Winter ozone fluctuations related with synoptic-scale waves over South America

Since long time ago it has been recognized the strong connection between tropospheric eddy activity and total ozone columnar content. To a large extent, synoptic-scale ozone fluctuations are produced by baroclinic disturbances in the troposphere, which are more susceptible to develop when large-scale meridional temperature gradients occur, specially during the winter season.

Using ECMWF analyses fields of standard meteorological variables and total ozone fields of Nimbus 7 TOMS version 7 data, we investigate ozone variations related with synoptic-scale wave activity during austral winter, for the period 1983-1988. In that sense, perturbations were defined as the difference between each value and the time mean for each season, thus removing any interannual variability from the series. The 300-hPa meridional wind perturbation (v') was chosen as the key variable to represent short-time scale perturbations, as its variability is concentrated in high frequencies, specially in the Southern Hemisphere.

The distribution of ozone perturbation standard deviation resembles that of mean total ozone content: maximum values are found near 55 S while decreasing to the Equator and polewards. Zonal asymmetries are also evident. A nearly-continuous band of 35 DU (Dobson units) extends over 50-60 S, with a relative minimum around 50 W while values above 40 DU locate over the southern portions of both Indian and East-Pacific Oceans.

A good agreement between regions of high (low) standard deviation of ozone perturbations and high (low) standard deviation of 300-hPa meridional wind perturbations (or storm tracks) is found at middle latitudes. In particular over the Indian Ocean, ozone standard deviation peak locates to the southeast of storm-track maximum, while over the Atlantic Ocean low ozone variability is observed over regions with low synoptic-scale wave activity.

A Rotated Extended Empirical Orthogonal Function (REEOF) analysis based on 300-hPa v' time series reveals that over southern South America and adjacent oceans, the leading winter synoptic-scale mode corresponds to a wave pattern that moves along subpolar jet latitudes. This pattern display the typical horizontal structure of baroclinic waves, with wave numbers between 4 and 5 and eastward-phase velocities of the order of 8 ms-1.

Composite fields of standard meteorological variables and total ozone exhibit the typical relationship between troughs (ridges) with the corresponding maximum (minimum) signature in total ozone fields. Upper-tropospheric circulation seems to account for the distribution of ozone mainly through horizontal advection; that is in part because of the equivalent-barotropic vertical structure of subpolar waves. In addition, vertical advection seems also important in explaining ozone distribution. Upward (downward) motions occur behind of the ridges (troughs), while downward (upward) motions are observed at the front side. Then air at the rear of the anticyclonic system is advected southward and upward, together with the uplifting of surfaces of constant ozone-mixing ratio. This effect causes ozone number density to decrease within air parcels. On the top of the ridge, vertical velocities become null and parcels undergo maximum vertical altitude, all consequently with minimum ozone ocurrence there. Once downstream, air is conduced to the northeast and downward, which leads to ozone enhacement over the trough due to horizontal advection and compression effects.