Long term analysis of the Polar Front position and thermohaline characteristics in Drake Passage, Antarctic.

Several studies have observed a consistent warming of the Southern Ocean with the migration of the Antarctic Circumpolar Current and the associated frontal systems southwards. This shift, is thought to be related to the predominantly positive phase of the Southern Annular Mode (SAM). This study use the long term salinity and temperature measurements from the High Resolution XBT Program (HR-XBT) in Drake Passage to analyze the temporal variability of the thermohaline properties of the Polar Front (PF). The measurements span from 1996 to 2014. The analysis of the variability of the thermohaline properties along both the isobars and isopycnals in the PF was carried out using Bindoff & McDougall’s (1994) method to account for warming, freshening and heaving of the water column. The main results indicate a southward migratory trend of the PF of 0.0229°/year (~2.5km) which is consistent with a positive SAM influence. Freshening dominates changes in the Drake Passage region and heaving is the least important of all three processes. Freshening occurs throughout the water column south of the PF while that signal is limited to the surface towards the north of the PF. The most significant freshening seems to be occurring along the isopycnals 27.2 to 27.5 kg/m³, values that correspond to ~100 and ~400m depth, respectively. In terms of T/S anomalies with depth, the results show positive anomalies for Pure Warming and negative anomalies for Pure Freshening in both isobaric and neutral surfaces, indicating also that the region is getting fresher with time. Overall, these results endorse the hypothesis that freshening plays a major role in the observed subsurface heating signal occurring along isopycnals.