The Area of the Stratospheric Polar Vortex as a Diagnostic for Tracer Transport on an Isentropic Surface

Abstract

Data retrieved from the LIMS (Limb lnfrared Monitor of the Stratosphere) experiment are used 10 calculate daily isentropic distributions of Ertel's potential vorticity, ozone, water vapor and nitric acid at the 850 K level in the Northern Hemisphere stratosphere for the period 25 October 1978 through 2 April 1979. Systematic redistributions of the quasi-conservative tracers are investigated by following the evolutions of the horizontal projection of the areas enclosed by isopleths of tracer on the isentropic surface. If the horizontal velocity is nondivergent on an isentropic surface, the areas change in response to nonconservative processes and /or irreversible mixing to unresolvable scales and so provide a diagnostic for quantifying the net cited of these two processes. The effects of the seasonal variation of the solar heating on the areas are identified from the evolutions of the hemispheric means and, for the potential vorticity, from a comparison with an annual Mile integration of a zonally symmetric, general circulation model. Superimposed on the seasonal trends are changes in areas on shorter time scales, and the LIMS potential vorticity, ozone and water vapor distributions each show the distinctive ?surf-zone, main?,vortex structure? described by McIntyre and Palmer. As winter progresses the main vortex decreases in size while the surf zone expands. The evidence of the observations, combined with estimates of the strength of the radiative processes acting on the potential vorticity field, indicates fairly convincingly that irreversible mixing is an important mechanism involved in the formation of the surf-zone, main-vortex structure, and the subsequent erosion in size of the vortex. In addition, there is evidence of strong diabatic cross-isentropic transport of air parcels in the surf zone acting to restore the large-scale gradients destroyed by the mixing. The only LIMS measured constituent for which mixing was not always the dominant mechanism of redistribution was nitric acid, and it is speculated that the effects of dynamically induced changes to the effective sources and sinks of nitric acid on the 850 K surface are overshadowing other processes, at least in late January and February. Implications to tracer transport studies are examined by using the isentropic potential vorticity field as a basis for calculating low resolution approximations to the Lagrangian-mean tracer mixing ratios. The results demonstrate the feasibility of the approach to the longer-species but indicate a need for further research to distinguish between dynamical and radiactive/photochemical effects.