Abstract
AbstractGravity waves (GWs) are important for coupling the mesosphere to the lower atmosphere during sudden stratospheric warmings (SSWs). Here, a minor SSW is internally generated in a simulation with the upper‐atmosphere configuration of the ICOsahedral Nonhydrostatic model. At a horizontal resolution of 20 km the simulation uses no GW drag parameterizations but resolves large fractions of the GW spectrum explicitly, including orographic and nonorographic sources. Consistent with previous studies, the simulated zonal‐mean stratospheric warming is accompanied by zonal‐mean mesospheric cooling. During the course of the SSW the mesospheric GW momentum flux (GWMF) turns from mainly westward to mainly eastward. Waves of large phase speed (40–80 m s ) dominate the eastward GWMF during the peak phase of the warming. The GWMF is strongest along the polar night jet axis. Parameterizations of GWs usually assume straight upward propagation, but this assumption is often not satisfied. In the case studied here, a substantial amount of the GWMF is significantly displaced horizontally between the source region and the dissipation region, implying that the local impact of GWs on the mesosphere does not need to be above their local transmission through the stratosphere. The simulation produces significant vertically misaligned anomalies between the stratosphere and mesosphere. Observations by the Microwave Limb Sounder confirm the poleward tilt with height of the polar night jet and horizontal displacements between mesospheric cooling and stratospheric warming patterns. Thus, lateral GW propagation may be required to explain the middle‐atmosphere temperature evolution in SSW events with significant zonally asymmetric anomalies.
Highlights
Sudden stratospheric warmings (SSWs) are marked by a sharp temperature increase of several tens of kelvins in the middle to upper stratosphere (30–50 km) over the course of several days (Butler et al, 2015)
The causal chain for this stratosphere-mesosphere coupling is well understood: under undisturbed winter conditions Gravity waves (GWs) propagate through stratospheric westerlies, which filter the GW pseudo-momentum flux (GWMF) to be predominantly westward in the mesosphere
By analyzing a simulated minor SSW we provide evidence that oblique propagation, which is typically neglected in GW parameterizations, is responsible for a substantial fraction of the mesospheric GWMF
Summary
Sudden stratospheric warmings (SSWs) are marked by a sharp temperature increase of several tens of kelvins in the middle to upper stratosphere (30–50 km) over the course of several days (Butler et al, 2015). TIME-GCM/CCM3 model simulations of self-generated SSWs with mesospheric cooling were analyzed by Liu and Roble (2002) They found GW-induced circulation changes in the mesosphere and lower thermosphere to have substantial impacts on the abundance of atomic oxygen. They pointed out, that the parameterization of GWs introduces some uncertainties, for instance, in the wave breaking altitudes, as GW specifications are often tuned so that the model results agree with reference climatologies. Yamashita et al (2013) argued that poleward propagation of GWs from low latitudes is important for GW variations during SSWs, even though, unlike our study, they focused on the recovery phases of elevated stratopause events Their ray tracing results are based on observed background conditions but homogeneous distributions of sources, as there was no information available on the true GW sources.
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