The ocean and its circulation

9.1. Physical characteristics of the ocean

9.1.1. The ocean basins

9.1.2. The cryosphere

9.1.3. Properties of seawater; equation of state

9.1.4. Temperature, salinity, and temperature structure

9.1.5. The mixed layer and thermocline

9.2. The observed mean circulation

9.3. Inferences from geostrophic and hydrostatic balance

9.3.1. Ocean surface structure and geostrophic flow

9.3.2. Geostrophic flow at depth

9.3.3. Steric effects

9.3.4. The dynamic method

9.4. Ocean eddies

9.4.1. Observations of ocean eddies

9.5. Further reading

9.6. Problems

We now begin our discussion of the circulation of the ocean. In this introductory chapter we describe the physical characteristics of the ocean, the properties of sea water, its global-scale temperature and salinity distributions, and the geography of the basins in which it is contained. We go on to describe, and interpret in terms of the balanced—geostrophic and hydrostatic—dynamics of Chapter 7, the observed pattern of ocean currents. Those readers who have a primary interest in ocean circulation and have not read all the way through to here, will find essential dynamical background reviewed in Chapters 6 and 7, to which frequent reference is made.

The ocean, like the atmosphere, is a stratified fluid on the rotating Earth. The two fluids have many similarities in their behavior and, especially, in their fluid dynamics. However, there are some important differences:

• The fluids are physically different. Water is (almost) incompressible and ocean thermodynamics has no counterpart to atmospheric moisture (as a source of latent heat).

• Unlike the atmosphere, all oceans are laterally bounded by continents (see Fig. 9.1) except in the Southern Ocean where the ocean extends all the way around the globe and fluid can pass

7T ~'—r—r—x—i—i—i—'—T—!—i—i—i—*—i—p—*—c—i—■—i _ i -j__; ' ! ' ' 1 I I—L i — i -. i— i—*—r

FIGURE 9.1. World relief showing elevations over land and ocean depth. White areas over the continents mark the presence of ice at altitudes that exceed 2km. The mean depth of the ocean is 3.7km, but depths sometimes exceed 6 km. The thin white line meandering around the ocean basins marks a depth of 4 km.

FIGURE 9.1. World relief showing elevations over land and ocean depth. White areas over the continents mark the presence of ice at altitudes that exceed 2km. The mean depth of the ocean is 3.7km, but depths sometimes exceed 6 km. The thin white line meandering around the ocean basins marks a depth of 4 km.

through Drake Passage, the narrow (600 km) gap between the tip of South America and the Antarctic peninsula.

• The ocean circulation is forced in a different way than the atmosphere. We have seen that the atmosphere is largely transparent to solar radiation and is heated from below by convection. By contrast, the ocean exchanges heat and moisture with the atmosphere at its upper surface; convection in the ocean is driven by buoyancy loss from above.1 In addition, there is a very important process forcing the ocean circulation that has no counterpart in the atmosphere. Winds blowing over the ocean surface exert a stress on it. The wind is a major driver of ocean circulation, particularly in the upper kilometer or so.

The wind-driven and buoyancy-driven circulations are intimately intertwined. Nevertheless, for pedagogical reasons here we discuss them separately, devoting Chapter 10 to the former and Chapter 11 to the latter. Finally, in Chapter 12, we discuss the role of the ocean in climate and paleo-climate.

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