2) Stratosphere

Global estimates of lower-stratospheric temperatures are derived from channel-4 of the MSU. During 1997, the estimated global mean temperature was 0.66°C below the 197995 base period means (Fig. 7). This was the second lowest value in the 19 year historical record, surpassed only by the record low value recorded in 1996. Mean annual stratospheric temperatures have grown progressively colder during the 19-yr record, except for two 2-yr periods in which temperatures were above-normal. These periods coincide with the eruption of El Chichon in 1982 and the eruption Mt. Pinatubo in 1991. Also, global stratospheric temperatures have been below the long-term mean for the past five years, likely in response to an overall decrease in global ozone concentrations during the period.

During JanuaryJune, lower-stratospheric temperatures were below average over most of the globe (Fig. 8a). Regions with the largest negative temperature anomalies included the Northern Hemisphere polar region, the subtropical North Pacific extending eastward across the southern United States, and the Southern Hemisphere middle latitudes. Substantially warmer than normal temperatures during the period were confined to the high latitudes of the South Pacific and portions of Antarctica.

Over the high latitudes of the Northern Hemisphere, wintertime temperatures have averaged well-below normal for the past three years (Fig. 9). During 1997 extremely low temperatures were observed during FebruaryApril, when area-mean temperatures for the region north of 60°N averaged more than 9.0°C below normal. These below-normal temperatures were associated with below normal geopotential heights at 50-hPa everywhere north of 60°N. The largest anomalies were centered directly over the polar region (Fig. 10a), where temperatures averaged more than 12.0°C below normal and heights averaged more than 600 m below normal. Collectively, these conditions were associated with an enhanced polar night jet (Fig. 10b). Features of this jet included an exceptionally strong gradient in the geopotential height field between 60°N and 80°N around the entire hemisphere, and average zonal wind speeds of 25-35 m s-1. These conditions contrast with the relatively weak height gradient and ill-defined zonal wind maximum normally observed throughout the polar region at this time of the year (Fig. 10c).

An additional feature of the polar region during February-March was the large-scale formation of polar stratospheric clouds in response to extremely cold conditions temperatures. The presence of these clouds leads to the chemical destruction of stratospheric ozone, and during March contributed to below-normal ozone concentrations throughout the Arctic [see section 2c(1)].

During JulyDecember, the primary features of the lower stratosphere included above-normal temperatures over much of the polar region of the Northern Hemisphere and below-normal temperatures over most of Antarctica. This anomaly pattern is nearly opposite in sign to that observed during January-June, and is consistent with increased ozone destruction due to polar stratospheric clouds in the Southern Hemisphere polar region, particularly during July-October.

In the extratropics, many of the anomaly features evident in the lower-stratospheric temperature field during July-December reflected the stratospheric signature of large-scale circulation anomalies that extended well into the middle and lower troposphere. For example, the very low temperatures over the high latitudes of the North Atlantic and Scandinavia, combined with above-average temperatures in the lower troposphere (Fig. 8b), reflected above-normal tropospheric heights and recurring high-latitude blocking activity throughout the region [see section 2b(1)]. Similarly, the patterns of above-normal temperatures in the lower stratosphere over the Gulf of Alaska and eastern Siberia reflected the high-altitude signature of persistent, large-amplitude troughs previously noted in these regions. This overall anomaly pattern across the higher latitudes of the Northern Hemisphere was remarkably similar to that observed during 1996, a year which also featured recurring blocking activity across the high latitudes of the North Atlantic (Halpert and Bell 1997, see their Fig. 9).

Over the subtropical North and South Pacific, negative temperature anomalies in the lower stratosphere during July-December were heavily linked to strong El Niño conditions, and were consistent with abnormally warm tropospheric temperatures and well-defined anticyclonic circulation anomalies at upper-tropospheric levels. In the Southern Hemisphere, the pattern of negative lower stratospheric temperature anomalies also delineated the anticyclonic-shear side of a very strong and extended South Pacific jet stream throughout the period [see section 3a(4)].

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