Competing energy cascades associated with seasonally-varying submesocale turbulence in the North Pacific Subtropical Countercurrent
Abstract
One of the main motivations for understanding the physics of submesoscale turbulence is its role in closing the kinetic energy (KE) budget of the ocean circulation by driving a forward cascade of KE out of the mesoscale, opening an energy pathway to dissipation. W hile certain types of submesoscale instabilities are known to trigger a forward cascade of KE, there are others which energize rathe r than damp mesoscale flows through an inverse cascade. Both types of instabilities often develop jointly since they form at fronts, and can coincide with other submesoscale processes that transfer energy downscale such as frontogenesis and lateral-shear instabili ties. In a turbulent, submesoscale flow field where these various processes are active, it is not known what sets the strengths of the forward and inverse cascades, or even whether the submesoscale is a net sink or source of KE for the mesoscale on the whole. The overarching goal of the research proposed here is to understand the factors and underlying physics that control the magnitudes of t hese competing KE cascades. The research will be studied in the setting of the North Pacific Subtropical Countercurrent (STCC), whe re there is a pronounced seasonal cycle in mesoscale eddy KE that appears to be linked to the submesoscale turbulence that develops in its fronts. The hypothesis is that the competing KE cascades are a function of the degree of flow imbalance in the submesoscale t urbulence which is controlled by the potential vorticity and its modulations across seasons.The seasonal variability of submesoscale s from the ARCTERX DRI field campaigns. The numerical work will involve high-resolution regional simulations of the STCC run using t he state-of-the-art CROCO ocean model. CROCO is designed for nesting higher-resolution "child" grids within coarser-resolution "par resolve the submesoscale processes that are predicted to be at play h a climatology that captures the annual cycle in atmospheric forcing to probe the seasonal variations in submesoscale turbulence. D iagnostics of key dynamical quantities such as KE spectra and the terms governing their evolution, the degree of flow imbalance, and the potential vorticity will be calculated to test the guiding hypothesis of the proposed research. The simulations will also be us ed to design, prior to the DRI s field campaigns, sampling strategies for the long-term autonomous and ship-based process measuremen ts by moving virtual vehicles, floats, etc., through the simulated STCC and optimizing the sampling to best test the guiding hypothe sis. After the campaigns and recovery of the data from the autonomous measurements, statistics of the stratification, mixed layer de pth, vorticity, strain, divergence, and lateral density gradients will be calculated from the observations and regional simulations and compared. Through this comparison we will be able to infer which processes the model is able to capture. Additional in-depth ana lyses aided by the insights gained from the diagnostics of the simulations will be performed on the ship-based observations to delve more deeply into the governing physics. It is anticipated that this research will improve our understanding of the processes that s et the relative strengths of forward and inverse energy cascades in submesoscale turbulence and their dependence on season, using th e STCC as a natural laboratory. The high-resolution simulations will yield insights into this physics to aid in the interpretation o f the observations and additionally result in strategies to optimally sample submesoscale turbulence using autonomous platforms. App roved for Public Release.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Sep 07, 2021
- Source ID
- N000142112886
Entities
People
- Leif Thomas
Organizations
- Office of Naval Research
- Stanford University
- United States Navy