The global climate during 1998 was affected by opposite extremes of the ENSO cycle, with one of the strongest Pacific warm episodes (El Nio) in the historical record continuing during Januaryearly May and Pacific cold episode (La Nia) conditions occurring from JulyDecember. In both periods, regional temperature, rainfall, and atmospheric circulation patterns across the Pacific Ocean and the Americas were generally consistent with those observed during past warm and cold episodes.Some of the most dramatic impacts from both episodes were observed in the Tropics, where anomalous convection was evident across the entire tropical Pacific and in most major monsoon regions of the world. Over the Americas, many of the El Nio (La Nia) related rainfall anomalies in the subtropical and extratropical latitudes were linked to an extension (retraction) of the jet streams and their attendant circulation features typically located over the subtropical latitudes of both the North Pacific and South Pacific.The regions most affected by excessive El Niorelated rainfall included 1) the eastern half of the tropical Pacific, including western Ecuador and northwestern Peru, which experienced significant flooding and mudslides; 2) southeastern South America, where substantial flooding was also observed; and 3) California and much of the central and southern United States during JanuaryMarch, and the central United States during AprilJune.El Niorelated rainfall deficits during 1998 included 1) Indonesia and portions of northern Australia; 2) the Amazon Basin, in association with a substantially weaker-than-normal South American monsoon circulation; 3) Mexico, which experienced extreme drought throughout the El Nio episode; and 4) the Gulf Coast states of the United States, which experienced extreme drought during AprilJune 1998. The El Nio also contributed to extreme warmth across North America during JanuaryMay.The primary La Niarelated precipitation anomalies included 1) increased rainfall across Indonesia, and a nearly complete disappearance of rainfall across the east-central equatorial Pacific; 2) above-normal rains across northwestern, eastern, and northern Australia; 3) increased monsoon rains across central America and Mexico during OctoberDecember; and 4) dryness across equatorial eastern Africa.The active 1998 North Atlantic hurricane season featured 14 named storms (9 of which became hurricanes) and the strongest October hurricane (Mitch) in the historical record. In Honduras and Nicaragua extreme flooding and mudslides associated with Hurricane Mitch claimed more than 11 000 lives. During the peak of activity in AugustSeptember, the vertical wind shear across the western Atlantic, along with both the structure and location of the African easterly jet, were typical of other active seasons.Other regional aspects of the short-term climate included 1) record rainfall and massive flooding in the Yangtze River Basin of central China during JuneJuly; 2) a drier and shorter-than-normal 1997/98 rainy season in southern Africa; 3) above-normal rains across the northern section of the African Sahel during JuneSeptember 1998; and 4) a continuation of record warmth across Canada during JuneNovember.Global annual mean surface temperatures during 1998 for land and marine areas were 0.56C above the 196190 base period means. This record warmth surpasses the previous highest anomaly of 0.43C set in 1997. Record warmth was also observed in the global Tropics and Northern Hemisphere extratropics during the year, and is partly linked to the strong El Nino conditions during Januaryearly May.
Bulletin of the American Meteorological Society – American Meteorological Society
Published: May 1, 1999
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