ALBERT

Description: Downward wave coupling (DWC) is an important process that characterizes the dynamical coupling between the stratosphere and troposphere via planetary wave reflection. A recent modeling study indicated that natural forcing factors, including sea-surface temperature variability and quasi-biennial oscillation, influence DWC and the associated surface impact in the Northern Hemisphere (NH). In light of this, we further investigate how DWC in the NH is affected by anthropogenic forcings, using a fully coupled chemistry-climate model CESM1 (WACCM). The results indicate that the occurrence of DWC is significantly suppressed in the future, starting later in the seasonal cycle, with more events concentrated in late winter (February-March). The future decrease in DWC events is associated with enhanced wave absorption in the stratosphere due to increased greenhouse gases. The enhanced wave absorption is manifest as more absorbing types of stratospheric sudden warmings, with more events concentrated in early winter. This early winter condition leads to a delay in the development of the upper stratospheric reflecting surface, resulting in a shift in the seasonal cycle of DWC towards late winter. The tropospheric responses to DWC events in the future exhibit different spatial patterns compared to those of the past. In the North Atlantic sector, DWC-induced circulation changes are characterized by a poleward shift and an eastward extension of the tropospheric jet, while in the North Pacific sector, the circulation changes are characterized by a weakening of the tropospheric jet. These responses are consistent with a change in the pattern of DWC-induced synoptic-scale eddy-mean flow interaction.