UNITThe Circulation of the OceansLearning Objectives- Know how ocean currents form- Know the role of ocean circulation in the global climate system- Understand why the movement of deep-ocean water is independent of the surface-ocean circulation- Know how differences in water density play an important role in deep-ocean circulation- Understand the Coriolis Effect on ocean circulation.- Know how the circulation of the deep ocean is affected by water salinity- Know what drives surface circulation- Understand the effect of Ekman spiral/Ekman transport on water motion.- Understand the nature of wind-induced vertical circulation: coastal upwelling and downwelling and equatorial upwelling.- Know what is meant by thermohaline circulation.- Know the general vertical structure of the oceanReview Questions 1.) What effects does the surface-wind pattern have on the circulation of the oceans?The movement of the wind over the ocean causes friction at the surface. As aresult of this friction, the wind drags the ocean surface with it as it blows, thussetting up a pattern of surface-ocean wind-drift currents. 2.) Why do ocean currents not move in exactly the same direction as the wind?Because of the Coriolis Effect ocean currents do not move in exactly the same direction as the wind. The Coriolis Effect influences ocean currents just as it doeswinds, so the water is deflected to the right of the path of the wind in the NorthernHemisphere (and to the left of the wind’s path in the Southern Hemisphere). Observations show that this deflection tends to be approximately 20–25° from thewind direction.3.) What is the Ekman spiral? Explain why Ekman transport occurs.Due to friction between wind and the water surface, some of the kinetic energy ofthe air is transferred to the top layer of the water. As that layer moves, it dragsalong the water just below it, which in turn drags along the water just below that,36UNIT5and so on. The water can be thought to move as many thin, coupled layers, andkinetic energy is transferred down the water column. However, as the energy istransferred downward, friction causes some of the energy to be dissipated in theform of heat, so each level moves more slowly than the level above. At somedepth below the surface, the effects of the wind-induced movement disappear.However, as each layer moves, it is again subject to the Coriolis Effect. Once alayer starts to move, the water is deflected to the right of the path of the layerabove (or the wind path, for the surface layer). This movement is to the right inthe Northern Hemisphere and to the left in the Southern Hemisphere. The deeperbelow the surface, the farther each layer is deflected to the right or left of thesurface layer, producing a spiraling effect known as the Ekman spiral. When themovements of all the individual layers of water in the spiral are added, the netdirection of transport within the water column is at a right angle (90°) to the winddirection. This net movement of water is referred to as Ekman transport.4.) What is upwelling? Where does upwelling occur?Where divergence occurs at the surface, water must rise from below to replace it.Water at depth is cooler than water at the surface. The rising of cooler water to thesurface to replace warm, divergent surface water is referred to as upwelling.5.) What is mean by a geostrophic current?There is a force due to gravity, acting down the gradient of the surface slope thatis opposed by the Coriolis Effect. The net effect is a flow of water atapproximately 90o to the slope. The result is a geostrophic current that flowsapproximately perpendicular to the slope of the sea surface around the gyre.6.) Where does the salt in the oceans originate? Are the oceans getting saltier andsaltier with time? If not, then why not?The salts contained in seawater are largely the result of the weathering of crustalrocks. The oceans are not getting saltier, because many processes also removesalts from seawater. These processes include the following:i. Evaporation of seawater from shallow seas. The remaining salts areconcentrated and precipitate from solution as evaporite deposits, such ashalite (table salt, NaCl) and gypsum (CaSO4∙2H2O).ii. Biological processes. For example, some marine organisms remove theelements calcium or silicon from seawater to form their shells, some of whichare eventually deposited in ocean sediments.iii. Chemical reactions between seawater and newly formed volcanic rocks on thesea floor.iv. The formation of sea spray. As small droplets of seawater become airborne,salts, especially sodium and chlorine, are removed when the spray is depositedon land. These salts are eventually returned to the oceans via rivers.37Overall, salts are removed from seawater at a rate that essentially equals the rate of input, when averaged over geologic time scales (millions of years). 7.) Define the thermohaline circulation. What are the processes that drive thecirculation of the deep oceans?Because deep-ocean circulation depends on temperature and salinity, thiscirculation is referred to as thermohaline circulation (thermo is Greek for“heat,” and haline comes from the Greek hals, for “salt”). In the deep oceans,horizontal changes in density are small, whereas vertical changes can be larger.But the densest water is at the bottom, so the structure is very stable.Consequently, the movement of water through the deep ocean is relatively slow.8.) Explain the differences among the pycnocline, the halocline, and the thermocline.The transition zone between the surface zone and the deep ocean is on the orderof a kilometer in thickness and is characterized by a rapid increase in density withincreasing water depth. The very sharp increase in density is called thepycnocline; the transition zone is referred to as the pycnocline zone. In someregions this density gradient is dominated by salinity changes, and salinity risesrapidly with increasing depth. In this case, the salinity gradient is specificallyreferred to as the halocline. In most other regions, temperature changes dominatethe density gradient, and temperature drops rapidly with increasing depth. Therethe transition is called the thermocline. In any of these cases, a steep densitygradient forms that makes this