Orography and SSWs: Which Mountains Matter Most?

Sudden stratospheric warmings (SSWs), when the wintertime stratospheric westerly jet reverses direction, coincident with rapid warming of high-latitude stratospheric air, are associated with tropospheric wave activity propagating upwards into the stratosphere. Orography can be a strong source of tropospheric planetary wave activity and so in this paper we study how different orographic regions contribute to the frequency of simulated SSWs. Using the Whole Atmosphere Community Climate Model (WACCM), we study the relative importance of specific regions of orography in the Northern hemisphere. The abstract figure highlights the orographic regions we focus on: the Mongolian mountains (solid black box outline) the Rocky mountains (long-dashed box outline) and the Himalaya/Tibetan plateau (short-dashed box outline). We find that Asian orography has much more impact on SSWs than North American orography. Removing just the 'Mongolian mountains' reduces the frequency of SSWs from 0.45 per year in a control simulation, to 0.1 per year, whilst removing the similar-altitude Rocky mountains has no significant impact. We present mechanisms for this substantial difference, with a focus on both the strength and wave-number of planetary waves initiated by the different topography, as well as the conditions for vertical wave propagation downstream of the different orography.