Impact of Planetary Wave Reflection on Polar Stratospheric Ozone

The dynamical linkage between the stratosphere and troposphere is dominated by planetary Rossby waves, which are generated in the troposphere by orographic and/or non-orographic forcing. Planetary waves propagate upward into the stratosphere where they either dissipate (often manifested as a sudden stratospheric warming) and initiate downward zonal-mean response, or they are reflected downward toward the troposphere. It is well established that variable wintertime planetary wave forcing in the stratosphere controls the variability of Arctic stratospheric ozone through changes in the strength of the polar vortex and the residual circulation. While most previous studies focused on the variations in upward wave flux entering the lower stratosphere, the impact of downward planetary wave reflection on ozone remains unclear. In this study, we seek to more thoroughly investigate the mechanisms by which downward wave refletion affects polar stratospheric ozone. Utilizing the MERRA2 reanalysis and a fully coupled chemistry–climate simulation with the Community Earth System Model (CESM1(WACCM)) of the National Center for Atmospheric Research (NCAR), we find two downward wave reflection effects on ozone: (1) the direct effect in which the residual circulation is weakened during winter, reducing the typical increase of ozone due to upward planetary wave events and (2) the indirect effect in which the modification of polar temperature during winter affects the amount of ozone destruction in spring.

Winter seasons dominated by downward wave reflection events (i.e., reflective winters) are characterized by lower Arctic ozone concentration, while seasons dominated by increased upward wave events (i.e., absorptive winters) are characterized by relatively higher ozone concentration. This behavior is consistent with the cumulative effects of downward and upward planetary wave events on polar stratospheric ozone via the residual circulation and the polar temperature in winter. The results establish a new perspective on dynamical processes controlling stratospheric ozone variability in the Arctic by highlighting the key role of wave reflection.