Transport of QBO-induced middle stratospheric variability to the Antarctic lower stratosphere on a one year time scale

Observations of nitrous oxide (N2O) from the Microwave Limb Sounder (MLS) reveal anomalies in composition within the Antarctic winter polar vortex that are traced to transport of anomalies from the winter southern subtropical middle stratosphere one year prior. Using meteorological fields from the Modern-Era Retrospective Analysis for Research and Applications (MERRA), we show that late winter MLS N2O anomalies in the southern subtropics are primarily controlled by vertical advection anomalies. Interannual variations in that advection are induced by the secondary circulation of the Quasi-Biennial Oscillation (QBO) in the middle stratosphere. The subtropical QBO-induced anomalies, which are found from ~600-800K, are isentropically mixed throughout the southern hemisphere extratropics in late spring after vortex breakdown. Correlations greater than 0.8 are found between the low latitude winter anomalies and Antarctic summer anomalies on the same isentropic surface throughout this layer. The Antarctic anomalies are trapped in the vortex as it forms during fall (April/May) and in winter, descent transports the anomalies found near 700 K in late summer to 450 K by September. The Antarctic anomalies at 450 K have a correlation greater than 0.9 with the 700 K subtropical anomalies one year earlier.

The transport pathway between the subtropical middle stratosphere and the Antarctic lower stratosphere has consequences for inorganic chlorine levels inside the Antarctic ozone hole. Using the compact correlation between N2O and inorganic chlorine (Cly), we infer Cly variability from observed N2O variability in the Antarctic vortex at levels that affect O3 depletion. We find that Cly variability inside the vortex, driven by QBO-induced variability one year earlier, is up to ten times greater (~200 ppt) than the expected annual Cly decline due to the Montreal Protocol (~20 ppt). This variability was previously unquantified and represents an additional source of variability affecting ozone hole area.