12.2 The Dynamical Link between Frontogenesis and Convergence

Thursday, 29 June 2017: 10:45 AM
Salon F (Marriott Portland Downtown Waterfront)
Roy Barkan, University of California, Los Angeles, CA; and J. C. McWilliams

Oceanic submesoscale currents are characterized by elongated anisotropic flow structures with large magnitudes of lateral buoyancy gradients |∇b|2, lateral velocity gradients |∇U|2, enstrophy ζ2, and strain-rate variance S2. Historically, the horizontal advective tendency for the sharpening of lateral buoyancy gradients was used to define and quantify the frontogensis process [1], which is believed to be an important mechanisms for submesoscale generation in the ocean [2]. We introduce a scaling theory for the advective frontogentic tendency rates of |∇b|2, |∇U|2, ζ2, and S2 that suggests that, to leading order, all of the advective frontogentic tendency rates of anisotropic submesoscale currents within a surface mixed layer are the same, and equal to the convergence rate. The scaling theory is verified on a feature by feature basis (Figure 1), and statistically (Figure 2), using realistic submesoscale resolving simulations in the Northern Gulf of Mexico. This implies that the frontogensis of submesosale anisotorpic flows is largely governed by the divergent part of the velocity field. Furthermore, it suggests that the divergence field, a much easier quantity to measure than the various frontogentic tendency rates, may provide a valid assessment of the frontognesis process in oceanic field observations. Finally, because the available potential energy in the mixed-layer, which is the dominant energy source for other submesoscale generation mechanisms, is proportional to the lateral buoyancy gradient magnitude, the theory may be used more generally to interpret upper ocean processes other than frontogensis.


[1] Hoskins, Brian J., and Francis P. Bretherton. “Atmospheric frontogenesis models: Mathematical formulation and solution.” Journal of the Atmospheric Sciences 29.1 (1972): 11-37.
[2] McWilliams, James C. “Submesoscale currents in the ocean.” Proc. R. Soc. A. Vol. 472. No. 2189. The Royal Society, 2016.

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