E3 Turbulence and Mixing

Topic

Ocean mixing is the process by which properties such as heat, salt, and nutrients diffuse—both vertically and horizontally—between water masses of differing densities. It is precisely this diffusion of properties that prevents the ocean from remaining a mere stack of isolated layers, causing it instead to constantly exchange heat, salt, and nutrients between adjacent layers.

In the ocean interior—beyond the surface and bottom boundary layers—diapycnal mixing (mixing that cuts across density surfaces) is extremely weak, with a diffusivity of barely one hundred-thousandth of a square meter per second; yet, it is absolutely critical for sustaining global thermohaline circulation. Without this mixing—however weak—the deep ocean would eventually stagnate. The primary energy source for this interior mixing is the breaking of internal gravity waves, generated mainly by tides; this breaking ensures that some mixing persists even in the ocean's calmest depths.

In the surface mixed layer, by contrast, turbulence is far more intense: diffusivity reaches one-hundredth of a square meter per second—a thousand times higher than in the ocean interior—driven by wind stress on the surface, convection caused by surface heat loss, and wave action itself. It is this combination of three distinct forcing agents that causes the surface mixed layer to be agitated far more violently than any other part of the ocean.

Double diffusion is a mixing mechanism unique to the ocean in which heat and salt diffuse at vastly different rates—heat diffuses nearly a hundred times faster than salt—and this difference in rates can trigger convection even when the water's overall density is stable. So-called "salt fingers" are the best-known manifestation of this double diffusion: they occur when warm, salty water floats atop cold, less salty water, and the difference in the diffusion rates of heat and salt causes the formation of thin, vertical water filaments that sink while exchanging heat and salt with the surrounding water.

The standard scheme used by ocean models to calculate this vertical turbulent diffusivity constructs a profile that depends simultaneously on depth, stratification stability, and surface forcing; this profile allows a single, coherent scheme to represent both the intense mixing of the surface layer and the weak mixing of the interior. The expected behavior is for ocean mixing to be strong in the surface mixed layer, weak—though never exactly zero—in the interior, and even more intense over rougher seafloor topography (such as mid-ocean ridges and continental slopes), where internal gravity waves are generated and break more easily than over a smooth bottom.

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