As discussed in the last issue of Pacific ENSO Update, the occurrence of an El Niño-Southern Oscillation (ENSO) "warm event" is defined by a departure from "normal" sea surface temperatures and atmospheric surface pressure patterns in the tropical Pacific. In this region, increased sea surface temperatures and a reversal of normal atmospheric surface pressure patterns are conditions typical of an ENSO warm event. Throughout most of 1995, the Southern Oscillation Index (SOI - a measure of surface atmospheric pressure patterns in the Pacific) has been rising. This trend toward more positive values of the SOI is typical of a shift toward more normal conditions following an ENSO event, and is expected to continue. Also, sea surface temperatures (SSTs) in the eastern and central equatorial regions and near the date line have rapidly cooled to near-normal levels. Taken together, these trends toward near-normal conditions seem to indicate an apparent end to the multi-year ENSO warm event, which has been occurring in the Pacific since 1991. Historically, this long-lived period of ENSO warm conditions closely matches those of the years 1911-15 and 1939-42. At present, both the SOI and SSTs are expected to hover near normal values for the next 9-12 months. It should be noted, however, that the high variablity of springtime conditions have been known to reduce the skill levels of long-term predictions, including those involving ENSO, made at these times of year in the past.
The contents of the latest issue (dated 4/15/95) of the "ENSO Advisory," produced by the NOAA Climate Analysis Center (CAC) in Washington D.C., are shown in Appendix II. As discussed in the last issue of Pacific ENSO Update, this publication is produced monthly by CAC when ENSO warm conditions are occurring in the Pacific. Because of the recent return to near-normal conditions, this latest issue of the ENSO advisory will be the last in the series (see text in Appendix II). A new series of ENSO Advisories will be produced when ENSO warm conditions are expected to occur again in the Pacific. However, Pacific ENSO Update will continue to be issued quarterly, to summarize the ongoing climate conditions in the Pacific region and inform on the impacts of "normal," as well as "warm" and "cold," phases of the ENSO climate cycle.
While ENSO warm events are typically defined by increased sea surface temperatures (SSTs), and a weakening or reversal of normal atmospheric surface pressure patterns (that is, negative values of the SOI), the opposite conditions sometimes occur, resulting in the other extreme of the ENSO climate cycle, often called a "cold event," or sometimes, "La Niña." These kind of events are typically defined by cooler-than-normal SSTs , and strong positive values of the SOI, and sometimes follow the occurrence of ENSO warm events. La Niña events most recently occurred in years 1988-89 and 1974-75. While both La Niña and ENSO can cause drought conditions in Micronesia, with La Niña droughts being especially severe, other impacts are different, such as the shifting of typhoon activity in the western tropical Pacific to the north and west with La Niña, compared to ENSO-related shifts to the south and east. If the present trends following this latest ENSO warm event were to be extended, toward even cooler SSTs and a higher positive value of SOI, a La Niña event could develop. However - most of the climate prediction models, which are monitored by the Pacific ENSO Applications Center, do not indicate this trend. Therefore, a La Niña event is not presently expected to develop; instead, closer-to-normal climate conditions are anticipated for the next 9 to 12 months.
As noted in the last issue of Pacific ENSO Update, the following summaries of expected local climate variability for the island areas indicated are not based on official forecasts (unless otherwise noted), but rather draw on scientific knowledge of past associations between local climate variabilities following ENSO warm events, the advice of research staff of the Pacific ENSO Applications Center, various experimental forecasts, and other sources. Note that credits for individual contributions are included in this issue of Pacific ENSO Update.
HAWAII: The effects of the recent ENSO warm event may be slow to dissipate in the coming months as Hawaii enters its dry season. Temperatures may tend to be slightly above normal, and the most recent Long-Lead Outlook for the Hawaiian Islands, issued by the NOAA Climate Prediction Center (shown in Appendix I) generally reflects this expectation for the coming months. Users are cautioned to make note of the possible longer-term impact of abnormal conditions experienced due to ENSO, as even with a possible return of a "normal" dry season rainfall this summer, the problems associated with the ENSO-related dryness experienced during much of the last winter would likely be exacerbated.
With regard to the approaching hurricane season, recent research has shown that longer-than-normal track storms have been more likely to pass through the central Pacific near the Hawaiian islands in post-ENSO years. This may result in a slightly elevated risk of hurricane-related impacts in the coming season.
- sources: NOAA-CPC and U.H. Dep't of Meteorology
AMERICAN SAMOA: With the recent disappearance of the long-lived period of ENSO warm conditions, the higher-than-normal chance of tropical cyclone and storm activity in the region, and the occurrence of wetter-than-normal conditions, have similarly abated. Climate conditions in American Samoa and neighboring islands are expected to return to normal in the coming months. Tropical cyclone and storm activity in the1995-1996 season should shift westward, toward more normal longitudes between Vanuatu and Australia. Sea surface temperatures along the equator north of Samoa are presently near their normal values (see graph along the 170W longitude line, Figure 1), and are expected to remain so in the coming months.
- source: UOG-WERI
GUAM/CNMI: Consistent with ENSO-related patterns, rainfall from November 1994 through March 1995 was about two-thirds of normal on Guam and in the CNMI. This dryness on Guam has contributed to a steep decline in the water level of the Fena Reservoir, deterioration of fairways at golf courses, and to an increase in the number of wildfires. In Saipan, where much of the public water supply is obtained from wells, the impact of the recent dryness has further aggravated the ongoing problems associated with saltwater intrusion into the major well-field on the south side of the island.
Slightly-below-normal rainfall is expected to persist for the region into June, and perhaps into early-to-mid-July. However, by mid-July, rainfall is expected to return to normal. Note that this is a modification of the outlook for extended dry conditions reported in the last issue of Pacific ENSO Update, due to the fact that a "La Niña" event is no longer expected to occur.
At the request of water resource managers on Guam, for use in their management models, the following expectations of monthly rainfall totals are provided:
MONTH(S): RAINFALL: May, 1995 - 66% of normal June-July, 1995 - 75% of normal Aug-Dec, 1995 - 100% of normal Extended outlook: Jan-June, 1996 - 100% of normalThe outlook for the coming typhoon season is for normal conditions, with respect to when and where they occur. As per the normal pattern, typhoon activity will begin in the Philippine Sea in June, and slowly move eastward as the year progresses. Typhoon threats to Guam and CNMI will be greatest from late September until mid-December. The low levels of Guam's Fena reservoir, due to the dry conditions of late 1994 and early 1995, could be eliminated by a single tropical storm or typhoon event.
MICRONESIA: The local variability summary for the FSM is split into two regions south and north of 9 degrees N latitude (see map).
Region 1 (south of 9 degrees N): From satellite images, the cloud patterns associated with a persistent low-pressure zone called the "near equatorial trough" appears to have returned to normal during April. As a result, near-normal rainfall is expected in this region for the remainder of 1995, with wet conditions into early summer, dry conditions during summer, and a resumption of rainfall in late fall. Rainfall totals for spring may be slightly below normal, due to the late onset of rains which occurred this season. A small risk of typhoon activity exists for Chuuk and Yap States during May and June, and normally higher activity can be expected for all FSM states during the months of October, November, and possibly December. However, with the disappearance of ENSO warm conditions, typhoon threats in Kosrae State are expected to be greatly reduced. The extended outlook for January - June 1996 rainfall is a continuation of normal conditions.
Region 2 (north of 9 degrees N): The outlook for this region has been modified from that given in the last issue of Pacific ENSO Update. The severity and duration of dry conditions are now expected to be reduced. The islands and atolls in the northern part of the FSM may experience drier-than-normal conditions (about 75% of normal rainfall) until mid-to-late June. After this time, conditions are expected to return to normal. Typhoon activity may occur in the northern parts of Chuuk and Yap States during May and June, and from mid-September to Mid-December. The extended outlook for January -June 1996 rainfall is a continuation of normal conditions.
- source: UOG-WERI
PALAU: The outlook for this region has been modified from that given in the last issue of Pacific ENSO Update. Typhoon activity in the summer is no longer expected to be pushed significantly to the north and west. Therefore, normal episodes of heavy summer rainfall from occasional west and southwest monsoonal winds can be expected. Rainfall and storm threat conditions are now expected to return to normal by late May for Palau. While there is some possibility of tropical storm or typhoon activity in June, more serious threats will resume in October, November, and/or the first half of December. The extended outlook for January to June 1996 is for normal rainfall and storm threat conditions.
- source: UOG-WERI
MARSHALL ISLANDS: Winter and spring rainfall totals have been below normal in the southern RMI, while warmer-than-normal sea surface temperatures (SSTs) and their associated convective (thunderstorm-type) rainfall have been occurring farther east near the date line. Now that the ENSO warm conditions are ending and SSTs are cooling in this region, rainfall patterns in the RMI should return to normal. Higher levels of rainfall can occur due to the influence of single events. For example, the occurrence of tropical storm "Chuck" is responsible for a large amount of the total rainfall recorded for region in April, 1995.
Since the ENSO warm conditions now appear to have ended, typhoon threats in the RMI should be much lower in the coming season, especially for the eastern islands. The western islands may see some westerly winds and tropical storm activity in October and November, but by mid-December and January, trade winds should strengthen and prevent late-season typhoon development. Enewetok and Wake Island may experience some typhoon threats in September and October. The extended outlook for January through June 1996 is for near-normal rainfall conditions.
- source: UOG-WERI
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 the individuals and institutions listed below:
NOAA National Weather Service CLIMATE ANALYSIS CENTER (CAC)
Diagnostics Branch, and CLIMATE PREDICTION CENTER (CPC):
World Weather Building, Washington D.C. 20233.
Contact CAC at 301-763-8227 for more information on the ENSO Advisory.
Contact CPC at 301-763-8167 for more information on the Long-Lead Outlook for the Hawaiian Islands.
NOAA National Weather Service PACIFIC REGION HEADQUARTERS (PRH):
Prince Kuhio Federal Building, Suite #4110
Box 50027, Honolulu, Hawaii 96850
Contact J. Partain at 808-541-1671 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)734-3132 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, Chairman, 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.
Site visits are an important component of the Pacific ENSO Applications Center project organization (see diagram on back cover of this issue). These site visits are designed to promote better awareness about the ENSO climate cycle, its impacts, and the potential usefulness of climate forecasts based on ENSO for the benefit of local government agencies and other organizations involved with climate-sensitive resource management issues in the various island areas.
The Pacific ENSO Applications Center conducted its first site visit to American Samoa in October, 1994. More recently, site visits to Guam and Saipan were completed in April, 1995. Introductory meetings with representatives of several agencies and organizations were held, which focused on:
Appendix I: CPC Long Lead Outlook for the Hawaiian Islands
Appendix II: CAC ENSO Advisory, dated 4/13/95
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
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.