Transport of near-tropopause air into the lower midlatitude stratosphere

During the last week of January 1992 ozonesonde observations over Europe revealed a layer of very low ozone concentrations in the stratosphere - below 100 parts per billion (199) by volume in the potential-temperature range 360-380 K. A coincident lidar observation revealed that the air was virtually free of volcanic aerosol, which filled the lower stratosphere at the time. The layer corresponded well with low potential vorticity (PV) in ECMWF analysis fields. Trajectory calculations confirmed a subtropical origin for the layer, and PV fields suggest that it was formed in a streamer of low-PV air drawn from the troposphere over North America as the sub-tropical jet stream turned northwards on 25 January. Ozone profiles on the equatorward flank of the subtropical jet stream contain similar mixing ratios to those seen in the layer over Europe in the same potential-temperature range. The mass of the layer at its maximum extent is estimated as 6 × 1015 kg. Most of the air in the layer eventually returned to the subtropics after mixing with ambient midlatitude air.