Stratospheric temperature trends: Observations and model simulations

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 temper atures consist of radiosonde, satellite, lidar, and rocketsonde measurement s; meteorological analyses based on radiosonde and/or satellite data; and p roducts based on assimilating observations using a general circulation mode l. Each of these contain varying degrees of uncertainties that influence th e 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 (similar to 16-21 km) cooli ng over the 1979-1994 period is strikingly coherent among the various data sets with regard to magnitude and statistical significance. A substantial c ooling 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 differ ent data sets, with similar to0.75 K/decade cooling in the similar to 20- t o 35-km region and increasing cooling above (e.g., similar to2.5 K/decade a t 50 km). Model investigations into the cause or causes of the observed tem perature trends are also reviewed, Simulations based on the known changes i n species' concentrations indicate that the depiction of lower stratospheri c ozone is the major radiative factor in accounting for the 1979-1990 cooli ng trend in the global, annual-mean lower stratosphere (similar to0.5 to 0. 6 K/decade), with a substantially lesser contribution by the well-mixed gre enhouse gases. Ozone loss is also an important causal factor in the latitud e-month pattern of the lower stratospheric cooling trend. Uncertainties ari se due to incomplete knowledge of the vertical profile of ozone loss near t he tropopause. In the middle and upper stratosphere, both well-mixed greenh ouse gases and ozone changes contribute in an important manner to the cooli ng, but model simulations underestimate the observed decadal-scale trend. W hile there is a lack of reliable information on water vapor changes over th e 1980s decade, satellite measurements in the early to middle 1990s indicat e increases in water vapor that could be a significant contributor to the c ooling of the global lower stratosphere.