Ocean structure and circulation

The aspect of the ocean that perhaps most affects the climate is the temperature distribution at the surface—the

Figure 2.2. The annual average temperature at the ocean surface, in degrees centigrade. Adapted from World Ocean Atlas, 2009 of the National Oceanic and Atmospheric Administration (http://www.nodc .noaa.gov/OC5/WOA09/pr_woa09.html).

Figure 2.2. The annual average temperature at the ocean surface, in degrees centigrade. Adapted from World Ocean Atlas, 2009 of the National Oceanic and Atmospheric Administration (http://www.nodc .noaa.gov/OC5/WOA09/pr_woa09.html).

sea-surface temperature or SST—which is shown in figure 2.2. We see, as expected, a decrease of temperatures poleward but also a noticeable east-west variation in the midlatitudes. Let us first describe the circulation giving rise to this in a rather basic way, and to that end it is useful, if perhaps a slight oversimplification, to divide the ocean circulation into two components:

1. a quasi-horizontal circulation that is largely driven by the effects of the wind and that gives rise most noticeably to the great ocean gyres and the oceans: a descríptíve overvíew

2. a meridional overturning circulation (MOC), in which cold, dense water sinks at high latitudes before moving equatorward and rising in low latitudes and/ or in the opposite hemisphere. (Meridional generi-cally means in the north-south direction.)

How the winds and density gradients produce these currents is the subject of chapter 4; here we are less ambitious and seek simply to describe the circulation and the horizontal and vertical structure of the ocean.

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