Mixing Processes

The ocean’s effect on weather and climate is governed largely by processes occurring in the few tens of meters of water bordering the ocean surface. For example, water warmed at the surface on a sunny afternoon may remain available to warm the atmosphere that evening, or it may be mixed deeper into the ocean not to emerge for many years, depending on near-surface mixing processes. Local mixing of the upper ocean is predominantly forced from the state of the atmosphere directly above it. The daily cycle of heating and cooling, wind, rain, and changes in temperature and humidity associated with mesoscale weather features produce a hierarchy of physical processes that act and interact to stir the upper ocean. Some of these are well understood, whereas others have deRed both observational description and theoretical understanding. This article begins with an example of in situ measurements of upper ocean properties. These observations illustrate the tremendous complexity of the physics, and at the same time reveal some intriguing regularities. We then describe a set of idealized model processes that appear relevant to the observations and in which the underlying physics is understood, at least at a rudimentary level. These idealized processes are Rrst summarized, then discussed individually in greater detail. The article closes with a brief survey of methods for representing upper ocean mixing processes in large-scale ocean models. Over the past 20 years it has become possible to make intensive turbulence proRling observations that reveal the structure and evolution of upper ocean mixing. An example is shown in Figure 1, which illustrates mixed-layer evolution, temperature structure and small-scale turbulence.The small white dots in Figure 1 indicate the depth above which stratiRcation is neutral or unstable and mixing is intense, and below which stratiRcation is stable and mixing is suppressed. This represents a means of determining the vertical extent of the mixed layer directly forced by local atmospheric conditions. (We will call the mixed-layer depth D.) Following the change in sign from negative (surface heating) to positive (surface cooling) of the surface buoyancy Sux, Jb, the mixed layer deepens. Jb represents the Sux of density (mass per unit volume) across the sea surface due to the combination of heating/cooling and evaporation./precipitation. The mixed layer shown in Figure 1 deepens each night, but the rate of deepening and Rnal depth vary. Each day, following the onset of daytime heating, the mixed layer becomes shallower. SigniRcant vertical structure is evident within the nocturnal mixed layer. The maximum potential temperature (h) is found at mid-depth. Above this, h is smaller and decreases toward the surface at the rate of about 2 mK in 10 m. The adiabatic change in temperature, that due to compression of Suid parcels with increasing depth, is 1 mK in 10 m. The region above the temperature maximum is superadiabatic, and hence prone to convective instability. Below this superadiabatic surface layer is a layer of depth 10}30 m in which the temperature change is less than 1 mK. Within this mixed layer, the intensity of turbulence, as quantiRed by the turbulent kinetic energy dissipation rate e, is relatively uniform and approximately equal to Jb. e represents the rate at which turbulent motions in a Suid are dissipated to heat. It is an important term in the evolution equation for turbulent kinetic energy, signifying the tendency for turbulence to decay in the absence of forcing. Below the mixed layer, e generally (but not always) decreases, whereas above, e increases by 1}2 factors of 10. Below the mixed layer is a region of stable stratiRcation that partially insulates the upper ocean from the ocean interior. Heat, momentum, and chemical species exchanged between the atmosphere and the ocean interior must traverse the centimeters thick cool skin at the very surface, the surface layer, and the mixed layer to modify the stable layer below. These vertical transports are governed by a combination of processes, including those that affect only the surface itself (rainfall, breaking surface gravity waves), those that communicate directly from the surface throughout the entire mixed layer (convective plumes) or a good portion of it (Langmuir circulations) and also those processes that are forced at the surface but have effects concentrated at the mixed-layer base (inertial shear, Kelvin}Helmholtz instability, propagating internal gravity waves). Several of these processes are represented in schematic form in Figure 2. Whereas Figure 1 represents the 0001