Pacific ENSO Update

1st Quarter 1999 - Vol. 5 No. 1



SSTs point toward drier than normal winter and early spring rainfall conditions in near-equatorial regions east of 160° East.

By July 1998, the equatorial sea surface temperatures (SSTs) in the central Pacific indicated that weak La Niña (cold episode) conditions had developed. The rapidness of the cooling suggested that a strong La Niña event could develop. To date equatorial SSTs have continued to fall much more slowly, and in January 1999 the temperatures had become 1.5-2°C (2.7-3.6°F) cooler than normal between 120° West and 170° West. This indicates a moderate La Niña event and agrees with our predictions in the last two Newsletters. The equatorial cold tongue associated with the La Niña now extends from the west coast of South America westward to about 160° East. Some climate models are predicting La Niña conditions to peak in early 1999, while others suggest that the La Niña will not peak until the August-October 1999 timeframe. Some models also predict that cooler than normal equatorial SSTs will continue into 2000, while others suggest that SSTs will return to near normal values in late 1999. We expect to see a peak around February-March of 1999 and a return to near normal conditions by the last quarter of the year. The colder than normal equatorial SSTs should maintain stronger than normal easterly low level equatorial winds and drier than normal atmospheric conditions along the equatorial belt east of 160° East. However, conditions become wet only 200 miles north and south of the dry equatorial zone.


The SOI points to near normal to above normal rainfall for most of Micronesia, but less than normal rainfall will prevail during winter and early spring in the Marshall Islands and in near-equatorial regions. Tropical cyclone activity will return to normal as the La Niña weakens and allows tropical cyclone development to occur in central and eastern parts of Micronesia.

The Southern Oscillation Index (SOI) began to rise and became positive in May of 1998. It has hovered between +1 and + 1.5 since July 1988, but it jumped to near +2.0 in January 1999. It will likely not rise much more, and will begin to return to more normal values by September. This behavior of the SOI supports moderate La Niña conditions through the Northern Hemisphere spring. The higher than normal SOI indicates higher than normal pressures over Tahiti and continued lower than normal pressures over northern Australia, reflecting the very active Australian monsoon. The monsoon has, in fact, been very active from the South China Sea to northwestern Australia and across the southwest Pacific to the Solamon Islands and Vanauatu. Episodes of monsoon surges have extended to Fiji, bringing some periods of heavy rains to the parched island nation. La Niña conditions have shifted the upward branch of the Walker Circulation (the primary east-west tropical circulation pattern) to the west, bringing heavy rains to Indonesia, the southern South China Sea, the southern Philippines, western Micronesia, and northern Australia. While good rains have returned to the Samoa region, the region will likely see slightly below normal rainfall as the South Pacific Convergence Zone (SPCZ) is frequently anchored to the west by the strong Australian monsoon.


Sea level heights should rise slightly in the western Pacific as water temperatures warm to deeper levels and increase the volume of basin water. Sea levels should be lower than normal in the central and eastern equatorial Pacific where La Niña waters are colder than normal, the thermocline is shallow, and water volume is reduced. Sea level heights should return to near normal values throughout the Pacific basin by early 2000. These heights can be temporarily and locally modulated by stronger than normal trade winds from the east, from monsoon surges from the west, and from tropical cyclones.


Tropical cyclone activity is expected to be heavy from northwest Australia and across northern Australia eastward into the Southwest Pacific. Significant activity is expected to remain west of the international date line. A rare February tropical storm (TS Iris) spun up in central Micronesia, as westerly winds from the Australian monsoon spread northward across the equator. Since 1970, there have been two tropical storms, two typhoons, and one tropical depression in the western North Pacific during February.

For nearly four days as Iris tracked westward from south of Guam to the north of Palau, cloud clusters associated with the tropical cyclone and cloud bands in its outer circulation contributed the major part of near-record February rains across the Mariana Islands.

Northern-Southern Hemisphere twin cyclones could form in April or May, affecting central and western Micronesia and northern Australia. By summer, the Northern Hemisphere monsoon is expected to become active, bringing episodes of southwesterly wind and heavier than normal rains to most of Micronesia. In 1999, tropical cyclone activity should return to normal in terms of numbers and locations of development.

Mahalo Nui Loa to the Environmental Verification and Analysis Center

On behalf Chip Guard of the University of Guam, Tom Schroeder, Cheryl Anderson, and Ray Tanabe of the University of Hawaii and Jerry Norris of the Pacific Basin Development Council, I would like to thank Mark Morrissey, Andrew Wood, Brad McGavock, and their staff at the Environmental Verification and Analysis Center (EVAC) for serving as editor of the Pacific ENSO Update. Mark assumed the role as guest editor when Alan Hilton, the former editor of the update, reported in April 1998 for an extended assignment as ship captain for the NOAA Corps.

The EVAC folks edited and produced three issues of the newsletter and did an outstanding job. Production of the Pacific ENSO Update occurred in addition to their center project, Schools of the Pacific Rainfall Climate Experiment (SPaRCE), which educates students in climate information throughout the Pacific. Details of EVAC's ongoing projects appeared in the September 1998 newsletter of the Pacific ENSO Update. EVAC's letter SPaRCE can be read at their website < >. We sincerely thank all of them for their assistance. The Pacific ENSO Applications Center hopes to continue to work cooperatively with EVAC in furthering education and information dissemination on climate variability.

Ray Tanabe has taken over as editor as of this issue and we congratulate and thank him for his expanded role in the Pacific ENSO Application Center.

Mahalo for all the effort supporting a better understanding of ENSO in the Pacific region!

- Mike Hamnett Michael P. Hamnett
Social Science Research Institute

A Short Assessment of the Pacific ENSO Applications Center Rainfall Predictions For Micronesia and American Samoa During the 1997-98 Drought

Figure 1 compares the predicted, the observed, and the normal rainfall values in inches at the major Micronesian Islands and at Pago Pago, American Samoa during the 1997-1998 El Niño-induced drought. Figure 2 shows the predicted and observed rainfall in terms of percent of normal rainfall. Overall, for the 12-month period, the predictions were very accurate. However, at certain locations, there were some problems in predicting the onset of the drought and in anticipating the return of rains that broke the drought. For example, the onset of the drought was predicted a few weeks too early for Guam, Saipan, Palau, and Yap. And, the drought onset was predicted to start a few weeks too late for Kosrae and American Samoa. It was even more difficult to predict the reestablisment of the rains. For example, the drought was forecast to end several weeks too early for Guam, Saipan, and American Samoa, and several weeks too late for Yap, Palau, and the Marshall Islands. Part of the problem in determining the end of the drought is that there is no real definition of the “end”. Defining the “end of the drought” is an area that requires a great deal of work ince different locations will likely have different criteria for the definition. A forthcoming WERI Technical Report will break down the 1997-1998 rainfall predictions in more detail.

NOAA's Climate Prediction Center in Washington D.C. routinely monitors the latest forecast results from several ENSO models. The PROGNOSTIC DISCUSSION FOR LONG-LEAD OUTLOOKS outlines the these results.


Hawaii: State: The trade winds, strong at times, marked the first ten days of the month across the Hawaiian Islands. These trades brought occasional periods of heavy showers over the windward side of the island of Hawaii, but no damage was reported. Rainfall totals for the rest of the state were relatively modest.

The initial period of trade wind weather was followed by the passage of a weak cold front across the island chain on 11 and 12 February. This system was accompanied by rainfall totals of less than 0.50 inches at most locations across the state and a few reports of 1.00 to 1.50 inches from the windward side of the island of Hawaii. The trailing high pressure ridge settled in just to the north of the state bringing stable weather conditions with land and sea breezes through 18 February.

On 19 February, a weak shear line pushed across the state and lined itself up with the island of Hawaii the following day. An upper level shortwave trough then dropped in from the northwest and became superposed over the shear line. This combination resulted in an unstable airmass over the eastern end of the island chain and helped trigger heavy showers and thunderstorms on the morning of 20 February. Urban and small stream flood advisories were subsequently issued for the eastern districts of the island of Hawaii. Showers and thunderstorms continued to develop through the day, which prompted the issuance of a flash flood watch for the windward districts of the island. The watch was later expanded to cover the entire island of Hawaii and all of Maui. The axis of the upper level trough moved east of the island chain during the early morning hours of 22 February and the subsiding air to the west of the trough stabilized the airmass. As much as 13 inches of rain fell over the windward side of the island of Hawaii during the three-day event leaving some low-lying areas flooded, though no major damage was reported.

Minimal rainfall was observed statewide through 25 February where light to moderate trade winds were accompanied by stable conditions aloft. A weak shear line stalled over the island of Kauai on 26 February and dropped up to 1.50 inches on the windward slopes of the island. The rest of the state remained dry through the end of the month.

For a more complete summary, and the county by county wrap-ups, please see the February 1999 Precipication Summary from the National Weather Service Honolulu Weather Forecast Office.

American Samoa: Although rains returned to the islands after several months of severe drought conditions, they were less than expected. We are anticipating a continued increase in rainfall, but values will likely be slightly drier than normal for the Samoa region.

In the last Pacific ENSO Update, we anticipated rainfall amounts to reach near normal or slightly above normal values from November through February as the South Pacific Convergence Zone (SPCZ) became anchored over or near the islands. While the SPCZ has increased rainfall in the region, the very strong Australian monsoon has frequently anchored the SPCZ west of the islands. This has reduced rainfall amounts below the levels predicted in the last Newsletter for November through February. In fact, November was the hottest November ever recorded at Pago Pago. Pago Pago rainfall values for October were 4.30 inches (64%) and for November were 3.22 (30%). Amounts increased in December to 10.46 inches (72%) and in January to 11.36 inches (90%), still well-below the values we anticipated in our last Newsletter. The moderate La Niña event will likely keep the remainder of the rainy season and the upcoming dry season somewhat drier than normal. However, we expect the following rainy season to be near normal. Rainfall predictions from the Climate Prediction Center support this line of thinking as well. As anticipated, tropical cyclone activity in the Southern Hemisphere moved west of the international date line, reducing the threat to the Samoa region. Early season activity was somewhat closer to the date line than expected, affecting Fiji.

The rainfall predictions for American Samoa and the region are:

Inclusive Period         % of long-term average
                              Samoa Region
Feb-Oct 99                         85%
Nov 99 - Mar 2000                 100%

- sources: UOG-WERI and PEAC

Guam/CNMI: With an active monsoon expected across the Northwest Pacific, we expect Guam and the CNMI to be wetter than normal in the summer of 1999.

The last quarter of 1998 was significantly drier than expected, especially in the CNMI. In October, November, and December, rainfall at Guam International Airport (GIA) was 9.27 (77%), 6.79 inches (83%), and 3.94 inches (73%) respectively, while that at AAFB was 8.02 inches (62%), 6.37 inches (70%), and 4.18 inches (70%). At Saipan International Airport (SIA), the respective values for October, November, and December were 4.68 inches (43%), 2.95 inches (51%), and 1.21 inches (31%). Capitol Hill was wetter with 6.52 inches (54%), 5.24 (72%), and 2.86 inches (60%), respectively. Tinian rainfall for the 3-month period was wetter than SIA but likely drier than Capitol Hill. The NASA rain gage network located at the Rota Resort and Country Club on Rota and the Rota Airport showed October amounts of around 5.7 inches (44%), 4.8 inches (53%) and 2.8 inches (47%). The Sabana area was probably a little wetter than the Airport and Golf Resort areas, but not significantly so.

Rainfall picked up in the first quarter of 1999 throughout the region, especially in February when a rare tropical cyclone dropped more than 10 inches of rain in 3-4 days. January rainfall was highly variable on Guam, with 4.70 inches (106%) at GIA and 3.46 inches (61%) at Andersen AFB. Some tropical cyclone activity in April or May should render the spring wetter than normal. We expect a very active monsoon across the western Pacific from July through September, and this should bring significantly wetter than normal conditions to the Mariana Islands. Rainfall will become slightly wetter than normal to normal in the last quarter of 1999.

Tropical cyclone activity could affect the Mariana Islands as early as March, but it is more likely in April or May. The greatest threat of tropical cyclone activity will occur from September to mid-December.

Our rainfall predictions through March 2000 for Guam/Rota and Saipan/Tinian are:

Inclusive Period         % of long-term average
                      Guam/Rota       Saipan/Tinian
Jan-Mar 99              130%              120%
Apr-Sep 99              120%              120%
Oct 99 - Mar 2000       100%              100%

- sources: UOG-WERI

Federated States of Micronesia: We anticipate somewhat wetter than normal conditions for most of the FSM. Near-equatorial regions east of 160° East will be drier than normal through the winter and early spring of 1999.

Rainfall over most of the FSM was above normal for the period October 1998-January 1999. This was largely the result of westward-propagating tropical disturbances developing in the trade wind convergence zone near 5-8° North. In southwestern portions of the FSM, heavy rains occasionally spread northward from the active Australian monsoon that extended from the South China Sea, southeastward across northern Australia and into the Solamon Islands. A portion of the western FSM, extending from Woleai through Polowat, was about 15-20% drier than normal during the period. This dryness extended northward into Yap and Ulithi during November and December. Easterly equatorial winds brought dry conditions to Nauru and equatorial areas east of 160°East.

Yap State: October, November, and December rainfall at the Yap Airport was 16.11 inches (135%), 7.95 inches (88%), and 17.65 inches (85%). Over the 3-month period, both Ulithi Atoll and Woleai Atoll had about 20-25% less rainfall than Yap Island. In January, rainfall rose to near normal amounts at Yap (7.65 inches--104%). Woleai and Ulithi had somewhat below normal amounts with 10.04 inches (94%) and 5.97 inches (90%), respectively. During February, rainfall increased over the western FSM as the periphery of Tropical Storm Iris dropped several inches of rain on Yap. Rainfall is expected to be above average through spring. By summer, rainfall will become near normal to slightly below normal through the summer as the normal monsoon activity moves well to the north. During fall and winter, rainfall is expected to approach more normal amounts. Yap State islands could be subjected to tropical storms and typhoons in April and May, with a larger threat from September until mid-December.

Rainfall for Yap and its atolls is expected to be:

Inclusive Period                % of long-term average
                             Yap               Outer Atolls:
                            Island       S.of 8·N     N. of 8·N
Jan-Jun 99                   110%          105%         105%
Jul-Sep 99                   100%           95%         100%
Oct 99-Mar 2000              100%          100%         100%

- sources: UOG-WERI

Chuuk State: October, November, and December rainfall at Weno Island 12.62 inches (94%), 15.02 inches (145%), and 13.83 inches (128%), respectively. At Lukunoch, October-December rainfall averaged about 12.7 inches (114%). These amounts were considerably higher than anticipated in our last predictions. Conditions were significantly drier at Polowat where October-December rainfall averaged 6.33 inches (64%), but this amount was less than we predicted. In January, Weno Island saw 13.79 inches of rain (129%), Lukunoch observerd 16.83 inches (158%), and Polowat measured 7.71 (96%). These were above the values anticipated in our last Newsletter. The Chuuk Islands are expected to see near to above normal rainfall through spring. By summer, rainfall will then become slightly below normal through the summer as the normal monsoon activity moves well to the north. During fall and winter, rainfall is expected to approach more normal amounts. Although tropical cyclone activity was much reduced in 1998, Chuuk State islands could be subjected to a tropical storm or typhoon in April or May and again from October-mid December.

Predicted rainfall for Chuuk State is as follows:

Inclusive Period               % of long-term average
                        Chuuk         Outer Atolls
                        Lagoon    Southern    Northern
Jan-Jun 99              110%       105%        100%
Jul-Sep 99               95%        95%         90%
Oct 99-Mar 2000         100%       100%        100%

- sources: UOG-WERI

Pohnpei State: October, November, and December rainfall values were14.81 inches (89%), 17.35 inches (110%), and 19.91 (131%). For Pingalap, rainfall averaged 14 inches (88%) over the period, and for Nukuoro rainfall averaged 11 inches (93%). Most of Pohnpei State's rainfall came tropical disturbances that developed in the trade wind convergence zone that meandered between 4° North and 8° North. Nukuoro rainfall was higher than expected as the strong Australian monsoon occasionally replaced the equatorial easterlies with moist northwesterly winds and intermittant heavy rains. In fact, Kapingamarangi likely received some relief from the monsoonal rains. In January, Pohnpei had 18.22 inches of rain (139%), Pingelap measured 18.33 inches (136%), and Nukuoro was drenched with 36.63 inches (312%). Wetter than normal conditions are expected to continue until late spring in all but near-equatorial locations. By summer, rainfall will then become slightly below normal through the summer as the normal monsoon activity moves well to the north. During fall and winter, rainfall is expected to approach more normal amounts.

Rainfall predictions for Pohnpei State are as follows:

Inclusive Period          % of long-term average
                     Pohnpei           Outer Atolls:
                     Island    Eastern    Southern    Equatorial
Jan-May 99            125%      120%        140%        165%
Jun-Sep 99             90%       90%         90%         90% 
Oct 99-Mar 2000       100%      100%        100%        100%

- sources: UOG-WERI

Kosrae State: October (7.75 inches -- 48%) and November (8.36 – 53%) rainfall at the Kosrae Airport was dry as expected due to increased equatorial easterly winds resulting from the current La Niña event. Rainfall amounts at Tafunsak (57%) and Utwa (58%) were slightly greater during the period. However, rainfall amounts increased earlier than expected, as tropical disturbances developed in the trade wind convergence zone closer to the equator than expected. December amounts at the Airport increased to 23.41 inches (161%) and January amounts were even higher at 29.76 inches (207%). At Tafunsak and Utwa, December values were 18.88 inches (130%) and 24.54 inches (169%), respectively, and January values were 26.56 inches (185%) and 32.16 inches (223%), respectively. Rainfall is expected to remain above normal through the spring, but will then become below normal through the summer as the normal monsoon activity moves well to the north. During fall and winter, rainfall is expected to approach more normal amounts. Kosrae has a low risk of tropical cyclone activity, especially in non-El Niño years.

The anticipated rainfall in Kosrae for the next year is as follows:

Inclusive Period     % of long-term average
Dec 98-Mar 99                160% 
Apr-Jun 99                   130%
Jul-Oct 99                    90%
Nov 99-Mar 2000              100%

- sources: UOG-WERI

Palau: Palau can expect much above normal rainfall until summer and then above normal rainfall for the remainder of the year.

Rainfall at Koror in October, November and December was 19.05 inches (137%), 17.43 inches (154%), and 13.57 inches (113%), respectively. At Peleliu, October-December rainfall averaged 15.46 inches (120%). January 1999 rainfall at Koror and Peleliu was even greater at 24.83 inches (232%) and 24.69 inches (231%), respectively. The northern atolls (Kayangel, etc.) were slightly drier than Koror, and the southern atolls (Tobi, etc.) were likely similar to Koror. The heavy rainfall for Palau was the result of the combination of westward moving tropical disturbances that developed in the trade wind convergence zone and the active Australian monsoon that occasionally spread northward into the southern Palau region. Tropical Storm Iris passed north of Palau in February 1999, keeping regional rainfall way above normal. Rainfall is expected to remain above or near normal through spring. Then rainfall will become slightly below normal through the summer as the normal monsoon activity moves well to the north. During fall and winter, rainfall is expected to approach more normal amounts. Palau could experience a tropical cyclone in April or May and again from October-December 1999.

The expected rainfall for the next 12 months for Palau is:

Inclusive Period              % of long-term average
                    Koror and           Outer Atolls
                    Babelthaup     S. of 8ºN    N. of 8ºN
Jan-Mar 99            160%            130%        150%
Apr-Jun 99            130%            115%        130%
Jul-Oct 99             95%             95%         90%
Nov 99-Mar 2000       100%            100%        100%

- sources: UOG-WERI

Marshall Islands: Despite a relatively wet summer and fall in the Marshall Islands, the development of a moderate La Niña is expected to reduce winter and early spring rains. At Majuro (representative of southern atolls), rainfall for October, November, and December was 14.45 inches (104%), 13.57 (106%), and 11.48 (97%), respectively. At Kwajalein (representative of northern atolls) rainfall for the last three months of the year was 14.69 inches (123%), 7.95 inches (75%), and 5.85 inches (72%). In January, Majuro and Kwajalein had rainfall amounts of 7.23 inches (86%) and 4.55 (100%). Enhanced easterly winds associated with the moderate La Niña is expected to create drier than normal conditions in the Marshalls until about April-May 1999. The increased monsoon activity expected across the western North Pacific during summer is not likely to increase rainfall in the Marshall Islands.

Tropical cyclone activity could be a threat to the Marshall Islands in October and November 1999.

We anticipate the following rainfall amounts for the Marshall Islands:

Inclusive Period           % of long-term average
                                  RMI Atolls
                            Southern     Northern
Jan-Apr 99                     80%          75%
May-Sep 99                     95%          90%
Oct 99-Mar 2000               100%         100%

- sources: UOG-WERI


Long-Lead Outlook for Hawaiian Islands issue dated 17 December 1998, from NOAA's Climate Prediction Center (CPC).

ENSO ADVISORY of December 11 ,1998

SPECIAL SECTION - Experimental Forecasts for Pacific Island Rainfall


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
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
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.

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
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.

HIG #331, 2525 Correa Road, Honolulu, Hawaii 96822
Contact R. Tanabe at 808-956-2324 for more information on ENSO-related climate data for the Pacific Islands.
Contact A. Wood at 405-447-8412 for more information about this issue of the Pacific ENSO Update

For further information, please contact:

Raymond Tanabe
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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