Pacific ENSO Update - 1st Quarter 1997 - Vol.3 No.1


CURRENT CONDITIONS

Climatic conditions in the equatorial Pacific over recent months have remained close-to-normal, though with some variability. Sea surface temperatures (SSTs) to the west of the equator and 170°W (north of Samoa) have warmed somewhat, to levels slightly above their long-term average values. SSTs to the east are still slightly below average, though less so than in earlier months. October-January values for the Southern Oscillation Index or SOI (based on Tahiti and Darwin atmospheric surface pressures) ranged from near-zero to +1. Ongoing positive values would usually suggest continued La Niña-like conditions (as were experienced in 1995-96). However, while the SOI has remained weakly positive, changes in absolute pressures at Tahiti and Darwin have also occurred. The stronger-than-normal easterly surface winds (which have affected the region since 1995) have finally weakened to near-normal wind speeds. Impacts of these changes, and activity of the Australian monsoon, seem to have had greater impact on the recent local variability in the islands of Micronesia and the region around Samoa.

Long-term ENSO predictions made by climate models at this time of year are generally limited in their skill, due to the so-called "spring barrier". Some models are pointing toward continuation of near-normal to slightly cool equatorial SST conditions, while others suggest a possible warming of SSTs by the summer or fall of 1997. Conditions at present do not seem to indicate a rapid rate of change. Therefore, rainfall and tropical cyclone activity in the region over coming months is expected to be near-normal. Model results may give a better indication of possible changes later in the year, and will be reported in future issues of Pacific ENSO Update.


Year in Review - 1996:

Consistent with outlooks reported in the four 1996 issues of Pacific ENSO Update, no significant El Niño (warm event) developed, and conditions over the year changed periodically from weakly La Niña (cold event) to near-normal. Western North Pacific sea surface temperatures (SSTs) were near-normal for the year, while those in the central and eastern Pacific were slightly cool (La Niña-like). The Southern Oscillation Index (SOI - described below) hovered between zero and +1.


SOUTHERN OSCILLATION INDEX (SOI):

The ENSO cycle is defined by trends in oceanic and atmospheric conditions, which in turn affect climate by changing normal weather patterns. One important measure of ENSO activity - the Southern Oscillation Index or SOI - is based on the differences in atmospheric surface pressure between Tahiti (in the south central Pacific) and Darwin (in northwest Australia). As can be seen in the top graph at left, higher pressures in one location are generally associated with lower pressures at the other. The differences are used to determine the SOI, which appears in the graph below. Negative values of SOI are associated with El Niño or ENSO warm events, while positive values of SOI are associated with La Niña cold-events. However, the SOI value can be misleading if pressures at both locations rise or fall at the same rate. Usually, positive values of SOI occur when pressure is higher-than-normal at Tahiti and lower-than-normal at Darwin, as was the case in 1988-89, a strong La Niña or cold-event year. The SOI also switched to positive in 1995-96, and has remained weakly positive into 1997. In this case however, positive SOI values have been maintained even when pressures are changing at Tahiti and Darwin at approximately the same rate. Regional variability can be affected by this. For example, the "normal" positive SOI with high pressure at Tahiti will cause drier-than-normal conditions in the Samoa region (higher regional pressure, La Niña-like condition), but with pressures at Tahiti (and Darwin) both falling, wetter-than-normal conditions can result in the Samoa region (lower regional pressure) even though SOI remains positive. This seems to have been the case in the last few months, with short-term pressure changes occurring with high monthly rainfall variability at Samoa.


The slightly positive SOI kept low-level easterly winds stronger than normal, causing western North Pacific tropical cyclone development to occur farther west and north than usual --much like a La Niña year. Many tropical cyclones developed at high latitudes. Overall, the outlooks for 1996 tropical cyclone activity reported in Pacific ENSO Update were accurate, except that the activity continued into late December and early January, about 3 weeks longer than anticipated. Overall rainfall predictions were for wetter than normal conditions for Yap, Guam the CNMI, and Chuuk; near normal conditions for Palau and Wake; slightly drier than normal conditions for Kosrae, Pohnpei, Majuro, and American Samoa; and drier than normal conditions for Kwajalein. Table 1 (below) shows the 1996 rainfall, and compares average predictions from the LOCAL VARIABILITY SUMMARIES section of the newsletter with the actual rainfall. Annual rainfall predictions for Guam, Palau, Chuuk, Pohnpei, and Wake were close to that observed. Conditions at Yap, Kosrae, Majuro, Kwajalein, and American Samoa were wetter than anticipated, for reasons further explained in the text below. The southern CNMI (Saipan, Tinian, and Rota) was drier than anticipated, primarily due to the lower than normal tropical cyclone activity in the area. Based on satellite imagery, the uninhabited northern islands of the CNMI appear to have been wetter than normal.
Table 1. Comparison of long-term averages, actual & % of average rainfall for 1996, and predicted 1996 rainfall deviations from the long-term averages. Predicted deviations from the yearly totals were averaged from information in the LOCAL VARIABILITY SUMMARIES secions within the previous six issues of Pacific ENSO Update . - source: UOG-WERI
Location
Avg. Rainfall
1996 Rainfall
Avg. Prediction
1996 % of Avg
Andersen, Guam
98.83
103.61
+10 to +15
+5
Tiyan, Guam
91.02
108.80
+10 to +15
+20
Saipan IAP CNMI
68.90
66.93
+10 to +15
-3
Koror, Palau
147.97
147.36
~0 to +5
0
WSO, Yap
120.06
156.01
+15 to +20
+30
Weno, Chuuk
133.11
146.74
+15 to +20
+10
Kolonia, Pohnpei
187.76
180.78
-10
-4
SARS, Kosrae
208.04
232.24
-10
+12
Majuro, RMI
131.34
153.13
-10 to -15
+17
Kwajalein, RMI
102.09
98.59
-20 to -25
-4
Wake, RMI
36.58
39.04
~0
+7
Pago Pago, Am. Samoa
121.80
122.50
-10
+1

The wetter than expected conditions at Yap, Kosrae, Majuro, Kwajalein, and American Samoa appeared to be associated with a weaker-than-normal "inactive" Australian monsoon. Like ENSO, the Asian-Australian monsoon is an important feature of the climate system in the western Pacific, and its strength can vary from year to year (relationships between monsoon strength and ENSO are not well understood, and are a topic of current study). In normal conditions during the northern hemisphere winter and early spring, relatively dry northeasterly trade winds flow across the equator into an "active" Australian monsoon, with a pattern as shown below in Figure 1. During much of 1996, this flow instead became easterly and merged with southern hemisphere southeast trade winds, creating an unseasonable trade wind trough of low pressure across the RMI and eastern Caroline Islands. Numerous tropical disturbances developed in this region, then moved to the west and northwest, affecting most islands in Micronesia. These disturbances often merged with remnant cold-front shearlines at higher latitudes, causing heavier than normal winter season rainfall over Guam and the CNMI. The process is illustrated in Figure 2 below, showing the flow patterns, cloudiness, and associated rainfall which occurred during parts of the 1995-1996 and 1996-1997 Northern Hemisphere dry seasons (October - March).

Typhoon Fern (Dec 96) and Tropical Storm Hannah (Jan 97) dropped large amounts of rain over Yap State, showing the impact that a single tropical cyclone event can have on the rainfall at a particular location. In the southern hemisphere, the weak Australian monsoon failed to anchor the South Pacific Convergence Zone (SPCZ) in its normal position (west of the date line, between Fiji and Australia). As a result, the SPCZ frequently drifted eastward and become active over the region around Samoa. This resulted in high month-to-month variability and wetter than expected conditions, with rainfall activity in the region periodically similar to that more typical of El Niño warm events, but for shorter periods of time, and without increased tropical cyclone activity.


ENSO FORECASTS:

NOAA's Climate Prediction Center in Washington D.C. routinely monitors the latest forecast results from several ENSO models. A discussion on the latest forecast results appears below:

PROGNOSTIC DISCUSSION OF SST FORECASTS
CLIMATE PREDICTION CENTER NCEP 
NATIONAL WEATHER SERVICE  WASHINGTON DC             
3 PM EST THURSDAY FEBRUARY 13 1997

The official SST forecast for the east-central equatorial Pacific 
(120-170W longitude - also called Niño 3.4) calls for SST 
anomalies to be near zero through the middle of the year.  The SSTs 
in Niño 3.4 are now near to slightly below normal.  The SSTs 
near the South American coast are still below normal - but have 
warmed considerably in the last month.  

This forecast is based on a combination of several statistical and 
dynamic forecasting tools.  This is the most difficult time of year to 
forecast SSTs in the equatorial Pacific.  The accuracy of the forecasts
improves when the early summer SST patterns become known.  A 
statistical model based on a constructed analogue (CA) and one based 
on canonical correlation analysis (CCA) indicate positive SST anomalies 
by late spring and becoming substantially above normal by mid 1997.  
The dynamic forecast from the NCEP model indicates a similar evolution 
of Niño 3.4 SSTs.  Another dynamic model - a new version of the 
Cane and Zebiak model (LDEO2) indicates continued negative anomalies
throughout 1997.  A statistical blend of these four tools based on 
multiple linear regression on historical forecasts indicates near normal
conditions throughout 1997.  In view of the uncertainty in the SST 
predictions initialized in this season and the disagreement among the 
dynamic models - a forecast of near normal through mid-1997 is 
indicated.  It is still too early to make a reliable prediction of 
Niño 3.4 SSTs for winter 1997-98 - however - because the 
majority of the tools used here indicate significant positive anomalies 
in place by early fall - positive anomalies for next winter seem more 
likely than negative ones at this time. 

Results from climate forecasting models are often mixed at this time of year, due to high variability in boreal (northern) springtime conditions. The dynamical models noted above are split in their predictions of long-term trends toward warm (El Niño) or a continuation of near-normal to cold conditions. The statistical models predict trends toward warm conditions over the coming months, but a statistical blend of all model results indicates near-normal conditions for the year. Summer and fall conditions should allow for more skillful long-term predictions from the models. Given this range of results in the models, and the limits of their skill at longer lead times, the following LOCAL VARIABILITY SUMMARIES are based on an expectation that conditions will be near-normal through 1997. Any changes in trends, and their impact on the outlooks in these LOCAL VARIABILITY SUMMARIES, will be reported in future issues of Pacific ENSO Update.


LOCAL VARIABILITY SUMMARIES:

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: From November through January, automated rain gauges recorded a general pattern of above-normal rainfall in leeward areas and below-normal rainfall in windward areas throughout the state. This pattern was especially strong over the Big Island, where Kona and North Kohala gauges consistently recorded 1-3 times normal rainfall, while North Hilo and Puna district gauges saw as little as 10% of normal. Similar conditions occurred in Maui County, where leeward and mauka gauges on Molokai, Lanai, and Maui recorded 2-4 times normal rainfall, while Hana and some other windward areas received only about half of normal for the period. Leeward/windward areas of Kauai were also generally above/below normal, but conditions were more variable on Oahu. The extremely wet conditions of November affected the entire island, with a gauge in Waianae recording over 12 times its normal rainfall (1218%!) for the month, while windward areas received 2-4 times normal rainfall. By contrast, December rainfall ranged between about 40%-100% of normal throughout the island. January saw a pattern more like that on Maui.

These unusual rainfall conditions occurred with several instances of rapidly moving mid-latitude winter storm systems passing to the north, causing weakened or southerly wind flow over the islands and subsequent rainfall in normally "leeward" areas. Associated with this pattern of rapidly moving storms was a continued weakening/displacement of normal pressure patterns (e.g. the North Pacific High), preventing normal delivery of trade winds and associated rainfall to windward island areas. The more southern location of the Big Island (and to a lesser extent, Maui) relative to the other islands means somewhat lesser exposure to northern winter storm activity, and the virtual lack of trade winds has allowed for extended drought conditions in windward regions, especially on the Big Island.

See page 10 for the latest issue of the Long-Lead Outlook for the Hawaiian Islands from NOAA-CPC.
- sources: NWS-PR and PEAC


AMERICAN SAMOA: Rainy season rainfall at Pago Pago from October through January was 55% above average, due to an exceptionally wet October and January. However, rainfall for all of 1996 was very close to normal, at just 1% above the long-term average. For 1997, rainfall in American Samoa and immediately surrounding regions is expected to be about 10% below normal levels, since the annual rainfall in this region is about 20-25% heavier during ENSO warm events. Some climate models are indicating the possible return of warm equatorial SSTs and ENSO warm event conditions by October of 1997. However, the skill level of the models is relatively low at this time of year, so expectation of a new ENSO event may be pre-mature. Weakening of the monsoon over Australia during the southern hemisphere rainy season could lead to temporary episodes of heavy rainfall in the region around Samoa, similar to the wet conditions which occurred over recent months.

The locations of the development and movement of tropical cyclones in both the northern and southern hemispheres of the western Pacific are significantly influenced by the meteorological conditions that evolve during ENSO warm events. With continued cool conditions and no development of a warm event, tropical cyclone activity for the 1997-98 season (generally beginning in November) should be limited to more normal areas to the west, e.g. Vanuatu and the Solomon Islands. Subsequent movement of cyclones should also be primarily west of the date line, toward areas like Australia's east coast, New Caledonia, Vanuatu, and possibly Fiji. Tropical cyclone activity east of the date line is expected to be below-average.
- source: UOG-WERI


GUAM/CNMI: A weak Australian monsoon in the first part of the winter in 1996 led to much wetter-than-normal conditions in January-early February . A similar situation is also occurring during the winter of 1997. On Guam, above-normal rainfall has been occurring since November. The three-month total for November-January was about 50% above normal.

Some climate models are predicting drier-than-average conditions for the next few months in Guam and the CNMI. The outlooks (table, below) reflect this prediction for March through June. However, the wetter-than-normal January rainfall will likely keep total dry-season rainfall on Guam near 100% of average. Wetter-than-average conditions are expected for July-November, then slightly drier conditions thereafter. These outlooks could change if a significant ENSO warm event develops before the end of the year, but near-normal conditions are expected for now. Any changes to these rainfall outlooks will be reported in future issues of Pacific ENSO Update.

MONTHS:              % OF LONG-TERM AVERAGE:
                       - CNMI -    - GUAM -
 Feb 97 - Jun 97          95         100
 Jul 97 - Nov 97         115         115
 Dec 97 - Feb 98          90          95
The typhoon season is expected to return to normal in terms of when and where tropical storms and typhoons normally develop. During 1996, tropical cyclone activity in the western North Pacific was at near record high activity, but much of this activity was west and north of Micronesia. Weak La Niña conditions in 1996 brought stronger-than-normal low level easterly winds into the western North Pacific, shifting tropical cyclone development to the west and north. With near-normal winds and SSTs in 1997, more tropical cyclone development can be expected to occur east and south of the Marianas, increasing the threat of storms that may pass near or over the islands. Typhoon threats to Guam and the CNMI can occur in April and May, but greatest levels of threat are from mid-September to mid-December.
- source: UOG-WERI


FEDERATED STATES OF MICRONESIA (FSM): Although some climate models are indicating the possible return of warm equatorial SSTs and ENSO warm event conditions by late 1997, the skill level of the models is relatively low at this time of year, so an expectation for a 1997-98 warm event may be pre-mature. Conditions at present are near-normal and are expected to remain so for coming months. Rainfall for 1997 is expected to be near or slightly above average over western FSM states and slightly below average over eastern states. Tropical cyclone activity for the FSM is expected to be near-normal, with greatest threats primarily to western FSM states from October to mid-December. Individual summaries for FSM states appear below:

Chuuk State: Rainfall from October 1996 through January 1997 was 5% above the average for Weno Island, Polowat, and Lukunoch. Rainfall over the next 12 months for all of Chuuk State islands is expected to be 5-10% higher than average. Some rainfall models are predicting significantly drier conditions for late 1997 (which would be consistent with an ENSO warm event), but near-normal climate conditions should allow for above-average overall rainfall, with high month-to-month variability.

Topical cyclone activity during 1997 is expected to be near-normal, with threats to Chuuk State islands being greatest during October-December. Some developing tropical depressions, tropical storms or a minimal typhoon could affect the area during April, May, and/or June as well. The region around Chuuk is a favored area for tropical cyclone development.

Kosrae State: Kosrae was one of the wettest locations in Micronesia during 1996, with 232.24 inches recorded for the year. That was 17% above the annual average, and resulted primarily from a very wet January-February and April-May. Rainfall for 1997 is expected to be slightly wetter than average (+5%) through April and slightly drier than average (-10%) through the remainder of the year. Fall and winter rainfall should be slightly below average, unless ENSO conditions reappear. The November 1997-March 1998 rainfall could be wet as with the past two winters if the monsoon is weak over and east of Australia.

Topical cyclone activity during 1997 is expected to be near-normal. For Kosrae this means occasional episodes of strong westerly winds and heavy rains that can last up to a week, as developing cyclones pass by to the north of the island during the months of October through December. In general, cyclone threats to Kosrae will be low compared to what can occur during ENSO warm event years.

Pohnpei State: 1996 rainfall at Kolonia was slightly drier than average, but the higher elevations of Pohnpei and the other state islands appeared to have been slightly wetter than average. Higher elevations on Pohnpei are estimated to have received over 250 inches of rain. For Pohnpei Island and the surrounding state islands from Pingalap to Nukuoro, above-normal rainfall is expected to continue through April (about 5% above average), then to drop to levels about 10% below average through September. With continued near-normal climate conditions expected, rainfall should be about average from October 1997 to March 1998, and overall annual rainfall should be about normal.

Topical cyclone activity during 1997 is expected to be near-normal. Impacts on Pohnpei should be similar to Kosrae, meaning occasional episodes of strong westerly winds and heavy rains that can last up to a week as developing cyclones pass by to the north of the island, during the months of October through December. In general, cyclone threats to the region should be low compared to what can occur during ENSO warm event years. Only developing cyclones should significantly affect the area, compared to potentially more damaging mature cyclones.

Yap State: Rainfall on Yap and northern Yap State islands (Ulithi, Fais, etc.) was significantly greater than previously expected--up to 30% more rainfall than average for 1996. This was primarily due to heavy rains from numerous tropical disturbances in January 96, and from Typhoon Fern in December 96. Tropical Storm Hannah also left January 1997 wetter than normal. Southern Yap State islands were slightly drier than average during spring and fall, but were slightly wetter than average for the entire year. For 1997, rainfall is expected to be near-average across the State until April, and thereafter about 20% wetter-than-average for northern islands and about 10% wetter-than-average for southern islands for the remainder of the year. January-March 1998 rainfall should be near-normal. High month-to-month variability in rainfall can also be expected.

Tropical cyclone activity for Yap State is expected to return to near-normal. This means that Yap, Ulithi, and Fais can expect their greatest threats from September through December, with lesser threats in May and June. Islands to the south can expect their greatest threats from October through December, with lesser threats during April and May.
- source: UOG-WERI


MARSHALL ISLANDS (RMI): ENSO warm events and La Niña cold events have profound effects on rainfall and tropical cyclone activity in the RMI. Some climate models are currently predicting a trend toward warm equatorial sea surface temperatures (SSTs) in the central and eastern Pacific (possibly leading to ENSO warm event conditions) between now and late 1997. MARSHALL ISLANDS (continued): However, the skill level of model predictions at this time of year is relatively low, so the expectation of a 1997/98 ENSO warm event may be pre-mature. A small amount (less than 1°C) of warming of SSTs in the region of the date line, combined with the eastward extension of the monsoon trough, could bring above-normal rain to the RMI in late 1997 (October, November, and December). Conditions for coming months (from February through August) should be near-normal, which is relatively dry for the RMI, especially the northern islands.

Without an ENSO warm event, typhoon threats will be low for the RMI, especially for eastern parts of the island nation. The greatest tropical cyclone threats for the RMI are from September through November for the northern islands, from October through December for the southern islands, and from August through October for far northern atolls like Enewetak , Bikini, and Wake Island.
- source: UOG-WERI


PALAU: The 1996 rainfall for Koror, at 147.36 inches, was within 1 inch of the long-term annual average of 147.97 inches. While this is very "near-normal" rainfall, there was high month-to-month variability. January 1996 was very wet due to the passage of several tropical disturbances. April had over 18 inches of rain and May had over 16 inches, due to the respective passages of Tropical Storm Ann and Typhoon Bart. Southwesterly monsoon winds and rains from Super Typhoons Sally and Violet brought nearly 16 inches of rain in September. For 1997, rainfall totals are expected to be about 5% above the annual average for 1997 and for early 1998.

Tropical storm and typhoon activity for 1997 should be about normal, with some threats to Palau during March -April, and more serious threats expected during October-December. Typhoons can also activate southwest monsoon winds and rains from July to October, as the tropical storms and typhoons pass to the north of the islands.
- source: UOG-WERI



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)735-2695 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 #331, 2525 Correa Road, Honolulu, Hawaii 96822 Contact Dr. T. Schroeder at 808-956-7476 for more information on hurricanes and climate in Hawaii.

PACIFIC EL NINO-SOUTHERN OSCILLATION (ENSO) APPLICATIONS CENTER: HIG #331, 2525 Correa Road, Honolulu, Hawaii 96822 Contact C. Yu at 808-956-7110 for more information on ENSO-related climate data for the Pacific Islands. Contact A. Hilton at 808-956-2324 for more information on Pacific ENSO Update and applications.

APPENDICES:

Long-Lead Outlook for the Hawaiian Islands issue dated February 13, 1997 from NOAA's Climate Prediction Center (CPC).

SPECIAL SECTION: Experimental Forecasts for Pacific Island Rainfall

SPECIAL SECTION: Climate Data from the TOGA-TAO Buoy Array

ENSO Advisory of January 13, 1997


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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