To a large extent, horizontal movement of ocean surface waters mirrors the long-term average planetary circulation of the atmosphere. Three surface wind belts encircle each hemisphere: trade winds (equator to 30 degrees latitude), westerlies (30 to 60 degrees), and polar easterlies (60 to 90 degrees). The westerlies of middle latitudes and the trade winds of the tropics drive the most prominent features of ocean surface motion, large-scale roughly circular current systems elongated in the east-west direction known as gyres. Subtropical gyres are centered near 30 degrees latitude in the North and South Atlantic, the North and South Pacific, and the Indian Ocean. Gyres in the Northern and Southern Hemispheres are similar except that they rotate in opposite directions because the Coriolis effect acts in opposite directions in the two hemispheres. Viewed from above, subtropical gyres rotate in a clockwise direction in the Northern Hemisphere but in a counterclockwise direction in the Southern Hemisphere. 
Ekman transport causes surface waters to move toward the central region of a subtropical gyre from all sides, producing a broad mound of water. Surface water begins flowing downhill. A balance develops between the Coriolis force and the force arising from the horizontal water pressure gradient such that surface currents flow parallel to the contours of elevation of sea level. This current is known as geostrophic flow.
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Driven by the long-term average winds in the subtropical highs, Ekman transport causes surface waters to move toward the central region of a subtropical gyre. This transport produces a broad mounding of water as high as 1 m (3 ft) above mean sea level near the center of the gyre (Figure 6.5). As more water is transported toward the center of the gyre, the surface slope of the mound becomes steeper. At the same time, the horizontal water pressure gradient produced under the sloping sea surface increases. In response to the horizontal gradient in water pressure, water moves from where the pressure is higher toward where the pressure is lower, that is, downhill. Surface water parcels flow outward and down slope from the center of the gyre. The Coriolis effect causes these parcels to shift direction to the right in the Northern Hemisphere (to the left in the Southern Hemisphere). Eventually, the outward-directed pressure gradient force balances the apparent force due to the Coriolis effect and the water parcels flow around the gyre and parallel to contours of elevation of sea level. The horizontal movement of surface water arising from a balance between the pressure gradient force and the Coriolis force is known as geostrophic flow. As noted earlier, viewed from above, geostrophic flow in a subtropical gyre is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Adapted from DataStreme Ocean and |