What Causes Tidal Amplitude and Phase Changes During Sudden Stratospheric Warming?

During sudden stratospheric warming (SSW), large-scale circulation in the stratosphere undergoes dramatic changes. In undisturbed conditions, the prevailing wind system in the stratosphere is westerly (eastward) in the winter hemisphere and easterly (westward) in the summer hemisphere. During a major warming, such as that occurred in January 2009, the stratospheric zonal-mean temperature increases poleward from 60N and zonal-mean zonal wind reverses. These circulation changes in the stratosphere not only reduce the background/zonal-mean zonal wind asymmetry between the two hemispheres (through the reversal of zonal wind in the winter hemisphere), but can also redistribute the stratospheric ozone concentrations. Both changes in zonal wind and ozone have been invoked to explain the tidal wave changes during SSW, but the details and the relative contributions remain unknown. The purpose of this study is to elucidate the details of the two mechanisms and their relative contributions.

In the numerical simulation of the January 2009 SSW using the whole atmosphere model (WAM) coupled with an ionosphere model, our previous study showed that the ionosphere response corresponds well to the simulated semidiurnal tidal wave amplitude and phase changes. Therefore, detailed analysis of mechanisms causing the tidal wave changes during SSW can advance our understanding of the dynamical forcing of the lower atmosphere on the upper atmosphere and ionosphere and guide our model development for improving model forecast capability.