Climate Assessment for 1999

Abstract

The global climate during 1999 was impacted by Pacific cold episode (La Niña) conditions throughout theyear, which resulted in regional precipitation and atmospheric circulation patterns across the Pacific Ocean and theAmericas that are generally consistent with those observed during past cold episodes. The primary La Niña_relatedprecipitation anomalies included 1) increased rainfall across Indonesia, and a nearly complete disappearance of rainfall acrossthe east-central and eastern equatorial Pacific; 2) above-normal rains across northwestern and northern Australia; 3)increased monsoon rains across the Sahel region of western Africa; 4) above-average rains over southeastern Africa, 5)above-average rains over the Caribbean Sea and portions of Central America, and 6) below-average rains in southeastern South America. The La Niña also contributed to persistent cyclonic circulation anomalies in the subtropics of bothhemispheres, which flanked the area of suppressed convective activity over the eastern half of the equatorial Pacific. In theNorthern Hemisphere this anomaly feature contributed to a pronounced westward retraction of the wintertime East Asian jetstream, which subsequently impacted precipitation and storm patterns across the eastern North Pacific and western NorthAmerica. The La Niña_related pattern of tropical rainfall also contributed to a very persistent pattern of anticycloniccirculation anomalies in the middle latitude of both hemispheres, extending from the eastern Pacific across the Atlantic andAfrica eastward to Australasia. This anomaly pattern was associated with an active Atlantic hurricane season, an inactiveeastern North Pacific hurricane season, above-average rains in the African Sahel, and an overall amplification of theentire southeast Asian summer monsoon complex. The active 1999 North Atlantic hurricane season featured 12 named storms, 8 of which became hurricanes, and 5of which became intense hurricanes. The peak of activity during mid-AugustOctober was accompanied by lowvertical wind shear across the central and western Atlantic, along with both a favorable structure and location of theAfrican easterly jet. In contrast, only 9 tropical storms formed over the eastern North Pacific during the year, making it one ofthe most inactive years for that region in the historical record. This relative inactivity was linked to a persistent patternof high vertical wind shear that covered much of the main development region of the eastern North Pacific. Other regional aspects of the short-term climate included: 1) above-average wintertime precipitation andincreased storminess in the Pacific Northwest, United States; 2) above-average monsoonal rainfall across the southwesternUnited States; 3) drought over the northeastern quadrant of the United States during April_mid-August; 4)hurricane-related flooding in the Carolinas during September; 5) drought over the south-central United States during July_November;6) below-average rainfall in the Hawaiian Islands throughout the year, with long-term dryness affecting some parts ofthe islands since October 1997; 7) a continuation of long-term drought conditions in southeastern Australia, with mostof Victoria experiencing below-average rainfall since late 1996; and 8) above-average rainfall in central China duringApril_August. Global annual mean surface temperatures during 1999 for land and marine areas were 0.41% C above the1880_1998 long-term mean, making it the fifth warmest year in the record. However, significant cooling was evident in theTropics during 1999 in association with a continuation of La Niña conditions. In contrast, temperatures in both theNorthern Hemisphere and Southern Hemisphere extratropics were the second warmest in the historical record during 1999,and only slightly below the record 1998 anomalies. The areal extent of the Antarctic ozone hole remained near record levels during 1999. The ozone hole alsolasted longer than has been observed in past years.