It is known from small sets of tide gauges that sub‐surface pressure (sea level corrected for the inverse barometer effect) around Antarctica varies coherently around about half of the continent, and that this coherent signal is related to atmospheric forcing in the form of the Antarctic Oscillation, or Southern Hemisphere Annular Mode. We here confirm that this coherence extends to a more extensive network of tide gauges, and to parts of the continental shelf far from the shore, as measured by bottom pressure gauges. We use time series from an eddy‐permitting ocean model with realistic forcing to relate the coherent mode to fluctuations in transport through Drake Passage, and confirm, using a 1° resolution barotropic model, that the fluctuations are predominantly due to barotropic dynamics, although baroclinic dynamics are expected to play an increasing role at interannual timescales.

We present the first direct evidence that interannual changes in ocean transport through Drake Passage are forced by variability in the Southern Annular Mode (SAM). This evidence is derived from two decades (1980s and 1990s) of subsurface pressure measurements from the tide gauge at Faraday station (western Antarctic Peninsula), combined with the output of an ocean general circulation model. In recent decades, the SAM has moved toward a higher‐index state (stronger circumpolar winds); this trend is not simply monotonic, but is the product of a long‐term change in the seasonality of the SAM. Whilst we cannot address directly the effect of the long‐term trend on circumpolar transport, bottom pressure data from Drake Passage during the 1990s demonstrate that ocean transport showed the same changes in seasonality as did the SAM. This offers a mechanism for atmospheric climate change to influence directly the large‐scale ocean circulation.

[1] The effect of ocean self-attraction and loading (SAL) is considered in a global, barotropic ocean model forced by atmospheric wind stress, atmospheric pressure, and tidal forcing. Periods shorter than 7 days are considered. The model is integrated with a proper calculation of these effects in terms of a (very expensive) Green's function convolution at each time step. SAL effects produce a perturbation of typically about 10% of the computed ocean bottom pressure, but much more in places, for both tidal and atmospherically forced motions. An investigation into simple parameterizations of these effects by means of a term proportional to local bottom pressure reveals the following results: For tides, the best coefficient is systematically dependent on depth, ranging from less than 0.08 in shallow water to about 0.12 in the deepest water, and incorporating this effect improves the parameterization. For atmospherically forced motions, there is still some effect of depth but more an effect of latitude (increasingly so for longer periods), regional variation is greater, and correlation between the SAL effect and bottom pressure is weaker. Parameterization with a constant coefficient of 0.1 reduces errors due to failing to include SAL by about 30%. To do better than this, a proper scale-dependent representation of SAL must be used.

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[1] The exchange of atmospheric plus oceanic mass between ocean basins is investigated using a global barotropic ocean model. We find two particular cases of exchange between two basins. At periods of 4-6 days, the exchange is between the Atlantic and Pacific basins, and represents a known oscillation forced by atmospheric pressure. This mode represents a failure of the inverse-barometer relationship due to the large scale and high frequency of atmospheric forcing, and the presence of continents. Significant exchange between Atlantic and Pacific also occurs at longer periods. The second case is most prominent at periods longer than 30 days (strongest at periods longer than 100 days), and represents a mass exchange between the Southern Ocean and the Pacific. The Southern Ocean part of this exchange is clearly related to the Southern Mode of fluctuations in Antarctic circumpolar transport, forced by Southern Ocean wind stress. The reason for the exchange being with the Pacific rather than other basins is explored, and is found to be related to the balance of wind stress by form stress in Drake Passage: exchange with the Atlantic and Indian oceans becomes dominant if Drake Passage topography is removed. While recognizing the limitations of a barotropic model, we contend that it is necessary to understand the barotropic adjustment process in order to make sense of longer timescale processes. Accordingly, we end with speculation on the possible importance of the barotropic results for global sea level and tropical dynamics.