Chapter 1: INTRODUCTION
and winter. Wind plays a large role in the development of the mixed layer
because of the vertical shear resulting from wind-induced surface currents.
Seasonal wind variations can account somewhat for seasonal mixed layer
depth since winds are stronger in the winter and weaker in the summer. In
addition, heating and cooling influence mixed layer depth. Stronger heating
during the summer leads to greater stratification, stability, and lower
potential energy. On the other hand, cooling destabilizes the water column in
the winter, allowing mixing to occur even in the absence of winds. For this
reason winds and surface heat fluxes combine to affect formation and
seasonal variation of the mixed layer.
In addition to seasonal differences, latitudinal and hemispheric trends are
present in mixed layer depth. Seasonal variation in mixed layer depth
generally increases poleward. This is a result of increased winds and cooling
towards higher latitudes. Mixed layers are generally deeper in the Southern
Hemisphere than in the Northern hemisphere, especially during the summer
(Soloviev and Klinger, 2001). This may reflect the fact that winds are
generally stronger in the Southern Hemisphere and there is a greater input of
heat in the Northern Hemisphere summer.
There are also significant differences in mixed layer depth between
oceans. During the winter, the high latitude North Atlantic develops very
deep (>1000 m) mixed layers. The winter deepening is not nearly as
intensive in the North Pacific. There is net evaporation and hence higher
salinity in the North Atlantic, which may lead to a greater destabilization of
the water column, accounting for the difference.
The mixed layer of the ocean is an important part of the global climate
system. It effectively exchanges momentum, energy, and greenhouse gases
with the atmosphere. Turbulent transport in the mixed layer also controls the
supply of nutrients to the upper, sunlit layers, greatly affecting the
phytoplankton grows and, consequently, the overall biological productivity
of the ocean.
1.7.4 Barrier layer
The depth of the surface mixed layer was traditionally determined as the
depth over which the temperature is uniform. However, in the presence of
strong freshwater sources (rainfalls, ice melting, river runoff) there is often a
halocline within the isothermal layer, which results in a change in density.
Lukas and Lindstrom (1991) refer to the layer between the tops of the
halocline and the thermocline as the barrier layer because of its impact on
the heat budget of the upper ocean. An example of the barrier layer from the
Indian Ocean is shown in Figure 1-18. The temperature within the barrier
63
and winter. Wind plays a large role in the development of the mixed layer
because of the vertical shear resulting from wind-induced surface currents.
Seasonal wind variations can account somewhat for seasonal mixed layer
depth since winds are stronger in the winter and weaker in the summer. In
addition, heating and cooling influence mixed layer depth. Stronger heating
during the summer leads to greater stratification, stability, and lower
potential energy. On the other hand, cooling destabilizes the water column in
the winter, allowing mixing to occur even in the absence of winds. For this
reason winds and surface heat fluxes combine to affect formation and
seasonal variation of the mixed layer.
In addition to seasonal differences, latitudinal and hemispheric trends are
present in mixed layer depth. Seasonal variation in mixed layer depth
generally increases poleward. This is a result of increased winds and cooling
towards higher latitudes. Mixed layers are generally deeper in the Southern
Hemisphere than in the Northern hemisphere, especially during the summer
(Soloviev and Klinger, 2001). This may reflect the fact that winds are
generally stronger in the Southern Hemisphere and there is a greater input of
heat in the Northern Hemisphere summer.
There are also significant differences in mixed layer depth between
oceans. During the winter, the high latitude North Atlantic develops very
deep (>1000 m) mixed layers. The winter deepening is not nearly as
intensive in the North Pacific. There is net evaporation and hence higher
salinity in the North Atlantic, which may lead to a greater destabilization of
the water column, accounting for the difference.
The mixed layer of the ocean is an important part of the global climate
system. It effectively exchanges momentum, energy, and greenhouse gases
with the atmosphere. Turbulent transport in the mixed layer also controls the
supply of nutrients to the upper, sunlit layers, greatly affecting the
phytoplankton grows and, consequently, the overall biological productivity
of the ocean.
1.7.4 Barrier layer
The depth of the surface mixed layer was traditionally determined as the
depth over which the temperature is uniform. However, in the presence of
strong freshwater sources (rainfalls, ice melting, river runoff) there is often a
halocline within the isothermal layer, which results in a change in density.
Lukas and Lindstrom (1991) refer to the layer between the tops of the
halocline and the thermocline as the barrier layer because of its impact on
the heat budget of the upper ocean. An example of the barrier layer from the
Indian Ocean is shown in Figure 1-18. The temperature within the barrier
63