How turbulent mountain stress influences sudden stratospheric warming ocurrence in WACCM

The implementation of the Turbulent Mountain Stress (TMS) parametrization in the Whole Atmospheric Community Climate Model (WACCM) is found to be critical to obtain a realistic Sudden Stratospheric Warming (SSW) frequency in the Northern Hemisphere. Comparing two 50-year simluations, one with TMS (TMS-on) and one without (TMS-off) reveals lower than observed SSW frequency in TMS-off from December to February, while in March both simulations show SSW frequencies comparable to reanalysis. Meridional eddy heat fluxes in the lower stratosphere are stronger in TMS-on than in TMS-off, except in March. These differences are accompanied by increased orographic gravity wave drag (OGWD) in TMS-off that comes mainly from the Himalayas and the Rocky Mountains in response to stronger surface winds. Two different mechanisms of how planetary and GWs interact are identified in the simulations. In the lower stratosphere, enhanced dissipation of GWs in TMS-off modifies the subtropical jet and thus the conditions for refraction of planetary waves. In early winter, wave geometry diagnostics shows waveguides formation from 55N to 75N in TMS-on, enhancing wave propagation to the polar vortex. On the contrary, vertical propagation in TMS-off is in inhibited above the lower stratosphere and confined to latitudes south of 50N. Compensation between resolved and parametrized GWs is also observed, leading to weaker Eliassen-Palm flux divergence in response to stronger OGWD in TMS-off. In late winter, conditions for propagation are similar in both simulations by late winter, which explains the reduced TMS-off bias in the frequency of March SSWs.