Diagnosing observed extratropical stationary wave changes in boreal winter

Stationary waves are time-averaged zonally asymmetric component of the climatological mean atmospheric circulation, primarily due to the unevenly distributed topography and diabatic heating. Stationary waves are subject to influence from long-term external forcing. In this study, the temporal evolution of the winter (January) Northern Hemisphere stationary waves during 1961-2020 is diagnosed with the fifth generation European Centre for Medium-range Weather Forecasts reanalysis data (ERA5), which shows an overall strengthening in amplitude and an eastward shift in phase. A stationary wave model is used to attribute the stationary wave response to changes in the zonal mean basic state (Delta ZM) and the zonally asymmetric diabatic heating forcing ( Delta q* ). The pattern of stationary wave changes is well captured by the response to Delta ZM alone, whereas the contribution of Delta q* to the amplitude increases in height and becomes dominant in the stratosphere. Delta q* is also found to be important in driving stationary wave changes in the North Pacific and Western Europe regions. Furthermore, changes in tropospheric stationary waves are probably a result of internal variability, whereas stratospheric changes are more likely to be driven by external forcing.

Automated summary

This study diagnosed the temporal evolution of winter Northern Hemisphere stationary waves during 1961-2020 using ERA5 data, showing an overall strengthening in amplitude and an eastward shift in phase. The changes in stationary waves are primarily driven by the zonal mean basic state and the zonally asymmetric diabatic heating forcing.