ATMOSPHERIC
CHEMISTRY RESEARCH
The Impact of Mixing within the Antarctic Stratospheric Vortex on Springtime
Ozone Loss
The wintertime, extra-tropical, stratospheric circulation is dominated by a cold, cyclonic polar vortex, disturbed by planetary-scale Rossby waves. Its isolation from the middle latitudes is due in part to the Rossby-restoring mechanism in the large scale and in part to horizontal wind shear acting at smaller scales (Juckes & McIntyre 1987; Dritschel et al. 1998).
The importance of the vortex edge as a transport barrier has motivated many studies of atmospheric transport. In this study, a new approach is taken (Haynes & Shuckburgh 1998): one adapted from a technique used to analyse satellite observations of chemical species (Nakamura & Ma 1997). Transport barriers are generally associated with weak mixing, separating regions of strong mixing. In a region of strong mixing a tracer will develop a complex geometrical structure. The approach here uses the diagnostic quantity "effective diffusivity", which characterises the geometric structure of a tracer, to deduce the mixing ability of a two-dimensional flow field. In our case, the effective diffusivity is diagnosed from a numerical simulation of a conserved tracer field. The tracer field was initialised with the potential-vorticity distribution and advected using the horizontal wind field available from meteorological analyses. Regions of strong mixing have large values of effective diffusivity and regions of weak mixing (those associated with barriers to transport) have small values.
Figure 1 shows the evolution of the effective diffusivity as a function of equivalent latitude on the 480 K isentropic surface during the 1996 winter and spring. During the winter, a broad region of weak mixing between equivalent latitudes of 58S and 68S separates regions of comparatively strong mixing in the middle latitudes and the vortex core. Air masses within this region of weak mixing will remain isolated from either the middle latitudes or the vortex core. It is appropriate to identify the equatorward edge of the region of weak mixing with the vortex edge. Thus, within the polar vortex there are two distinct regions of approximately equal area identified by their mixing characteristics. The region of strongest mixing is associated with the so-called "surf zone" in the middle latitudes. As spring progresses, the region of weak mixing persists. It is worth noting that both the width and position of this region are not fixed and change over the season. After mid-October the region starts to narrow and in late November there is a distinct increase in effective diffusivity in the region, indicating that in late November the ability of this region to act as a barrier to transport has been reduced.
The impact these mixing properties has on the distribution of ozone loss is studied using the SLIMCAT three-dimensional chemical transport model. Figure 1 shows that the 6065S equivalent latitude band is a region of weak mixing throughout winter and spring. The evolution of ozone loss which occurs within the 6065S equivalent latitude band is tracked by integrating the ozone loss tendencies within this equivalent latitude band and advecting the accumulated tracer.
Figure 2 shows the evolution of ozone loss that occurs within the 6065S equivalent latitude band. Ozone loss remains confined to the region of weak mixing until mid-October. Similar tracers for air masses which experience ozone loss in the strongly-mixed vortex core are rapidly mixed throughout the vortex core and are not confined to the equivalent latitude bands within which the ozone loss originally occurred.
Conclusions
Calculations of effective diffusivity have provided new insight into the transport processes occurring within the Antarctic stratospheric polar vortex. These calculations showed two distinct regions of approximately equal mass: a strongly-mixed vortex core; and a broad ring of weakly- mixed air extending out to the vortex edge. This broad ring of vortex air remained isolated from the vortex core between late winter and mid-spring. The weakly-mixed ring is important because ozone loss there is isolated from ozone loss in the vortex core. A simulation of the Antarctic ozone hole by a three-dimensional chemical transport model quantified the ozone loss within this ring and its isolation. It is the ozone-poor air in this isolated ring that sometimes passes over southern South America in spring, when the solar elevation becomes sufficiently high for the extra ultraviolet radiation to cause damage. Unlike the colder core, ozone loss in this isolated ring will be limited by polar stratospheric cloud amount, which may well increase as greenhouse gases increase, thereby delaying the reduction of ozone depletion episodes over southern South America beyond that expected from the Montreal Protocol.
References
M. N. Juckes & M. E. McIntyre, Nature 328, 590 (1987)
D. G. Dritschel, P. H. Haynes, M. E. McIntyre, Permeability of the stratospheric vortex edge: vortex scattering experiments and the role of shear, submitted to J. Geophys. Res., (1998)
P. H. Haynes & E. F. Shuckburgh, Effective diffusivity as a diagnostic of tropospheric and stratospheric transport, to be submitted, (1999).
N. Nakamura & J. Ma, J. Geophys. Res. 102, 25,721 (1997).
Adrian M. Lee, Peter H. Haynes and Emily F. Shuckburgh
University of Cambridge
Adrian.Lee@atm.ch.cam.ac.uk
