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A model simulation forced by prescribed ozone depletion shows strong dynamical modulation of the springtime cooling of the polar stratosphere associated with the Antarctic ozone hole. The authors find that in late spring the anomalous radiative cooling in response to ozone depletion is almost canceled above ~100 hPa by an increase in dynamical heating. Between ~300 and ~100 hPa, however, it is enhanced by a reduction in dynamical heating, resulting in the descent of the cold anomaly down to the tropopause. In early summer increased dynamical heating dominates as the radiative cooling diminishes so that the cold anomaly associated with the delayed breakup of the stratospheric vortex is reduced. The anomalous dynamical heating is driven by changes in the Brewer-Dobson circulation arising primarily from the dissipation of resolved-scale waves. The model changes are broadly consistent with trends from reanalysis and offline diagnoses of heating rates using a radiation scheme. These results help one to understand dynamically induced change in the evolution and timing of the stratospheric vortex in recent decades and will help to enable improved simulation of the Southern Hemisphere climate. [ABSTRACT FROM AUTHOR]

More than 40 years of radiosonde data from two Antarctic stations are examined for changes in the date of the final stratospheric warming that occurs each year as the vortex breaks up in spring/summer. A new measure of this date is derived that does not rely on specification of a threshold, as has been common previously. The date of final warming takes between 10 and 40 days to progress from 30 to 100 hPa and occurs 20–30 days later in the 1990s than in the 1960s. Multiple linear regression analyses of these final warming dates, and also of the vertical profile of the southern annular mode (SAM), are presented. Only a weak signal is found for a linear trend, but a significant response is found throughout the atmosphere to ozone mass deficit (OMD), representing stratospheric ozone loss. In the SAM a significant response to the combined influence of solar variability and the quasi-biennial oscillation (QBO) is also found. The seasonal evolution of these signals in the NCEP Reanalysis zonal mean temperatures is examined and their influences on final warming dates is calculated. This confirms that ozone loss is primarily responsible for the delayed warming in the lower stratosphere in recent years, but suggests that the phase of solar activity and the QBO also have an effect. The apparent downward progression of the signal of OMD, and of the combined solar activity and QBO, extends well into the upper troposphere and appears to be caused simply by a delay in the top-down breakup of the vortex. [ABSTRACT FROM AUTHOR]

The role of eddies in modulating the Southern Ocean response to the southern annular mode (SAM) is examined, using an ocean model run at multiple resolutions from coarse to eddy resolving. The high-resolution versions of the model show an increase in eddy kinetic energy that peaks 2–3 yr after a positive anomaly in the SAM index. Previous work has shown that the instantaneous temperature response to the SAM is characterized by predominant cooling south of 45°S and warming to the north. At all resolutions the model captures this temperature response. This response is also evident in the coarse-resolution implementation of the model with no eddy mixing parameterization, showing that eddies do not play an important role in the instantaneous response. On the longer time scales, an intensification of the mesoscale eddy field occurs, which causes enhanced poleward heat flux and drives warming south of the oceanic Polar Front. This warming is of greater magnitude and occurs for a longer period than the initial cooling response. The results demonstrate that this warming is surface intensified and strongest in the mixed layer. Non-eddy-resolving models are unable to capture the delayed eddy-driven temperature response to the SAM. The authors therefore question the ability of coarse-resolution models, such as those commonly used in climate simulations, to accurately represent the full impacts of the SAM on the Southern Ocean. [ABSTRACT FROM AUTHOR]

Historic upper-air observations from Antarctica are inspected to investigate the uniqueness of the vortex split in the Antarctic spring of 2002. No comprehensive meteorological observations are available prior to the International Geophysical Year (IGY) 1957/58, but stratospheric temperature observations from single stations provide evidence of varying strengths against the possibility of a split vortex in 1940 and in the early and middle 1950s. [ABSTRACT FROM AUTHOR]