The cold event or "La Niña" conditions reported in the last issue of Pacific ENSO Update have weakened over the last three months. Equatorial sea surface temperatures (SSTs) across most of the Pacific have warmed, and are now only slightly below their normal values (except near South America, where SSTs near the west coast of the continent have been observed as much as 3°C below normal). In the atmosphere, surface pressure patterns (indicated by the Southern Oscillation Index or SOI, which measures the difference in atmospheric pressure between Tahiti and Darwin, Australia) are close to normal, with five-month values for the SOI remaining near the long-term average of zero for the past few months. More details in these trends are given in the next section.
The ENSO Advisory, published monthly by NOAA's Climate Analysis Center when ENSO conditions are significant, was last issued in April, and no new issues are planned until conditions begin to change toward those of a developing "El Niño" warm event, or a recurrence of cold "La Niña" conditions. Conditions in coming months are expected to be near-normal.
Much of the region has received higher-than normal rainfall in recent months (except American Samoa, which generally receives less rainfall in non-El Niño years). Further discussion on trends and the outlooks for coming months are in the LOCAL VARIABILITY SUMMARIES section. Because of high variability in (northern) springtime climate conditions at this time of year, latest results from computer models for ENSO forecasts can be mixed and in some cases less skillful in their predictions. These forecasts, along with recent results from experimental predictions for Pacific Island rainfall, are further described in this issue.
The ENSO cycle is defined by trends in oceanic and atmospheric conditions, which in turn affect climate by changing normal weather patterns. The figures below show the trends in recent months and years for two important measures of ENSO activity - the Southern Oscillation Index or SOI (defined in the preceding section and shown in Figure 1 below), and the anomalies, or differences from long-term averages, of sea surface temperatures (SSTs) in the equatorial Pacific. Positive values of SOI, or a rising trend over time, are linked to cooler-than-normal SSTs and together define "La Niña" or cold-event conditions. The opposite trends apply to "El Niño" or ENSO warm-event conditions. SSTs have warmed over recent months, and the rise in SOI is leveling off near its long-term average value of zero. Easterlies and trade winds were stronger-than-normal when SST's were cooler and the SOI was rising, but are also now near-normal (see Figure 2 below). The rise from negative SOI values of the 1994-95 ENSO warm event is typical of La Niña trend, though not as extreme as past La Niña events (like 1988-89) which usually result in higher positive values of SOI. This less-than-normal change in SOI reduced the impact of extended drought conditions that can affect Micronesia when La Niña closely follows an ENSO warm event. Outlook for the tropical cyclone activity in 1996 and early 1997 is for near-normal conditions, as was seen with the mid-May development of Typhoon Bart in the western North Pacific and a "twin" depression in the Southern Hemisphere, in association with near-equatorial westerly winds pushing out to 160°E.
FIGURE 1: Southern Oscillation Index (SOI). Monthly values of SOI appear as grey bars, while the longer-term 5-month average is shown with the heavy dark line. El Niño warm events and their relative strength are evident from the negative "peaks" in 1982-83, 1986-87, 1991-92 and 1994-95, with unusual warm conditions during 1992-94. A strong La Niña cold event is evident in 1988-89. The recent rise in SOI during 1995-96 is consistent with La Niña conditions, though not as strong. (source: NOAA-CPC)
FIGURE 2: The frames below show regions of sub-normal sea surface temperatures (SSTs) and their change over recent months in the equatorial Pacific (see map area above). Also shown are differences in wind speed and direction (small arrows) compared to long-term averages for the region. SST and wind patterns affect each other, and are related to the SOI trend shown in figure 1 above. Rising SOI and stronger-than-normal easterly winds accompany colder-than-normal SSTs. Less cloudiness (measured from satellites in terms of outgoing longwave radiation or "OLR") and less rainfall occurs over the cold and windy areas, while more cloudiness and rainfall occurs where winds converge or pass over regions of warmer SST. The conditions shown, and their change over the last four months, are typical of a weakening La Niña event. Compared to February 1996, conditions in May 1996 are close-to-normal for most of the region. This data is collected by a network of buoys called the "TAO Array" that transmit their data via satellites to NOAA's Pacific Marine Environmental Lab (PMEL) in Seattle, where the data is analyzed and used to produce images like these and as important inputs to climate models. TAO buoys also collect sub-surface data on temperatures and ocean currents, which are also important in the analysis and prediction of ENSO.
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 March 1996 ELFB appears below:
From EXPERIMENTAL LONG-LEAD FORECAST BULLETIN (ELFB); March 1996:
For ENSO Condition: DYNAMICAL METHODS: The standard and the new Lamont Doherty Earth Observatory models predict continuing cold conditions through winter 1996-97. The NCEP coupled model calls for some weakening of the current cold SSTA east-central Pacific between now and boreal fall 1996. The Australian BMRC low order coupled model forecasts current cool Niño 3 to switch to warm for the second half of '96, further strengthening by late winter and spring '97. The Oxford coupled model calls for cool SST to continue for remainder of '96. The COLA coupled model predicts dissipation of cool SST conditions by summer 1996, warming to >1°C by winter '96-'97.
STATISTICAL METHODS: CDC's inverse modeling predicts dissipation of below normal east-central Pacific SST by summer '96, with possible positive anomalies emerging in the southeastern tropical Pacific. The Australian BMRC non-linear analogue predicts somewhat decreasing SOI by May 1996. UCLA's SSA-MEM predicts near to slightly below normal SST through '96, with near normal SOI. JPL/Utah State's CSSA/MARS "analog" system predicts a La Niña period peaking in early 1997. Univ. of British Columbia's neural net model predicts a warm episode for Niño 3 for fall-winter '96-'97. NCEP/CPC's CCA predicts current below normal Niño 3.4 (120°W-170°W) SST dissipating by June and becoming moderately warm by Jan '97. NOAA's constructed analog predicts dissipation of cool Niño 3.4 SST by late spring 1996, warming to ~0.5°C for fall and winter 1996-97. NCEP's 4-model consolidated forecast projects cool SST through 1996, switching to warm after March 1997.
Results from climate forecasting models are often mixed at this time of year, due to high variability in boreal (northern) springtime conditions. The dynamical models noted above are split (3 to 3) in their predictions of long-term trends toward warm (El Niño) or cold (La Niña) conditions. The statistical models are similarly split for predictions of trends toward warm, near-normal, or cold conditions over the coming months. Most of the predictions are for changes in ENSO condition that are fairly small compared to normal variability. Summer and fall conditions usually allow for more skillful long-term predictions from the models.
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: Rainfall amounts recorded at the primary National Weather Service stations in Hawaii continued an upward trend through the end of the wet (Hoo'ilo) season, bringing late-season totals in at above-average amounts, except at Honolulu. For the three-month period of January-February-March, rainfall totals measured at Lihue, Kahului, and Hilo were 102%, 138%, and 126% of average, respectively, versus 89% at Honolulu. April totals however were more reflective of the approaching dry (Kau) season, with less than half of average April rainfall amounts recorded at those locations. 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 at Pago Pago for January through March was about 95% of the average for those months. April rainfall dropped off to 62% of the average. Lingering La Niña-like conditions, with slightly cooler-than-normal eastern Pacific SSTs and positive SOI, suggest that an ENSO warm event is unlikely for the 1996-1997 season. Since warm events produce more rainfall (20-25% higher) in the Samoa region, annual rainfall for 1996 is expected to be about 10-15% below the long-term average. In response to short-term (monthly) fluctuations in the SOI, there may be high month-to-month variability in rainfall amounts.
Without the occurrence of an ENSO warm event, tropical cyclone activity in the southwest Pacific (beginning in November 96) will likely be limited to areas west of the date line, occuring primarily in the belt from the Gulf of Carpentaria (NE Australia) to the Solomon Islands and Vanautu. Cyclone activity is expected to increase over the low 1995-1996 values, with the return to near-normal SOI and SST conditions following the recent La Niña event.
- source: UOG-WERI
GUAM/CNMI: As indicated in the last issue of Pacific ENSO Update, dry season (Dec-May) rainfall for Guam and Rota has been greater than normal. January through April rainfall values at the Guam International Airport and at Andersen Air Force Base averaged about about 35% above the long-term norm. Wetter-than-average conditions are expected to continue through the rainy season and into next year's dry season. On Tinian and Saipan, dry season conditions have been drier than those on Guam and Rota, but rainfall is expected to increase in the Northern Marianas to wetter-than-average values.
When strong ENSO warm events are followed by La Niña cold event conditions, extended droughts can occur in the Mariana Islands region. Because of this pattern, more "normal" years are generally wetter than the long-term average. The recent La Niña conditions, however, have not been intense enough to influence the Mariana Islands with drought-like conditions, and also suggest that an ENSO warm event is not likely for the 1996-1997 season. Therefore, above-normal rainfall conditions are expected for the coming months. The following outlooks for monthly rainfall are provided for the use of water resource manages on Guam and Saipan:
MONTH(S): RAINFALL: May 96 - Nov 96: 110% of long-term average Dec 96 - May 97: 120% of long-term average
Tropical cyclone activity during 1996 is expected to return to normal. This means that development and intensification will occur farther east than in 1995. This also means an increased threat of tropical cyclone conditions for the Mariana Islands. While trade winds across the region have been stronger than normal, equatorial westerlies have been more prevalent than in 1995, allowing tropical cyclone development to occur in the central FSM. Typhoon Bart developed there in May, and additional tropical cyclone development is possible in that area for the remainder of May and early June. These cyclones often threaten the Marianas. By mid-June, development will move to the west, and by mid-July and August, to the northwest. By mid-September, activity will spread east of the Marianas, with the greatest threats to the area occurring in October, November, and early December.
- source: UOG-WERI
FEDERATED STATES OF MICRONESIA (FSM): Since no ENSO warm event is expected to develop during the northern winter of 1996-1997, rainfall over the eastern FSM area should be near the long-term average and rainfall over the western FSM should be above the long-term average. Due to the influence of the 1995-96 La Niña cold event, tropical cyclone development for that season was pushed far to the west, with most significant storm activity occurring near the Philippines. This year, tropical cyclone activity for the FSM is expected to return to normal, with greatest threats to western FSM States in May to mid-June, and to all four FSM States in October to mid-December, although least so for Kosrae. Summaries for the individual FSM States appear below:
Chuuk State: Rainfall for Weno Island from January through April was about 35% above the long-term average for those months. This was due, in part, to the weakness of Australian monsoon during the northern winter,and the lack of normally strong and relatively dry low-level northeasterly winds that usually affect Chuuk giving way to weaker easterly winds with deeper, rain-producing clouds. When strong ENSO warm events are followed by La Niña cold event conditions, extended droughts can occur in Chuuk, and because of this pattern, more "normal" years are generally wetter than the long-term average. The recent La Niña conditions, however, have not been intense enough to affect Chuuk with drought-like conditions, and also suggest that an ENSO warm event is not likely for the 1996-1997 season. Therefore, above-normal rainfall--perhaps 20% above average--is expected for Chuuk over the next 12 months.
La Niña conditions influenced tropical cyclone activity during 1995, and caused storms to develop and spread further to the west than normal. This spared Chuuk from significant tropical cyclone threats during that year. However, tropical cyclone activity for the remainder of 1996 is expected to return to normal. This means that Chuuk will experience more normal levels of threats from tropical cyclones until mid-June, and again from October to late November (for the northern islands of the State) and mid October to mid-December (for the southern islands). In 1997, threats from tropical cyclone activity for Chuuk can be expected to reappear in March.
Kosrae State: January through April rainfall at the airport was 63% greater than average. The weak Australian monsoon during the 1995-1996 northern winter allowed a stronger-than-normal trade wind convergence zone to develop over the Marshall Islands. This caused rain-producing weather patterns to move westward over Kosrae. Conditions were drier during March, but were extremely wet during April, with over 33 inches for that month. Rainfall for Kosrae is expected to be above normal until October 1996, then drier until February 1997, and again wetter through to May 1997.
Tropical cyclone threats to Kosrae are greatest from late October to mid-January during years with ENSO warm conditions. Since no ENSO warm event expected to develop for 1996/97, tropical cyclone threats to Kosrae should be reduced and limited to the period from late October to mid-December.
Pohnpei State: January through April rainfall at Kolonia was wetter-than-normal, though only 17% above the average for those months. This low value of excess rainfall (compared to nearby Kosrae) was primarily due to dry conditions during February and March. As with Kosrae, rainfall for Pohnpei is expected to be above normal until October 1996, then drier until February 1997, and again wetter through to May 1997.
Tropical cyclone threats to Pohnpei are greatest from late October to mid-January during ENSO warm events. Since no ENSO event is expected for 1996/97, tropical cyclone threats to Pohnpei should be reduced and generally be limited to the period from late October to mid-December.
Yap State: January through April rainfall at the Yap airport was 74% greater than the long-term average for those months. This was primarily due to the passage of several tropical disturbances that developed in the Marshall Islands during January and February. Rainfall for March and April was nearer to normal. Over the southern islands of Yap State, rainfall was about 35% above the average for the same 4-month period. Since no ENSO warm event, which would cause below-average rainfall in Yap, is expected to occur in 1996/97, annual rainfall should be about 20% above the average values. Much of this additional rainfall will be the result of a more active monsoon season across the region.
Of all FSM states, Yap is the most vulnerable to threats from tropical cyclones. Due in part to the La Niña conditions of late 1995, most tropical cyclone activity last season was safely west of Yap. With more normal conditions expected for 1996, tropical cyclone activity is expected to shift back to the east. The northern islands of Yap State will be vulnerable to tropical cyclones from May to mid-June and from late September to late November, then again in 1997 from March to mid-June. Those occurring in the fall of 1996 could be very intense. Threats for the southern islands will primarily be from mid-October to mid-December, then again in 1997 from March to June.
- source: UOG-WERI
MARSHALL ISLANDS (RMI): At the present time, no ENSO warm event is expected for the winter of 1996-97. The La Niña cold event of 1995-96 has gradually weaked, and conditions are expected to return to near-normal. This has several implications for the Marshall Islands in terms of tropical cyclone activity and rainfall.
With the 1995-96 La Nina cold event, tropical cyclone development for last season was pushed far to the west. Now that sea surface temperatures and atmospheric pressure patterns in the Pacific are returning to normal, conditions should return to near-normal in the Marshall Islands. This means that the monsoon trough should build eastward into the northern Marshalls by October, creating tropical depressions and perhaps one or two tropical storms in the area. In November and early December, the trough should become anchored over the southern Marshall Islands, creating some tropical depressions and perhaps one or two tropical storms, but probably no typhoons, without an ENSO warm event.
January through April rainfall values at Majuro were 82% above the long-term average, and those at Kwajalein were 73% above average. This was primarily the result of a very weak Australian monsoon, and a much stronger-than-normal trade wind convergence zone over the Marshall Islands, which caused the development of several wintertime tropical disturbances in the area. In March and April, the northern Marshall Islands were drier, with rainfall at 65% of the long-term average for those months. For the remainder of the year, rainfall is expected to be 90% to 95% of the long-term average.
- source: UOG-WERI
PALAU: January through April rainfall at Koror was about 24% greater than the long-term average. However, rainfall was highly variable from month to month, with March being dry and April being very wet. The continuation of the weak La Niña-like conditions, with cooler-than-normal SSTs in the central and eastern Pacific, and a near-normal value for SOI, suggest that no ENSO warm event will occur during the 1996-1997 winter season. Thus, without an El Niño or significant La Niña conditions, an active monsoon season should return, bringing rainfall in above-average amounts from Kayangel Island in the north to Tobi Island in the south (see map below).
Tropical cyclone activity should return to normal for the coming season. For Palau, this means that the greatest threats will be from May to mid-June and from late October to mid-December 1996. While tropical cyclones are infrequent for Palau, they can be devastating. During July, August, and September, Palau will experience episodes of strong southwesterly monsoon winds caused by development and movement of tropical cyclones several hundred miles to the northeast.
- source: UOG-WERI
The Republic of Palau is a
newly independent nation,
and as a former U.S. Trust
Territory is now under a
compact of free association
with the United States. It is
of the U.S.-Affiliated Pacific
Islands (USAPI), and is located
between Guam, Irian Jaya and
the Phillipines, between the
latitudes of 3°N - 9°N and the
longitudes of 131°E - 135°E.
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.