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We characterize the internal wave field at a standing meander of the Antarctic Circumpolar Current (ACC) where strong winds, bathymetry, and a strong eddy field combine to form a dynamic environment for the generation and dissipation of internal waves. We use Electromagnetic Autonomous Profiling Explorer float data spanning 0-1600 m depth collected from a meander near the Macquarie Ridge, south of Australia. Of the 112 internal waves identified, 69% are associated with upward energy propagation. Most of the upward propagating waves (35%) are found near the Polar Front and are likely generated by mean flow-topography interactions. Generation by wind forcing at the sea surface is likely responsible for more than 40% of the downward propagating waves. Our results highlight advection of the waves and wave-mean flow interactions within the ACC as the dominant processes affecting the wave dynamics. The larger dissipation time scales of the waves compared to advection suggests they are likely to dissipate away from the generation site. We find that about 79% (66%) of the waves in cyclonic eddies (the Subantarctic Front) are influenced by horizontal strain, whereas 92% of the waves in the slower Polar Front are influenced by the relative vorticity of the background flow. There is energy exchange between internal waves and the mean flow, in both directions. The mean energy transfer (1.4 ± 1.0 x 10-11 m² s-3 is from the mean flow to the waves in all dynamic regions except in anticyclonic eddies. The strongest energy exchange (5.0± 3.7 x 10-11 m² s-3 is associated with waves in cyclonic eddies. [ABSTRACT FROM AUTHOR]

This study presents novel observational estimates of turbulent dissipation and mixing in a standing meander between the Southeast Indian Ridge and the Macquarie Ridge in the Southern Ocean. By applying a finescale parameterization on the temperature, salinity, and velocity profiles collected from Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats in the upper 1600 m, we estimated the intensity and spatial distribution of dissipation rate and diapycnal mixing along the float tracks and investigated the sources. The indirect estimates indicate strong spatial and temporal variability of turbulent mixing varying from O(10−6) to O(10−3) m2 s−1 in the upper 1600 m. Elevated turbulent mixing is mostly associated with the Subantarctic Front (SAF) and mesoscale eddies. In the upper 500 m, enhanced mixing is associated with downward-propagating wind-generated near-inertial waves as well as the interaction between cyclonic eddies and upward-propagating internal waves. In the study region, the local topography does not play a role in turbulent mixing in the upper part of the water column, which has similar values in profiles over rough and smooth topography. However, both remotely generated internal tides and lee waves could contribute to the upward-propagating energy. Our results point strongly to the generation of turbulent mixing through the interaction of internal waves and the intense mesoscale eddy field. [ABSTRACT FROM AUTHOR]

This study presents the characteristics and spatiotemporal structure of near-inertial waves and their interaction with Leeuwin Current eddies in the eastern south Indian Ocean as observed by Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats. The floats sampled the upper ocean during July–October 2013 with a frequency of eight profiles per day down to 1200 m. Near-inertial waves (NIWs) are found to be the dominant signal in the frequency spectra. Complex demodulation is used to estimate the amplitude and phase of the NIWs from the velocity profiles. The NIW energy propagated from the base of the mixed layer downward into the ocean interior, following beam characteristics of linear wave theory. We visually identified a total of 15 near-inertial internal wave packets from the wave amplitudes and phases with a mean vertical wavelength of 89 ± 63 m, a mean horizontal wavelength of 69 ± 85 km, a mean horizontal group velocity of 3 ± 2 cm s−1, and a mean vertical group velocity of 9 ± 7 m day−1. A strong near-inertial packet with a kinetic energy of 20–30 J m−3 found propagating below 700 m suggests that the NIWs can contribute to deep ocean mixing. A blue shift of 10%–15% in the energy spectrum of the NIWs is observed in the upper 1200 m as the floats move toward the equator. The impacts of mesoscale eddies on the characteristics and propagation of the observed NIWs are also investigated. The elevated near-inertial shear variance in anticyclonic eddies suggests trapping of NIWs near the surface. Cyclonic eddies, in contrast, were associated with weak near-inertial shear variance in the upper 400 m. [ABSTRACT FROM AUTHOR]

This study investigates the spatiotemporal variability of turbulent mixing in the eastern south Indian Ocean using a collection of data from electromagnetic autonomous profiling explorer (EM-APEX) profiling floats, shipboard CTD, and microstructure profilers. The floats collected 1566 profiles of temperature, salinity, and horizontal velocity data down to 1200 m over a period of about four months. A finescale parameterization is applied to the float and CTD data to estimate turbulent mixing. Elevated mixing is observed in the upper ocean, over bottom topography, and in mesoscale eddies. Mixing is enhanced in the anticyclonic eddies due to trapped near-inertial waves within the eddy. We found that cyclonic eddies contribute to turbulent mixing in the depth range of 500–1000 m, which is associated with downward-propagating internal waves. The mean diapycnal diffusivity over 250–500-m depth is O(10−6) m2 s−1, and it increases to O(10−5) m2 s−1 in 500–1000 m in cyclonic eddies. The turbulent mixing in this region has implications for water-mass transformation and large-scale circulation. Higher diffusivity [O(10−5) m2 s−1] is observed in the Antarctic Intermediate Water (AAIW) layer in cyclonic eddies, whereas weak diffusivity is observed in the Subantarctic Mode Water (SAMW) layer [O(10−6) m2 s−1]. Counterintuitively, then, the SAMW water-mass properties are strongly affected in cyclonic eddies, whereas the AAIW layer is less affected. Comparatively high diffusivity at the location of the South Indian Countercurrent (SICC) jets suggests there are wave–mean flow interactions in addition to the wave–eddy interactions that warrant further investigation. [ABSTRACT FROM AUTHOR]

Turbulent mixing plays an important role in large-scale oceanic processes such as watermass transformation, global overturning circulation and stratification. The sensitivity of ocean models to the spatial and temporal variability of mixing has implications on ocean heat uptake in climate models and ocean circulation in regional models. Partly because of the lack of observations, the turbulent mixing in the southeast Indian Ocean is not well described. This study investigates the spatial and temporal variability of turbulent mixing in the eastern South Indian Ocean (SIO) using Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats, vertical microstructure profilers, and shipboard CTD data during 2013. The EM-APEX floats sampled the upper ocean with a frequency of 8 profiles per day which is high enough to resolve the internal waves who change the local mixing when they break in the ocean interior. The floats measured current velocities and water mass properties up to a depth of 1200 m with a resolution of 3 - 4 m in the vertical. By applying a shear-strain parameterization on the float data, we estimated the diapycnal diffusivity in the upper 1200 m of the ocean. We observe elevated diffusivity associated with wind near the surface, bathymetry and mesoscale eddies. Both warm core and cold core eddies impact the distribution of turbulent mixing. High mixing observed in the warm core eddy suggests trapping of near-inertial waves. We also observe elevated mixing below cold core eddy associated with deep reaching near-inertial waves. By applying a strain-only parameterization on the shipboard data, we observe elevated diffusivity near bathymetry associated with bottom-generated internal waves. Elevated mixing is observed at depths corresponding to Antarctic Intermediate Water and Subantarctic Mode Water suggesting that turbulent mixing impacts the modification of watermasses in this region. This work improves our understanding of turbulent mixing variability in the eastern SIO, a climatically important region for Australia. [ABSTRACT FROM AUTHOR]