Co-occurrence of gravity waves, vertical wind shear and turbulence in the lowermost stratosphere over the North Atlantic

Abstract. This study focuses on the spatial and temporal co-occurrence of gravity waves (GWs) and of vertical shear and turbulence in the extratropical lowermost stratosphere (LMS). For this, one year of ERA5 reanalysis data is used to analyse the occurrence and variability of resolved GW momentum fluxes, vertical shear, and turbulence indices in the region of the North Atlantic. In the tropopause region GWs can lead to strong vertical wind shear and ultimately to the generation of turbulence, which may result in troposphere-stratosphere exchange and mixing and is a risk for commercial aviation. The occurrences of GWs, shear, and turbulence are analysed in terms of vertical, geographical, and seasonal occurrence frequency distribution and put in relation to the large-scale circulation and to processes related to GW appearances. The contribution of resolved GWs to shear is found to be notable, especially in the North Atlantic winter lowermost stratosphere, where vertical momentum flux convergence provides a peak zonal GW forcing up to −2 m s^-1 day^-1 around 45° N at tropopause altitudes. The prominent vertical propagation in the wintertime mid-latitudes substantially leads to the formation of belt-like structures of GW activity, as evident by momentum fluxes, and further contributes to the pronounced occurrence of shear in the LMS. Ultimately, this study discusses the role of small-scale dynamics in shaping a quasi-permanent layer of elevated shear above the extratropical tropopause and its potential to generate turbulence in this region.