An Analysis of the Southern Ocean's Barotropic Response to the Wind During the Past 20 Years: Contributions to Sea-Surface Height Variations

The Southern Ocean is a major compartment of the meridional overturning circulation where large volumes of mode and intermediate waters are formed and exported from the surface. Substantial changes have occurred in the Southern Hemisphere atmospheric circulation over the past few decades with measurable impact on the properties of these water masses. A full understanding of the physical processes involved in the variability of the formation rate and properties of mode and intermediate water, and thus of the MOC is however still missing. These depend on exchanges with the atmosphere through ocean mixed-layer processes.

Although data coverage has much improved in the Southern Ocean with the development of the Argo program, Sea Surface Height (SSH) satellite-borne observations, with high space-time coverage and an archive now exceeding twenty years, remain a key source of information. However, SSH reflects changes affecting the whole water column, and is thus difficult to interpret in terms of upper ocean processes only. Furthermore, it does not provide a direct measure of heat/freshwater storage variations since convergence of mass (barotropic fluctuations) also contribute to SSH variations, the latter being particularly important at high latitudes. While we expect the barotropic signal to be larger at subseasonal timescales, it may not be negligible at longer timescales.

Here we examine the Southern Ocean’s response to the wind at timescales ranging from seasonal to interannual, using simulations from a finite-element barotropic model, assessed against altimeter observations. Simulations are in particular used to analyze the variability of the branch of Antarctic Circumpolar Current (ACC) that passes across the Kerguelen Plateau through the Fawn Trough, the transport of which was directly measured during 1 year, a series subsequently extended to 20 years thanks to the Topex/Jason altimeter archive.