Role of tides on the formation of the Antarctic Slope Front at the Weddell‐Scotia Confluence

Abstract

The structure of the Antarctic Slope Front (ASF) and the associated Antarctic Slope Current (ASC) on the Scotia Sea side of the Weddell-Scotia Confluence (WSC) is described using data from a hydrographic survey and three 1 year long moorings across the continental slope. The ASC in this region flows westward along isobaths with an annual mean speed of $0.2 m s 21 , with time variability dominated by the K 1 and O 1 tidal diurnal constituents, a narrowband oscillation with $2-week period attributable to the spring/neap tidal cycle, and seasonal variability. Realistic and idealized high-resolution numerical simulations are used to determine the contribution of tides to the structure of the ASF and the speed of the ASC. Two simulations forced by realistic atmospheric forcing and boundary conditions integrated with and without tidal forcing show that tidal forcing is essential to reproduce the measured ASF/ASC cross-slope structure, the time variability at our moorings, and the reduced stratification within the WSC. Two idealized simulations run with tide-only forcing, one with a homogeneous ocean and the other with initial vertical stratification that is laterally homogeneous, show that tides can generate the ASC and ASF through volume flux convergence along the slope initiated by effects including the Lagrangian component of tidal rectification and mixing at the seabed and in the stratified ocean interior. Climate models that exclude the effects of tides will not correctly represent the ASF and ASC or their influence on the injection of intermediate and dense waters from the WSC to the deep ocean.

Eddy Generation and Jet Formation via Dense Water Outflows across the Antarctic Continental Slope

“…For example, our model does not include a representation of tides, which contribute to AABW production via water mass transport by tidal residual flows and via diapycnal mixing due to breaking internal waves (Padman et al 2009). Flexas et al (2015) have shown that tides can generate ASF-like structures at the shelf break, even without surface wind forcing. Our model configuration also cannot accurately represent down-slope gravity currents, by which newly formed dense shelf water spills out onto the continental slope (e.g., Gordon et al 2004Gordon et al , 2009).…”

Stratified tidal flow over a tall ridge above and below the turning latitude

“…However, the influence of nearfield mixing on locally reducing stratification, combined with time-dependent changes in shear and stratification has the potential to reduce the Richardson number during some phases of the tide such that instabilities in the along-ridge flow may play an important role. In addition, the Lagrangian transport associated with these jets may be important for high-latitude coastal processes, transporting and mixing water masses across the shelf-break and providing a pathway for the regional transport of tracers and biological material (Flexas et al 2015).…”

Bottom Boundary Potential Vorticity Injection from an Oscillating Flow: A PV Pump

“…These PV gradients are linked to the Antarctic marginal overturning circulation, which helps to bring the warm middepth water over the shelf break toward the ice shelf grounding line. In another numerical experiment, tidal forcing is shown to be critical in reproducing the crossslope structure and time variability of the Antarctic Slope Front (ASF) and Antarctic Slope Current (ASC) along the South Scotia Ridge (Flexas et al 2015). This study showed that even without atmospheric forcing, tides can generate the observed structure and variability of the ASF and ASC through tidal rectification over the continental slope.…”

“…However, the resonance that occurs in the Ekman layer (Fig. 4) significantly amplifies the secondary circulation, which in turn leads to a peak in the PV flux for v ' f. The frequency associated with the peak PV injection falls near the inertial frequency, which suggests that diurnal and semidiurnal tides, as well as inertial oscillations, may be critical for understanding water mass structure and fluid transport in coastal regions (Flexas et al 2015). For v .…”

“…In the ocean, boundary currents over continental shelves and slopes can influence water mass modification (Whitworth et al 1998;Orsi et al 2002;Flexas et al 2015), eddy formation Gula et al 2015), and the generation of potential vorticity (PV) anomalies (Williams and Roussenov 2003). In coastal regions, interactions between ocean boundaries and mean flows influence the distribution of nutrients and other biogeochemical tracers (Gruber et al 2006;Dulaiova et al 2009).…”