Pacific ENSO Update

2nd Quarter 1998 - Vol.4 No.2


While the current El Niño has been the most intense on record, data now suggest that it is winding down. There has been a considerable reduction of eastern and central equatorial Pacific sea surface temperature (SST) anomalies since the last ENSO Pacific Update. In December and January, the SST anomalies in the eastern equatorial Pacific peaked at over 9°F (5°C) warmer than normal. In April, the anomalies had fallen to about 5°F (3°C), and they are expected to continue to fall to near normal values over the next several months. In addition, the area covered by the warmer than normal ocean temperatures has become significantly smaller in size. The next concern is whether or not the equatorial SSTs in the eastern and central Pacific will return to normal and hover there, or whether they will become cooler than normal, creating La Niña conditions (a cold event). Statistical climate models suggest that eastern Pacific SSTs will become colder than normal by the July time frame, while purely dynamic climate models are divided between a return to normal temperatures and cooler than normal SST conditions. We are now leaning toward the La Niña (cold event) scenario, and this will be reflected in our subsequent predictions.

For Micronesia, the SST conditions point toward an extended and very dry dry-season in 1998, a shortened rainy season in 1998, and a slightly drier than normal dry-season for 1999. For American Samoa, SST conditions point toward a very dry dry-season for 1998, a wetter than normal wet season in 1998-99, and a slightly drier than normal dry-season in 1999.

From January through March, the Southern Oscillation Index (SOI) averaged more than 3 units below normal (-3), with a minimum value of -3.4 in March. The low SOI values have been primarily a result of higher than normal pressures in northern Australia region (Darwin) and lower than normal pressures around French Polynesia (Tahiti) due to the tropical cyclone activity. As anticipated, the South Pacific Convergence Zone (SPCZ) shifted well east of the date line to the region of Samoa and Tahiti, producing considerable cyclone activity in January (Cyclones Tui, Ursula, Veli, and Wes). The Samoa region saw a significant increase in rainfall in January as tropical disturbances and cyclones developed in the region. TC Ron ran over Swains Island and TC Tui passed near Tutuila. It is of value to note that the Southern Oscillation, when measured by pressure anomalies along the equator (between the Galapagos and New Guinea or Indonesia), was the strongest ever seen. This kept the monsoon trough anchored closer to the equator than expected, keeping most of the February, March, and April cyclone activity equatorward and east of Samoa. We anticipate that the SOI will begin to rise sharply in May, and will become slightly positive by year's end as La Niña conditions develop. When the SOI begins to rise sharply, equatorial easterly winds and trade winds will strengthen. This will reinforce dry conditions over Micronesia and American Samoa. The positive SOI will bring very dry conditions to equatorial regions such as Nauru and Kapingamarangi Atoll, and will likely make 1999 dry seasons in Micronesia and Samoa slightly drier than normal.

SOI conditions, in general, point toward both a very dry and an extended dry-season over Micronesia, and indicate a much drier than normal rainy-season and dry-season for American Samoa.

Early season tropical cyclone activity in the western North Pacific is expected to be forced westward to the South China Sea, and should not affect Micronesia. Tropical cyclone activity for the western North Pacific islands is not likely until September or October. La Niña conditions could also restrict early tropical cyclone activity in 1999 to areas west of Micronesia. In the Southern Hemisphere, tropical cyclone activity will shift to regions west of the date line, and American Samoa will not likely be significantly affected until the next El Niño event.

Low level winds across Micronesia are from the east, while upper level winds are from the west all the way to the equator. This highly unusual upper level wind profile is creating tremendous vertical wind shear across the region. This will prevent tropical storm development. While the strong upper level winds will help create some areas of strong thunderstorms and heavy rain, the shear will make the shower activity relatively short-lived. This wind regime will also subject most of the area to strong subsidence (downward motion that prevents heavy rain development), keeping most of Micronesia dry. This will likely last until early to mid-June.

Greetings from the Environmental Verification and Analysis Center

EVACThe current issue of the Pacific ENSO Update was developed by Mark Morrissey, Andrew Wood, and their staff at the the Environmental Verificationand Analysis Center (EVAC). Mark assumed the role as guest editor when Alan Hilton, the regular editor of the update, reported in April for an extended assignment onboard the NOAA ship Ron Brown. Alan and Ray Tanabe at PEAC have been very helpful during production of the newsletter. The EVAC staff is very grateful to have the opportunity to help PEAC produce the update. Please take the time to find out more about our research center.

Update on Southern Hemisphere Tropical Cyclones

In the previous issue (1st Quarter, 1998) we suggested that tropical cyclone activity would persist east of the dateline. This has been bourne out. A tropical depression developed near the northern Cook Islands on April 21 and intensified into Tropical Cyclone Alan. Alan eventually struck Tahiti and Raiatea in the Society Islands. At least 10 people died, 26 were seriously injured and 750 buildings destroyed. the damages resulted from a mixture of high winds, heavy rains, and landslides.

As we write this report (May 15) the southern hemisphere season is finally near an end. It has been a rough one for the Cooks and French Polynesia.


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.

The consensus of current model predictions indicate that a cooling trend in the sea surface temperatures (SSTs) is likely over the coming months. There is less of a consensus, however, on the extent of this coolind. The predicted cooling trend in SSTs, which is noticeably stronger that in previous forecasts, suggests the current El Nino ending between now and September and the development of a cold event by the end of the year. The LOCAL VARIABILITY SUMMARIES in the following section provide the outlooks based on this forecast.


Hawaii: improving in some areas. The National Weather Service automated rain gauge network maintained throughout Hawaii, along with the four main airport weather stations, represent a total of 73 stations statewide, and during April, most of these gauges recorded below-normal rainfall. 62 of the 73 gauges recorded less than normal rainfall. Windward sides of all the islands and some of the central portions of the islands saw significant increases in the amount of rainfall. For example, Hanapepe and Kamuela both experienced above average rainfall in April with 2.84 inches (129% of normal) and 14.26 inches (207% of normal), respectively. Although many locations were still below average, any rainfall helps relieve the extremely dry conditions.

The increase in rainfall had many causes. The weather patterns across the central Pacific made a transition toward spring like conditions beginning in March and continued into April. The persistent high pressure ridge located over the islands during the winter months moved north of the islands during April. Movement of the ridge northward led to the development of a more seasonal trade wind pattern near the islands. All windward stations had a significant increase in rainfall during April compared to previous months, even though rainfall totals were still below normal at most locations. Nearly half of the rain that fell over the last three month in many windward locations (such as Hana, Hanalei, and Hilo) fell during April. These trade wind showers should continue into June for windward locations, providing more relief for those drought stricken areas. However, rainfall totals are still on a trend for below normal levels overall.

Even though conditions in other island regions are improving, the drought conditions on the leeward sides of the islands should continue into summer now that the climatological wet season has come to a close.

American Samoa: We are expecting near record drought conditions for American Samoa. As anticipated in the last Pacific ENSO Update, rainfall for the first 4 months of 1998 was highly variable. January rainfall was 10.64 inches (85% of normal), February had 2.02 inches or 16%, March 12.70 inches (113% of normal), and April rainfall dipped to 0.74 inches (6%). The strong low level equatorial westerly winds in the eastern Pacific often kept the South Pacific Convergence Zone (SPCZ) north and east of Samoa, giving the Samoa region extended periods of dry weather. After April, conditions will be very dry as trade winds strengthen over the region. This will extend the dry season and also make it significantly drier than normal. Wet weather is not expected to return to the Samoan Islands until around October. Historically, La Niña has produced wetter than normal wet season followed by a slightly drier than normal dry season over Samoa. This is reflected in our rainfall predictions for American Samoa and the region, which follow:

Inclusive Period       % of long-term average                      

                          Samoa Region 
May-Aug 98                     20%
Sep-Oct 98                     80%
Nov-Feb 98                    110%
Mar-Jun 99                     90%

- sources: NWS-PR and PEAC

Guam/CNMI: We are expecting near record drought conditions for Guam and the CNMI. January rainfall at Guam International Airport was 1.99 inches (45% of normal values), February was 1.22 inches (33%), March was 0.97 inches (33%), and April was 1.37 inches(35%). Conditions at Andersen Air Force Base were similar. This rain came mostly from shearlines and trade wind showers that transited the area. Guam has had over 800 ENSO-related grass and forest fires. For January and February, Saipan International Airport had 1.00 inch (25% of normal) and 1.10 inches (37% of normal), respectively. March and April rainfall in Saipan has been about 2 inches or 70% in March and 55%, respectively, in April. Rota received only about 30-35% of its normal rain. Tinian was slightly wetter than Saipan in January and February, and similar in March and April. Our rainfall predictions through June 1999 for Guam and the CNMI are as follows:

Inclusive Period        % of long-term average

                          Guam            CNMI
May-Jun 98                 25%             30%
Jul-Sep 98                 60%             55%
Oct-Feb 98                 90%             90%
Mar-Jun 99                 85%             85%

- source: UOG-WERI

Micronesia (FSM): We anticipate a record drought for all of the FSM. For the first 4 months of 1998, rainfall amounts were very low for the FSM states. Drought conditions are very serious in most of the FSM, and the US President declared the FSM a "disaster area", making it eligible for US aid. Water on many atolls is too salty to drink, and shipments of fresh water is occurring.

Thw atypical upper level wind profile over Micronesia will cause some areas of thunderstorms and heavy rain, although the strong vertical wind shear will make the shower activity relatively short lived. This wind regime will also subject most of the area to strong subsidence (downward motion that prevents heavy rain development), keeping most of Micronesia dry. this will likely last until early to mid-June.

The area of below normal sea level heights that has been prominent over Micronesia during the past 6-8 months has moved south and east of Micronesia and has decreased in size. As a result of that movement and the strong and persistent trade winds affecting Micronesia over the last 2 months, sea level heights around the islands have risen to levels slightly above normal. Higher than normal sea levels in the eastern Pacific have migrated northward toward the west coast of Central American and southern Mexico. Sea level heights are expected to become lower than normal in the Samoa region for month or two as sub-surface SSTs cool for a while there.

Yap State: Yap, like Guam and the CNMI, had some January and February rainfall from passing shearlines and trade wind showers. As a result, at the Yap airport, rainfall for January it was 4.45 inches or 61% of the average. However, rainfall in the next 3 months was considerably lower: February had only 1.34 inches (22%), March only 0.54 inches (4%), and April only 0.21 inches (3%). March and April rainfall for Ulithi Atoll were 0.72 inches (12%) and 0.44 inches (7%), respectively, while March and April rain at Woleai Atoll was 0.54 inches (6%) and 1.41 inches (11%). Water rationing has been imposed in Colonia, and significant rain will likely not return to Yap and the northern Yap State islands until mid to late June. The southern Yap State islands will begin to see some episodes of heavy rainfall in May, but will not get frequent rain until June. La Ni?a conditions could also make the southern atolls slightly drier than normal from November 1998-March 1999. 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
May-Jun 98                   25%         35%         25%
Jul-Sep 98                   70%         75%         70%
Oct-Feb 98                   85%         90%         85%
Mar-Jun 99                   85%         85%         85%

- source: UOG-WERI

Chuuk State: In January and February, Weno Island received only 1.25 inches (12%) of rainfall and 1.74 inches (28%), respectively. In March and April, the values at Weno were 0.78 inches (9%) and 1.13 inches (9%), respectively. January-April rainfall at Lukunoch averaged 1.37 inches(17%) and at Polowat averaged 0.8 inches (7%). Drought conditions are expected to continue into June for the southern islands and into July for the northern islands. However, there will be episodes of heavy rain, and residents should be prepared to fill their catchment tanks. La Ni?a could also make the southern atolls slightly drier than normal from November 1998-March 1999. Predicted rainfall for Chuuk Lagoon and outer atolls is as follows:

Inclusive Period        % of long-term average

			 Chuuk       Outer Atolls
                         Lagoon   Southern   Northern
May-Jun 98               25%       35%        25%
Jul-Sep 98               70%       75%        70%
Oct-Dec 98               90%       90%        85%
Jan-Jun 99               85%       85%        85%

- source: UOG-WERI

Pohnpei State: In January and February, Kolonia received only 0.64 inches (5% of its normal) and was 1.98 inches (18%), respectively. In March and April, the rainfall values were 2.95 (22%) and 4.96 (30%), respectively. For the first 3 months of 1998, Pingalap averaged only 0.75 inches (6% of normal) and Nukuoro averaged 1.30 inches (10% of normal). In April, rainfall increased to 3.21 inches (20%) in Pingalap and to 7.14 inches (42%) in Nukuoro. As a result of the low rainfall in Pohnpei, water rationing was imposed in Kolonia, most streams dried up, and rivers were running at very low levels. The increased rainfall in late April and early May provided some relief, and has reduced the fire potential. Water is being transported to the atolls of Pohnpei State. While observations have not been received from Kapingamarangi Atoll, it is believed to be very dry there. With the predicted La Ni?a, we anticipate that rainfall amounts are likely to remain well below normal until the summer of 1999. La Ni?a could also make the southern atolls slightly drier than normal from November 1998-March 1999. Rainfall estimates for Pohnpei State are as follows:

Inclusive Period      % of long-term average

                      Pohnpei      Outer Atolls:
                      Island    Eastern   Southern*
May 98                  25%       15%       30%
Jun-Aug 98              70%       60%       70%
Sep-Dec 98              95%       85%       85%
Jan-Jun 99              85%       85%       80%

* This does not include Kapingamarangi. This atoll will be very dry from now until after June 1999 if La Ni?a conditions materialize. If La Ni?a conditions do not appear and equatorial central Pacific SSTs revert to normal, then conditions will be dry until November 1998.
- source: UOG-WERI

Kosrae State: Rainfall at the Kosrae Airport during January and February was 1.29 inches (9% of normal) and 1.67 inches (10% of normal), respectively. Amounts had shown some increase in March and April with 3.87 inches (21%) and 8.02 inches (37%), respectively. At Utwa, Tofol, and Tafunsak rainfall amounts were between 1.28 and 1.17 inches (8-9%) in January, and they were between 1.5 and 1.83 inches (8-10%) in February. In April, rainfall ranged from 4.11 inches (19%) at Tafunsak to 8.20 inches (38%) at Utwa. This rainfall was sufficient to help restore some stream flow and to significantly reduce the fire threat. Despite the April rainfall, the drought is expected to continue for Kosrae into June. Significant rain should return in July. The anticipated rainfall in Kosrae for the next year is as follows:

Inclusive Period    % of long-term average

                        Kosrae Island
May 98                       30%
Jun-Aug 98                   75%
Sep-Dec 98                   95%
Jan-Jun 99                   85%

Palau: We are expecting a record drought in Palau. At Koror, rainfall was 4.72 inches (44% of normal) In January and 2.40 inches (26% of normal) in February. March and April were drier, with 0.50 inches (6%) and 2.17 (22%), respectively. At Peleliu, 6.64 inches (62% of normal) of rain fell in January. In March, values fell to 0.66 inches (8%), but rose to 2.91 inches (34%) in April. We suspect that Kayangel has had less rain than Koror, while Tobi and Sonsorel have had slightly more rain than Koror. Dry conditions are expected to continue through May into early June, although there will be some episodes of heavy rain. The expected rainfall for Palau is as follows:

Inclusive Period       % of long-term average

                   Koror and       Outer Atolls:
                   Babelthaup   N. of 8°N   S. of 8°N
May-Jun 98            25-30%        25%         20%
Jul-Sep 98             85%          85%         75%
Oct-Dec 98             95%         100%         90%
Jan-Jun 99            100%         100%        100%

- source: UOG-WERI

Marshall Islands: We are expecting a record drought in the Marshall Islands. Rainfall in the Marshall Islands for the first 4 month of 1998 was extremely dry. At Majuro (representative of southern atolls), rainfall was: for January, 1.57 inches (19% of normal); for February, 0.34 inches (6%); for March, 0.27 inches (3%); and for April, 0.64 (6%). At Kwajalein (representative of northern atolls) and Ebeye rainfall was: for January, 0.66 inches (8% of normal); for February, 0.91 (28%); for March, 0.75 (18%); and for April, 0.72 (10%). Conditions are expected to remain very dry through May in the southern islands and through June in the northern islands. Water resources on the smaller atolls are salty or have dried up, but islands are now getting drinking water produced from reverse osmosis systems, then delivered by ship. Several US agencies are providing food and water relief in the Marshall Islands. La Ni?a conditions are expected to create dry conditions in the spring of 1999. We anticipate the following rainfall amounts for the Marshall Islands:

Inclusive Period     % of long-term average

                             RMI Atolls:
                        Southern    Northern
May-Jun 98                 30%         25%
Jul-Aug 98                 70%         60%
Sep 98-Feb 99             100%         90%
Mar-Jun 99                 65%         55%


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


SPECIAL SECTION: Experimental Forecasts for Pacific Island Rainfall using data through April, 1998.

Long-Lead Outlook for Hawaiian Islands issue dated 14 May 1998, from NOAA's Climate Prediction Center (CPC).

ENSO ADVISORY of May 11,1998

For further information, please contact:

Mark Morrissey
Guest Editor, Pacific ENSO Update,
c/o Environmental Verification and Analysis Center (EVAC)
710 Asp Ave., Suite 8
Norman, OK 73069

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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