A comparison of Northern and Southern Hemisphere cross‐tropopause ozone flux

The downward flux of ozone across the midlatitude (30°–60°) tropopause is calculated using changes in potential vorticity (PV) and a statistical relationship between the PV changes and ozone flux. This approach separates dynamical aspects of transport from the seasonal cycle of ozone in the lowermost stratosphere. The Northern Hemisphere (NH) ozone flux is larger (∼24%) than that calculated in the Southern Hemisphere (SH) during the year 2000. The midlatitude SH total horizontal area of exchange is equal to or slightly greater than the area of exchange in the NH throughout an annual cycle. The mean changes in potential vorticity of parcels near the tropopause are also similar or slightly greater in the SH, suggesting that NH and SH downward air mass transport to the troposphere are comparable. These results show that the greater NH midlatitude ozone flux is mostly due to the amount of ozone available for exchange rather than dynamical differences near the tropopause level at these latitudes.