The thermohaline circulation of the ocean

11.1. Air-sea fluxes and surface property distributions

11.1.1. Heat, freshwater, and buoyancy fluxes

11.1.2. Interpretation of surface temperature distributions

11.1.3. Sites of deep convection

11.2. The observed thermohaline circulation

11.2.1. Inferences from interior tracer distributions

11.2.2. Time scales and intensity of thermohaline circulation

11.3. Dynamical models of the thermohaline circulation

11.3.1. Abyssal circulation schematic deduced from Taylor-Proudman on the sphere

11.3.2. GFD Lab XIV: The abyssal circulation

11.3.3. Why western boundary currents?

11.3.4. GFD Lab XV: Source sink flow in a rotating basin

11.4. Observations of abyssal ocean circulation

11.5. The ocean heat budget and transport

11.5.1. Meridional heat transport

11.5.2. Mechanisms of ocean heat transport and the partition of heat transport between the atmosphere and ocean

11.6. Freshwater transport by the ocean

11.7. Further reading

11.8. Problems

The thermohaline circulation is that part of the ocean circulation induced by deep-reaching convection driven by surface buoyancy loss in polar latitudes, as sketched in Fig. 11.1.1 As we shall see, deep convection in the ocean is highly localized in space and only occurs in a few key locations; in particular, the northern North Atlantic Ocean and around Antarctica. However, the response of the ocean to this localized forcing is global in scale. Giant patterns of meridional overturning circulation are set up that cross the equator and connect the hemispheres together. Unlike the faster wind-driven circulation, which is confined to the top kilometer or so, the thermohaline circulation plays a major role in setting properties of the abyssal ocean. Both

*The phrase thermohaline circulation is widely used but not precisely defined (see Wunsch, 2002). It means different things to different people. Perhaps its most literal interpretation is the circulation of heat and salt in the ocean and thus involves both wind-driven and buoyancy-driven circulation. Here, however, we adopt its more common, narrow usage, to mean the circulation induced by polar convection.

* buovancv

* buovancv

Abyssal Circulation

Pole Equator Pole

FIGURE 11.1. The deep ocean is ventilated by localized convection at polar latitudes, induced by loss of buoyancy (due to cooling and/or salt input), causing surface waters to sink to depth. Compensating upwelling is thought to occur on the large-scale, as indicated by the vertical arrows at mid-depth.

Pole Equator Pole

FIGURE 11.1. The deep ocean is ventilated by localized convection at polar latitudes, induced by loss of buoyancy (due to cooling and/or salt input), causing surface waters to sink to depth. Compensating upwelling is thought to occur on the large-scale, as indicated by the vertical arrows at mid-depth.

wind-driven (Chapter 10) and buoyancy-driven circulations play an important role in meridional ocean heat transport. However, because of the very long timescales and very weak currents involved, the thermohaline circulation is much less well observed or understood. We shall see that theory has played a central role in shaping our conception of the likely circulation patterns and mechanisms.

In this chapter, then, we describe the patterns of air-sea heat and fresh water fluxes that drive the thermohaline circulation and go on to discuss inferences of abyssal flow patterns and rates from observations of interior tracer distributions such as salinity and oxygen. We then develop a simple conceptual model and associated laboratory experiments of the thermohaline circulation employing the same dynamical framework, Taylor-Proudman on the sphere, used to discuss the wind-driven circulation in Chapter 10. This model predicts the existence of deep western boundary currents carrying fluid away from their source regions whose existence is confirmed by observations. Finally we discuss the role of the wind-driven and thermohaline circulations in the meridional flux of heat and freshwater.

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