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Climate Assessment Table of Contents

Stratospheric Temperatures


Global estimates of lower-stratospheric temperatures are derived from channel 4 of the MSU. The peak in the channel 4 weighting function varies from 70 hPa at extreme scan position to 100 hPa at nadir. During 1999, the estimated global mean temperature in the lower stratosphere was 0.45°C below the 1979–98 base period mean (Fig. 9), which is the fourth lowest value in the 21-yr record. The overall character of the time series has been dominated by major volcanic eruptions (i.e. El Chichon in 1982 and Mount Pinatubo in 1991), with an increase in temperatures observed immediately after these eruptions, followed by a rapid decrease in temperatures over the next several years. For the past seven years global lower-stratospheric temperatures have been below the long term mean, although they have increased slightly during the past few years from the record low values observed in 1996.

Regionally, lower stratospheric temperatures were above-average throughout the global tropics during 1999 (Fig. 10), with the largest positive anomalies observed over the eastern half of the tropical Pacific Ocean in association with La Niņa conditions. These positive anomalies were associated with a lowered tropopause and a cooler-than-normal troposphere (Fig. 8), in response to the La Niņa-related suppressed convection and the accompanying subtropical cyclonic circulation anomalies flanking the region of suppressed convection in both hemispheres. An anomalously warm lower stratosphere was also evident elsewhere in the global Tropics throughout the year, and contrasts with the anomalously cool tropical stratosphere that accompanies a major El Niņo episode (see Bell and Halpert 1999, their Fig. 10a).

In the middle latitudes, lower stratospheric temperatures were generally below average in both hemispheres during 1999 (Fig. 10), with the largest negative anomalies observed over the North Pacific, the North Atlantic, and eastern Asia. In each of these areas, the anomalies were accompanied by above-average tropospheric temperatures (Fig. 7) and above-average upper-tropospheric heights (see section 6, Figs. 78, 80, 82, 84). A reduced north-south slope of the tropopause and reduced jet stream winds existed in the area equatorward of all of these anomalies (see section 3e(1), Fig. 17), while an increased slope of the tropopause and enhanced westerly winds existed in the area poleward of these anomalies.

Lower-stratospheric temperatures have generally been below average in the middle latitudes of both hemispheres since early 1998 (Figs. 11a, 12a), with record negative anomalies observed in late 1998–early 1999. In the Northern Hemisphere, this cooling contrasts with the near-normal to above-normal temperatures observed during the previous three winters. At higher latitudes, above-normal temperatures were observed in the lower stratosphere of the Northern Hemisphere polar region (60°–90°N) during the first half of the year (Fig. 10a), primarily in response to significant warming during DJF (Fig. 11b). This is the second consecutive winter with well-above-normal temperatures in this region, following a string of five consecutive winters with colder-than-normal temperatures (Fig. 11b).

Lower-stratospheric temperatures were significantly cooler than average over Antarctica during the second half of 1999 (Fig. 10b), with near-record low values evident during November 1999 (Fig. 12b). For the past five years, October–November temperatures have been below-average across Antarctica, which has favored the persistence of polar stratospheric clouds that act to deplete Antarctic ozone [see section 5a(2)].

During October–November 1999 the anomalously low stratospheric temperatures were associated with below normal geopotential heights at 50-hPa everywhere poleward of 60°S (Fig. 13a). The largest anomalies were centered directly over the polar region, where heights averaged more than 300 m below normal and temperatures averaged more than 6°C below normal. Collectively, these conditions were associated with an exceptionally strong gradient in the geopotential height field between 60°–80°S (Fig. 13b). This height field reflected an enhanced polar vortex, and was accompanied by an enhanced subpolar jet stream which extended around the entire hemisphere with average zonal wind speeds exceeding 30 m s-1. These conditions reflect the positive phase of the Antarctic Oscillation (e.g., Thompson and Wallace 2000), and contrast with the weaker height gradient and weaker zonal wind maximum that characterize the climatological mean polar vortex (Fig. 13c).