In general, the timing and strengthening observed for this event in recent months has been well ahead of that seen in other warm episodes over recent decades (e.g. since the early 1950s). Although the 1982-83 warm event is still regarded as the strongest this century, the present event has already exceeded it by some measures, and may yet develop into a record-breaking event. For this reason, a review of some regional impacts in 1982/83 are discussed in this issue of Pacific ENSO Update. Also, a comparison of timing and strength of the present event with others of recent decades, maries of local variability, results from long-term model predictions, and the contents of the most recent issue of the ENSO Advisory from NOAA, are included in this issue.
SPECIAL SECTION - Comparision of ENSO Events
PROGNOSTIC DISCUSSION OF SST FORECASTS CLIMATE PREDICTION CENTER NCEP NATIONAL WEATHER SERVICE WASHINGTON DC 3 PM EDT THURSDAY AUGUST 14 1997 The official SST forecast for the east-central equatorial Pacific (120-170°W longitude - also called Niño 3.4) is for strong warm ENSO conditions continuing throughout the year and into early 1998. Equatorial SSTs are now more than 2°C above normal east of about 160°W and are 5°C above normal in some areas close to the South American coast. SSTs in record warm levels (for the period since 1950) for this time of year. SSTs are currently above 29°C along the equator in the central Pacific near the date line. The SST forecast is based on a combination of two statistical models (Constructed Analogue - CA - and Canonical Correlation Analysis - CCA) and one dynamical model (the NCEP coupled ocean-atmosphere model) and indicates that Niño 3.4 temperature anomalies will remain at or near their current levels throughout the winter. The NCEP coupled model suggests that SSTs will remain close to 3 standard deviations (sigma) above normal through the remainder of the year. The statistical models are slightly more conservative - but they also predict anomalies of near or exceeding 2 sigma. An objective regression combination of the three forecasts shows an anomaly in the 2 to 3 sigma range into early 1998 - suggesting that SSTs in the east central equatorial Pacific will continue to exceed the critical 28.5°C threshold for convection through the spring of 1998. This will support anomalously strong convection over a large area of the eastern equatorial Pacific this fall and winter. Temperature anomalies are expected to decrease early in 1998 but remain above normal through next summer. Confidence in the forecast this time of year is high and it is likely that temperatures for this fall and winter will be near record warm levels.
These recent results from the models are consistent in their outlook for continued development and persistence of a significant ENSO warm event over the remainder of the year and in to early 1998. Several ENSO Advisories summarizing this rapidly developing ENSO event have been issued by NOAA since the last issue of Pacific ENSO Update; The contents of the latest Advisory from August 13th, 1997, are shown in the Appendix on pages 11-12, and give more details about recent conditions in the tropical Pacific, and further expected developments.
In the last issue of Pacific ENSO Update, two possible scenarios were discussed with respect to the development of an ENSO warm event in 1997/98, one in which there was a rapid strengthening toward El Niño conditions, then a reversal trends (the Rapid Recovery scenario) and a second in which the transition occurs more slowly over two or more years (the Lingering Warm scenario). The current ENSO warm (El Niño) event has become significantly stronger than expected over the ensuing period, and may eventually equal or exceed the record event of 82-83. Therefore, the Rapid Recovery scenario seems more likely at this time, briefly reviewed as follows:
Rapid Recovery scenario: The ENSO warm event continues to develop until the end of 1997, when a rapid reversal of oceanic and atmospheric conditions sets in. SSTs in the central and eastern equatorial Pacific SSTs should be warmest in the October-December time frame, then begin to cool, while the SOI reaches its lowest point in the May-August time period, then begins to rise. However, as with the 1982-83 ENSO event, the SOI could make further drops, into early 1998. During the northern hemisphere fall, equatorial westerly winds will strengthen across the central Pacific. Unusually heavy rainfall and tropical cyclone development should affect the central Pacific tropics in both hemispheres. By late 1997, the easterly trade winds that had weakened during the warm phase of the ENSO, will rapidly re-strengthen and begin push the warm ocean waters back to the west, eventually re-establishing the "warm pool" in the western Pacific. This westward surge of strong easterly trade winds will bring unusually dry conditions to the central and western Pacific during the winter, spring and early summer of 1998. The strong trade winds will likely constrain 1998 tropical cyclone development to the South China Sea until summer.
While SSTs are presently warmer than those at this time in the 1982-83 event, the SOI is somewhat less negative. It remains to be seen whether the SOI has reached its lowest value and will begin to rise or whether it will continue to fall (into early 1998) as with the 1982-83 event. In either case, it seems likely that during October and November, that strong equatorial westerly winds will be shifted further into the eastern Pacific. This could reduce much of the increased rainfall and tropical cyclone activity often seen in the Marshall Islands and Samoa region during less intense ENSO warm events.
By December, a well-developed monsoon trough (the South Pacific Convergence Zone) will develop between Tokelau and the Marquesas Islands in the south central Pacific. The trough could occasionally extend as far west as Tuvalu and as far east as the Galapagos Is. As the trough moves to the south, the equatorial westerly winds are expected to be replaced by strong easterly trade winds induced by recovery from the warm phase. A similar monsoon trough system in 1982-83 became a very prolific producer of tropical cyclones in the south central Pacific.
Information in the LOCAL VARIABILITY SUMMARIES section is based on an expectation that conditions will be influenced by a strong warm event through 1997 and early 1998, with a reversal toward a cooling trend thereafter. 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: Windward areas of the state gererally received above-average precipitation from May through July, as a result of relatively moist trade wind flow. Leeward areas receive little precipitation in these conditions, and generally received below-average rainfall for the period. Although the Kona section of the Big Island would normally be an exception, receiveing greater rainfall in summer months, totals for the period there appear to have been about 15-25% below-normal. The strong ENSO warm event is expected to result in below-normal wintertime rainfall across much of hte state. Research conducted at UH found that 18 of 20 winter seasons during ENSO periods since 1905 experienced below-normal rainfall, as well as 16 of 20 spring seasons. This year, leeward areas may be particularly prone to droughts since recent months have been drier than normal. The Long Lead Outlook for the Hawaiian Islands from NOAA-CPC, shown on page 10, anticipates below-median rainfall for seasonal (3-mo.) periods from January-June 1998, due to ENSO.
For hurricane activity, history has also suggested some association with ENSO warm events, with warm SSTs and favorable atmospheric conditions combining for increased activity, particularly in the late season, both for local development (e.g. south of Hawaii) as well as for longer-lived systems moving in from the east. As yet, no significant systems have threatened the islands this season, though some hurricane activity in the Eastern Pacific has occured somewhat closer to 140°W this season.
- sources: NWS-PR, U.H. Meteorology Dept. and PEAC
AMERICAN SAMOA: The current ENSO warm (El Niño) event has become significantly stronger than expected, and may eventually equal or exceed the record event of 1982-83. In that case, rainfall and tropical cyclone outlooks for American Samoa will be significantly different than previously reported in the last issue of Pacific ENSO Update. The major heavy rainfall and tropical cyclone zone may shift even further east than the Samoa region, putting Samoa on the edge of the activity rather than in the middle. The major part of the monsoon trough will likely lie between Tokelau and the Marquesas Islands, and the increased tropical cyclone threats would be to eastern island groups like the Cook Islands and the Society Islands in French Polynesia. This was the case in 1982/83, when six tropical cyclones developed and passed through those areas east of Samoa, while Pago Pago recorded some of the lowest rainfall amounts on record, from late 1982 and well into 1983, which was the second driest year (after 1974) in the record since 1966.
Rainfall for Pago Pago for April, May, and June of 1997 was only 64% of normal, and July saw only 59% of normal values. Some increase in the dry season rainfall is expected, but total rainfall may be in the 15-20% below-normal range until November. Rainfall should be near normal from November to February, and should increase to 10-15% above normal from February through April. Around May, the monsoon trough will weaken and disappear as strong easterly trade winds, in association with the expected weakening of the El Niño at that time, dominate much of the tropical central Pacific. Rainfall after May in Samoa is expected to be about 20% below normal as a result.
The rapidly developing ENSO conditions were likely a factor in the occurrence of Tropical Cyclone Keli, an unusual late-season system which developed in June of this year, severely impacting islands in Tuvalu (for the first time since TC Bebe, at the time of the 1972/73 El Niño) , and passing through Wallis and Futuna, before approaching Samoa as a weakening tropical storm. In the coming season, the region around Samoa will likely see some increased tropical cyclone activity from December through March, but the South Pacific Convergence Zone will probably propagate to the east in April and May, reducing late-season threats to American Samoa and the surrounding areas.
- source: UOG-WERI and PEAC
GUAM/CNMI: The current El Niño is becoming one of the strongest of the century, and could equal or exceed the record 1982-1983 El Niño event. As a result, dry conditions are expected to develop for Guam and the CNMI, beginning in the fall of 1997, as the southwest monsoon disappears, and moisture-laden westerly winds become limited to equatorial regions between the Marshall Islands and the eastern Pacific. The winter of 1997 and the spring of 1998 are expected to be extremely dry, with dry conditions extending into the early summer, as post-ENSO warm conditions reverse trends. This prediction is consistent with statistical model predictions shown on page 9, with the more extreme dry forecasts indicative of what was experienced under similar conditions in 1983.
Recent rainfall variability in the area has been highly localized. April, May, and June for Guam was 40% above normal, while that for Saipan was about 25% below normal. Overall rainfall for the region was expected to be about 5% above normal for the period. For July through September the last outlook for the region was for 20% above normal rainfall. To date, the summer rainfall for Guam has been about 66% above normal, while that for Saipan and Tinian has been about 28% below normal. Rota rainfall for the summer has been about 35% above normal. On Guam, rainfall to date for August is record-setting, amounting to around 35 inches (150% above normal), due to narrow belt of persistent westerly winds on the south side of the monsoon trough. Saipan, although only 120 miles north of Guam, spent most of its time in the clearer, more northern part of the trough.
The monsoon activity should cease by late September, with the westerly winds moving to more equatorial regions near or east of the date line. However, by January, the westerlies will disappear as easterly trade winds become dominate over the entire tropical Pacific. Conditions in the Mariana Islands will, thus, become progressively drier through the fall, winter, and spring, and drier-than-normal conditions are expected to continue into July of 1998. Fall rainfall for the Marianas is predicted to be near normal for Guam and somewhat drier for the CNMI, but values could range from 15-20% below normal to 15% above normal for a specific island, depending on whether or not a tropical cyclone hits the island.
During the late winter and spring, conditions will be extremely dry as strong easterly trade winds dominate the region into summer. Some months could be more than 75% below normal.
At the request of water resource managers on Guam and in the CNMI for use in their water management models, we provide the following expectations of monthly rainfall totals:
Inclusive Period % of long-term average
Aug-Sept 1997 130% 100%
Oct-Dec 1997 95% 90%
Jan-Jun 1998 40% 40%
Jul-Aug 1998 90% 90%
Tropical cyclone activity in the western North Pacific during 1997 has been fairly normal to date. The greatest typhoon threats to Guam and the CNMI will be from now until early December as activity is expected to increase to the southeast of the island chain. With the very strong El Niño, fall-season development could be east of the Marshall Islands, reducing the threats to the Mariana Islands. In 1998, due to the expected strong trade winds, tropical cyclone development will be displaced far to the west, and the tropical cyclone season for Micronesia will be delayed until late July.
As a result of the earlier movement of warm sea water from the western Pacific to the eastern Pacific, sea levels are about 6-8 inches lower than normal in the Mariana Islands. This lower sea level has led to some bleaching of reefs in the region, and has also lowered the level of the narrow aquifers on the larger islands. In 1983, this effect, coupled with low rainfall, caused some wells to cease pumping in Saipan and Tinian, until the sea levels returned to normal in mid-summer. Also on Saipan, because of the low springflow and sources of low-chloride ground water, the quality of water decreased to the limits of potability. Rota was able to maintain ample spring water to irrigate crops. Both Guam and Saipan had numerous grass fires during the 1983 drought. On Guam, Merizo and Umatac experienced severe water shortages, as springs supplying their water dried up.
- source : UOG-WERI
MICRONESIA (FSM): The current ENSO warm (El Niño) event has become significantly stronger than expected at the time of the last Pacific ENSO Update. and could eventually equal or exceed the record event of 1982-83. In this case, the near-equatorial westerly winds and region of active rainfall and storm formation may extend well east of the date line this fall, causing the beginning of dry conditions for both eastern and western FSM states. Also, because of the rapid development of this event, a strong reversal toward La Niña-like conditions (similar to what occurred at the end of the end of the 1982/83 event) is expected in early 1998, which could intensify dry conditions through the first half of 1998 and strain water resources on many of the islands. Recent results from the seasonal rainfall prediction models, shown on page 9, generally support this outlook. Summaries and outlooks for individual regions of the FSM appear below, along with a review of some impacts from the 1982/83 ENSO event.
Yap & northern Chuuk State islands: Rainfall in April, May, and June 1997 was 66% of the average for Yap, its northern islands (Ulithi, Fais, etc.), and the northern islands of Chuuk. July rainfall was about 5% above average, primarily the result of infrequent, but heavy rains from tropical disturbances and an enhanced monsoon. For the year, the regional rainfall has been about 5% above normal. For August and September, rainfall is expected to be about average to 10% below average across the region. Monsoon activity should cease by late September, with the moisture-laden westerly winds moving away to more equatorial regions near and east of the date line. Conditions in Yap and northern Chuuk States will become progressively drier through fall, winter, and spring, with drier-than-normal conditions continuing into July 1998. Fall rainfall for the region is predicted to be about 10% below normal, but values could range from 25% below to 10% above normal for any island, depending on whether or not a tropical cyclone hits the island. During the late winter and spring, conditions will be extremely dry as strong easterly trade winds dominate the region into summer. Some months could see more than 75% below normal rainfall.
Tropical cyclone activity for Yap State is expected to be below normal after September. This is because we expect the development regions to shift eastward toward the date line. Thus, the storms will have more time to gain latitude and move away from the northern Yap and Chuuk regions. Tropical cyclone activity in 1998 will be delayed until mid-summer as strong easterly trade winds will force tropical cyclone activity westward to the South China Sea.
During the 1983 drought, reservoirs supplying water to Colonia dried up by the middle of February. The Airport Swamp also dried up due to over use as a source of drinking water and evaporation. Reduction in the yield of coconut and breadfruit was about 50%, losses of banana was about 25%, and losses of taro and yam were 60-70%.
Chuuk & southern Yap State islands: During April, May, and June, rainfall at Weno Island and Woleai Atoll was 28% below normal, while that at Lukunoch was 9% above normal for the same period. Rainfall activity increased drastically during July, with that at Chuuk being 27% above normal, that at Woleai being 69% above normal, and that at Lukunoch being a whopping 212% (37.78") above normal. Polowat had only 60% of its normal rainfall during April, May, and June, and July rainfall was still deficient at around 95% of average values. By September, rainfall in the region is expected to drop off considerably as moisture-laden monsoon westerly winds recede to the west.
With the very strong El Niño conditions and the expected trend in coming months, we expect rainfall from October through January to be at least 40% below normal for the region. February through May rainfall is expected to be even more deficient, with monthly amounts running about 60% below normal. Rain fall should begin to increase in June and reach near-normal values by August 1998. Such large rainfall deficits could significantly lower water tables and increase the chloride concentration of the available well water.
This fall, tropical cyclone activity in the region should be greatly reduced as tropical cyclone development is shifted eastward to the region of Pohnpei, the Marshall Islands, and the date line. It will also be reduced in the late spring and early summer of 1998, since strong trade winds will restrict early development to the South China Sea and delay the onset of the tropical cyclone season in the Philippine Sea. During the 1982-83 event, Chuuk had eight continuous months of dry weather (Oct 82-May 83).
On Weno Island, water resources were severely taxed, and stringent water rationing was required. On the smaller islands, many of the wells dried up or became too brackish to drink. There were large losses of coconut and breadfruit, especially where the soil was shallow. On the coral islands, salt water intrusion killed much of the taro and yam crops, but despite the severity of drought, Woleai seemed to fare well.
Pohnpei & Kosrae States: Rainfall for the 3-month period of April- June was somewhat drier than normal, though April was wet from Pohnpei eastward to the date line as several tropical cyclones developed northeast of Pohnpei. For the period, Pohnpei had 98% of its normal rainfall, Kosrae had 73%, and Pingalap had 86%. Nukuoro had only about half of its normal rain. Precipitation increased over the region in July, where rainfall was 25% above normal at Kosrae, 32% above at Nukuoro, 20% above at Pingalap, and 5% below normal at Pohnpei.
With the very strong ENSO and the expected trend in coming months, rainfall for the region should be near to slightly below-normal through mid-October. After this time, the rain-bearing westerly winds will shift well to the east, leaving the region relatively dry. We expect November-December rainfall to be 20-30% below normal for most sites and 30-40% below normal for Nukuoro. From January through May, rainfall for the entire region could be as much as 70% below normal. The rains will again return to the region in June, and rainfall values should slowly return to near normal values by September 1998.
This extended period of dryness could greatly strain water resources on the islands, especially the atolls, and would likely have a big impact on agriculture as well. During the 1983 drought, agricultural losses in Pohnpei were heavy--coconut 55%, banana and other fruits 75%, breadfruit 80%, and vegetables 85-90%. In Kosrae the losses were approximately--coconut 25-30%, fruits and vegetables 30%, and taro and yams 60%. Wild fires, which are very rare in the region, did occur during this drought.
With respect to tropical cyclones, we now expect activity to fall below normal. While there is some threat of a November or an early-December cyclone, we expect most of the activity to move east of the date line. With the strong El Niño, we also see less risk of episodes of strong westerly winds as occurred in March and April.
- source: UOG-WERI
MARSHALL ISLANDS (RMI) : In our last Pacific ENSO Update, we anticipated the development of a moderate to strong El Niño, The current ENSO warm (El Niño) event has become significantly stronger than expected since that time, and may eventually equal or exceed the record event of 1982-83. Because of this intense event, rainfall and tropical cyclone outlooks for the Marshall Islands are significantly different than previously reported. The heavy rainfall and strong tropical cyclone activity initially expected for this fall and early winter (similar to that of the moderate ENSO conditions in late 1992-93) will now likely be displaced farther to the east toward the central Pacific. As a result, a much drier regime should become dominant in the Marshall Islands, similar to that which occurred with the 1982-83 ENSO event.
Rainfall at Majuro for April, May, and June was 30% above normal and that at Kwajalein was about 50% above normal. In May, normally dry Uttirik Atoll experienced what may well be its highest recorded monthly rainfall ever--39.94 inches. Conditions were drier in July, with rainfall at Majuro only 38% of normal, and at Kwajalein only 49% of normal.
We expect some increase in rainfall values during October as tropical cyclone development moves south and west to the region of the western Marshall Islands. However, because of the strong El Niño, cyclone development may be somewhat suppressed in the Marshalls and enhanced to the east of the date line. Thus, we expect the region to become progressively drier through the fall and winter, much as it did during the 1982-83 ENSO event. Following the peak of this event, a trend toward the cold phase of the ENSO (i. e., La Niña) is expected, causing strong, dry trade winds to dominate the region by early 1998, making conditions very dry in the Marshall Islands through to July. Rainfall values during this period are expected to be 40-50% below normal, and could be lower than 70% below normal during March and April.
During the 1982-83 ENSO event, many of the atolls lost a significant portion of their coconut and breadfruit trees and their subsistence crops due to the drought. Wells became brackish, and many could not be used. Severe water rationing was necessary on Majuro, and water had to be transported to many of the outer islands.
- sources: PEAC and UOG-WERI
PALAU: As a result of the developing El Niño, early season tropical cyclone development and related rainfall was displaced to the east toward Pohnpei and the Marshall Islands. Thus, Palau experienced only about 75% of its normal April, May, and June rainfall. In July and August, Palau was sandwiched in an area of suppressed rainfall between two areas of strong monsoon flow, one to the south over Chuuk and one to the north over the northern Philippines. As a result, July rainfall in Palau was only 51% of its normal value. We anticipate some increase in rainfall during September due to monsoon activity, but still expect precipitation to be about 20% below normal. Monsoon surges will disappear by October, and tropical cyclone activity will shift progressively toward the date line and east of it during fall as a result of the El Niño warm event. By early 1998, a reversal in trends following the peak of El Niño conditions is expected, bringing strong, dry trade winds through the region toward the Asian coast.
We expect rainfall in Belau to be 30% below normal from October-December and 50-60% below normal from January through May. By June, rainfall should increase to near normal values.
This very strong El Niño event which has been developing could equal or exceed the record 1982-1983 event, which brought drought-like conditions to all of Micronesia. During the 1982-83 event, January-May stream flow on Babelthuap was less than 20% of the normal volume, and by March 1983, the Gihmel Reservoir ran dry.
Tropical cyclone activity for Palau is expected be well below normal, both in the fall of 1997 due to displacement of storm activity further to the east with the El Niño conditions, and in the late spring of 1998, with the expected strong trade winds that will likely prevent tropical cyclone development in the Philippine Sea region until late June.
- source: UOG-WERI
SPECIAL SECTION: Experimental Forecasts for Pacific Island Rainfall using data through July, 1997.
Long-Lead Outlook for Hawaiian Islands issue dated 14 August 1997 from NOAA's Climate Prediction Center (CPC).
ENSO ADVISORY of August 13, 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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