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Handout9 - Handout for Chapter 9 Ocean Circulation There...

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Handout for Chapter 9 Ocean Circulation * There are two types of circulation in the ocean . Wind-driven (or wind-induced) circula- tion and thermohaline circulation . Wind blowing at the sea surface directly drives wind-driven circulation. Thermohaline circulation is driven by changing the density of water, i. e., by chang- ing temperature and/or salinity. Evaporation and precipitation, heating and cooling, and freezing and melting of sea ice, at the sea surface could drive thermohaline circulation. Wind-driven cir- culation is mostly limited to near-surface layer of the ocean (~ top several hundred meters). While, thermohaline circulation is responsible for ventilating the global ocean, and its impact is felt all the way to the bottom. It can be said that below surface layer, thermohaline circulation dominates. Wind-driven Circulation * Ekman transport moves near-surface water (~ top 100 m) to the right (90 o ) of the wind di- rection in the Northern Hemisphere , it moves water to the left (90 o ) of the wind direction in the Southern Hemisphere . In Ekman layer, wind stress acting on the water particle is balanced by Coriolis Force. * If a low (high) atmospheric pressure system is located over the open ocean, the resulting Ekman transport moves near-surface water away from (toward) the center of the low pressure causing divergence (convergence) . Consequently, upwelling (downwelling) will result, bring- ing sub-surface water toward surface (pushing down surface water toward subsurface). * Coastal upwelling and downwelling : Winds blowing along a coast cause either upwelling or downwelling depending on the wind direction relative to the orientation of the coast. Looking into the downwind direction, if the land is located to your left (right), the resulting Ekman trans- port moves the warm near-surface water away from (toward) the coast, causing coastal upwelling (downwelling) in the Northern Hemisphere. In the Southern Hemisphere, if the land is located to your right (left), the resulting Ekman transport moves the warmer near-surface water away from (toward) the coast, causing coastal upwelling (downwelling). Coastal upwelling brings colder nutrient-rich sub-surface water closer to the surface where sufficient sun light is available, thus bringing about higher biological productivity.
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