This leads to the spiral affect seen in the above diagram. The net movement of water averaged over the entire upper meters of the ocean is 90o to the right of the wind direction in the northern hemisphere. When the water is pushed to the right it forms the hill we described above. So, when water is pushed along by the wind it wants to be turned to the right by the Coriolis force in the northern hemisphere but it must fight against gravity trying to move up the hill of water formed by Ekman transport.
A balance is met between the Coriolis and the gravity pressure gradient force. This balance produces a balanced flow called a Geostrophic current. Note the difference is strength Sv between the western and eastern boundary currents. This is caused by the effect of the rotating Earth which tends to move the "hill" of water to the western sides of the ocean basins. The effect of winds on the vertical movement of water.
Upwelling along the coast caused by Ekman transport of waters waters move to the right of the wind. The waters moved offshore are replaced by waters from below. This brings cold, nutrient rich waters to the surface. The thickness of the surface layer entrained by wind is of the order of meters about the thickness of the thermocline at low- and mid-latitudes , up to a maximum of m.
Due to earth rotation the main ocean current system consists of large anticyclonic gyres clockwise rotating in the northern hemisphere and anticlockwise in the southern hemisphere [1]. The Antarctic Circumpolar Current is situated in the Southern Ocean and constantly circles around Antarctica because there are no land masses to interrupt the currents. It is an eastward-flowing current driven by the dominant western winds at this latitude. The most famous ocean current, the Gulf Stream , is a vast moving mass of water, transporting an enormous amount of heat from the Caribbean across the ocean to Europe.
It passes by the US east coast as a narrow jet, due to the northward increase of the Coriolis effect [3] and then spreads out as a meandering current over the ocean while generating a series of meso-scale eddies and whirls.
The North Atlantic Gyre is completed by the Canary Current in the Eastern Atlantic that transports relatively cold water south and west. The Kuroshio is a warm boundary current in the north-western Pacific, similar to the Gulf Stream.
In regions where Ekman transport deflects the boundary current from the coast, water from the deep ocean rises to the ocean surface, see figure 2. This phenomenon is called 'upwelling' and is very important for enrichment of surface waters with organic matter and nutrients.
Upwelling zones are characterized by a very rich marine life with abundant resources for fishery. Upwelling also occurs at the equator at the Pacific Ocean Equatorial upwelling. The North Equatorial Current is deflected to the north and the South Equatorial current to the south as a consequence of the Coriolis effect.
This produces upwelling of nutrient rich water and cooling of the surface water near the equator of the Pacific, see figure 3. Downwelling zones exist north and south of the equator. Instability of the coupled ocean-atmosphere dynamics produces large fluctuations in the climate of the Pacific region, which are felt at the global scale. Weakening of the easterly trade winds allows warm water from the Western Pacific to flow back with the Equatorial Counter Current to the eastern South American boundary, where upwelling currents of cold deep ocean water are shut off.
This results in relative warming of the Eastern Pacific lowering the sea surface atmospheric pressure and relative cooling of the Western Pacific increasing the sea surface atmospheric pressure and hence induces a further weakening of the easterly trade winds.
This feedback strengthens the so-called El Nino phase of the oscillation [5] [6]. The shut-off of the food-rich upwelling currents has major consequences for marine life and fisheries. After a number of years three on average, but variable the system sweeps back to the opposite phase, called La Nina. The onset and offset of the oscillation are still not fully understood. Deep ocean circulation is primarily driven by density differences.
It is called thermohaline circulation , because density differences are due to temperature and salinity. However, the water masses moving around by thermohaline circulation are huge.
Density gradients alone are not sufficient for sustaining the deep ocean circulation. Upwelling and mixing processes, to bring deep ocean water back to the surface, are required too [8]. The density of surface water increases when frigid air blows during winter across the ocean at high latitudes. The water density increases further by evaporation and by salt expulsion when sea ice is formed.
From these regions, a cold deep water layer spreads over the entire ocean basins. The thermohaline circulation moves water masses around between the different ocean basins [9] [10]. The conveyor belt is fed in the northern North Atlantic with high-salinity water due to evaporation supplied by the Gulf Stream , which sinks to great depth after cooling down in the Arctic region, forming the North Atlantic Deep Water NADW.
The replacement of this dense sinking water generates a continuous surface flow feeding the conveyor belt. This current compensates for the net northward surface flow in the Atlantic Ocean. The cold dense water from the Antarctic zone fills the deep water layer in these oceans and then gradually rises and mixes with the surface waters of the Indian and Pacific oceans.
Surface currents in the ocean are driven by global wind systems that are fueled by energy from the sun. Surface wind-driven currents generate upwelling currents in conjunction with landforms, creating deepwater currents. Currents may also be caused by density differences in water masses due to temperature thermo and salinity haline variations via a process known as thermohaline circulation. These currents move water masses through the deep ocean—taking nutrients, oxygen, and heat with them.
Occasional events such as huge storms and underwater earthquakes can also trigger serious ocean currents, moving masses of water inland when they reach shallow water and coastlines.
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