The Antarctic Slope Front Position Estimated from Hydrography

Abstract:

The Antarctic Slope Front (ASF) is a fundamental feature of the subpolar Southern Ocean that is still poorly observed, due to the remotness of its location and cloud and ice cover. In this study we build a statistical climatology of the temperature and salinity (T-S) fields of the Antarctic margin using T-S profiles, and derive the position of the Antarctic Slope Front (ASF). We use a comprehensive compilation of observational datasets including the profiles gathered by instrumented marine mammals. The mapping method takes into account the covariance of temperature and salinity and includes front-sharpening and bathymetry-respecting components, providing an accurate representation of the ocean.

First we decompose the observed T-S profiles into vertical modes of variability, using a functional principal component analysis, between 10 and 380 m to maximise the number of profiles and the depth range analysed. Second we interpolate optimally the principal components on a 0.5x0.5^° grid, and reconstruct T-S profiles for each grid point and each months, providing a coherent climatology of the Antarctic margin, using the correlation between T and S at any depth. The first vertical mode (46% of the T-S variance) captures the temperature shift opposing the warm profiles north of the ASF from the cold one over the Antarctic shelf. It is well correlated with temperature at 300 m (0.97). Hence we use temperature at 300 m to locate the ASF, with a contour method and a gradient method, two complementary approaches that provide a more complete view of the front location and circumpolarity (Figure 1). The ASF follows approximately the 1000 m isobath, consistent with previous work. Finally we discuss the results, especially the seasonal variation of the gradient across the front, the seasonal branching and merging of multiple fronts in the Weddell Sea and the emergence of the ASF in the Amundsen Sea. This is the first hydrography-based circumpolar mapping of the ASF to our knowledge.