THE NEAR-SURFACE LAYER OF THE OCEAN
Stable stratification inhibits turbulent mixing below the relatively thin
near-surface convection layer. Vertical mixing of momentum is confined to
the shallow diurnal mixed layer, so that during the day, flow driven directly
by the wind stress is confined to a similarly thin current known as the diurnal
jet. In the evening, when convection is no longer confined by the solar
radiation effect, convective thermals penetrate deeper into the mixed layer,
increasing the turbulent mixing of momentum at the bottom of the diurnal
jet. The diurnal jet then releases its kinetic energy during a relatively short
time period. This process is so intense that the kinetic energy released
cannot be dissipated locally. As a result, a Kelvin-Helmholtz type of
instability is formed, which generates billows – another type of organized
structure. The billows intensify the deepening of the diurnal mixed layer.
The theoretical basis for this mechanism is described in Section 5.5.3.
Although the energy of convective elements is relatively small, it serves
as a catalyst for the release of the kinetic energy by the mean flow. In most
of the equatorial ocean, the Equatorial Undercurrent intensifies the shear in
the upper ocean; the evening deepening of the diurnal jet is therefore
sometimes so intense that it resembles a shock wave, which radiates very
intense high frequency internal waves in the underlying thermocline.
The diurnal cycle is often omitted from numerical ocean models for
reasons of computational cost. However, the mixed layer response to dailyaveraged surface fluxes is not necessarily the same as the average response
to the diurnal cycle (Shinoda et al., 1998; McCreary et al., 2001).Neglecting
the diurnal cycle replaces periodic nightly convective pulses with chronic
mixing that does not reach as deep.
Upper ocean convection is a mechanism effectively controlling the
seasonal cycle in the ocean as well (Woods and Barkman, 1986b).
Resolution of diurnal changes is usually uneconomical when the seasonal
cycle is considered. Because of the nonlinear response of the upper ocean to
atmospheric forcing, simply averaged heat fluxes cannot be used to estimate
the contribution of convection on the seasonal scale. The sharp transition
between the nocturnal period, when convection dominates mixing in the
surface layer, and the daytime period, when the Sun severely limits the depth
of convection leaving the wind stress to control mixing, may in fact simplify
the design of models for the seasonal cycle of the upper ocean.
Parameterization of the convection on the seasonal and global scale is
therefore an important task for the prediction of climate and its changes.
Though free convection is probably one of the most studied types of
organized motion, many intriguing questions regarding convection in the
open ocean still remain. Some of them, like the role of penetrative
convection in mixed layer dynamics, are of crucial importance for
improvement of global ocean circulation modeling. Others, like the role of
392
Stable stratification inhibits turbulent mixing below the relatively thin
near-surface convection layer. Vertical mixing of momentum is confined to
the shallow diurnal mixed layer, so that during the day, flow driven directly
by the wind stress is confined to a similarly thin current known as the diurnal
jet. In the evening, when convection is no longer confined by the solar
radiation effect, convective thermals penetrate deeper into the mixed layer,
increasing the turbulent mixing of momentum at the bottom of the diurnal
jet. The diurnal jet then releases its kinetic energy during a relatively short
time period. This process is so intense that the kinetic energy released
cannot be dissipated locally. As a result, a Kelvin-Helmholtz type of
instability is formed, which generates billows – another type of organized
structure. The billows intensify the deepening of the diurnal mixed layer.
The theoretical basis for this mechanism is described in Section 5.5.3.
Although the energy of convective elements is relatively small, it serves
as a catalyst for the release of the kinetic energy by the mean flow. In most
of the equatorial ocean, the Equatorial Undercurrent intensifies the shear in
the upper ocean; the evening deepening of the diurnal jet is therefore
sometimes so intense that it resembles a shock wave, which radiates very
intense high frequency internal waves in the underlying thermocline.
The diurnal cycle is often omitted from numerical ocean models for
reasons of computational cost. However, the mixed layer response to dailyaveraged surface fluxes is not necessarily the same as the average response
to the diurnal cycle (Shinoda et al., 1998; McCreary et al., 2001).Neglecting
the diurnal cycle replaces periodic nightly convective pulses with chronic
mixing that does not reach as deep.
Upper ocean convection is a mechanism effectively controlling the
seasonal cycle in the ocean as well (Woods and Barkman, 1986b).
Resolution of diurnal changes is usually uneconomical when the seasonal
cycle is considered. Because of the nonlinear response of the upper ocean to
atmospheric forcing, simply averaged heat fluxes cannot be used to estimate
the contribution of convection on the seasonal scale. The sharp transition
between the nocturnal period, when convection dominates mixing in the
surface layer, and the daytime period, when the Sun severely limits the depth
of convection leaving the wind stress to control mixing, may in fact simplify
the design of models for the seasonal cycle of the upper ocean.
Parameterization of the convection on the seasonal and global scale is
therefore an important task for the prediction of climate and its changes.
Though free convection is probably one of the most studied types of
organized motion, many intriguing questions regarding convection in the
open ocean still remain. Some of them, like the role of penetrative
convection in mixed layer dynamics, are of crucial importance for
improvement of global ocean circulation modeling. Others, like the role of
392