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Title: Numerical Mixing Suppresses Submesoscale Baroclinic Instabilities Over Sloping Bathymetry

Journal Article · · Journal of Advances in Modeling Earth Systems
ORCiD logo [1]; ORCiD logo [2]; ORCiD logo [3]
  1. Texas A & M Univ., College Station, TX (United States); Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
  2. Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
  3. Texas A & M Univ., College Station, TX (United States)
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The impacts of spurious numerical salinity mixing (Mnum) on the larger-scale flow and tracer fields are characterized using idealized simulations. The idealized model is motivated by realistic simulations of the Texas-Louisiana shelf and features oscillatory near-inertial wind forcing. Mnum can exceed the physical mixing from the turbulence closure (Mnum) in frontal zones and within the mixed layer. This suggests that simulated mixing processes in frontal zones are driven largely by Mnum. Near-inertial alongshore wind stress amplitude is varied to identify a base case that maximizes the ratio of Mnum to Mphy in simulations with no prescribed horizontal mixing. We then test the sensitivity of the base case with three tracer advection schemes (MPDATA, U3HC4, and HSIMT) and conduct ensemble runs with perturbed bathymetry. Instability growth is evaluated using the volume-integrated eddy kinetic energy (EKE) and available potential energy (APE). While all schemes have similar total mixing, the HSIMT simulations have over double the volume-integrated Mnum and 20% less Mphy relative to other schemes, which suppresses the release of APE and reduces the EKE by roughly 25%. This results in reduced isohaline variability and steeper isopycnals, evidence that enhanced Mnum suppresses instability growth. Differences in EKE and APE between the MPDATA and U3HC4 simulations are marginal. However, the U3HC4 simulations have 25% more Mnum. Experiments with variable horizontal viscosity and diffusivity coefficients show that small amounts of prescribed horizontal mixing improve the representation of the ocean state for all advection schemes by reducing the Mnum and increasing the EKE.

Research Organization:
Pacific Northwest National Laboratory (PNNL), Richland, WA (United States)
Sponsoring Organization:
National Science Foundation (NSF); USDOE; USDOE Office of Science (SC), Advanced Scientific Computing Research (ASCR). Scientific Discovery through Advanced Computing (SciDAC)
Grant/Contract Number:
AC05-76RL01830
OSTI ID:
2479820
Report Number(s):
PNNL-SA--206849
Journal Information:
Journal of Advances in Modeling Earth Systems, Journal Name: Journal of Advances in Modeling Earth Systems Journal Issue: 12 Vol. 16; ISSN 1942-2466
Publisher:
American Geophysical Union (AGU)Copyright Statement
Country of Publication:
United States
Language:
English

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54 ENVIRONMENTAL SCIENCES
baroclinic instabilities
fronts
numerical mixing
physical mixing
submesoscales