The Mixed Layer

An important feature of the oceans is the top surface layer of constant(!?) density, salinity and temperature known as the mixed layer. The following figure demonstrates the variety of processes taking place on top, below and within the mixed layer. It is for this reason that a good understanding of the properties of the mixed layer is important if we wish to accurately determine the transfer of properties such as heat, light, momentum and salinity between the atmosphere and the ocean.

The mixed layer maintains its constant profile through turbulent mixing at the surface, and as such is especially important around Antarctica, where strong winds are prevalent. As it is the part of the ocean affected most directly by changes in atmospheric conditions, the mixed layer can show strong seasonal variability. Through a release of dense salty water (sea ice formation), we can see the mixed layer deepen in the winter, and through the addition of freshwater in the summer (melting) we expect the mixed layer depth to reduce. This simple idea is shown in the following figure.

A higher wind velocity will increase the ocean surface stress which, as well as driving more turbulent mixing at the surface, can also provide a constant source of open water generation as the ice is forced away from the ice shelf edge. This ‘sea ice factory' can thus lead to a constant supply of cold, dense salty water, potentially giving rise to a very deep mixed layer during the winter months.

Sea Ice

Sea ice covers a large fraction of the earths surface. It is found in both the northern and southern hemispheres and its extent shows strong seasonal (and potentially annual) variability.

Sea ice formation is a complex process and is in itself still an active area of research. The turbulence of the ocean and the salinity of the sea water means sea ice forms very differently to that of pure ice.

The salinity of sea water lowers the freezing point of the the water. For the kinds of salinities we find at the ocean surface (34 parts per thousand) we have a freezing temperature of around -1.8 degrees C (pure water freezes at 0 degrees C!). The sea ice then retains some of this salt in brine pockets which significantly affect the thermal properties of the sea ice. Some of this brine is then drained away (mainly through a process of gravity drainage, although other mechanisms play a role too). We always expect the sea ice to maintain some kind of bulk salinity (a rough approximation of 5 parts per thousand are often used in models). Photo of sea ice courtesy of Rosie Willatt, CPOM.