No abstract

Estimates of stratospheric age from observations of long‐lived trace gases with increasing tropospheric concentrations invoke the implicit assumption that an air parcel has been transported intact from the tropopical tropopause. However, because of rapid and irreversible mixing in the stratosphere, a particular air parcel cannot be identified with one that left the troposphere at some prior time. The parcel contains a mix of air with a range of transit times, and the mean value over this range is the most appropriate definition of age. The measured tracer concentration is also a mean over the parcel, but its value depends both on the transit time distribution and the past history of the tracer in the troposphere. In principle, only if the tropospheric concentration is increasing linearly can the age be directly inferred. We illustrate these points by employing both a one‐dimensional diffusive analog of stratospheric transport, and the general circulation model (GCM) of the Goddard Institute for Space Studies (GISS). Within the limits of the GCM, we estimate the time over which tropospheric tracer concentrations must be approximately linear in order to determine stratospheric age unambiguously; the concentration of an exponentially increasing tracer is a function only of age if the growth time constant is greater than about 7 years, which is true for all the chlorofluorocarbons. More rapid source variations (for example, the annual cycle in CO2) have no such direct relationship with age.

Recent analyses have revealed simple relationships between the simultaneously measured mixing ratios of certain stratospheric constituents. In some cases, the relationship appears to be nearly linear, so that measured concentrations of one can be used to predict the other. We argue here that such relationships are to be expected for species of sufficiently long lifetime. Species whose local lifetimes are longer than quasihorizontal transport time scales are in climatological slope equilibrium, i.e., they share surfaces of constant mixing ratio, and a scatter plot of the mixing ratio of one versus that of the other collapses to a compact curve whose slope at any point is the ratio of the net global fluxes of the two species through the corresponding surface of constant mixing ratio. Species whose local lifetimes are greater than vertical transport time scales are in gradient equilibrium and their mixing ratios display a linear relationship. For species whose atmospheric lifetimes are determined by removal in the stratosphere, the slope of this relationship in the lower stratosphere can be related to the ratio of their atmospheric lifetimes. These statements are illustrated using results from a two-dimensional chemistrytransport model. IntroductionInterpretation of the observed distributions of atmospheric constituent concentrations is often confused by the complexity of transport processes. Concentrations at a fixed location, for example, may fluctuate rapidly in response to horizontal and vertical displacements of air during the passage of meteorological disturbances. Recent work has shown that much simplification can be achieved by using almost-conserved quantities to replace Eulerian coordinates. Thus Schoeberl et al. [1989] and Lair et al. 1990] were able to collate observations taken in different ocations and at different times during the recent airborne stratospheric polar missions by binning the data with respect to potential temperature and potential vorticity [see also Schoeberl and Hartmann, 1991; Schoeberl et al., 1992]. Resulting reconstructions maintained, for example, the sharp tracer gradients near the edge of the polar vortex; these gradients would have been greatly smoothed out by conventional averaging, Paper number 92JD00450 0148-0227 / 92 / 92JD-00450 $ 05.00 given the large latitudinal and temporal fluctuations in the location of this edge. The effectiveness of this technique appears to rest on the fact that potential temperature and potential vorticity may be assumed conserved for long enough to act as Lagrangian tracers during the short-term fluctuations associated with Rossby waves on the edge of the polar vortex. Then, the short-term fluctuations of the mixing ratio isopleths of long-lived tracers are almost coincident with those of potential vorticity and potential temperature. Another useful viewpoint can be obtained by using as reference a very well conserved quantity such as the mixing ratio of N20; the surfaces of constant N20 mixing ratio follow the air motions more precisely than, s...

Improvements in our understanding of transport processes in the stratosphere have progressed hand in hand with advances in understanding of stratospheric dynamics and with accumulating remote and in situ observations of the distributions of, and relationships between, stratospheric tracers. It is convenient to regard the stratosphere as being separated into four regions: the summer hemisphere, the tropics, the wintertime midlatitude ''surf zone'', and the winter polar vortex. Stratospheric transport is dominated by mean diabatic advection (upwelling in the tropics, downwelling in the surf zone and the vortex) and, especially, by rapid isentropic stirring within the surf zone. These characteristics determine the global-scale distributions of tracers, and their mutual relationships. Despite our much-improved understanding of these processes, many chemical transport models still appear to exhibit significant shortcomings in simulating stratospheric transport, as is evidenced by their tendency to underestimate the age of stratospheric air.

Abstract.We evaluate transport characteristics of two-and three-dimensional chemical transport models of the stratosphere by comparing their simulations of the mean age of stratospheric air and the propagation of annually periodic oscillations in tracer mixing ratio at the tropical tropopause into the stratosphere to inferences from in situ and satellite observations of CO2, SFe, and water vapor. The models, participants in the recent NASA "Models and Measurements II" study, display a wide range of performance. Most models propagate annual oscillations too rapidly in the vertical and overattenuate the signal. Most models also significantly underestimate mean age throughout the stratosphere, and most have at least one of several unrealistic features in their mean age contour shapes. In the lower stratosphere, model-to-model variation in N20, NO•, and CI• is well correlated with variation in mean age, and the magnitude of N O• and Cly variation is large. We conclude that model transport inaccuracies significantly affect simulations of important long-lived chemical species in the lower stratosphere.

The response of the Southern Ocean to a repeating seasonal cycle of ozone loss is studied in two coupled climate models and is found to comprise both fast and slow processes. The fast response is similar to the interannual signature of the southern annular mode (SAM) on sea surface temperature (SST), onto which the ozone hole forcing projects in the summer. It comprises enhanced northward Ekman drift, inducing negative summertime SST anomalies around Antarctica, earlier sea ice freeze-up the following winter, and northward expansion of the sea ice edge year-round. The enhanced northward Ekman drift, however, results in upwelling of warm waters from below the mixed layer in the region of seasonal sea ice. With sustained bursts of westerly winds induced by ozone hole depletion, this warming from below eventually dominates over the cooling from anomalous Ekman drift. The resulting slow time-scale response (years to decades) leads to warming of SSTs around Antarctica and ultimately a reduction in sea ice cover year-round. This two-time-scale behavior-rapid cooling followed by slow but persistent warming-is found in the two coupled models analyzed: one with an idealized geometry and the other with a complex global climate model with realistic geometry. Processes that control the time scale of the transition from cooling to warming and their uncertainties are described. Finally the implications of these results are discussed for rationalizing previous studies of the effect of the ozone hole on SST and sea ice extent.

A conceptual model of global stratospheric transport is described, based on the assumption of rapid isentropic mixing within midlatitude “surf zones” but weak mixing into the tropics. Thus the tropical region is isolated from middle latitudes, and trace species budgets there are balances between mean upwelling and local chemical sources and sinks. In middle latitudes, where long‐lived species are assumed to be in “slope equilibrium,” the budgets are more complex, being influenced by isentropic mixing, mean downwelling, entrainment from the tropics, and local chemistry. The one‐dimensional vertical flux‐gradient relation obtained in previous studies in which mixing was assumed to be global is lost in this model. Tracer correlations are compact separately in each region, with substantial differences between tropical and midlatitude relationships. The result discussed by Plumb and Ko linking the slope of the correlation diagram to net global fluxes (and thus to lifetimes) is valid in this model only at the midlatitude tropopause.

No abstract