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).
