Pacific ENSO Update - 4th Quarter 1996 - Vol.2 No.4


CURRENT CONDITIONS

Sea surface temperatures (SSTs) in the equatorial Pacific during recent months have remained at near-normal levels throughout most of the region. From August through October, equatorial SSTs from the date line to 90°W longitude (near the Galapagos) were just slightly below their long-term average values for those months, while SSTs west of the date line were just slightly above. Eastern Pacific SSTs near the South American coast have remained about 1°C below the long-term average. Surface pressure patterns in the atmosphere over the tropical Pacific, however, have experienced some recent changes: The Southern Oscillation Index or SOI (which measures the relationship in atmospheric pressure between Tahiti and Darwin, Australia) has remained positive throughout the year, primarily due to above-normal pressures at Tahiti. In September, pressures began to drop at both locations, resulting in near-normal levels at Tahiti and below-normal levels at Darwin. Although the SOI remains positive, regional changes accompanied this shift, including stronger-than-normal easterlies and trade winds occurring further to the west, increased rainfall in the region around Samoa, and a northward and westward shift in westerly monsoon wind patterns near Micronesia. Reduced rainfall and tropical cyclone activity was seen in central and western Micronesia until late October, when more normal monsoon conditions became established and Typhoon Dale developed and moved through the region.

A consensus of recent results from the various computer model predictions for ENSO suggests that near-normal, to slightly-cool, SST conditions will continue until mid-1997. As a result, near-normal climate conditions are expected for the region. More details on local climate conditions are described in the following pages of this issue...


LOCAL VARIABILITY SUMMARIES:

As noted in each issue of Pacific ENSO Update, the following summaries of expected local climate variability for the various island areas indicated are not based on official forecasts (unless otherwise stated). Further information is available from your local National Weather Service office, or from the sources listed in the acknowledgements section.


HAWAII: Rainfall across the state from August to October was quite variable. Automated rain gauges recorded rainfall in above-normal amounts on most of Kauai and Oahu's Waianae coast, near-normal amounts on the Kona coast and Oahu's North Shore and central plains, and below-normal amounts in Honolulu, windward Oahu, most of the Big Island and virtually all of Maui.

Over the summer, an unusually strong Tropical Upper Tropospheric Trough or TUTT (described in Pacific ENSO Update, 8/95) developed and extended from west of the islands into the northeast Pacific, producing persistent low surface pressures north and west of the islands. This prevented the North Pacific High (a pressure system that normally develops to the northeast of Hawaii, delivering trade winds and related rainfall to the area at this time of year) from gaining effective strength. The resulting lack of winds and rain led to reductions in surface (soil) moisture, allowing island temperatures to rise and even drier conditions to develop, leading to drought conditions on the Big Island and on Maui that were particularly apparent. Such summertime drought events in Hawaii are due to the lack of seasonal trades, and are unrelated to the wintertime droughts that can occur here with ENSO events. For example, a non-ENSO summertime drought occurred in 1981.

The extremely wet conditions of early November were a dramatic shift, though they were generally focussed on the south and west sides of the islands. Oahu had the heaviest rain by far, with over 30 inches recorded at Waipahu between the 1st and 18th of the month. In comparison, greatest rainfall on other islands for the same period was 16.83 inches at Kokee on Kauai, 7.95 inches at Ulupalakua on Maui, and 12.34 inches in Kau on the Big Island. Some northern island areas, such as the Hana coast on Maui, received considerably less rainfall, and still remain below normal for the season since August.

See page 7 for the latest issue of the Long-Lead Outlook for the Hawaiian Islands from NOAA-CPC.
- sources: NWS-PR and PEAC


AMERICAN SAMOA: At Pago Pago, rainfall for January-October of 1996 is nearly normal, at 3.5% above average, but rainfall for the last three months of this period (August-October) was 76% above normal levels. This surplus was primarily due to the early onset of the rainy season and an extremely wet October, with almost 30 inches of rainfall recorded for the month. This appears to be related to the drop in surface pressure which occurred across the South Pacific, from Tahiti to Darwin (see page 1). As noted in the last issue of Pacific ENSO Update, such changes can cause month-to-month variability in rainfall. Although drier conditions were expected to continue for the area at that time, rainfall is now expected to be near-normal or slightly above normal for American Samoa and for neighboring Western Samoa. The PEAC model forecasts for Pago Pago (shown on page 6) support this prediction. Conditions may be even wetter if pressures remain comparatively low over the region, entrenching the South Pacific Convergence Zone in the region and bringing annual rainfall for 1996 up to perhaps 10% above normal, and wet season rainfall for October-April perhaps 35% or more above normal. However, since SSTs in the eastern Pacific are slightly cooler-than-normal and are predicted to remain so, the low pressure over the region is expected to be short-lived, and seasonal rainfall levels should be closer to normal as a result.

Tropical cyclone activity in the Southwest Pacific was unusually low in 1995-96, apparently due to a relatively weak northwest monsoon over northern and eastern Australia. Tropical cyclone activity in the 1996-97 season is expected to increase to more normal levels. However, with the predicted continuation of slightly cooler-than-normal or near-normal SSTs, and a return to more normal atmospheric pressure patterns, no ENSO warm event is expected to develop and influence tropical cyclone activity in the region. For Samoa, this means that cyclone activity will likely be limited to areas west of the islands, with cyclones developing and moving through areas west of the international date line, such as Vanuatu, the Solomon Islands, New Caledonia, Australia's east coast, and possibly Fiji. Tropical cyclone activity for areas east of the international date line, including American Samoa, Western Samoa, the Cook Islands, and French Polynesia, should be fairly low.
- source: UOG-WERI


GUAM/CNMI: As with the outlook in the last issue of Pacific ENSO Update, no ENSO warm event or La Niña cold event is expected during the next year. A consensus of the various statistical and dynamic climate model predictions suggests that such events are not likely for the 1996-1997 season. The models point toward near-normal or slightly cooler equatorial SSTs. Since no ENSO warm event is expected for 1997, rainfall in Guam and the CNMI is likely to be slightly above the long-term average for the next 12 months. To date, 1996 rainfall at Guam International Airport (drier station on Guam) and at Andersen Air Force Base (wetter station on Guam) has been 114% and 95% (respectively) of averages for the 10-month period of January-October. There has been slightly more rainfall for the recent four month period of July-October, when rainfall for those sites at 116% and 97% of average, respectively. While rainfall data were not received from CNMI, satellite imagery observed lush vegetation on Saipan, indicating that rainfall has at least been near-normal there as well. For the coming year, December-May dry season rainfall is expected to be near-normal (though drier than the 1996 dry season), and June-November wet season rainfall is expected to be slightly above normal. High month-to-month variability can be expected, particularly for the wet season. The PEAC and CPC statistical models (see page 6) indicate close-to-normal conditions for Guam in the coming months, and this should hold true for the CNMI as well. The following expectations of monthly rainfall totals are provided for the use of water resource manages on Guam and Saipan:

MONTHS:              RAINFALL:
  Dec 96 - May 97      100% of long-term average
  Jun 97 - Nov 97      115% of long-term average

Tropical cyclone activity during 1996 increased from 1995 levels for Guam, with Typhoon Dale impacting the island in November. Saipan and Tinian were hit by Tropical Storm Yates in October, but the northern islands of Pagan, Alamagan and Agrihan experienced even higher levels of activity during the season. Storm activity should increase in 1997 as the development region extends eastward to more normal longitudes. Tropical cyclones will affect northern islands by August, Saipan and Tinian by September, and Rota and Guam by October. Activity should subside in early December.
- source: UOG-WERI


FEDERATED STATES OF MICRONESIA (FSM): The current SST and SOI conditions and their recent trends suggest that an ENSO warm event is not likely for 1996-1997. A consensus of the predictions from statistical and dynamic climate models support a continuation of near-normal to slightly cooler-than-normal SSTs. Since El Niño warm event conditions are not expected to develop, rainfall for 1997 should be near or slightly below the long-term average over the eastern FSM states, and somewhat above the long-term average over the western states. Tropical cyclone activity for the FSM is expected to be near normal, with increasing threats to all four states, primarily from October to mid-December, although threats should be minimal for Kosrae. Summaries for the individual states of the FSM appear below:

Chuuk State: January through October rainfall for Weno Island has been 19% above the long-term average for the 10-month period. This is consistent with rainfall for the last four months of July-October, which was 20% above average. Since no La Niña or El Niño event is expected to develop, above-average rainfall is expected to continue for the islands of Chuuk State at about 15% to 20% above average for 1997.

During 1996, La Niña conditions kept tropical cyclone activity well north and west of the region until late in the season. In November, Typhoon Dale developed between Weno Island and Naminuito Atoll, causing wind and wave damage at those locations. More activity is expected in the region for 1997, as tropical cyclone conditions return to near-normal for Chuuk State. This means that Chuuk State islands north of 8°N may experience tropical cyclone threats from mid-September to late November, and islands south of 8°N from mid-October to mid-December. Threats from tropical cyclone activity may also occur from March to May of 1997.

Kosrae State: While Kosrae appeared to be the wettest location in Micronesia during the first half of 1996, averaging about 25 inches per month, conditions since August have been drier-than-normal. Since an El Niño warm event (which usually brings wetter-than-normal conditions to Kosrae) is not expected, rainfall should be about 10% below the annual average of about 170 inches. Although less rainfall is expected, month-to-month variability can be high in Kosrae.

Tropical cyclone distribution is expected to return to normal. Since no El Niño warm event is expected for 1996-1997, tropical cyclone threats to Kosrae will be few and will be limited to the period from late October to mid-December. Tropical cyclones can generally be expected to pass north of Kosrae, with greatest local effects coming from winds on the weak side of storm systems as they pass.

Pohnpei State: Rainfall at Kolonia, Pohnpei for the first six months of 1996 was well above average, but since August, conditions have been drier, with rainfall about 26% below average. As a result, overall rainfall for the 10-month period of January-October 1996 has been near-normal. Since no ENSO warm event (which would bring wet conditions to Pohnpei) is expected during 1997, drier-than-average rainfall conditions are more likely for the coming year, at overall levels perhaps 10% below normal. In particular, winter and spring rainfall in 1997 is expected to be significantly less than the wet conditions which were experienced during those seasons in 1996.

Tropical cyclone threats to Pohnpei are greatest from late October to mid-January during El Niño events. Since no El Niño warm event is expected for 1996-1997, tropical cyclone threats to Pohnpei State will generally be limited to the period from late October to mid-December. These tropical cyclones can most often be expected to pass north of Pohnpei, with greatest local effects coming from winds on the weak side of storm systems as they pass.

Yap State: Rainfall for the 10-month period of January-October 1996 in the northern islands of Yap State (north of 8°N) was about 20% above normal. This surplus was primarily due to extremely wet conditions during spring and early summer. While overall rainfall remains above-normal for the year, conditions since August have been significantly drier. August-October rainfall at Yap Airport was 24% below the average for the 3-month period. This was primarily due to the lack of tropical cyclone activity, which was north and west of the normal late-summer and early-fall locations. Southern islands of Yap State (south of 8°N) have had similar above-normal rainfall conditions - about 15-20% above average - during the ten months of January to October, 1996. Since no ENSO event is expected to occur during 1997, the southern islands should continue to have rainfall amounts about 15% above average. The northern islands should have near-normal to slightly wetter-than-normal conditions in 1997. This outlook is supported by the PEAC and CPC model forecasts for Yap (see page 6). If the southwest monsoon activity in the region returns to normal levels (it has been low over the past two years), then the northern islands of Yap State may get greater rainfall amounts - perhaps 10 to 15% above the long-term average.

While tropical cyclone activity in the region was below normal in 1996, Typhoon Dale in November had a very large circulation that generated huge ocean swells. Despite Dale's passage north of Yap and Ulithi, these massive swells affected many of the Yap State islands, causing considerable erosion and flooding of taro patches as far south as Woleai Atoll. Tropical cyclone activity in 1997 is expected to increase to more normal levels. Typhoons should develop farther east, and may be stronger as they traverse the region. Threats to islands in northern Yap State will be greatest from mid-September to late November, and for the southern islands from mid-October to mid-December.
- source: UOG-WERI


MARSHALL ISLANDS (RMI): While both the northern and southern RMI were wetter-than-average during early 1996, conditions in the northern RMI have been relatively dry since March, and in the southern RMI since July. Overall rainfall at Kwajalien for January-October 1996 is about 10% below the 10-month average, but rainfall over the months since March was almost 30% below average. Wet conditions in the southern areas during the first half of 1996 were apparent with Majuro's January to June rainfall, which was 21% above average. In the months since July, however, rainfall at Majuro has generally been below normal, with the overall amount about 20% below the 4-month average. These dry conditions are likely to continue into 1997, since no ENSO warm event (which would bring more rain to the region) is expected to develop. As a result, northern islands of RMI are expected to receive rainfall about 20-25% below long-term averages, and southern islands can expect rainfall about 10-15% below long-term averages.

Tropical cyclone threats increase dramatically for the RMI during ENSO warm events. The warmer SSTs extend the zone of formation for tropical cyclones to the east, allowing more cyclones to develop and intensify before tracking through the RMI. Since no ENSO warm event is expected for 1997, tropical cyclone risk should be minimal, at least until September of 1997. Enewetak, Bikini, and Wake Island, however, can be affected by strong typhoons during the fall of any year. Weaker tropical storm activity may affect the northern RMI from early October to mid-November, and the southern RMI from late October to early December.
- source: UOG-WERI


PALAU: Overall rainfall at Koror during 1996 has been very close to normal, with rainfall for January-September rainfall just 3% above the long-term average for those months. As with other parts of Micronesia, recent months have been somewhat drier, with July-September rainfall about 12% below the long-term average for the period. Deflection of the monsoon trough to the north and west reduced August rainfall to less than 7 inches. This appears to be the result of stronger-than-normal easterly winds in the western north Pacific, apparently related to the shift in SOI conditions discussed on page 1. Since no ENSO warm event or La Niña cold event is expected in 1997, rainfall at Koror should be near-normal or slightly wetter-than-normal, with the monsoon trough expected to return to a more normal position in 1997. Conditions at Peliliu will be drier than those at Koror. High month-to-month rainfall variability may also be expected in the region.

Topical cyclone activity in 1997 should return to more normal levels, with the greatest tropical cyclone risks for Palau from mid-October to mid-December. Since no ENSO event is expected, the likelihood for late December or early January tropical cyclones in the area will be reduced.
- source: UOG-WERI



ACKNOWLEDGEMENTS and FURTHER INFORMATION:

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 Technology (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 C. Yu at 808-956-7110 for more information on ENSO-related climate data for the Pacific Islands. Contact A. Hilton at 808-956-2324 for more information on Pacific ENSO Update and applications.

APPENDICES:

Long-Lead Outlook for the Hawaiian Islands issue dated November 14, 1996 from NOAA's Climate Prediction Center (CPC).

SPECIAL SECTION: Experimental Forecasts for Pacific Island Rainfall


For further information, please contact:

Alan C. Hilton, LT/NOAA
Editor, Pacific ENSO Update,
Pacific ENSO Applications Center
c/o Dept. of Meteorology, HIG Room 331
University of Hawaii - Manoa Campus
2525 Correa Road - Honolulu, HI 96822
Tel: 808-956-2324 Fax: 808-956-2877
E-mail: hilton@soest.hawaii.edu

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.