Observations of subtropical air in the European mid-latitude lower stratosphere

Evidence of two wave-breaking events, during which subtropical air was tran sported into the northern mid latitudes, is presented. The first event took place in January 1997 when ozone mixing ratios of the order of 120 parts p er billion (10(9)) by volume (p.p.b.v.) were measured in the lower stratosp here above Aberystwyth (52.4 degrees N, 4.1 degrees W). The amount of subtr opical air involved was 13 x 10(14) kg. A second, more dramatic, wave-break ing event took place in March 1997 involving 36 x 10(14) kg of air. This ca se contained a region with potential vorticity less than or equal to 2 pote ntial vorticity units (1 PVU = 1 x 10(-6) K kg(-1) m(2)s(-1)), consistent w ith ozone mixing ratios of 80 p.p.b.v. observed at Aberystwyth at 365 K. Tr ajectory analysis showed that this air was advected northwards from near th e subtropical tropopause. The extent to which irreversible transport into t he mid-latitude lower stratosphere followed bath events is evaluated using trajectory analysis and potential-vorticity evolution.