Millimeter wave spectroscopic measurements over the South Pole. 2. An 11-month cycle of stratospheric ozone observations during 1993-1994

A quasi-continuous record of ozone profiles throughout the stratosphere over the South Pole has been obtained over an 11-month cycle, from February 1993 to January 1994. This record includes the first winter measurements of ozone profiles in the altitude region above ∼30 km. Observations were made approximately every 3 days, using a high-sensitivity millimeter wave spectrometer to quantitatively measure the pressure-broadened ozone rotational emission line at 276.923 GHz. Vertical mixing ratio profiles have been derived from pressure-broadened lineshapes by a deconvolution technique. A number of interesting features are present. We find a persistent double-peaked structure in the mixing ratio profiles, lasting through most of the winter period until the remains of the lower peak are destroyed by spring “ozone hole” chemistry. A new low-altitude peak is reformed in December as the vortex breaks up. With the aid of circumpolar UARS/MLS ozone maps, we interpret the lower peak as due to transport from ozone-rich regions near the edge of the continent, while the profile from ∼30 km upward, composing the “trough” region and upper peak, appears to be the result of normal polar summer photochemistry. This double-peaked structure then becomes “fossilized” within the strong, isolated, fall-winter vortex. The mixing ratio of the upper peak increases after polar sunset, which we interpret as due to poleward mixing causing an erasure of the negative poleward gradient maintained by photochemistry before sunset. Mixing ratio isopleths show a relatively steady downward trend for a 3-month period after the winter vortex pattern is established, preceded by rapid variations in ozone mixing ratios over the 20- to 40-km range. Downward transport rates derived from isopleth slopes in the upper stratosphere are significantly smaller than vertical transport derived from theoretical studies, and we propose an explanation for this discrepancy based on ozone flow from the mesosphere. Descent rates determined from ozone isopleths in the midstratosphere (25 to 35 km) are shown to be in good agreement with recent model estimates of downward transport in the winter vortex, and with the mid to lower stratospheric descent rate inferred from our own South Pole measurements of N2O. Total column measurements are in generally good agreement with those derived from a Dobson photospectrometer at the pole and from local ozonesonde measurements. All three indicate there was no significant increase in total ozone over the pole during the winter of 1993. The onset of the spring ozone hole over the pole was evident by mid- to late-August, well before local stratospheric sunrise on September 11, indicating relatively rapid poleward transport of ozone-depleted air from sunlit regions of the vortex during this period.