2. Climate and global change issues

a. Surface temperature

Annual mean global surface temperatures for land and marine areas combined are based on data collected from over 1000 land-based weather stations, and from approximately 7000 ships and 1000 ocean buoys. The 1997 estimated global mean temperature anomaly over land and marine areas combined was +0.43°C above the 196190 base period mean (Fig. 1a ), with an approximate error of 0.06°C (Jones et al. 1997). This was the highest value recorded dating back to 1860, which slightly surpassed the previous record estimate of +0.38°C (±0.06°C) set in 1995. In the Northern Hemisphere, 1997 was the second warmest year on record, with annual mean temperatures reaching +0.51°C above average (Fig. 1b). In the Southern Hemisphere, 1997 was the warmest year on record, with the annual mean temperature reaching 0.35°C above normal (Fig. 1c).

Six of the warmest ten years in the historical record have been observed since 1990. During this period, the larger positive anomalies have been recorded in the Tropics (30°N30°S) (Fig. 2a) and in the Northern Hemisphere extratropics (30°N90°N) (Fig. 2b), where departures in both regions approximately doubled those observed in the Southern Hemisphere extratropics (Fig. 2c). The warmth in the Tropics has been strongly linked to recurring Pacific warm episode conditions. Since 1990, there have been four distinct periods of mature-phase El Niño/Southern Oscillation (ENSO) conditions, including the very strong 1997/98 warm episode. This latter episode was a major contributor to record 1997 warmth in the global Tropics, where temperatures averaged 0.6°C above the 196190 base period means, and was also an important contributor to the record global mean temperature set during the year (Fig. 1a). Annual mean temperatures during 1997 in both the Northern and Southern Hemisphere extratropics were also much above the 196190 base period means, with the 1997 anomaly in the Northern Hemisphere extratropics ranking third behind 1995 and 1990 (Fig. 2b), and the 1997 anomaly in the Southern Hemisphere extratropics equaling the previous record value set in 1993 (Fig. 2c).

The estimated global mean surface temperature during 1997 for land regions only was also well above the 196190 mean (+0.31°C), resulting in the fifth warmest year in the historical record dating back to 1950 (Fig. 3a). All five of the warmest years have occurred during the 1990s. The tropical (20°S20°N) land areas during 1997 recorded their largest mean annual temperature anomaly (+0.56°C) in the historical record dating back to 1950 (Fig. 3b), surpassing the previous record of +0.50°C set in 1987. Temperatures in the global tropics were especially warm during OctoberDecember, when the three-month mean value averaged +0.97°C above the 1961-90 base period mean. In the Northern Hemisphere extratropics, annual mean land-only temperatures were the seventh warmest in the past 48 years (Fig. 3c), rebounding from the slightly negative anomaly observed during 1996. In the Southern Hemisphere extratropics, annual mean land-only temperatures during 1997 were +0.28°C above normal, which is comparable to the largest values in the record dating back to 1950 (Fig. 3d).

The pattern of annual-average temperature anomalies during 1997 (Fig. 4) was dominated by well above normal temperatures across the tropical Pacific. Annual anomalies across the central and eastern tropical Pacific averaged 1°3.5°C above the 1961-90 mean, with monthly anomalies exceeding +5°C during OctoberDecember (not shown). These anomalies were larger than those recorded during the 1982/83 ENSO.

Abnormally warm SSTs were also observed across much of the tropical Indian Ocean during the year, where annual mean temperatures averaged 0.5°1.0°C above normal. Much of this warmth was observed during JuneDecember, partly in association with a weaker-than-normal Indian monsoon circulation during JuneSeptember [see section 4c(1) ], and with strong El Niño conditions throughout the period. During OctoberDecember these abnormally warm SSTs contributed to very large rainfall totals throughout equatorial eastern Africa and the western Indian ocean [see section 4b(1)].

In the Northern Hemisphere extratropics, the most prominent annual mean temperature anomalies during 1997 included above-normal temperatures over most of Siberia, over the eastern North Atlantic/western Europe, and across the eastern Pacific/western North America (including Alaska), and below-normal temperatures over the eastern two-thirds of North America, across the central North Pacific, northern India, and the Middle East (Fig. 4). The largest annual anomalies were observed over the eastern half of Russia and Siberia, where temperatures averaged 1°2.5°C above normal. In these regions, temperatures were above-normal in all seasons except June-August (JJA), with the largest seasonal anomalies exceeding +5°C during March-May (MAM) (see section 5, Fig. 69). This abnormal warmth was linked to an anomalous large-scale circulation pattern featuring above-normal heights and broad southwesterly flow throughout the region (see section 5, Fig. 70). A similar circula
tion pattern during JanuaryMay 1995 also resulted in excessive warmth (temperatures 6°8°C above average) and substantially reduced snow cover over Russia and Siberia (Halpert et al. 1996).

Elsewhere during 1997, a pattern of above-average temperatures over the eastern Atlantic/western Europe and below-average temperatures over eastern North America was observed during March-November (see section 5, Figs. 69, 71, 73 ). This temperature pattern was partly linked to a recurring negative phase of the North Atlantic Oscillation (NAO). This negative NAO pattern was particularly prominent during April-June, as indicated by the largest negative NAO index for this three-month period dating back to 1950 (not shown). During this 3-month period, temperatures averaged 1°-3°C below normal across eastern North America and 1°-2°C above normal across southwestern Europe (not shown). This anomalous circulation pattern also brought heavy rainfall to large parts of eastern Europe during April-June, and contributed to large-scale flooding throughout the region in July (see section 4d).

Over the Pacific sector, negative SST anomalies in the central north Pacific and positive temperature anomalies over Alaska and the eastern Pacific prevailed throughout much of the year (Fig. 4, also see section 5, Figs. 67, 69, 71 , 73). In the Middle East and northern India, annual temperatures averaged near 1°C below the 1961-90 means. In both of these regions temperatures during MAM and September-November (SON) were 0.5°-2°C below normal, with smaller negative anomalies observed during JJA. Temperatures over India were also below normal during December-February (DJF).

In the Southern Hemisphere extratropics, annual mean temperatures were above-normal over most of South America, Africa and southeastern Australia, and below-normal over western Australia (Fig. 4). In South America the anomalous warmth was most evident during June-December in association with strong El Niño conditions (see section 5, Figs. 71, 73 ). In Africa, temperatures were generally above normal in all four seasons. In Australia, seasonal temperatures were more variable, with above-normal temperatures observed in southern Australia during DJF (see section 5, Fig. 67) and over the eastern half of the continent during SON (see section 5, Fig. 73). Elsewhere, below-normal temperatures dominated northern and western Australia during the first three seasons of the year (see section 5, Figs. 67, 69 , 71).

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