Climate assessment for 1999

The global climate during 1999 was impacted by Pacific cold episode (La Nin a) conditions throughout the year, which resulted in regional precipitation and atmospheric circulation patterns across the Pacific Ocean and the Amer icas that are generally consistent with those observed during past cold epi sodes. The primary La Nina-related precipitation anomalies included 1) incr eased rainfall across Indonesia, and a nearly complete disappearance of rai nfall across the east-central and eastern equatorial Pacific; 2) above-norm al rains across northwestern and northern Australia; 3) increased monsoon r ains across the Sahel region of western Africa; 4) above-average rains over southeastern Africa, 5) above-average rains over the Caribbean Sea and por tions of Central America, and 6) below-average rains in southeastern South America. The La Nina also contributed to persistent cyclonic circulation anomalies i n the subtropics of both hemispheres, which Banked the area of suppressed c onvective activity over the eastern half of the equatorial Pacific. In the Northern Hemisphere this anomaly feature contributed to a pronounced westwa rd retraction of the wintertime East Asian jet stream, which subsequently i mpacted precipitation and storm patterns across the eastern North Pacific a nd western North America. The La Nina-related pattern of tropical rainfall also contributed to a very persistent pattern of anticyclonic circulation a nomalies in the middle latitude of both hemispheres, extending from the eas tern Pacific across the Atlantic and Africa eastward to Australasia. This a nomaly pattern was associated with an active Atlantic hurricane season, an inactive eastern North Pacific hurricane season, above-average rains in the African Sahel, and an overall amplification of the entire southeast Asian summer monsoon complex. The active 1999 North Atlantic hurricane season featured 12 named storms, 8 of which became hurricanes, and 5 of which became intense hurricanes. The peak of activity during mid-August-October was accompanied by low vertical wind shear across the central and western Atlantic, along with both a favor able structure and location of the African easterly jet. In contrast, only 9 tropical storms formed over the eastern North Pacific during the year, ma king it one of the most inactive years for that region in the historical re cord. This relative inactivity was linked to a persistent pattern of high v ertical 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 and increased storminess in the Pacific Northwest , United States; 2) above-average monsoonal rainfall across the southwester n United States; 3) drought over the northeastern quadrant of the United St ates during April-mid-August; 4) hurricane-related flooding in the Carolina s 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 of the islands since October 1997; 7) a continuation of long-term drought conditions in southea stern Australia, with most of Victoria experiencing below-average rainfall since late 1996; and 8) above-average rainfall in central China during Apri l-August. Global annual mean surface temperatures during 1999 for land and marine are as were 0.41 degrees C above the 1880-1998 long-term mean, making it the fi fth warmest year in the record. However, significant cooling was evident in the Tropics during 1999 in association with a continuation of La Nina cond itions. In contrast, temperatures in both the Northern Hemisphere and South ern Hemisphere extratropics were the second wannest in the historical recor d during 1999, and only slightly below the record 1998 anomalies. The areal extent of the Antarctic ozone hole remained near record levels du ring 1999. The ozone hole also lasted longer than has been observed in past years.