Climate assessment for 1998

The global climate during 1998 was affected by opposite extremes of the ENS O cycle, with one of the strongest Pacific warm episodes (El Nifio) in the historical record continuing during January-early May and Pacific cold epis ode (La Nina) conditions occurring from July-December. In bath periods, reg ional temperature, rainfall, and atmospheric circulation patterns across th e Pacific Ocean and the Americas were generally consistent with those obser ved during past warm and cold episodes. Some of the most dramatic impacts from both episodes were observed in the T ropics, where anomalous convection was evident across the entire tropical P acific and in most major monsoon regions of the world. Over the Americas, m any of the El Nino- (La Nina-) related rainfall anomalies in the subtropica l and extratropical latitudes were linked to an extension (retraction) of t he jet streams and their attendant circulation features typically located o ver the subtropical latitudes of both the North Pacific and South Pacific. The regions most affected by excessive El Nino-related rainfall included 1) the eastern half of the tropical Pacific, including western Ecuador and no rthwestern Peru, which experienced significant flooding and mudslides; 2) s outheastern South America, where substantial flooding was also observed; an d 3) California and much of the central and southern United States during J anuary-March, and the central United States during April-June. EI Nino-related rainfall deficits during 1998 included 1) Indonesia and por tions of northern Australia; 2) the Amazon Basin, in association with a sub stantially weaker-than-normal South American monsoon circulation; 3) Mexico , which experienced extreme drought throughout the El Nino episode; and 4) the Gulf Coast states of the United States, which experienced extreme droug ht during April-June 1998. The El Nino also contributed to extreme warmth a cross North America during January-May. The primary La Nina-related precipitation anomalies included I) increased r ainfall across Indonesia, and a nearly complete disappearance of rainfall a cross the east-central equatorial Pacific; 2) above-normal rains across nor thwestern, eastern, and northern Australia; 3) increased monsoon rains acro ss central America and Mexico during October-December; and 4) dryness acros s 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 muds lides associated with Hurricane Mitch claimed more than 11 000 lives. Durin g the peak of activity in August-September, the vertical wind sheer across the western Atlantic, along with both the structure and location of the Afr ican easterly jet, were typical of other active seasons. Other regional aspects of the short-term climate included 1) record rainfal l and massive flooding in the Yangtze River Basin of central China during J une-July; 2) a drier and shorter-than-normal 1997/98 rainy season in southe rn Africa; 3) above-normal rains across the northern section of the African Sahel during June-September 1998; and 4) a continuation of record warmth a cross Canada during June-November. Global annual mean surface temperatures during 1998 far land and marine are as were 0.56 degrees C above the 1961-90 base period means. This record war mth surpasses the previous highest anomaly of +0.43 degrees C set in 1997. Record warmth was also observed in the global Tropics and Northern Hemisphe re extratropics during the year, and is partly linked to the strong El Nino conditions during January-early May.