Sources and sinks of momentum in the Southern Ocean State Estimate

Abstract:

Strong, persistent winds over the Southern Ocean drive the Antarctic Circumpolar Current (ACC) on an unblocked eastward path around Antarctica. Observations and reanalyses have shown that Southern Ocean winds have increased over the past 60 years [Thompson, 2002; Swart and Fyfe, 2012]. ACC baroclinic transport appears to remain stable [Böning et al., 2008], however, suggesting that the interior mechanisms that output momentum from the Antarctic Circumpolar Current (ACC) system are counterbalancing changes in the input wind stress. Here we describe where and how momentum exits the ACC system in a six-year, eddy permitting state estimate of the Southern Ocean.

We find that 95% of the zonal momentum input via wind stress at the surface is balanced by topographic form stress across ocean ridges, while the remaining 5% is balanced via bottom friction and momentum flux divergences at the northern and southern boundaries of the analysis domain. While the time-mean zonal wind stress field exhibits a relatively uniform spatial distribution, time-mean topographic form stress concentrates at shallow ridges and across the continents that lie within the ACC latitudes -- primarily Kerguelen Plateau, the Macquarie Ridge region, and South America and the Drake Passage fracture zones -- as well as across deep basins separated by basin-scale plains. Topographic form stress can be divided into shallow and deep regimes: the shallow regime contributes most of the westward form stress that serves as a momentum sink for the ACC system, while the deep regime consists of strong eastward and westward form stresses that largely cancel in the zonal integral. The time-varying form stress signal, integrated longitudinally and over the ACC latitudes, tracks closely with the wind stress signal integrated over the same domain; at zero lag, 88% of the variance in the six-year form stress time series can be explained by the wind stress signal, indicating that changes in the integrated wind stress signal are communicated via rapid barotropic response down to the level of bottom topography.

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