A Linear Decomposition of the Southern Ocean Thermohaline Structure in only two Modes with Application to Frontal Detection

The Southern Ocean is a critical component of the World Ocean as it connects the three major ocean basins. Its thermohaline structure is deeply influenced by the presence of the Antarctic Circumpolar Current (ACC), where water masses of the World Ocean are advected, transformed and redistributed to the other basins. It remains challenging to describe and visualize the complex 3D pattern of this circulation and its associated tracer distribution. Here, we propose an objective method for decomposing the Southern Ocean thermohaline structure. A principal component analysis (PCA, which is a variant of the EOF method) is applied directly on the shape of temperature and salinity profiles to determine the main spatial patterns of their variations. Using the Southern Ocean state estimate (SOSE), we determined the vertical modes describing the Southern Ocean thermohaline structure between 0 and 2000 m. We found that the two first modes explain more than 90% of the combined temperature-salinity variance, thus providing a surprisingly good approximation of the thermohaline properties in the Southern Ocean. The first mode (72% of total variance) describes accurately temperature variations, as well as surface and deep salinity variations. The second mode (20%) mostly improves salinity at 500 m in the region of Antarctic Intermediate Water formation. These two modes present circumpolar patterns that can be closely related with standard frontal definitions, especially for the ACC fronts (Subtropical, Subantarctic, Polar and Southern ACC fronts). By projecting hydrographic profiles on the SOSE-derived modes, it is possible to determine their position relative to the fronts. This provides a new, objective, way to define ACC fronts and spatial variability. The concept is successfully applied on the WOCE-SR3 hydrographic section. This method can also provide a useful metric to compare model outputs with available observations and to better assess heat and salt content changes.