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 th is mechanism are found in the literature. Other studies show that the cooli ng producing PSCs is of synoptic scale. In this paper we use data from Pola r Ozone Aerosol Measurement (POAM) II and from TIROS Operational Vertical S ounder (TOVS) showing coincident occurrences of PSCs and ozone miniholes ov er the sea and land. Using European Centre for Medium-Range Weather Forecas ts (ECMWF) analyses, we show that when both PSCs and localized ozone minima appear, they are associated primarily with anticyclonic potential vorticit y anomalies near the tropopause. The flow anomaly penetrates upward and dow nward, inducing an upward displacement of isentropic surfaces above and a d ownward displacement below, the upward and downward penetration being consi stent with the deformation scale. These flow anomalies result in synoptic-s cale quasi-adiabatic uplift through the lower stratosphere. The adiabatic c ooling of the air masses creates the conditions for PSC formation. Coincide ntally, the ozone partial pressure decreases, and the localized ozone minim um appears. Our purpose in this paper is to show that the primary PSC forma tion mechanism in the Arctic is the same as for minihole formation: synopti c-scale dynamics. We show three multiday sequences of PSCs and localized oz one minima. We reveal the robustness of the PSC/dynamics link by showing mu ltiyear, monthly statistics of POAM II PSC sighting fraction compared with PSC formation temperature and isentropic geopotential.