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Abstract
Internal tides can generate ocean mixing, surface convergences and near-bottom currents, so their predictability is of interest. Internal tides can become temporally incoherent when they interact with topographic features or mesoscale ocean dynamics such as eddies and boundary currents, making prediction challenging. Using in situ observations of temperature, salinity and velocities from an array of deep ocean moorings, this study characterises the strength and variability of the internal tides, in both time and space, off eastern Australia (~ 27°S) during 2012-2013. The data are unique as the mooring array spans the East Australian Current (EAC), a dynamic and eddy-rich Western Boundary Current.
The internal tide energy in the EAC is, generally, greater in the diurnal frequency band than the semidiurnal band (2.73 kW m-1 and 0.46 kW m-1, respectively). Internal tide variability is compared to local barotropic tidal forcing, stratification and eddy kinetic energy to determine the influence of and interaction with mesoscale EAC circulation. The results reveal that the diurnal internal tide is locally generated above the continental slope and the strength is modulated by the passing of cyclonic fronts and eddies that alter the local stratification and velocity field. Peaks in semidiurnal internal tide energy are sporadic and may result from a combination of remote and local generation.
We show for the first time that cyclonic frontal eddies along the inshore edge of the Western Boundary Current enhance the near bottom stratification above the continental slope and modulate the diurnal internal tide at the study area. The intensification of the internal tides modulates the vertical displacement of the water column (as observed in temperature and vertical velocity).
A case study of a mesoscale cyclonic eddy passing through the study site reveals that the eddy acts as a catalyst triggering interaction (energy transfer) between the barotropic tide and the internal tide. We show that the response manifests horizontally (increasing horizontal velocities), rather than vertically (vertical temperature and velocities remain stable) during the passage of the eddy.
The results provide insight into the origin and variability of the internal tide off eastern Australia and the interaction with the western boundary current and the passage of cyclonic fronts. Studying the characteristics of internal tides and their interaction with mesoscale ocean circulation helps us quantify their variability and improve their predictability particularly in dynamic regions, which is non-trivial in contrast to the deterministic barotropic tides that generate them.
