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Sebastian Essink

Postdoctoral Scholar

Email

sessink@apl.washington.edu

Phone

206-543-1391

Research Interests

I use observations and modeling to investigate ocean physics that cascade energy and properties from the balanced circulation to turbulence and mixing, such as submesoscale flows and internal waves. I am also interested in the impact of these flows on biology.

Department Affiliation

Ocean Physics

Education

B.Sc. Oceanography/Geophysics, University of Hamburg, 2012

Ph.D. Physical Oceanography, MIT-WHOI Joint Program, 2019

Publications

2000-present and while at APL-UW

Island Arc Turbulent Eddy Regional Exchange (ARCTERX): Science and Experiment Plan

The ARCTERX Team, "Island Arc Turbulent Eddy Regional Exchange (ARCTERX): Science and Experiment Plan," Technical Report, APL-UW TR 2201. Applied Physics Laboratory, University of Washington, July 2022, 49 pp.

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15 Jul 2022

Submesoscale flows such as fronts, eddies, filaments, and instabilities with lateral dimensions between 100 m and 10 km are ubiquitous features of the ocean. They act as an intermediary between the mesoscale and small-scale turbulence and are thought to have a critical role in closing the ocean's kinetic budget by facilitating a forward energy cascade, where energy is transferred to small scales and dissipated.

The initiative uses a suite of measurements from autonomous platforms and ships combined with regional simulations to characterize the submesoscale flows in the western Pacific Ocean between Luzon and Mariana Island arcs &$151; the ARCTERX region.

Program goals are to characterize the strength and spectral properties of the turbulent cascade of kinetic energy on the submesoscales in the ARCTERX study region and understand the processes that control energy transfers across scales and their seasonal variability.

Near-inertial wave interactions and turbulence production in a Kuroshio anticyclonic eddy

Essink, S., E. Kunze, R.-C. Lien, R. Inoue, and S. Ito, "Near-inertial wave interactions and turbulence production in a Kuroshio anticyclonic eddy," J. Phys. Oceanogr., 52, 2687-2704, doi:10.1175/JPO-D-21-0278.1, 2022.

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21 Jun 2022

Interactions between near-inertial waves and the balanced eddy field modulate the intensity and location of turbulent dissipation and mixing. Two EM-APEX profiling floats measured near-inertial waves generated by typhoons (i) Mindulle, 22 August 2016, and (ii) Lionrock, 30 August 2016, near the radius of maximum velocity of a mesoscale anticyclonic eddy in the Kuroshio–Oyashio Confluence east of Japan. High-vertical-wavenumber near-inertial waves exhibit energy-fluxes inward toward eddy center, consistent with wave refraction/reflection at the eddy perimeter. Near-inertial kinetic energy tendencies are nearly two orders of magnitude greater than observed turbulent dissipation rates ε, indicating propagation/advection of wave packets in and out of the measurement windows. Between 50–150 m, ε ~ O(10-10 W kg-1) , more than an order of magnitude weaker than outside the eddy, pointing to near-inertial wave breaking at different depths or eddy radii. Between 150–300 m, small-scale inertial-period patches of intense turbulence with near-critical Ri occur where comparable near-inertial and eddy shears are superposed. Three-dimensional ray-tracing simulations show that wave dynamics at the eddy perimeter are controlled by radial gradients in vorticity and Doppler-shifting with much weaker contributions from vertical gradients, stratification and sloping isopycnals. Surface-forced waves are initially refracted downward and inward, consistent with the observed energy-flux. A turning-point shadow zone is found in the upper pycnocline, consistent with weak observed dissipation rates. In summary, the geometry of wave/mean flow interaction creates a shadow zone of weaker near-inertial waves and turbulence in the upper part while turning-point reflections amplify wave shear leading to enhanced dissipation rates in the lower part of the eddy.

On characterizing ocean kinematics from surface drifters

Essink, S., V. Hormann, L.R. Centurioni, and A. Mahadevan, "On characterizing ocean kinematics from surface drifters," J. Atmos. Ocean. Technol., 39, 1183-1198, doi:10.1175/JTECH-D-21-0068.1, 2022.

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12 May 2022

Horizontal kinematic properties, such as vorticity, divergence, and lateral strain rate, are estimated from drifter clusters using three approaches. At submesoscale horizontal length scales O(1–10)-km, kinematic properties become as large as planetary vorticity f, but are challenging to observe because they evolve on short O(hours–days) timescales. By simulating surface drifters in a model flow field, we quantify the sources of uncertainty in the kinematic property calculations due to the deformation of cluster shape. Uncertainties arise primarily due to (i) violation of the linear estimation methods and (ii) aliasing of unresolved scales. Systematic uncertainties (iii) due to GPS errors, are secondary but can become as large as (i) and (ii) when aspect ratios are small. Ideal cluster parameters (number of drifters, length scale, and aspect ratio) are determined and error functions due to cluster deformations are estimated empirically and theoretically. We apply the most robust method — a two-dimensional, linear least-squares fit — to the first few days of a drifter data set from the Bay of Bengal. Application of the length-scale and aspect-ratio criteria minimizes errors (i) and (ii), and reduces the number of clusters and so computational cost. The drifter-estimated kinematic properties map out a cyclonic mesoscale eddy with a surface, submesoscale fronts at its perimeter. Our analyses suggest methodological guidance for computing the two-dimensional kinematic properties in submesoscale flows, given the recently increasing quantity and quality of drifter observations, while also highlighting challenges and limitations.

More Publications

Acoustics Air-Sea Interaction & Remote Sensing Center for Environmental & Information Systems Center for Industrial & Medical Ultrasound Electronic & Photonic Systems Ocean Engineering Ocean Physics Polar Science Center
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