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Frank Lunkeit and Yorck von Detten

Abstract

A large number of uncoupled integrations with an idealized general circulation model are carried out to investigate the linearity of the atmospheric quasi-equilibrium response to North Atlantic sea surface temperature (SST) anomalies related to interdecadal variability. A statistical approach is used to judge the linearity of the temporally averaged global atmospheric response with respect to the amplitudes of the SST anomaly patterns. The results indicate that the assumption of a linear correlation of the corresponding magnitudes is, in general, not valid. Since the model, though idealized, provides all the characteristics of a full general circulation model (GCM), some important conclusions can be drawn for the design and interpretation of GCM sensitivity experiments.

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Yorck von Detten and Joseph Egger

Abstract

It has been demonstrated in Part I of this paper that synoptic-scale waves propagating around Antarctica are modified by the Antarctic topography such that they transport westerly angular momentum out of Antarctica. These transports are necessary to maintain a vigorous slope wind regime, which generates westerly angular momentum through surface friction. The results of Part I are based on a quasi-barotropic wave-mean flow interaction model where the mean flow equations are averaged over the Antarctic domain. Frequency and wave-length of the waves are prescribed according to observations. Here, the authors relax many of the constraints imposed in Part I. The model atmosphere is now fully three-dimensional and extends from the South Pole to 30°S. A wave perturbation is inserted in the baroclinic zone to the north of Antarctica. Initially the wave grows due to baroclinic instability, while cooling at the slopes induces downslope winds. After an initial phase of intensification the slope wind regime begins to decay, while the maturing depression moves toward Antarctica. However, the circulation of this occluding low is altered by the Antarctic topography such that substantial northward transport of angular momentum is induced over Antarctica. The slope winds intensify as soon as these transient fluxes affect the upper-tropospheric polar vortex. This demonstrates that topographic wave modification can be quite effective in supporting the Antarctic downslope wind regime in a fully three-dimensional baroclinic atmosphere.

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