Global wind circulation Global wind circulation. (a) unequal heating of the Earth’s surface and the Coriolis deflection cause a zonal wind system to develop, arranged in three circulation cells-Hadley, Ferrel, and polor cells. (b) The plotting of prevailing winds of air-pressure map of the world reveals that winds flow from high-pressure zones to low-pressure zones at an angle to the regional pressure gradients.

Surface ocean currents Surface ocean currents. Global wind-driven ocean circulation consists of gyres that rotate clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Compare the flow direction of the water currents with the zonal wind patterns shown in figure 6-3. In the above map, C. stands for current.

An El Nino Year. (c) When the Southern Oscillation develops, the trade winds diminish and then reverse, leading to an eastward movement of warm water along the equator. The surface waters of the Central and Eastern Pacific become warmer, and storms over land may increase.

An El Nino year (d) sea-surface temperatures on 13 May 1992, a time of El Nino conditions. The thermocline was deeper than normal, and a equatorial upwelling was suppressed. Note the absence of coastal upwelling along the coast and the lack of the tongue of recently upwelled water extending westward along the equator.

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Figure 9. 13 Equatorial upwelling Figure 9.13 Equatorial upwelling. (a) The South Equatorial Current, especially in the Pacific, straddles the geographic equator. Water north of the equator veers to the right (northward), and water to the south veers to the left (southward). Surface water therefore diverges, causing upwelling. Most of the upwelled water comes from the area above the Equatorial Undercurrent, at depths of 100 meters (330 feet) or less.

Figure 9. 14 Coastal upwelling and downwelling Figure 9.14 Coastal upwelling and downwelling. (a) In the Northern Hemisphere, coastal upwelling can be caused by winds from the north blowing along the west coast of a continent. Water moved offshore by Ekman transport is replaced by cold, deep, nutrient-laden water. (b) A prolonged southerlywind along a Northern Hemisphere west coast can result in downwelling.

Indian Ocean Dipole