Stratospheric temperature trends: Observations and model simulations

Abstract

Trends and variations in global stratospheric temperatures are an integral part of the changes occurring in the Earth's climate system. Data sets for analyzing long‐term (a decade and more) changes in the stratospheric temperatures consist of radiosonde, satellite, lidar, and rocketsonde measurements; meteorological analyses based on radiosonde and/or satellite data; and products based on assimilating observations using a general circulation model. Each of these contain varying degrees of uncertainties that influence the interpretation and significance of trends. We review the long‐term trends from approximately the mid‐1960s to the mid‐1990s period. The stratosphere has, in general, undergone considerable cooling over the past 3 decades. At northern midlatitudes the lower stratosphere (∼16–21 km) cooling over the 1979–1994 period is strikingly coherent among the various data sets with regard to magnitude and statistical significance. A substantial cooling occurs in the polar lower stratosphere during winter‐spring; however, there is a large dynamical variability in the northern polar region. The vertical profile of the annual‐mean stratospheric temperature change in the northern midlatitudes over the 1979–1994 period is robust among the different data sets, with ∼0.75 K/decade cooling in the ∼20‐ to 35‐km region and increasing cooling above (e.g., ∼2.5 K/decade at 50 km). Model investigations into the cause or causes of the observed temperature trends are also reviewed. Simulations based on the known changes in species' concentrations indicate that the depletion of lower stratospheric ozone is the major radiative factor in accounting for the 1979–1990 cooling trend in the global, annual‐mean lower stratosphere (∼0.5 to 0.6 K/decade), with a substantially lesser contribution by the well‐mixed greenhouse gases. Ozone loss is also an important causal factor in the latitude‐month pattern of the lower stratospheric cooling trend. Uncertainties arise due to incomplete knowledge of the vertical profile of ozone loss near the tropopause. In the middle and upper stratosphere, both well‐mixed greenhouse gases and ozone changes contribute in an important manner to the cooling, but model simulations underestimate the observed decadal‐scale trend. While there is a lack of reliable information on water vapor changes over the 1980s decade, satellite measurements in the early to middle 1990s indicate increases in water vapor that could be a significant contributor to the cooling of the global lower stratosphere.