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DYNAMICAL OCEANOGRAPHY
In this chapter, we focus on the understanding of the wind-driven ocean
circulation in the North Atlantic. After an introduction on this flow in
section 5.1, an idealized model of a constant density flow (the barotropic
midlatitude β-plane model) is derived in section 5.2. Local solutions of
this model are constructed using asymptotic methods (inner- and outer
expansions) which will naturally lead to the concepts of geostrophic equilibrium and the Ekman boundary layers (section 5.3) and the formulation
of the barotropic quasi-geostrophic vorticity equation (section 5.4).
5.1. The North Atlantic surface circulation
A view of the surface circulation in the Atlantic basin can be obtained from an
inspection of Fig. 2.4a. In the North Atlantic, the circulation is characterized by
two cells, usually called ‘gyres’. The smallest one, the subpolar gyre, is formed
by currents south of Greenland and in the Labrador Sea such as the Irminger
Current, the West Greenland Current and the Labrador Current. The largest one,
the subtropical gyre, is formed by currents around the Sargasso Sea, such as the
North Equatorial Current to the south. Near the western boundary, the latter current joins part of the South Equatorial current and part of the combined currents
flows northward as the Antilles Current, while the other part flows into the Gulf
of Mexico. The latter water eventually ‘escapes’ between Florida and Cuba as the
Florida Current.
A merger of the Florida and Antilles current leaves the east coast of the US at
Cape Hatteras and is then called the Gulf Stream. Snapshots of the circulation in
the Gulf Stream region from a high resolution model are plotted in Fig. 5.1. The
Gulf Stream flows northeastwards towards the Grand Banks of Newfoundland
near (40 ◦ N, 50 ◦ W). The current that exists north-eastward of the Gulf Stream is
called the North Atlantic Current. This current bifurcates into a part that contributes to the flows in the Norwegian Sea and a part that deflects southward as
the Canary current.
Ex. 5.1
Characteristic of the flow in the North Atlantic as sketched in Fig. 5.1 are strong
boundary currents (Florida Current and Gulf Stream) which exist at the western
side of the basin. The Florida Current is situated above the continental slope,
but the Gulf Stream is situated in the open ocean. The horizontal velocities in
the Gulf Stream (up to 2.5 ms −1 ) are among the largest measured in the ocean.
Its mean velocity is about 1.5 ms −1 and its average width is about 115 km. The
average volume transport of the Florida Current is 30 Sv (1 Sv = 10 6 m 3 s −1 ). This
transport increases towards Cape Hatteras up to 150 Sv near 65 ◦ W, but decreases
againto35Svnear40 ◦ W. In comparison, the average surface current velocity in
the subtropical gyre is about 0.1 ms −1 .
DYNAMICAL OCEANOGRAPHY
In this chapter, we focus on the understanding of the wind-driven ocean
circulation in the North Atlantic. After an introduction on this flow in
section 5.1, an idealized model of a constant density flow (the barotropic
midlatitude β-plane model) is derived in section 5.2. Local solutions of
this model are constructed using asymptotic methods (inner- and outer
expansions) which will naturally lead to the concepts of geostrophic equilibrium and the Ekman boundary layers (section 5.3) and the formulation
of the barotropic quasi-geostrophic vorticity equation (section 5.4).
5.1. The North Atlantic surface circulation
A view of the surface circulation in the Atlantic basin can be obtained from an
inspection of Fig. 2.4a. In the North Atlantic, the circulation is characterized by
two cells, usually called ‘gyres’. The smallest one, the subpolar gyre, is formed
by currents south of Greenland and in the Labrador Sea such as the Irminger
Current, the West Greenland Current and the Labrador Current. The largest one,
the subtropical gyre, is formed by currents around the Sargasso Sea, such as the
North Equatorial Current to the south. Near the western boundary, the latter current joins part of the South Equatorial current and part of the combined currents
flows northward as the Antilles Current, while the other part flows into the Gulf
of Mexico. The latter water eventually ‘escapes’ between Florida and Cuba as the
Florida Current.
A merger of the Florida and Antilles current leaves the east coast of the US at
Cape Hatteras and is then called the Gulf Stream. Snapshots of the circulation in
the Gulf Stream region from a high resolution model are plotted in Fig. 5.1. The
Gulf Stream flows northeastwards towards the Grand Banks of Newfoundland
near (40 ◦ N, 50 ◦ W). The current that exists north-eastward of the Gulf Stream is
called the North Atlantic Current. This current bifurcates into a part that contributes to the flows in the Norwegian Sea and a part that deflects southward as
the Canary current.
Ex. 5.1
Characteristic of the flow in the North Atlantic as sketched in Fig. 5.1 are strong
boundary currents (Florida Current and Gulf Stream) which exist at the western
side of the basin. The Florida Current is situated above the continental slope,
but the Gulf Stream is situated in the open ocean. The horizontal velocities in
the Gulf Stream (up to 2.5 ms −1 ) are among the largest measured in the ocean.
Its mean velocity is about 1.5 ms −1 and its average width is about 115 km. The
average volume transport of the Florida Current is 30 Sv (1 Sv = 10 6 m 3 s −1 ). This
transport increases towards Cape Hatteras up to 150 Sv near 65 ◦ W, but decreases
againto35Svnear40 ◦ W. In comparison, the average surface current velocity in
the subtropical gyre is about 0.1 ms −1 .