The cooler-than-normal sea surface temperatures (SSTs) in the equatorial Pacific have warmed in recent months to near-normal levels throughout most of the region. Equatorial SSTs over a large area, from 160°E longitude (near Kosrae) to 90°W longitude (near the Galapagos) were almost exactly at their long-term average values for July, having warmed slightly from June values. Eastern Pacific SSTs near the South American coast have warmed somewhat since April, but remain slightly (about 1°C) below their long-term average for May-July. In the atmosphere, surface pressure patterns (indicated by the Southern Oscillation Index or SOI, which measures the relationship in atmospheric pressure between Tahiti and Darwin, Australia) have remained somewhat elevated, with positive values of SOI in June (+1.0) and July (+0.7). This has been primarily due to above-normal pressures at Tahiti. Stronger-than-normal easterlies and trade winds have accompanied the elevated SOI, although the present trend is toward near-normal wind patterns throughout the region.
A consensus of recent results from the various computer model predictions for ENSO suggests that near-normal, to slightly-cool, SST conditions will continue for the coming months. As a result, near-normal climate conditions are expected for the region. Rainfall in the region has been shifting toward a pattern with wetter-than-average conditions for island areas to the west of 160°E, and drier-than-average conditions for those to the east (except at Hawaii, which has had wetter conditions due in part to its more northern location). This pattern is consistent with more normal conditions, and is essentially reversed during ENSO warm events. More details on ENSO trends, computer predictions, and local climate conditions are described in the following pages of this issue...
NOAA's Climate Prediction Center in Washington D.C. produces the Climate Diagnostics Bulletin each month, with technical information on many features of global climate, including latest results from several computer models that predict ENSO conditions. The July 1996 issue shows model results that, taken together, give an outlook for near-normal conditions, or possibly slightly-cool conditions, for now through early 1997. This information, along with current SOI and SST conditions, suggests that no "El Nño" or ENSO warm event is likely to occur this year.
Last year at this time, SSTs were cooling and the SOI was rising, consistent with the development of "La Nña" or ENSO cold-event conditions. This combination allowed for stronger-than-normal easterly surface wind flow to develop and persist over the near-equatorial Pacific region. Normal formation of tropical storms and cyclones in much of the region was suppressed, with most storm activity displaced far to the west (e.g. in the Philippine Sea), and a below-normal tropical cyclone season overall.
Although the SOI remains elevated at present, and stronger-than-normal easterly surface winds have been occurring in the region, SSTs are now very close to normal, and high-altitude winds are also near-normal. A trend toward SOI values near zero and a more normal surface wind pattern is expected, and conditions for tropical cyclone and storm formation in the region will return to normal under these conditions. Activity for the present season may therefore increase over the below-normal levels seen last year. However, this activity should have a near-normal distribution, meaning that eastern island regions like the Marshall Islands and the eastern FSM (and Samoa, for its coming storm season) should be at their normally low levels of risk from tropical cyclones, instead of the increased threats for those areas that can occur with ENSO warm events.
Monsoon activity and associated rainfall patterns are also expected to return to normal. Satellite measurements of Outgoing Longwave Radiation (OLR) observed a northward shift in July of cloudiness and associated rainfall with the monsoon trough, moving over the island regions of CNMI, Guam, and Yap. Cloud cover and rainfall over southern and eastern island groups (Palau, Chuuk, Pohnpei, Kosrae and RMI) are reduced, though periods of heavy rain and bursts of westerly winds may be experienced in those areas with the monsoon during October and November. The overall pattern however will be for dry conditions east of 160°E and wet conditions west of 160°E. This is an essentially normal pattern, which is reversed during ENSO warm event years.
Given the present conditions and the trend in computer model predictions, the following LOCAL VARIABILITY SUMMARIES are based on an expectation that conditions will be near-normal until mid-1997. Any changes in ENSO trends and their impact on these outlooks will be reported in future issues of Pacific ENSO Update.
As noted in each issue of Pacific ENSO Update, the following summaries of expected local climate variability for the various island areas indicated are not based on official forecasts (unless otherwise stated). Further information is available from your local National Weather Service office, or from the sources listed in the acknowledgements section.
HAWAII: Overall dry season rainfall recorded at the primary National Weather Service stations in Hawaii has been close to normal. Although rainfall was below-normal early in the season, with April and May amounts being well below the long-term averages for Lihue, Honolulu, Kahului, and Hilo, rainfall in June was above-normal at all locations except Kahului, and rainfall in July was also above-normal for all locations. At present, rainfall for the four-month period of April-July ranges from lowest at 82% of the long-term average for Kahului, to highest at 121% of the long-term average for Lihue. Mauka stations on Oahu have also recorded above-average rainfall for the season.
Forecasts for rainfall in the coming months are available from the Long-Lead Outlook for the Hawaiian Islands, produced monthly by NOAA's Climate Prediction Center (CPC) in Washington D.C. The most recent issue is shown on page 7, indicating normal climatological probabilities and some small chances of above-normal rainfall in coming seasons. No development of ENSO conditions, which might otherwise affect rainfall in Hawaii, are apparent.
As with 1995, the 1996 hurricane season to date has seen below-normal activity in the central tropical Pacific. Near-normal activity for tropical storms and hurricanes is expected for the remainder of the season, particularly since no ENSO warm event is expected for 1996-97.
- sources: NWS-PR and PEAC
AMERICAN SAMOA: An El Nño warm event is not likely for the 1996-1997 season. A consensus of the various statistical and dynamic climate models support this prediction, and even suggest a possibility of slightly cooler equatorial SSTs. Since El Nño events produce increased rainfall (20-25% higher) in the Samoa region, annual rainfall for 1996 is expected to be about 10-15% below the long term average. Rainfall at Pago Pago for May, June and July of 1996 averaged at 6.99 inches per month, or about 89% of the long-term average. There may be high month-to-month variability in the rainfall, partly in response to short-term fluctuations in the SOI. For example, May rainfall at Pago Pago was 10% above average and July rainfall was 45% below average.
Without the occurrence of an ENSO warm event, tropical cyclone development in the Southern Hemisphere (beginning October 1996) will likely be limited to areas west of the international date line, occurring primarily in the belt from the Gulf of Carpentaria (NE Australia) to the Solomon Islands, and tracking between eastern Australia and the date line. With the return to near-normal SST and SOI conditions, cyclone activity is expected to increase over the low values seen in 1995-1996. However, without an ENSO warm event, tropical cyclone activity for American Samoa and areas east of the international date line should be fairly low.
- source: UOG-WERI
GUAM/CNMI: When cold "La Nña" events follow ENSO warm events, extended dry conditions can occur in Guam and the CNMI. Because of this, non-cold event years are expected to be wetter than the long-term average. A consensus of the various statistical and dynamic climate model predictions suggests that an El Nño warm event is not likely for the 1996-1997 season, and even suggest a possibility of slightly cooler equatorial SSTs. Since no ENSO warm event is expected this year, rainfall in Guam and the CNMI is likely to be somewhat above the long-term average for at least the next 12 months. The drier-than-average conditions associated with the relatively weak 1995-1996 La Nña event have ended, and wetter-than-average rainfall conditions are now expected for the region. Although May-June rainfall values at Guam International Airport (drier station on Guam) and Andersen Air Force Base (wetter station on Guam) were only 64% and 45% of average for the two-month period (respectively), July rainfall rebounded to values of 39% and 76% above the respective averages for the month. On Saipan, similar rainfall occurred at the International Airport (drier location) and at Capitol Hill (wetter location).
Above-average rainfall is expected to continue for Guam and the CNMI through the 1996 wet season and into the 1997 dry and wet seasons. The PEAC and CPC statistical rainfall models (see page 6) also have some hints of wetter-than-average conditions on Guam in the coming months, and this should hold true for the CNMI as well. The following expectations of monthly rainfall totals are provided for the use of water resource manages on Guam and Saipan:
MONTH(S): RAINFALL: Aug 96 - Jan 97: 110% of long-term average Feb 97 - May 97: 120% of long-term average Jun 97 - Sep 97: 110% of long-term average
Tropical cyclone activity during 1996 is expected to return to near normal. This means that development and intensification will occur farther east than in 1995, and suggests an increased threat of tropical cyclone conditions for the Mariana Islands. Northern islands in the CNMI have already experienced some tropical cyclone passages earlier this year. By mid-September, storm development will spread south and east of the Marianas, with greatest threats to the area occurring from October through early December.
- source: UOG-WERI
FEDERATED STATES OF MICRONESIA (FSM): The current SST and SOI conditions and their recent trends suggest that no ENSO warm event will occur in 1996-1997. A consensus of the predictions from statistical and dynamic climate models support a continuation of near-normal to slightly cooler SSTs. Since El Nño warm event conditions are not expected to develop, rainfall for 1996-1997 should be near the long-term average over the eastern FSM states, and somewhat above the long-term average over the western states. Tropical cyclone activity for the FSM is expected to become more normal, with increasing threats to all four states, primarily from October to mid-December, although minimally so for Kosrae. Summaries for the individual states of the FSM appear below:
Chuuk State: Rainfall for Weno Island from January through April was about 35% above the long-term average for those months, but dropped to about 10% below average for May through July. The wet conditions of early 1996 were apparently due to a relatively weak Australian monsoon during the Northern Hemisphere winter. This allowed a trade wind convergence zone to develop and persist over the Marshall Islands to the east, forming tropical disturbances which tracked westward across Chuuk State. Now that La Nña conditions have weakened and no El Nño event is expected to develop, slightly greater-than-average rainfall is expected for the islands of Chuuk State, perhaps 10% to 15% above the average for the next 12 months. However, total rainfall for 1996 should be well above average, especially due to the wet conditions that occurred early in the year.
During late 1995 and early 1996, La Nña conditions displaced tropical storm development and intensification far to the north and west, near 20°N - 130°E. This spared Chuuk from significant tropical cyclone threats. However, tropical cyclone activity for the remainder of 1996 is expected to return to normal. This means that Chuuk State islands north of 8°N may experience tropical cyclone threats from mid-September to late November, and islands south of 8°N from mid-October to mid-December. Threats from tropical cyclone activity may also occur from March to May of 1997.
Kosrae State: Kosrae has been the wettest location in Micronesia during 1996. The wet winter and spring of 1996 was apparently due to a relatively weak Australian monsoon, which allowed for the development and persistence of a much stronger-than-normal trade wind convergence zone east of Kosrae, near the date line. Tropical disturbances developed there and tracked westward, through the region around Kosrae. The convergence zone then shifted west, virtually over Kosrae, and rainfall for April, May, and June averaged over 31 inches per month, nearly twice the long-term average. July rainfall fell to about 80% of the average. Since El Nño conditions result in greater-than-average rainfall in Kosrae, and no El Nño event is expected for 1996-1997, overall rainfall from August 1996 to September 1997 should be slightly below the long-term average. However, annual rainfall for 1996 will likely be above-average, due to the extremely wet winter and spring. Although less rainfall is expected, month-to-month variability can be high in Kosrae.
Tropical cyclone distribution is expected to return to normal. Since no El Nño warm event is expected for 1996-1997, tropical cyclone threats to Kosrae will be few and will be limited to the period from late October to mid-December. Tropical cyclones can be generally expected to pass north of Kosrae, with greatest local effects coming from winds on the weak side of storm systems as they pass.
Pohnpei State: April and May rainfall at Kolonia, Pohnpei averaged over 26 inches per month, nearly 50% above the average for those months. June and July conditions were considerably drier, with rainfall at a little over 12 inches for those months--about 30% below the average. These dry conditions were due to the earlier than normal northward shift of the low-level monsoon trough and its associated cloudiness and rainfall. As usual, rainfall amounts at higher elevations in the interior of the island were greater than at Kolonia. El Nño conditions result in strong equatorial westerly winds and greater-than-average rainfall in Pohnpei, especially from October to the following May. Since no ENSO warm event is expected to develop in 1996-1997, drier-than-average rainfall is expected for the remainder of 1996 through September of 1997. Due to the abnormally wet winter and spring of 1996, total rainfall for the year will likely be near the long-term average.
Tropical cyclone threats to Pohnpei are greatest from late October to mid-January during El Nño events. Since no El Nño warm event is expected for 1996-1997, tropical cyclone threats to Pohnpei State will generally be limited to the period from late October to mid-December. These tropical cyclones can be generally expected to pass north of Pohnpei, with greatest local effects coming from winds on the weak side of storm systems as they pass.
Yap State: Rainfall over northern Yap State has been significantly wetter-than-average during 1996--over 75% above the long-term average. This was due to a very wet winter and spring, apparently because of the weaker-than-normal Southern Hemisphere monsoon, which allowed the normally dry northeasterly winds typical of the season to give way to more tropical east-southeasterly winds, bringing tropical disturbances and heavy rainfall over the region. Rainfall over the southern islands was about 20% above the average for the same period. July rainfall at the Yap airport was 38% above normal. Below-normal rainfall for Yap State primarily occurs when a La Nña cold event follows an ENSO warm event. This sequence is not expected to begin for at least the next 12 months, and probably later. As a result, rainfall in northern Yap State is expected to be about 20% above the average from now through September 1997. Southern Yap State rainfall should be 10% to 15% above average. Much of this additional rainfall will be the result of more active southwesterly winds surging across the region in association with the monsoon, a feature that normally occurs from July through mid-September. Annual rainfall for 1996 at Yap will likely be well above the long-term average because of the wet conditions in the first half of the year.
Yap is the most vulnerable of the FSM states to tropical cyclones. While tropical cyclone activity in 1995 was mainly west of Yap, storm activity in 1996 is expected to shift back to the east and may pose greater threats to Yap State. Typhoons developing farther to the east have greater chance to strengthen as they traverse the region. Threats to northern Yap State will be greatest from mid-September to late November, and for southern Yap State from mid-October to mid-December.
- source: UOG-WERI
MARSHALL ISLANDS (RMI): Both the northern and southern RMI were significantly wetter-than-average during January and February of this year. Kwajalein averaged over 10.5 inches per month and Majuro over 15 inches per month for the two-month period. This was nearly three times the norm at Kwajalein and twice the norm at Majuro. The wet winter and spring was due, in part, to a weak Southern Hemisphere Australian monsoon that allowed for development of a strong trade wind convergence zone east of and over the southern RMI. This spawned several tropical disturbances that tracked near and over the southern RMI atolls. Since March, rainfall at Kwajalein and Ebeye has been about 30% below average. Rainfall at Wake Island to the northwest has also been mostly below-normal. Dry conditions appeared in July for Majuro, with rainfall at 43% below the average. Conditions are expected to remain relatively dry in the northern RMI until September and in the southern RMI until October, when the monsoon trough moves into the region. Wettest conditions normally occur in the RMI with ENSO warm events, when SSTs are warm near the date line. Since no warm event is expected, rainfall may be below-average in 1997.
Tropical cyclone threats increase dramatically for the RMI during ENSO warm events. The warmer SSTs extend the zone of formation for tropical cyclones to the east, allowing more cyclones to develop and intensify before tracking through the RMI. Since no ENSO warm event is expected this year, tropical cyclone risk should be minimal, at least until September of 1997. Enewetak, Bikini, and Wake Island, however, can be affected by strong typhoons during the fall of any year. Weaker tropical storm activity may affect the northern RMI from early October to mid-November, and the southern RMI from late October to early December.
- source: UOG-WERI
PALAU: To date, 1996 rainfall has been about 30% above average at Koror. As with most of Micronesia, the wet winter and spring was due to a weak Southern Hemisphere monsoon that allowed development of trade wind convergence zone to the east, producing several tropical disturbances that tracked westward over Palau. Rainfall in June and July was less--about 25% below normal--primarily due to an early northward shift of the monsoon trough and its associated cloudiness and rainfall away from Palau. Extended dry conditions in Palau primarily occur when a La Nña cold event follows an ENSO warm event. This sequence is not expected to begin for at least the next 12 months. By mid-September, rainfall at Palau is expected to again be above-average, with more active southwesterly winds surging across the region with the monsoon.
Conditions for 1996-1997 tropical cyclone activity are expected to be closer to normal. The 1995-1996 La Nña conditions shifted tropical cyclone activity to the north and west of Palau. Under normal conditions, the greatest tropical cyclone risks for Palau are from mid-October to mid-December. Since no ENSO warm event is expected during 1996-1997, late-season threats in late December or early January are not very likely, but can occasionally occur.
- 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 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 #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.
Long-Lead Outlook for the Hawaiian Islands issue dated August 15, 1996 from NOAA's Climate Prediction Center (CPC).
SPECIAL SECTION: Experimental Forecasts for Pacific Island Rainfall
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.