JOHN M. WALLACE1
Extended records of surface air temperature at land stations and tropospheric temperature are examined, with emphasis on horizontal structure, seasonality, and detection of specific signals such as El Niño and volcanic eruptions. Interdecadal trends in hemispheric mean temperature show up clearly in data for summertime and the transition seasons, even without any filtering of monthly mean data. In wintertime they tend to be obscured by large local temperature fluctuations associated with regional teleconnection patterns. A distinctive El Niño/Southern Oscillation (ENSO) signature is evident in averages over the tropical belt, where surface- and upper-air temperatures exhibit a remarkably consistent pattern of interannual variability. However, the ENSO cycle is not evident in time series of spatially averaged temperatures in the extratropical regions of the globe. The El Chichon and Pinatubo eruptions show up clearly in globally averaged lower-stratospheric temperatures based on MSU Channel 4 data, but they tend to be obscured by dynamical phenomena in hemispheric averages or averages over particular latitude belts. Short-lived cooling signatures of a few of the major volcanic eruptions are evident in the mean summertime land surface air temperatures of the Northern Hemisphere extratropics. Eruptions that took place in high northern latitudes during spring and summer produced the strongest signatures. The warming that has been observed in surface air temperatures in both hemispheres since the late 1970s is not as clearly evident in temperatures aloft. The observed pattern of temperature trends is suggestive of a secular decrease in the mean static stability of the extratropical regions of both hemispheres, perhaps in response to an increase in the optical thickness of the atmosphere.
Much of the empirical work on interdecadal climate variability has been directed toward the production of time series of surface-air temperature and sea surface temperature averaged over the globe and over the Northern and Southern Hemispheres separately. Through an extended effort at data collection and quality control, the reliability of these time series has been substantially improved, and the record length has been extended beyond a century. Gridded analyses of