The infuence of wind stress and topogrpahy on the ACC frontal locations in Drake Passage

Bin Zhang, IMCS, Rutgers University
John M Klinck, CCPO, Old Dominion University

The Antarctic Circumpolar Current (ACC) features three major fronts: the Subantarctic Front (SAF), the Polar Front (PF), the Southern ACC Front (SACCF). The locations of these fronts are not stable. The PF can shift away from its historical mean locations on the order of
100 km. The ACC transport in Drake Passage varies over a large range (50 to 60 Sv). Numerical simulations with the Regional Ocean Modeling System are carried out to study the frontal variability
under the influence of local wind stress and bottom topography in Drake Passage.

Different surface wind stress are applied in the model, the 6 hourly QSCAT/NCEP blended wind stress,monthly running mean filtered wind stress and no wind situtation. With 6 hourly wind stress, the PF location is more variable than that with the monthly running mean filtered wind stress. The mean PF location changes with different wind stress. This change is different at different locations in the model.
The surface elevation to each side of PF changes with the wind forcing. The peak frequencies at which the wind stress is correlated to the surface elevation above 95% confidence level in the south are the 8 and 30 days with the wind stress change leading the surface elevation change. The peak frequencies to the north of the PF are 8, 15 and 40 days. The positive phase lag at some frequencies might be due to the contamination from the local baroclinic instabilities.

The mean 500~m temperature tracked PF location is consistent with mean surface PF location. The surface PF tends to be south of the 500~m PF front. This difference between the surface and 500~m PF locations is modulated by the wind stress and the topography. With stronger wind stress, the difference is reduced.

The Shackleton Ridge is reproduced in the model using a Gaussian type function. Compared to model results with smoothed topography, the SACCF path behaves more realistic with the northward intrusion. It indicates, in the middle of drake passage, the Shackleton ridge affects the pathway of the SACCF, among other reasons, such as the cold water intrusion and the PF instability. This tells us the importance of realistic representation of bottom topography in simulating the ACC fronts.