive Period % of long-term average Guam/Rota Saipan/Tinian Jul-Oct 98 70% 52% Nov 98-Feb 99 90% 80% Mar-Jun 99 90% 90% Jul-Dec 99 100% 100%
- sources: UOG-WERI
Federated States of Micronesia: We anticipate normal to somewhat wetter than normal conditions for most of the north-western FSM. Most of the eastern FSM will be drier than normal, and near- equatorial regions will be much drier than normal through the winter and early spring of 1999. Rainfall over most of the eastern and southern FSM was below normal for the 4-month period July-October, largely as a result of enhanced equatorial easterly winds associated with the current weak La Niña event. These winds also brought very dry conditions to Nauru and the more equatorial atolls of Pohnpei State. Rainfall in northern Yap State was above normal.
Yap State: July, August, September, and October rainfall at the Yap Airport was 11.99 inches (82%), 12.03 inches (79%), 18.13 inches (134%), and 16.11 inches (135%). Both Ulithi and Woleai Atolls had about 20-25% less rainfall than Yap Island. In September, rainfall associated with disturbances induced by upper level low pressure systems in the Tropical Upper Tropospheric Trough (TUTT) moved rapidly southward from a positon along 20ø North latitude of the Mariana Islands to a position near 10ø North latitude and 130ø-140ø East longitude. These disturbances stagnated over Yap and Palau, bringing heavier than anticipated rainfall, despite the fact that monsoon activity we expected in late summer and early fall did not materialize. Tropical Storms Zeb and Babs did drop several inches of rain on Yap in the first half of October. Yap State islands could be subjected to tropical storms and typhoons until mid-December. This threat will resume in May 1999 and again from September to December 1999.
Rainfall for Yap and its atolls is expected to be:
Inclusive Period % of long-term average Yap Outer Atolls: Island S.of 8·N N. of 8·N Jul-Aug 98 81% 50% 48% Sep-Dec 98 120% 95% 100% Jan-Mar 99 100% 80% 100% Apr-Dec 99 100% 100% 100%
- sources: UOG-WERI
Chuuk State: July, August, September, and October rainfall at Weno Island was 5.43 inches (45%), 12.89 inches (89%), 6.49 inches (56%), and 12.62 inches (94%), respectively. At Lukunoch, July and August rainfall averaged 7.40 inches (65%) and September and October rainfall averaged 9.26 inches (75%). Conditions were drier at Polowat where July and August rainfall averaged 5.83 inches (41%) and September and October rainfall averaged 6.88 inches (55%). Although tropical cyclone activity has been much reduced in 1998, Chuuk State islands could be subjected to a tropical storm or typhoon until mid-December.
Predicted rainfall for Chuuk State is as follows:
Inclusive Period % of long-term average Chuuk Outer Atolls Lagoon Southern Northern Jul-Sep 98 63% 75% 70% Oct-Dec 98 90% 90% 85% Jan-Mar 99 80% 75% 80% Apr-Dec 99 100% 100% 100%
- sources: UOG-WERI
Pohnpei State: July and August rainfall for Kolonia, Pohnpei were 8.53 inches (46%) and 14.93 inches (90%), respectively. September and October values were 10.75 inches (67%) and 14.81 inches (89%). July and August rainfall amounts for Pingalap averaged 11.83 inches (63%), while those in September and October averaged 15.77 inches (97%). Nukuoro received an average of only 6.04 inches (35%) in July and August and an average of 6.89 inches (46%) in September and October. Most of Pohnpei State's rainfall came from upper level disturbances initiated by the Tropical Upper Tropospheric Trough. In Pohnpei, the mountains also played an important role in producing rain-bearing clouds. Nukuoro rainfall was diminished by the stronger than normal easterly equatorial winds associated with the ongoing weak La Niña event. Pohnpei State islands will have a small threat from tropical cyclones from October to December 1999.
Islands very close to the equator are expected to remain considerably drier than normal as a result of the ongoing La Niña event. This will include such islands as Kapingamarangi, Nauru, Banaba (Ocean Island), and Tarawa.
Rainfall predictions for Pohnpei State are as follows:
Inclusive Period % of long-term average Pohnpei Outer Atolls: Island Eastern Southern Equatorial Jul-Oct 98 73% 80% 41% 15% Nov 98-Mar 99 80% 75% 50% 20% Apr-Jul 99 90% 85% 75% 50% Aug-Dec 99 100% 100% 100% 90%
- sources: UOG-WERI
Kosrae State: July and August rainfall for Kosrae Airport was 12.20 inches (72%) and 13.56 inches (82%). Rainfall amounts decreased substantially in September (6.64 inches -- 39%) and October (7.75 inches -- 48%) due to increased equatorial easterly winds as a result of the current La Niña event. Rainfall at Tafunsak averaged 60% of normal for the 4-month period and that at Utwa averaged 68% of normal during the period. Rainfall is expected to remain below normal through March. By June, rainfall is expected to rise to near normal values as the La Niña phases out. Kosrae has a low risk of tropical cyclone activity, especially in non-El Niño years.
The anticipated rainfall in Kosrae for the next year is as follows:
Inclusive Period % of long-term average Jul-Aug 98 77% Sep-Oct 98 44% Nov 98-Mar 99 55% Apr-Jun 99 85% Jul-Dec 99 95%
- sources: UOG-WERI
Palau: Palau can expect near normal to above normal rainfall for the next year. Rainfall at Koror in July and August was 7.52 inches (42%) and 10.16 inches (68%). In September and October values rose to 10.09 inches (85%) and 19.05 inches (137%), as upper level low pressure areas associated with the Tropical Upper Tropospheric Trough (TUTT) brought disturbances that stagnated over the Palau area. This resulted in above normal rainfall, despite the lack of an active monsoon over the region. However, farther south in Peleliu, conditions were drier. July-September rainfall there averaged a little over 7 inches or slightly less than 50% of normal. In October, rainfall at Peleliu rose to 14.98 inches (108%) with some effects of the TUTT-induced rains. Rainfall for Koror should be slightly above normal until January, then near normal thereafter. More southerly islands should be slightly drier than normal until March, then near normal thereafter. Palau could experience a tropical cyclone from now until mid-December and in the fall of 1999.
The expected rainfall for the next 12 months for Palau is as follows:
Inclusive Period % of long-term average Koror and Outer Atolls Babelthaup S. of 8ºN N. of 8ºN Jul-Sep 98 65% 60% 49% Oct-Dec 98 115% 115% 100% Jan-Mar 99 100% 95% 80% Apr-Dec 99 100% 100% 100%
- sources: UOG-WERI
Marshall Islands: Despite a relatively wet summer and fall in the Marshall Islands, the development of a moderate La Niña is expected to reduce winter and early spring rains. At Majuro (representative of southern atolls), rainfall for July and August was 16.29 inches (125%) and 12.05 inches (105%). Rainfall was more variable in September and October with the respective months experiencing 9.80 inches (79%) and 19.45 inches (141%). Kwajalein (representative of northern atolls) had 12.63 inches (121%) and 11.18 inches (111%) in July and August. September and October rainfall was also more variable at Kwajalein with 9.04 inches (76%) and 14.69 inches (123%). Enhanced easterly winds associated with the anticipated moderate La Niña is expected to create drier than normal conditions in the Marshalls from December to about April 1999.
Tropical cyclone activity will not be a threat to the Marshall Islands until at least September 1999.
We anticipate the following rainfall amounts for the Marshall Islands:
Inclusive Period % of long-term average RMI Atolls Southern Northern Jul-Aug 98 115% 116% Sep-Nov 98 100% 95% Dec 98-Apr 99 70% 60% May-Dec 99 95% 90%
- sources: UOG-WERI
Long-Lead Outlook for Hawaiian Islands issue dated 17 December 1998, from NOAA's Climate Prediction Center (CPC).
ENSO ADVISORY of December 11 ,1998
SPECIAL SECTION - Experimental Forecasts for Pacific Island Rainfall
The information contained in the LOCAL VARIABILITY SUMMARIES section and elsewhere in this issue of the Pacific ENSO Update has been drawn from many sources. Further information may be obtained by contacting your local National Weather Service office, or the individuals and institutions listed below:
NOAA National Weather Service - National Centers for Environmental
Prediction (NCEP) - CLIMATE PREDICTION CENTER (CPC):
World Weather Building, Washington D.C. 20233.
Contact CPC at 301-763-8155 for more information on the ENSO Advisory, the Long-Lead Outlook for the Hawaiian Islands, and other publications discussed in this bulletin.
NOAA National Weather Service - Pacific Region
WEATHER SERVICE FORECAST OFFICE (WSFO)
University of Hawaii - Manoa Campus
HIG #225, 2525 Correa Road, Honolulu, Hawaii 96822
Contact the WSFO at 808-973-5270 for more information on NWS-PR sources of climate information.
University of Guam (UOG) WATER AND ENERGY RESEARCH INSTITUTE (WERI):
Lower campus, University of Guam
UOG Station, Mangilao, Guam 96923
Contact C. Guard or M. Lander at (671)735-2685 for more info on tropical cyclones and climate in the Pacific Islands.
University of Hawaii (UH) School of Ocean and Earth Science and
(SOEST) DEPARTMENT OF METEOROLOGY:
HIG #331, 2525 Correa Road, Honolulu, Hawaii 96822
Contact Dr. T. Schroeder at 808-956-7476 for more information on hurricanes and climate in Hawaii.
PACIFIC EL NINO-SOUTHERN OSCILLATION (ENSO) APPLICATIONS CENTER:
HIG #331, 2525 Correa Road, Honolulu, Hawaii 96822
Contact R. Tanabe at 808-956-2324 for more information on ENSO-related climate data for the Pacific Islands.
Contact A. Wood at 405-447-8412 for more information about this issue of the Pacific ENSO Update
For further information, please contact:
Guest Editor, Pacific ENSO Update,
c/o Environmental Verification and Analysis Center (EVAC)
710 Asp Ave., Suite 8
Norman, OK 73069
Publication of the
Pacific ENSO Update is funded in part
by Grant Number NA46GP0410 from the National Oceanic
and Atmospheric Administration (NOAA) Office of Global
Programs. The views expressed herein are those of the
author(s) and do not necessarily reflect the views of NOAA
or any of its sub-agencies.
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