On the Origin of the Southern Hemisphere Ozone Croissant

During Austral winter and spring monthly mean column ozone near 40-60S exhibits

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.

Extratropical travelling

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

circulation.