Pacific ENSO Update - 4th Quarter 1997 - Vol.3 No.4


CURRENT CONDITIONS

The 1997 "El Niño" warm event has strengthened to record levels over recent months, and is expected to persist well in to early 1998. Monthly values for sea surface temperatures (SSTs) in the eastern and central equatorial Pacific were warmest on record for October, from the period 1950-present. SSTs during the El Niño of 1982-83 reached similar levels, though some 2-3 months later than the present event. Since some warming is expected as part of the normal annual cycle, SSTs will likely remain well above normal for the coming months.

In the atmosphere, surface wind patterns in the central and eastern equatorial Pacific have continued to reflect the strong El Niño. Weakened easterlies have given way to reversed, westerly wind flow, which first appeared briefly in June, and now again in October and November. Consistent with the atmospheric conditions, the Southern Oscillation Index (SOI) has stayed negative, and could still drop further as the pattern of atmospheric convection remains predominantly shifted to the east, over the region of warm SSTs. Because of the strength of this El Niño event, the westward zone of strong subsident (sinking) flow in the atmosphere (suppressing the development of deep convective, rain-bearing clouds) will also spread further east, as it did in 1982-83, extending from Hawaii in the north and Samoa in the south westward to the Philippines and Australia. This, along with brisk easterlies that will push back in to the region with the expected weakening of the El Niño in the spring and early summer, will suppress deep convective rainfall, extending dry conditions and potentially severe droughts in much of the region. Samoa and surrounding areas may see some periods of heavy rainfall during January-March due to developing tropical cyclones, which have already exhibited an El Niño-related eastward shift in seasonal activity.

Refer to the LOCAL VARIABILITY SUMMARIES section, and the various advisories included with this issue of Pacific ENSO Update, for more information about the El Niño event and the outlooks for coming months.


ENSO FORECASTS

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 DISCUSSIONS FOR LONG-LEAD OUTLOOKS 
CLIMATE PREDICTION CENTER NCEP 
NATIONAL WEATHER SERVICE  WASHINGTON DC             
3 PM EST THURSDAY NOVEMBER 13 1997
PROGNOSTIC DISCUSSION OF SST FORECASTS

The official SST forecast for the east-central equatorial  
Pacific (120-170°W longitude - also called Niño 3.4) is
for very strong warm ENSO conditions throughout the remainder 
of the year and into spring 1998.  Equatorial SSTs are at least 
2°C above normal east of about 170°W and are 4-5°C above 
normal from about 120°W towards the South American coast near 
and just south of the equator.  SSTs in this region have been 
steady in the past few weeks and are at record warm levels (for 
the period since 1950) for this time of year.  North of the 
equator SSTs are currently 29°C or higher from about 160°W to 
120°W.

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 April 1998.  
The statistical models are only slightly more conservative - they 
too predict anomalies of 2 to 3 sigma through March 1998.  An 
objective regression-based combination of the three forecasts 
shows an anomaly in the 2.5 to 3 sigma range into spring 1998 - 
suggesting that SSTs in the east central equatorial Pacific will 
continue to exceed the critical 28.5°C threshold for deep convection 
through that time.  This will support anomalously strong convection 
over a large area of the eastern equatorial Pacific this winter.  
It is noteworthy that Niño 3.4 SSTs for this winter will be 
at or near record warm levels. Confidence in forecasts is very high 
until March but begins decreasing substantially for projections 
beyond early spring and becomes low for forecasts for the summer 
and beyond.  Temperature anomalies are expected to decrease 
rapidly between April and August 1998 - returning to zero by the 
summer.  There are hints for mild cold conditions developing in 
late summer and fall 1998.  This possible development needs to 
be monitored - however confidence in the forecast made at this 
time of year for next autumn is rather low. 

In addition to the model results available from NOAA, the Experimental Climate Forecast Division of the International Research Institute (IRI) for climate prediction (described in the 2nd Quarter 1997 Pacific ENSO Update) monitors several ENSO models, and generates predictions of expected regional climate variations. A new development from the IRI is the preparation of net assessment forecasts for several areas of the globe, giving probablistic outlooks for seasonal rainfall totals in coming seasons. PEAC and IRI will be cooperating on preparation of such net assessments for the tropical Pacific region in the near future, and when these are available they will be included in future issues of Pacific ENSO Update.

Several ENSO Advisories summarizing general conditions and outlooks have been issued by NOAA in recent months. Text from the ENSO Advisory dated November 10, 1997 is shown here.


LOCAL VARIABILITY SUMMARIES

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: Hydrometeorological data summaries from the National Weather Service noted rainfall amounts for the August-October period that were generally below normal throughout much of the state. Wind data from Hilo, Honolulu, and Lihue airports recorded below-average wind speeds during much of the period, indicating weakend trade wind flow which was especially evident in the below-average rainfall values for most windward areas. Some localized episodes of land and sea breeze convergence did cause downpours in some locations, bringing rainfall to above-normal totals compared to surrounding areas. Examples are Hilo and Glenwood on the Big Island, Mahinahina and Pukalani on Maui, Maunawili and the Waianae Coast on Oahu, and Lihue on Kauai. Most other areas received below-normal rainfall at 50%-80% of averages for the period. Statewide, Kihei was lowest, with only 15% of normal rainfall.

Early winter indications from a preliminary analysis of November rainfall did find a few more localized events of heavy downpours, with Kamuela and Honokaa on the Big Island (and Pukalani in upcountry Maui) receiving about double of average for the month, however most of the rest of the state received near- or below-normal rainfall. Strong El Niño conditions are historically associated with below-normal wintertime rainfall across much of the state, especially in the December-February period, and possibly extending into spring, with drought implications. The Long Lead Outlook for the Hawaiian Islands from NOAA-CPC anticipates below-median rainfall for several seasonal (3-mo.) periods throughout the first half of 1998.

Hawaii's hurricane season traditionally ends on November 30th, and an interesting summary of this year's season from the National Weather Service is included in this issue. The historical association of greater regional storm activity with El Niño was evident this year, though fortunately none of the systems (which remained at storm intensity or less) posed serious threats to the Hawaiian Islands. Of note, however, is also the increased chance of "late season" storm activity with El Niño, and the development of Tropical Storm Paka to the southwest of the islands - on December 2nd - is particularly interesting in this regard. December cyclones in the central tropical North Pacific are extremely rare - this may be the first case of storm development in the month of December since a report dating to 1904. Other notable late season events included Nina in 1957 and Iwa in 1982, systems which initially formed in late November of those notably strong El Niño years, and later intensified to hurricanes that threatened the Hawaiian Islands - with significant consequences to Kauai and Oahu, in the case of Iwa.
- sources: NWS-PR and PEAC


AMERICAN SAMOA: July-September rainfall for Pago Pago was about 90% of normal, but October-November was only 53% of normal values. With the early timing and strength of this El Niño event, we had expected closer-to-normal rainfall beginning in November, in association with local activity of the South Pacific Convergence Zone (SPCZ) and early season tropical cyclone activity in the region. However, rainfall seems to be responding as it did in late 1982, under the influence of those strong (though somewhat later) El Niño conditions, which saw dry conditions for October thru December (about 37% of normal). Conditions through December may remain dry, but January through April may see greater rainfall with local influence of the SPCZ and tropical cyclone season activity. Whether conditions are wet for Samoa will depend on tropical cyclone activity in the region, either in the developing or mature stage, but if the activity misses Samoa, the area will be drier than normal. With this very strong El Niño, it is possible that most tropical cyclone activity will be shifted well to the east of Samoa, posing continuing threats to the Cook Islands and French Polynesia, where some islands have already experienced devestating impacts, due to Tropical Cyclones Martin and Osea in November. Along with Tropical Cyclone Keli last June, this eastward shift in seasonal activity of tropical cyclones is already exhibiting the strong influence of this El Niño event. The National Institute for Water and Atmospheric Research Ltd. (NIWA) in New Zealand recently issued a press release describing expected shifts in tropical cyclone activity for the southern hemisphere this season, including general indications of changes in risk for Samoa and surrounding island regions. The NIWA press release is included in this issue of Pacific ENSO Update.

Following the cyclone season, El Niño conditions are expected to weaken, and reverse trends toward ENSO cold or "La Niña" conditions. Easterly winds will strengthen and dominate much of the tropical Pacific, with characteristic rainfall being light and of short duration. As a result, conditions in Samoa may be much drier than normal from April through August or September 1998. By October or November 1998, rainfall should be back to near-normal. Total rainfall for the year is expected to be 70-75% of normal. The following are our fourth quarter rainfall predictions for the Samoa region:

Inclusive Period        % of long-term average
                            Samoa Region
Oct-Dec 1997                    65%
Jan-Apr 1998                    80%
May-Sep 1998                    30%
Oct-Dec 1998                   100%
- source: UOG-WERI and PEAC


GUAM/CNMI: July-September rainfall for Tiyan was 147% of normal, while that for October was 84%. For the comparable periods at Andersen Air Force Base, the values were 107% and 90%, respectively. Record rainfall fell at Tiyan in August. The 38.49 inches of rain was primarily due to heavy monsoon surge activity. Typhoons Joan and Ivan also contributed to the October rainfall. To date in November, rainfall has exceeded the monthly average at both locations, primarily due to the passage of Typhoon Keith.

In Saipan, July-September rainfall for Capitol Hill was only 63% of normal, while that for October was 73%. For the same time periods at Saipan International Airport, the values were 103% and 114%, respectively. This is unusual in that Capitol Hill is normally wetter than the Airport. Typhoon Joan kept the CNMI relatively wet in October and Typhoon Keith made the first half of November wet in the Commonwealth.

With the strength of the current El Niño event, very dry conditions are still expected for Guam and the CNMI, beginning about now and extending into the early summer, as strengthening easterly winds follow the peak of this warm event. Since the middle of November, conditions have been extremely dry for Guam and the CNMI, and the conditions are expected to become even drier by January and continue that way until summer. A typhoon could develop in the Marshall Islands between now and mid-December and bring heavy rain to Guam and the CNMI, however, most activity has already shifted east of the date line, and conditions for further 1997 typhoon development are not looking favorable. Some shearlines could cause a day or two of light rain in the region during the winter and early spring, but most shearlines are expected to pass to the north and east. The outlooks for rainfall totals in the coming months are as follows:

Inclusive Period        % of long-term average
                         Guam          Saipan
Oct-Dec 1997              70%           70%
Jan-Jun 1998              25%           25%
Jul-Aug 1998              75%           75%
Oct-Dec 1998              95%           95%
- source : UOG-WERI


MICRONESIA (FSM): Because of the near-record strength of the current ENSO warm (El Niño) event, the equatorial westerly winds and the region of active rainfall and storm formation have migrated well east of the date line, and dry conditions have set in for both eastern and western FSM states. 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 to develop during spring and summer of 1998, and the accompanying strong easterly wind flow will further extend dry conditions in the FSM through the first half of 1998. As a result, very low levels of rainfall are expected for most parts of the FSM, and the resulting drought may be at near-record levels, possibly exceeding prior records on some islands. Summaries for individual FSM regions and outlooks for rainfall amounts are given in the following sections.


Yap & northern Chuuk State islands: Rainfall for July-September was 90% of the average at the Yap airport and 94% of normal at Ulithi Atoll. For October, rainfall was 87% of normal at the airport and 42% at Ulithi. At Woleai Atoll, July-September rainfall was 118% of normal and during October it was 97% of the norm. However, since the end of October, conditions have been considerably drier across Micronesia. While there is an outside chance that Yap could receive a day or two of heavy rain from a tropical cyclone until mid-December, it is not likely based on the extreme eastward displacement of convective activity beyond the Marshall Islands. We are revising our earlier November-December predictions to lower the expected amounts of rain. Based on the strength of the current El Niño event, we expect conditions from January through June to be very dry--comparable to or drier than 1983 values. Dry conditions could extend into July. Outlooks for rainfall totals in coming months are as follows:

Inclusive Period        % of long-term average
                     Yap             Outer Atolls:
                   Island      S. of 8°N      N. of 8°N
Oct-Dec 97           50%          40%            50%
Jan-May 98           25%          30%            25%
Jun-Aug 98           70%          80%            70%
Sep-Dec 98           95%          95%            95%	
- source : UOG-WERI


Chuuk & southern Yap State islands: Rainfall during July-September at Weno Island was 118% of normal, while that for October was only 62%. At Lukunoch, July-September rainfall was 157% of normal, but values dropped to 69% in October. To the west at Polowat, conditions have been much drier--July-September rainfall was 72% of normal, falling to 28% of normal in October, and reported rainfall in November was only 2% of normal! While there is an outside chance that a tropical cyclone could develop in the southern Marshall Islands, move west and bring some heavy rain to northern parts of Chuuk State, it appears that most activity has already shifted out to the east of the date line, so that such rainfall is not likely for this area. While some short episodes of equatorial convective activity could develop and move northward until the end of December, prolonged periods of rain are not expected. We are therefore revising our earlier predictions for the November-December period to reflect lower rainfall. Based on the strength of the current El Niño event, we expect conditions from January through May to be very dry--comparable to or drier than those which occurred in 1983. The drought conditions could also extend into June. Outlooks for rainfall totals in this area for coming months are as follows:

Inclusive Period        % of long-term average
                    Chuuk            Outer Atolls:
                   Lagoon       Southern       Northern
Oct-Dec 97           40%          40%            50%
Jan-May 98           30%          30%            25%
Jun-Aug 98           75%          80%            75%
Sep-Dec 98           95%          95%            95%	
- source : UOG-WERI


Pohnpei State: At Kolonia, July-September rainfall was 94% of normal, while that for October was 82%, and for November, 71%. Through October, rainfall was greater at Metalanim, at Paies-Kitti, and at higher elevations, but in November those locations received about the same amount of rainfall as Kolonia. At Pingalap to the east and Nukuoro to the south, October rainfall was about 80% of normal. However, Pingalap was considerably drier in November, with about 40% of normal rainfall (less than 4 inches), though Nukuoro appeared to fare better with over 10 inches for the month.

While there is an outside chance that a tropical cyclone could develop in the southern Marshall Islands, move west and bring some heavy rain to Pingalap, Mokil, and Pohnpei, it appears that most convective activity has already shifted to the east of the date line, and that tropical cyclone-induced rainfall is not likely for this area. We expect conditions to be very dry in Pohnpei State from late December through May-comparable to or drier than rainfall values of the 1983 dry period, and these dry conditions could extend into June. Outlooks for rainfall totals in Pohnpei State for coming months are as follows:

Inclusive Period        % of long-term average
                  Pohnpei            Outer Atolls:
                   Island       Eastern       Southern
Oct-Dec 97           55%          60%            55%
Jan-May 98           15%          15%            15%
Jun-Aug 98           80%          80%            85%
Sep-Dec 98           95%          95%            95%	
- source : UOG-WERI


Kosrae State: Rainfall at the Kosrae Airport for July-September was 104% of normal, while that for the same period at Utwa was 90% of normal. For October, the airport had 117% of normal rainfall and Tafunsak had 115% of the norm. However, since the end of October, conditions over Micronesia have become very dry. Kosrae SAWRS station recorded 6.41 inches on rainfall in November - less than half of normal for the month. While there is an outside chance that a tropical cyclone could develop in the southern Marshall Islands, move west and bring some heavy rain to Kosrae, it appears that most activity has already shifted to the east of the date line, and that tropical cyclone-induced rainfall is not likely. We expect conditions to be very dry in Kosrae from late December through April-comparable to or drier than rainfall values of the 1983 dry period. The dry period could run into May. Outlooks for rainfall totals in Kosrae for coming months are as follows:

Inclusive Period        % of long-term average
                            Kosrae Island
Oct-Nov 1997                    65%
Dec-Apr 1998                    15%
May-Aug 1998                    75%
Sep-Dec 1998                    95%
- source: UOG-WERI


MARSHALL ISLANDS (RMI) : At Majuro (representative of the southern RMI atolls), rainfall for July-September was 98% of normal, while in October it fell off to 80% of normal amounts. At Kwajalein (representative of northern atolls), rainfall from July-September was 117% of normal, while October values rose to 130% of average amounts. These heavy rainfall amounts came primarily from the development of typhoons Ivan, Joan, and Keith. However, starting in November, rainfall in the Marshall Islands has dropped off considerably. Rainfall at Kwajalien was 49% of normal and at Majuro, 62% of normal. Rainfall is expected to continue to be light across the Marshall Islands as most convective activity has now shifted east of the date line, in response to the very strong El Niño conditions. While there could be one or two more episodes of tropical cyclone development in the Marshall Islands, these would probably last only 2-3 days, and then clear skies will return. Also because of the strong El Niño conditions, some possibility of late season tropical cyclone activity still remains. There is a chance of a tropical cyclone developing east of the date line, attaining typhoon intensity, and moving westward through the Marshall Islands. We expect very dry conditions to affect the Marshall Islands from December 1997 into June 1998 for the southern atolls and from December 1997 into July 1998 for the northern atolls. Rainfall deficits are expected to match or possibly exceed those of 1983 dry period:

Inclusive Period        % of long-term average
                              RMI Atolls:
                        Southern      Northern
Oct-Dec 1997              70%           85%
Jan-Jun 1998              15%           15%
Jul-Aug 1998              80%           70%
Oct-Dec 1998              90%           85%
- source: UOG-WERI


PALAU: Koror rainfall totals continued to be well below normal. From April-June they were 74% of normal, from July-September they were 56% of normal, for October they were 83% and for November, 37% of normal. Farther south at Peleliu, July-September rainfall was 58% of normal. Peleliu's October rainfall was 49% of normal. Being located in the far western Pacific basin, the early onset of these dry conditions due to the very strong El Niño is typical for Palau as well as the Philippines. While some short episodes of convective activity could develop until the end of December, prolonged periods of rainfall are not expected. In fact, we are revising our earlier November-December predictions to reflect lesser amounts of expected rain. We expect January through May to be very dry--comparable to or perhaps even drier than 1983. Since the monsoon trough will be deflected to the north of the Philippines by the strong easterly wind flow expected to develop across Micronesia in spring, low rainfall could continue well into June:

Inclusive Period         % of long-term average
                        Koror,         Atolls
                      Babelthuap      S. of 6°N
Oct-Dec 1997              55%           50%
Jan-Jun 1998              25%           30%
Jul-Aug 1998              80%           85%
Oct-Dec 1998              95%           95%
- source: UOG-WERI


ACKNOWLEDGEMENTS and FURTHER INFORMATION


The information contained in the LOCAL VARIABILITY SUMMARIES section and elsewhere in this issue of the Pacific ENSO Update has been drawn from many sources. Further information may be obtained by contacting your local National Weather Service office, or the individuals and institutions listed below:

NOAA National Weather Service - National Centers for Environmental
Prediction (NCEP) - CLIMATE PREDICTION CENTER (CPC):
World Weather Building, Washington D.C. 20233.
Contact CPC at 301-763-8155 for more information on the ENSO Advisory, the Long-Lead Outlook for the Hawaiian Islands, and other publications discussed in this bulletin.

NOAA National Weather Service - Pacific Region
WEATHER SERVICE FORECAST OFFICE (WSFO)
University of Hawaii - Manoa Campus
HIG #225, 2525 Correa Road, Honolulu, Hawaii 96822
Contact the WSFO at 808-973-5270 for more information on NWS-PR sources of climate information.

University of Guam (UOG) WATER AND ENERGY RESEARCH INSTITUTE (WERI):
Lower campus, University of Guam
UOG Station, Mangilao, Guam 96923
Contact C. Guard or M. Lander at (671)735-2685 for more info on tropical cyclones and climate in the Pacific Islands.

University of Hawaii (UH) School of Ocean and Earth Science and Technology
(SOEST) DEPARTMENT OF METEOROLOGY:
HIG #331, 2525 Correa Road, Honolulu, Hawaii 96822
Contact Dr. T. Schroeder at 808-956-7476 for more information on hurricanes and climate in Hawaii.

PACIFIC EL NINO-SOUTHERN OSCILLATION (ENSO) APPLICATIONS CENTER:
HIG #331, 2525 Correa Road, Honolulu, Hawaii 96822
Contact R. Tanabe at 808-956-7110 for more information on ENSO-related climate data for the Pacific Islands.
Contact A. Hilton at 808-956-2324 for more information on Pacific ENSO Update and applications.


APPENDICES:

SPECIAL SECTION: Experimental Forecasts for Pacific Island Rainfall using data through October, 1997.

Summary of 1997 hurricane season in the central tropical North Pacific, from the National Weather Service-Pacific Region, Honolulu WFO.

"El Niño predicted to cause more frequent tropical cyclones in the South Pacific", press release dated 27 November 1997 from the National Institute for Water and Atmospheric Research Ltd. (NIWA) in New Zealand.

Long-Lead Outlook for Hawaiian Islands issue dated 13 November 1997 from NOAA's Climate Prediction Center (CPC).

ENSO ADVISORY of November 10, 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
E-mail: hilton@soest.hawaii.edu

Publication of the Pacific ENSO Update is funded in part
by Grant Number NA46GP0410 from the National Oceanic
and Atmospheric Administration (NOAA) Office of Global
Programs. The views expressed herein are those of the
author(s) and do not necessarily reflect the views of NOAA
or any of its sub-agencies.


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