Abstract. We have developed objective criteria for choosing the location of the northern hemisphere polar vortex boundary region and the onset and breakup dates of the vortex. By determining the distribution of Ertel's potential vorticity (Epv) on equivalent latitudes, we define the vortex edge as the location of maximum gradient of Epv constrained by the location of the maximum wind jet calculated along Epv isolines. We define the vortex boundary region to be at the local maximum convex and concave curvature in the Epv distribution surrounding the edge. We have determined that the onset and breakup dates of the vortex on the 450 K isentropic surface occur when the maximum wind speed calculated along Epv isolines rises above and falls below approximately 15.2 m s-•. We use 1992-1993 as a test case to study the onset and breakup periods, and we find that the increase of polar vortex Epv values is associated with the dominance of the term in the potential vorticity equation involving the movement of air through the surface due to the diabatic circulation. We also find that the decrease is associated with the dominance of the term involving radiatively induced changes in the stability of the atmosphere.

Abstract. The temperature of the polar lower stratosphere during spring is the key factor in changing the magnitude of ozone loss in the Arctic polar vortex. In this paper, we quantitatively demonstrate that the polar lower stratospheric temperature is primarily controlled by planetary-scale waves. We use National Centers for Environmental Prediction/National Center for Atmospheric Research reanalysis data coveting the last two decades to investigate how these planetary waves are connected to polar lower stratospheric temperatures. In particular, we show that the tropospheric eddy heat flux in middle to late winter (January-February) is highly correlated with the mean polar stratospheric temperature during March. These planetary waves are forced by both thermal and orographic processes in the troposphere and propagate into the stratosphere in the middle and high latitudes. Strong midwinter planetary wave forcing leads to a warmer spring Arctic lower stratosphere in early spring, while weak midwinter forcing leads to cooler spring Arctic temperatures. In addition, this planetary wave driving also has a strong impact on the strength of the polar vortex.

Reconstruction of the Airborne Antarctic Ozone Experiment and Airborne Arctic Stratosphere Expedition aircraft constituent observations, radiative heating rate computations, and trajectory calculations are used to generate comparative pictures of the 1987 southern hemisphere (SH) late winter and 1989 northern hemisphere (NH) mid‐winter, lower stratospheric, polar vortices. Overall, both polar vortices define a region of highly isolated air, where the exchange of trace gases occurs principally at the vortex edge through erosional wave activity. Aircraft measurement showed that (1) between 50 and 100 mbar, horizontally stratified long‐lived tracers such as N2O are displaced downward 2–3 km on the cyclonic (poleward) side of the jet with the meridional tracer gradient sharpest at the jet core. (2) Eddy mixing rates, computed using parcel ensemble statistics, are an order of magnitude or more lower on the cyclonic side of the jet compared to those on the anticyclonic side. (3) Poleward zonal mean meridional flow on the anticyclonic side of the jet terminates in a descent zone at the jet core. Despite the similarities between the SH and NH winter vortices, there are important differences. During the aircraft campaign periods, the SH vortex jet core was located roughly 8°–10° equatorward of its NH counterpart after pole centering. As a result of the larger size of the SH vortex, the dynamical heating associated with the jet core descent zone is displaced further from the pole. The SH polar vortex can therefore approach radiative equilibrium temperatures over a comparatively larger area than the NH vortex. The subsequent widespread formation of polar stratospheric clouds within the much colder SH vortex core gives rise to the interhemispheric differences in the reconstructed H2O, NOy, ClO, and O3, species which are affected by polar stratospheric clouds.

A radiation model, together with National Meteorological Center temperature observations, was used to compute daily net heating rates in the northern hemisphere (NH) for the Arctic late fall and winter periods of both 1988–1989 and 1991–1992 and in the southern hemisphere (SH) for the Antarctic fall and winters of 1987 and 1992. The heating rates were interpolated to potential temperature (θ) surfaces between 400 K and 2000 K and averaged within the polar vortex, the boundary of which was determined by the maximum gradient in potential vorticity. The averaged heating rates were used in a one‐dimensional vortex interior descent model to compute the change in potential temperature with time of air parcels initialized at various θ values, as well as to compute the descent in log pressure coordinates. In the NH vortex, air parcels which were initialized at 18 km on November 1, descended about 6 km by March 21, while air initially at 25 km descended 9 km in the same time period. This represents an average descent rate in the lower stratosphere of 1.3 to 2 km per month. Air initialized at 50 km descended 27 km between November 1 and March 21. In the SH vortex, parcels initialized at 18 km on March 1, descended 3 km, while air at 25 km descended 5–7 km by the end of October. This is equivalent to an average descent in the lower stratosphere of 0.4 to 0.9 km per month during this 8‐month period. Air initialized at 52 km descended 26–29 km between March 1 and October 31. In both the NH and the SH, computed descent rates increased markedly with height. The descent for the NH winter of 1992–1993 and the SH winter of 1992 computed with a three‐dimensional trajectory model using the same radiation code was within 1 to 2 km of that calculated by the one‐dimensional model, thus validating the vortex averaging procedure. The computed descent rates generally agree well with observations of long‐lived tracers, thus validating the radiative transfer model.

The NASA Goddard Space Flight Center (GSFC) two‐dimensional (2‐D) model of stratospheric transport and photochemistry has been used to predict ozone changes that have occurred in the past 20 years from anthropogenic chlorine and bromine emissions, solar cycle ultraviolet flux variations, the changing sulfate aerosol abundance due to several volcanic eruptions including the major eruptions of El Chichón and Mount Pinatubo, solar proton events (SPEs), and galactic cosmic rays (GCRs). The same linear regression technique has been used to derive profile and total ozone trends from both measurements and the GSFC model. Derived 2‐D model ozone profile trends are similar in shape to the Solar Backscattered Ultraviolet (SBUV) and SBUV/2 trends with highest percentage decreases in the upper stratosphere at the highest latitudes. The general magnitude of the derived 2‐D model upper stratospheric negative ozone trend is larger than the trends derived from the observations, especially in the northern hemisphere. The derived 2‐D model negative trend in the lower stratosphere at middle northern latitudes is less than the measured trend. The derived 2‐D model total ozone trends are small in the tropics and larger at middle and high latitudes, a pattern that is very similar to the Total Ozone Mapping Spectrometer (TOMS) derived trends. The differences between the derived 2‐D model and TOMS trends are generally within 1–2% in the northern hemisphere and the tropics. The derived 2‐D model trends are generally more in southern middle and high latitudes by 2–4%. Our 2‐D model predictions are also compared with the temporal variations in total ozone averaged between 65°S and 65°N over the TOMS observing period (1979–1993). Inclusion of anthropogenic chlorine and bromine increases, solar cycle ultraviolet flux variations, and the changing sulfate aerosol area abundance into our model captures much of the observed TOMS global total ozone changes. The model simulations predict a decrease in ozone of about 4% from 1979 to 1995 due to the chlorine and bromine increases. The changing sulfate aerosol abundances were computed to significantly affect ozone and result in a maximum decrease of about 2.8% in 1992 in the annually averaged almost global total ozone (AAGTO) computed between 65°S and 65°N. Solar ultraviolet flux variations are calculated to provide a moderate perturbation to the AAGTO over the solar cycle by a maximum of ±0.6% (about 1.2% from solar maximum to minimum). Effects from SPEs are relatively small, with a predicted maximum AAGTO decrease of 0.22% in 1990 after the extremely large events of October 1989. GCRs are computed to cause relatively minuscule variations of a maximum of + 0.02% in AAGTO over a solar cycle.