Mixing and Turbulence in the Ocean

Turbulence at scales of centimetres and below quietly accomplishes what the great currents cannot: it stirs heat, salt, nutrients, and momentum across density surfaces and ultimately sustains the deep overturning of the entire ocean.

Definition

Ocean mixing is the irreversible transfer of properties such as heat, salt, and momentum by small-scale turbulent motions, while turbulence is the chaotic, three-dimensional fluid motion that produces this transfer.

Scope

This topic covers the generation of turbulence by wind, convection, and shear; the structure and entrainment of the surface mixed layer; diapycnal (cross-isopycnal) mixing in the interior driven largely by breaking internal waves; double-diffusive processes; and the parameterization of mixing in ocean and climate models.

Core questions

• What processes generate turbulence in the surface layer and in the stratified interior?
• How does turbulence set the depth and properties of the mixed layer?
• What controls the rate of diapycnal mixing that allows dense deep water to return to the surface?
• How can mixing, which acts at unresolved scales, be represented in large-scale ocean models?

Key theories

Shear instability and the Richardson number

Stratified shear flow becomes turbulent when the destabilizing effect of velocity shear overcomes the stabilizing effect of stratification, a transition predicted by a low gradient Richardson number.

Energetics of abyssal mixing

Sustaining the observed deep stratification and overturning requires a global supply of mixing energy, traced largely to winds and to internal tides breaking over rough topography.

Mechanisms

Wind stress and surface cooling generate convection and shear that mix the upper ocean into a near-homogeneous layer; in the interior, internal waves grow, steepen, and break where the Richardson number falls low enough for shear instability, producing patches of turbulence that mix water across density surfaces. The cumulative effect raises dense deep water and closes the overturning circulation.

Clinical relevance

Mixing sets the supply of nutrients to sunlit surface waters and thus primary production, controls how quickly the ocean absorbs heat and carbon, and is one of the largest sources of uncertainty in climate projections because it must be parameterized rather than resolved.

History

Munk's 1966 Abyssal Recipes posed the problem of how much mixing is needed to maintain the deep ocean's stratification; microstructure measurements from the 1970s onward, culminating in tracer-release experiments and the Munk-Wunsch energetics framework of 1998, established mixing as a central, energy-limited control on global circulation.

Key figures

• Walter Munk
• Carl Wunsch
• Lewis Fry Richardson

Seminal works

• thorpe2005
• munkWunsch1998

Frequently asked questions

Why does deep-ocean mixing matter for climate?

Mixing allows cold, dense deep water to slowly return upward, closing the global overturning circulation that redistributes heat and carbon; its rate strongly influences how the ocean buffers climate change.

Where does the energy for ocean mixing come from?

Most of it comes from the wind blowing on the surface and from tides driving internal waves that break over rough seafloor topography, converting large-scale energy into small-scale turbulence.