What Makes The Ocean CiRCULATE

As we discussed in chapter 2, it is useful to think of the large-scale ocean circulation as having two main components:

a quasi-horizontal circulation consisting of the gyres and other surface-enhanced currents, and a deeper overturning circulation, the meridional overturning circulation. What makes the ocean go around this way? What "drives" the ocean, if anything? Bypassing the ambiguous term "drive," there are three main distinct physical phenomena that lead to the circulation of the ocean:1

1. The mechanical force of the wind on the surface of the ocean provides a stress that produces a quasihorizontal circulation that includes, most noticeably, the wind-driven gyres. The predominantly horizontal currents of the world's ocean, shown in figure 2.3 in chapter 2, are primarily a consequence of wind forcing. Less obviously, the wind also plays a role in producing a deep, interhemispheric meridional overturning circulation, a circulation in which the water sinks near one pole and rises near the other.

2. Buoyancy effects, caused mainly by the cooling of the oceans at high latitudes and heating at low latitudes, generally produce denser water at high latitudes. Salinity is a secondary source of density gradients in today's climate. An overturning circulation arises in response to these density gradients with cool, dense water sinking at high latitudes, moving equatorward and rising at lower latitudes and/or in the opposite hemisphere.

3. The mixing of fluid properties, and in particular heat, by small-scale turbulent motions (sometimes called turbulent diffusion) brings heat down into the abyss and enables an overturning circulation to be maintained.

The gyres and other quasi-horizontal currents are mainly a response to winds, and although they are affected by buoyancy effects and mixing, we can safely call them wind driven. The meridional overturning circulation (MOC), on the other hand, involves all three effects in an essential way. Most obviously, the MOC arises as a response to the surface density gradients (item 2 in our list) and is sometimes called the thermohaline circulation, so-called because it is enabled by the buoyancy effects of heat and salt leading to the sinking of dense water. However, we will see that the MOC can only be maintained if either mixing or wind is present, for they enable the deep water to rise to the surface to begin circulating anew. Without them, the deep circulation would stagnate.

Let's first discuss the wind-driven circulation, the great gyres, and western intensification, and follow that with a discussion of the MOC. The equatorial currents are different again, and we defer discussing them until chapter 6.

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