Quasi‐horizontal transport and mixing in the Antarctic stratosphere

Abstract

The quasi‐horizontal transport and mixing properties of the Antarctic stratosphere are investigated with a semi‐Lagrangian transport model and a “contour advection” technique for the winter and spring of 1992 using analyzed winds from the United Kingdom Meteorological Office data assimilation system. Transport calculations show that passive tracers are well mixed inside the polar vortex as well as in the midlatitude “surf zone.” At the vortex edge, strong radial gradients in the tracer fields are well preserved, and their evolutions follow that of the potential vorticity until some time after the breakdown of the polar vortex. In the middle stratosphere there is little tracer exchange across the vortex edge in August and September. Some vortex air is eroded into the surf zone in filamentary form in October, and very strong exchange of air occurs between high and middle latitudes in November. In the lower stratosphere the vortex is not so isolated from the midlatitudes as in the middle stratosphere, and there is more mass exchange across the vortex edge. Calculations of the lengthening of material contours using the contour advection technique show that in the middle stratosphere, strong stirring (i.e., stretching and folding of material elements) occurs in the inner vortex, while the strongest stirring occurs in the midlatitude surf zone and the weakest occurs at the vortex edge. In the lower stratosphere, strong stirring occurs in the inner vortex. Stirring is moderate at the vortex edge and in the midlatitudes.