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Internal waves, turbulent mixing and upper ocean heat balances in the southeast Indian Ocean

Cyriac, A ORCID: 0000-0001-6149-740X 2020 , 'Internal waves, turbulent mixing and upper ocean heat balances in the southeast Indian Ocean', PhD thesis, University of Tasmania.

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A unique feature of the circulation in the eastern south Indian Ocean (SIO) is the eastward-flowing, near-surface geostrophic currents, known as the South Indian Countercurrent (SICC) and Eastern Gyral Current (EGC). They act as a source of water for the only poleward-flowing eastern boundary current of the mid-latitude global ocean called the Leeuwin Current (LC). The downwelled waters beneath the LC also supply the intermediate-depth, equatorward-flowing Leeuwin Undercurrent and the anticyclonic subtropical gyre. Instabilities of the Leeuwin Current System generate mesoscale eddies that propagate westward into the SICC jets, which can themselves be unstable. This interaction of the zonal currents and eddy field occurs in a region of strong air-sea exchange and subduction of high-salinity surface waters that contributes to the meridional overturning circulation of the Indian Ocean. Superimposed on the large and mesoscale environment are internal waves which are responsible for most of the turbulent mixing in the stratified ocean through wave breaking. In the interior of the ocean, this turbulent mixing plays an important role in setting the large-scale stratification and consequently, the large-scale circulation where mixing from surface processes cannot provide energy directly.
At present, the global products from in situ and remotely sensed observations provide a good understanding of the large-scale, near-surface circulation and air-sea exchanges. From hydrographic transects, we understand the vertical structure of the zonal currents along three repeat lines, but none close to Australia. Recently, global analyses from the Argo array and a collection of sparse observations provide estimates of vertical mixing in the eastern Indian Ocean, but they differ by an order of magnitude. Moreover, until this study, there were no direct long-term measurements of air-sea fluxes with which to examine the interactions that are central to the upper ocean variability in this region, where the reanalysis products disagree in their magnitudes of the surface fluxes. Thus, a better understanding of the interactions between these different scales of motion and air-sea interface in this region is essential to further our understanding of the Indian Ocean’s influence on Australian and regional climate.
This study aims to provide a detailed picture of the geostrophic currents and eddies, air-sea exchange, internal waves and turbulent mixing in the eastern SIO by exploiting high resolution in situ observations, reanalysis products, and satellite altimetry. Specific goals of this thesis include: i) characterize the spatial and temporal variability of the near-inertial internal wave field in the SIO and estimate the wave properties and potential sources; ii) quantify the turbulent mixing associated with the breaking of internal waves; iii) investigate potential relationships between the geostrophic circulation, internal waves and mixing in the regional context; and iv) determine the relative roles of atmospheric forcing and ocean processes on the evolution of observed mixed layer temperature.
To achieve these goals, we made use of a collection of in situ observations from two hydrographic and microstructure surveys across the strongest SICC jet between 25◦S – 32◦S, a two-year RAMA flux mooring deployment at 25◦S, 100◦E, and five Electromagnetic Autonomous Profiling Explorer (EMAPEX) floats which gave 3726 collocated profiles of temperature, salinity and velocity with 8 profiles per day. The floats provided high spatial resolution data with 3 – 4 dbar in the vertical and 3 – 5 km in the horizontal during July – October 2013 up to a depth of 300 m (2160 profiles) and 1200 m (1566 profiles). We also took advantage of additional data from the Argo array, reanalysis products and ocean climatology to provide broader spatial and temporal context for our study. The near-inertial waves are identified by applying a complex demodulation of the high-resolution velocity data provided by the floats. To estimate turbulent mixing, we applied a fine-scale parameterization on the EM-APEX and shipboard data. Using mixed layer heat budget diagnostic models, we analysed the mixed layer heat balance from the mooring data.
We identify many near-inertial internal waves in the EM-APEX records and examine a representative subset of 15 internal waves with near-inertial frequency in the upper 1200 m. The observed near-inertial wave field in the southeast Indian Ocean has a mean vertical wavelength of 89 ± 63 m, a mean horizontal wavelength of 69 ± 85 km, a horizontal group velocity of 3 ± 2 cm s−1 and a mean vertical group velocity of 9 ± 7 m day−1 . The mean vertical energy flux of the downward propagating beams is more than 40% of the wind work, with the potential to reach the deep ocean. The generation and propagation of the observed near-inertial waves are dependent on the regional dynamics. High energy near-inertial waves, that are consistent with having been generated from a remote region from an earlier wind event and not having generated locally, are observed at a depth of 700 m with kinetic energy of 20 – 30 J m−3 , providing energy for the deep ocean mixing. High near-inertial shear variance is observed in the warm core eddies near the surface consistent with trapping of near-inertial waves by the anticyclonic vorticity field. Large near-inertial wave amplitudes associated with patches of high near-inertial shear variance are often observed near the surface following strong wind events suggesting that some waves are generated locally. Most of the observed near-inertial beams are found to be propagating downward and equatorward with a blue-shift of 10 – 15%, suggesting that wind is the main source of energy for the observed beams.
The inferred turbulent mixing in the eastern SIO from EM-APEX floats shows substantial spatial and temporal variability. The mean diapycnal diffusivity for this region is at background levels (O(10−6 m2 s −1 )) in the upper 250 – 500 m whereas it is elevated between 500 – 1000 m in cyclonic eddies. We find that elevated mixing (O(10−3 m2 s −1 )) in this region occurs in association with strong wind events, mesoscale eddies and rough bottom topography. Within warm core eddies, near-inertial wave breaking results in elevated mixing in the upper 400 m. Whereas, higher mixing within cyclonic eddies is associated with downward propagating internal waves with frequencies higher than inertial, possibly due to wave capturing by the strain field of the eddy. From a strain parameterization of CTD data, elevated mixing levels were observed near the sea floor, suggesting a possible role for internal wave generation due to tidal motions or strong geostrophic flows over rough bathymetry. Enhanced mixing is often found where elevated near-inertial wave amplitudes occur, suggesting that the near-inertial waves play an important role in the turbulent mixing distribution of the upper 1000 m. The mixing estimates show that higher diffusivities are found in the Antarctic Intermediate Water (AAIW) layer (O(10−3 m2 s −1 )) and very low diffusivities (O(10−6 m2 s −1 )) are found in the Subantarctic Mode Water (SAMW) layer in this region, suggesting a role for internal wave breaking in the modification of AAIW on its pathway northward. Enhanced mixing in mesoscale eddies is also found to be important for the maintenance of the SICC where its jet-like structure is thought to be associated with potential vorticity gradients generated by the mixing of potential vorticity like a tracer.
We find that on seasonal timescales in the subtropical southeast Indian Ocean, the primary balance in the mixed layer heat budget is between the surface net heat flux, and turbulent entrainment with contribution from horizontal advection at times. Both zonal and meridional advection terms appear to be dominated by the presence of mesoscale eddies and possibly annual and semi-annual Rossby waves propagating from the eastern boundary. During austral summer, all heat flux terms tend to warm the mixed layer, with more contribution from surface net heat flux and meridional advection. During austral winter, horizontal advection warms the mixed layer whereas surface net heat flux and vertical processes cool the mixed layer. Turbulent entrainment is in good agreement with the heat budget residual for most of the year. This analysis is complemented by a 12-year regional ocean heat budget analysis around the mooring using reanalysis products. The seasonal cycle of the heat storage and surface net heating at the mooring location from the long-term analysis is in reasonably good agreement with the 2-year mooring analysis suggesting that this heat budget analysis provides a longer-term context for understanding the processes that drive the surface layer heat budget in this region.
This study shows that the interaction between internal waves and mesoscale eddies is important for the mixing budget and in setting the stratification of the eastern SIO. This thesis improves our understanding of wave-eddy-mean flow interactions and its implications on the large-scale circulation and air-sea exchanges in the eastern SIO. It also prompts us of the importance of the high-resolution ocean and atmosphere observations in understanding multi-scale processes that can have a profound impact on the large-scale circulation and climate system.

Item Type: Thesis - PhD
Authors/Creators:Cyriac, A
Keywords: Internal waves, turbulent mixing, mesoscale eddies, south Indian Ocean, watermass, mooring. EM-APREX floats
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