1 Sverdrup Theory
1.1 Introduction
Pressure maps of the flow in the upper ocean, such as that shown in Fig. 1.1.1
(a, Stommel et al. 1978; b, Reid 1994) indicate a large-scale clockwise circulation in the subtropical North Atlantic, i.e., in the region south of 45°N but
north of a few degrees of the equator. Similarly deduced patterns of circulation
have been found in each of the oceans in both northern and southern
hemispheres, and the reader is referred to any number of introductory texts
(for example, Pickard and Emery 1982) dealing with descriptive oceanography
for a review of the overall circulation patterns deduced from a painstaking
accumulation of oceanographic data over the past 100 years. Such large-scale
patterns of flow comprise the general circulation of the ocean, and this book
presents some recent theoretical advances in our understanding of the
dynamics of the circulation. We refer at times to observations pertinent to
particular theoretical issues, but we do not attempt to discuss comprehensively
the observational description of the general circulation.
In each basin of the great world oceans the overall pattern of flow is
notably similar within the first 1-2 km in depth. A broad flow, sweeping
clockwise across the entire breadth of the basin is returned within a remarkably
narrow western boundary current; in the case of the North Atlantic this is the
Gulf Stream, and its presence is apparent in the closely packed isolines of
pressure in the western part of the gyre. Although it is difficult even today to
give a precise numerical value for the magnitudes of the various parts of this
upper ocean circulation, its broad outlines have become increasingly clear.
Schmitz and McCartney (1993), for example, have attempted to synthesize
available data and to assign transport values to the component parts of the
circulation, as shown in Fig. 1.1.2. Each isoline in the figure is labeled by a
circled number indicating the estimated transport on each such branch of the
circulation. The units are in sverdrups (1 sverdrup = I million m 3 /s). To obtain
transports of this magnitude over scales of the order of a few thousand
kilometers and a depth of about 1 km requires an average velocity in the region
of the broad southward flow of the order of 1 cmjs.
This glimpse of the general circulation of the ocean, limited as it is to the
upper ocean and to the subtropical gyres, is still sufficient to alert us to one of
1.1 Introduction
Pressure maps of the flow in the upper ocean, such as that shown in Fig. 1.1.1
(a, Stommel et al. 1978; b, Reid 1994) indicate a large-scale clockwise circulation in the subtropical North Atlantic, i.e., in the region south of 45°N but
north of a few degrees of the equator. Similarly deduced patterns of circulation
have been found in each of the oceans in both northern and southern
hemispheres, and the reader is referred to any number of introductory texts
(for example, Pickard and Emery 1982) dealing with descriptive oceanography
for a review of the overall circulation patterns deduced from a painstaking
accumulation of oceanographic data over the past 100 years. Such large-scale
patterns of flow comprise the general circulation of the ocean, and this book
presents some recent theoretical advances in our understanding of the
dynamics of the circulation. We refer at times to observations pertinent to
particular theoretical issues, but we do not attempt to discuss comprehensively
the observational description of the general circulation.
In each basin of the great world oceans the overall pattern of flow is
notably similar within the first 1-2 km in depth. A broad flow, sweeping
clockwise across the entire breadth of the basin is returned within a remarkably
narrow western boundary current; in the case of the North Atlantic this is the
Gulf Stream, and its presence is apparent in the closely packed isolines of
pressure in the western part of the gyre. Although it is difficult even today to
give a precise numerical value for the magnitudes of the various parts of this
upper ocean circulation, its broad outlines have become increasingly clear.
Schmitz and McCartney (1993), for example, have attempted to synthesize
available data and to assign transport values to the component parts of the
circulation, as shown in Fig. 1.1.2. Each isoline in the figure is labeled by a
circled number indicating the estimated transport on each such branch of the
circulation. The units are in sverdrups (1 sverdrup = I million m 3 /s). To obtain
transports of this magnitude over scales of the order of a few thousand
kilometers and a depth of about 1 km requires an average velocity in the region
of the broad southward flow of the order of 1 cmjs.
This glimpse of the general circulation of the ocean, limited as it is to the
upper ocean and to the subtropical gyres, is still sufficient to alert us to one of