Recent measurements of total ozone at British Antarctic Survey stations

The deep minimum in total ozone, the ‘ozone hole’, which is now observed each year in early spring in Antarctica, appears to be confined to the cold core of the polar vortex. The breakdown of the vortex in 1986 followed an unusual course, and produced an atypical variation of total ozone at Halley Bay. Upper-air data show that the cold core in this year was subjected to large displacement and rapid deformations. Despite these dynamic perturbations, the ozone amount within the core seems to have been scarcely affected. This is consistent with the observed behaviour of other tracers at high latitudes. The observations are most readily explained by supposing that the core of the polar vortex is maintained as a material entity or isolated air mass, even when it suffers erosion of material from its edge (as happens throughout the period leading up to the final warming). It has recently been suggested, solely on dynamical grounds, that the process of erosion is essentially one sided; the eroded material is readily mixed into middle latitudes, but the main vortex is remarkably impervious to even small-scale incursions of the surrounding air. Formerly, the timing of the final warming was very variable from year to year. Since 1979, no early final warming has occurred. The effect on monthly mean temperatures has been described by some workers as a cooling of the lower stratosphere. W hat should be said is that temperature in the core of the vortex rises more slowly now than formerly. This is not unexpected, because the heating rate in the lower stratosphere is strongly dependent on ozone amount. It is suggested that this slower warming can, by its influence on the diabatic mass circulation, affect ozone amounts outside the vortex. The limited evidence available suggests that no significant depletion of ozone occurs in darkness. However, by the time that the Sun is high enough to permit frequent measurements of ozone the loss rate is high and remains so until the equinox. It then diminishes rapidly, and the minimum value of ozone is attained by mid-October, well before the final warming of the lower stratosphere. This suggests a greater degree of solar control than is evinced by other aspects of vortex behaviour. It is inferred that the depletion of ozone arises largely from chemical sinks, and some reactions likely to be important are discussed. Attention is drawn to the photochemically labile reservoirs HOCl and NO3. Competitive reactions between their photoproducts determine critically the persistence, or otherwise, of an HOx, catalytic cycle in early spring.

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