5 UNIT The Circulation of the Oceans Learning 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 deepocean 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 ocean Review 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 a result of this friction the wind drags the ocean surface with it as it blows thus setting 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 does winds so the water is deflected to the right of the path of the wind in the Northern Hemisphere 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 the wind 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 of the air is transferred to the top layer of the water As that layer moves it drags along the water just below it which in turn drags along the water just below that 36 and so on The water can be thought to move as many thin coupled layers and kinetic energy is transferred down the water column However as the energy is transferred downward friction causes some of the energy to be dissipated in the form of heat so each level moves more slowly than the level above At some depth 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 a layer starts to move the water is deflected to the right of the path of the layer above or the wind path for the surface layer This movement is to the right in the Northern Hemisphere and to the left in the Southern Hemisphere The deeper below the surface the farther each layer is deflected to the right or left of the surface layer producing a spiraling effect known as the Ekman spiral When the movements of all the individual layers of water in the spiral are added the net direction of transport within the water column is at a right angle 90 to the wind direction 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 the surface 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 that is opposed by the Coriolis Effect The net effect is a flow of water at approximately 90o to the slope The result is a geostrophic current that flows approximately 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 and saltier with time If not then why not The salts contained in seawater are largely the result of the weathering of crustal rocks The oceans are not getting saltier because many processes also remove salts from seawater These processes include the following i Evaporation of seawater from shallow seas The remaining salts are concentrated and precipitate from solution as evaporite deposits such as halite table salt NaCl and gypsum CaSO4 2H2O ii Biological processes For example some marine organisms remove the elements calcium or silicon from seawater to form their shells some of which are eventually deposited in ocean sediments iii Chemical reactions between seawater and newly formed volcanic rocks on the sea 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 deposited on land These salts are eventually returned to the oceans via rivers 37 Overall 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 the circulation of the deep oceans Because deep ocean circulation depends on temperature and salinity this circulation 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 order of a kilometer in thickness and is characterized by a rapid increase in density with increasing water depth The very sharp increase in density is called the pycnocline the transition zone is referred to as the pycnocline zone In some regions this density gradient is dominated by salinity changes and salinity rises rapidly with increasing depth In this case the salinity gradient is specifically referred to as the halocline In most other regions temperature changes dominate the density gradient and temperature drops rapidly with increasing depth There the transition is called the thermocline In any of these cases a steep density gradient forms that makes this stratification very stable 9 What is bottom water Where and how does bottom water form Bottom water constitutes the densest water produced in the oceans Near the poles the surface waters are cooled below the normal freezing point 1 9 C in some areas by contact with the cold overlying atmosphere The freezing point is lower