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Climate Assessment Table of Contents

Surface Temperatures

Global surface temperatures are based on data collected from over 1000 land-based weather stations, combined with a blended sea-surface temperature (SST) data set derived from satellite, ship, and ocean buoy measurements. The estimated 1999 global mean temperature for land and ocean combined was 0.41C above the 1880–1998 long-term mean (Fig. 1a), which was the fifth warmest year in the record. However, the 1999 temperature was 0.25C cooler than the record-breaking anomaly of 0.66C recorded in 1998. A similar decrease in global mean temperatures between 1998 and 1999 was also reported by the Hadley Centre of the U.K. Meteorological Office and the Climatic Research Unit of the University of East Anglia, U.K. (WMO press release)

The anomalous warmth during 1999 continues a period of well above-average global temperatures that began in 1977. Since that time global temperatures have risen at a rate of 0.16C/decade, which is much larger than the rate of 0.04C/decade recorded prior to 1977. This rapid rise in temperatures in recent years made the decade of the 1990s the warmest in the record, with seven of the ten warmest years occurring since 1990.

The global mean SST during 1999 was 0.26C above the 1880–1998 mean (Fig. 1b), which is 0.25C lower than that recorded in 1998. The global mean land-only temperature during 1999 was 0.76C above the 1880-1998 mean (Fig. 1c), and was the second warmest in the record. However, this value was still considerably lower than the record departure of 1.02C set in 1998.

The most significant cooling during 1999 occurred in the Tropics (20N–20S), where the combined land–ocean anomaly of 0.21C (Fig. 2b) was well below the record value of 0.72C set in 1998. This drop in temperatures was evident in both the land-only (0.14C) and SST-only (0.43C) anomalies (not shown), both of which were more than 0.5C below the record 1998 values. This relative cooling throughout the Tropics was linked to the persistence of La Nia conditions throughout the year (Fig. 3, also see section 3).

In contrast, temperatures during 1999 in both the Northern (20–90N; Fig. 2a) and Southern (20–90S; Fig. 2c) Hemisphere extratropics were the second warmest in the record dating back to 1880, and were only slightly below the record 1998 values. In the Northern Hemisphere extratropics, the 1999 mean land-only temperature anomaly was 1.12C, while the mean SST anomaly was 0.40C (not shown). These values were only 0.05C and 0.1C below the record 1998 values, respectively.

The spatial pattern of mean annual temperature anomalies during 1999 (Fig. 3) featured below-average temperatures across the eastern half of the tropical Pacific, with temperatures ranging between 1–2C below average across the equatorial Pacific east of 170E. This anomalous cooling reflected the persistence of La Nia conditions throughout the year. Below-normal SSTs also prevailed along the west coast of the United States, across the higher latitudes of the North Pacific, and over the southeastern South Pacific. In contrast, above-normal SSTs covered much of the North Atlantic and South Atlantic Ocean basins during the year, with the largest positive anomalies (exceeding 1C) found over the central North Atlantic. Large areas of positive SST anomalies were also observed in the far western Pacific from 20–45N, and along the west coast of Australia.

Most land areas recorded above-average temperatures for the year as a whole. This anomalous warmth was particularly prominent across North America, where temperatures averaged 1–3C above normal across the eastern United States and central Canada, and more than 3C above normal in northeastern Canada. Positive temperature anomalies were observed in these areas during much of the year (see section 6, Figs. 77a, 79a, 81a, 83a), in association with a persistent pattern of above-normal heights (see section 6, Figs. 78, 80, 82, 84) and anomalous anticyclonic circulations at upper levels that covered central North America and Canada (see section 3, Figs. 20a, 23). These conditions were accompanied by reduced snow cover duration across the central and eastern United States during December 1998–February 1999 (DJF) (Fig. 4a), and across the intermountain region of the western U. S. and large portions of central Canada during March–May 1999 (MAM) (Fig. 4b).

Annual mean temperatures during 1999 for Canada as a whole averaged 1.7C above the 1961–90 mean (Fig. 5a), making it the third warmest year in the last 40 years. This warmth was surpassed only by the record anomaly of 2.5C set in 1998 and by the anomaly of 2.0C observed in 1981.

According to preliminary estimates, annual mean temperatures during 1999 for the contiguous United States averaged 1.07C above the 1961–90 mean (Fig. 5b), making it the second warmest year since 1900. This warmth was also surpassed only by the record anomaly of 1.28C set in 1998.

This anomalous warmth across North America was particularly prominent during DJF, when temperatures were 2–4C above average from the United States Gulf Coast to northern Canada (see section 6, Fig. 77a). Temperatures in the eastern two-thirds of Canada also remained 2–4C above average during MAM; (see section 6, Fig. 79a), and were 1–2C above average during June–August (JJA) (see section 6, Fig. 81a). This anomalous warmth redeveloped across nearly all of North America during September–November (SON) (see section 6, Fig. 83a), and was particularly evident in the United States during November when more than 70% of the country reported monthly mean temperatures in the top 10th percentile of the 1961–90 distribution (not shown).

A second extensive region of anomalous warmth extended across Europe, northern Africa, Russia, and much of Asia during 1999 (Fig. 3), with annual mean temperatures averaging more than 1C above-average in these regions. This anomalous warmth was associated with an anomalous upper-level anticyclonic circulation extending from the eastern Pacific eastward across the Atlantic Ocean to Asia throughout the year [see section 3e(2), Fig. 23]. It was also associated with a reduced southward extent of the snowpack during DJF and MAM (Fig. 4).

In Europe and Asia (excluding Russia), annual mean temperatures during 1999 averaged 1.1C (Fig. 5c) and 1.2C (Fig. 5d) above the 1961–90 base period mean, respectively. In Europe, this value was only slightly below the record anomaly observed during both 1989 and 1990. Similarly, the 1999 warmth in Asia was second only to the record anomaly of 1.4C established in 1998. In contrast, the 1999 anomaly in Russia was +0.52C (Fig. 5e), which is well below the record value established in 1995. Overall, this anomalous warmth across Eurasia was evident in all four seasons (see section 6, Figs. 77a, 79a, 81a, 83a), with only the MAM season featuring substantially below-average temperatures over large portions of Russia and northeastern Asia (see section 6, Fig. 79a).

Annual mean temperatures were also above-normal in Africa during 1999 (Fig. 3), with values of 0.5–2C above average observed over much of the continent. In South Africa, this anomalous warmth was evident in all seasons (see section 6, Figs. 77a, 79a, 81a, 83a), while in northern Africa it was evident in all seasons except DJF.

In contrast to the near-record breaking temperatures observed over much of the Northern Hemisphere and Africa, annual mean temperatures in Australia during 1999 were only 0.11C above the 1961–90 mean (Fig. 5f). This value is well below the record positive anomaly of +0.62C set in 1998. Regionally, annual mean temperatures in Australia averaged 1–2C above normal in the interior regions, but were below average along the northwest coast and along the east coast near Brisbane.

Below-average temperatures also prevailed along the western interior of South America during 1999, in a band stretching southward from northern Brazil to northern Argentina (Fig. 3). Farther east, temperatures in the eastern half of the continent were only slightly warmer than normal during the year, with annual anomalies generally less than +1C.