Pacific ENSO Update

3rd Quarter 1999 - Vol. 5 No. 3



Equatorial SST values indicate that the La Niña is rapidly waning. This suggests that normal rainfall patterns and normal trop ical cyclone behavior will soon return to Micronesia and American Samoa. While most climate forecast models lean toward a continuation o f weak La Niña conditions, we are expecting a return to near normal SSTs. The Current Conditions and Predictions in this 3rd Quar ter Newsletter have not changed significantly from those in the 1st and 2nd Quarter Newsletters.

By May 1999, most equatorial sea surface temperatures (SSTs) in the Eastern and Central Pacific had warmed to values within 0.5\06 0C (1°F) of normal. Significant pockets of below normal SSTs remained primarily near the date line and off the west coast of Ecuador and Peru. Radar images from the TOPEX-Poseidon Satellite indicate that sub-surface ocean temperature anomalies have shrunk considerably and that only a few areas of greater than 2°C (3.6°F) still remain. SST information now indicates that the moderate-strong La Ni&n tilde;a event of 1998-99 likely peaked in the January-February 1999 timeframe. Many climate models predict a continuation of current LaNiña conditions through the boreal winter and spring. Some are even predicting a strengthening of cold conditions by the boreal f all. The NECP Canonical Correlation ENSO Forecast suggests a return to normal to slightly above normal equatorial SSTs. Our prediction is for continued weakening of the La Niña and a return to near normal sea surface temperatures and patterns by fall. However, eve n if the La Niña (cold event) were to continue into the spring and summer of 2000, its drought-producing effects would be less sev ere than in 1998-99.


The drop in the SOI to near normal values suggests an eventual return to normal rainfall activity. However, as the atmosphere adjusts, we expect conditions to set up over the western North Pacific that will temporarily make southern and eastern parts of Micronesi a somewhat drier than normal and northern and western parts somewhat wetter than normal. Rainfall should return to normal by fall.

The Southern Oscillation Index (SOI) appears to have peaked at +2.0 standard deviations above normal in January, but after falling bel ow 1.0 standard deviation in February and March, the SOI rose to about +1.4 in April, before falling to near zero in May. This behavior in the SOI fits well with the SST changes and the observed weakening in the low level easterly winds along the equator throughout the Pac ific. As a result, we expect to see continued movement back to normal atmospheric patterns, and as a result, a re-emergence of monsoon a ctivity in the western North Pacific from July-October 1999. We still expect to see some reverse orientation to the monsoon trough, that is to say, with its axis oriented from the WSW near Asia to the ENE in the vicinities of Wake Island and Marcus Island (near 25°N 155 °E), southeast of Japan. This will likely result in a higher than normal latitude of development for tropical cyclones and monsoon ac tivity. This should also lead to somewhat less than normal rainfall amounts at more eastward (e.g., RMI) and equatorward (e.g., Palau an d the southern FSM) Micronesian islands. For American Samoa, both dry season and wet season rainfall values should return to near normal . The SOI is expected to hover between 0 and \061 0.5 standard deviations for the next year.

The western Pacific has had two persistent near-equatorial troughs (NETs), one in the northern hemisphere at about 5-8°N and one in the southern hemisphere at about 7-10°S ( Figure 1 ). However, over the last 2-3 months, the northern hemisphere trough has gained pro minence. Upper level cyclones in the very active Tropical Upper Tropospheric Trough (TUTT) have been very persistent and have penetrated more equatorward than normal. This has led to the development of tropical disturbances, greatly enhanced convection, and rainfall in th e NET in central and western Micronesia, south of 10°N. North of 10°N, rain was also closely associated with the cyclonic cells i n the TUTT. However, this rain was more in the form of heavy, relatively short-lived thunderstorms. Figure 1 illustrates the relationsh ip between the upper-level TUTT, cyclonic cells in the TUTT, the NET (ITCZ), and cloudiness and rainfall over Micronesia. Figure 1 also shows the southern hemisphere NET and rainfall over American Samoa.

Another change that occurred over this period was the return of rainfall to the equatorial band west of the date line. The very stron g rainfall gradients observed in January-March from 2-8° north of the equator, weakened from April-June, and the axis of the heaviest rainfall shifted slightly to the north and west. The 3-monthly maximum rainfall values in April-June were considerably less than those i n January-March. Very dry equatorial areas have receded eastward to east of the date line. In the southern hemisphere, the near-equator ial trough (NET) was most persistent from 160°E to 160°W, from 7-11°S ( Figure 1 ). Persistent disturbances associated with the e astern periphery of the NET brought very wet conditions to American Samoa in May. Conditions for American Samoa should revert back to no rmal by July or August, with normal activity in both the dry-season trade winds and the wet-season South Pacific Convergence Zone (SPCZ). During "normal" years, locations frequently experience high month-to-month rainfall variability.


Tropical cyclone distribution is expected to return to normal by mid-summer. In the northern hemisphere, we expect a shift in the location of development back toward more eastern longitudes. This will give tropical cyclones more time to intensify and threaten more of Micronesia. American Samoa and southern hemisphere locations will see a shift in tropical cyclone development back to the west, general ly west of the date line, to a more normal status.

Tropical cyclone activity in the northern hemisphere has been light to-date. However, we expect activity to increase significantly in late July or early August. As indicated above, we expect the northern hemisphere monsoon trough to acquire a "reverse" orient ation. Therefore, during the boreal summer, most tropical cyclone activity is expected to be significantly north of Micronesia, but from September into December, tropical cyclone development is expected to move southward into Micronesia and eastward toward the western Marsh all Islands.

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: No flash flood warnings were required or issued during June. Trade winds prevailed throughout the entire month of June at mostly light to moderate intensities. Trade wind showers were present across the region on a consistent basis, but totals were generally light statewide. The lack of significant rains over portions of the islands of Hawaii and Maui helped worsen an ongoing rainfall deficit that can be traced back to the 1997/98 El Nino event.

Kevin Kodama - Senior Service Hydrologist, NWSFO Honolulu, HI

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, precipitation amounts are expected to return to near normal values by Jun e. Tropical cyclone activity is not expected to be significant in the Samoa region until the next El Niño event develops.

As anticipated in the last ENSO Newsletter, rainfall increased significantly in American Samoa from April-June. At Pago Pago, A pril, May, and June rainfall was measured at 5.02 inches (42%), 29.10 inches (293%), and 6.29 inches (85%). The 3-month average was 140% of n ormal, primarily due to the extremely wet May. The SPCZ was very active in May, extending from west of Vanuatu, eastward to French Polynesia. A large area of deep convection sat over the Samoa region during late May ( Figure 1 ). With the waning of the La Niña, we expect meteor ological patterns to return to normal, and with that, rainfall should become near-normal (which includes the probability of high month-to-mont h variability).

A few developing tropical cyclones may affect American Samoa from November to March, but the occurrence of strong tropical storm-force or h urricane-force winds is not likely until the next El Niño event occurs.

The rainfall predictions for American Samoa and the region through September 2000 are:

Inclusive Period         % of long-term average
                              Samoa Region
Jul-Sep 99                         90%
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 a bove normal from late spring to mid fall. Rainfall and tropical cyclone activity should return to near normal modes. For rainfall, month-to- month variability can be great.

April through June rainfall on Guam showed high variability, with rainfall strongly related to the passage of cyclonic systems in the T ropical Upper Tropospheric Trough (TUTT) and to a few tropical disturbances associated with the TUTT. April, May, and June rainfall at the Gu am International Airport (GIA) was 3.68 inches (94%), 3.53 inches (58%), and 10.41 inches (161%), respectively. For the same months at Anders en Air Force Base (AAFB), rainfall was 6.64 inches (136%), 4.04 inches (61%), and 9.19 inches (145%). At Saipan International Airport, April, May, and June rainfall amounted to 3.01 inches (108%), 3.08 inches (70%), and 4.32 inches (93%). Values at Capital Hill were 4.04 inches (11 5%), 3.11 inches (57%), and 7.08 inches (122%). This period was slightly drier than the first three months of the year. Rota Airport measure d 6.04 inches (124%), 5.86 inches (89%), and about 7.50 inches (118%) during the respective three months. Rainfall was similar at the NASA Tr opical Rainfall Measuring Mission rain gauge network located on the Rota Resort and Country Club on the north side of Rota. Tinian rainfall w as not available, but was likely similar to that at Capitol Hill, Saipan.

Our last ENSO Update Newsletter predicted slightly wetter than normal conditions for the Mariana Islands in the April-June perio d; Guam and Rota were slightly wetter (~110%), while Saipan was slightly drier (~94%). The cyclonic circulations in the TUTT attributed to m ost of the rainfall in the Mariana Islands, and also created many episodes of electrical (lightning/thunder) storms. We do expect rainfall to increase in the region during the summer in association with expected strong monsoon activity. Near-normal conditions should return in the fall. Tropical cyclones could threaten the area from September through early December.

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

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

- sources: UOG-WERI

Yap State: Rainfall for Yap and the other northern islands is expected to be above normal into the fall of 1999. At the southern islands, summer rainfall is expected to be slightly below normal to near normal. Tropical cyclone activity will be significantly greater in the fall of 1999 than during the last 18 months.

While we predicted above normal rainfall for Yap State during April, May, and June in the last Pacific ENSO Update, most areas w ere even wetter than expected. In fact, Yap State had the highest percentage of rainfall of any area in Micronesia. Yap Airport recorded 14. 96 inches (260%) in April, 15.31 inches (169%) in May, and 15.99 inches (126%) in June. Conditions were significantly drier at Ulithi with 8. 64 inches (176%), 5.15 inches (67%), and 8.99 inches (83%) for the same three months. Conditions were also very wet in the southern Yap islan ds, where Woleai Atoll experienced rainfall amounts of 18.58 inches (269%), 18.47 inches (151%), and 19.00 inches (146%) for the respective th ree months. This illustrates the persistent very strong gradient of rain that occurred over a distance of only 100 miles. This was i ndicative of the type of rain induced by cyclonic cells in the TUTT. North of 10°N, most rain was caused by smaller, more-isolated, yet he avy thunderstorm-type rainfall. South of 10°N, upper-level winds associated with the TUTT induced disturbances within near-equatorial trou gh that created broader, heavier, longer-lasting areas of rain. Yap State also received some rain from an enhanced monsoon associated with Ty phoon Kate in late April. Northern Yap State islands are expected to have above normal rainfall through the summer. Tropical cyclones will affect northern Yap State from September-December and southern parts of the State from October-December.

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

Inclusive Period                % of long-term average
                             Yap               Outer Atolls:
                            Island       S.of 8·N     N. of 8·N
Jul-Sep 99                   130%           95%         120%
Oct 99-Jun 2000              100%          100%         100%

- sources: UOG-WERI

Chuuk State: Rainfall for Chuuk State is expected to be slightly below normal into early fall when normal rainfall amounts are ex pected to return. Tropical cyclone activity around Chuuk State will begin in October, and will be significantly greater than during 1998.

Rainfall at Weno Island during April, May, and June was 22.03 inches (178%), 8.90 inches (73%), and 11.06 inches (94%). Luchonor had simil ar rainfall with 23.03 inches (186%), 8.98 inches (73%), and 11.70 inches (100%) for the same months. Polowat was considerably wetter with 31 .41 inches (524%), 2.69 inches (30%), and 8.47 inches (68%) during the period. While the April rainfall at Polowat seems very high, many trop ical disturbances moved out of southern Pohnpei State, south of Chuuk and the Mortlock Islands, then turned to the west-northwest over the reg ion of western Chuuk State. In April, the developing Tropical Storm Kate contributed to heavy rains over the State. We expect that Chuuk Sta te will be somewhat drier than normal through late summer. Then, normal rainfall amounts are expected to return. Chuuk State could experienc e a developing tropical cyclone from October to December.

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

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

- sources: UOG-WERI

Pohnpei State: After a very wet spring, rainfall for Pohnpei State is expected to be 15-20% drier than normal until early fall. Some tr opical cyclone activity around Pohnpei State\022s northern islands could occur in October and November.

Pohnpei State was considerably wet in April, but most areas became drier than normal in May and June. A exception was Kapingamarangi, which after a lengthy drought, became wetter. For April, May, and June, Kolonia recorded 21.21 inches (129%), 12.13 inches (63%), and 11.55 inches (67%). While rainfall varied across the island, it was very heavy at all locations. Pingalap was somewhat drier than Pohnpei, with 16 .77 inches (98%), 7.95 inches (47%), and 11.72 inches (72%) during the respective three months. Nukuoro, which was extremely wet in the first three months of the year, saw monthly rainfall amounts of 34.27 inches (228%), 12.52 inches (85%), and 11.76 inches (96%). Significant rains finally returned to Kapingamarangi, which recorded 13.38 inches (98%) in April, 5.26 inches (51%) in May, and 13.79 inches (190%) in June. Mo st of Pohnpei State rainfall came from tropical disturbances that flared up in response to the passage of cyclonic cells in the TUTT (see Figu re 1). Most of Pohnpei State is expected to have slightly below normal rainfall until late fall, when we expect rainfall to return to normal.

Pohnpei State and the eastern islands will not be very susceptible to a direct hit by a tropical cyclone until the next El Niño even t begins to occur. However, Pohnpei could see some effects of tropical cyclones that pass to the north in October and November.

Rainfall predictions for Pohnpei State through September 2000 are as follows:

Inclusive Period          % of long-term average
                     Pohnpei           Outer Atolls:
                     Island    Eastern    Southern    Equatorial
Jul-Sep 99             80%       80%         85%         90% 
Oct 99-Jun 2000       100%      100%        100%        100%

- sources: UOG-WERI

Kosrae State: After very wet conditions in the first 4 months of the year, Kosrae State is expected to be 15-20% drier than normal unti l early fall. Tropical cyclone activity around Kosrae is not likely until the next El Niño occurs.

Kosrae was very wet from January-April as the northern and southern hemisphere trade winds converged, creating a band of deep convectio n over the area. Cyclonic cells in the TUTT (see Figure 1 ) increased the convection over the area as the cells moved north of Kosrae on their trek to the west. During the 3-month period of April-June, Kosrae Airport recorded 29.15 inches (135%), 9.04 inches (48%), and 13.63 inches ( 72%), respectively. Tafunsak and Tofol were likely slightly drier than the airport, although no rainfall reports were received from these two sites. Utwa was wetter than the Airport, especially in June when the village had 18.92 inches of rain.

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 through September 2000 is as follows:

Inclusive Period     % of long-term average
Jul-Oct 99                    80%
Nov 99-Mar 2000              100%

- sources: UOG-WERI

Republic of Palau: Palau is expected to be slightly drier than normal until the fall, and then have near-normal rainfall until onset of th e next El Niño event. Tropical cyclone threats will increase in November and December.

The Republic of Palau experienced a significant reduction in rainfall during April-June, after a very wet January-March. In Koror, Ap ril rainfall was 14.69 inches (169%), that in May was 10.66 (89%), and that in June was 11.93 (69%). Farther south in Peleliu, rainfall was s lightly less, with monthly accumulations of 5.87 inches (65%), 16.09 inches (134%), and 13.42 inches (79%). Amounts at Tobi Island far to the south-southwest were likely a little less, while those at Kayangel to the north were probably somewhat more. This change in rainfall occurre d as the monsoon trough moved to the north in a "reverse- oriented" mode from Southeast Asia to southeast of Japan. Palau did recei ve some enhanced rain in late April and early June as Typhoons Kate and Maggie developed east of the Philippines. However, for the most part , rainfall for Palau resulted from enhanced convection associated with tropical disturbances that flared up in response to cyclonic cells in t he TUTT that moved north of the Island Nation (see Figure 1 ).

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 some tropical cyclone activity from late October to early December.

The expected rainfall through September 2000 for Palau is:

Inclusive Period              % of long-term average
                    Koror and           Outer Atolls
                    Babelthaup     S. of 8ºN    N. of 8ºN
Jul-Sep 99             90%             90%         80%
Oct 99-Jun 2000       100%            100%        100%

- sources: UOG-WERI

Republic of the Marshall Islands: With the waning of the La Niña, we expect conditions to be drier than normal in the RMI until lat e summer or early fall, when normal rainfall patterns should return. Tropical cyclones could threaten Wake Island and Enewetok by September a nd other western sections of the RMI by October.

While we indicated drier than normal conditions in the spring and summer for the RMI in our last Pacific ENSO Update Newsletter, rainfall was even lighter than expected. Majuro (representative of the southern islands) had both more rainfall and a higher percentage of r ainfall than Kwajalein (representative of the northern islands). At Majuro, April, May, and June rain amounts were 5.32 inches (53%), 8.20 in ches (73%), and 13.07 inches (113%), respectively. Arno had similar rainfall to Majuro, while Mili, Jaluit, Maleolap, and Ailinglapalap had a little less over the 3-month period. Rain measured at Kwajalein was 2.18 inches (29%), 3.58 (36%), and 12.34 inches (128%) for the respectiv e months. Ebeye had amounts similar to Kwajalein, but Wotje, Utirik, and Mejit were likely a little drier. Satellite data indicated that Wak e, Enewetok, Ujae, and Ujelang were significantly drier than Kwajalein. Most of the rain in the RMI resulted from weak disturbances in the NE T (see Figure 1 ) that flared in response to favorable upper-level winds associated with cyclonic cells in the TUTT that moved to the west, nor th of Kwajalein. At Wake, most rain came directly from the cyclonic cells in the TUTT.

Conditions in the RMI are expected to remain 20-30% drier than normal, until late summer or early fall. Normal rainfall patterns should re turn to the islands by that time. The RMI could experience a tropical cyclone from September to November at Wake and Enewetok and from Octobe r to December at locations farther south. Intense typhoons are not likely in the main RMI island chain until the next El Niño event.

Expected rainfall amounts for the RMI through September 2000 are as follows:

Inclusive Period           % of long-term average
                                  RMI Atolls
                            Southern     Northern
Jul-Sep 99                     85%          75%
Oct 99-Mar 2000               100%         100%

- sources: UOG-WERI


In light of recent drought conditions on Nauru and in Kiribati, and at the request of Nauru and SOPAC, we are providing the following gener al information for those Micronesian areas. Since we are not receiving any rainfall data from those locations, we must base the predictions o n historical rainfall data, meteorological satellite imagery, and sea surface temperature and SOI predictions.

With the expected continued weakening of La Niña, we expect near-normal rains to return to Nauru and much of Kiribati. In fact, considerable rain has already fallen in the equatorial regions west of the date line in the last 1-2 months. Heavier and more frequent rains also appear to have returned to the northern Gilbert Islands (Matkin, Butaritari, and Tarawa). Rainfall should return to near-normal values by the October-November timeframe. Beru and Arorae will not likely experience significantly wet weather until the next El Niño event o ccurs. Some relief for these two island regions is probable by November, but it will not likely last longer than 3-4 months. The rain will b e useful for catchments and for partial restoration of the fresh water lens. Christmas Island will continue to have very dry weather until th e next El Niño event. Fanning Island should see increased rainfall in July.

This will be the last of these special predictions, unless conditions change drastically. Without monthly rainfall data for these location s, we cannot make credible predictions as conditions return to normal. We welcome daily and/or monthly rainfall data from these locations. D ata can be sent via e-mail to: or or FAX to: (1-671) 734-8890.


Long-Lead Outlook for Hawaiian Islands issue dated 15 July 1999, from NOAA's Climate Prediction Center (CPC).

ENSO ADVISORY of 12 July 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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