b. Tropospheric/stratospheric temperatures

1) Troposphere

Global fields of mean lower-tropospheric temperature are derived from channel-2R of the Microwave Sounding Unit (MSU) flown aboard the NOAA series of polar-orbiting satellites (Spencer et al. 1990). Most of the earth is sampled twice daily from each of two MSU instruments flying concurrently on different satellites. The MSU data record began in 1979 and has continued uninterrupted since that time. During 1997, the annual-mean global lower-tropospheric temperature from the MSU (Fig. 5) was slightly below the 1979-95 base period mean (­0.15°C). This value is the seventh coldest in the 19-yr record, and is comparable to that observed in 1996. The MSU-derived temperatures have averaged below the 17-yr average in five of the past six years. During this 6-yr period temperatures averaged 0.2-0.4°C below the warmer period of 1987-1991. Note that the very low temperatures recorded by the MSU instruments in 1992 and 1993 were strongly influenced by the eruption of Mt. Pinatubo in June 1991. [In previous annual climate assessments lower-tropospheric temperatures derived from the MSU have been compared to estimates obtained from the global radiosonde network (e.g., Halpert and Bell 1997). However, radiosonde estimates for 1997 were unavailable for this Assessment.]

The regional patterns of lower­tropospheric temperature anomalies show marked differences between the first and second halves of the year (Fig. 6). In the Tropics and subtropics, colder than normal conditions prevailed during January-June (Fig. 6a ), partly in association with a continuation of weak Pacific cold episode conditions into early 1997. In contrast, tropical temperatures were generally above-normal during July-December (Fig. 6b), and below-normal temperatures were primarily confined to Africa. During this period the largest positive anomalies were found over the eastern half of the Pacific Ocean in association with an El Niño-related extension of deep tropical convection to well east of the date line (see section 3a, Figs. 26c, d). This warmth is thermodynamically consistent with another well-known El Niño-related feature: subtropical anticyclonic circulation anomalies at upper levels of both hemispheres, flanking the region of enhanced tropical convection.

The July-December period also featured an eastward extension of abnormally warm temperatures to the northern half of South America and the tropical North Atlantic. This warmth was partly a result of advective processes, and in the Amazonian region was also the result of anomalous sinking motion in association with a weaker-than-normal upper-level anticyclonic circulation (see section 3a, Figs 27c, d).

In the extratropics, the January-June period featured below-normal temperatures over most of both hemispheres, with positive anomalies confined to western Europe, central and eastern Asia, the eastern North Atlantic and the high latitudes of the South Atlantic and South Indian oceans (Fig. 6a). In Eurasia, the pattern of abnormally warm temperatures over central Russia and colder-than-normal conditions over northern Europe was also evident in the seasonal surface temperature and snow cover fields (see section 5, Figs. 67, 69 , and 19a, b). These conditions were associated with an anomalous atmospheric circulation, characterized by a pronounced upper-level trough throughout western Russia and broad southwesterly flow farther downstream (see section 5, Figs. 68, 70).

In the Pacific/ North America region, the pattern of below-normal lower­troposphere temperatures throughout Canada and the northern tier of the United States, and above-normal temperatures over the high latitudes of the North Pacific, reflected recurring high-latitude blocking over the central North Pacific. This blocking pattern contributed to increased storminess and a series of major cold-air outbreaks across central North America. In contrast, the July-December period featured abnormally warm temperatures throughout Canada, and abnormally cool conditions across most of the United States (Fig. 6b). These differences in temperature between the first and second halves of the year are consistent with the transition from Pacific cold to warm episode conditions.

Over the North Atlantic, lower-tropospheric temperatures during July-December were abnormally warm at high latitudes and in the subtropics, and close to normal in the middle latitudes. These conditions reflected recurring high latitude blocking activity during the period, and an overall southward shift of the main belt of westerlies to the central North Atlantic (see section 5, Figs. 72, 74 ). These circulation features are consistent with a recurring negative phase of the North Atlantic Oscillation (NAO), especially during October-December. Farther east, temperatures were generally below-normal across southeastern Europe during the period, in association with a persistent upper-level trough across the region. This trough was associated with an eastward extension of enhanced geostrophic westerlies across southern and central Europe, which contributed to increased storminess and above-normal rainfall throughout the region (see section 4d).

In the Southern Hemisphere extratropics, one of the most prominent features during July-December was persistent below-normal lower­tropospheric temperatures across the central latitudes of the South Pacific. This pattern is consistent with the presence of strong El Niño conditions throughout the period and, in combination with abnormally warm temperatures in the subtropics, was associated with a pronounced eastward extension of the South Pacific jet stream to well east of the date line [see section 3a(4)]. Farther south, above-normal temperatures covered the high latitudes of the eastern South Pacific during the period, in association with a persistent pattern of above-normal heights and recurring blocking activity. Increased cool-season blocking activity is also favored in this region during strong Pacific warm episodes (Karoly 1989).

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