THE NEAR-SURFACE LAYER OF THE OCEAN
that of Q E or Q L (except perhaps in some extreme situations such as cold air
outbreaks over warm western boundary currents). The top of the water
column becomes colder and denser than the water below, and convection
begins. In this way, cooling is associated with the homogenization of the
water column and the deepening of the mixed layer. Warming due to solar
radiation occurs in the surface layer of the ocean and is associated with restratification and reduction in mixed layer depth (see Chapter 4). The most
prominent examples of this mixing/re-stratification process are the diurnal
cycle (night-time cooling and day-time warming) and the seasonal cycle
(winter cooling and summer warming).
There are also important geographical variations in convection, with net
cooling of relatively warm water occurring more at higher latitudes and a net
warming of water occurring closer to the equator. For this reason, mixed
layer depth generally increases towards the poles, though at very high
latitudes ice-melt can lower the surface salinity enough to inhibit convection.
Over most of the ocean, annual average mixed layer depths are in the range
of 30 to 100 m, though very dramatic convection in such places as the
Labrador Sea, Greenland Sea, and western Mediterranean Sea can deepen
the mixed layer to thousands of meters. The deep convection can be
significantly affected by the rotation of the Earth.
Convection directly affects several aspects of the near-surface ocean.
Most obviously, the velocity patterns of the turbulent flow are influenced by
the presence of convection, as is the velocity scale. The convective velocity
field then controls the vertical transport of heat (or more correctly, internal
energy), salinity, momentum, dissolved gases, and other properties, and the
vertical gradients of these properties within the mixed layer. Convection
helps to determine property exchanges between the atmosphere and ocean
and the upper ocean and the deep ocean. The importance of convection for
heat and gas exchange has implications for climate studies, while convective
influence on the biologically productive euphotic zone has biological
implications as well.
5.8.1 Phenomenology
The classical problem of free convection in the ocean is to determine the
motion in a layer of fluid in which the top surface is kept colder than the
bottom surface (Soloviev and Kilnger, 2001). This is an idealization of such
geophysical examples as an ocean being cooled from above or the
atmosphere being heated from below. The classical problem ignores such
complications as wind stress on the surface, waves, topographic
irregularities, and the presence of a stably stratified region below the
convection region. The study of convection started in the early twentieth
386
that of Q E or Q L (except perhaps in some extreme situations such as cold air
outbreaks over warm western boundary currents). The top of the water
column becomes colder and denser than the water below, and convection
begins. In this way, cooling is associated with the homogenization of the
water column and the deepening of the mixed layer. Warming due to solar
radiation occurs in the surface layer of the ocean and is associated with restratification and reduction in mixed layer depth (see Chapter 4). The most
prominent examples of this mixing/re-stratification process are the diurnal
cycle (night-time cooling and day-time warming) and the seasonal cycle
(winter cooling and summer warming).
There are also important geographical variations in convection, with net
cooling of relatively warm water occurring more at higher latitudes and a net
warming of water occurring closer to the equator. For this reason, mixed
layer depth generally increases towards the poles, though at very high
latitudes ice-melt can lower the surface salinity enough to inhibit convection.
Over most of the ocean, annual average mixed layer depths are in the range
of 30 to 100 m, though very dramatic convection in such places as the
Labrador Sea, Greenland Sea, and western Mediterranean Sea can deepen
the mixed layer to thousands of meters. The deep convection can be
significantly affected by the rotation of the Earth.
Convection directly affects several aspects of the near-surface ocean.
Most obviously, the velocity patterns of the turbulent flow are influenced by
the presence of convection, as is the velocity scale. The convective velocity
field then controls the vertical transport of heat (or more correctly, internal
energy), salinity, momentum, dissolved gases, and other properties, and the
vertical gradients of these properties within the mixed layer. Convection
helps to determine property exchanges between the atmosphere and ocean
and the upper ocean and the deep ocean. The importance of convection for
heat and gas exchange has implications for climate studies, while convective
influence on the biologically productive euphotic zone has biological
implications as well.
5.8.1 Phenomenology
The classical problem of free convection in the ocean is to determine the
motion in a layer of fluid in which the top surface is kept colder than the
bottom surface (Soloviev and Kilnger, 2001). This is an idealization of such
geophysical examples as an ocean being cooled from above or the
atmosphere being heated from below. The classical problem ignores such
complications as wind stress on the surface, waves, topographic
irregularities, and the presence of a stably stratified region below the
convection region. The study of convection started in the early twentieth
386