Modulation of sea surface roughness by oceanic fronts: The observed quadripolar principle

Under light to moderate wind conditions, fine scale (O (100 m)) oceanic fronts are revealed by anomalies on sea surface roughness images. As interpreted, oceanic current gradients around the fronts modulate short waves to create such surface roughness anomalies. In this paper, we report the quadripolar principle: an observed recurrent correlation between the sign of the roughness anomaly and the orientation between the front and the wind. The sign of the anomaly exhibits a quadripolar dependency on the wind / front relative angle. As observed, this quadripole does not depend on the sign of the sea surface temperature jump across the front but reverses sign from the Northern to the Southern hemisphere. Different explanations of such phenomenon are discussed. The first one involves interactions between Ekman currents and the front to create areas of divergence and convergence. This most likely creates dipoles rather than quadripoles. The second explanation involves favoured cyclonic vorticity of the horizontal currents around submesoscale fronts. This creates strain within the wind direction, thereby creating quadripoles in agreement with the observations.