Observations of Eddy - Internal Wave Interactions in the Tropics

Abstract

Based on an extensive dataset obtained from multiple individual ship-based samplings of mesoscale eddies and a long-term mooring (2006-2023), interactions between mesoscale eddies and internal waves are analyzed. Theory predicts that anticyclonic mesoscale eddies shift the effective Coriolis force for near-inertial waves (NIW) locally in regions of strong relative vorticity towards subinertial frequencies, leading to trapping of NIW in their core and accelerated downward propagation to a critical layer at the eddy base where mixing is eventually enhanced. In contrast, cyclones might expel NIW through the same but reverse effect. In both cases, and independent of their relative vorticity, increased mixing is expected in regions of strong vertical geostrophic shear at the rims due to critical layer processes. We are able to confirm these theoretical predictions in the observed dataset for several eddies (located mostly in the eddy-rich eastern boundary upwelling systems and waters surrounding the Cape Verde Archipelago). Velocity measurements in coherent anticyclonic eddies repeatedly show pronounced alternating current bands with amplitudes up to 15 cm/s, likely associated with convergence of downward propagated NIW. Microstructure measurements, supplemented by finescale parameterizations, show enhanced mixing in both, the base of anticyclones and slightly elevated dissipation rates at the rims of cyclones, where geostrophic shear is strong. Counterintuitive to the aforementioned theory, enhanced downward propagation was also observed below the center of cyclones. Frequency spectra from the mooring data indicate that the NIW frequency band is altered by the relative vorticity. Overall, a high variability of the internal wave field is observed and impacts of mesoscale eddies are readily apparent in the case of anticyclones, representing a path of energy into the deeper ocean and a key process for local mixing.