Pacific ENSO Update

4th Quarter 2000 - Vol. 6 No. 4


During the last 3 months, sea surface temperature (SST) distribution in the equatorial Pacific and the Southern Oscillation Index (SOI) have continued to hover at near normal values. La Niña conditions that persisted from the boreal (Northern Hemisphere) fall of 1998 until the boreal spring of 2000 have virtually disappeared, though there are some small residual oceanic and atmospheric La Niña effects still evident. Sub-surface sea temperatures in the western equatorial Pacific have increased over the past year, setting the stage for the next El Niño event.


Over the last six months, cold SST anomalies in the equatorial central and extreme eastern part of the Western Pacific have fallen to -0.4°C and 0°C respectively, well within the "normal" range. The characteristic "cold tongue" that extended from the equatorial Eastern Pacific into the equatorial Western Pacific has virtually disappeared. This indicates the demise of the 2-year 1998-2000 La Niña, and signals the recurrence of normal oceanic conditions. Most Global Climate Models (GCMs) now support the rapid return to near zero SST anomalies, but now with a bias toward slightly warmer than normal SSTs by the boreal (Northern Hemisphere) spring. This is illustrated in Figure 1, which shows the individual and average SST predictions for the Central Pacific (170ºW-120ºW) region produced by the National Centers for Environmental Prediction coupled model. A few other models suggest a slower change of the cooler than normal temperature to positive SST anomalies. At this time, it is too early to accurately predict when Central Pacific SST anomalies will become positive enough to declare the next El Niño event. Thus, the current and predicted SSTs suggest that the next El Niño will occur in 2002 or later. A recurrence of significant La Niña conditions in 2000-2001 is now deemed small, but some residual La Niña effects are still apparent.

In our last newsletter, sub-surface sea temperatures in equatorial and near-equatorial parts of the Western North Pacific had warmed to more than 5°C above normal at the 150-foot depth between 160°E-170°E. This anomaly had fallen to only 3°C above normal in September and only 2ºC in October. At the same time, the depth of the warmer than normal sub-surface temperatures has increased, in some places to more than 400 feet. Warm anomalies up to 2°C extend down to 250 feet. Warm anomalies greater than 2°C are not extending eastward much beyond the Date Line. Since our last newsletter, some colder than normal sub-surface SSTs have reappeared in the Central and Eastern Pacific. Sub-surface SST profiles now suggest that another El Niño is not likely until 2002 or later.

Sea level heights should be back to near normal values over the entire region. Local storm conditions, strong and persistent trade winds, nearby tropical cyclones, and favorable spring and fall astronomical conditions could all lead to short periods (lasting from days to weeks) of above normal sea levels and tides for all locations.


After a 2-3 month period of negative SOI values, the index began to rise again and has hovered near +1 standard deviation for the last 2 months. This has manifested itself in some residual La Niña effects in the atmosphere. For example, equatorial regions from 135ºE to 180º are more subsident than normal, suppressing normal cloudiness and rainfall. Also, equatorial easterlies have remained stronger than normal, although the anomaly is half its value in June. Despite these conditions, other atmospheric conditions point toward a move to normalcy. Adequate, if not normal, rains have returned to the equatorial areas. Tropical cyclones are developing much farther to the east than in the previous two years. Monsoon westerly winds have extended east of 145ºE on three occasions, three more than in 1998 and 1999. We expect the SOI to slowly return to normal in response to normal SST distribution. This should lead to a further weakening in equatorial easterly winds.

With the SOI back to near neutral, atmospheric circulation patterns should exhibit normal behavior. The positions (and normal variability) of the monsoon troughs, the trade wind troughs (ITCZs), the South Pacific Convergence Zone (SPCZ), and the mid-latitude jet streams should become normal, with fairly normal distributions of clouds and rainfall. It should be understood that "normal behavior" often exhibits high month-to-month variability in cloudiness and rainfall. In much of Micronesia, the rainfall surplus from the heavy dry-season La Niña-induced rains balanced the rainfall deficit from the earlier El Niño-induced drought. As a result, at these locations, normal rainfall is about 100% of the long-term average. This is not the case for the Mariana Islands, northern Yap State, the northern Marshall Islands, and near-equatorial areas between 150ºE and the Date Line. At these locations, normal rainfall will be somewhat wetter than the long-term average. It is also not the case for American Samoa, which will likely be slightly drier than normal. There, wet conditions from weak and moderate El Niño events more than balance the drought conditions associated with intense El Niño events.


As of late November, only 22 named tropical cyclones (by the Japan Meteorological Agency (JMA)) had formed in the Western North Pacific. The Joint Typhoon Warning Center (JTWC) had identified 32 total tropical cyclones (named and unnamed) over the same period. Thus, the activity is about 20% below normal for the season. Most of the tropical cyclones developed in western longitudes or at high latitudes in response to La Niña conditions earlier in the year. By August, cyclone genesis had shifted more to the east with the development of Typhoons Jelawat, Ewiniar, Saomai, Shanshan, and Yagi. And Typhoon Xangsane developed from a large monsoon depression in the monsoon trough-the first such classical development in two years. Based on the expected resumption of normal SST and SOI conditions, normal tropical cyclone activity is anticipated for the rest of this year and likely through 2001. This means that western and central Micronesia could see typhoon activity until late-December. Eastern Micronesia could see a few tropical storms and weak typhoons develop in the area until mid-December. American Samoa is not expected to experience a significant tropical cyclone until the next El Niño, although it could see a tropical depression or weak tropical storm in the January-March 2001 timeframe.

A new tropical cyclone name list has been inaugurated for the Western North Pacific that uses names provided by countries of the Western Pacific and East Asia. Tropical cyclone names are now issued by the Regional Specialized Meteorological Center in Tokyo, Japan. A list of the names was given in Table 1 of our last (3rd Quarter) Pacific ENSO Update. A list of the storm names, their pronunciations, and their meanings is available at

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:October marked the start of the Hawaiian cool/wet season and the 2000-2001 cool season started quite dramatically with several heavy rain events during the last week of the month. These events were generated by an upper level low that passed north of the island chain from 26 through 30 October. The low helped destabilize the underlying trade wind marine layer and spawned heavy showers and thunderstorms over the Big Island on 26 and 27 October, and very strong thunderstorms over eastern Maui on 28 and 29 October. The east Maui storms generated tremendous amounts of runoff that resulted in several road closures and stranded tourists in the Hana area overnight. Several individuals also required rescue from floodwaters. Fortunately, no fatalities occurred and roads were reopened the next day following debris removal. Strong thunderstorms also occurred over eastern Moloka'i during the afternoon of 29 October and resulted in the rescue of one individual from a vehicle trapped in floodwaters. The final heavy rain event of the month was marked by heavy shower development over the windward sections of Kaua'i that resulted in some stream flooding.

Prior to these end of the month events, October's weather mainly consisted of moderate to fresh trade winds. These trades brought considerable amounts of rainfall to the windward sides of the Big Island and Maui but were rather unremarkable over the rest of the island chain. An upper level low that passed north of the islands on 11 and 12 October helped trigger afternoon thunderstorms over the higher elevations of Mauna Loa on the Big Island. These storms were short-lived and did not produce any significant flooding.

Kevin Kodama - Senior Service Hydrologist, NWSFO Honolulu, HI

For a more complete summary, and the county by county wrap-ups, please see the October 2000 Precipication Summary from the National Weather Service Honolulu Weather Forecast Office.

American Samoa: Rainfall at Pago Pago for the first 9 months of 2000 was 86.71 inches or 101% of the norm. At Pago Pago, July, August, and September rainfall was 3.34 (53%), 3.21 (48%), and 6.77 inches (101%), respectively, amounting to only 66% for the 3-month period. In October, rainfall increased to 11.08 inches (103%). With fairly persistent southeast trade winds, east and southeast locations on Tutuila likely received more rain.

The below normal rainfall experienced in the Samoa region for the July and August timeframe was the result of stronger than anticipated subsidence associated with the lingering effects of the La Niña. With equatorial SSTs and the SOI expected to return to normal values, rainfall should also be normal for the next year, but it will continue to exhibit high month-to-month variability. "Normal" rainfall for Pago Pago is slightly below the long-term average; that is to say that normal is 90-95% of the long-term average. Significant tropical cyclone activity is not expected in the Samoa region until the next El Niño event occurs, but some tropical depressions and weak tropical storms could bring periods of heavy rains as they develop in the east-west orie nted portion of the SPCZ and move through the region.

Predicted rainfall for American Samoa from Nov 2000 through Dec 2001 is as follows:

Inclusive Period         % of long-term average
                              Samoa Region
Nov 2000 - Dec 2001                95%

- sources: UOG-WERI and PEAC

Guam/CNMI: Spring and early summer on Guam were considerably drier than normal. Rainfall at Guam International Airport (GIA) during July, August, and September was 6.17 inches (59%), 18.40 inches (134%), and 12.69 inches (94%), respectively. This amounted to 96% of normal values. October measurements were 11.40 inches (95%). Over the same period, Andersen Air Force Base (AAFB) measured 14.07 inches (129%), 15.45 inches (115%), and 12.46 inches (94%), or 113% of normal for the period. October rainfall was 15.74 inches (122%). Heavy rains also drenched southern and western parts of the island for days favorable for thunderstorm development in these areas.

July, August, and September rainfall at Saipan International Airport (SIA) was 7.50 inches (93%), 9.20 inches (74%), and 20.50 inches (152%), or 106% of normal for the period. Precipitation in October was 10.96 inches (101%). Capitol Hill was somewhat wetter, as usual, with 7.43 inches (83%), 12.97 inches (104%), and 15.49 inches (115%). October rain fell to 9.92 inches (83%). Three-month rainfall amounts at the Tinian Airport were 13.68 inches (152%), 15.31 inches (122%), and 10.33 inches (77%). The 3-month average rainfall for Tinian Airport was 117%. October rainfall there was 16.76 (140%). At Rota Airport, July, August, and September saw rain amounts of 10.94 inches (105%), 14.63 inches (111%), and 15.02 inches (112%), or 109% for the 3 months. In October, the amount was 16.18 inches (128%). On the northern part of the island, at the Rota Resort and Country Club, the NASA TRMM network there recorded 98% of that at the Airport in July-September, but 104% of that at the Airport in October.

Rainfall for Guam and the CNMI is expected to return to normal. Unlike much of Micronesia, La Niña-induced dry season rains do not quite compensate for the rainfall deficit created from the El Niño-induced drought. Thus, normal rainfall for the Mariana Islands is slightly greater than 100% of the long-term average; that is to say that normal is 105-110% above the long-term average. This additional rain is due to the active monsoon and the increased tropical cyclone activity expected during years not affected by El Niño-induced drought and wet and dry La Niña conditions. However, as is common during "normal" periods, there will be high month-to-month variability in the rainfall. Tropical cyclone activity should also return to normal. Thus, the Mariana Islands can expect typhoon threats primarily from September through December. Most will approach from the east-southeast. A secondary period of occurrence is in April and May. While typhoons in these months are more rare, they can be very intense, as was the case with Pamela in May1976 and Andy in April 1989.

Predicted rainfall for the Mariana Islands from Nov 2000 through Dec 2001 is as follows:

Inclusive Period         % of long-term average
                      Guam/Rota       Saipan/Tinian
Nov 2000 - Dec 2001     110%              110%

- sources: UOG-WERI

Yap State: Yap Island was drier for the three months, July through September, than for the previous 3-month period. The weather station at the Yap Airport recorded 20.43 inches (141%) in July, 15.40 inches (101%) in August, and 9.14 inches (68%) in September, or 103 % of normal rainfall for the three months. In October, 18.01 inches (150%) fell there. Ulithi, 100 miles to the northeast had significantly less rain than did Yap. July, August, and September rainfall amounts on Ulithi were 18.01 inches (146%), 17.02 inches (146%), and 2.63 inches (23%) or 105% for the 3-month period. October was wetter with 11.02 inches (108%). Farther south at Woleai Atoll, rainfall totaled 15.12 inches (108%) in July, 18.96 inches (129%) in August, and 4.16 inches (36%) in September, for a 3-month average of 91%. Rain on the atoll in October was measured at 11.86 inches (87%).

Rainfall for Yap State is expected to be about 100%-110% of the long-term average, depending on location. Unlike much of Micronesia, La Niña-induced dry-season rains do not compensate for the rainfall deficit created from the El Niño-induced droughts. Thus, normal rainfall for northern sections of Yap State is greater than 100% of the long-term average. This additional rain is due to the active monsoon and the increased tropical cyclone occurrence. Tropical cyclone activity should be normal, and this means that the northern Yap State islands could be affected by tropical storms and typhoons from late September through mid-December, while southern islands can be affected from mid-October until late-December. There could also be a threat of storms in May and June.

Predicted rainfall for Yap State from Nov 2000 through Dec 2001 is as follows:

Inclusive Period                % of long-term average
                             Yap               Outer Atolls:
                            Island       S.of 8·N     N. of 8·N
Nov 2000 - Dec 2001          105%          100%         105%

- sources: UOG-WERI

Chuuk State: During the third quarter of the year, Chuuk lagoon islands were wetter than normal, while the outer islands to the south and west were drier than normal. During July, August, and September, the weather station at Weno Island measured 10.13 inches (84%), 22.73 inches (156%), and 13.50 inches (117%). This amounted to119% of the norm for the period. In October, 15.89 inches (118%) were recorded. At Lukunoch, rainfall for the respective months was 9.46 inches (78%), 19.41 inches (133%), and 4.63 inches (40%). October rain there was 11.42 inches (85%). Rain in Polowat was even less with 8.35 inches (60%), 12.89 inches (86%), and a very dry 1.80 inches (14%). October saw increased, but continued below normal rainfall return to Polowat with 7.04 inches ( 59%).

Rainfall for all of Chuuk State is expected to be below normal until September or October. After that time, rainfall should return to normal, but with high month-to-month variability. Tropical cyclone activity should also return to normal. This means that Chuuk will have a higher risk of getting a tropical storm or typhoon, primarily during the months of October, November, and December. Some risk will also occur in April and May as well.

Predicted rainfall for Chuuk State from Nov 2000 through Dec 2001 is as follows:

Inclusive Period               % of long-term average
                        Chuuk         Outer Atolls
                        Lagoon    Southern    Western
Nov 2000 - Dec 2001     105%        95%        100%

- sources: UOG-WERI

Pohnpei State: The third quarter of 2000 for Pohnpei State was slightly drier than normal to near normal for the entire state. Observations from Pohnpei Island suggest that coastal areas of the island were wetter than the interior locations, but this is not normally the case during the summer months. Thus, for whatever reason, some of the interior observations may reflect lower values of rainfall than actually occurred. July, August, and September rainfall measured at the weather station at Kolonia was 13.58 inches (97%), 19.56 inches (118%), and 10.44 inches (65%). The weather station also measured 15.14 inches (91%) in October. At Pingalap, wet conditions resumed in July and August with 16.55 inches (104%) and 23.43 inches (158%), respectively, but drier conditions returned in September, which had only 6.71 inches (45%). October saw a resumption of rain with 14 74 inches (100%). At Nukuoro, precipitation amounts were 11.73 inches (81%), 17.34 inches (153%), and 9.84 inches (61%), respectively. October rainfall was 13.83 inches (129%) for the island. With the disappearance of the La Niña, rainfall increases seen in May and June at Kapingamarangi continued with measurements of 5.73 inches (55%), 7.69 inches (125%), and 5.58 inches (95%), respectively. October rainfall for Kapingamarangi was not available.

Rainfall for all areas of Pohnpei State is expected to be near normal throughout the forecast period. However, high month-to-month variability can be expected. While tropical cyclones are not expected to pose a serious threat to Pohnpei State until the next El Niño event, the islands could experience some periods of high surf from typhoons passing to the north.

Predicted rainfall for Pohnpei State from Nov 2000 through Dec 2001 is as follows:

Inclusive Period          % of long-term average
                     Pohnpei           Outer Atolls:
                     Island    Eastern    Southern    Equatorial
Nov 2000 - Dec 2001   100%      100%        100%         95%

- sources: UOG-WERI

Kosrae: The drier than normal conditions that replaced La Niña-induced heavy rains in May, continued into September. The automated weather station at Kosrae Airport recorded 12.55 inches (74%), 19.39 inches (118%), and 9.40 inches (56%) for July, August, and September. This amounted to 83% of normal rainfall for the period as compared to 84% for the previous quarter. In October, the Airport had a considerable increase with 19.92 inches (123%). In August, Utwa recorded 25.78 inches, making it the wettest location during the period on Kosrae. Conditions at Tafunsak seemed to be slightly drier than at other locations on the island. In October, the Airport had the wettest measurement on the Island, with Utwa and Tafunsak measuring about one-half inch less durin g the month.

Rainfall for Kosrae is expected to be near normal through the remainder of the forecast period. However, high variability should be expected in the month-to-month rainfall. Kosrae should not experience any tropical cyclones until the next El Niño event begins.

Predicted rainfall for Kosrae State from Nov 2000 through Dec 2001 is as follows:

Inclusive Period     % of long-term average
Nov 2000 - Dec 2001            100%

- sources: UOG-WERI

Republic of Palau: Palau was wetter than normal during July and August, but significantly drier than normal in September. Heavy rains in July and August were the result of the re-emergence of the southwest monsoon into the western North Pacific. Stronger than normal trade winds in September increased subsidence over the nation, reducing cloudiness and rainfall. The weather station at Koror recorded 20.38 inches (113%) in July, 17.16 inches (115%) in August, and a meager 3.10 inches (26%) in September. This amounted to a 3-month average of 85%. In October, rainfall increased to 13.49 inches (97%). For the three month, rainfall at the Mariculture Center and at Nekken Forestry was greater than at the weather station. October rainfall at Mariculture was identical to that at the weather station, while Nekken Forestry was significantly wetter with 16.83 inches. Farther south at Peleliu, conditions were drier with 14.44 inches (81%) in July, 16.02 inches (105%) in August, and 3.56 inches (30%) in September. This produced a 3-month average of only 72%. In October, the island had 15.38 inches or 111% of that expected.

For the entire Palau island chain, we expect rainfall to return to near normal. Characteristic of "normal" conditions is the likelihood of high month-to-month variability in rainfall. This rainfall behavior should last until the next El Niño event begins. Tropical cyclone activity should also return to normal, meaning that all islands from Angaur northward could experience a typhoon from October through December. Islands south of Angaur are generally not directly threatened by typhoons, although swell from typhoons passing to the north could cause dangerous surf.

Predicted rainfall for Palau from Nov 2000 through Dec 2001 is as follows:

Inclusive Period              % of long-term average
                    Koror and           Outer Atolls
                    Mountain Is.  S. of 8ºN    N. of 8ºN
Nov 2000 - Dec 2001   100%            100%        100%

- sources: UOG-WERI

Republic of the Marshall Islands (RMI): Despite the increase in rainfall from the second quarter to the third quarter of 2000, the Republic of the Marshall Islands (RMI) continued to be drier than normal. Most of the increased rainfall was the result of increased convection and rainfall associated with the frequent passage of upper level cyclonic cells in the Tropical Upper Tropospheric Trough (TUTT). During July, August, and September, the Majuro weather station (representative of the southern islands) measured 11.31 inches (87%), 11.06 inches (96%), and 7.02 inches (57%). For the period, the average was 80%, a substantial increase over the 55% of the previous quarter. In October, rainfall was 12.27 inches, or 89% of normal. Kwajalein rainfall (representative of the central islands) during the third quarter increased to near normal values, after a very dry second quarter. Rainfall at Kwajalein (and nearby Ebeye), was 9.94 inches (95%) in July, 12.46 inches (123%) in August, and 9.03 inches (76%) in September. This was an average of 98% for the period. In October, amounts rose to 13.85 inches or 116% of that normally expected. Rainfall amounts at Laura were similar to those inMajuro. Ailingalaplap had somewhat less rainfall than Majuro, with only 65% of normal rainfall for the period. Jaluit's rainfall was similar to that in Kwajalein. Rainfall increased at Wotje during the July-September timeframe, and amounted to a 3-month average of 82%. In October, rainfall ranged from 73% of normal at Ailingalaplap, to 67% at Jaluit, to 61% at Wotje. Observations were not available from Utirik, however, satellite imagery indicated that rainfall increased dramatically over that received during the first six months of 2000. In fact, from January-June, Utirik received a total of only 2.3 inches (5% of normal). This was likely as dry as the 1998 El Niño-induced drought.

Rainfall for the Marshall Islands is expected to be somewhat drier than normal until about March, and then normal for the remainder of the forecast period. Tropical cyclone activity for the Marshalls should return to normal, meaning that Wake and Enewetok could see typhoons from September into November. The remainder of the islands south of 12°N could experience a tropical storm or minimal typhoon in October and November. The eastern Marshall Islands will not likely experience an intense typhoon until the next El Niño event.

Predicted rainfall for the RMI from Nov 2000 through Dec 2001 is as follows:

Inclusive Period           % of long-term average
                                  RMI Atolls
                            Southern     Central   Northern 
Nov 2000 - Feb 2001            90%          90%       65%
Mar 2000 - Dec 2001           100%         100%      100%

- sources: UOG-WERI


Long-Lead Outlook for Hawaiian Islands issue dated 14 December 2000, from NOAA's Climate Prediction Center (CPC).


SPECIAL SECTION - Markov model for Pacific SST and sea level

SPECIAL SECTION - Experimental Forecasts for Pacific Island Rainfall


The information contained in the LOCAL VARIABILITY SUMMARIES section and elsewhere in this issue of the Pacific ENSO Update has been drawn from many sources. Further information may be obtained by contacting your local National Weather Service office, or the individuals and institutions listed below:

NOAA National Weather Service - National Centers for Environmental
World Weather Building, Washington D.C. 20233.
Contact CPC at 301-763-8155 for more information on the ENSO Advisory, the Long-Lead Outlook for the Hawaiian Islands, and other publications discussed in this bulletin.

NOAA National Weather Service - Pacific Region
University of Hawaii - Manoa Campus
HIG #225, 2525 Correa Road, Honolulu, Hawaii 96822
Contact the WSFO at 808-973-5270 for more information on NWS-PR sources of climate information.

Lower campus, University of Guam
UOG Station, Mangilao, Guam 96923
Contact C. Guard or M. Lander at (671)735-2685 for more info on tropical cyclones and climate in the Pacific Islands.

University of Hawaii (UH) School of Ocean and Earth Science and Technology
HIG #331, 2525 Correa Road, Honolulu, Hawaii 96822
Contact Dr. T. Schroeder at 808-956-7476 for more information on hurricanes and climate in Hawaii.

HIG #331, 2525 Correa Road, Honolulu, Hawaii 96822
Contact C. Palmer 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:

Cynthia Palmer
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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