Evaluating the impact of changes in stratospheric circulation on ozone trends and recovery

Stratospheric ozone amounts are sensitive to changes in composition and to changes in the atmospheric circulation. Trends in total ozone may be chemical or dynamical in origin. An uncertainty in deducing the ozone trend due to changes in stratospheric chlorine and bromine from total ozone observations arises from the difficulty of untangling the ozone changes due to changes in the stratospheric residual circulation and transport from the contribution of photochemical processes. We use model simulations to address this issue and develop metrics of circulation change that can be used to separate chemical and transport trends in ozone.

We analyze fully-coupled simulations using the GEOS-4 Chemistry Climate Model (GEOS CCM), and off-line simulations using the GSFC Chemistry and Transport Model (CTM) with input meteorological fields from the GEOS-4 general circulation model (GEOS-4 GCM). Simulations with GEOS-4 CCM and with the CTM were made with time varying chlorine sources and also with constant chlorine at 1960 values. The residual circulation speeds up in these simulations, including CTM simulations with GEOS-4 GCM fields calculated with fixed ozone climatology and the GEOS CCM simulation with constant, low chlorine.

The speed-up in the residual circulation is accompanied by a decrease in the mean age and could cause a trend in stratospheric ozone. In addition to better characterizing the errors in ozone trend determination due to the tangling of chemical and dynamical contributions, this analysis will seek signatures of the speed-up in the residual circulation arising from changes in greenhouse gases and sea surface temperatures. This speed-up is a common feature in chemistry-climate models.