Surface layer

Surface layers are characterized by large normal gradients of tangential velocity and large concentration gradients of any substances (temperature, moisture, sediments et cetera) transported to or from the interface.

In non-neutral conditions the mixing length is also affected by buoyancy forces and Monin-Obukhov similarity theory is required to describe the horizontal-wind profile.

Once formed at a particular source, a water mass will travel some distance via large-scale ocean circulation.

Typically, the flow of water in the ocean is described as turbulent (i.e. it doesn't follow straight lines).

The logarithmic flow profile has long been observed in the ocean, but recent, highly sensitive measurements reveal a sublayer within the surface layer in which turbulent eddies are enhanced by the action of surface waves.

[5] It is becoming clear that the surface layer of the ocean is only poorly modeled as being up against the "wall" of the air-sea interaction.

[6] The depth of the surface mixed layer is affected by solar insolation and thus is related to the diurnal cycle.

After nighttime convection over the ocean, the turbulent surface layer is found to completely decay and restratify.

The decay is caused by the decrease in solar insolation, divergence of turbulent flux and relaxation of lateral gradients.

[7] During the nighttime, the surface ocean cools because the atmospheric circulation is reduced due to the change in heat with the setting of the sun each day.

[8] In general, the surface mixed layer only occupies the first 100 meters of the ocean but can reach 150 m in the end of winter.

The diurnal cycle does not change the depth of the mixed layer significantly relative to the seasonal cycle which produces much larger changes in sea surface temperature and buoyancy.

With several vertical profiles, one can estimate the depth of the mixed layer by assigning a set temperature or density difference in water between surface and deep ocean observations – this is known as the “threshold method”.

One study explored diurnal variability of the mixed layer depth in the Western Equatorial Pacific Ocean.

Results suggested no appreciable change in the mixed layer depth with the time of day.

The significant precipitation in this tropical area would lead to further stratification of the mixed layer.

[9] Another study which instead focused on the Central Equatorial Pacific Ocean found a tendency for increased depths of the mixed layer during nighttime.

Over a 15-day study period in Australia, the diurnal mixed layer cycle repeated in a consistent manner with decaying turbulence throughout the day.