After 3 years of El Niņo and La Niņa conditions, sea surface temperature (SST) distribution in the equatorial Pacific and the Southern Oscillation Index (SOI) have returned to near normal vaules. La Niņa conditions that persisted from the boreal (Northern Hemisphere) fall of 1998 until the boreal spring of 2000 have virtually disappeared. Sub-surface sea temperatures in the western equatorial Pacific have increased since last year, setting the stage for the next El Niņo event.
Over the last two months, cold SST anomalies in the equatorial Central and extreme eastern part of the Western Pacific have fallen to -0.3°C, within the "normal" range. The characteristic "cold tongue" that extended from the equatorial Eastern Pacific into the equatorial Western Pacific has virtually disappeared. This signals the demise of the La Niņa, and a likely recurrence of normal oceanic conditions. Several Global Climate Models (GCMs) support the return to near zero SST anomalies, but with slow decay of the existing slightly cooler than normal equatorial SSTs. Other models suggest a more rapid decay of the cooler than normal temperatures with slight positive SST anomalies developing by December. At this time, it is too early to accurately predict when Central Pacific SST anomalies will become positive enough to declare the next El Niņo event. While this could occur as early as the boreal spring of 2001, it will more likely occur in 2002 or later. There is some chance of a recurrence of La Niņa conditions by the boreal fall of 2000 as occurred last year, but this likelihood is now deemed small.
Sub-surface sea temperatures in equatorial and near-equatorial parts of the Western North Pacific warmed to more than 4°C above normal in July at the 150-foot depth between 160°E-170°E. Warm anomalies greater than 2°C extended down to 250 feet in the Western Pacific. In addition, warm anomalies greater than 2°C extended into the Central Pacific to 160°W. This sub-surface temperature profile is much greater than that in July 1996 and is similar to that in December 1996 prior to the onset of the 1997 El Niņo. These kinds of conditions are setting the stage for the next El Niņo.
Sea level heights should be back to near normal values, but could be slightly higher than normal given the current subsurface temperature profile. This could especially affect the near equatorial islands of western Kiribati and Pohnpei State, and the island of Nauru. Sea levels in the Samoa region are expected to be near normal. Local storm conditions, nearby tropical cyclones, and favorable spring and fall astronomical conditions could all lead to short periods of above normal sea levels and tides for all locations.
In June, the SOI became negative (-0.6°C) for the first time since September 1999 when the index temporarily attained a value of -0.2°C. Negative values have continued into August (-0.3°C), signifying that atmospheric conditions are falling in line with the waning La Niņa ocean conditions. Despite the fall of the SOI, stronger than normal equatorial easterly winds continued into July in the Central and Western Pacific. This delayed the extension of the monsoon into central portions of the Western Pacific. By August, monsoon flow developed in the eastern part of the Western Pacific and gave rise to the genesis of Typhoons Ewiniar and Bilis. With the SOI returning to normal, we expect equatorial easterly winds to weaken to normal values. In May, heavier, more normal rains returned to equatorial/near equatorial portions of the Western Pacific west of 160°E. Similar rains are expected to return to all equatorial/near equatorial areas west of the Date Line by October.
With the SOI back to normal, atmospheric circulation patterns should exhibit normal behavior. The positions (and normal variability) of the monsoon troughs, of the trade wind troughs (ITCZs), of the South Pacific Convergence Zone (SPCZ), and of the mid-latitude jet streams should become normal, with fairly normal distributions of clouds and rainfall. Figure 1, GMS imagery from 25 August, shows several features that are "normal" for the Western Pacific. West winds along the equator are creeping eastward torward the Date Line. Tropical cyclone and monsoon activity are also inching farther to the east, and convection is also developing in near-equatorial areas between 150°E and the Date Line. Finally, the austral (Southern Hemisphere) winter SPCZ is less active and has shifted westward, giving less rainfall to the Cook Islands, Tonga, and Samoa. 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 Niņa-induced rains balanced the rainfall deficit from the earlier El Niņo-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 150šE 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 Niņo events more than balance the drought conditions associated with intense El Niņo events.
As of mid-August, 11 named tropical cyclones (by the Japan Meteorological Agency) had formed in the Western North Pacific. This is near normal for the season. The Joint Typhoon Warning Center (JTWC) has identified 19 total tropical cyclones (named and unnamed) so far. Thus, the activity is near normal for the season. Most of the tropical cyclones developed in western longitudes or at high latitudes in response to La Niņa conditions earlier in the year. Based on a resumption of normal SST and SOI conditions, normal tropical cyclone activity is expected for the rest of this year and likely through 2001. This means that western and central Micronesia could see typhoon activity from late-September to late-December. Eastern Micronesia could see a few tropical storms and weak typhoons develop in the area from late September to December. Were an El Niņo to develop in 2001 (not expected at this time), then eastern and central Micronesia could see monsoon and possible tropical cyclone conditions through the boreal winter and spring. American Samoa is not expected to experience a significant tropical cyclone until the next El Niņo, although it could see a tropical depression or weak tropical storm in the January-March timeframe. Were an El Niņo to develop in 2001 (not expected at this time), then the Samoa region could expect some increased tropical cyclone activity from October 2001-March 2002.
New tropical cyclone name lists were 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 is given in Table 1 and the list, pronunciations, and meanings are available at http://www.weather.gov.hk/informtc/sound/tcname2000e.htm.
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:Meteorologically speaking, the highlight of the month was the passage of Tropical Storm Daniel to the north of the Big Island on 31 July. In terms of hydrology, Daniel was largely a non-event. Isolated thunderstorms flared-up over the Big Island in association with one of the trailing rain bands and dropped 1 to 3 inches of rain in a couple of areas. Otherwise, statewide rainfall totals attributed to Daniel's circulation were mainly less than 0.25 inches with a few spots between 0.25 and 0.50 inches.
Other than Tropical Storm Daniel, the weather pattern during July was dominated by trade winds in the moderate to fresh range. Active shower bands embedded within the trades brought rainfall to the windward sides of the island chain on a daily basis. This helped push rainfall totals to near to above normal levels at most of the windward gages. Upper level lows also moved near the state on 6 July, 10-12 July, and 25-27 July. These lows helped enhance the trade wind showers and the afternoon convection over the leeward areas of the Big Island and Maui.
Kevin Kodama - Senior Service Hydrologist, NWSFO Honolulu, HI
For a more complete summary, and the county by county wrap-ups, please see the July 2000 Precipication Summary from the National Weather Service Honolulu Weather Forecast Office.
American Samoa: Rainfall for the first 7 months of 2000 was 76.73 inches or 98% of the norm. At Pago Pago, April, May, and June rainfall was 4.85 inches (40%), 15.40 inches (155%), and 7.93 inches (107%), respectively. This amounted to an average of 9.39 inches (101%) per month for the 3-month period. In July, rainfall amounts there fell to 3.34 inches (53%). This high month-to-month rainfall variability is characteristic of "normal" periods in the Samoa region, where rainy season precipitation is closely tied to the location of the SPCZ. From January through May 2000, somewhat stronger than normal southeast trade winds associated with the last La Niņa event likely brought heavier rain amounts to eastern portions of the islands.
With equatorial SSTs and the SOI returning to normal values, we expect rainfall to return to normal, continuing to exhibit high month-to-month variability. We do expect the normal rainfall to be slightly below the long-term average. Significant tropical cyclone activity is not expected in the Samoa region until the next El Niņo event occurs, but some tropical depressions and weak tropical storms could bring periods of heavy rains as they develop in the east-west oriented portion of the SPCZ and move thr ough the region.
Predicted rainfall for American Samoa from Jul 2000 through Sep 2001 is as follows:
Inclusive Period % of long-term average Samoa Region Jul - Sep 2000 100% Oct 2000 - Sep 2001 90%
- 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 April, May, and June was 1.63 inches (42%), 7.44 inches (123%), and 4.74 inches (73%), respectively. This amounted to only 84% of normal for the three months. July also remained drier than normal with 6.17 inches (59%). Over the same period, Andersen Air Force Base (AAFB) measured 1.40 inches (29%), 7.07 inches (107%), and 4.45 inches (70%), or only 73% of normal for the period. However, July rainfall jumped to14.07 inches (129%) as thunderstorms favored the northern part of the island. Heavy rains also drenched southern and western parts of the island for days favorable for thunderstorm development in these areas.
April, May, and June rainfall at Saipan International Airport (SIA) was 1.37 inches (49%), 6.41 inches (146%), and 5.13 inches (110%), or 84% of normal for the period. Precipitation in July climbed to 7.13 inches (88%). Capitol Hill was somewhat wetter, as usual, with 2.69 inches (77%), 5.87 inches (107%), and 4.81 inches (83%). July rain jumped to 7.43 inches (83%). Four-month rainfall amounts at the Tinian Airport were 1.93 inches (77%), 2.73 inches (107%), 6.54 inches (113%), and 13.68 inches (152%). At Rota Airport, April, May, and June saw rain amounts of 1.83 inches (40%), 6.12 inches (97%), and 5.15 inches (83%). In July, the amount was 10.94 inches (105%). On the northern part of the island at the Rota Resort and Country Club, the NASA TRMM network there recorded 78% of that at the Airport in April-June, but 117% of that at the Airport in July.
Rainfall for Guam and the CNMI is expected to return to normal. Unlike much of Micronesia, La Niņa-induced dry season rains do not quite compensate for the rainfall deficit created from the El Niņo-induced drought. Thus, normal rainfall for the Mariana Islands is slightly greater than 100% of 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 Niņo-induced drought and wet and dry La Niņa 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. There could also be a threat of storms in April and May. Most approach from the east to southeast.
Predicted rainfall for the Mariana Islands from Jul 2000 through Sep 2001 is as follows:
Inclusive Period % of long-term average Guam/Rota Saipan/Tinian Jul 2000 - Sep 2001 110% 110%
- sources: UOG-WERI
Yap State: Yap Island was one of the wettest locations in Micronesia from April through July. The weather station at the Yap Airport recorded 7.39 inches (128%) in April, 22.01 inches (243%) in May, and 10.10 inches (80%) in June, or 144% of normal rainfall for the three months. In July, values again rose as the monsoon began to affect the region, with the Airport receiving 20.43 inches (141%). All other locations on Yap had similar rainfall to that at the Airport. Ulithi, 100 miles to the northeast had significantly less rain than did Yap. April, May, and June rainfall amounts on Ulithi were 5.09 inches (104%), 9.81 inches (127%), and 6.64 inches (62%) or 92% for the 3-month period. July was wetter with 12.39 inches (146%). Farther south at Woleai Atoll, rainfall totaled 14.80 inches (135%) in April, 15.11 inches (124%) in May, and 13.39 inches (102%) in June, for a 3-month average of 119%. Rain on the atoll in July was measured at 15.12 inches (108%).
Rainfall for Yap State is expected to be about 100%-110% of the long-term average, depending on location. Unlike much of Micronesia, La Niņa-induced dry season rains for Northern Yap State do not compensate for the rainfall deficit created from the El Niņo-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 Jul 2000 through Sep 2001 is as follows:
Inclusive Period % of long-term average Yap Outer Atolls: Island S.of 8·N N. of 8·N Jul - Sep 2000 110% 85% 105% Oct 2000 - Sep 2001 110% 100% 105%
- sources: UOG-WERI
Chuuk State: Southern parts of Chuuk State were drier than normal during the second quarter of the year, while western parts were wetter. During April, May, and June, the weather station at Weno Island measured 12.11 inches (98%), 9.97 inches (82%), and 12.10 inches (103%). This amounted to 84% of the norm for the period. In July, 10.13 inches (84%) were recorded. At Lukunoch, rainfall for the respective months was 10.03 inches (81%), 9.26 inches (76%), and 8.28 inches (71%) inches. July rain there was 9.46 inches (78%). Rain in Polowat was much higher with 10.21 inches (170%), 14.54 inches (162%), and 10.95 inches (88%). In July, Polowat saw only 8.35 inches (60%) of rain.
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 Jul 2000 through Sep 2001 is as follows:
Inclusive Period % of long-term average Chuuk Outer Atolls Lagoon Southern Western Jul - Sep 2000 90% 90% 85% Oct 2000 - Sep 2001 100% 100% 100%
- sources: UOG-WERI
Pohnpei State: The second quarter of 2000 was slightly wetter than normal for Pohnpei and Nukuoro, and drier than normal for the eastern islands of Pingalap and Mokil and the southern island of Kapingamarangi. The wet weather was an extension of La Niņa-induced rains seen in the first quarter of the year. April, May, and June rainfall measured at the weather station at Kolonia was 21.18 inches (129%), 20.09 inches (105%), and 13.33 inches (78%). The weather station also measured 13.58 inches (74%) in July. At Pingalap, wet conditions ended early with 12.33 inches (72%), 7.78 inches (46%), and 8.52 inches (52%) inches for April, May, and June. July saw a resumption of rain with 16.55 inches (104%). At Nukuoro, precipitation amounts of 25.16 inches (168%), 22.67 inches (154%), 16.87 inches (138%), and 11.73 inches (81%) were recorded for the 4-month period. Much less rain fell at Kapingamarangi during the 4-month period with measurements of 3.35 inches (25%), 9.80 inches (95%), 11.33 inches (156%), and 5.73 inches (55%) for the respective months.
Rainfall for all areas of Pohnpei State is expected to be slightly below normal until September. Afterwards, rainfall is expected to revert to normal amounts for the next year. High month-to-month variability can be expected. While tropical cyclones are not expected to pose a serious threat to Pohnpei until the next El Niņo event, Pohnpei could experience some periods of high waves from typhoons passing to the north.
Predicted rainfall for Pohnpei State from Jul 2000 through Sep 2001 is as follows:
Inclusive Period % of long-term average Pohnpei Outer Atolls: Island Eastern Southern Equatorial Jul - Sep 2001 95% 90% 90% 80% Oct 2000 - Sep 2001 100% 100% 100% 100%
- sources: UOG-WERI
Kosrae: Drier than normal conditions replaced La Niņa-induced heavy rains in May. The automated weather station at Kosrae Airport recorded 22.82 inches (105%), 15.19 inches (81%), and 11.85 inches (62%) for April, May, and June. This amounted to 84% of normal rainfall for the period. In July, the Airport had 12.55 inches (74%). Conditions at Tafunsak were drier than the Airport, while Tofol and Utwa were wetter. In April, Tofol had 28.72 inches (133%) and Utwa had 25.41 inches (117%). In July, Utwa was the driest part of the island with 9.53 inches (56%).
Rainfall for Kosrae is expected to be somewhat drier than normal through September, then revert back to normal amounts. However, high month-to-month variability should be expected in the rainfall. Kosrae should not experience any tropical cyclones until the next El Niņo event.
Predicted rainfall for Kosrae State from Jul 2000 through Sep 2001 is as follows:
Inclusive Period % of long-term average Jul - Sep 2000 90% Oct 2000 - Sep 2001 100%
- sources: UOG-WERI
Republic of Palau: Republic of Palau: Palau was drier than normal during the second quarter of the year. This resulted from the early demise of La Niņa-induced heavy rains seen earlier in the year. In addition, stronger than normal trade winds delayed the onset of Pacific monsoon westerly winds and rains. The weather station at Koror recorded 12.71 inches (147%) in April, 9.32 inches (78%) in May, and 11.95 inches (69%) in June. This amounted to a 3-month average of 90%. In July, rainfall increased to 20.38 inches or 113%, as monsoon rains arrived. Rainfalls at the Mariculture Center and at Nekken Forestry were similar to that at the weather station. Farther south at Peleliu, rainfall was a little wetter with 11.97 inches (132%), 13.16 inches (110%), and 11.58 inches (68%). This produced a near-normal 3-month average of 97%. In July, the island had 14.44 inches or 81% of that expected.
For the entire Palau island chain, we expect rainfall to return to 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 Niņo event begins. Tropical cyclone activity should also return to normal. This means that all islands from Angaur northward could experience a typhoon from October through December. Islands south of Angaur are generally not threatened by typhoons.
Predicted rainfall for Palau from Jul 2000 through Sep 2001 is as follows:
Inclusive Period % of long-term average Koror and Outer Atolls Mountain Is. S. of 8ºN N. of 8ºN Jul 2000 - Sep 2001 100% 100% 100%
- sources: UOG-WERI
Republic of the Marshall Islands (RMI): The RMI was quite dry during the second quarter of 2000, especially in the extreme northern islands. After a very wet first quarter due to La Niņa-induced rains, Majuro rainfall (representative of the southern islands) dropped off drastically as the trade wind trough to the east virtually disappeared. During April, May, and June, the Majuro weather station measured 8.76 inches (85%), 4.02 inches (36%), and 5.31 inches (46%) inches. For the period, the average was a meager 55%. In July, rainfall increased to 11.31 inches, or 87% of normal. Kwajalein rainfall (representative of the central islands) was also much drier than normal during the second quarter of 2000. Rainfall at Kwajalein and likely at nearby Ebeye, was 2.33 inches (31%) in April, 3.15 inches (32%) in May, and 4.04 inches (42%) in June. This was an average of only 35% for the period. In July, amounts rose to 9.94 inches or 95% of that normally expected. Rainfall amounts at Mili, Arno, and Laura were similar to that in Majuro. Ailinglaplap, Jaluit, and Wotje had rainfall similar to that at Kwajalein. Utirik was extremely dry with 0.11 inches (2%) in April, a trace (0%) in May, and 0.23 inches (3%) in June. Conditions in July improved little with 0.34 inches (2%). In fact, since January, Utirik has received a total of only 2.3 inches (5%). This was likely as dry as the 1998 drought. While these values seem extremely low, satellite data did indicate that very little rainfall fell in the area.
Rainfall for most of the Marshall Islands is expected to approach normal amounts by October. However, Utirik and the extreme northern islands could remain drier than normal until December. Tropical cyclone activity for the Marshalls should return to normal, meaning that Wake and Enewetok could see typhoons by September. The remainder of the islands south of 12 °N could experience a tropical storm or minimal typhoons in October and November. The eastern Marshall Islands will not likely experience an intense typhoon until the next El Niņo event.
Predicted rainfall for the RMI from Jul 2000 through Sep 2001 is as follows:
Inclusive Period % of long-term average RMI Atolls Southern Central Northern Jul - Sep 2000 90% 80% 15% Oct - Dec 2000 100% 100% 60% Dec 2000 - Sep 2001 100% 100% 100%
- sources: UOG-WERI
Long-Lead Outlook for Hawaiian Islands issue dated 15 June 2000, from NOAA's Climate Prediction Center (CPC).
ENSO ADVISORY of July 11 ,2000
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
Prediction (NCEP) - CLIMATE PREDICTION CENTER (CPC):
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
WEATHER SERVICE FORECAST OFFICE (WSFO)
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.
University of Guam (UOG) WATER AND ENERGY RESEARCH INSTITUTE (WERI):
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
(SOEST) DEPARTMENT OF METEOROLOGY:
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.
PACIFIC EL NINO-SOUTHERN OSCILLATION (ENSO) APPLICATIONS CENTER:
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:
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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