Strong Vertical Wind Shear as a Substantial Feature of the Tropopause Region: a 10-Year Northern Hemispheric ERA5 Study

The transition region between the troposphere and the stratosphere is a region of exceptional gradients; the temperature inversion which gives rise to the definition of the lape rate tropopause is associated with a sharp increase in static stability N2, along with restoring buoyant forces which inhibt stratosphere-troposphere exchange (STE). The existence of this transport barrier results in pronounced trace gas mixing ratio gradients of species with distinct tropospheric/stratospheric sources and/or sinks. The distribution of vertical wind gradients (vertical wind shear S2) is another substantial feature particularly in the tropopause region, because it controls the dynamic stability of the flow along with the potential for turbulent STE. Vice versa, dynamic stability limits the amount of wind shear that is sustainable by the flow. We present a climatology of the occurrence frequency distribution of strong vertical wind shear in the Northern Hemisphere UTLS region based on ECMWF ERA5 reanalysis data. The analysis identifies strong vertical wind shear above a certain treshold to be a largely tropopause-related feature which gives rise to defining a tropopause shear layer (TSL). The occurrence of the TSL is fundamentally consistent with 1) the fact that dynamic stability can be sustained by the static stability in the tropopause region and 2) the occurrence of processes in the tropopause region that result in momentum gradient sharpening. This involves large scale upper level fronts and the associated wind shear according to the thermal wind relation, vertically confined outflow of convection and large scale ascent like warm conveyor belts, and enhanced gravity wave activity associated with flow imbalances and cloud processes. We will present central features of the TSL, i.e., the relation of its geographical occurrence frequency distribution to planetary circulation features like baroclinic waves in the storm track regions, the subtropical and polar jet stream, and the tropicalWalker circulation cells. We discuss the function of the TSL concerning the efficiency of the tropopause as a transport barrier, and its potential role in the formation and maintenance of the chemical transition layer between the troposphere and the stratosphere.