The role of the winter residual circulation in the summer mesopause regions in WACCM

Abstract

Abstract. High winter planetary wave activity warms the summer polar mesopause via a link between the two hemispheres. Complex wave-mean-flow interactions take place on a global scale, involving sharpening and weakening of the summer zonal flow. Changes in the wind shear occasionally generate flow instabilities. Additionally, an altering zonal wind modifies the breaking of vertically propagating gravity waves. A crucial component for changes in the summer zonal flow is the equatorial temperature, as it modifies latitudinal gradients. Since several mechanisms drive variability in the summer zonal flow, it can be hard to distinguish which one is dominant. In the mechanism coined interhemispheric coupling, the mesospheric zonal flow is suggested to be a key player for how the summer polar mesosphere responds to planetary wave activity in the winter hemisphere. We here use the Whole Atmosphere Community Climate Model (WACCM) to investigate the role of the summer stratosphere in shaping the conditions of the summer polar mesosphere. Using composite analyses, we show that in the absence of an anomalous summer mesospheric temperature gradient between the equator and the polar region, weak planetary wave forcing in the winter would lead to a warming of the summer mesosphere region instead of a cooling, and vice versa. This is opposing the temperature signal of the interhemispheric coupling that takes place in the mesosphere, in which a cold and calm winter stratosphere goes together with a cold summer mesopause. We hereby strengthen the evidence that the variability in the summer mesopause region is mainly driven by changes in the summer mesosphere rather than in the summer stratosphere.

The Climatology of Elevated Stratopause Events in the UA‐ICON Model and the Contribution of Gravity Waves

“…First, a control (CTL) simulation was carried out where both the sub-grid scale orography (SSO) scheme and non-orographic GW scheme are used and two sensitivity tests where (a) the SSO scheme was disabled (no-SSO, hereafter) and (b) the non-orographic GW scheme was disabled (no-GWD, hereafter). Although the exclusion of GW drags results in a very strong forcing, this kind of simulation has been performed previously using other models (e.g., EMAC (Eichinger et al (2020)), WACCM (Kuilman & Karlsson, 2018) and CMAM (Ren et al, 2011)) and has been shown to be useful in understanding the importance of GW drags for middle atmospheric dynamics. In ICON, the SSO drag is treated after Lott and Miller (1997), and the non-orographic GW drag parameterization that treats the generation, propagation, and breaking of GWs from synoptic-scale flow (e.g., fronts, convection, jet streams) is based on the works of Warner and McIntyre (1996) and Scinocca (2003).…”