The impact of stratospheric ozone changes on downward wave coupling in the Southern Hemisphere

The impact of stratospheric ozone changes on downward wave coupling between the stratosphere and troposphere in the Southern Hemisphere is investigated using a suite of Goddard Earth Observing System chemistry-climate model (GEOS CCM) simulations. Downward wave coupling occurs when planetary waves reflected in the stratosphere impact the troposphere. In reanalysis data, the climatological coupling occurs from September to December when the stratospheric basic state has a well-defined high-latitude meridional waveguide in the lower stratosphere that is bounded above by a reflecting surface, called a bounded wave geometry. Reanalysis data suggests that downward wave coupling during November-December has increased during the last three decades. The GEOS CCM simulation of the recent past captures the main features of downward wave coupling in the Southern Hemisphere. Consistent with the Modern Era Retrospective-Analysis for Research and Application (MERRA) dataset, wave coupling in the model maximizes during October-November when there is a bounded wave geometry configuration. However, the wave coupling in the model is stronger than in the MERRA dataset, and starts earlier and ends later in the seasonal cycle. The late season bias is caused by a bias in the timing of the stratospheric polar vortex breakup. Temporal changes in stratospheric ozone associated with past depletion and future recovery significantly impact downward wave coupling in the model. During the period of ozone depletion, the spring bounded wave geometry, which is favorable for downward wave coupling, extends into early summer, due to a delay in the vortex breakup date, and leads to increased downward wave coupling during November-December. During the period of ozone recovery, the stratospheric basic state during November-December shifts from a spring configuration back to a summer configuration, where waves are trapped in the troposphere, and leads to a decrease in downward wave coupling. Model simulations with chlorine fixed at 1960 values and increasing greenhouse gases show no significant changes in downward wave coupling and confirm that the changes in downward wave coupling in the model are caused by ozone changes. The results reveal a new mechanism wherein stratospheric ozone changes can affect the tropospheric circulation.