Sack Lunch Seminar (SLS)

Louis-Philippe Nadeau, CIMS, NYU, presents "Dynamics and Transport of a Quasigeostrophic Circumpolar Current."

Attempts to describe what sets wind-driven circumpolar transport of the Antarctic Circumpolar Current (ACC) alternately appeal to either basin or channel dynamics. In the latter case, one typically tries to relate the transport to the strength of the wind stress; for example, averaged over the Drake Passage latitude band or along the path of the current. Basin theories, on the other hand, try to relate the eastward transport around Antarctica to the wind stress curl, e.g., to the southward transport into Drake Passage latitudes. Here we consider eddy-rich simulations of a wind-driven quasigeostrophic model in an idealized Southern Ocean. For weak forcing, the transport is well described as a sum of channel and basin components. Our main focus is on stronger forcing. In this regime, transport saturates due to an increasing eddy-driven recirculation. Moreover, the vertically integrated time mean circumpolar streamlines are found to stem from a Sverdrup-like interior. The Sverdrup flux into Drake Passage latitudes can then be thought of as one part that feeds the circumpolar current and another that is associated with the recirculation.

A simple analytic model is developed to predict the transport evolution with the wind stress amplitude. The vertical distribution of the flow is obtained using the geometry of the geostrophic contours (or characteristics), key to predicting the occurrence of the transport saturation.

Numerical simulations in large domains are carried over a wide range of parameters. The simulations using a reference zonal wind stress profile agree qualitatively with the analytic model. However, quantitative discrepancies are observed in the saturation regime. The addition of a topographic continental ridge along the western boundary increases the strength of the recirculation and reduces the circumpolar transport. A similar effect is observed for decreasing bottom drag. Increasing a zero-curl eastward wind stress reduces the upper layer expression of the recirculation and increases the transport. Increasing the curl-containing portion of the forcing (while holding the mid-channel stress constant) increases the recirculation and decreases the transport. Increasing the horizontal and vertical resolution give rise to topographically-driven inertial recirculation blocking Drake Passage and reduces the transport. This behavior disappears, however, when realistic topography is used. In this context, the numerical results agree well with the predictions of the analytic model.