The winddriven circulation

10.1. The wind stress and Ekman layers

10.1.1. Balance of forces and transport in the Ekman layer

10.1.2. Ekman pumping and suction and GFD Lab XII

10.1.3. Ekman pumping and suction induced by large-scale wind patterns

10.2. Response of the interior ocean to Ekman pumping

10.2.1. Interior balances

10.2.2. Wind-driven gyres and western boundary currents

10.2.3. Taylor-Proudman on the sphere

10.2.4. GFD Lab XIII: Wind-driven ocean gyres

10.3. The depth-integrated circulation: Sverdrup theory

10.3.1. Rationalization of position, sense of circulation, and volume transport of ocean gyres

10.4. Effects of stratification and topography 10.4.1. Taylor-Proudman in a layered ocean

10.5. Baroclinic instability in the ocean

10.6. Further reading

10.7. Problems

In Chapter 9 we saw that the ocean comprises a warm, salty, stratified lens of fluid, the thermocline, circulating on top of a cold, fresh, relatively well mixed abyss, as sketched in Fig. 10.1. The time-mean circulation of thermocline waters is rapid relative to the rather sluggish circulation of the abyss.

There are two processes driving the circulation of the ocean:

1. tangential stresses at the ocean's surface due to the prevailing wind systems, which impart momentum to the ocean—the wind-driven circulation, and

2. convection, induced by loss of buoyancy in polar latitudes, due to cooling and/or salt input, causing surface waters to sink to depth, ventilating the abyss—the thermohaline circulation.

This separation of the circulation into wind-driven and thermohaline components is somewhat artificial but provides a useful conceptual simplification. In this chapter we will be concerned with the circulation of the warm, salty thermocline waters sketched in Fig. 10.1 that are brought into motion by the wind. We shall see that the effects of the wind blowing over the ocean is to induce, through Ekman pumping or suction (see Section 10.2), a pattern of vertical motion indicated by the arrows on the figure. Pumping down of buoyant surface water in the subtropics and sucking up of

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Pole Equator Pole

FIGURE 10.1. The ocean comprises a warm, salty, stratified lens of fluid, the thermocline, circulating on top of a cold, fresh, relatively well mixed abyss. The surface layer, above the horizontal dotted line at a depth of about 100 m, is driven directly by the wind. The thermocline below is brought in to motion through a pattern of vertical velocity driven by the wind (Ekman pumping and suction), which induces flow in the ocean beneath.

Pole Equator Pole

FIGURE 10.1. The ocean comprises a warm, salty, stratified lens of fluid, the thermocline, circulating on top of a cold, fresh, relatively well mixed abyss. The surface layer, above the horizontal dotted line at a depth of about 100 m, is driven directly by the wind. The thermocline below is brought in to motion through a pattern of vertical velocity driven by the wind (Ekman pumping and suction), which induces flow in the ocean beneath.

heavier interior fluid at the pole and the equator, tilts density surfaces, as sketched in Fig. 10.1 and evident in Fig. 9.7, setting up a thermal wind shear and geostrophic motion. The presence of jagged topography acts to damp strong mean currents in the abyss. Vertical shears build up across the tilted thermocline, however, supporting strong surface currents. The tilting of the thermocline induced by the collusion of horizontal surface density gradients and vertical motion induced by the wind, leads to a vast store of available potential energy (recall our discussion in Section 8.3.2). We shall see in Section 10.5 that this potential energy is released by baroclinic instability, leading to an energetic eddy field, the ocean's analogue of atmospheric weather systems. Ocean eddies have a horizontal scale of typically 100 km, and as discussed in Chapter 9, are often much stronger than the mean flow, leading to a highly turbulent, chaotic flow (see Fig. 9.24). The mean pattern of currents mapped in Figs. 9.14 to 9.16 only emerges after averaging over many years.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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