Pacific ENSO Update

4th Quarter 2001 - Vol. 7 No. 4


CURRENT CONDITIONS


Sea surface temperatures (SSTs) across the tropical Pacific remain near normal, with a distribution of slightly warmer than normal SST in the western and central equatorial Pacific, and slightly cooler than normal SST in the eastern equatorial Pacific. The Southern Oscillation Index (SOI) has fallen to weak negative values for the last six months after being positive for almost three years. The August value of the SOI of -1.0 was the lowest value since it was at -1.9 in April 1998. In September, the SOI rose to near zero. Persistent low-level easterly wind anomalies in equatorial latitudes of the western and central equatorial Pacific have weakened in recent months. The SST distribution in the tropical Pacific, the status of the SOI, and the weakening of the low-level easterly winds are indicative of a climate state that is neither El Niņo nor La Niņa (designated by some as "El Niņo-neutral"). Over the past two years there has been a gradual eastward expansion of the area of positive subsurface temperature anomalies into the central Pacific supporting the current demise of the long-lived La Niņa and perhaps signaling the imminent start of the next warm episode.

The western Pacific monsoon circulation was well developed in July and August, and rainfall amounts at most islands were wetter than normal with notable exceptions of dry conditions in the northern RMI, Chuuk State, and Kosrae State. During September, the monsoon trough moved northward into subtropical latitudes, and a weak ridge of high pressure stretched through the Caroline island groups bringing clear, hot, and dry conditions to Palau, Chuuk, Pohnpei, and Kosrae for many days (Figure 1). Daily high temperatures at Pohnpei WSO reached the mid-90s on many days with blazing sun and very light winds. During summers with a well-developed monsoon circulation, the slow episodic migrations of the trough tend to cause high month-to-month variability of rainfall throughout all of Micronesia. On Guam, for example, August was extremely wet, as persistent gusty southwest monsoon winds accompanied heavy rainfall events that totaled 25-30 inches (twice normal) for the month. The wet August on Guam was followed by a dry September with rainfall amounts of ~7 inches (half of normal). At most islands, the rainfall in September was far less than the August rainfall (Figure 2). Very dry conditions continued to plague the northern Marshall Islands. At Kwajalein, it has been dry for many months, and water rationing is being considered. During September, Kwajalein received only 3.77 inches of rain while Majuro received 20.89 inches, providing a great illustration of the rainfall gradients that are possible in Micronesia (even without tropical cyclones).

SST

Sea surface temperatures (SSTs) across the tropical Pacific are near normal, with a distribution of slightly warmer than normal SST in the western and central equatorial Pacific, and slightly cooler that normal SST in the eastern equatorial Pacific. Persistent warmer than normal SST anomalies in the equatorial central Pacific and much of the equatorial Western Pacific have remained stable for the past few months. In the NINO 4 region (5°N-5°S ; 160°E-150°W), the SST during July and August was +0.5ē warmer than normal. SST values remain below normal in the eastern equatorial Pacific east of about 130°W. Most of the international Global Climate Models (GCMs) are backing off of earlier indications that a weak or moderate El Niņo would begin late in 2001 or early 2002. Now, ten of eleven favor "El Niņo-neutral" conditions through March 2002. Two of these models develop El Niņo warming of the central and eastern equatorial Pacific by June 2002, while five maintain a neutral forecast. The most likely scenario is a continuation of neutral conditions through mid-2002. Definitions of El Niņo developed in the early 1980s were contingent on persistent SST anomalies of at least +1ēC along the tropical Pacific coast of South America, while more recent definitions have focused on persistent SST anomalies of at least +0.4ēC in the central equatorial Pacific. At this time, it is too early to accurately predict when central Pacific SST anomalies, along with other climate indicators, will become large enough for sufficient time to declare the next El Niņo event, but based on current SST distribution, slightly negative values of the SOI, near-normal equatorial Pacific winds, and a near-normal distribution of tropical cyclones, it does not seem likely that this will occur until early 2002 or later.

The depth of the equatorial oceanic thermocline (as represented by the depth of the 20°C isotherm) continued to remain deeper than normal in the western Pacific and near normal in the extreme eastern Pacific. Over the past two years there has been a gradual eastward expansion of the area of positive subsurface sea temperature anomalies into the central Pacific supporting the current demise of the long-lived La Niņa. As recently as March 2001, colder than normal sub-surface temperatures in the central and eastern equatorial Pacific were -4°C and -2°C respectively. Since then, the central Pacific subsurface temperatures have warmed to as much as 3°C above normal at depths of 150 meters, while the subsurface temperatures in the far eastern part of the equatorial Pacific have remained colder than normal. The gradual eastward expansion of warmer than normal sub surface water is an indicator of a possible start of the next El Niņo in 2002.

SOI

During April through September 2001, SOI values were slightly negative (-0.1, -0.8, -0.1, -0.4, -1.0, and -0.1 respectively), indicating the demise of a prolonged period of weak to moderate La Niņa conditions associated with positive values of the SOI prior to April. The SOI value of -1.0 during August was the lowest value of the SOI since it was -1.9 in April 1998. With persistent weak negative values of the SOI, the easterly wind flow of the equatorial Pacific weakened and the monsoon trough pushed out to near normal locations in Micronesia by late June and July. During August, a vigorous monsoon trough pushed eastward through the Mariana Islands bringing gusty southwest winds and heavy rain squalls to Palau, Yap, Guam, and the CNMI. Disturbances in the weak trade wind trough across eastern Micronesia brought high rainfall to some islands (Majuro received over 20 inches in September), but missed other islands leaving them very dry (Kwajalein had only 3.77 inches during September). Cloudiness and showers associated with the South Pacific Convergence Zone (SPCZ) have been focused a bit southwestward of normal, leaving American Samoa in dry trade wind flow for much of the past few months - another place with a persistent lack of rainfall for the past few months.

We expect the SOI to remain slightly negative (El Niņo-neutral values) through mid 2002. With the SOI in weak negative territory, atmospheric circulation patterns should exhibit more normal behavior. The positions (and natural fluctuations) of the monsoon troughs, the trade wind troughs (ITCZs), the South Pacific Convergence Zone (SPCZ), and the mid-latitude jet streams should be near normal, with fairly standard distributions of clouds and rainfall. It should be understood that "normal behavior" characteristically exhibits high month-to-month variability in the cloudiness and rainfall.

TROPICAL CYCLONE ACTIVITY

Western North Pacific tropical cyclone activity from mid July through September, 2001 was near normal with 14 tropical cyclones: Kongrey, Yutu, Toraji, Man-Yi, Usagi, Pabuk, Wutip, Sepat, Fitow, Danas, Nari, Vipa, Francisco, and Lekima, named by the Japan Meteorological Agency (JMA). The Joint Typhoon Warning Center (JTWC) identified 15 total tropical cyclones including a tropical depression (TD 15W) in addition to those named by the JMA. Most of the western North Pacific tropical cyclones developed in western longitudes - forming near the Mariana Islands, but becoming tropical storms or typhoons after moving into the Philippine Sea or when northeast of the Mariana Islands. The only populated island in Micronesia to be directly affected by a tropical cyclone during July-September was Agrihan in the CNMI, which was hit by Typhoon Man-Yi in August. Based on the expected slightly warmer than normal SST in the western North Pacific and weakly negative SOI, normal tropical cyclone activity in the western North Pacific is anticipated for the second half of 2001. This means that much of Micronesia should be affected by tropical cyclone activity from October until late-December. In eastern Micronesia, this activity should be limited to a few developing tropical storms, and to the fringe effects of storms passing to the north for Pohnpei State and Kosrae State. American Samoa is not expected to experience a significant tropical cyclone until the next El Niņo event, although the region could see a tropical depression or weak tropical storm in the January-April 2002 timeframe.


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.


LOCAL VARIABILITY SUMMARIES:

State of Hawaii: September was largely a month of moderate to fresh trade winds with a mid-month breakdown (14 to 16 September) caused by the remnants of Hurricane Gil and a surface trough to the northeast of the islands. The most significant event of the month was an upper level trough southwest of the Hawaiian Islands that triggered thunderstorms over the islands of O'ahu and Kaua'i on 16 September. Thunderstorms appeared to be anchored over the east and central portions of the Ko'olau Range and produced minor stream flooding. Heavy showers and minor stream flooding also occurred over portions of Kaua'i. An earlier upper level low produced heavy showers and isolated thunderstorms over north and east Kaua'i on 6 September. No reports of flooding or damages were received from this event. A weak and ragged shear line affected the islands on 26 and 27 September, bringing enhanced windward showers to the islands

Kevin Kodama - Senior Service Hydrologist, NWSFO Honolulu, HI

For a more complete summary, and the county by county wrap-ups, please see the September 2001 Precipication Summary from the National Weather Service Honolulu Weather Forecast Office.

American Samoa: Rainfall at Pago Pago Airport for July, August, and September was 4.69 inches (75%), 3.03 inches (45%), and 4.72 inches (71%), respectively, amounting to 63% of normal for the 3-month period. The dry conditions in the Samoa region for the July-September were due to persistent dry trade wind flow. Cloudiness and showers associated with the South Pacific Convergence Zone (SPCZ) have been focused a bit southwestward of normal, leaving American Samoa in dry trade wind flow since April 2001. During September, however, some cloudiness and showers in the SPCZ moved over Samoa bringing a slight increase of rainfall during that month.

With central equatorial Pacific SST anomalies expected to be slightly warmer than normal and the SOI expected to be weakly negative through March of 2002, the rainfall from October through the end of the forecast period should average near normal. During the past three years of persistent La Niņa (the last half of 1998 to early 2001 -- years fairly similar in terms of SST and SOI values), the rainfall in American Samoa was near normal to slightly wetter than normal. Rainfall in American Samoa is not as closely tied to ENSO as it is in other regions. While some of the very dry periods (1982-83*, 1987*, 1990, 1993, and 1998*) at American Samoa have occurred in association with major El Niņo events (years shown with a "*"), and some of the very wet periods (1980-81!, 1985-86!, 1994, and 1999) have occurred in years prior to major El Niņo events (years shown with a "!"), there is not a consistent ENSO signal in the rainfall there on average. Thus, while it is too early to predict the next El Niņo, it does not appear that a significant event will occur before 2002. Such an event should not affect rainfall in American Samoa during this forecast period.

Significant tropical cyclone activity is not expected in the Samoa region until after the next El Niņo event occurs. Despite this, some tropical depressions and weak tropical storms could develop in the portion of the SPCZ that connects to the eastern reaches of the Australian northwest monsoon trough during November 2001 through April 2002, bringing a few periods of heavy rains to the Samoan Islands as these weak cyclones and monsoon squalls move through the region.

Predicted rainfall for American Samoa from October 2001 through September 2002 is as follows:


Inclusive Period         % of long-term average
                              Samoa Region
Oct 2001 - Apr 2002               90%
May 2002 - Sep 2002              100%

- sources: UOG-WERI and PEAC

Guam/CNMI: Guam/CNMI: Rainfall at Guam International Airport (GIA) during July, August, and September was 15.53 inches (147%), 24.83 inches (181%), and 7.40 inches (55%), respectively. This amounted to 127% of the normal value for the period July-September. Andersen Air Force Base (AAFB) measured 15.37 inches (141%), 21.98 inches (164%), and 5.58 inches (42%), or 114% of the average rainfall for the period. July rainfall on Guam was nearly 15 inches at most locations (partly the result of the near passage of the tropical disturbances that became tropical cyclones Utor, Kongrey, Yutu, and Toraji). During August, southwesterly monsoonal winds dominated the island weather and brought widespread heavy rains of ~20-30 inches to the island (double the normal value). Near gale force southwesterly winds during 13-18 of August accompanied large waves on the west side of the island that caused some coastal erosion. August 2001 was the wettest month for most of Guam since August 1997 (although some places on Guam may have had a similar monthly total rainfall in December 1997 when Typhoon Paka produced upwards of 20 inches in 24 hours on the northern half of the island). During September, the monsoon circulation weakened, and with light winds and higher pressure, hit-and-miss island thunderstorms produced only half of September's normal rainfall. High month-to-month variability of rainfall (Figure 2) is typical in summers that have episodic strong monsoon wind events.

July, August, and September rainfall at Saipan International Airport (SIA) was 8.11 inches (100%), 20.09 inches (161%), and 8.64 inches (64%), or 108% of the average for the period. For July, August, and September, Capitol Hill's measured rainfall was 11.47 inches (127%), 24.12 inches (193%), and 11.12 inches (82%). Thus, at Capitol Hill, quarterly rainfall was somewhat higher at 133%. As at Guam, southwesterly monsoonal winds dominated Saipan's weather during August and brought widespread heavy rains of ~20-25 inches to the island (double the normal value). In addition to the large-scale southwest monsoonal flow and accompanying heavy rains, gale force southwesterly winds and very heavy rainfall occurred during 16-17 of August as Tropical Storm Pabuk brushed past the island. August's active monsoon brought copious rains to all of the Mariana Islands. Rainfall amounts for July, August, and September at the Tinian Airport were 7.16 inches (80%), 22.87 inches (183%), and 6.87 inches (51%), respectively. The 3-month rainfall for Tinian Airport was above normal at 105%, largely due to the heavy monsoon rains during August. At Rota Airport, July, August and September rain amounts were 8.62 inches (83%), 20.19 inches (162%), and 15.73 inches (124%). This gave a 3-month average of 123%. On the northern part of Rota, at the beautiful Rota Resort and Country Club, the NASA TRMM network there recorded almost exactly the same amount of rain (99%) as the Rota Airport for the period July-September. Very wet conditions prevailed on the CNMI during August 2001, thanks to persistent southwest monsoon flow and the near passage of Tropical Storm Pabuk. The month of August 2001 was the wettest month at most locations since August of 1997.

Based on 30 years of rainfall data, non-ENSO years on Guam and the CNMI tend to be slightly wetter than the average. This should be expected since monsoon and tropical cyclone activity are normally greater during these years. Thus "El Niņo neutral" rainfall for the Mariana Islands is somewhat greater than the long-term average or 105-110% above the long-term average. As is common during "normal" periods, there can be high month-to-month variability in the rainfall. While it is too early to predict the next El Niņo, an event in 2002 would likely not affect Guam and the CNMI during this forecast period. During October-early January, tropical cyclone activity should return to normal after three years of well-below normal activity. Thus, the Mariana Islands can expect typhoon threats from October through early January, with October and November being the months of greatest threats. Most of these will approach from the east-southeast.

Predicted rainfall for the Mariana Islands from October 2001 through September 2002 is as follows:


Inclusive Period         % of long-term average
                      Guam/Rota       Saipan/Tinian
Oct 2001 - Dec 2001     105%              100%
Jan 2002 - Sep 2002     110%              110%

- sources: UOG-WERI

Yap State: Abundant rain occurred in Yap State during the months of July and August, with lesser amounts in September. As at Guam, southwesterly monsoonal winds dominated Yap's weather during August and brought widespread heavy rains of ~20 inches to the islands of Yap State (approximately 5 inches more than normal). During September, the monsoon circulation weakened, and with light winds and higher pressure, hit-and-miss thunderstorms produced below normal rainfall at most locations. High month-to-month variability of rainfall is typical in summers that have episodic strong monsoon wind events. The weather station at the Yap Airport recorded 18.83 inches (130%) in July, 21.06 inches (139%) in August, and 7.45 inches (55%) in September, or 109% of normal rainfall for the three months. July, August, and September rainfall amounts on Ulithi were 18.18 inches (147%), 22.18 inches (172%), and 13.40 inches (117%) or 146% for the 3-month period. Farther south at Woleai Atoll, rainfall was 11.92 inches (85%) in July, 18.00 inches (122%) in August, and 9.92 inches (85%) in September, for a 3-month average of 99%.

Rainfall for Yap State is forecast to be about 100%-110% of the long-term average, depending on location. When all ENSO events in the last 30 years are considered, the wetter than normal La Niņa conditions do not quite add up to the deficits caused during El Niņo-induced droughts. Therefore, "normal", or "non-ENSO year" rainfall for Yap State must make up for this deficit, and thus should be greater than 100% of the long-term average. This additional rain is due to increased monsoon activity and to increased tropical cyclone occurrence. Tropical cyclone activity should be normal, and this means that the northern and southern islands of Yap State islands could be affected by tropical storms and typhoons from mid-October until late-December.

Predicted rainfall for Yap State from October 2001 through September 2002 is as follows:


Inclusive Period                % of long-term average
                             Yap               Outer Atolls:
                            Island       S.of 8·N     N. of 8·N
Oct 2001 - Dec 2001          100%          100%         100%
Jan 2002 - Mar 2002           90%          100%          90%
Apr 2002 - Sep 2002          110%          100%         110%

- sources: UOG-WERI

Chuuk State: During July, August, and September, the weather station at Weno Island measured 8.57 inches (71%), 12.66 inches (87%), and 10.56 inches (92%). This amounted to 83% of normal amounts for the 3-month period. Weno has now been drier than normal for six months. During late August and September, the monsoon trough moved northward into subtropical latitudes, and a weak ridge of high pressure stretched through the Caroline island groups bringing periods of clear, hot, and dry conditions to Palau, Chuuk, Pohnpei, and Kosrae for many days. During summers with a well-developed monsoon circulation, the slow episodic migrations of the trough tend to cause high month-to-month variability of rainfall throughout all of Micronesia. Chuuk State islands to the north and south were also dry during July-September. In the Mortlocks at Lukunoch, rainfall for July, August, and September was 9.18 inches (76%), 15.42 inches (106%), and 2.39 inches (21%) for the respective months. The 3-month average was 71%. Rain for Polowat in July, August, and September was 5.49 inches (39%), 3.36 inches (22%), and 1.10 inches (8%) in the respective months, giving a net below-normal 3-month average of 24%. The extreme dryness at some of the islands of Chuuk State during July through September seems unrealistically low. Satellite imagery, however, revealed a sharp decrease (especially during September) in shower and thunderstorm activity associated with a weak ridge of high pressure that stretched through the Caroline island groups bringing periods of clear, hot, and dry conditions to Palau, Chuuk, Pohnpei, and Kosrae for many days. This ridge of high pressure often forms there when the monsoon trough axis migrates to higher latitudes. The Caroline Islands can experience prolonged periods of dry weather when the monsoon trough axis migrates to higher latitudes and a weak ridge of high pressure forms over them. During summers with a well-developed monsoon circulation, the slow episodic migrations of the trough tend to cause high month-to-month variability of rainfall throughout all of Micronesia.

Rainfall for all of Chuuk State is expected to be slightly wetter than normal as the monsoon trough moves back into the region during October-December, but with high month-to-month variability. Tropical cyclone activity should return to normal in Chuuk State for the second half of 2001. This means that several tropical disturbances will form or move over Chuuk that later move northwest and become tropical cyclones, and Chuuk will also have a chance of getting a tropical storm or typhoon, primarily during the months of October, November, and December, and again during April and May of 2002.

Predictions for Chuuk State from October 2001 through September 2002 are as follows:


Inclusive Period               % of long-term average
                        Chuuk         Outer Atolls
                        Lagoon    Southern    Western
Oct 2001 - Mar 2002     100%       100%        100%
Apr 2002 - Sep 2002     110%       115%        110%

- sources: UOG-WERI

Pohnpei State: As at Chuuk State, rainfall at Pohnpei State was generally drier than normal for the three-month period July-September (but especially during September). Also similar to Chuuk State, the dryness of July, August and September followed generally dry conditions in April, May and June. At the weather station at Kolonia, the July, August, and September rainfall totals were 15.68 inches 85%), 19.28 inches (117%), and 7.35 inches (46%), respectively. This amounted to a 3-month value of 83% of average precipitation. At Pingalap, observed rainfall in July, August, and September was 10.48 inches (66%), 15.49 inches (104%), and 9.17 inches (61%), respectively, for a 3-month amount of 77%. At Nukuoro, July through September precipitation amounts were 9.90 inches (69%), 22.40 inches (197%), and 6.89 inches (63%), respectively. Kapingamarangi continued to be wetter than normal. July, August, and September rainfall measurements there were 14.80 (142%), 15.46 (251%), and 9.01 (153%) respectively. This followed a wet April-June that had a 3-month average of 148%. The months of August through October are the "dry" season at Kapingamarangi, with average monthly rainfall totals of 6.16, 5.89, and 4.82 inches respectively. Although islands in Pohnpei State were generally dry during July-September, some periods of heavy showers occurred in the weak low-level easterly wind flow in the equatorial latitudes of Kapingamarangi. The pronounced dryness of September in most islands of Pohnpei State can be attributed to movement of the monsoon trough into subtropical latitudes, and a weak ridge of high pressure that stretched through the Caroline island groups bringing clear, hot, and dry conditions to Palau, Chuuk, Pohnpei, and Kosrae for many days. Daily high temperatures at Pohnpei WSO reached the mid-90s on many days with blazing sun and very light winds. During summers with a well developed monsoon circulation, the slow episodic migrations of the trough tend to cause high month-to-month variability of rainfall throughout all of Micronesia. Rainfall in Pohnpei State is expected to return to near normal as the monsoon trough axis moves back into Micronesia during October through December.

Over the last 30 years, the El Niņo-induced droughts have led to much larger rainfall deficits than the La Niņa-associated rainfall excesses. Thus, normal years and years leading into another El Niņo should be wetter to make up for the El Niņo-related deficit. As a result, rainfall for all areas of Pohnpei State is expected to be near normal through December, then somewhat above normal for the remainder of the forecast period. At all locations, high month-to-month variability in rainfall can be expected. Even though tropical cyclones are not expected to pose a serious threat to Pohnpei State until the next El Niņo event, some tropical disturbances will move over Pohnpei that later move northwest and become tropical cyclones. There is a slight risk that Pohnpei could experience one or two periods of high waves and heavy rains with strong westerly winds from tropical cyclones passing to the north during October through December and then again during April and May of 2002.

Predicted rainfall for Pohnpei State from October 2001 through September 2002 is as follows:


Inclusive Period          % of long-term average
                     Pohnpei           Outer Atolls:
                     Island    Eastern    Southern    Equatorial
Oct 2001 - Mar 2002   100%      100%        100%        110%
Apr 2002 - Sep 2002   110%      110%        110%        120%

- sources: UOG-WERI

Kosrae: Similar to Pohnpei, Kosrae was also dryer than normal during July, August and especially during September, with rainfall totals of 11.04 (65%), 12.53 (76%), and 9.76 (57%) respectively. The 3-month period July-September averaged 66%. Kosrae has been dryer than normal since April 2001. This dryness resulted first from a weakening of the persistent trade wind trough that was anchored in the area earlier in the year, and later (especially during September), when the monsoon trough moved northward into subtropical latitudes, and a weak ridge of high pressure stretched through the Caroline island groups bringing clear, hot, and dry conditions to Palau, Chuuk, Pohnpei, and Kosrae for many days. Utwa, Tafunsak, and Tofol were also dryer than normal. Rainfall in Kosrae State is expected to return to near normal values during October-December 2001 as the monsoon trough moves back into the Caroline Islands.

Over the last 30 years, the El Niņo-induced droughts have led to much larger rainfall deficits than the La Niņa-associated rainfall excesses. Thus, normal years and years leading into another El Niņo should be wetter to make up for the El Niņo-related deficit. Rainfall for Kosrae is expected to be near normal from October through December, and above normal for the remainder of the forecast period. However, high variability should be expected in the month-to-month rainfall amounts. Even though tropical cyclones are not expected to pose a serious threat to Kosrae State until the next El Niņo event, some tropical disturbances will move over Kosrae that later move northwest and become tropical cyclones. There is a slight risk that Kosrae could experience one or two periods of rough seas and heavy rains with strong westerly winds from developing tropical cyclones passing to the north during October through December 2001, and then again during April and May 2002.

Predicted rainfall for Kosrae State from October 2001 through September 2002 is as follows:


Inclusive Period     % of long-term average
Oct 2001 - Mar 2002            100%
Apr 2002 - Sep 2002            115%

- sources: UOG-WERI

Republic of Palau: Rainfall at Koror during July, August, and September was 21.77 inches (121%), 20.38 inches (136%), and 9.30 inches (78%), respectively. The 3-month average was 115% of normal. For 4 months in a row (June-August), Koror has received more than 20 inches of rain per month; then, in September, the island experienced a dry break. Southwesterly monsoonal winds dominated Palau's weather during June through August and brought widespread heavy rains to the island. During September, the monsoon circulation weakened, and with light winds and higher pressure, hit-and-miss showers and thunderstorms produced below normal rainfall. High month-to-month variability of rainfall is typical in summers that have episodic strong monsoon wind events. For the 3-months of July, August, and September, the monthly rainfall distribution at the Mariculture Center (20.05 inches, 14.57 inches, and 7.26 inches) was roughly the same as that at the Weather Station (wet in July, near normal in August, and dry in September); but at Nekken Forestry, the distribution was somewhat different: July with 12.47 inches (69%) was dry, August with 17.12 inches (115%) was wet, and September with 13.80 inches (116%) was also wet. Farther south at Peleliu, conditions were a bit drier overall, with 14.37 inches (81%) in July, 15.08 inches (101%) in August, and 6.52 inches (55%) in September. This produced a 3-month average of 81%.

Over the last 30 years, the El Niņo-induced droughts and the La Niņa-associated rainfall excesses have nearly cancelled each other. Thus, normal years and years leading into another El Niņo should be near average to slightly wetter than average. For the entire Palau island chain, we expect rainfall to be wetter than normal for the rest of 2001. 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, meaning that all islands from Angaur northward could experience a tropical cyclone from October through December. Islands south of Angaur are generally not directly threatened by typhoons, although typhoons passing to the north could cause dangerous surf, and heavy rainsqualls on gusty southwesterly wind.

Predicted rainfall for Palau from October 2001 through September 2002 is as follows:


Inclusive Period              % of long-term average
                    Koror and           Outer Atolls
                    Mountain Is.  S. of 8ºN    N. of 8ºN
Oct 2001 - Dec 2001   100%            100%        100%
Jan 2002 - Sep 2002   105%            110%        105%

- sources: UOG-WERI

Republic of the Marshall Islands (RMI): Quite dry conditions dominated the Republic of the Marshall Islands (RMI) during the first half of 2001, and continued to dominate the northern islands during July through September. The Majuro weather station (representative of the southern islands) measured 10.04 inches (77%), 11.72 inches (102%), and 20.89 inches (168%). For the 3-month period, the average for Majuro was 115% (thanks to abundant rain in September). At nearby Laura the net rainfall amount for July, August, and September (86%) was drier than at Majuro (this was primarily due to big difference in September when the Majuro weather station received 20.89 inches and Laura 12.46). Kwajalein (representative of the central islands) has been very dry since April. Rainfall at Kwajalein (and nearby Ebeye) was 2.17 inches (29%) in April, 4.68 inches (47%) in May, 4.63 inches (48%) in June, 7.73 inches (74%) in July, 9.27 inches (92%) in August, and only 3.77 inches (32%) in September. This gave an average of only 64% for the period July-September and only 54% for the 6-month period April-September.

Ailingalaplap continued to be much drier than normal with 3.06 inches (26%) in July, 6.18 inches (57%) in August, and 5.64 inches (46%) in September. This yielded a 3-month average of only 43%. Jaluit received about half of normal rainfall in the past few months. Farther north, Wotje continued to be the driest of the locations that measured rainfall during the year. In the first half of 2001 Wotje received only about 25% of its normal rainfall. During July, August, and September, the accumulated rainfall there was 15.01 inches or 49% of normal. Observations were not available from Utirik during January through April, June, August and September, but satellite imagery indicated that it was also likely very dry. The values of rainfall in the northern islands of the RMI seem very low, but are supported by satellite imagery showing most of the rainfall confined to showers along the trade wind trough, which tended to be south of these islands. Several large areas of showers associated with TUTT cells and some tropical disturbances moving westward along the trade wind trough moved through the Marshall Islands and produced rainfall at the southern islands, but left the northern islands still very dry.

The monsoon trough will move back into the Caroline Islands during October-December, and the trade wind trough eastward from the end of the monsoon trough should stretch through the Marshall Islands. Thus, rainfall for the Marshall Islands is expected to be near the long-term average from October through December and then above the long-term average for the remainder of the forecast period. The persistent dryness of the northern islands has been unforeseen, but with a normal trade wind trough stretching through the region, some increase of rainfall closer to normal values can be expected; however, January marks the beginning of the winter "dry" season in the northern islands, and even normal rainfall will not help much to alleviate the recent very dry conditions. Tropical cyclone activity for the Marshalls should return to normal, meaning that Wake and Enewetok could experience the near passage of a tropical storm from October into December. The rest of the Marshall Islands could experience gusty squalls from a developing tropical depression, but will not likely experience a typhoon until the fall of the next El Niņo year (not expected until 2002 or later).

Predicted rainfall for the RMI from October 2001 through September 2002 is as follows:


Inclusive Period           % of long-term average
                                  RMI Atolls
                            Southern     Central   Northern 
Oct 2001 - Dec 2001           100%         100%       90%
Jan 2002 - Sep 2002           110%         120%      100%

- sources: UOG-WERI


APPENDICES:

Long-Lead Outlook for Hawaiian Islands issue dated 18 October 2001, from NOAA's Climate Prediction Center (CPC).

ENSO ADVISORY

SPECIAL SECTION - Markov model for Pacific SST and sea level

SPECIAL SECTION - Experimental Forecasts for Pacific Island Rainfall


ACKNOWLEDGEMENTS and FURTHER INFORMATION

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 Technology
(SOEST) DEPARTMENT OF METEOROLOGY:
HIG #350, 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 #350, 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 350
University of Hawaii - Manoa Campus
2525 Correa Road - Honolulu, HI 96822
Tel: 808-956-2324 Fax: 808-956-2877
E-mail: cpalmer@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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