E6 Mesoscale and Submesoscale

Topic

The oceanic mesoscale—spanning spatial scales of ten to five hundred kilometers and time scales ranging from weeks to months—is dominated by eddies primarily generated by baroclinic and barotropic instabilities within major ocean currents, such as the Gulf Stream, the Antarctic Circumpolar Current, and the Kuroshio Current. These eddies significantly transport heat, salt, biochemical substances, and momentum throughout the global ocean; indeed, this intense transport causes the mesoscale to concentrate the vast majority of the ocean's kinetic energy—roughly one hundred times more than the mean circulation itself—rendering the ocean fundamentally turbulent at this scale.

Oceanic baroclinic instability—the same mechanism previously identified as responsible for atmospheric cyclogenesis—occurs when the ocean's horizontal density gradient stores available potential energy; this instability drives the conversion of that potential energy into the kinetic energy of the aforementioned eddies. The necessary condition for triggering this instability, known as the Charney-Stern condition, requires the quasi-geostrophic potential vorticity to change sign at some point within the vertical water column. The Rossby radius of deformation—approximately fifty kilometers at mid-latitudes—represents the characteristic horizontal scale of the eddies generated by this instability.

In contrast, the submesoscale operates on much smaller scales, ranging from just one hundred meters to ten kilometers, with time scales of hours to days and a Rossby number close to one. Oceanic fronts, filaments, and submesoscale vortices emerge at this scale; they are dynamically significant for three simultaneous reasons: they actively transport substances into the ocean interior through a process known as subduction; they enhance the supply of nutrients to the layer where photosynthesis occurs; ...and they contribute to tracer mixing to a degree disproportionate to their small size; consequently, the submesoscale—despite occupying a tiny fraction of the ocean—determines much of how nutrients are distributed.

The expected behavior involves energy flowing simultaneously in two opposite directions: an inverse cascade transporting energy from the submesoscale to the mesoscale, causing eddies to grow rather than dissipate; and a forward cascade carrying that same energy in the opposite direction, from the submesoscale down to the microscales, where it is ultimately fully dissipated. Thus, from the mesoscale eddies triggered by the baroclinic instability of major currents to the submesoscale fronts and filaments that redistribute nutrients and tracers, the ocean organizes its turbulence into a hierarchy of scales, ensuring that energy never ceases to move—either upward or downward—within that same spectrum.

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