Exploring polar stratospheric cloud and ozone minihole formation: The primary importance of synoptic-scale flow perturbations

The formation of polar stratospheric clouds (PSCs) is sometimes attributed to cooling induced by mountain waves. Some examples of PSCs explained by this mechanism are found in the literature. Other studies show that the cooling producing PSCs is of synoptic scale. In this paper we use data from Polar Ozone Aerosol Measurement (POAM) II and from TIROS Operational Vertical Sounder (TOVS) showing coincident occurrences of PSCs and ozone miniholes over the sea and land. Using European Centre for Medium-Range Weather Forecasts (ECMWF) analyses, we show that when both PSCs and localized ozone minima appear, they are associated primarily with anticyclonic potential vorticity anomalies near the tropopause. The flow anomaly penetrates upward and downward, inducing an upward displacement of isentropic surfaces above and a downward displacement below, the upward and downward penetration being consistent with the deformation scale. These flow anomalies result in synoptic-scale quasi-adiabatic uplift through the lower stratosphere. The adiabatic cooling of the air masses creates the conditions for PSC formation. Coincidentally, the ozone partial pressure decreases, and the localized ozone minimum appears. Our purpose in this paper is to show that the primary PSC formation mechanism in the Arctic is the same as for minihole formation: synoptic-scale dynamics. We show three multiday sequences of PSCs and localized ozone minima. We reveal the robustness of the PSC/dynamics link by showing multiyear, monthly statistics of POAM II PSC sighting fraction compared with PSC formation temperature and isentropic geopotential.