ATLANTIC SUBTROPICAL POTENTIAL VORTICITY BARRIER AS SEEN BY MEASUREMENTS OF OZONE BY AIRBUS IN-SERVICE AIRCRAFT (MOZAIC) FLIGHTS

The existence and the seasonal variability of the Atlantic subtropical potential vorticity barrier controlling the stratosphere-troposphere exchanges between the lowermost extratropical stratosphere and the upp er equatorial troposphere are investigated using Measurements of Ozone by Airbus In-Service Aircraft (MOZAIC). The methodology is based on t he relationship between strong gradients of potential vorticity and of ozone mixing ratio situated on the cyclonic-shear side of the subtrop ical jet stream. Episodes of high ozone mixing ratio sampled along MOZ AIC flight tracks over the subtropical Atlantic with mixing ratio exce eding 100 ppbv on length scales larger than 200 km on flight levels be tween 11 and 12 km are studied. A total of 154 high ozone episodes is extracted from the MOZAIC database over the period August 1994 to Apri l 1997. All these high ozone episodes are observed north of 15 degrees N and have lengths ranging from subsynoptic to synoptic scales. It is shown that this barrier effect at 15 degrees N over the central/easte rn Atlantic fits with the southernmost latitude of the subtropical jet stream during the period of interest. South of the subtropical jet st ream within the latitude band where the Intertropical Convergence Zone oscillates, tens of ozone-rich transients (high-ozone episodes with l ength scales smaller than 80 km) are sampled within the upper equatori al Atlantic troposphere (9 to 12 km). At present, the origin of these tropical ozone-rich transients is still not clear, Some outlooks are g iven to investigate the possibility that some of the ozone-rich transi ents may be interpreted as the final result of tropopause foldings and small scale mixing processes. The seasonal variability of the subtrop ical barrier is captured when determining the subtropical tropopause b reak (STB) point for each flight, that is, the southernmost latitude o f the southernmost high-ozone episode of a flight, and classifying STB points on a monthly basis. A sinusoidal evolution of STB points appea rs, reflecting the northernmost (southernmost) position of the dynamic al barrier in summer (winter) boreal months. This seasonal variability clearly agrees with that of the position of the subtropical jet strea m as derived from mean isotach analysis.