The cold event or "La Niña" conditions reported in the last issue of Pacific ENSO Update have continued to develop and influence weather patterns in the tropical Pacific during recent months. Sea surface temperatures (SSTs) along the equator east of 160°E longitude (between Pohnpei and the Solomon Islands) to the South American Coast have remained below normal. In the equatorial region between Hawaii and Tahiti (160°W to 140°W longitude), SSTs more than 1°C below normal were reported for January. In the Western Pacific (regions west of 160°E longitude), SSTs have remained near or slightly above their normal values. This SST pattern is typical of La Niña conditions, and essentially the opposite of that seen during ENSO warm events.
In the atmosphere, La Niña conditions are also apparent, with reduced cloudiness over the colder-than-normal equatorial regions (and increased cloudiness in the west), stronger-than-normal easterly winds, and a rising Southern Oscillation Index (SOI), or difference in surface atmospheric pressure between Tahiti and Darwin, Australia. In January, the SOI increased to a value over 1.0 for the first time since May, 1990.
The effect on weather patterns throughout the region has been toward higher-than-normal amounts of rainfall in most areas, except the Marshall Islands, American Samoa, and parts of Hawaii, which lie closer to the cool ocean regions. These trends are discussed further in the LOCAL VARIABILITY SUMMARIES section, and are expected to continue in coming months, until the current La Niña conditions begin shifting back toward near-normal in spring or summer. Most computer models are predicting this trend, and these are discussed futher in this issue of Pacific ENSO Update, along with more information about experimental forecasts for Pacific Island rainfall from statistical computer models.
Weather patterns in the tropical western North Pacific during the first half of 1995 were characteristic of the transition from an El Niño "warm event" to a La Niña "cold event", with a cooling of equatorial sea surface temperatures (SSTs) from 160°E to the South American coast, a rising of the Southern Oscillation Index (SOI), a delay in summertime tropical cyclone activity, and generally very dry conditions over the western North Pacific, well into summer. Weather patterns in late 1995 were largely characteristic of a mature La Niña, with colder equatorial SSTs and abnormally strong easterly trade winds dominating the region from June through December.
Strong easterly winds delayed the normal eastward advance of westerly monsoonal winds, and during the early summer, the monsoon was forced into a south-southwest to north-northeast orientation, from the South China Sea to southern Japan. This caused drier-than-normal conditions across Micronesia and into the Philippines. By September, the trade winds had finally weakened, but only two brief episodes of westerly monsoonal winds advanced as far east as 150°E (near Chuuk). In October, the trade winds again strengthened, preventing further advance of near-equatorial westerly winds that normally extend to the Marshall Islands in October and November, as the Southern Hemisphere (SH) monsoon develops and strengthens over Australia. Westerly winds in the Northern Hemisphere (NH) were constrained to the extreme western part of the Philippine Sea.
Equatorial sea surface temperatures (SSTs) from 150°W to the coast of South America experienced a significant cooling trend during spring and early summer, typical of a transition from El Niño to La Niña conditions. However, equatorial SSTs from 160°E-150°W cooled to levels only slightly below normal, indicating a La Niña event of only moderate strength.
In the atmosphere, La Niña conditions became apparent in 1995 with reduced cloudiness over the colder-than-normal equatorial regions (and increased cloudiness in the west), stronger-than-normal easterly winds, and a rising Southern Oscillation Index (SOI), or difference in surface atmospheric pressure between Tahiti and Darwin, Australia, and an important indicator of ENSO activity.
Consistent with the trend toward La Niña conditions, the SOI increased in value during most of 1995. A positive value of the SOI is generally associated with La Niña conditions. Though SOI values followed a rising trend throughout most of the year, they did not rise much above the long term average value of zero. However, a rapidly rising SOI (e.g. a change greater than 1.0 over about 6 months) has been shown to lead to significantly drier-than-normal conditions in Micronesia, whether or not the SOI actually becomes positive. Such a rapid change would be consistent with a strong La Niña event. Since this did not occur in 1995, this again indicates La Niña conditions of only moderate strength. Despite lower-than-normal monsoon and typhoon activity, rainfall in the latter half of 1995 was above-average in most Micronesian locations west of the Marshall Islands. Heavier-than-normal rainfall occurred primarily from tropical disturbances and developing tropical storms in these areas. Localized thunderstorms also made a major contribution to the total rainfall of the mountainous islands.
In eastern regions, including parts of the Marshall Islands, Hawaii, and American Samoa, drier-than-average conditions persisted through the end of the year. These island areas lie closest to the central tropical Pacific, which has been most greatly influenced by the development of La Niña conditions, including colder-than-normal SSTs pushing westward along the equator from South America, reduced cloudiness and rainfall, and stronger-than-normal easterly winds throughout the region.
Hawaii's 1995 hurricane season was the quietest since 1979. During the season, only one decaying tropical storm (remnant of Tropical Storm Barbara,) entered the region west of 140°W longitude. In the Southern Hemisphere, only five named storms occurred in the Pacific during the 1994/95 cyclone season, well below the yearly average of about ten.
In Hawaii, a rainfall gauge maintained since 1958 on the grounds of the UH-Manoa campus recorded its second-lowest annual rainfall in 1995: 20.98 inches, compared to lowest rainfall of 19.77 inches in 1983. Both years are similar in that they followed ENSO warm events.
The latest issue of the Experimental Long-lead Forecast Bulletin from NOAA gives the results of several experimental models for forecasting ENSO conditions. A summary from the December 1995 ELFB appears below:
From EXPERIMENTAL LONG-LEAD FORECAST BULLETIN (ELFB); December 1995:
For ENSO Condition:
DYNAMICAL METHODS: The Scripps/MPI HCM predicts dissipation of the current negative SSTA in eastern/central tropical Pacific by summer 1996. The NCEP coupled model calls for -1°C SSTA east-central Pacific this winter, weakening by summer 1996. The standard Cane-Zebiak (C-Z) model predicts current below normal Nino 3 SST dissipating by summer, some warming by fall; the new C-Z model predicts cold conditions through fall. The Australian BMRC low order coupled model forecasts current cool Nino 3 to normalize by late northern spring '96, increasing to 1.5°C by winter 1995-96. The Oxford coupled model calls for cool SST to continue or further amplify into summer 96. The COLA coupled model predicts dissipation of cool SST conditions by summer 1996, warming to >1°C SSTA central and eastern basin by winter '96-97.
STATISTICAL METHODS: The Penland inverse modeling predicts dissipation of current below normal east-central Pacific SST by summer '96. The Australian BMRC non-linear analogue predicts somewhat increasing SOI by February 1996. The UCLA/JPL SSA-MEM predicts near to slightly below normal SST through fall '96. The JPL CSSA/MARS "analog" system predicts a general La Nina period from late 1996 to 1997 or 1998. Zhang and Pan's SSA-like EOF method forecasts dissipation of current cold conditions, with mild warm conditions by summer 1996. The NCEP/CPC's CCA predicts current below normal Nino 3.4 (120°-170°W) SST weakening through summer, rising through normal in fall '96. NOAA's constructed analog predicts dissipation of cool Nino 3.4 SST by northern spring 1996, turning slightly warm summer through winter 1996-97.
In general, the dynamical models are in agreement for predicting near-normal conditions to continue through the middle of 1996. Two out of six models are predicting some warming by the end of the year, but at a magnitude less than earlier predicitons. The remaining dynamical models predict either near-normal conditions or slightly cool conditions over this period.
Most of the statistical models are predicting a trend toward normal conditions through the first half of 1996. Two out of seven models are predicting a slight warming trend for mid-to-late 1996. One model predicts continued cold conditions.
Given this range of results in the models, and the limits of their skill at longer lead times, the following LOCAL VARIABILITY SUMMARIES are based on an expectation that conditions will be near-normal through 1996. Any changes in trends, and their impact on the outlooks in these LOCAL VARIABILITY SUMMARIES, will be reported in future issues of Pacific ENSO Update.
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: With respect to monthly norms, rainfall amounts recorded at primary National Weather Service stations in Hawaii have followed an upward trend throughout the current wet (Hoo'ilo) season, yet to date still remain below seasonal norms, except at Lihue. January rainfalls measured at Honolulu, Kahului, and Hilo were 99%, 60%, and 145% of their monthly norms, respectively. Yet for the three-month period of November-December-January, total accumulations for those sites were only 55%, 53% and 79% of their respective averages. Lihue by contrast received 172% of normal for the same period, with nearly all the surplus delivered in the heavy Kona thunderstorm event of November 2-3. Without significant contributions from Kona storm activity in February or March, rainfall totals are likely to remain below seasonal norms in leeward island areas as the dry (Kau) season approaches. However, springtime tradewind showers generally deliver adequate rain to windward mountains, which comprise (for example, on Oahu) the major water source. The Long-Lead Outlook for the Hawaiian Islands, issued by NOAA's Climate Prediction Center, has more detailed forecasts. The most recent issue is shown on page 7.
- source: NWS-PR and PEAC
AMERICAN SAMOA: Rainfall declined significantly at Pago Pago, following a period of near-normal rainfall for the dry season months of July-September. From October through January, rainfall was only 41% of average values. December was extremely dry, with only 2.25 inches, or 16% of average for that month. Disregarding December, rainfall for the other months combined was about 88% of average. Because of La Niña conditions, a 10-15% reduction in rainfall during the rainy-season of 1995-1996 is expected, since rainy-season rainfall in the Samoa region is about 20-25% heavier during ENSO "warm event" years. Some increase in rainfall for Samoa is expected in the short-term--until about April.
Global and regional climate models are predicting slightly cooler than average equatorial SSTs for the next 9 months. This suggests that no ENSO warm event should occur in 1996. Therefore, drier-than-average conditions are expected to continue through early 1997 in the Samoa region.
With cold equatorial SSTs and with the SOI continuing to hover around zero, the development of Southwest Pacific tropical cyclones is expected to occur in more normal longitudes, toward Vanuatu and the Solomon Islands. Development of tropical cyclones should be west of the date line, and subsequent movement should be toward Australia's east coast, New Caledonia, Vanuatu, and occasionally, Fiji. Tropical cyclone activity east of the date line will be below average. To date, tropical cyclone activity in the entire Southwest Pacific region has been unusually low.
- source: UOG-WERI
GUAM/CNMI: During 1995, strong trade winds typical of the shift toward La Niña conditions extended below-normal rainfall from spring into mid-August. While trade winds remained dominant through to December, they temporarily gave way to two brief monsoon episodes and some developing tropical storms. These rain-producing systems brought rainfall in the Marianas back to near-normal levels in September and early October. Northeasterly trade winds again strengthened over the western North Pacific by mid-October, preventing the eastward advance of near-equatorial westerly (monsoon) winds that normally extend out to the Marshall Islands in October and November. Despite the lack of rain-producing monsoon westerlies, a few tropical disturbances and frequent episodes of local rainshowers and thunderstorms drenched Guam and Rota. Saipan and Tinian, a mere 100 miles to the north, were less influenced by these rainfall events.
Although the dry season is normally expected for the region from December through May, conditions in December were influenced by Tropical Storm Dan and periodic tropical disturbances near the Philippines, which brought heavier-than-normal rainfall to the Mariana Islands. Capital Hill on Saipan received over 14 inches of rain in December. Current wet conditions over the Mariana Islands could last into mid-March.
Most global and regional climate models are indicating a continuation of cooler-than-normal equatorial SSTs. Therefore, no El Niño warm event is expected for 1996, and rainfall over the Mariana Islands is expected to be slightly above average for the remainder of the year. The following expectations of monthly rainfall totals are provided for the use of water resource managers on Guam:
MONTH(S): RAINFALL: Feb 96 - Mar 96: 125% of long-term average Mar 96 - Mar 97: 110% of long-term average
The total number of western North Pacific tropical storm systems was near-normal for 1995, however many of these were weak, and intensification occurred much farther west than normal. The 1996 typhoon season is expected to be near-normal, with respect to time and location of tropical storm and typhoon events. This means that more tropical cyclones are expected to develop farther to the east during September, October, and November, allowing them to become more intense before possibly affecting the Mariana Island chain. There is also a greater chance for the southern four islands to be affected by some late-spring (April-May) tropical cyclone activity. In 1996, the greatest typhoon threats to Guam and Saipan-Rota-Tinian will be from mid-September through mid-December.
- source: UOG-WERI
FEDERATED STATES OF MICRONESIA (FSM): Since El Niño conditions are not expected to develop during 1996, rainfall over the entire island nation should be somewhat above the long-term average. Summaries for the individual states of the FSM appear below:
Chuuk State: While October and November rainfall over Weno Island, Lukunor, and Puluwat were about average, December rainfall was about 37% above normal. This resulted from the passage of tropical disturbances that developed in the eastern FSM. Rainfall for all Chuuk State islands is expected to continue to be about 20-25% above average until about March 1996. For the remainder of 1996 and early 1997, rainfall should continue to be about 10-15% above normal.
Tropical cyclone activity should return to normal during 1996. This means that threats will be greatest from October to mid-December, and that compared to 1995, there is a greater chance that tropical storm systems will be of typhoon strength. A developing tropical depression, tropical storm, or minimal typhoon could also affect the state during April or May as well. These systems often move erratically near Chuuk.
Kosrae State: Rainfall for Kosrae was about 20% less than average from June through October, about average in November, and more than 35% above average in December and January. These recent wet conditions are due to convergence of trade winds and stronger-than-normal easterly winds caused by La Niña conditions. This convergence led to the development of several tropical disturbances that affected Kosrae. These wet conditions are expected to continue until about mid-March 1996. From mid-March to September 1997, rainfall should be near the long-term average. Rainfall from October 1996 through January 1997 should drop to slightly below average. This is because rainfall is higher than average during the fall and early winter period during ENSO years, but ENSO conditions are not expected to develop in 1996.
Tropical cyclone activity is expected to return to normal for 1996. Threats will be greatest from October to mid-December 1996, but the chance of Kosrae being hit by a severe tropical storm or typhoon is much less than during ENSO years. There is some chance of tropical storm activity in the region during March or April, in association with the development of tropical cyclones in the Southern Hemisphere.
Pohnpei State: Strong easterly winds, possibly related to La Niña conditions, were reported to have caused higher-than-normal tides and some flooding on the north and east sides of Pohnpei. Rainfall totals for the 3-month period of October, November, and December (48.9 in) were near the long-term average (47.7 in). Rainfall is expected to be slightly above normal until late February or mid-March, and near-normal from mid-March through February 1997. This is based on the expectation that no significant El Niño event will develop in 1996, which would cause drier-than-normal conditions.
Tropical cyclone activity is expected to return to normal for 1996. As with Kosrae State, threats of tropical storm and cyclone activity will be greatest from October to mid-December 1996, but the chance of Pohnpei being hit by a severe tropical storm or typhoon is much less than during El Niño years. There is some chance of tropical storm activity in the region during March or April, in association with the development of tropical cyclones in the Southern Hemisphere.
Yap State: After a summer in which rainfall for the Yap State islands was about 50% below average, two brief monsoon episodes and several nearby tropical cyclones (Polly and Sibyl in September; Zack and Angela in October) brought much-needed rain to the islands. Fortunately, there were no direct typhoon hits in the region. The 3-month total rainfall for October, November, and December for Yap Island and Ulithi was more than 40% above average. Wet conditions of about 30% above average are expected to continue until about mid-March, and about 15-20% above average for the remainder of 1996. The wetter than average conditions are expected during the non-ENSO 1996, since ENSO and La Niña years are drier than average for Yap.
Tropical cyclone activity for Yap State is expected to return to near normal in 1996. This means that Yap, Ulithi, Ngulu, and Fais can expect their greatest threats of storm activity from September through early December, with lesser possible threats in May and June. Yap State islands farther to the south can expect their greatest threats from October through December, with lesser threats during April and May.
- source: UOG-WERI
MARSHALL ISLANDS (RMI): Since the last issue of Pacific ENSO Update, conditions have been wetter-than-normal over the southern RMI and drier-than-normal over the northern RMI. At Majuro, rainfall totals for October and November were about 25% and 13% below normal (respectively), due to the absence of monsoon conditions and near-equatorial westerly winds. However, rainfall for December and January was nearly 70% above normal, primarily due to a convergence near 7-8°N of northeasterly trade winds and stronger-than-normal equatorial easterly winds, related in part to La Niña conditions, and the development of several tropical disturbances in the region.
Above-average rainfall may last until April for the southern RMI. Afterwards, rainfall should be about 10-15% below-average for the southern regions through February 1997. Northern islands can expect rainfall to continue to be below-normal for the remainder of 1996.
Tropical storm and typhoon activity in the RMI should be near-normal for 1996, with only slight chances of activity in the region, especially for eastern areas. Greatest tropical cyclone threats will be from September through October for the Wake-Enewetok-Kwajalien region, and from October through November for the more southern islands.
- source: UOG-WERI
PALAU: Despite the lack of normal monsoon activity in the Philippine Sea, Palau experienced heavy rainfall during the fall and winter. From September to early-November, several tropical storms and typhoons (Polly, Sibyl, Ted, Yvette, Zack and Angela) brought increased cloudiness and rainfall to Palau. Fortunately, there were no direct typhoon strikes on the islands.
Tropical Storm Dan created heavy rainfall in late-December, and episodes of passing tropical disturbances extended wet conditions into early February. The 3-month rainfall at Koror for October-December was a wet 49.35 inches, or 33% above normal. A continuation of wet conditions--about 25-30% above average--is expected to continue into mid-March, and afterward, rainfall should be about 15-20% above average monthly values for the remainder of 1996 and into early 1997. No significant El Niño activity is expected for 1996 that would cause significantly drier-than-normal conditions for Palau.
Tropical storm and typhoon activity is expected to return to normal for 1996. By June, tropical storm activity will pick up in the Philippine Sea, and by mid-July, monsoon conditions should begin to periodically surge across the region, bringing wet summertime weather. Greatest threats of tropical storm and typhoon activity will occur during June and from October through December, 1996.
- source: UOG-WERI
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.
Long-Lead Outlook for the Hawaiian Islands issue dated January 18, 1996 from NOAA's Climate Prediction Center (CPC).
SPECIAL SECTION: Experimental Forecasts for Pacific Island Rainfall
Appendix I: ENSO Advisory of January 16, 1996
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
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.