Enhancing the Realism of MOM6-SIS2 Simulations with Ocean Tides

PI: Zaron, Edward (Oregon State University)
Start Year: 2022 | Duration: 3 years
Partners: NOAA, NASA, Oregon State University, University of Michigan, Florida State University

Project Abstract:

Ocean tides comprise a significant fraction of ocean variability – encompassing sea level, ocean currents, and stratification – involving both barotropic and baroclinic processes. The interactions of tidal phenomena with lower-frequency processes, such as seasonal stratification, and higher-frequency processes, such as internal waves and subsurface turbulence, lead to transports of energy and tracers that are not accurately represented in state-of-the-art earth system reanalyses and climate forecasting systems. 

This project aims to enhance the realism of global ocean and sea ice simulations based on the coupled MOM6-SIS2 ocean modeling system. We propose to run the MOM6-SIS2 system to simulate the joint atmospherically- and tidally-forced ocean circulation, and then refine the resolution of this system to create skillfully accurate predictions of the barotropic and low-mode baroclinic tides. Our targeted horizontal grid, a nominal (1/48)-degree tripolar grid, is chosen to capture the topographic gradients responsible for generating the low-mode baroclinic tides. While this system represents the state-of-the-art among global modeling components for Earth system prediction, future progress in computing capabilities, numerical methods, and subgrid-scale parameterizations will supersede MOM6-SIS2 with more capable systems. Our goal is to implement a methodology for calibrating the resolution-dependent parameterizations of MOM6-SIS2 to yield accurate predictions of barotropic and baroclinic tides, and to enable the efficient calibration of future systems after MOM6-SIS2 becomes obsolete.

Our approach is based on experience with data-assimilative tidal modeling systems in which it is axiomatic that the accuracy of predictions is proportional to the number of degrees of freedom available as control parameters. Using an ensemble method with 1000 or more degrees of freedom, chosen to reflect the uncertainties in seafloor topography and resolution-dependent parameterizations, we expect to produce MOM6-SIS2-based tide predictions that are comparable in accuracy to the widely-used barotropic TPXO solutions. We are confident that intercomparing and calibrating the simulated baroclinic currents and sea level with observational data – from satellite altimeters, surface drifters, and other high-resolution platforms – will reveal strengths and limitations of the MOM6-SIS2 system and demonstrate the extent to which our understanding of ocean physics fundamentally limits the predictability of baroclinic tidal phenomena.

In parallel with the efforts to model tides globally, we also plan to conduct studies within regional domains. Idealized simulations will be used initially to examine the grid-convergence of MOM6 in the context of barotropic-to-baroclinic conversion at topography. We will also implement the capability of prescribing baroclinic tidal boundary conditions on regional domains. This capability will be used for studying numerical convergence of internal wave scattering at topography, and it will enable more realistic regional MOM6 simulations.