Global estimates of mean tropospheric temperatures are obtained from satellite observations and from the NCEP/NCAR reanalysis system (Kalnay et al. 1996). The satellite estimates are obtained from channels 2 and 2R of the Microwave Sounding Unit (MSU) (Spencer et al. 1990, Spencer and Christy 1992) on the NOAA series of polar-orbiting satellites, and are updated and modified in real-time by Christy (personal communication). The peak in the channel 2 weighting function varies from 400 hPa at extreme scan position to 700 hPa at nadir position. In contrast, the peak in the channel 2R weighting function is near 750 hPa.
To facilitate comparisons between satellite-derived and reanalysis-derived global annual mean tropospheric temperatures (Fig. 7a ), the reanalysis data are subjected to a discretized vertical weighting function having the same shape as the channel 2 weighting function. The reanalysis data are used as a replacement for radiosonde-based estimates, which are no longer available in time for this assessment. Overall, time series of global annual mean temperature anomalies derived from both channel 2 and reanalysis data exhibit the same general behavior during the past twenty years. A more detailed intercomparison of these datasets can be found in Chelliah and Ropelewski (1999).
According to the MSU- (reanalysis-) based estimates, the annual mean global
tropospheric temperature during 1998 was 0.5°C (0.33°C) above the 197995 base period
mean. These are the largest positive anomalies observed in the record, and are the first
large positive anomalies observed since the eruption of
Mount Pinatubo in 1991. Both data sets also show a record increase in temperature of approximately 0.5°C between 1997 and 1998. The second largest year-to-year increase in global mean tropospheric temperatures (0.4oC) occurred from 1976 to 1977, coincident with major changes in the atmospheric circulation noted previously by several authors (e.g., Trenberth 1990).
The spatial pattern of annual tropospheric temperature anomalies during 1998 derived from channel 2R of the MSU (Fig. 8a) shows generally above-average temperatures between 30°S and 60°N, and below-normal temperatures at high latitudes in the Southern Hemisphere. Negative temperature anomalies were also observed over northeastern and northwestern Russia. This global pattern is very similar to the pattern of annual mean surface temperature anomalies (Fig 4).
Lower tropospheric temperature anomalies exhibited marked differences between the first and second halves of the year (Figs. 9a, b ). These differences largely reflect the influence of warm episode conditions during JanuaryMay and cold episode conditions during the second half of the year. During both periods there is considerable zonal symmetry to the anomaly patterns throughout the Tropics, subtropics, and extratropics, as well as considerable interhemispheric symmetry, which is also typical of extremes in the ENSO cycle.
During January-June (Fig. 9a ) the zonally symmetric features included abnormally warm temperatures throughout the Tropics and the subtropics of both hemispheres, and a local minimum in temperature anomalies near the 30° latitude band in both hemispheres. These anomalies reflected an increased meridional temperature gradient in the lower-midlatitudes of both hemispheres, and a decreased meridional temperature gradient in the 40°-55° latitude bands. These conditions are consistent with increased westerlies near 30° latitude in both hemispheres and with decreased westerlies in the 40°-55° latitude bands (see section 3, Figs. 27a, b). They are also consistent with an overall equatorward shift in strong westerly winds in both hemispheres during the period.
In contrast the July-December period (Fig. 9b) featured a relative minimum in mean tropospheric temperature anomalies in the Tropics and subtropics, and a relative maximum in the middle latitudes between 30°-40° latitude bands of both hemispheres. These features reflected both a decreased meridional temperature gradient in the lower-midlatitudes of both hemispheres and an increased meridional temperature gradient in the 40°-55° degree latitude bands. These conditions are consistent with decreased westerlies near 30° latitude in both hemispheres and with increased westerlies in the 40°-55° latitude bands (see section 3, Fig. 27d). They are also consistent with an overall poleward shift in strong westerly winds in both hemispheres during the period.
There is also considerable zonal asymmetry evident in the temperature anomaly patterns during both periods, with the largest anomalies observed over the Pacific sector of both hemispheres and over the Americas. These patterns strongly reflect the teleconnective response of the atmospheric circulation to opposite extremes in the ENSO cycle. For example, large positive temperature anomalies in the subtropics of both hemispheres during January-June were collocated with the ENSO-related subtropical anticyclonic circulation anomalies observed at upper-levels flanking the region of enhanced tropical convection (see section 3, Figs. 27a, b). These anomalies reflected a dramatic weakening of the upper-level mid-Pacific trough in both hemispheres, and a pronounced eastward extension of the subtropical ridges to well east of the date line (see section 3c). In contrast, the mean July-December conditions were consistent with La Niņa-related subtropical cyclonic circulation anomalies at upper levels flanking the region of suppressed tropical convection (see section 3, Fig. 27d ). These anomalies reflected a strengthening of the mid-Pacific trough in both hemispheres and a westward retraction of the subtropical ridges toward the western Pacific (see section 3d).
In the middle latitudes, the January-June period featured negative temperature anomalies over both the North and South Pacific, which is consistent with the observed pattern of below-normal heights in these regions (see section 5, Figs. 64 , 66). In contrast, positive temperature anomalies and above-normal heights were observed in these regions during July-December.
Large positive temperature anomalies were also observed over Canada and the northern
United States during January-June. In combination with the anomaly pattern over the North
Pacific, these conditions during winter are consistent with a substantially more
zonally-uniform temperature field across the eastern North Pacific and North America than
is evident in the climatological mean. More zonally uniform height and wind fields also
accompanied these conditions, consistent with the ongoing strong El Niņo episode during
period. Specific circulation features which accompanied these conditions (see also section 3b) included 1) a pronounced eastward extension and southward shift of the East Asian jet stream to the southwestern United States, 2) a strengthening of the subtropical jet stream across northern Mexico, the southern United States, and the Gulf of Mexico, 3) a pronounced weakening of the Hudson Bay Low, and 4) substantially reduced northwesterly flow throughout central North America.
In the Southern Hemisphere, the January-June temperature anomaly pattern is also consistent with increased jet stream winds across the eastern South Pacific and South America, which produced increased storminess and above-normal rainfall across central South America and the western South Atlantic (see section 4d). These conditions are similar to those observed during the July-December 1997 period, which also featured a pronounced extension of the wintertime jet stream from Australia to the west coast of South America.
In contrast, the temperature and circulation features over the South Pacific during July_December 1998 reflected a westward retraction of the jet steam to Australia (see section 3d , Fig. 31b). This jet structure is consistent with the La Niņa-related amplification of the mid-Pacific trough and westward retraction of the subtropical ridge toward the western South Pacific.
2) Lower stratosphere
Global estimates of lower-stratospheric temperatures are derived from channel 4 of the MSU and from the NCEP/NCAR reanalysis. The peak in the channel 4 weighting function varies from 70 hPa at extreme scan position to 100 hPa at nadir position. Both analyses show that 1998 was the sixth consecutive year with below-average temperatures (Fig. 7b). During the past twenty years the character of the two time series has been dominated by major volcanic eruptions (i.e., El Chichon in 1982 and Mt. Pinatubo in 1991), with above normal lower-stratospheric temperatures observed immediately after these eruptions followed by a rapid drop in temperatures for several years. Global lower-stratospheric temperatures reached a minimum in 1996 and subsequently increased slightly during 1997 and 1998.
During 1998 the MSU-estimated temperatures averaged 0.4°C below the 1979-95 means, which is the fourth lowest value in the 20-year record. The reanalysis-estimated temperatures averaged 0.63°C below the 1975-95 means, which is the seventh lowest value dating back to 1958.
The spatial pattern of lower-stratospheric annual temperature anomalies during 1998 (Fig. 8b) shows below-normal temperatures throughout the high latitudes of the Southern Hemisphere, much of the global tropics and subtropics, and the middle latitudes of the Northern Hemisphere. Large positive stratospheric temperature anomalies were evident only at high latitudes of the Northern Hemisphere. This overall anomaly pattern has many features similar to those observed during the July-December 1997 period (Bell and Halpert 1998).
During January-June 1998, lower-stratospheric temperatures were below average throughout the global Tropics and subtropics, over the North Atlantic and Europe, and over the high latitudes of the South Pacific and South Atlantic (Fig. 10a ). In the Tropics and subtropics (especially across the central Pacific), negative temperature anomalies were found above an elevated tropopause in association with the El Niņo-related enhanced convection and anomalous subtropical anticyclones (see section 3c). These temperature anomalies are opposite to those observed in the troposphere (Fig. 9a).
During July-December 1998, lower-stratospheric temperatures warmed considerably throughout the global Tropics, consistent with the demise of the El Niņo conditions and the subsequent development of La Niņa conditions. This warming was also associated with a transition to the easterly phase of the stratospheric quasi-biennial oscillation (QBO) during the year.
In the extratropics January-June temperatures in the lower stratosphere were above normal across the eastern Pacific in both hemispheres, reflecting a lowered tropopause along the cyclonic-shear side of the enhanced westerly jets. Between the Tropics and extratropics the reversal in the sense of the anomalous temperature gradient from that observed in the troposphere (Fig. 9a) is consistent with an increased tropopause slope, and with an anomalously strong decrease in westerly winds with height above the jet stream level. Opposite conditions were observed during July_December (Fig. 9b) over the eastern Pacific in association with the development of La Niņa conditions.
Over Antarctica above-average lower-stratospheric temperatures were observed during January-June (Fig. 10a), while significantly below-normal temperatures were observed during July-December 1998 (Fig. 10b). In both periods, these conditions are similar to those observed in 1997. During August-November 1998 these extremely low temperatures were associated with increased ozone destruction due to high chlorofluorocarbon concentrations in polar stratospheric clouds. A record size of the Antarctic ozone hole was measured during October-November [see section 2c(1)].
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