Stepwise changes in stratospheric temperature

Abstract

Northern hemispheric temperature analyses for the time period July 1964 until June 1997 are used to investigate the long-term changes of the lower and middle stratosphere. Use of annual, hemispheric means removes many of the dynamical effects and allows the thermal structure to be investigated with more certainty. There is a clear cooling over the 33 years considered. It is not well described by a linear trend because of the increased cooling since 1991. These changes are consistent with those derived from independent satellite-based soundings. It is shown that the low temperatures in the 1990s are not well described by a simple statistical model including the linear trend plus the interannual variability for the period 1964/1965–1980/1981, when the stratosphere was relatively undisturbed by volcanic debris and the O₃ changes were small. Possible links between the stratospheric changes reported here and apparent changes in the thermal structure in the 1990s (reported in other studies) are discussed.