Planetary-scale tropopause folds in the southern subtropics

Daily measurements of tropospheric ozone at La Reunion Island (55 E, 21 S) in July 1998 show that ozone layers are quasi-permanent in the troposphere between 6 and 10 km with values reaching 80 to 100 ppbv. Meridional cross sections of potential vorticity reveal that these layers are related to tropopause folds beneath the subtropical jet. Folding is persistent and extends over a considerable longitude range from mid-Atlantic to mid-Pacific. We suggest that this structure is due to the convergent flow associated to the descent branch of the Hadley circulation during austral winter.