The tropical stratopause in the UKMO stratospheric analyses: Evidence for a 2‐day wave and inertial circulations

Large‐scale atmospheric motions near the low‐latitude stratopause in the UK Meteorological Office stratospheric analyses during the northern hemisphere winter are investigated, focusing in particular on the 2‐day wave and inertial eddies. The 2‐day wave is a westward‐propagating planetary wave which appears recurrently in the summer subtropical upper stratosphere and mesosphere. Space‐time spectral analysis of the assimilated fields of temperature and winds, and high‐resolution transport of water vapour, reveal large‐amplitude breaking planetary waves in the summer subtropics, principally a 2‐day wave 3, and a wave 1 of a period near 7 days. A region of anomalous zonal‐mean meridional gradient of potential vorticity also exists, pointing to a barotropic instability mechanism for the origin of the 2‐day wave. By contrast, the winter subtropics are characterized by mixing on less‐than‐planetary scales during the period considered, occurring during the easterly phase of the stratospheric semi‐annual oscillation. Water vapour is mixed turbulently into gyres, and drawn in narrow filaments out of the southern hemisphere. The potential‐vorticity field displays vertically layered, zonally‐asymmetric variations. The behaviour of the model assimilated fields is supported by along‐track water vapour distribution observed by the Microwave Limb Sounder aboard the Upper Atmosphere Research Satellite which shows evidences for intermittent filamentary mixing in the northern subtropics, and a 2‐day wave signature in the summer subtropics. Based on these results, a possible mechanism connecting the initiation of the austral 2‐day wave events to strong planetary‐wave activity in the winter is proposed. The winter planetary waves provide the background conditions for development of inertial instabilities at low latitudes of the winter hemisphere, which in turn create conditions favourable for barotropic instability of the summer subtropical easterly jet.