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Kathy Gunn

acoustic & physical oceanographer
CERC fellow, CSIRO
kathy [dot] gunn [at] csiro [dot] au

Research

Observations of the structure, properties and variability of submesoscale (<10 km) to mesoscale oceanic processes using approaches from acoustic and physical oceanography.

Ventilation of Antarctic Bottom Water

with Steve Rintoul and Matthew England.

I am working as part of the Southern Ocean dynamics, circulation and water mass formation project team. As a member of this team, I will be using classic and state-of-the-art hydrographic observations to quantify the ventilation of bottom water over time and space.

Vertical Mixing and Heat Fluxes Conditioned by a Seismically Imaged Oceanic Front

with Alex Dickinson, Nicky White, and Colm-cille Caulfield.

read the paper here

Using seismic oceanography , we calculated spatial and temporal distributions of the dissipation rate of turbulent kinetic energy, ε, of diapycnal mixing rate, K, and of vertical diffusive heat flux, F. We found that estimates of ε, K, and F are elevated compared to regional and global mean values. Over six weeks, cross-sectional mean estimates vary little whilst smaller scale thermohaline structures appear to have a spatially localized effect. In contrast, a mesoscale front modifies ε and K to a depth of 1 km, across a region of O(100) km. This front clearly enhances mixing rates, both adjacent to its surface outcrop and beneath the mixed layer, whilst also locally suppressing ε and K to a depth of 1 km. As a result, estimates of F increase by a factor of two at the surface outcrop of the front.

eddyshelf-3d.png

Estimating diapycnal diffusivity from seismic images. (b) Example seismic reflection profile where red/blue stripes correspond to vertical displacements in density within water column. (c) Black lines = tracked reflections; thick black line = spectrally analysed tracked reflections shown in (d). (d) Power spectrucm of vertical displacement as function of horizontal wavenumber calculated using multi-taper Fourier Transform from linearly detrended tracked reflection shown in (c). Internal wave, turbulent and noise sub-ranges characterised by -2, -5/3 and 0 spectral slopes, respectively. Vertical dashed line = white noise cutoff. (e) Slope spectrum as function of horizontal wavenumber. Internal wave, turbulent and noise sub-ranges characterised by -1/2, +1/3 and +2 spectral slopes, respectively. Thin black line = model fit to sub-ranges; blue portion of spectra = identified turbulent sub-range; vertical dashed line = white noise cutoff.

Heave and Mixing in the Agulhas Current

with Lisa Beal, K McMonigal and Shane Elipot.

read the paper here read its partner here

By analyzing the variability of salinity -- a water mass tracer -- across a moored array in the Agulhas Current, we have determined the drivers of significant water mass changes in this western boundary current. Water mass properties are changed:

  1. reversibly by heave
  2. irreversibly by cross- and along-isopycnal mixing.
fig1.png

The position and strength of the current ultimately drives the observed property changes. The jet position changes due to meandering, whereby the current completely detaches from the continental slope, and through smaller submesoscale lateral shifts of the jet. Although the latter is associated with small salinity anomalies, submesoscale shifts affect the water column for a greater portion of the year (grey bar). Meanders drive the largest irreversible changes in water masses (black bar left-hand side). However, these large scale lateral shifts drive variability for an equal amount of time as changes in the jet strength (black bar right-hand side).

Seismic Imaging of Submesoscales and Stirring at a Front

with Nicky White and Colm-cille Caulfield.

read the paper here

Using seismic reflection imaging -- a low frequency acoustic technique often called Seismic Oceanography -- we mapped the evolution of an oceanic front from the sea-surface to 2 km depth across two weeks. These images maintain up to 10 m horizontal and vertical resolution in two- and three-dimensions for hundreds of km length and down to abyssal depths. Therefore there are significant observational advantages associated with this technique.

fig2.png

A set of seismic images (a-d; Days of year 39, 42, 43 and 45) revealed: (i) advection of a front, (ii) the rapid evolution of a lens that is deep, centered at 800 m depth, and large, up to 35 km length and 700 m height, and (iii) stirring at depths of over 1200 m depth.

The deep and rapidly evolving lens, which is banked against the front (e-h; interpretation of panels a-d), has not previously been detected using traditional physical oceanographic techniques. It is most likely the result of frontally induced ageostrophic circulation or near-inertial wave generation.

Seismic Imaging of Eddy-Dominated Warm-water Advection in Antarctica

with Nicky White, Colm-cille Caulfield and Rob Larter

read the paper here

Seismic reflection images of thermohaline circulation from the Bellingshausen Sea, adjacent to the West Antarctica Peninsula, show that distinctive lens-shaped patterns of reflectivity with lengths of 0.75–11.00 km and thicknesses of 100–150 m are in abundance in the Marguerite and Belgica Troughs (a). These troughs are known pathways for warm water to intrude onto the continental shelf. Thus, these warm water lenses (b,c) contribute to ice mass loss in West Antarctica.

bay-turbulence-summary.png

I developed an iterative procedure that can be used to convert seismic imagery into maps of temperature, in the absence of concurrent hydrographic data (b). Using this approach, we were able to identify the anomalous properties of the lenses, as well as their number and size. We find that these lenses surround anomalously warm water (c). Finally, analysis of the temporal and spatial information contained within the acoustic records suggests that these lenses are advecting southward onto the continental shelf.

High and Low Frequency Acoustics Map Same Features

Advisors: Rob Larter (British Antarctic Survey) and Nicky White (University of Cambridge)

During my investiagtion of thermohaline structures around Antarctica I found a feature that appeared in both low and high frequency acoustic techniques.

shelf-flow-summary.png

The upper portion of a lens is visible at a range of 25–35 km along a seismic profile (a). Here a bright convex-upward reflection appears to delineate the top of a lens which is just over 10 km in length. A coincident 38 kHz echo sounder profile also shows the same strcuture (d). Due to their frequency differences, seismic methods will record changes in the physical properties of the water column whilst an echosounder will record biology.

Publications

    1. Gunn, K. L., Dickinson, N. A., White, N. J. Caulfield, C. P. (2021). Conditioning of Vertical Mixing and Heat Flux by a Front in the Brazil-Falkland Confluence. Frontiers in Marine Science
    2. Gunn, K. L., Beal, L. M., Elipot, S., McMonigal, K. M. and Houk, A. (2020). Mixing of Subtropical, Central and Intermediate Waters Driven by Shifting and Pulsing of the Agulhas Current. Journal of Physical Oceanography
    3. McMonigal, K. M., Beal, L. M., Elipot, S., Gunn, K. L., Hermes, J., Morris, T. and Houk, A. (2020). The Impact of Meanders, Deepening and Broadening, and Seasonality on Agulhas Current Temperature Variability. Journal of Physical Oceanography.
    4. Gunn, K. L. , White, N. J. and Caulfield, C. P. (2020). Time-Lapse Seismic Imaging of Oceanic Fronts and Transient Lenses within South Atlantic Ocean. Journal of Geophysical Research: Oceans.
    5. Gunn, K. L. (2019). Thesis: Time-lapse acoustic imaging of oceanic fronts and eddies.
    6. Gunn, K. L., White, N. J., Larter, R. D. and Caulfield, C. P. (2018). Calibrated Seismic Imaging of Eddy-Dominated Warm-Water Transport across the Bellingshausen Sea, Southern Ocean. Journal of Geophysical Research: Oceans.

Contact

Email: kathy.gunn [@] csiro.au; kgunn.sc.tas [@] gmail.com

Acknowledgments

My work has been funded by the US National Science Foundation (NSF). I have also been supported by the Universities of Cambridge and Miami as well as the British Antarctic Survey. Website styling gratefully borrowed from Deepak Cherian.