Ocean Circulation

Volume 1, The Earth system: physical and chemical dimensions of global environmental change, pp 557–579 Ocean circulation is one of the major elements of climate, moving heat, freshwater, nutrients and dissolved gases. Ocean circulation at the largest spatial scales and long time scales contains many elements that are in common in the different oceans. Understanding the processes that drive these different elements has progressed a long way and is essential for verifying climate models. The ocean circulation is forced (1) by the winds, through a thin frictional layer called the Ekman layer and through the response of the interior ocean to mass convergences and divergences in this layer, and (2) by heating and cooling that change the temperature and processes that change the amount of fresh water and hence the salinity. The first type of circulation is wind driven and the second is the thermohaline circulation. The prevailing, large-scale winds of the Earth are trades (easterlies) in the tropics, westerlies at mid-latitudes. The wind-driven ocean circulation that results contains subtropical gyres around ocean high pressure regions and subpolar gyres around low pressure regions. These gyres are asymmetric, with strong western boundary currents (velocities of about 1 m s 1) that extend to great depth and relatively weak currents elsewhere. With increasing depth, each subtropical gyre shrinks poleward and westward towards its western boundary current. Shallow eastern boundary currents are created by local upwelling along eastern boundaries, driven by equatorward winds. In the tropics, the trade winds drive westward surface flow and create a strong and very thin eastward-flowing undercurrent on the equator. At the latitude of Drake Passage, between South America and Antarctica, the ocean is open all the way around the Earth. The westerly winds here create the strong, deep-reaching eastward flowing Antarctic Circumpolar Current. Except in the strong western boundary currents and the Antarctic Circumpolar Current, the deep flow is driven by thermohaline forcing, in which dense waters are made in isolated locations at high latitudes and spread through the oceans via relatively strong deep western boundary currents , which differ in mechanism and often direction from the wind-driven western boundary currents. Abyssal flow away from the deep western boundary currents is dominated by topography, and is often found to move parallel to the topography, with low pressure in the center. Ocean currents transport heat from the tropics to the poles. Most of the heat is lost at …