On the occurrence of enhanced vertical wind shear in the tropopause region: A ten year ERA5 northern hemispheric study

Abstract. A climatology of the occurrence of enhanced wind shear in the UTLS is presented, which gives rise to define a tropopause shear layer (TSL). Enhanced wind shear in the tropopause region is of interest because it can generate turbulence which can lead to cross-tropopause mixing. The analysis is based on ten years of daily northern hemispheric ECMWF ERA-5 reanalysis data. The vertical extent of the region analysed is limited to the altitudes from 1.5 km above the surface up to 25 km, to exclude the planetary boundary layer as well as enhanced wind shear in higher atmospheric layers like the mesosphere/lower thermosphere. A threshold value of S2t = 4 · 10−4 s−2 is applied, which marks the top end of the spectrum of atmospheric wind shear to focus on situations which cannot be sustained by the mean static stability in the troposphere according to linear wave theory. This subset of the vertical wind shear spectrum is analysed for its vertical, geographical, and seasonal occurrence frequency distribution. A set of metrics is defined to narrow down the relation to planetary circulation features, as well as indicators for momentum gradient sharpening mechanisms. The vertical distribution reveals that large shear values occur almost exclusively at tropopause altitudes, within a vertically confined layer of about 1–2 km extent directly above the local lapse rate tropopause (LRT). The TSL emerges as a distinct feature in the tropopause-based 10 year temporal and zonal mean climatology, spanning from the tropics to latitudes around 70° N, with average occurrence frequencies of the order of 1 %–10 %. The horizontal distribution of the tropopause based enhanced vertical wind shear exhibits distinctly separated regions of occurrence, which are generally associated with jet streams and their seasonality. At midlatitudes, enhanced wind shear values occur most frequently in regions with an elevated tropopause and at latitudes around 50° N, associated with jet streaks within northward reaching ridges of baroclinic waves. At lower latitudes in the region of the subtropical jet stream, which is mainly apparent over the East Asian continent, the occurrence frequency of enhanced tropopause-based wind shear reaches maximum values of about 30 % during winter and is tightly linked to the jet stream seasonality. The interannual variability of the occurrence frequency for enhanced wind shear might furthermore be linked to the variability of the zonal location and strength of the jet. The east-equatorial region features a bi-annual seasonality in the occurrence frequencies of tropopause based enhanced vertical wind shear. During the summer months, large areas of the tropopause region over the Indian ocean are up to 70 % of the time exposed to large values of wind shear, which can be attributed to the emergence of the tropical easterly jet. During winter, this occurrence frequency maximum shifts eastward over the maritime continent, where it is exceptionally pronounced during the 2011 la Niña year, as well as quite weak during the El Niño phases of 2010 and 2015/2016. This agrees with the atmospheric response of the Pacific Walker circulation cell in the ENSO ocean-atmosphere coupling.