Mesoscale eddies are ubiquitous in the World Ocean. A considerable amount of knowledge about mesoscale eddies has been gained, especially since the advent of satellite altimetry. However, most previous work has focused on signals near the sea-surface and, more recently, down to about 2000 m (thanks to Argo). The signals of mesoscale eddies in the deep ocean and their influence on the deep-ocean dynamics have not yet been intensively studied.

In this study, the connections between mesoscale eddies and deep ocean dynamical processes, including low-frequency flows, internal waves, and ocean mixing, are examined by combing the sea surface height anomaly data and a collection of deep ocean measurement near the crest of the East Pacific Rise (EPR) between 9 and 10 N. First, the relationship between mesoscale eddies and subinertial flows in the deep ocean over the EPR were examined. The subinertial velocities at depth are significantly correlated with geostrophic near-surface currents, which are dominated by westward-propagating mesoscale eddies. It is concluded that the subinertial velocity near the EPR crest is a super-position of velocities associated with eddies propagating westward across the ridge and "topographic flows." Second, the relationship between subinertial flows and internal waves were investigated. The observations reveal subinertial modulations of internal waves, particularly near-inertial oscillations and internal tides. These subinertial modulations are highly correlated with the subinertial flows in the deep ocean. Third, based on a finescale parameterization model, the deep ocean diapycnal diffusivity over the ridge crest was estimated. The estimated diapycnal diffusivity shows a variation on the subinertial time scale. In particular, the measurements imply a significant increase in diapycnal diffusivity near the seafloor during episodes of increased subinertial flow. Fourth, combined with previous numerical and theoretical studies, the observations imply energy transfer near the crest of the EPR from low-frequency flows, including mesoscale eddies, to near-inertial oscillations, turbulence, and mixing.

Considering the ubiquitousness of mesoscale eddies in the ocean, it is expected that the circulation near other portions of the global mid-ocean ridge system is similarly dominated by mesoscale variability and topographic effects. This is particularly important for dispersal of larvae and geochemical tracers associated with hydrothermal sources that are found primarily along the crest of mid-ocean ridges. Also, the observed eddy-modulated mixing is expected to be useful for validating and improving numerical-model parameterizations of turbulence and mixing in the ocean. Furthermore, since the frequency and intensity of mesoscale eddies depend on the state of the climate, the observed eddy modulation of deep ocean mixing connects climate change and climate variability to physical and biogeochemical dynamics in the deep ocean and implies an unexplored feedback mechanism potentially affecting the global climate system.