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8 Transport in the Oceans and Coastal Zone
Measurements of the vertical and horizontal temperature gradients in the salt
fingers showed that the average ratio of the width to the length of salt fingers
was about 0.58, so that the fingers are apparently a little less than twice as long
as they are wide. In the ocean there are many areas where the temperature
and salinity profiles are suitable for double diffusion to occur. Examples are
the Strait of Gibraltar where as many as 20 steps of decreasing temperature
and salinity extends horizontally for 30 miles. A greater number of steps have
been discovered in the Arctic Ocean above a layer of warm saline water entering
from the Atlantic.
Salt fingers may also produce significant fluxes of salt and heat in the central
waters of the World Ocean. Such water masses comprise the upper few, to
several hundred meters, of the huge subtropical gyres in which excess of heat
and salt, produced by strong surface heating, are redistributed both laterally
and vertically. When salt fingering dominates and turbulent mixing is negligible, the coefficient of vertical diffusion of salt is higher than that for turbulent
mixing regimes. According to Hamilton et al. (1993) estimation for the Canary
Basin these two values are 2.8x 10- 5 m 2 /s and 13x 10- 5 m 2 /s, respectively.
Gargett and Schmitt (1982) presented direct measurements of the temperature signature of salt fingers in the eastern North Pacific. Salt-fingering signature were observed throughout the mid-gyre towing station. Similarly to
the North Pacific gyre, salt fingers should be endemic to the subtropical gyres
of the South Pacific and Indian Oceans and to the entire Atlantic between
about 400N and 40 0 S (Gargett and Schmitt, 1982). However, a stratification
characterized by the presence of double diffusion of heat and salt, when the
temperature and salinity gradients make opposing contribution to the density
gradient, are not typical in the ocean. Usually a temperature decreases with
depth while salinity increases, and stratification becomes stable. The reader
should consult Monin and Ozmidov (1985) for an in-depth discussion.
8.5 Diffusion and Mixing in Estuaries
8.5.1 Introduction and Classification
The term 'estuary' is derived from the Latin word estuarium which means tidal.
A typical estuary can be defined as a semi-enclosed coastal basin that receives
an inflow of both freshwater and salt water, and is affected by tidal oscillations.
There are various classifications of estuaries depending on the phenomena which
are taken into account. In this section we will mostly concentrate on water
dynamics, stratification and mixing in estuaries.
Under this assumption, estuaries can be assigned to one of three general types:
the salt-wedge estuary, the partly mixed estuary, and the well-mixed estuary
(Fig. 8.12). The relative contributions of river flow to tidal currents are largely
responsible for the chemical, circulatory and sedimentologic characteristics of
all these types of estuaries (Pinet, 1992).
8 Transport in the Oceans and Coastal Zone
Measurements of the vertical and horizontal temperature gradients in the salt
fingers showed that the average ratio of the width to the length of salt fingers
was about 0.58, so that the fingers are apparently a little less than twice as long
as they are wide. In the ocean there are many areas where the temperature
and salinity profiles are suitable for double diffusion to occur. Examples are
the Strait of Gibraltar where as many as 20 steps of decreasing temperature
and salinity extends horizontally for 30 miles. A greater number of steps have
been discovered in the Arctic Ocean above a layer of warm saline water entering
from the Atlantic.
Salt fingers may also produce significant fluxes of salt and heat in the central
waters of the World Ocean. Such water masses comprise the upper few, to
several hundred meters, of the huge subtropical gyres in which excess of heat
and salt, produced by strong surface heating, are redistributed both laterally
and vertically. When salt fingering dominates and turbulent mixing is negligible, the coefficient of vertical diffusion of salt is higher than that for turbulent
mixing regimes. According to Hamilton et al. (1993) estimation for the Canary
Basin these two values are 2.8x 10- 5 m 2 /s and 13x 10- 5 m 2 /s, respectively.
Gargett and Schmitt (1982) presented direct measurements of the temperature signature of salt fingers in the eastern North Pacific. Salt-fingering signature were observed throughout the mid-gyre towing station. Similarly to
the North Pacific gyre, salt fingers should be endemic to the subtropical gyres
of the South Pacific and Indian Oceans and to the entire Atlantic between
about 400N and 40 0 S (Gargett and Schmitt, 1982). However, a stratification
characterized by the presence of double diffusion of heat and salt, when the
temperature and salinity gradients make opposing contribution to the density
gradient, are not typical in the ocean. Usually a temperature decreases with
depth while salinity increases, and stratification becomes stable. The reader
should consult Monin and Ozmidov (1985) for an in-depth discussion.
8.5 Diffusion and Mixing in Estuaries
8.5.1 Introduction and Classification
The term 'estuary' is derived from the Latin word estuarium which means tidal.
A typical estuary can be defined as a semi-enclosed coastal basin that receives
an inflow of both freshwater and salt water, and is affected by tidal oscillations.
There are various classifications of estuaries depending on the phenomena which
are taken into account. In this section we will mostly concentrate on water
dynamics, stratification and mixing in estuaries.
Under this assumption, estuaries can be assigned to one of three general types:
the salt-wedge estuary, the partly mixed estuary, and the well-mixed estuary
(Fig. 8.12). The relative contributions of river flow to tidal currents are largely
responsible for the chemical, circulatory and sedimentologic characteristics of
all these types of estuaries (Pinet, 1992).