The global-mean tropospheric temperature is monitored by two independent observing systems: radiosondes and polar-orbiting satellites. The global radiosonde record began in 1958 and has the advantage of a relatively long time series (39 yr) of directly measured temperatures. The satellite record [derived from measurements taken by the Microwave Sounding Unit (MSU) channel 2R flown aboard the polar-orbiting satellites] began in 1979 and has the advantages of 1) sampling the atmosphere with one type of instrument at any given time and 2) global sampling, with most of the earth sampled twice daily from each of two instruments flying concurrently on different satellites (Spencer et al. 1990). However, potential problems with the satellite data include calibration, diurnal sampling aliasing, orbital drift, etc. (Hurrell and Trenberth 1997).
The overall time series of the satellite-derived and radiosonde-derived tropospheric temperature anomalies are similar during the period from 1979 through 1996 (Fig. 6). The radiosonde observations (blue bars) indicate a mean tropospheric temperature 0.16°C below normal for 1996, while the satellite-based temperature anomalies (red bars) show an annual global-mean anomaly of -0.09°C, with both anomalies computed with respect to the 1982-91 base period. This is the fourth time in 5 yr that temperatures in the global troposphere have averaged below the 10-yr means. However when the last 5 yr of radiosonde data are compared to the entire 39-yr period, it is evident that global tropospheric temperatures remain above the cooler 1960-78 period, but below the 1987-91 warm period.
The spatial pattern of annual mean tropospheric temperature anomalies during 1996 shows the largest negative anomalies across North America, Europe, and western Asia (Fig. 7). This pattern is similar to the surface temperature anomaly pattern during 1996 (Fig. 3) and is opposite to that observed during 1995 (see Halpert et al. 1996, their Fig. 9). Colder than normal tropospheric temperatures were also observed throughout most of the global Tropics and over the land masses of the Southern Hemisphere during 1996, while positive anomalies were observed over Greenland, extreme eastern and northern Canada, the southwestern United States, and extreme eastern Siberia.
Estimates of global lower-stratospheric temperatures (Fig. 8) from both radiosonde observations (blue bars) and satellite observations (red bars) show 1996 to be the coldest year on record. However, the magnitude of the anomalies is substantially larger in the radiosonde network (1.4°C) than in the satellite record (0.52°C). At least some of this difference can be related to changes in the radiosonde sensors throughout the period (IPCC 1995). For the radiosonde network, 1996 is the 11th yr out of the past 12 with negative temperature anomalies relative to the 1982-91 base period mean. In contrast, annual stratospheric temperatures averaged above the 1982-91 mean in all years prior to 1979.
The spatial pattern of annual stratospheric temperature anomalies was dominated by negative anomalies over much of the globe (Fig. 9). The only exceptions were over eastern Siberia, Alaska, and northwestern Canada in the Northern Hemisphere, and in a band extending from south of Australia and New Zealand to near 60°E in the Southern Hemisphere. At high Northern Hemisphere latitudes, a strong wavenumber-1 pattern showed below-normal temperatures (more than 1.5°-2.5oC below normal) extending across the North Atlantic from eastern Canada to Scandinavia. This feature coincided with below-normal heights throughout the middle stratosphere across the high latitudes of the North Atlantic, overlaying a vertically deep pattern of above-normal heights and temperatures in the troposphere (Fig. 10). These conditions were consistent with a strong negative phase of the North Atlantic oscillation (see section 3c) and with recurrent blocking episodes across the high latitudes of the North Atlantic. Negative anomalies in tropical and subtropical regions were related to the quasi-biennial oscillation of stratospheric temperatures. In the Southern Hemisphere, the largest negative temperatures anomalies (1.5°-2.5oC below normal) were observed over Antarctica, consistent with the enhanced ozone depletion during September-December throughout the polar region [see section 2d(1)].
