Oceanic Circulation

Thermohaline Circulation

Compared with the circulation patterns generated by atmosphere-ocean coupling and the Coriolis force, thermohaline circulation is extremely slow, on the order of millimeters per second (mm/s), compared with wind-driven current systems (centimeters per second [cm/s]). Despite its slow pace, thermohaline circulation results in a horizontal range of temperature nearly matching the vertical gradient (30 to −1 °C). For the vertical gradient, this results in two distinct zones: (1) the mixed layer (generally the upper 50–200 m [meters] depending on latitude) in which temperatures approximate the surface, and (2) the thermocline (200–800 m in tropics, 50–100 m in summer midlatitudes, nonexistent in polar latitudes and winter midlatitudes). The horizontal gradient is best examined by looking at the gyres, since in this form of heat circulation, they exert the most influence.

Gyres

Subtropical circulation in the Atlantic is dominated by the Atlantic gyre. This is a large, asymmetrical, clockwise-rotating current system. This system is overlain by a large, high-pressure atmospheric system. This high-pressure atmospheric system sets up a wind-stress field that generates outward-flowing surface currents. In addition to wind-stress-generated rotational momentum (the winds are being deflected clockwise by the Corio-lis force), these currents become deflected themselves by Ekman transport (the 45° deflection of a wind-generated surface current to the right of wind direction in the Northern Hemisphere by the [Page 2059]Coriolis force). The current shear generated by wind stress, frictional forces, and Ekman transport enables the system to acquire relative vortic-ity, an expression of the tendency of a system to rotate. This is different from the absolute vortic-ity, which takes into account the vorticity relative to a fixed point and hence includes planetary vorticity—vorticity due to the fact that the Earth rotates. In addition, there is potential vorticity, which factors in the thickness of the rotating body of water and causes the system to shallow (and widen) as it approaches the equator and become narrow and deep as it swings back toward the pole. This is the main way in which topography affects large rotating systems such as the oceanic gyres (seafloor depth controls how deep and hence narrow the current can become). Like all forms of momentum, vorticity is conserved, and its conservation is the mechanism maintaining the gyre.

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