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A major warming of the ocean waters across the eastern and central tropical Pacific Ocean, known as the El Niño/ Southern Oscillation (ENSO), has developed since March 1997. The El Niño developed very rapidly during April-May, and reached strong intensity by June. This event is currently comparable in magnitude and extent to the 1982/83 episode, which was the strongest ENSO of the century. The official NOAA forecast is for a continuation of strong warm episode conditions through the remainder of the year, and a likely persistence of these conditions through February-April 1998. Based on past warm episodes and on current long-range forecasts, the El Niño is expected to weaken during May-July 1998. Forecasts beyond this time have little skill and should be deemed unreliable.
Features accompanying the current El Niño include: abnormal patterns of rainfall and cloudiness over most of the global tropics, a nearly complete shut-down of the normal easterly winds across the entire tropical Pacific, and abnormal air pressure patterns throughout the global tropics. Thus far, the primary El Niño impacts have been in the Tropics and subtropics, and across the eastern South Pacific and central South America (Figure 1). There has also been a dramatic decrease in tropical storm and hurricane activity across the subtropical North Atlantic and an expanded area of favorable conditions for tropical cyclone activity over the eastern North Pacific (see Special Climate Summary #97/4 to be issued on 20 November). The major impacts in the United States are not expected until this winter.
A description of the normal pattern of ocean temperatures in the tropical Pacific is presented in section 2a, followed by how these temperatures affect global weather patterns in section 2b. The evolution of a typical El Niño is described in section 2c, and some basic reasons why weather patterns vary from one El Niño to the next are addressed in section 2d. The ocean temperatures accompanying the current El Niño episode are then described in section 3. The primary ENSO-related impacts to date and expectations for the next several months are discussed in section 4. The official long-range temperature and precipitation outlooks for the United States during the next two seasons are presented in section 5, and the long-range El Niño outlook is discussed in section 6. An overall summary is reserved for section 7.
2. El Niño
a) Normal conditions
Throughout the year the ocean waters in the tropical Pacific are normally warmest in the west and coldest in the east, with the largest differences between the two regions observed during September and October (Figure 2a ). During these months temperatures in the eastern Pacific reach their annual minimum, with the coldest waters found along the west coast of South America and extending westward along the equator to just east of the date line. Temperatures in this cold tongue range from 22 - 27°C, generally well below the temperature of 28°C needed to fuel deep tropical convection and heavy rainfall. Temperatures across the central and east-central tropical Pacific normally begin to increase during December and peak in April (Figure 3), while temperatures in the extreme eastern Pacific normally begin to increase during November and peak in March (Figure 3, bottom). In contrast, the ocean waters across the western Pacific and Indonesia remain above 28°C and normally support tropical rainfall throughout the year.
b) Why the tropical Pacific ocean temperatures impact global weather patterns
The variations in tropical Pacific ocean temperatures contribute significantly to the large-scale patterns of tropical rainfall throughout the year, with heavy rainfall typically coinciding with ocean waters greater than 28°C, and very little tropical rainfall coinciding with cooler waters. Thus, tropical rainfall normally extends farthest east [from Indonesia to approximately 160°W] and is most expansive during January-April when ocean waters in the central Pacific are warmest. Tropical rainfall is confined to the western Pacific during September and October, when the ocean waters are below 28°C east of the date line.
The ocean temperatures and distribution of tropical rainfall affect the atmospheric heating across the tropics and subtropics, with the largest heating coinciding with warmest ocean waters and heaviest rainfall. This atmospheric heating helps to determine the overall north-south temperature differences in both hemispheres, which significantly affects the strength and location of the mid-latitude jet stream winds over both the North and South Pacific, primarily during the respective hemisphere's winter season. In these seasons, the jet stream is normally strongest over the western Pacific, and weaker east of the date line. These jet streams are a major factor controlling the weather patterns and storm tracks in the middle latitudes of both North and South America, with the largest impacts also occurring during the respective hemisphere's winter and spring seasons.
c) How does a typical El Niño evolve?
The El Niño phenomenon basically refers to an abnormal warming of ocean temperatures across the eastern and central tropical Pacific which, during a strong episode, extends from the date line to the west coast of South America. These abnormally warm waters are accompanied by large-scale changes in tropical rainfall, wind patterns and air pressure, which ultimately interact to affect the midlatitude jet streams and weather patterns through processes described above.
During an El Niño the ocean waters typically become warmer than normal during March-August, with continued abnormal warming during September-November. In many episodes, the actual ocean waters in the central Pacific become warm enough by mid-November to affect tropical rainfall. The increase in tropical rainfall east of the date line often triggers an evolution to strong or "mature" El Niño conditions by early December. This mature phase of the El Niño typically lasts until April-May, with many episodes decaying by May or June.
d) Why are the mid-latitude weather patterns somewhat variable from one El Niño to the next?
In response to the more uniform pattern of heating in the tropics during an El Niño the wintertime jet streams in each hemisphere tend to be more zonally uniform and extend farther east than normal. However, the timing, location and magnitude of the ocean warming varies from one El Niño to the next, which results in variations in the patterns of tropical rainfall and deep tropical heating. These conditions contribute to variations in the precise location, strength and structure of the mid-latitude jet streams over both the North and South Pacific from one El Niño to the next, and thus to the variability in weather patterns and storm tracks over North and South America.
A second major reason for the variability in weather patterns from one El Niño to the next is simply that El Niño is not the only factor influencing the weather and climate. In particular, the atmosphere exhibits considerable variability on time scales ranging from days to seasons to years, and this variability often reflects nothing more than the normal chaotic behavior of the atmosphere. This description is particularly applicable to areas such as eastern North America, the North Atlantic, Europe, etc., which are heavily influenced by features such as the North Atlantic jet stream.
3. 1997 El Niño conditions
Ocean temperatures across the central and east-central equatorial Pacific have averaged at least 28°C since April 1997 (Figure 3), with the normal cooling of ocean waters typical of June-October notably absent. During September tropical ocean temperatures averaged more than 29°C from 170°E to 130°W, and more than 28°C eastward to the west coast of South America in the area just north of the equator (Figure 2b). These values are 2°-3°C (4°-6°F) above normal in the central and east-central Pacific and more than 4°C (8°F) above normal in the eastern Pacific (Figure 2c). This abnormally warm water spans more than one-quarter of the distance around the globe, and during September covered an area exceeding 24 million square kilometers (9.5 million square miles). These waters have been sufficiently warm to significantly affect tropical rainfall patterns since June (Figure 4). There is no indication that El Niño conditions have begun to weaken.
4. Regional El Niño impacts during June-October
A schematic depiction of the primary El Niño impacts during June-October is shown in Figure 1. The most pronounced impacts have been in the Tropics and subtropics, and across the eastern South Pacific and central South America. Dramatic changes in rainfall are evident over much of the global tropics (see also Figure 4 ), with significantly increased rainfall across the eastern Pacific and well below-normal rainfall throughout Indonesia and the western tropical Pacific. In Indonesia, many areas have experienced rainfall deficits of more than 400-500 mm (16-20 inches) during the past several months, with area-averaged totals of only 60 mm (2.25 inches) compared to the normal of 180-200 mm (7-8 inches) recorded during August and September (Figure 5). This dryness has contributed to large-scale burning by uncontrolled wildfires in the tropical rainforests of Sumatra and Borneo.
Elsewhere, ENSO-related dryness during June-October included virtually all of central America and northern South America, and an area extending eastward across the tropical South Atlantic. Farther east, the southern part of western Africa has experienced substantially below-normal rainfall during July-September. Other ENSO impacts during the period include 1) a dramatic decrease in tropical storm and hurricane activity across the subtropical North Atlantic, 2) an expanded area of favorable conditions for tropical cyclone activity over the eastern North Pacific and an extension of that activity well into October, and 3) an overall weaker than normal monsoonal circulation in the upper atmosphere across the Indian subcontinent, despite generally near-normal rainfall across India this summer.
A summary of regional impacts for specific areas, along with expected temperature and rainfall patterns in the upcoming months is provided below.
Indonesia/ Eastern Australia/ New Guinea: The region has been very dry since June, with large-scale wildfires and drought. Many areas have already experienced rainfall deficits of 16-20 inches in the last several months. Significantly below-normal rainfall is expected across the region into early next year.
Southern part of west Africa: The region has also received abnormally low rainfall since July, with below-normal rainfall expected to continue into early next year. The region also received an extended period of suppressed rainfall during the 1982-83 El Niño.
Tropical storm and hurricane activity: There was a nearly complete shut-down of Atlantic tropical storm and hurricane activity after July, and an expanded area of favorable conditions for hurricane activity over the eastern North Pacific. This is due in part to an ENSO-related pattern of abnormally strong upper-level westerly winds extending from the eastern North Pacific to the southwestern Sahel area of Africa.
Central America: Abnormally dry conditions covered the region during June-October. The major El Niño impact in the region is nearly over as their rainy season has ended.
Northern South America: Abnormally dry conditions have developed across the region, with dryness extending northeastward to western Africa. We expect dryness across Northern Brazil to continue and perhaps intensify into early next year.
Southern South America: Much of central and southern South America has been wetter than normal during June-October, with most of the central portion of the continent also warmer than normal. In several instances, some locations in central Chile received their normal annual rainfall total in a single day. These conditions have resulted from an El Niño-related increase in jet stream winds and storminess across the central and eastern South Pacific, and a pronounced eastward extension of this storminess into the continent.
Ecuador, northwest Peru, southwest Colombia: Typically above-normal rainfall occurs during El Niño from February-May. In light of the current strong El Niño conditions, we expect very heavy rainfall to begin as early as December, and to continue into March-May.
North America typically receives its strongest ENSO impacts during the winter and early spring. However, the current episode has already affected the continent in several ways. The persistence of abnormally warm ocean waters off the west coast has resulted in the appearance of unusual marine species from the Baja Peninsula to the Pacific Northwest. Another impact has been reduced tropical storm and hurricane activity across the eastern seaboard and Gulf Coast of the United States, with only one system (Danny) entering the country this year.
Western Canada and Pacific Northwestern United States: The region has been experiencing abnormally wet conditions due to increased storminess and enhanced onshore flow. However, it is not clear if these conditions are related to ENSO. Typically, this region experiences warmer than normal temperatures during December-March when El Niño conditions are present.
Southern Alaska: This area has been experiencing abnormally warm and dry conditions due to the same southward shift in storminess and westerly winds that has brought increased rainfall to western Canada and northwestern United States. Also, very warm ocean waters are present along the entire southern coast of Alaska. Again, it is not clear if these conditions are related to ENSO. El Nino normally contributes to warmer than normal temperatures across southern Alaska in the winter season.
California: There have been no major El Niño impacts yet other than locally warmer than normal temperatures along the immediate coast. Abnormally wet weather is expected across the state this winter, with potentially excessive snowfall through the Sierra Nevada Mountains and other mountain ranges due to an increase in winter storm activity. These conditions result from an overall southward shift in winter storms from the Pacific Northwest to the approximate latitude of central California during ENSO.
Southeastern United States: This area has begun to experience wetter than normal conditions that may be related to ENSO. It can expect an overall pattern of wetter than normal conditions during October-March, with cooler than normal temperatures across the Gulf Coast States.
Southwestern United States: This region can expect enhanced wintertime rainfall at lower elevations and increased snowfall at higher elevations. The chances of wintertime drought are significantly reduced across the region.
Central Plains and Texas: This area often begins receiving above-normal precipitation in the fall season during El Niño. Thus, the enhanced precipitation observed across the region during October 1997 may be related to El Niño. We expect a continuation of enhanced precipitation across Texas this winter.
Northern tier of United States: This area has not experienced any well-defined ENSO-related impacts yet. It can expect warmer than normal conditions this winter.
Ohio Valley States: This area has not experienced any ENSO-related impacts yet. It can expect drier than normal conditions during January-March.
5. Official long-range temperature and precipitation outlook
The National Weather Service does not predict actual temperature or rainfall departures for long-lead monthly and seasonal outlooks. Instead, the outlooks indicate areas in which there is an increased likelihood of a particular rainfall or temperature departure (Figure 6). We caution that any forecasts or statements of actual rainfall totals for the upcoming winter season should be deemed unreliable and viewed with extreme caution.
The peak El Niño impacts on the United States are expected to be during the winter and early spring seasons. Based on NCEP predictions and on observations of past strong warm episodes we can expect that during the fall and winter seasons the southern United States is likely to be wetter than normal, while the northern High Plains, Montana and the Ohio Valley region are likely to be drier than normal (Figure 6, right). Temperatures are more likely to be warmer than normal across the northern tier of the United States and California during November-April (Figure 6, Left), and cooler than normal across the Southeast and Gulf Coast during February-April.
6. Forecast of tropical Pacific ocean temperatures
The NCEP coupled ocean-atmosphere model has consistently indicated the continuation of strong warm episode conditions for this upcoming winter and spring. Normally, the confidence in these forecasts is relatively high for the first season, and diminishes thereafter. The confidence in any forecast beyond two or three seasons is generally low, and forecasts beyond the July 1998 period are unreliable and should be used with extreme caution.
When interpreting any El Niño forecasts, it is crucial to recognize that the actual ocean temperatures are the single most critical component of the ENSO phenomenon due to their direct impact on the patterns of tropical rainfall. The seasonal forecasts of Pacific ocean temperatures for the next three seasons (Figure 7 ) indicate the continuation of strong warm episode conditions through February-April 1998, with ocean temperatures greater than 28°- 29°C across the entire eastern half of the Pacific (Figure 7, Left). Thereafter, the model indicates a fairly rapid weakening of the warm episode during May-July 1998, with tropical ocean temperatures dropping below 28°C over the eastern Pacific east of 150°W. This evolution is consistent with past strong warm episodes. An official forecast for the tropical Pacific ocean temperatures will be issued by the National Weather Service on November 13, 1997 based on this model and two other forecast techniques.
When interpreting forecasts for ocean temperature departures from normal (Figure 7, right), it is important to keep in mind that the smaller departures predicted for February-April 1998 merely reflect the normal increase in ocean temperatures at that time of the year, and do not indicate a weakening of the El Niño. In fact, the areal extent of ocean waters greater than 28°C is forecast to be larger during that period than during November-January, and these conditions will likely have a larger impact on the global atmospheric circulation than the very large anomalies currently observed.
There is currently a major El Niño episode in place, which is comparable in amplitude and areal extent to the strongest warm episodes of the century. Forecasts of tropical Pacific ocean temperatures from the NCEP coupled ocean-atmosphere model indicate the continuation of strong warm episode conditions through this upcoming winter and spring. Thereafter, the model indicates a fairly rapid weakening of the El Niño during May-July 1998, with tropical ocean temperatures dropping below 28°C over the eastern Pacific east of 150°W. This evolution is consistent with past strong warm episodes.
The primary impacts of the El Niño to date have been in the Tropics, the subtropics, and over the middle latitudes of the South Pacific and South America. The El Niño has also virtually shut-down Atlantic tropical storm and hurricane activity since August. A more detailed comparison of the atmospheric flow between the inactive 1997 hurricane season and the active 1995 season will appear in Special Climate Summary #97/4 to be issued on 20 November.
The major El Niño impacts on the United States are not expected until this winter. Long-range outlooks provided by the National Weather Service (Figure 6) indicate an increased likelihood of 1) milder than normal conditions across the northern tier of the country this winter, 2) cooler and wetter than normal conditions across the Gulf Coast states including Florida, 3) a potentially very wet and stormy winter in California, and 4) drier than normal conditions in the Ohio Valley and Montana.
Finally, when interpreting the climate information linked to El Niño it is important to note that while abnormal temperature and rainfall patterns can and sometimes do result in severe climate conditions, they do not imply calamitous conditions in many instances. For example, the above-normal precipitation expected in the Southwest and southern Plains states implies a reduced chance for wintertime drought such as occurred during November 1995- May 1996. In Florida the above-normal rainfall expected this winter indicates reduced chances of wildfires. In California, the potential impacts from El Niño can be severe. However, the above-normal rainfall across the state during the 1992-93 El Niño resulted in an end to severe long-term drought conditions that had persisted since 1986/87, and to a much-needed replenishment of water reserves.