Pacific ENSO Update

2nd Quarter 1999 - Vol. 5 No. 2



SSTs suggest that the La Niña is waning. This points toward a return to normal rainfall patterns and normal tropical cyclone behavior for both Micronesia and American Samoa. The Current Conditions and Predictions in this 2nd Quarter Newsletter have not changed much from our 1st Quarter Newsletter.

In January 1999, equatorial sea surface temperatures (SSTs) were 1.5-2°C (2.7-3.6°F) cooler than normal between 115° West and 165° East. Over the last two months, the Central and Eastern Pacific equatorial areas covered by the cooler than normal SSTs have begun to shrink. In addition, the depth of cold waters has become shallower. This suggests that the current moderate to occasionally strong La Niña event has either peaked or is likely near its peak, and will slowly wane. The western Pacific has had two persistent near-equatorial troughs, one in the northern hemisphere and one in the southern hemisphere. These troughs have produced east-west bands of wet weather north and south of the equator, but have accentuated dry conditions within 125 miles of the equator through increased subsidence. Figure 1 illustrates the very intense western North Pacific January-through-March rainfall gradients from 2° to 8° North of the equator, with maxima near Nukuoro and Mindinao. Very dry areas are shown in near equatorial regions east of 150° East, from 1-2° North to 8° South. The dotted lines in the southern hemisphere indicate that the analysis there was not based on actual rainfall values but on the interpretation of daily satellite imagery and the 31.34 inches of rain (Jan-Mar) for American Samoa. Some climate models are predicting a continuation fo cool equatorial SSTs into the January to March 2000 time frame. Others suggest the return to noemal conditions by late September 1999. We believe that the La Niñ:a has peaked, and will phase out by October, The cooler than normal equatorial SSTs should still maintain stronger that normal easterly winds along the equator and drier than normal conditions over the equatorial belt east of 160 E into June through the boreal (northern hemisphere) spring. In fact, these winds are already beginning to weaken west of the date line.


The SOI points to normal to above normal rainfall for Micronesia and American Samoa, but less than normal rainfall will prevail during the boreal spring in the Marshall Islands and in near-equatorial regions. 1

The Southern Oscillation Index (SOI) jumped to near +2.0 (moderate-strong La Niña) in January 1999. Since July 1998, the SOI had been equal to or above +1.0, but in March 1999, it fell to +0.9 (weak to moderate La Niña). The positive SOI has been associated with much wetter than normal rainfall in much of Micronesia, especially Palau, southern Yap State, Chuuk State, Pohnpei State, and Kosrae State. Figure 1 shows the extremely sharp rainfall gradients that occurred across Micronesia during the first three months of 1999. We believe that the SOI will move back to within 0.5 units of zero (normal values) by the August-September time frame. With this change, we expect the equatorial low level easterly winds, which have been significantly stronger than normal since April 1998, to weaken, allowing monsoon westerly winds to push farther eastward. Rains should begin returning to equatorial areas west of the date line by summer and become near normal by late fall. We still believe that there is a good chance that the monsoon trough across the western North Pacific will undergo episodes of reverse orientation, that is to say, oriented from the west-southwest near the Philippines to the east-northeast north of Wake Island. This should bring above normal rainfall to most of Micronesia, but southeastern and eastern areas (Pohnpei State, Kosrae State, and the Marshall Islands) will likely be slightly drier than normal during the summer. American Samoa is expected to see near normal rainfall return by June after a somewhat drier than normal rainy season.


Tropical cyclone activity will return to normal as the La Nina weakens and allows tropical cyclone development to extend into the eastern half of Micronesia. Significant tropical cyclones should not affect American Samoa.

Tropical cyclone activity in the northern hemisphere started unusually early in 1999. Tropical Storm Hilda developed in January in the South China Sea, Tropical Storm Iris occurred in February in the Philippine Sea north of Yap and Palau, and Tropical Storm Jacob and Tropical Storm Kate had their births in the Philippine Sea in early and late April, respectively. Typhoon Leo also developed in the South China Sea near the end of April . Each of these tropical cyclones had a concurrent weak twin cyclone in the southern hemisphere, although these circulations were not picked up a s official tropical cyclones. Chuuk State, Yap State, and the Mariana Islands could see a strong tropical storm or a typhoon in May, but most are as west of 150\047 East could be affected by a typhoon from October through early December.


From April 26-28, 1999, the Economics and Human Dimensions (EHD) program within the Office of Global Programs (OGP), NOAA, hosted the "Human Di mensions Meeting" in Tucson, Arizona. The purpose of the meeting was "to provide a forum for researchers to share information and experiences, to demonstrate in concrete terms the value of NOAA human dimensions research in terms of advancing the knowledge base and improving societal welfare , and to identify specific future priorities for research" (EHD/OGP 1999). Four participants from the University of Hawaii attended the meeting: Dr. Michael Hamnett and Ms. Cheryl Anderson to discuss the assessment of the Pacific ENSO Applications Center and the1997-98 ENSO warm event and t he Impact of Climate Variability on the Pelagic Fisheries in the Pacific Islands; and Drs. Nancy Davis Lewis and Liem Tran to discuss the Impact o f Climate Variability on Health in the Pacific Islands.

Please Click Here to see the full report.

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.


State of Hawaii: Trade winds were uninterrupted through the month of April across the Hawaiian Islands. Strong trades destabilized by an upper level low just to the northeast of the island chain brought heavy showers to portions of the island of Hawaii during 10 to 13 April. An Urban and Small Stream Flood Advisory was issued on 11 and 13 April due to reports of minor flooding over the North and South Kona Districts (11 Apr) and the Hilo area (13 Apr) of that island. Following this rain event, the entire state experienced a general drying trend in light to moderate trades that persisted through the remainder of the month.

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

American Samoa: After more than 1 year of below normal rainfall, amounts are expected to return to near normal values by June. Tropical cyclone activity is not expected to be significant in the Samoa region until the next El Ni\044o event develops.

As anticipated in the last ENSO Newsletter, rainfall was slightly below normal for American Samoa. At Pago Pago, January, February, and March measuredd rainfall was 11.36 inches (90%), 14.50 inches (114%), and 5.48 inches (49%). The 3-month average was 84% of normal (10.45 inches). While the South Pacific Convergence Zone (SPCZ) was normally active in January and February, it shifted to the east of the date line in March and early Ap ril. This may have been associated with the very high level of tropical cyclone activity west of Australia and into the Gulf of Carpentaria. With the waning of the La Ni\044a and the development of symmetrical near-equatorial troughs in the northern and southern hemispheres, conditions are bec oming more normal. As a result, we expect rainfall to become near normal, which includes the probability of high month-to-month variability.

Developing tropical cyclones may affect American Samoa from November to March, but the occurrence of strong tropical storm-forc e or hurricane-force winds is not likely until the next El Ni\044o event occurs.

The rainfall predictions for American Samoa and the region through June 2000 are;

Inclusive Period         % of long-term average
                              Samoa Region
Apr-Sep 99                         95%
Oct 99 - Jun 2000                 100%

- sources: UOG-WERI and PEAC

Guam/CNMI: With the anticipation of strong monsoon activity across the Northwest Pacific, we expect rainfall for Guam and the CNMI to be above normal from late spring to mid fall.

While January and early February were wetter than normal in the Mariana Islands, the second half of February, March, and early April were dry. Rains increased in late April. Rainfall at the Guam International Airport (GIA) was 4.70 inches (106%), 12.59 inches (337%), 1.91 inches (64%), and 3.68 inches (94%) for January, February, March, and April respectively. For the same months at Andersen Air Force Base (AAFB), rainfall was 3.46 inches (61%), 15.35 inches (294%), 1.76 inches (43%), and 6.64 inches (136%). At Saipan International Airport, January, February, March, and April rainfall amounted to 4.77 inches (149%), 5.71 inches (238%),1.48 inches (74%), and 3.01 inches (108%). Values at Capitol Hill were 4.07 inches (102%), 9.94 inches (331%), and 1.70 inches (68%). Rota Airport measured 4.04 inches (71%), 12.26 inches (235%), and 1.54 inches (38%) during the respective three months. Rainfall was similar at the NASA Tropical Rainfall Measurement Mission rain gauge network located on the Rota Resort and Country Club on the north side of Rota. Tinian rainfall was slightly wetter than that at Capitol Hill. In February, 90% of the rainfall at these locations occurred over a 5-day period-13-16 February-in an otherwise dry month.

While our last ENSO Update Newsletter predicted slightly drier than normal conditions for the period, the heavy rain event during February actually made the period wetter than normal. Despite this, Saipan and Tinian experienced drought-like conditions for most of the period. The northern hemisphere monsoon trough became well established in March, and during the fourth week of April, the monsoon temporarily surged as far east as 150ñ East as Tropical Storm Kate developed. This placed the Mariana

Islands on the eastern edge of the surge, producing some heavy rain episodes over the islands. We do expect rainfall to increase in Guam and Rota by late April to early May and for Saipan and Tinian by early to mid-May. This will occur as the result of the northward movement and intensification of an already well-developed near-equatorial trough. Summer over the entire region should be wetter than normal, with near-normal conditions returning by fall. There could be a tropical cyclone affecting the area in May, but the greatest chance of a threat will occur from September through early December.

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

Inclusive Period         % of long-term average
                      Guam/Rota       Saipan/Tinian
Apr-Jun 99              110%              105%
July-Sep 99             125%              130%
Oct 99 - Jun 2000       100%              100%

- sources: UOG-WERI

Yap State: Rainfall for Yap State is expected to be above normal into the fall of 1999. Tropical cyclone activity around Yap State will be significantly greater than during 1998.

Yap was somewhat wetter than normal during the first three months of the year, but it was drier than Chuuk to the south and drier than Palau to the west. Yap Airport recorded 7.65 inches (104%) in January, 5.47 inches (91%) in February, 9.13 inches (153%) in March, and 14.96 inches (260%) in April. Rainfall at Ulithi was similar with 5.97 inches (96%), 5.07 inches (100%), 7.66 inches (151%), and 8.64 inches (176%). Much more rain fell in Woleai, where 10.04 inches (94%), 10.17 inches (136%), 23.66

inches (285%), and 18.58 inches (169%) were recorded. In late February and March, the near-equatorial trough, in which tropical disturbances and tropical storms develop, developed much earlier than normal. It sat over the region of Woleai Atoll much of

the time, while the more northern islands of Yap State were just north of the heaviest rains. By the fourth week of April, the monsoon had temporarily surged as far east as 150ñE as Tropical Storm Kate developed, placing Yap and its northern islands within monsoonal flow and periods of very heavy rain.

Rainfall for Yap and its atolls through June 2000 is expected to be:

Inclusive Period                % of long-term average
                             Yap               Outer Atolls:
                            Island       S.of 8·N     N. of 8·N
Apr-Jun 99                   115%          115%         110%
Jul-Sep 99                   120%          120%          95%
Oct 99-Jun 2000              100%          100%         100%

- sources: UOG-WERI

Chuuk State: Rainfall for Chuuk State is expected to be slightly above normal through June, then slightly below normal into early fall when normal rainfall amounts are expected. Tropical cyclone activity around Chuuk State will be significantly greater than during 1998.

Rainfall at Weno Island during January, February, March, and April was 13.79 inches (129%), 4.73 inches (77%), 18.10 inches (217%), and 22.03 inches (178%). Luchonor was wetter with 16.83 inches (158%), 8.24 inches (118%), 27.82 inches (336%), and 23.03 inches (178%) for the same months. Polowat had more rainfall than Weno with 7.71 inches (96%), 8.37 inches (134%), 20.22 inches (324%), and 31.41 inches (520%) during the period. The rain was the result of disturbances that developed in the active near-equatorial trough that sat nearly over Chuuk State in February and March. In April, tropical deep convection (much of it associated with the developing Tropical Storm Kate) dropped heavy rains on the State. We expect that Chuuk State will be somewhat wetter than normal through late summer when normal rainfall amounts are expected to return. Chuuk State could experience a developing tropical cyclone in May, and will again be vulnerable to tropical storms and typhoons from October to December.

Predicted rainfall for Chuuk State through June 2000 is as follows:

Inclusive Period               % of long-term average
                        Chuuk         Outer Atolls
                        Lagoon    Southern    Northern
Apr-Jun 99              110%       100%        115%
Jul-Sep 99               90%        85%         95%
Oct 99-Jun 2000         100%       100%        100%

- sources: UOG-WERI

Pohnpei State: After a very wet winter and spring, rainfall for Pohnpei State is expected to be slightly drier than normal until early fall. Tropical cyclone activity around Pohnpei State's northern islands could occur in October and November.

All of Pohnpei State was considerably wetter than normal except for Kapingamarangi. For January, February, March, and April, Kolonia recorded 18.22 inches (139%), 18.05 inches (167%), 27.33 inches (202%), and 21.20 inches (129%). While rainfall varied across the island, it was very heavy at all locations. Pingalap was even wetter with 18.33 inches (148%), 22.94 inches (188%), 23.66 inches (163%), and 16.77 inches (98%) during the respective four months. Most surprising was Nukuoro, which saw monthly rainfall amounts of 36.63 inches (312%), 13.39 inches (127%), 32.35 inches (238%), and 34.27 inches (228%). Areas within two degrees latitude of the equator were very dry, a manifestation of the colder than normal equatorial SSTs accompanying the La Niña event. Being on the edge of this region, Kapingamarangi continued to be drier than normal, but with some episodes of heavy rain. April rainfall there jumped to 13.38 inches (98%).

The northern islands of Pohnpei State will become susceptible to tropical cyclones during October and November. Pohnpei and the eastern islands will not be very susceptible to a direct hit until the next El Niño event begins to occur.

Rainfall predictions for Pohnpei State are as follows:

Inclusive Period          % of long-term average
                     Pohnpei           Outer Atolls:
                     Island    Eastern    Southern    Equatorial
Apr-Jun 99            100%       95%         90%         85%
Jul-Sep 99             90%       85%         85%         85% 
Oct 99-Jun 2000       100%      100%        100%        100%

- sources: UOG-WERI

Kosrae State: After a very wet winter and spring, rainfall for Kosrae State is expected to be slightly drier than normal until early fall. Tropical cyclone activity around Kosrae is not likely until the next El Niño occurs.

Kosrae was very wet in January as the northern and southern hemisphere trade winds converged, creating a thin band of deep convection over the area. As the narrow band of convection moved north during February and March, slightly drier than normal conditions prevailed over Kosrae. Wet conditions returned in April. During the 4-month period, Kosrae Airport recorded 29.76 inches (207%), 11.83 inches (72%), 17.05 inches (91%), and 29.15 inches (135%). Tafunsak and Tofol were slightly drier than the airport, while Utwa was slightly wetter. Utwa measured 32.16 inches in January.

Kosrae has very little chance of being affected by a tropical cyclone until the next El Niño event begins to occur. That is likely a few years off.

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

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

- sources: UOG-WERI

Republic of Palau: Palau is expected to remain very wet until summer when the islands will be slightly drier than normal until the fall. Tropical cyclone threats will increase in November and December.

The Republic of Palau was very wet during January, February, March and April. In Koror, rainfall was 24.83 inches (232%), 6.44 (71%), 19.24 (235%), and 14.69 inches (169%) for the respective months. Farther south in Peleliu, rainfall was equally heavy until April, with monthly accumulations of 24.69 inches (231%), 10.64 inches (118%), 17.24 inches (210%), and 5.87 inches (65%). Amounts at Tobi Island far to the south were likely a little greater until April, while those at Kayangel to the north were somewhat drier. Early in the period, this heavy rainfall resulted from an active Australian monsoon that drew monsoon rains across the southern Philippines and parts of Palau. Later in the period, the northern hemisphere monsoon trough became established over the same regions. By the fourth week of April, the monsoon had temporarily moved to the north and had surged as far east as 150ñE in association with Tropical Storm Kate, placing Palau under strong monsoonal flow.

While Palau does not have as large a tropical cyclone threat as most other areas of Micronesia, islands north of Anguar will be susceptible to tropical cyclones from late October to early December.

The expected rainfall for the next year for Palau is:

Inclusive Period              % of long-term average
                    Koror and           Outer Atolls
                    Babelthaup     S. of 8ºN    N. of 8ºN
Apr-Jun 99            120%            105%        125%
Jul-Sep 99             90%             85%         90%
Oct 99-Jun 2000       100%            100%        100%

- sources: UOG-WERI

Republic of the Marshall Islands: The slightly below normal rainfall for the Marshall Islands during the winter and spring will continue until fall when rainfall amounts are expected to return to normal. The area from Majuro to Kwajalein could experience a weak, developing tropical cyclone in October and November.

As indicated in our last newsletter, the Marshall Islands were generally drier than normal. As usual, Majuro (representative of the southern atolls) had more rain than Kwajalein (representative of the northern atolls). At Majuro, January, February, March, and April rainfall were 7.23 inches (86%), 3.82 inches (62%), 10.15 inches (123%), and 5.32 inches (53%). Arno had similar rainfall, while Mili, Jaluit, Maleolap, and Alingalaplap had somewhat less rainfall. Rain amounts at Kwajalein for each of the four months was 4.55 inches (100%), 3.82 inches (118%), 1.84 inches (42%), and 2.18 inches (29%). Rainfall at Ebeye was similar to that at Kwajalein, while that at Wotje, Utirik, and Mijet were likely somewhat wetter. Wake, Enewetok, Ujelang, and Ujai were drier than Kwajalein.

The Marshall Islands are expected to be 10-15% drier than normal through summer, with normal rainfall returning by early fall. Typhoons could effect Wake and Enewetok in September and October. A weak tropical storm could effect the main Marshall Islands

from October to December, but typhoons are not likely there until the next El Niño event, likely a few years away.

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 13 May 1999, from NOAA's Climate Prediction Center (CPC).

ENSO ADVISORY of 10 May 1999

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 #250, 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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