Sea surface temperatures (SSTs) in the eastern and central equatorial Pacific have remained almost unchanged since the last Pacific ENSO Update. Present values are comparable to those observed during the 1982-83 event, and are most likely at their peak levels. Conditions in the atmosphere continue to reflect this, with deep convection shifted to the eastern tropical Pacific, and strong subsident (sinking) flow suppressing deep convective, rain-bearing cloud development over Micronesia and other central and western areas. South of the equator, the present cyclone season continues to exhibit an eastward shift in activity, with numerous storms affecting Samoa, the Cook Islands, and French Polynesia. A comparison of present season activity to the 1982-83 season appears in this issue.
Most climate models are predicting that equatorial SSTs will begin to cool from about May onward, reaching near-normal or possibly below-normal levels by the end of the year. With this trend, brisk easterly winds will push back in to the central and western Pacific, continuing to limit deep convective rainfall in the region. Such conditions will extend the length of the dry season in Micronesia, and limit the total rainfall. South of the equator, Samoa and surrounding areas will see below-average rainfall for the current wet season and the coming dry season. With the dry conditions already underway, record droughts are expected to result in many areas. Refer to the following pages in this issue of Pacific ENSO Update for further details and outlooks.
In the last issue of Pacific ENSO Update, the early development of an unusual late-season storm system southwest of Hawaii was noted. This system eventually developed in to Typhoon Paka. In early December, strong convective activity and westerly low-level winds along the equator in the central Pacific led to the formation of "twin" cyclones, Typhoon Paka in the northern hemisphere and Cyclone Pam in the southern hemisphere (see section on facing page describing "twin" cyclones). Paka eventually moved into the Marshall Islands, depositing substantial rain in the southern islands. It later intensified to a super typhoon, and on 16 and 17 December, it blasted Guam with heavy rains and 125 kt sustained winds, inflicting extensive damage there and also on Rota in the CNMI. Elsewhere in the region, stronger than expected convective activity over northern Australia did kick off some small, weak, short-lived equatorial disturbances that drifted into the southern Micronesia area and dropped considerable rain on Koror and surrounding islands in December and early January. Aside from the greater than normal December rainfall received locally in Guam, the CNMI, and the southern Marshall Islands due to Typhoon Paka, and the near-normal rainfall in Palau due to the tropical disturbances, the rest of Micronesia received very little rainfall and the beginnings of drought, due to the larger-scale influence of El Niño on atmospheric conditions in the region. Since early January, dry conditions have prevailed on all islands and also developed in the region around Hawaii, where Maui and the Big Island have been particularly dry, leading to serious drought conditions on parts of the islands. Further tropical cyclone activity in Micronesia is not likely until June or July, and then only in the extreme western sections, as strengthening easterly winds are expected to push through the region.
During December and most of January, the conventional atmospheric indicator of "El Niño" , the Tahiti-Darwin Southern Oscillation Index (SOI), exhibited only moderately negative values (-1.2 to -1.5) instead of more strongly negative values as were expected to develop. This was primarily a result of lower than expected pressures at Darwin due to localized weather systems in the region. The Australian monsoon trough became very active over northern and northeastern Australia and eastward to Samoa . Tropical Cyclones Katrina, and Susan resulted, and along with Pam from earlier in December, affected areas from the Coral Sea to Vanuatu and Fiji in the west, and the Cook Islands in the east. In late December and in January, cyclone activity in the South Indian Ocean was very low. Cyclone activity was noted primarily in the eastern portion of the basin (Northwestern Australian region) and extending to the east as far as Darwin, keeping pressures low there. Cyclone Les meandered in the area of Darwin for nearly two weeks. In spite of the unusual activity around Darwin, most South Pacific activity has been shifted well east of the date line to the region of Samoa and Tahiti, producing cyclone activity in December (Cyclones Pam and Ron) and in January (Cyclones Tui, Ursula, Veli, and Wes). By the end of January, pressures had fallen sharply to their lowest values since February 1983 in the Tahiti area, and had risen in the Darwin area, yielding a strong negative monthly value for SOI at -3.3 as reported by NOAA. It is of value to note that the "equatorial" Southern Oscillation, measured by pressure anomalies over larger areas along the equator between the Galapagos and the New Guinea/Indonesia region instead of Tahiti-Darwin, has more consistently exhibited as strong a negative condition as ever seen. Taken together, the regional cyclone and Southern Oscillation conditions reflect on the peaking strength of the major El Niño event which is underway.
Between March and May, the SOI is expected to rise sharply, as pressures rise in the south Central Pacific and easterly winds return to the equatorial latitudes. The Micronesia drought will be reinforced when this occurs. These conditions, in general, point toward both a longer than usual and drier than usual dry season over Micronesia, and a drier than usual wet season and subsequent dry-season for Hawaii and for Samoa and other southern hemisphere locations.
- sources: UOG-WERI and PEAC
In extreme ENSO events such as 1982-83 and the current 1997-1998 event, the southern hemisphere cyclone development region extends far east of the date line, to French Polynesia. Figures 1 and 2 below are presentations of storm tracks during the very strong ENSO events of 1982-83 and the present year. Regions shown are bounded by the equator to 30°S latitude and 170°E to 110°W longitude. Storm tracks are shown for activity noted from December through May 1982-83, and from October through early February of the current 1997-98 season. Track data for 1997-98 has been extracted from operational warnings and is only approximate for named systems (Storm or Cyclone intensity). The 1982-1983 track data were collected by meteorologists at the University of Hawaii and also include weak systems (T.D.'s = Tropical Depressions) only identified by the UH staff and thus, "unofficial" systems. Prior to 1982, it had been asserted that Tahiti had experienced no tropical cyclones since 1907. Therefore, the activity seen during the 1982-1983 ENSO event was a shock to many of the island residents. Increased activity for this year (1997-98) was expected, and unfortunately has resulted in substantial devastation to several of the Cook Islands and the Society Islands. Thus far for the 1997-98 season, both the Cook Islands and French Polynesia have each had three significantly destructive storm episodes.
In 1983, weak systems (T.D.'s) developed far to the east of French Polynesia. Fortunately these are isolated waters and the storms were primarily of scientific interest. The "unofficial" weak system of May 11, 1983 is the easternmost South Pacific system ever observed by experienced meteorologists in the satellite era.
As we write this section, the Southern Hemisphere cyclone season has three more months to run. Events to watch for are more storms in the eastern regions of the South Pacific and the remote possibility of twins forming in the North Pacific, such as Ekeka (1992) and Hali (1992).
- source: U.H. Dep't of Meteorology
PROGNOSTIC DISCUSSIONS FOR LONG-LEAD OUTLOOKS
CLIMATE PREDICTION CENTER NCEP
NATIONAL WEATHER SERVICE WASHINGTON DC
3 PM EST THURSDAY FEBRUARY 12 1998
PROGNOSTIC DISCUSSION OF SST FORECASTS
The official SST forecast for the east-central equatorial Pacific (120-170°W
longitude - also called Niño 3.4) is for very strong warm ENSO conditions to
continue through April. Equatorial SSTs are currently at least 2°C above
normal
east of about 170°W and over 4°C above normal from about 120°W towards the
South American coast near and just south of the equator. While the SSTs
anomalies are expected to begin to soon decrease in magnitude (anomalies
have already lowered slightly in the past few weeks east of about 140°W) - the
SSTs will will remain well above the 28.5°C threshold for convection throughout
the eastern equatorial Pacific and therefore will continue to heavily influence
the circulation patterns affecting the U.S. through the spring.
There is substantial disagreement among various models over the equatorial
Pacific SSTs beyond mid-1998. The two statistical models - one model based
on a Constructed Analogue (CA) - and the other on Canonical Correlation
Analysis (CCA) - predict a steady and rapid decrease in temperature throughout
the summer with substantial cool anomalies developing by September. The
NCEP dynamical model (the coupled ocean-atmosphere model) predicts that
Niño 3.4 temperature anomalies will fall rapidly to about 1°C above normal by
June then remain at about that level until late in the year when anomalies will
slowly fall again toward normal. A statistical consolidation forecast based on
the past performance of these three models follows the NCEP model through
the fall - then begins a rapid decrease to normal - or slightly below - by the end
of the year. A majority of models run at other institutes (notably the Scripps
model) also predict that ENSO will be in the cold phase by the end of 1998.
The confidence in the predictions of the equatorial Pacific SSTs at this time of
year is quite low - so the official forecast reflects the agreement among all
models - that the SSTs anomalies will decrease substantially through the
spring. The official forecast then compromises between the conflicting signals
and calls for a slow decrease to zero anomaly sometime late in the year. It is
still too early to call for a cold event for next winter at this time - however it does
look more likely that the Niño 3.4 SSTs will be below normal by the end of the
year than above. The expected accuracy of the Niño 3.4 SST forecast for next
winter will greatly improve in a few months, once the so-called springtime
barrier in predictability is passed.
Although the model predictions discussed above are in general agreement regarding a cooling trend in the sea surface temperatures (SSTs) from about May onward, skill in long-term predictions made at this time of year is limited. Therefore some models anticipate cooling to near-normal SSTs while others predict below-normal SSTs (possible La Niña event) by the end of 1998. The nearer-term (and higher confidence) weakening of the present El Niño event is, however, the more important consideration for the outlooks given in the LOCAL VARIABILITY SUMMARIES following this section.
Several ENSO Advisories summarizing general conditions and outlooks have been issued by NOAA in recent months. The ENSO Advisory dated February 11, 1998 is shown here.
Information in the LOCAL VARIABILITY SUMMARIES section is based on an expectation that conditions will be influenced by a strong warm event through early 1998, with a cooling SST and rising SOI trend thereafter. Any changes in trends, and their impact on these outlooks, will be reported in future issues of Pacific ENSO Update.
As noted in each issue of Pacific ENSO Update, the following summaries of expected local climate variability for the various island areas indicated are not based on official forecasts (unless otherwise stated). Further information is available from your local National Weather Service office, or from the sources listed in the acknowledgements section.
HAWAII:
Last month, Hawaii joined Micronesia in the development of drought conditions. The National Weather Service automated rain gauge network maintained throughout Hawaii, along with the four main airport weather stations, represent a total of 73 stations state-wide, and during January, not a single one of these gauges recorded above-normal rainfall. 63 of the 73 gauges recorded less than half of normal rainfall, while 36 recorded less than 25%. This winter looks to be no exception to the tendency toward wintertime drought in Hawaii associated with El Niño events. Hilo and nearby locations in the Puna area of the Big Island recorded extremely low rainfall - on the order of 1-2% of normal values. The 0.14 inches recorded at Hilo is the lowest monthly rainfall ever observed for any month since recordkeeping began there. Rainfall was similarly very low over much of Maui. Reports of water supply, wild fire, and agriculture problems on both islands have been making local news. Kauai and Oahu fared better with less severe shortfalls.
In general, the dry conditions have been associated with persistent zones of high pressure ridging over the islands. This feature in the region of Hawaii is typical of the larger-scale effects on atmospheric circulation in the tropical Pacific related to El Niño. Winter storms in the north Pacific are prevented from brushing by Hawaii - particularly for the southern islands. Sunny days with few clouds and chilly nighttime temperatures have been typical, as well as light winds and buildup of volcanic "vog" around the islands. Although most storm activity has stayed to the north, large storm swells have reached the islands bringing high surf to north and west facing beaches. The below-normal winter rainfall historically associated with strong El Niño conditions often extends in to the following spring.
Both the Long Lead Outlook for the Hawaiian Islands and the latest experimental forecasts from NOAA-CPC anticipate this trend.
- sources: NWS-PR and PEAC
AMERICAN SAMOA:
December rainfall for Pago Pago was 7.84 inches or 54% of that normally expected, while that during January was 10.64 inches or 85% of normal amounts. About 5 inches of the January total was due to the near passage of Tropical Storm Tui near the end of the month - the closest encounter with the main islands to date for the current cyclone season, which has been active from Samoa eastward. Tui caused some damage on the neighboring islands in western Samoa. Cyclone Ron ran over Swains Island in late December and went on to affect nearby islands in Wallis and northern Tonga, but missed the main islands in the Samoa region.
The record strength of this El Niño will make rainfall in the Samoa region highly variable. In the next two months, the South Pacific Convergence Zone (SPCZ) will meander considerably, from the equator southward to 15°S latitude. Samoa will be near the western extent of the SPCZ, which is expected to move eastward toward French Polynesia, leaving Samoa drier and with a reduced chance of getting a tropical cyclone. As a result, we have reduced the predicted January through April rainfall for the region from 80% down to 70% of the average. Keep in mind that a direct or near-direct hit by a tropical cyclone would make the location very wet for a few days and would make the month a wet one. After April, conditions will be very dry as trade winds strengthen over the region. This will extend the dry season and also make it significantly drier than normal. Wet weather is not expected to return until October. The rainfall predictions for American Samoa and islands in the surrounding region are:
Inclusive Period % of long-term average
Samoa Region
Feb-Apr 1998 70%
May-Sep 1998 25%
Oct-Dec 1998 90%
Jan-Mar 1999 100%
- source: UOG-WERI and PEAC
GUAM/CNMI:
We are expecting a record drought for Guam and the CNMI. December rainfall for Tiyan was estimated at 23.48 inches or more than 4 times the normal December rainfall. However, in only 2 days, Typhoon Paka dropped 21 of the 23.48 inches. Thus, without the typhoon, rainfall would have been about 50% of normal. We indicated in the last Pacific ENSO Update that the region would be wetter than normal if hit by a tropical cyclone and dryer than normal without a tropical cyclone.
December rainfall for the Saipan International Airport was 5.30 inches, or 10% above normal, and for Capitol Hill was 4.46 inches. Typhoon Paka was also responsible for the excess rainfall at Saipan. Rota is estimated to have received 10-12 inches from Typhoon Paka. January rainfall for the Guam airport was 1.99 inches or 45% of normal values for the month, and that for Andersen Air Force Base was 2.98 inches or 52% of normal. At Saipan, January rainfall was highly variable. The airport received 1.0 inch of rain or 25% of normal, while Capital Hill received 3.98 inches or about 85% of the monthly norm. Tinian airport got 1.3 inches. Most of the January rain came from shearlines that transited the area. Our rainfall predictions for January 1998 through March 1999 for Guam and the CNMI are as follows:
Inclusive Period % of long-term average
Guam CNMI
Feb-May 1998 20% 20%
Jun-Aug 1998 50% 45%
Sep-Dec 1998 90% 85%
Jan-Mar 1999 100% 100%
- source : UOG-WERI
MICRONESIA (FSM):
We anticipate a record drought for all of the FSM, and anticipate that water resources will deplete a month earlier than they did in 1983. In December, some of the more equatorial islands, such as Nukuoro, Lukunoch, and Kosrae, received more rainfall than other islands. However, as predicted, January rainfall dropped drastically for the FSM states. In fact, for all islands except northern Yap State, conditions were even drier than predicted, and drought conditions are already serious. Wells on many atolls are already getting salty and surface water on high islands is drying up. The FSM President has issued a disaster declaration, authorizing emergency relief actions.
Yap State: Yap, like Guam and the CNMI, had some December and January rainfall from passing shearlines. As a result, at the Yap airport, rainfall for December was 5.93 inches or 66% of the average and in January it was 4.45 inches or 61% of the average. At Ulithi, December rainfall was 3.94 inches (about 50% of normal), while at Woleai Atoll it was 3.35 inches (29% of normal). The January values for Ulithi and Woleai were 2.82 (38%) and 3.26 inches (31%), respectively. Conditions for the Yap State islands are expected to be much drier in February, March, and April. The reservoir on Yap is expect to dry up in March and will probably not receive significant rain until late June or July. Rainfall for Yap and its atolls is expected to be:
Inclusive Period % of long-term average
Yap Outer Atolls:
Island S. of 8°N N. of 8°N
Feb-May 98 20% 15% 20%
Jun-Aug 98 65% 75% 60%
Sep-Dec 98 85% 90% 85%
Jan-Mar 99 100% 100% 100%
- source : UOG-WERI
Chuuk State: December rainfall for Weno Island was 2.88 inches or 27% of normal, while that at Lukunoch was a wet 11.51 inches or about 5% above normal. To the west at Polowat conditions continued to be extremely dry with 1% (one percent) of normal rainfall occurring in December. Since October, Polowat rainfall has only averaged 12% of normal, while that at Weno was only 40% of normal. January rainfall was, in general, much drier. In January, Weno received only 1.25 inches or 12% of normal rainfall. Values were 0.31 inches (3% of normal) at Lukunoch and 1.12 inches (12% of normal) at Polowat. January rainfall at Namanuito Atoll and the Hall Islands is estimated to have been about 3.0-3.5 inches. Predicted rainfall for Chuuk Lagoon and outer atolls is as follows:
Inclusive Period % of long-term average
Chuuk Outer Atolls:
Lagoon Southern Northern
Feb-May 98 15% 15% 20%
Jun-Aug 98 70% 75% 60%
Sep-Dec 98 90% 90% 85%
Jan-Mar 99 100% 100% 100%
- source : UOG-WERI
Pohnpei State: At Kolonia, December rainfall was 22% of normal, while rainfall for Pingalap was about 30% and for Nukuoro, 68% of normal. While we expected that rainfall would decrease drastically in January, rainfall amounts for the month were even drier than anticipated. Kolonia received only 0.64 inches, or 5% of normal monthly rainfall. Pingalap measured only 0.47 inches (4% of normal) and Nukuoro had only 0.74 inches (5% of normal). As a result of the low rainfall in Pohnpei, water rationing has been imposed in Kolonia, streams have already dried up, and rivers are already running at very low levels. The danger of high concentrations of water-borne diseases is very high. All water should be boiled or chlorinated. Forest areas are reported to be drying up and the fire potential for Pohnpei is also very high as it was during the 1982-83 El Niño. Water wells at many of the atolls could soon become salty. Rainfall outlooks for Pohnpei are as follows:
Inclusive Period % of long-term average
Pohnpei Outer Atolls:
Island Eastern Southern
Feb-May 98 10% 10% 10%
Jun-Aug 98 50% 55% 60%
Sep-Dec 98 85% 80% 85%
Jan-Mar 99 100% 100% 100%
- source : UOG-WERI
Kosrae State: Rainfall at the Kosrae Airport for December was 8.61 inches or 59% of normal, while that for the same period at Utwa was 11.33 inches or about 75% of normal. However, in January, rainfall dropped off sharply to 1.29 inches or only 9% of normal at the airport. January values at Utwa, Tofol, and Tafunsak were between 1.28 and 1.17 inches (8-9%). As a result of the very low rainfall in Kosrae, streamflow is expected to decrease rapidly, which will increase the danger of high concentrations of water-borne diseases. All water should be boiled or chlorinated. The fire potential for Kosrae will also be high. Anticipated rainfall for Kosrae is as follows:
Inclusive Period % of long-term average
Kosrae Island
Feb-Apr 1998 10%
May-Aug 1998 65%
Sep-Dec 1998 90%
Jan-Mar 1999 100%
- source: UOG-WERI
MARSHALL ISLANDS (RMI)
:
The December rainfall at Majuro (representing southern atolls) was 7.63 inches and at Kwajalien (representing northern atolls) was 5.03 inches - both about 64% of normal. However, Typoon Paka was responsible for about 5 inches at Majuro and 3.59 inches at Kwajalien. Without Paka, rainfall would have been just 22% at Majuro and 18% at Kwajalien. January rainfall at Majuro was only 1.57 inches or 19% of normal, and for Kwajalien and Ebeye, only 0.66 inches, or 8% of normal. Elsewhere, rainfall was 0.55 inches (5%) at Mili, 1.21 inches (12%) at Jaluit, and 1.43 inches (18%) at Wotje. Record drought conditions are expected for RMI in coming months. Cyclone activity is unlikely until at least late 1998, possibly not until the fall of 1999:
Inclusive Period % of long-term average
RMI Atolls:
Southern Northern
Feb-May 1998 10% 10%
Jun-Aug 1998 70% 60%
Sep-Dec 1998 90% 80%
Jan-Mar 1999 100% 95%
- source: UOG-WERI
PALAU:
We are expecting a record drought in Palau. In December at Koror, rainfall totals were near normal for the first time since March 1997. At the Koror weather station, December rainfall was 11.35 inches or 5% below normal. Farther south at Peleliu, values were 30% above normal values. In January, rainfall at Koror fell to 4.72 inches or 44% of normal amounts, while 6.64 inches or 62% of normal amounts fell at Peleliu. We suspect that Kayangel has had less rain than Koror, while Tobi and Sonsorel have had slightly more rain than Koror. Conditions are expected to be significantly drier in February, March and April. Surface water sources on Babeldaob should be monitored closely as they could deplete quite rapidly. The expected rainfall for Palau is as follows:
Inclusive Period % of long-term average
Koror & Outer Atolls:
Babeldaob S. of 8°N N. of 8°N
Feb-Apr 98 20% 20% 20%
May-Aug 98 70% 65% 70%
Sep-Dec 98 85% 80% 90%
Jan-Mar 99 100% 100% 100%
- source: UOG-WERI
SPECIAL SECTION: Experimental Forecasts for Pacific Island Rainfall using data through January, 1998.
Long-Lead Outlook for Hawaiian Islands issue dated 12 February 1998, from NOAA's Climate Prediction Center (CPC).
ENSO ADVISORY of February 11, 1998
For further information, please contact:
Alan C. Hilton, LT/NOAA
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
E-mail: hilton@soest.hawaii.edu
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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