Effect of surface mesoscale eddies on deep-sea currents and mixing in the northeastern South China Sea

Recent studies suggest that deep-reaching surface-generated eddies result in anomalous current velocities in the deep sea, and ultimately lead to energy transfer from mesoscale to small-scale motions. Here we examine the influence of mesoscale eddies on deep-sea subinertial and near-inertial currents, and on possible enhanced oceanic mixing in the deep South China Sea (SCS). We analyzed current velocity data for nearly a full water column. Data were obtained using acoustic Doppler current profilers and recording current meters on a deep-sea mooring system at a depth of 2100 m in the northeastern SCS from October 2012 to May 2013. A highly nonlinear southwestward-propagating anticyclonic eddy was detected via a resolved sea-surface-level anomaly. This eddy induced pronounced subinertial currents with a characteristic time scale of 1-2 months and a maximum velocity of up to 0.2 m s(-1) at the subsurface and 0.1 m s(-1) at great depth. Near-inertial energy co-occurring with subinertial flows showed a distinctive vertical propagation trend during strong subinertial oscillations in the deep sea. During periods of strong subinertial and near-inertial kinetic energy, estimates of diapycnal diffusivity in the deep ocean showed approximately 10-fold enhancement, with a mean value of 1.2 x 10(-3) m(2) s(-4) compared to the background value of 1.4 x 10(-4) m(2) s(-1). The results provide observational evidence of the effect of surface-observed mesoscale motions on benthic currents and ocean mixing in the deep SCS. (C) 2015 Elsevier Ltd. All rights reserved.