Session G33: Geophysical Fluid Dynamics: Oceanography I

Interactions Between Physical Processes and Carbonate Chemistry in the Oceanic Mixed Layer

Ocean tracers such as carbon dioxide, play critical roles in the global carbon cycle and climate. These tracers evolve primarily in the oceanic mixed layer where gas exchange occurs and light is plentiful. There is substantial heterogeneity in tracer spatial distributions, and the effects of submesoscale turbulence remain incompletely understood, particularly in the sub-kilometer range. In this study, large eddy simulations are used to examine the effects of wind- and wave-driven turbulence, diurnal forcing, and wave breaking on carbonate chemistry in the oceanic mixed layer at submesoscales. Simulations are performed for various ocean conditions to determine the effects of these physical processes on the air-sea carbon dioxide flux and amount of total dissolved inorganic carbon. The wave-averaged Boussinesq equations are solved in the simulations using pseudo-spectral and finite difference methods in the horizontal and vertical directions, respectively. The carbonate chemistry system consists of seven reacting species and is solved using a second-order Runge-Kutta-Chebyshev integration scheme. Results are presented for the evolution and steady-state properties of each chemical species for the varied ocean conditions. Non-dimensional parameters based on chemical and mixing timescales are used to classify and parameterize the results. Implications of these results for Earth system models are outlined, and an outlook for future research directions is also provided.   

LES, DES and RANS simulations of Langmuir circulation in shallow water

Wind and wave-driven Langmuir circulation (LC) in the upper ocean mixed layer has been extensively studied via field measurements and numerical simulations. However, studies of LC under the influence of coastal boundaries and its interactions with other coastal ocean processes are lacking. Large-eddy simulations (LES) of LC in inner coastal shelves have been limited to periodicity over horizontal directions, characteristic of a water column unaffected by coastal boundaries. As such, these simulations have been performed with pseudo-spectral solvers employing a highly accurate spectral discretization in the horizontal directions. These simulations may be deemed as "single water column LES", in which the vertical turbulent mixing induced by the cells is well-resolved, but their interaction with lateral boundaries and lateral flows are left unresolved. To address this deficiency, it is necessary to extend eddy-resolving simulations to non-spectral discretizations capable of handling non-periodic boundary conditions in lateral directions. A second-order accurate finite volume discretization will be used employing LES, DES (detached eddy simulation) and RANS (Reynold-averaged Navier-Stokes) methodologies, characterized by different forms of subgrid-scale modeling and near-wall treatment. It is found that careful consideration must be given to these components, as various combinations of the subgrid-scale model and the near-wall treatment can lead to excessive damping of the cells near the bottom of the water column. This adverse numerical effect can prevent the simulation from (1) accurately capturing the interaction between LC with the bottom boundary layer and (2) accurately predicting the lateral length scales of LC. The best performing subgrid-scale models and near-wall treatments will be identified. Performance of the simulations will also be assessed against recently published data of full-depth LC obtained from field measurements.   

Eulerian flows induced by internal tides

Tidal flow over bottom topography in the ocean generates beams in the near field that evolve to become dominantly horizontally propagating vertical mode-1 internal tides. Previous work has shown that two-dimensional (spanwise-infinite) tides self-interact to excite mode-1 superharmonics with double the horizontal wavenumber, and that successive near-resonant interactions between the parent tide and superharmonics can lead to a cascade which results in the formation of a solitary wave train, in good agreement with the predictions of the KdV and Ostrovsky equations. In this work we focus on the self-interaction of the parent tide and superharmonics that induce an Eulerian mean flow. Whether or not superharmonics grow to substantial amplitude, the forcing of the induced Eulerian mean flow is constant. However, the forcing is not resonant resulting in structures which are a superposition of horizontally long, vertical modes. On the f-plane, the structure is the same as the Stokes drift, but oscillates at frequency f at amplitudes between zero and minus twice the Stokes drift amplitude. If f=0, only horizontally modulated internal tides induce a mean flow that gradually grows to be larger than the magnitude of the Stokes drift.   

An Exploration of the Vertical Distribution of Mixing in a Submarine Canyon

Ocean mixing is a vital process that supports the global overturning circulation and biogeochemical processes. Despite this importance, significant questions remain about the underlying processes and the vertical distribution of mixing. Here, we consider the dissipation due to internal tides (internal waves at the tidal frequency) interacting with a submarine canyon, which has previously been shown to be a hotspot for mixing. Using both observations and a high-resolution simulation of the Eel Canyon-Mendocino Ridge system, we examine the magnitude of and processes that support internal wave-driven mixing over the region. We have found that the simulations match well with observations, suggesting that the internal tides present in the model are the primary driver of observed dissipation. Given the agreement between the observations and model, we have additionally used the model to understand the dynamics of the internal tides, and subsequent dissipation, at the edges of the canyon, as well as the vertical extent of mixing throughout the canyon. Our developing understanding of the processes by which the internal tides deposit their energy to mixing, as well as the vertical structure of this mixing, will inform future parameterizations of mixing in next-generation ocean models.   

Evaluation of the reduced Craik-Leibovich equations

Langmuir turbulence, engendered by the interaction of surface gravity waves and mean currents, drives various climatologically important mixing processes within the upper ocean and across the air-sea interface. The dynamics are well-described by the Craik-Leibovich (CL) equations, a wave-averaged version of the Navier--Stokes equations. Nevertheless, simulations of these equations in spatially extended domains remain computationally expensive. An asymptotic reduction of the CL equations termed the reduced CL (rCL) equations renders such simulations feasible by self-consistently filtering out small-scale downwind motions and exploiting the highly anisotropic dynamics that emerges in the strong surface-wave-forcing limit. In this investigation, we assess the accuracy and efficiency of simulations of the rCL equations via direct comparisons of computed flow structures and statistics with those obtained by simulating the full CL equations. Comparisons are made both within the formal regime of validity of the rCL equations, i.e. in the extreme wave-forcing limit, and outside it, with parameter values reflecting Langmuir turbulence in the presence of swell waves.