A positive eddy in the South Atlantic (20th June 2011)

Float 73063: Caught in a positive eddy in the South Atlantic

Our attention was drawn to this float by an anomalously warm and saline intrusion appearing in its profiles during April/May 2011. There was little evidence of this water mass in surrounding floats but a comparison with satellite-derived altimetry suggested it was accurately depicting the water column. It appears that this float was caught in a well defined eddy, as indicated in the satellite imagery below. The diameter of the loop made by the float is of the order ~200 km which corresponds to a large mesoscale eddy structure.

Eddies have a number of causes, notably the convergence of currents but wind and seafloor topography have also been implicated in specific conditions. Once formed they can remain stable for long periods of time and carry bodies of water far from their usual tracks. So it seems to be in this case. The properties of the intrusion are similar to those found in water formed by overturning (convection) in the upper Southern Ocean. As it travels north it undergoes warming and mixing but the eddy structure seems to have allowed it to retain discrete properties from surrounding water masses.

Temperature-salinity plot for float 73063 with anomalous data points circled. Data is colour-coded for time; red represents early profiles and the most recent profiles are coloured blue.

Temperature-salinity plot for float 73063 with anomalous data points circled. Data is colour-coded for time; red represents early profiles and the most recent profiles are coloured blue.

Recent trajectory for float 73063 (1901054)

Recent trajectory for float 73063 (1901054).

Satellite-derived sea surface height plot for mid-May 2011 showing the ~40 cm bulge generated by the positive eddy associated with float 73063

Satellite-derived sea surface height plot for mid-May 2011 showing the ~40 cm bulge generated by the positive eddy associated with float 73063.

 

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