Changes in oceanic and atmospheric conditions in the equatorial Pacific over the last three months have been consistent with the early development of an ENSO warm event. The Southern Oscillation Index - or SOI (described on page 3) made a significant drop from a high February value, and has continued dropping in negative territory since March. This "springtime drop" pattern is similar to conditions observed for the beginnings of notable ENSO warm events in 1957-58, 1972-73, and 1982-83. Associated with the change in SOI, periods of westerly winds in the western tropical Pacific have accompanied an eastward shift in warm surface waters, into the central and eastern tropical Pacific. Sea level observations (see page 2) and sea surface temperatures (SSTs) reflecting this shift are higher-than-normal along the coast of South America, and near the date line. In response, wet conditions and tropical storm development has already affected the Marshall Islands and eastern Micronesia.
Most long-term ENSO predictions made by climate models at this time are consistent with further development of a warm event over the remainder of the year. Experimental rainfall predictions for the Pacific islands (see page 9) are also generally consistent with this outlook. Although a warm event is expected, the intensity and duration of the event is difficult to forecast. A moderately strong event followed by a rapid reversal of SOI and SST trends (typical of events in the '70s and '80s) would affect the region differently than an extended weak-to-moderate period of warm conditions that does not quickly dissapate (as occurred in the early 1990s). However, near-term variability in island climates is likely to be typical of a developing warm event. Details are given in the LOCAL VARIABILITY SUMMARIES section of this issue of Pacific ENSO Update...
SPECIAL SECTION: Pacific Sea Level and ENSO
NOAA's Climate Prediction Center in Washington D.C. routinely monitors the latest forecast results from several ENSO models. A discussion on the latest forecast results appears below:
PROGNOSTIC DISCUSSION OF SST FORECASTS CLIMATE PREDICTION CENTER NCEP NATIONAL WEATHER SERVICE WASHINGTON DC 3 PM EDT THURSDAY MAY 15 1997 The official SST forecast for the east-central equatorial Pacific (120-170 degrees W longitude - also called Nino 3.4) calls for SST anomalies to continue the warming trend which has been observed since late January. The SSTs from the date line to the South American coast have been more than 1 degree C above normal in the last week - and have warmed most rapidly in the last 6 weeks. The SST forecast is based on a combination of several statistical and dynamic forecasting tools and the most recent observations indicating very sudden warming. A statistical model based on a constructed analogue (CA) and one based on canonical correlation analysis (CCA) indicate positive SST anomalies by summer and becoming more strongly above normal by fall 1997 and winter 97/98. The dynamic forecast from the NCEP model indicates a similar evolution of positive Nino 3.4 SST anomaly. Although these three models agree - they do not forecast the suddenness of the warming as it appears to take place right now. This is in part because the latest initial conditions for these models are the seasonal February-March-April (FMA) average or some other low frequency description that includes the cool conditions in the past season and last year. The sudden warming of the last weeks is thus not known to any of these models. Opinions differ as to whether the sudden warming is just a "blip" or an early onset of a warm event to stay. Another dynamic model - a new version of the Cane and Zebiak model (LDEO2) indicates continued negative anomalies throughout 1997. This model is the only one among the four which continues to predict cold event conditions. In view of the recent upward trend in SSTs to decidedly above-normal temperatures in the east-central tropical Pacific, temperatures would have to decline from current values to be in agreement with LDEO2. However, atmospheric conditions for such a drop in temperature currently do not exist. In fact - low level equatorial Pacific winds and the Southern Oscillation Index itself have been suggestive of a developing warm episode during March, April, and May. For these reasons the consensus forecast among the three models which are in agreement is the scenario currently considered to be the most likely to occur. Thus - a forecast of positive SST anomalies and warm event conditions through at least winter 97/98 is indicated.
These recent results from the models are more consistent, compared to those from the last issue of Pacific ENSO Update, in their combined outlook for warmer-than-normal sea surface temperatures (SSTs), and further development of an ENSO warm event over the remainder of the year. Although the early and rapid warming of SSTs in the central and eastern tropical Pacific over the last three months was not anticipated, the falling SOI condition and recurrent westerly "wind bursts" along the equator in the western tropical Pacific further supports the likelihood that a warm event is already developing.
The latest ENSO Advisory from NOAA (shown in the Appendix on page 11) was issued on May 9, 1997. The ENSO Advisory gives more details about recent conditions in the tropical Pacific, and the expected development of an ENSO warm event in the months ahead.
Near-term outlooks for local variability in the islands are based on a continuation of warm conditions and further development of an ENSO warm event in the coming months. Longer term outlooks, however, are highly dependent on whether the warm event ends quickly with a developing cold event, or lingers for some time. These possibilities are described in two scenarios, which are also referred to in the LOCAL VARIABILITY SUMMARIES section...
Scenario 1 - Rapid Recovery: Most warm events which have occured over the last few decades occur on a one-year time scale, and are followed by a rapid reversal of trends in oceanic and atmospheric conditions, which can lead to the development of a cold (La Niña) event. In this scenario, SOI reaches its lowest point in the May-August time frame, and begins to rise thereafter. The warming trend in central and eastern equatorial Pacific SSTs follows by several months, and is greatest in the October-December time frame, then begins to cool. A shift of heavy rainfall and cyclone development to eastern Micronesia and the Marshall Islands occurs, with drier conditions in western Micronesia during the developing warm event in the first year (1997, in this case). Following this, strong easterly (trade) winds return, pushing low level westerly winds back toward Asia. Warm waters are re-established in the western Pacific "warm pool" and cooler waters reappear in the east. Widespread drought conditions can occur in Micronesia with the rising SOI/cooling SST conditions in the first half of the second year (1998 in this case). In the Southern Hemisphere, a similar pattern occurs, with heavy rains and cyclone development in the September-April time frame shifted eastward toward Samoa and Tahiti, and drier conditions west of the date line and in northeastern Australia. The 1972/73 ENSO warm event, followed by a La Niña cold event in 1973/74, was typical of this sequence.
Scenario 2 - Lingering Warm: Less often, but typical of more recent events, trade winds strengthen slowly and equatorial SSTs do not significantly cool after the first year, but linger into the following year . The eastward shift in heavy rains and cyclone devel-opment lasts for at least two years (e.g. 1997 and 1998 in this case), with drought conditions delayed until the reversal in trends toward cold conditions (e.g. at least until 1999). Such extended warm conditions occured in 1986-88 and 1991-94. La Niña cold events eventually followed in 1989 and 1995.
The scenarios described in the previous section will affect the long-term outlook for climatic variability in the islands. Information in the LOCAL VARIABILITY SUMMARIES section is based on an expectation that conditions will be influenced by a developing warm event through 1997, with further development as described in Scenario 1. Any changes in trends, and their impact on these outlooks, 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: The transition from winter to summer season in the state has seen some high variability in rainfall. February and April were comparatively dry, while March was quite wet. Leeward areas generally received greater-than normal rainfall, and windward areas received less, although some episodes of "wet" trade winds following storm front passages in March delivered significant rainfall to windward areas. In contrast to this general pattern, Kauai had wetter-than-normal conditions over most of the island during the last three months. On Maui, Kula was notably dry, with only 36% of normal rainfall for the 3-month period.
In Hawaii, ENSO warm events are associated with below-normal wintertime rainfall. Droughts can result, since this occurs immediately following the normally dry summer months. With ENSO warm conditions under development at this time, the winter of 97/98 may see below-normal rainfall in the state. This outlook is further discussed in the latest issue of the Long-Lead Outlook for the Hawaiian Islands from NOAA-CPC, shown on page 10.
For hurricane activity, history has also suggested some association with ENSO warm events, with warm SSTs and favorable atmospheric conditions combining for increased activity, both for local development (e.g. south of Hawaii) as well as for longer-lived systems moving in from the east. In May, some tropical disturbance development was already observed, occurring southwest of the state.
- sources: NWS-PR and PEAC
AMERICAN SAMOA: January-April rainfall at Pago Pago was 10% above average, even though January alone saw almost 28 inches, due to the temporary eastward movement of the South Pacific Convergence Zone (SPCZ) over the region around Samoa. Since late January, rainfall has declined, with April getting less than half of the normal monthly amounts.
With warm ENSO conditions continuing to devlop as described in Scenario 1, rainfall is expected to be about 10% above normal through June, then 10-15% above normal from July to October, and about 20% above normal from October 1997-April 1998. With the expected return of stronger easterly (trade) winds, rainfall is expected to be about 10% below normal after April 1998.
Under the influence of an ENSO warm event, tropical cyclone activity is expected to increase in the region around Samoa, as the development area shifts eastward to tropical latitudes north of the Samoan Islands. Greater activity may increase the frequency of tropical cyclone threats to Samoa and other tropical island areas east of the date line during the 1997-1998 rainy/cyclone season. If extended warm conditions as described in Scenario 2 were to occur, then wetter-than-normal conditions and higher-than-normal tropical cyclone activity would extend into the next season in late 1998-early 1999.
- source: UOG-WERI
GUAM/CNMI: January-April 1997 rainfall was 19% above normal at Andersen Air Force Base , and 45% above normal at Guam International Airport (Tiyan) for the same period. This surplus rainfall was primarily the result of Typhoon Isa, which passed south of Guam in April, and brought rainfall totals for the month in excess of 10 inches at each of the two stations. However, Isa did not affect Tinian or Saipan in the CNMI, and dropped only small amounts of rain on Rota. January through April rainfall at Saipan International Airport was only 1% above normal, and was 20% below normal at Capital Hill, for the same period.
Early development of the ENSO warm event over recent months is apperent in sea level data from the Marianas Islands. Sea level deviations of about -10cm observed at Saipan are consistent with the eastward migration of warm waters from the "warm pool" in the western tropical Pacific.
With the continued development of an ENSO warm event as described in Scenario 1, rainfall for Guam and the CNMI during the first year (e.g. 1997) could be highly variable, ranging from 15% below-normal to 15% above-normal. Generally, conditions in the Marianas are wetter in early summer through early fall, due to enhanced monsoon activity and more frequent moist, unstable westerly winds. In the fall, westerly winds move south of the island chain, and tropical cyclone development shifts eastward, from the normal region near Chuuk, out toward the Marshall Islands. This increases the number of potential tropical cyclone threats to the Mariana Islands (though not necessarily the number of cyclones that actually hit the islands). If one or more of the resulting cyclones hits or get close to the islands, rainfall totals for fall through early winter will be above normal; likewise, if they miss the islands, the period will be drier-than-normal.
Since actual storm tracks cannot be predicted more than a few days in advance (but their expected increase is a factor), the rainfall outlook for Guam and CNMI is based on the likelihood that one or more tropical cyclones will get close enough to drop significant rainfall on the islands. Late season events are also more likely with ENSO warm conditions. Thus, slightly wetter-than-normal rainfall is expected. By January, however, with warm conditions ending and a cooling trend underway (as described in Scenario 1), below-normal rainfall is likely, with the spring and early summer being especially dry. Tropical cyclone development could be delayed until mid-summer, and be pushed to the west of the Mariana Islands, with the possible result of severe drought conditions in Guam and/or CNMI well into 1998. The following outlooks for seasonal rainfall are provided for the use of regional water resources managers in Guam and the CNMI:
MONTHS: % OF LONG-TERM AVERAGE: May 97 - Jun 97 105 Jul 97 - Sep 97 120 Sep 97 - Dec 98 105 Sep 97 - Dec 98 80If an extended period warm conditions as described in Scenario 2 occurs, then the dry conditions anticipated for 1998 would be delayed, at least until the spring of 1999.
FEDERATED STATES OF MICRONESIA (FSM): The possibility of an ENSO warm event development in 1997 noted in the last issue of Pacific ENSO Update is now more apparent. January - April sea level data from Yap and Pohnpei (with Yap levels down by -15cm, and Pohnpei values falling from +10cm to -2cm over the period) both reflect the easward movement of warm water from the western tropical Pacific toward the date line. Rainfall variability has also been consistent with the developing El Niño. Individual summaries for FSM states and islands (from west to east) appear below:
Yap and northern Chuuk State islands: Rainfall at the Yap weather station from January through April was 22% above normal, primarily due to wet January and February rainfall at 75% above normal. March and April saw only 62% of the normal rainfall, as the region of active rainfall and storm development zones shifted eastward with the warm surface waters and the developing ENSO warm event. With continued development in ENSO conditions as described in Scenario 1, the southwest monsoon is expected to become very active in the region from late June through September. Rainfall in the northern Yap and Chuuk State islands could be about 15-20% above normal at that time. From October 1997 to January 1998, rainfall is expected to be about 10% below normal, although the close passage of a single tropical cyclone could produce above-normal rainfall tor the period. From February through June 1998, rainfall about 15-20% below normal is expected,as the ENSO warm event ends with a cooling trend and strong, comparatively dry trade winds push active rainfall and storm development zones out to the west.
Tropical cyclone activity is expected to be slightly below normal during the entire period. Normal activity from September through November will be pushed to the east near the date line, and most of the resulting tropical storms and typhoons will pass to the north and east of Yap. With the expected cooling trend, spring and early summer development in 1998 will be pushed westward into the South China Sea.
Chuuk and southern Yap State islands: January through April rainfall at Weno Island was 12% below normal. April was especially dry at 5.47 inches, or 44% of the normal amount. This is typical of these months during an El Niño warm event, as convective activity normally around Chuuk is shifted to the east.
Because of the developing ENSO warm event, the southwest monsoon should be very active and rainfall should be about 10-15% above normal from June through September. By October, conditions are expected to dry to about 85% of normal values as monsoon activity wanes. Conditions will remain dry through June 1998, if strong easterly trade winds associated the cooling trend in ENSO conditions described in Scenario 1 do occur. Tropical cyclone activity is expected to be below-normal during 1997, as the primary development area near Chuuk will be shifted to the east, especially after September 1997. Springtime (1998) tropical cyclone development may be forced to the west by strong trade winds.
Pohnpei and Kosrae States: Rainfall from January through April was 45% above normal at Kolonia on Pohnpei. April was especially wet, with rainfall at 73% above normal for the month, in association with the development of Typhoon Isa and Tropical Storm Jimmy. Most of this rain fell in the first half of the month, leading to tragic landslides on Pohnpei. Rainfall on Kosrae was also above normal, but somewhat less so with 12% above the average for January-April. These wet conditions in the area around Pohnpei and Kosrae follow from the early development of ENSO warm conditions, with active rainfall and storm development zones moving into the region along with the warm waters from the west. Rainfall should be slightly above normal from May through June, then perhaps 10-15% above normal from July through September, with the enhanced southwest monsoon activity expected in association with ENSO. Once the monsoon recedes to the west, rainfall and storm activity will be concentrated to the east, in the region of the Marshall Islands and the date line. Rainfall for Pohnpei and Kosrae will return to near-normal levels in October - December, but a single tropical cyclone passing close enough to the islands could produce a significant rainfall surplus, as a result of increased development in the region of the date line and Marshall Islands. With the cooling trend expected in early 1998 as described in Scenario 1, strong and comparatively dry easterly (trade) winds may reduce rainfall in the January - June 1998 time frame to slightly below-normal levels. Tropical cyclone activity in the area during this time would also be suppressed, by being displaced to the west toward the South China Sea.
- source: UOG-WERI
MARSHALL ISLANDS (RMI): The rainfall for January-April in southern RMI atolls was near-normal. Majuro received 101% of normal for the period. However, April was particularly wet: Majuro got 14.54 inches, Mili, 22.01; Jaluit, 18.88; and Alingalaplap, 16.11. The greater rainfall was likely due to ENSO warm conditions developing near the date line. Typhoon Jimmy and Tropical Storm Isa developed nearby to the west. Kwajalein rainfall was 57% above normal and Wake Island reported 32% above normal rainfall for January-April. With an active southwest monsoon expected, rainfall at northern atolls like Wake and Enewetak should be above normal from July through September. After the monsoon recedes, warm ENSO conditions near the dateline will again favor active rainfall and storm development in the southern RMI. Rainfall on Kwajalein, Majuro, and surrounding atolls could be well above normal from October 1997 through February 1998.
With the expected monsoon and ENSO conditions, tropical cyclone activity in the RMI and surrounding areas will likely be higher than normal. In August and September, northern and western atolls (Wake, Enewetak) may see increased potential for local development of tropical storms and cyclones, with stronger and greater eastward extent of the southwest monsoon. By October, ENSO warm conditions are expected to favor strong westerly winds, enhanced atmospheric convection, and potential for development of tropical storms and cyclones nearby and to the east of the southern RMI. The resulting cyclones can reach typhoon intensity in the southern islands, with winds approaching 100 knots as they pass the northern islands (Kwajalein, Wotje, etc.), and even reaching super-typhoon intensity in the western islands (Ujelang, Enewetak). While the maximum intensity of tropical cyclones affecting the RMI is variable from one ENSO warm event to the next, the likelihood of tropical storm and typhoon activity in the region for late 1997 is higher than usual.
If conditions develop as described in Scenario 1, ENSO warm conditions will give way to a cooling trend and easterly (trade) winds will affect the region through the first half of 1998, reducing rainfall and storm activity. However, if ENSO warm conditions linger (Scenario 2), wet and stormy conditions similar to early 1997 may again affect the RMI in early 1998.
- sources: PEAC and UOG-WERI
PALAU: Overall rainfall at Koror from January through April was 39% above normal, primarily the result of a very wet February. Rainfall in February was 27.13 inches, or nearly three times the normal for the month. March was close-to-normal with 9.10 inches, and both January and April were similar, with about 2 inches below normal, or 75% of normal rainfall for both months. This trend for January through April rainfall apparently reflected the west-to-east passage of active tropical convection and associated rainfall, from the Indonesia /Indian Ocean region in the west, out to the east toward the date line, in association with the developing ENSO warm event. Also, below-normal sea levels observed at Malakal in March and April (-15cm and -16cm, respectively) look to indicate the eastward movement of warm water out of the western tropical Pacific, as well as local reports from Palau of cooler-than-normal seawater temperatures both at the surface and at depth. All of these indications are consistent with the early development of an ENSO warm event.
Rainfall should be 10-15% above normal for the period June through September, with an active southwest monsoon expected. By October, the monsoon should recede to the South China Sea. Tropical convection and storm development should become localized far to the east near the date line, due to ENSO warm conditions. Rainfall in Palau may be below-normal, as eastward displacement of tropical storm and cyclone activity will cause most systems (and associated rainfall) to pass northeast of Palau. As a result, October through January 1998 rainfall is expected to be about 15% below normal, and perhaps drier in the southern islands.
If ENSO conditions develop as described in Scenario 1, the warm event will give way to a cooling trend and easterly (trade) winds will affect much of the western tropical Pacific through the first half of 1998. Tropical storm and cyclone development will be pushed west into the South China Sea. This may extend below-normal rainfall conditions in the region around Palau, with rainfall perhaps 5-10% below normal for February-June 1998.
- source:s PEAC and 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)735-2695 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 May 15, 1997 from NOAA's Climate Prediction Center (CPC).
SPECIAL SECTION: Experimental Forecasts for Pacific Island Rainfall
ENSO Advisory of May 9, 1997
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.
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