Pacific ENSO Update

2nd Quarter 2001 - Vol. 7 No. 2


CURRENT CONDITIONS


During the last few months, sea surface temperature (SST) distribution in the equatorial Pacific has slowly warmed to near El Niño-neutral values. The Southern Oscillation Index (SOI) has also fallen over the last few months and is approaching neutral values. Sub-surface sea temperatures in the eastern and central equatorial Pacific have also warmed over the last few months, further supporting the demise of La Niña conditions. In the western equatorial Pacific, surface sea temperatures have remained relatively constant over the past 6 months, slowly setting the stage for the next El Niño event. The absence of westerly low-level equatorial winds in eastern parts of the western Pacific suggests that an El Niño will not occur until at least 2002. [At the time of this publication, there is evidence of a westerly wind burst in the western equatorial Pacific. While the majority view precludes a 2001 event, the timing of the westerly burst coincides with a similar event in May 1986. To view the westerly wind burst, please see the Tropical Atmosphere Ocean project. Stay tuned for further developments. Schroeder] Rainfall amounts at most islands decreased during April 2001, with some extremely dry amounts reported on Guam, the CNMI, and the RMI (Figure 1).

SST

After several months of "cooler than normal" SST anomalies in the equatorial Central and extreme eastern part of the equatorial Western Pacific, SST anomalies rose to near normal values (-0.3 in the NINO 3.4 region). The La Niña "cold tongue" that characteristically extends from the equatorial Eastern Pacific into the equatorial Western Pacific further weakened over the period. Above normal SST values have appeared in the Eastern Pacific (+1.3ºC in March), but this rise appears to be very shallow based on sub-surface temperature analyses in the same area. The Global Climate Models (GCMs) continue to predict warming of Central Pacific SSTs through the boreal fall and winter, but are now reducing the speed and maximum values of the warming. In fact, of nine climate models, four are predicting neutral conditions by September, four are predicting warm conditions of 1 ºC or less, and one continues cold conditions. By December, of eight available models, four are predicting neutral and four are predicting warm conditions. 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, but based on current SST distribution, it does not seem likely that this will occur until 2002 or later. We will continue to monitor the unusual SST warming in the Eastern equatorial Pacific near the South American coast (NINO 1&2 regions).

Sub-surface sea temperatures in equatorial and near-equatorial parts of the Northwest Pacific have remained above normal over the past 6 months, but some warming to nearly 5ºC above normal has occurred over the past few months. The depth of the warmer than normal sub-surface temperatures in some places is more than 400 feet, and warm anomalies up to 2°C extend down to 250 feet. Warm anomalies greater than 2°C also extend eastward to 170ºW. In March, colder than normal sub-surface SSTs in the Central and Eastern Pacific were -4°C and -2°C, respectively, substantiating the continuance of La Niña conditions into the boreal spring of 2001. Sub-surface SST profiles also suggest that another El Niño is not likely until 2002 or later.

SOI

January through March 2001 SOI values hovered between +0.7 and +1.5, indicative of weak to moderate La Niña conditions. By mid-April they had fallen to +0.4, and by the third week of April, they had fallen to +0.2. Despite the +1.5 SOI value, much of Micronesia did not reflect the moderate La Niña rainfall distribution seen in the boreal spring seasons of 1999 and 2000. The trade wind trough that normally accompanies such La Niña conditions did not persist, especially in the Marshall Islands. As a result, the long east-west band of persistently heavier than normal rainfall characteristic of the 1999 and 2000 La Niña events was much shorter in length and narrower in breadth during the spring of 2001. In fact, many of the Micronesian rainfall stations showed high month-to-month rainfall variability more indicative of El Niño-neutral conditions. Over the period, easterly low-level winds dominated the low latitudes (5ºN-5ºS), and were as much as 10 knots stronger than normal. In the upper troposphere (30,000-50,000 ft), winds over Micronesia tended to be more westerly than normal. These wind patterns accompanied a westward shift of the Pacific Walker Circulation; however, the downward (sinking air) branch along the equator did not extend as far west as in previous La Niña years. Figure 2 shows that Kapingamarangi rainfall increased with each succeeding La Niña event. On average, equatorial westerly winds did not extend east of 140ºE, and tropical cyclones developed farther west than normal (although not as far west as in the previous two years). While monsoon and equatorial westerly winds occasionally extended east of 145°E, the duration of the events was short lived. Southern Hemisphere tropical cyclone activity has been well below normal, with most activity occurring in the Southwest Indian Ocean. On April 12, a short-lived burst of westerly low-level winds along the equator between 130-145ºE preceded the development of tropical cyclone twins (Alistaire in the Southern Hemisphere and Tropical Depression 02W in the Northern Hemisphere). So far in 2001, no tropical cyclones occurring in the Northwest Pacific have been named. An abundance of named tropical cyclones during the first half of the year in the Northwest Pacific is a sign of El Niño. The lack of named tropical cyclones there through April 2001 is another indication that the next El Niño may hold off until 2002 or later.

We expect the SOI to move back toward zero (El Niño-neutral values) by summer. With the SOI near zero, 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 become more 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.

Figure 2 compares rainfall at selected Micronesian Islands and at American Samoa for January-March 1999, January-March 2000, January-March 2001, and the January-March long-term (30-year) average. The figure illustrates that La Niña years are wetter than average at some locations, drier than average at other locations, and near average or a mix of wet and dry at still other locations. Some aspects of the specific La Niña rainfall behavior will be further discussed in the individual Local Variability Summaries.

TROPICAL CYCLONE ACTIVITY

Northwest Pacific tropical cyclone activity during 2000 was below normal with 23 tropical cyclones named by the Japan Meteorological Agency (JMA). The Joint Typhoon Warning Center (JTWC) identified 34 total tropical cyclones including tropical depressions. Most of the Northern Hemisphere tropical cyclones developed in western longitudes or at high latitudes in response to La Niña conditions early in the year and late in the year. Tropical cyclone activity from July 2000-April 2001 in the Southern Hemisphere was characterized by low activity in the Southwest Pacific (eight named cyclones) and higher activity in the Southwest Indian Ocean (12 named cyclones). This westward shift in activity is characteristic of La Niña influences. Tropical cyclone activity in the Northwest Pacific is off to a slow start in 2001 with only two tropical depressions identified by the JTWC as of April. Based on the expected resumption of near normal SST and SOI conditions by the boreal summer, normal tropical cyclone activity in the Northwest Pacific is anticipated for the second half of 2001. This means that Micronesia could see tropical cyclone activity from September until late-December. In eastern Micronesia, these should be limited to a few tropical storms and weak typhoons in the Marshall Islands 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 Niño 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: A kona storm that developed west of Kaua'i at the end of March continued to affect the island chain through 2 April. Rainfall was especially heavy over Kaua'i during the morning hours of 2 April where thunderstorms dumped 6 to 8 inches of rain (by radar estimates) over the western half of the island. Flash flooding occurred in the Waimea, Makaweli, and Wainiha Rivers with some damages to businesses reported in the town of Waimea. No injuries or deaths were reported in this event.

A second kona storm developed far northwest of the state on 5 to 6 April and helped destabilize that airmass enough to produce thunderstorms near Kaua'i. Although flash flood watches were posted, thunderstorm activity managed to remain offshore for this event.

After this second kona storm, the trade winds became firmly entrenched for the remainder of the month. Trade strength was mainly in the moderate to fresh range with a bout of strong trades during the period from 9 through 12 April. Ample trade wind moisture with contributions from shear line remnants (17 to 18 April and 29 April) brought frequent showers over the windward areas of the state.

Kevin Kodama - Senior Service Hydrologist, NWSFO Honolulu, HI

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

American Samoa: Rainfall at Pago Pago Airport for January, February, and March was 6.39 inches (51%), 13.66 inches (107%), and 15.21 inches (135%), respectively, amounting to 98% of normal for the 3-month period. In April 2001, the 6.62 inches of rainfall was only 53% of normal.

The near normal rainfall experienced in the Samoa region for the January-February-March timeframe was due to periodic eastward (wet for Samoa) and westward (dry for Samoa) shifts of the SPCZ. The high month-to-month variability was indicative of weakening La Niña conditions and a return to an El Niño-neutral atmosphere. With equatorial SST anomalies and the SOI expected to return to near zero values by June, the rainfall from May through the end of the forecast period should average near normal. Figure 2 compares the differences in rainfall among the 1999, 2000, and 2001 La Niña events. These events were fairly similar in terms of SST and SOI values, and for American Samoa, they were near normal to slightly wetter than normal. Thus, "normal" rainfall for Pago Pago is actually slightly below (90-95%) the long-term average, as reflected in the new predictions. While it is too early to predict the next El Niño, 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 Niño event occurs. Despite this, some tropical depressions and weak tropical storms could develop in the east-west oriented portion of the SPCZ during the November 2001 - April 2002 timeframe, bringing heavy rains to the Samoan Islands as the cyclones move through the region.

Predicted rainfall for American Samoa from May 2001 through Jun 2002 is as follows:


Inclusive Period         % of long-term average
                              Samoa Region
May 2001 - Jun 2002             90% - 95%

- sources: UOG-WERI and PEAC

Guam/CNMI: Rainfall at Guam International Airport (GIA) during January, February, and March was 2.90 inches (65%), 3.26 inches (87%), and 2.13 inches (71%), respectively. This amounted to 74% of the normal values. The April measurement was only 1.15 inches (26%). Andersen Air Force Base (AAFB) measured 3.27 inches (57%), 1.91 inches (37%), and 2.52 inches (62%), or 52% of the average rainfall for the period. April rainfall there was 2.28 inches, or only 40% of the average. Long spells of nearly cloudless, hot, dry weather have begun to cause defoliation of some trees, and desiccation of grasslands. Wildfires have become a problem. Water levels in scenic streams and popular waterfalls have fallen to very low levels, and debris and algal growth have proliferated in the stream beds and swimming holes. The water level in the Fena Reservoir (supplying 20% of Guam's residential and commercial water needs) has fallen below its 10-year average.

January, February, and March rainfall at Saipan International Airport (SIA) was 1.39 inches (43%), 1.92 inches (80%), and 1.29 inches (65%), or 63% of the average for the period. Precipitation in April was a very dry .80 inches (25%). For January, February, and March, Capitol Hill's measured rainfall was 4.56 inches (114%), 4.13 inches (138%), and 1.47 inches (59%). Thus, at Capitol Hill, quarterly rainfall was considerably higher at 104%. However, April rainfall at Capitol Hill was only 1.82 inches (46%) as the dry conditions widened their grip. Rainfall amounts for the 3-month period at the Tinian Airport were 2.62 inches (66%), 5.51 inches (184%), and 1.00 inch (40%), respectively. The 3-month rainfall for Tinian Airport was a near normal 97%. April rainfall at the Tinian Airport was a meager 1.47 inches (37%), and (as on Guam), numerous small wildfires have become a problem. At Rota Airport, January, February, and March rain amounts were 6.01 inches (114%), 2.45 inches (52%), and 1.94 inches (53%). This gave a 3-month average of only 73%. In April, dry conditions continued with only 2.53 inches (48%). On the northern part of Rota at the beautiful Rota Resort and Country Club, the NASA TRMM network recorded 80% of the amount at the Rota Airport for the period January-March. As very dry conditions tighten their grip on Guam and the CNMI, wildfires and desiccation of vegetation have become a problem. A lack of early season tropical cyclone development has also contributed to this dryness. It is hoped that the Tropical Upper Tropospheric Trough (TUTT) develops in a normal (climatological) manner in May and June to initiate heavier showers and help to bring deep convection associated with tropical cyclones and tropical disturbances at low latitudes northward to these island groups. A cautionary note: the people in Guam and in the CNMI should consider the consequences of a possible continuation of very dry conditions through mid-July.

Figure 2 compares the differences in rainfall among the 1999, 2000, and 2001 La Niña events. While these events were fairly similar in terms of SST and SOI values, the rainfall tended to decrease with each succeeding event. 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 "normal" 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 Niño, an event in 2002 would likely not affect Guam and the CNMI during this forecast period. During the second half of 2001, tropical cyclone activity should return to normal after three years of well-below normal activity. Thus, the Mariana Islands can expect typhoon threats primarily from September through December, and most of these will approach from the east-southeast. Time is running out for a spring typhoon, and the likelihood of a typhoon (or tropical storm) passing over or near Guam, the CNMI, or other islands of the Marianas now through June is low.

Predicted rainfall for the Mariana Islands from May 2001 through Jun 2002 is as follows:


Inclusive Period         % of long-term average
                      Guam/Rota       Saipan/Tinian
May 2001 - Jun 2001      70%               80%
Jul 2001 - Jun 2002     110%              110%

- sources: UOG-WERI

Yap State: Northern Yap State dried out quite a bit for the three months of January, February, and March. The weather station at the Yap Airport recorded 5.02 inches (68%) in January, 6.84 inches (114%) in February, and 5.98 inches (100%) in March, or 94% of normal rainfall for the three months. This was considerably less than the 132% experienced during the previous quarter. In April, only 2.30 inches of rain (31%), fell there. January, February, and March rainfall amounts on Ulithi were 1.98 inches (32%), 3.07 inches (60%), and 6.56 inches (129%), or 74% for the 3-month period. This was less than half of the 155% of the previous quarter. April rainfall was 5.35 inches (86%). Farther south at Woleai Atoll, rainfall was 10.98 inches (103%) in January, 5.49 inches (73%) in February, and 11.41 inches (137%) in March, for a near normal 3-month average of 104%. Rain on the atoll in April was measured at 8.90 inches (83%). Figure 2 compares the differences in rainfall among the 1999, 2000, and 2001 La Niña events. These events were fairly similar in terms of SST and SOI values, and for Yap, the rainfall was very similar.

Rainfall for Yap State is expected 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 Niña conditions do not quite add up to the deficits caused during El Niño-induced droughts. Thus, normal 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 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. Yap State could see some rain from the outer showers and rainbands of tropical cyclones developing to their south and west in May and June.

Predicted rainfall for Yap State from May 2001 through Jun 2002 is as follows:


Inclusive Period                % of long-term average
                             Yap               Outer Atolls:
                            Island       S.of 8·N     N. of 8·N
May 2001 - Sep 2001           95%          100%          95%
Oct 2001 - Jun 2002          110%          115%         110%

- sources: UOG-WERI

Chuuk State: During January, February, and March, the weather station at Weno Island measured 12.61 inches (118%), 9.60 inches (155%), and 13.60 inches (163%). This amounted to145% of normal amounts for the 3-month period. In April, only 6.31 (59%) inches was recorded. Wet La Niña conditions in Micronesia during 2001 covered a much smaller area than in 1999 and 2000. While Weno caught the brunt of the rainfall, Chuuk State islands to the north and south were considerably drier. In the Mortlocks at Lukunoch, rainfall for January, February, and March was 14.64 inches (137%), 11.95 inches (115%), and 7.13 inches (85%) for the respective months. The 3-month average was 112%. April rain was 14.33 (134%). Rain for Polowat in January, February, and March was 7.58 inches (95%), 4.50 inches (72%), and a wetter 9.11 inches (145%) in the respective months, giving a near-normal 3-month average of 104%. April saw a below normal rainfall of 6.74 (84%). Figure 2 compares the differences in rainfall among the 1999, 2000, and 2001 La Niña events. These events were fairly similar in terms of SST and SOI values, and for Weno Island, the spring rainfall was very similar.

Rainfall for all of Chuuk State is expected to be slightly wetter than normal, but with high month-to-month variability. Over the last 30 years, the El Niño-induced droughts have led too much larger rainfall deficits than the La Niña-associated rainfall excesses. Thus, normal years and years leading into another El Niño should be wetter to make up for the El Niño-related deficit. This is due to increased monsoon activity and to some increased tropical cyclone activity. 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.

Predictions for Chuuk State from May 2001 through Jun 2002 are as follows:


Inclusive Period               % of long-term average
                        Chuuk         Outer Atolls
                        Lagoon    Southern    Western
May 2001 - Sep 2001     100%       100%         90%
Oct 2001 - Jun 2002     110%       115%        110%

- sources: UOG-WERI

Pohnpei State: Coastal areas of Pohnpei Island were wetter than the interior locations, but this is not normally the case. Thus, for whatever reason, some of the interior observations may reflect lower values of rainfall than actually occurred. January, February, and March rainfall measured at the weather station at Kolonia were 21.71 inches (166%), 12.51 inches (116%), and 9.00 inches (66%), respectively. This amounted to a 3-month value of 116% of average precipitation. The weather station measured 12.17 inches (93%) in April 2001. At Pingalap, observed rainfall in January, February, and March was 26.69 inches (216%), 9.77 inches (80%), and 2.03 inches (14%), respectively, for a 3-month amount of 103%. April produced 7.27 inches (59%) there. At Nukuoro, January through March precipitation amounts were 17.71 inches (151%), 26.00 inches (246%), and 20.05 inches (147%), respectively. The 3-month period was 181% of the normal value, making Nukuoro the wettest location in Micronesia in terms of percent of rainfall. The 3-month total of 63.76 inches was less than 2 inches below the wettest location that was on Kosrae. Rainfall in April 2001 was 13.55 inches (115%) for the island. Kapingamarangi continued to see below normal boreal spring rainfall, but amounts were more than double those for the same period in 2000. January, February, and March measurements there were 5.53 inches (53%), 10.08 inches (98%), and 6.37 inches (46%), respectively, giving a 3-month average of 66%. April rainfall for Kapingamarangi rose substantially to 14.33 inches (137%) -- a sign that lingering La Niña effects are ending there. Figure 2 compares the differences in rainfall among the 1999, 2000, and 2001 La Niña events. These events were fairly similar in terms of SST and SOI values, and while Pohnpei is normally wet, there was as much as a 20-inch difference in rainfall.

Over the last 30 years, the El Niño-induced droughts have led to much larger rainfall deficits than the La Niña-associated rainfall excesses. Thus, normal years and years leading into another El Niño should be wetter to make up for the El Niño-related deficit. As a result, rainfall for all areas of Pohnpei State is expected to be near normal through September, then somewhat above normal for the remainder of the forecast period. At all locations, high month-to-month variability in rainfall can be expected. While it is too early to predict the next El Niño, an event in 2002 would likely produce heavier than normal rainfall for Pohnpei State during the winter of 2001-2002 and the spring of 2002. October 2001-March 2002 rainfall predictions reflect this possibility. Even though 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 waves from typhoons passing to the north.

Predicted rainfall for Pohnpei State from May 2001 through Jun 2002 is as follows:


Inclusive Period          % of long-term average
                     Pohnpei           Outer Atolls:
                     Island    Eastern    Southern    Equatorial
May 2001 - Sep 2001   110%      110%        130%         90%
Oct 2001 - Jun 2002   110%      115%        120%        130%

- sources: UOG-WERI

Kosrae: Observers at Kosrae Airport recorded 31.01 inches (215%), 26.74 inches (164%), and 6.15 inches (33%) for January, February, and March, which amounted to 137% (63.90 inches) of the long-term average rainfall for the period. This heavy rainfall resulted from a combination of a persistent trade wind trough anchored between 4ºN and 5ºN across central Micronesia and the effects of periods of an active Australian monsoon. In April, the Airport measured 9.74 inches (68%). Tofol was the wettest Kosrae State location that measures rain with 65.41 inches for the three months (2% wetter than the Airport), while Tafunsak was the driest with 54.11 inches (15% drier than the Airport). Utwa fell between the other stations with 62.50 inches (2% drier than the Airport). Figure 2 compares the differences in rainfall among the 1999, 2000, and 2001 La Niña events. These events were fairly similar in terms of SST and SOI values, and Kosrae was one of the wettest locations every year.

Over the last 30 years, the El Niño-induced droughts have led to much larger rainfall deficits than the La Niña-associated rainfall excesses. Thus, normal years and years leading into another El Niño should be wetter to make up for the El Niño-related deficit. Rainfall for Kosrae is expected to be above normal through March, near normal from April through September, and above normal for the remainder of the forecast period. However, high variability should be expected in the month-to-month rainfall amounts. While it is too early to predict the next El Niño, an event in 2002 would likely produce heavier than normal rainfall for Kosrae State during the winter of 2001-2002 and the spring of 2002. Oct 2001-Mar 2002 rainfall predictions reflect this possibility. Kosrae should not experience any tropical cyclones until the next El Niño event begins.

Predicted rainfall for Kosrae State from May 2001 through Jun 2002 is as follows:


Inclusive Period     % of long-term average
May 2001 - Sep 2001            105%
Oct 2001 - Jun 2002            115%

- sources: UOG-WERI

Republic of Palau: Rainfall at Koror during January, February, and March was 6.21 inches (58%), 11.93 inches (131%), and 7.15 inches (87%), respectively. The 3-month average was 92% of normal compared with 127% for the previous quarter. In April, there was abundant rainfall of 13.27 (124%). For the 3-months of January, February, and March, rainfall at the Mariculture Center was 9% more than that at the Weather Station and at Nekken Forestry was 12% more than that at the Weather Station. April rainfall at Mariculture and Nekken Forestry was wetter with 10.60 and 10.59 inches respectively. Farther south at Peleliu, conditions were wetter with 12.38 inches (116%) in January, 10.89 inches (121%) in February, and 12.60 inches (154%) in March. This produced a 3-month average of 130%. In April, the island had 11.80 inches or 111% of that expected. Figure 2 compares the differences in rainfall among the 1999, 2000, and 2001 La Niña events. These events were fairly similar in terms of SST and SOI values, but Koror showed decreasing rainfall with each succeeding La Niña event.

Over the last 30 years, the El Niño-induced droughts and the La Niña-associated rainfall excesses have nearly cancelled each other. Thus, normal years and years leading into another El Niño should be near average to slightly wetter than average. 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 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 windy rain squalls.

Predicted rainfall for Palau from May 2001 through Jun 2002 is as follows:


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

- sources: UOG-WERI

Republic of the Marshall Islands (RMI): After a near normal fall and winter, drier than normal conditions returned to the Republic of the Marshall Islands (RMI). The trade wind trough that often occurs in the spring across the RMI during La Niña events only occurred sporadically. Generally, it was more persistent to the west. During January, February, and March, the Majuro weather station (representative of the southern islands) measured 5.73 inches (68%), 5.10 inches (83%), and 0.46 inches (6%). (There is some question as to the accuracy of the 0.46 inch March value, since Laura (30 nm west) and Jaluit (120 nm west-southwest) had rainfall amounts of 8.80 inches and 6.47 inches, respectively.) For the 3-month period, the average for Majuro was 52%. In April, rainfall was 4.07 inches, or 48% of normal. Kwajalein rainfall (representative of the central islands), after near normal fourth quarter values, was very dry during the first three months of 2001. Rainfall at Kwajalein (and nearby Ebeye) was 3.53 inches (77%) in January, 1.75 inches (54%) in February, and 1.13 inches (28%) in March. This was an average of 53% for the period. In April, very dry conditions continued with rainfall of 2.17 inches, or 47% of that normally expected. Strangely, 3-month rainfall amounts at Laura were considerably higher than those at Majuro, giving Laura 130% of normal. This seems high. April rainfall at Laura fell to 4.51 inches. Given the regional rainfall distribution and the synoptic weather patterns over the 3-month period, we feel that an average of the Majuro and Laura rainfall values are most representative for the atoll. This rationale would make the January to March rainfall value about 90% of normal.

Ailingalaplap was also much drier than normal. In January, February, and March, Ailingalaplap experienced 7.92 inches (122%), 2.01 inches (43%), and 2.20 inches (36%), respectively, for a 3-month value of 67%. Even Jaluit, which was very wet during the last few months of 2000, became significantly drier than normal. For the first three months of the year, Jaluit had 6.60 inches (77%), 3.81 inches (62%), and 6.48 inches (78%), respectively, for a 3-month value of 72%. Farther north, Wotje continued to be the driest of the locations that measured rainfall during the year. In the January-March timeframe, rainfall amounted to a 3-month amount of only 3.63 inches (30%). Monthly values were 2.33 inches (54%) in January, 0.45 inches (15%) in February, and 0.85 inches (22%) in March. In April, rainfall at some of the Marshall Islands was extremely low. Ailingalaplap recorded only 0.51 inches, and Wotje reported an astonishingly dry value of only 0.13 inches. Observations were not available from Utirik; however, satellite imagery indicated that it was also likely very dry.

The Tropical Upper Tropospheric Trough (TUTT) is developing in a climatological manner, and should act to bring a return of near normal rainfall to the RMI in the late spring and summer of 2001. Thus, rainfall for the Marshall Islands is expected to be near the long-term average through October and then above the long-term average for the remainder of the forecast period. While it is too early to predict the next El Niño, an event in 2002 would likely produce heavier than normal rainfall for the south and central Marshall Islands during the winter of 2001-2002 and the spring of 2002. May 2001-Jun 2002 rainfall predictions reflect this possibility. 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 fall of next El Niño year (not expected until 2002 or later).

Predicted rainfall for the RMI from May 2001 through Jun 2002 is as follows:


Inclusive Period           % of long-term average
                                  RMI Atolls
                            Southern     Central   Northern 
May 2001 - Oct 2001            90%          90%       90%
Nov 2001 - Jun 2002           110%         120%      130%

- sources: UOG-WERI


APPENDICES:

Long-Lead Outlook for Hawaiian Islands issue dated 12 April 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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