a zonally asymmetric maximum centered near Australia, the Ozone Croissant (OC).
This asymmetry amplifies from August to October. Here we seek to explain the
preferred longitudinal position and late winter intensification of the OC.
Lagrangian trajectories and a detailed momentum
budget show that the Australian subtropical westerly jet (ASWJ) is due to outflow from
convection over SE Asia and Indonesia.
Air crossing the equator in this sector is strongly diluted by detrainment from thunderstorms.
This inertially neutral outflow penetrates
far into the Southern Indian Ocean (SIO), curving
counterclockwise into the entrance of the ASWJ.
The longitudinal configuration of monsoon outflow determines the zonal extent
of the ASWJ and the position and strength
of the Southern Indian Ocean High (SIO High) near its entrance.
The OC lies immediately
poleward of the ASWJ, where poleward and downward ozone transport by synoptic waves
is most vigorous.
Seasonal sharpening of the OC from August to October occurs as convection shifts to a
smaller region over Indonesia. Numerical simulations of case studies illustrate
air mass motions within synoptic waves near the tropopause.
planetary wave ridges stall preferentially in
the Australian sector, co-amplifying with the SIO High, providing a rich source of ozone
for subsidence into synoptic troughs near the tropopause.
It is argued that convection controls the strength and position of the
SIO High, which is uniquely situated to modulate the SH extratropical stratosphere.
This region seems to be a "sweet spot" for forcing the SH circulation, analogous to
North Atlantic ridges and boreal sudden warmings.
The shape of the OC is intimately linked to
the structure of the ozone hole, polar vortex temperatures, and polar stratospheric
clouds (PSCs). A brief overview of changes associated with
the El Nino Southern Oscillation (ENSO) is given.
During La Nina the polar vortex is stronger and
displaced farther off the pole, the OC is more zonally asymmetric, and there are more
PSCs and colder temperatures in the polar vortex above 16 km.
During El Nino the polar vortex is weaker, more centered on the pole, midlatitude ozone
is arranged in an annulus, and there are fewer PSCs and warmer temperatures in the polar
vortex above 16 km.
Conditions during October and during La Nina favor stalling of travelling wave two ridges,
which amplify and merge with the SIO High within SH westerlies.
This provides a regional dynamical
conduit by which tropical convection affects the SH stratospheric