8.4 Diffusion and Mixing in the Ocean
281
8.4 Diffusion and Mixing in the Ocean
8.4.1 Introduction
In the open ocean, the vertical structure of the water mass is characterized by
the presence of one or more thermoclines, which are regions of large temperature
gradient within the upper few hundred meters. The thermocline structure
is strongest in equatorial waters where there is a great contrast between the
warmed surface layer and the cold deeper water. This contrast weakens at
increasing latitudes.
In the upper ocean, from the thermocline to the surface, there is a large variety
of water motion such as currents, turbulence, internal and surface waves. All
of these motions are responsible for the exchange of matter, momentum and
heat between atmosphere and the underlying deep ocean water. Some of these
mechanisms have been discussed in previous chapters; in this section we will
concentrate on diffusion processes and the resulting mixing in stratified ocean
waters.
From the perspective of this book, one of the most important transport processes is the diffusion of gases in the water column and the dispersion of various
biogenic elements. For consistency, these problems are left to Part III, dedicated to the application of various physical results, reported in Parts I and II,
to marine ecology.
The upper ocean layer is the subject of constant bombardment by a large
variety of particles coming from the atmosphere. Matter of atmospheric origin
penetrates the ocean in different ways. In order to track the intensity of substance penetration, tracers which are easy to detect are required. One type of
such tracers are radioactive isotopes. Generally, in the ocean we observe three
types of isotopes: terrigenic isotopes, which are part of the Earth's core, cosmic
isotopes coming from the cosmos and surrounding atmosphere, and isotopes resulting from human activity. The first two types of isotopes are supplied to the
ocean surface at an almost constant rate, however, some seasonal variation is
observed for cosmic isotopes.
Radioactive isotopes are not the only tracers used to study mixing in the
ocean. In the previous section, we described the use of fluoresceine dye for
mixing in experiments in the Black Sea. During the North Atlantic Tracer
Release Experiment (NATRE) in 1992, sulfur hexafluoride was released within
a few metres of the (Jt = 28.036 isopycnal surface, near 300 m depth, in the
Canary Basin in the Atlantic Ocean (Ruddick and Walsh, 1997). The lateral
dispersion and vertical spreading were observed during the following year.
In the next section we describe the main features of the mixing processes in
the upper ocean using the results of tracer releases. We start with radioactive
isotope experiments.
281
8.4 Diffusion and Mixing in the Ocean
8.4.1 Introduction
In the open ocean, the vertical structure of the water mass is characterized by
the presence of one or more thermoclines, which are regions of large temperature
gradient within the upper few hundred meters. The thermocline structure
is strongest in equatorial waters where there is a great contrast between the
warmed surface layer and the cold deeper water. This contrast weakens at
increasing latitudes.
In the upper ocean, from the thermocline to the surface, there is a large variety
of water motion such as currents, turbulence, internal and surface waves. All
of these motions are responsible for the exchange of matter, momentum and
heat between atmosphere and the underlying deep ocean water. Some of these
mechanisms have been discussed in previous chapters; in this section we will
concentrate on diffusion processes and the resulting mixing in stratified ocean
waters.
From the perspective of this book, one of the most important transport processes is the diffusion of gases in the water column and the dispersion of various
biogenic elements. For consistency, these problems are left to Part III, dedicated to the application of various physical results, reported in Parts I and II,
to marine ecology.
The upper ocean layer is the subject of constant bombardment by a large
variety of particles coming from the atmosphere. Matter of atmospheric origin
penetrates the ocean in different ways. In order to track the intensity of substance penetration, tracers which are easy to detect are required. One type of
such tracers are radioactive isotopes. Generally, in the ocean we observe three
types of isotopes: terrigenic isotopes, which are part of the Earth's core, cosmic
isotopes coming from the cosmos and surrounding atmosphere, and isotopes resulting from human activity. The first two types of isotopes are supplied to the
ocean surface at an almost constant rate, however, some seasonal variation is
observed for cosmic isotopes.
Radioactive isotopes are not the only tracers used to study mixing in the
ocean. In the previous section, we described the use of fluoresceine dye for
mixing in experiments in the Black Sea. During the North Atlantic Tracer
Release Experiment (NATRE) in 1992, sulfur hexafluoride was released within
a few metres of the (Jt = 28.036 isopycnal surface, near 300 m depth, in the
Canary Basin in the Atlantic Ocean (Ruddick and Walsh, 1997). The lateral
dispersion and vertical spreading were observed during the following year.
In the next section we describe the main features of the mixing processes in
the upper ocean using the results of tracer releases. We start with radioactive
isotope experiments.