Pacific ENSO Update

4th Quarter 1999 - Vol. 5 No. 4



Equatorial SST values indicate that the La Niña has fluctuated between neutral-to- weak intensity and moderate intensity over the past 3-month period. Equatorial SSTs from 120W-160W remain about 1ºC (1.9ºF) below normal, although the area is quite narrow. Coldest anomalies have recently appeared to the east, near 110W-120W. These "La Niña" conditions appear to have been sufficient to keep rainfall values somewhat drier than normal in some areas, especially near the equator, in northern and eastern parts of Micronesia and in American Samoa. Now, most climate forecast models lean toward a continuation of weak to even moderate La Niña conditions well into the NH spring of 2000. This suggests a continuation of somewhat drier than normal conditions in near equatorial areas east of 150ºE, in eastern Micronesia, in the Mariana Islands, and in American Samoa.

After showing a general warming trend from January through May 1999 [most anomalies had warmed to values within 0.5°C (1°F) of normal], SSTs and sub-surface sea temperatures in the Central and Eastern equatorial Pacific began to cool. SST anomalies in the Niño 3.4 region (170W-120W) reached –1.1ºC (2ºF) in August. Radar images from the TOPEX-Poseidon Satellite indicate that sub-surface ocean temperature anomalies that shrunk in the spring, have stabilized with some areas greater than -2°C (-3.6ºF). SST information now indicates that the moderate-strong La Niña event of 1998-99 peaked in the December 1998-January 1999 timeframe, but suggests that we are entering a multi-year event. Most climate models now predict a strengthening of La Niña conditions into the boreal spring or summer of 2000. However, there is disagreement among the models as to when the conditions will return to normal. Some models indicate that this will occur as early as January 2000, while others suggest moderate conditions as late as July 2000. Our predictions split the difference, with SSTs returning to near normal values by June. Even if the La Niña (cold event) were to continue into middle or late 2000, its drought-producing effects would likely not be as severe as in 1998-99. In addition, the heavy rainfall amounts observed in early 1999 from 2N-10N would likely not be as large. Sub-surface sea temperatures in the western Pacific are beginning to increase, setting the stage for a future El Niño event.


The continued drop in the SOI to near normal values suggests a return to near normal pressure patterns in the Southwest Pacific. However, to date, vertical motion fields do not seem to have yet returned to normal. This was seen from July-October in the western North Pacific where surface pressures were much higher than normal and the monsoon was nearly non-existent. With cooler SSTs stubbornly remaining along the equator, reduced evaporation coupled with strong subsidence will likely make conditions drier than normal in equatorial regions. Rainfall in other parts of Micronesia and in American Samoa will be near normal.

The Southern Oscillation Index (SOI) peaked at +2.0 standard deviations above normal in January, but since May, SOI values have been near normal (5-month running mean was 0.3 through mid October). This behavior in the SOI is somewhat out of phase with the SST behavior, but is consistent with the observed weakening of the low level easterly winds along the equator throughout the Eastern and Central Pacific. Vertical motion analyses over the western North Pacific indicate much weaker than normal upward motion from the date line westward to 130ºE. Some tropical cyclone genesis has occurred in the western Philippine Sea and the South China Sea. The equatorial SOI between the Eastern Pacific and Indonesia is still positive, indicating some residual La Niña-like atmospheric conditions. Figure 1 shows the normal August vertical motion field for the region (top diagram) and the anomaly (bottom diagram) during August 1999. The anomaly indicates that the upward motion is much weaker than normal. Upper level circulation patterns from 15N to15S over the central and western Pacific tropics have been more convergent than normal, which has been conducive to below normal rainfall. In the Northern Hemisphere, upper level westerly winds associated with the Tropical Upper Tropospheric Trough (TUTT) have been more equatorward than normal, producing the more convergent patterns. Even with the strengthening of the La Niña (in the SST field), we expect to see a continued, albeit slow, movement back to more normal atmospheric patterns.

The re-emergence of monsoon activity in the western North Pacific from July-October 1999 did not develop as expected. While the monsoon trough was oriented in a reverse fashion (NE-SW) in July as predicted, it was much less active than anticipated. Weak to non-existent monsoon conditions persisted through August and September, keeping rainfall amounts over Yap and the Mariana Islands considerably less than expected. As anticipated, tropical cyclones built farther to the north, but they were very small in size. Most of Micronesia will have below normal to near normal rainfall for the remainder of 1999. For American Samoa, we now believe that both dry season and wet season rainfall values will be slightly below normal, although there is some sign that the SPCZ is becoming more active around 15S. The SOI is expected to continue to hover at near-normal values for the next year.

Despite the inactivity of the monsoon, divergent winds associated with the periodic passage of upper cyclones in the Tropical Upper Tropospheric Trough (TUTT) initiated several tropical disturbances, which contributed significantly to Micronesian rainfall during the period. However, between these disturbances, the TUTT produced convergent upper level winds, suppressing rainfall. Surface heating also helped to initiate thunderstorms over the mountain islands of Micronesia. The combination of the two rain sources produced adequate rainfall over the islands.


Tropical cyclone (TC) activity has been below normal in the western North Pacific. To date, there have been 30 numbered TCs. Only 23 named tropical storms and typhoons have occurred and most of these have been weaker and smaller than normal. September was quite active with six named tropical cyclones, but all activity was west of 132ºE. Only one super typhoon has developed so far this year, —Super Typhoon Bart in September. A As predicted, many of the tropical cyclones developed to the north, affecting Japan. A Although we expected a shift in the location of tropical cyclone development back toward more eastern longitudes, a more La Niña-like pattern developed, with a westward shift in the region of development. Subdued monsoon activity and a poorly developed near-equatorial trough may delay development east of 150E until the fall of 2000. American Samoa and southern hemisphere locations will see a shift in tropical cyclone development back to the west, generally west of the date line, to a more normal status.

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: The 1999-2000 cool season for the Hawaiian Islands kicked off dramatically during October with the development of a large kona storm to the northwest of the island chain. The storm brought periods of heavy rainfall and varying degrees of flooding across the state from 18 through 21 October. Flash flood warnings were issued for portions of Kauai (18 and 19 October) and Oahu (19 October) where two to five inches of rainfall fell within a 24-hour period. Flash flooding also stranded 12 tourists in Ohe'o Gulch on the island of Maui during the afternoon of 20 October. Eyewitness reports stated that stream levels rose 15 to 20 feet in less than one minute. Fortunately, there were no injuries or fatalities.

Conditions prior to and after the kona storm mainly involved moderate trade winds over the island chain. Shower areas embedded within this trade wind flow brought intermittent light to moderate rains to the all the islands with daily totals mostly under one inch.

Kevin Kodama - Senior Service Hydrologist, NWSFO Honolulu, HI

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

American Samoa: Rainfall in the region was slightly below normal as anticipated. The continuation of La Niña conditions kept rainfall somewhat below, and this is likely to continue until June or July. Tropical cyclone activity is not expected to be significant in the Samoa region until the next El Niño event develops.

Rainfall in American Samoa was slightly below normal as anticipated in the last ENSO Newsletter. The extension of La Niña conditions shifted SPCZ activity to the west and kept the region drier. An east-west oriented portion of the SPCZ did stagnate over the Samoa region in September, more than doubling the normal rainfall. At Pago Pago, July, August, and September rainfall was measured at 3.41 inches (54%), 5.71 inches (85%), and 14.16 inches (212%). The 3-month average was about 117% of normal, primarily due to the very wet September. Most climate models suggest that La Niña will continue until the Jun-July timeframe. This should continue to keep rainfall slightly below normal to near normal until the next dry season. However, high month-to-month variability will likely accompany the return to near normal values.

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

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

Inclusive Period         % of long-term average
                              Samoa Region
Nov 1999 - Jun 2000                95%
Jul - Dec 2000                    100%

- sources: UOG-WERI and PEAC

Guam/CNMI: Strong monsoon activity did not develop across the Northwest Pacific as anticipated. Continuing La Niña conditions brought in stronger than expected trade winds, and this kept the monsoon and tropical cyclone activity well west of its normal location. We expect rainfall for Guam and the CNMI to be slightly below normal until summer. Then, rainfall and tropical cyclone activity should return to near normal modes. For rainfall, month-to-month variability can be great.

July through September rainfall on Guam and in the CNMI showed high variability, with rainfall strongly related to the passage of cyclonic systems in the Tropical Upper Tropospheric Trough (TUTT) and to a few tropical disturbances associated with the TUTT. July, August, and September rainfall at the Guam International Airport (GIA) was 12.81 inches (122%), 8.39 inches (61%), and 11.94 inches (89%), respectively. For the same months at Andersen Air Force Base (AAFB), rainfall was 15.12 inches (138%), 5.66 inches (42%), and 13.16 inches (99%). At Saipan International Airport, July, August, and September rainfall amounted to 6.08 inches (58%), 12.75 inches (102%), and 14.12 inches (105%). Values at Capital Hill were 11.15 inches (79%), 11.26 inches (90%), and 11.25 inches (83%). Rota Airport measured 10.54 inches (101%), 12.53 inches (95%), and 11.73 inches (88%) during the respective three months. Rainfall was similar at the NASA Tropical Rainfall Measuring Mission rain gauge network located on the Rota Resort and Country Club on the north side of Rota. Tinian was drier than other locations, averaging 9.32 inches (79%) for the 3-month period.

Our last ENSO Update Newsletter predicted wetter than normal conditions for the Mariana Islands during the summer period. With the lack of a strong monsoon, cyclonic circulations in the TUTT attributed to most 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 of 2000 in association with the return of stronger monsoon activity. However, until then, La Niña conditions will likely keep rainfall slightly below normal. Tropical cyclones could threaten the area from October through early December.

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

Inclusive Period         % of long-term average
                      Guam/Rota       Saipan/Tinian
Nov - Dec 1999           90%               85%
Jan - Jun 2000           95%               95%
Jul - Dec 2000          100%              100%

- sources: UOG-WERI

Yap State: Rainfall for Yap was lower than expected due to feeble monsoon activity. Although lingering La Niña conditions kept stronger than anticipated trade winds in the region, rainfall is expected to return to normal by summer. Tropical cyclone activity could occur in November and December of 1999.

While we predicted above normal rainfall for Yap State during July, August, and September in the last Pacific ENSO Update, most areas experienced near-normal to below normal rainfall. Three-month rainfall amounts were 95% of normal for Yap, 99% of normal for Ulithi, and 86% of normal for Woleai. Yap Airport recorded 12.96 inches (89%) in July, 19.74 inches (130%) in August, and 8.76 inches (65%) in September. Conditions were similar at Ulithi with 9.33 inches (76%), 13.33 inches (103%), and 11.37 inches (99%) for the same three months. As expected, conditions were slightly drier in the southern Yap islands, where Woleai Atoll experienced rainfall amounts of 8.38 inches (60%), 14.27 inches (97%), and 11.91 inches (102%) for the respective three months. An inactive monsoon and a weak monsoon trough over the region kept conditions drier than normal during the summer. Yap State islands are expected to have near normal to slightly above normal rainfall into the summer as a result of residual La Niña effects. Rainfall should return to normal by summer. Tropical cyclones could affect Yap State from October-December.

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

Inclusive Period                % of long-term average
                             Yap               Outer Atolls:
                            Island       S.of 8·N     N. of 8·N
Nov-Dec 1999                  90%           85%          95% 
Jan-Jun 2000                 110%          115%         110%
Jul-Dec 2000                 100%          100%         100%

- sources: UOG-WERI

Chuuk State: As expected, Chuuk was drier than normal during the last quarter, primarily due to a very dry August. Rainfall for Chuuk State is expected to be slightly below normal until January, then a little above normal until summer when normal rainfall returns. Tropical cyclone activity around Chuuk State could occur in November and December 1999, and could also occur in April and May 2000.

Rainfall at Weno Island during July, August, and September was 13.55 inches (112%), 4.02 inches (28%), and 13.76 inches (119%) - —86% for the period. Luchonor was wetter with 19.38 inches (160%), 6.82 inches (47%), and 12.99 inches (113%) for the same months. T The dry conditions that began in Polowat in May continued through the summer with 8.02 inches (57%), 5.47 inches (36%), and 11.15 inches (84%) during the period. This amounts to only 54% of the 3-month normal. We expect that Chuuk State will be somewhat drier than normal until January. Then, slightly above normal rainfall should occur in response to La Niña conditions. Normal rainfall amounts are expected to return by summer. Chuuk State could experience a tropical cyclone in November and December. Tropical cyclone activity could resume in April and May.

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

Inclusive Period               % of long-term average
                        Chuuk         Outer Atolls
                        Lagoon    Southern    Western
Nov-Dec 1999             90%        85%         80%
Jan-Jun 2000            110%       115%        110%
Jul-Dec 2000            100%       100%        100%

- sources: UOG-WERI

Pohnpei State: As predicted in our last Pacific ENSO Update, after a very wet winter and early spring, summertime rainfall for Pohnpei Island was drier than normal - —in fact 30% below normal for the 3-month period. Pohnpei State will be somewhat drier than normal until January, then near normal. Tropical cyclone activity around Pohnpei will likely not occur until October and November 2000.

For July, August, and September, Kolonia recorded 13.93 inches (76%), 8.46 inches (51%), and 13.57 inches (84%). While rainfall varied across the island, it was below normal at all locations. Pingalap was much wetter than Pohnpei, with 18.18 inches (114%), 21.55 inches (145%), and 28.72 inches (192%) during the respective three months. Nukuoro was also wetter than Pohnpei, with monthly rainfall amounts of 23.88 inches (164%), 20.20 inches (178%), and 9.22 inches (84%). Kapingamarangi exhibited variable rainfall with 9.31 inches (89%) in July, 13.82 inches (224%) in August, and only 1.29 inches (22%) in September. Most of Pohnpei State rainfall came from tropical disturbances that flared up in response to the passage of cyclonic cells in the TUTT. The center of the disturbances moved in an arc over Pingalap, then south of Pohnpei, and across Nukuoro. Most of Pohnpei State is expected to have slightly below normal rainfall until January, then slightly above normal rainfall to June, when we expect rainfall to return to normal. Kapingamarangi is expected to be much drier than normal until July due to continued La Niña conditions.

Pohnpei State and the eastern islands will not be very susceptible to a direct hit by a tropical cyclone until the next El Niño event begins to occur. However, Pohnpei could see some effects of tropical cyclones that pass to the north in October and November 2000.

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

Inclusive Period          % of long-term average
                     Pohnpei           Outer Atolls:
                     Island    Eastern    Southern    Equatorial
Nov-Dec 1999           85%       90%         85%         70%
Jan-Jun 2000          110%      110%        105%         50%
Jul-Dec 2000          100%      100%        100%        100%

- sources: UOG-WERI

Kosrae State: Kosrae State was only slightly drier than normal for the 3-month period with 98% of its normal rainfall. Kosrae is expected to be slightly wetter than normal from January to June. Tropical cyclone activity around Kosrae is not likely until the next El Niño event occurs.

During July, August, and September, rainfall at Kosrae Airport was variable with 19.89 inches (117%), 13.27 inches (80%), and 16.76 inches (97%), respectively. Tafunsak, Tofol, and Utwa were slightly wetter than the airport. Utwa was wetter than any of the locations in September, when 18.58 inches (108% of normal) of rain fell there. Most of Kosrae’s rainfall came from disturbances associated with cycloni cells in the Tropical Upper Tropospheric Trough and from local, mountain-induced thunderstorms.

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 December 2000 is as follows:

Inclusive Period     % of long-term average
Nov-Dec 1999                  90%
Jan-May 2000                 110%
Jun-Dec 2000                 100%

- sources: UOG-WERI

Republic of Palau: Koror had 114% of its normal rainfall from July –September. Palau is expected to be slightly drier than normal until January, then wetter until May. Monsoon activity and rainfall should return to normal by June. A tropical cyclone could affect Palau in November or December.

Continuing La Niña conditions restricted monsoon activity to the western part of the basin with heavy rainfall over the South China Sea and the southern Philippines. Even though a "reverse oriented" monsoon trough developed south of Japan, it was relatively weak and narrow. As a result, the Republic of Palau experienced greater than normal rainfall in July and August. Conditions became dry in September. In Koror, July rainfall was 18.33 inches (102%), August rainfall was 26.00 (174%), and that in September was only 8.00 inches (67%). Farther south in Peleliu, total 3-month rainfall was slightly less, with monthly accumulations of 12.54 inches (70%), 29.29 inches (197%), and 7.64 inches (64%). Amounts at Tobi Island far to the south-southwest were likely less, while those at Kayangel to the north were probably a little drier than Koror.

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 December 2000 for Palau is:

Inclusive Period              % of long-term average
                    Koror and           Outer Atolls
                    Babelthaup     S. of 8ºN    N. of 8ºN
Nov-Dec 1999           90%             90%         80%
Jan-May 2000          110%            110%        100%
Jun-Dec 2000          100%            100%        100%

- sources: UOG-WERI

Republic of the Marshall Islands: With the forecast of continuing La Niña conditions into June, we expect conditions to be drier than normal until January, then slightly wetter than normal until June. Normal rainfall patterns should return by June or July. Tropical cyclones will not likely affect the RMI until September of 2000.

While we indicated drier than normal conditions in the summer and fall for the RMI in our last Pacific ENSO Update Newsletter, rainfall was somewhat greater than expected. This was primarily the result of the combination of persistent disturbances associated with cyclonic cells in the Tropical Upper Tropospheric trough and a well developed trade wind trough. Majuro (representative of the southern islands) had both more rainfall and a higher percentage of rainfall than Kwajalein (representative of the northern islands). At Majuro, July, August, and September rain amounts were 13.07 inches (101%), 10.99 inches (95%), and 11.35 inches (91%), respectively. Arno had similar rainfall to Majuro, while Mili, Ebon, Maleolap, and Ailinglapalap had a little less over the 3-month period, while Jaluit had more. Rain measured at Kwajalein was 10.61 inches (102%), 8.93 (88%), and 8.93 inches (95%) for the respective months. Ebeye had amounts similar to Kwajalein, but Wotje, Utirik, and Mejit were likely a little drier. Satellite data indicated that Wake, Enewetok, Ujae, and Ujelang were significantly drier than Kwajalein.

Conditions in the RMI are expected to remain 10-15% drier than normal until January, then should be somewhat wetter until summer due to the development of a well-developed trade wind trough. Normal rainfall patterns should return to the islands by that time. The RMI could experience a tropical cyclone from September to November 2000 at Wake and Enewetok and from October to December 2000 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 December 2000 are as follows:

Inclusive Period           % of long-term average
                                  RMI Atolls
                            Southern     Northern
Nov-Dec 1999                   90%          85%
Jan-May 2000                  115%         105%
Jun-Dec 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 general information for those Micronesian areas. Since we are not receiving any rainfall data from those locations, we must base the predictions on historical rainfall data, meteorological satellite imagery, and sea surface temperature and SOI predictions.

With the now predicted continuation of La Niña conditions until June, we expect Nauru, equatorial and southern hemisphere portions of Kiribati and northern parts of Tuvalu to be significantly drier than normal. While heavier and more frequent rains appear to have returned to the northwestern Kiribati (Matkin, Butaritari, and Tarawa), a revitalized La Niña brought dry conditions back to equatorial areas. Rainfall may not return to near-normal values until the July-September timeframe. Beru and Arorae will not likely experience significantly wet weather until the next El Niño event occurs. Some relief for these two island regions is possible by December, but it will not likely last longer than 2-3 months. The rain will be useful for catchments, but will not do much to restore the fresh water lens. Christmas Island will continue t o have very dry weather until the next El Niño event.

This was expected to be the last of the last of these special predictions, however, we are continuing them in light of the revitalized La Niña. Without monthly rainfall data for these locations, we cannot make credible predictions as conditions return to normal. We welcome daily and/or monthly rainfall data from these locations. Data can be sent via e-mail to: 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

SPECIAL SECTION - New Markov model for Pacific SST and sea level


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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