Ocean Circulation‬ Study Materials

‭OCEAN CIRCULATION‬ ‭Currents are movements and movements of sea water which involve the transport of masses of‬ ‭water. This transport can be horizontal or vertical. In the first case we will have horizontal‬ ‭currents which can be superficial and deep, in the second case vertical currents which can be‬ ‭ascending and descending. The causes of the formation of currents are mainly three:‬ ‭‬ c • ‭ ombined action of winds and pressure difference‬ ‭•‬ ‭different heating of water masses‬ ‭•‬ ‭salinity variations determined by precipitation, evaporation and river inputs‬ ‭However, even further upstream than these causes we can find more general primary causes‬ ‭represented by radiation with its effects and by the effects of the earth's rotation. All these causes‬ ‭are somehow linked to each other and converge in the direction of producing current motions. In‬ ‭fact, the sun heats the atmosphere and this heating is at the origin of atmospheric circulation, and‬ ‭therefore of the formation of winds. The energy of the wind is transferred to the upper layer of the‬ ‭ocean which is set in motion. Solar energy, causing variations in temperature and salinity,‬ ‭influences the density of water and gives rise to a thermohaline circulation. Finally, the Earth's‬ ‭rotation through the Coriolis force, through the rotation of the wind and Ekman effects, determines‬ ‭the direction of the currents in the upper layers. At depth, the direction of the currents will however‬ ‭also be influenced by the morphology of the seabed.‬ ‭Radiation can be considered the main driver of both atmospheric and oceanic circulation. It acts‬ ‭differently on the atmosphere and the ocean, and determines different conditions in different‬ ‭areas in both. The relationship between these two large compartments, atmosphere and sea,‬ ‭causes the establishment of seasonal and local meteorological conditions, characterized by a‬ ‭regime of pressure and consequently wind. Surface currents are caused by the wind, which blows‬ ‭constantly in the direction and persists over time. But the winds are produced by different heating‬ ‭conditions of the water masses.‬ ‭At an equatorial level, strong radiation causes the air‬ ‭masses to heat up in the lower layers of the atmosphere.‬ ‭These warm and therefore lighter air masses move‬ ‭towards higher layers of the atmosphere. Once they reach‬ ‭the top they are diverted north and south, progressively‬ ‭cool down, become heavier and descend back to high‬ ‭latitudes. Thus returning to head both from the north and‬ ‭from the south towards the equator. The equator,‬ ‭characterized by ascending movements of air masses, is‬ ‭configured as a low pressure area. While the‬ ‭areas of subpolar descent are areas of high pressure. The movement of air masses from areas‬ ‭of high pressure to areas of low pressure is the origin of the winds, which always blow from‬ ‭areas of high to areas of low pressure.‬ ‭The movement of air masses gives rise to a convection cell that extends between areas of high and‬ ‭low pressure: one in the northern hemisphere and one in the southern hemisphere. The arrows, which‬ ‭delimit the cell, mark the direction of circulation of the air masses. In reality the system is more‬ ‭complex: 6 cells are formed in the atmospheric circulation, three to the north and three to the south of‬ ‭the equator. Starting from the equator, both towards the north and towards the south, a direct cell‬ ‭alternates, an indirect one and then a direct one again. In indirect cells, called Ferrel cells, the direction‬ ‭of circulation of the air masses is the opposite of that of direct cells. In summary, alternating areas of‬ ‭high and low pressure form on the Earth's surface.‬ ‭at the equator, where warming is maximum, the warmest and lightest air masses move to the‬ ‭highest layers of the atmosphere, where they split up heading north and south. During this journey‬ ‭they give off heat, cool down, become heavier and then, at around 30° north and south latitude,‬ ‭they descend downwards again. Once on land they set off again both north and south.‬ ‭This closes the first cell, located between 30° north and south latitude and the equator. Of the‬ ‭cooled air that has fallen to the ground at 30° latitude, the branch that goes north in the northern‬ ‭hemisphere and goes south in the southern hemisphere collides with cold air of polar origin. This‬ ‭happens around 60° latitude: an area of low pressure is thus recreated, the warmer area rises‬ ‭into the highest layers of the atmosphere and there it divides again towards the north and‬ ‭towards the south and, as we described before, they complete both the indirect cell and the‬ ‭direct polar cell, both north and south. Everything would stop here if we didn't have the effect of‬ ‭the earth's rotation. The earth rotates around its axis counterclockwise with a movement from‬ ‭west to east.‬ ‭This means that if a particle is on land at high latitudes, it moves eastwards with a certain speed‬ ‭and at the equator it moves faster because it travels more distance in the same unit of time. The‬ ‭speed decreases going from low to high altitudes. If the particle moves in a fluid, the movement‬ ‭will be different depending on the hemisphere. If we are in the northern hemisphere and the‬ ‭particle moves from the equator towards the pole, it moves from an area where the eastward‬ ‭velocity is greater to one where it is smaller and we will say that the particle has an eastward‬ ‭angular momentum greater than that of the water in which it moves: the particle precedes the‬ ‭earth's surface and for this reason undergoes a deflection towards the east, i.e. towards its right.‬ ‭If, however, the particle moves from the pole towards the equator, it moves towards an area‬ ‭where the velocity towards the east increases, therefore the particle remains behind the‬ ‭movement of the earth's surface and thus undergoes a deflection towards the west, this time too‬ ‭towards his right. In the southern hemisphere we can make a completely similar reasoning,‬ ‭except that in both directions of the particle's movement, from the equator towards the pole or‬ ‭from the pole towards the equator, the particle will always be deflected towards its left. All that is‬ ‭described is the Coriolis effect, which produces a rightward depression of the waters moving in‬ ‭the northern hemisphere and a leftward depression of the waters moving in the southern‬ ‭hemisphere.‬ ‭Here we see the two directions of equator-pole flexion, polo equator, represented for both‬ ‭hemispheres.‬ ‭On the left we see the movement of air masses represented as we had previously described‬ ‭it, without taking into account the Coriolis effect; on the right we see the effect of the‬ ‭deviation of air masses near the earth's surface. The air masses that generate constant‬ ‭winds that blow from high pressure areas towards low pressure areas undergo a deviation‬ ‭towards their right in the northern hemisphere and towards their left in the southern‬ ‭hemisphere. Thus on the left we have the trade winds that blow from east to west between‬ ‭30° latitude and the equator. Then we have the westerly winds that blow from west to east‬ ‭at latitudes between 30° and 60°. Both north and south latitudes. Finally, we have the polar‬ ‭winds that blow again from east to west, between the poles and 60° both north and south.‬ ‭The action of the winds determines the formation of surface currents which affect the‬ ‭surface water layer up to a maximum depth of 200 m. They are constant winds that always‬ ‭blow in the same direction, but also seasonal winds like the Monsoons‬ ‭can influence surface currents on a seasonal basis. The effect of the Coriolis force not only‬ ‭defines the direction of the currents but is also transmitted to the mass of water and‬ ‭determines the direction of the current. But the mass of water moved by the wind is actually‬ ‭composed of multiple layers of water at gradually increasing depths: the wind acts at a‬ ‭superficial level‬ ‭The behavior of currents as a function of depth is described by the Ekman spiral. We can represent‬ ‭the water column divided into several layers: the surface layer hit by the wind moves but its‬ ‭direction, due to the Coriolis force, is deviated by 45° compared to the direction of the wind. The‬ ‭movement is then transmitted to the layer of water below but the deviation will be to the right if the‬ ‭current moves in the northern hemisphere and to the left if it moves in the southern hemisphere. The‬ ‭movement will therefore be transmitted from layer to layer but always being further deviated.‬ ‭Therefore in each layer the direction of the current changes and progressively decreases in speed‬ ‭due to the friction of the water layers. In the image, the black arrow indicates the direction of motion‬ ‭in the layer above while the gray arrow indicates friction and the blue one indicates the direction of‬ ‭motion in that layer of water. Proceeding in this way, from the surface towards the bottom, you will‬ ‭reach a depth at which the speed of the current cancels out and therefore you will no longer be‬ ‭affected by the effect of the wind blowing on the surface.‬ ‭The blue arrows are the same as the previous image. The image represents the spiral of currents that‬ ‭originates through the process just described: this is the Ekman spiral, a spiral of currents with‬ ‭different directions and intensities that goes from the surface towards the bottom. The depth at which‬ ‭the influence of the wind on the motion of the water ceases varies, but generally does not go beyond‬ ‭the first 100 m. Overall, the layer of water affected by this movement of currents affected by the‬ ‭Ekman spiral is called the Ekman layer. While the Ekman depth is the one at which the effect of the‬ ‭wind ends, the current motion extinguishes. The net movement of the water, or average flow, is also‬ ‭called Ekman transport and has a direction that is deviated by 90° compared to that of the wind that‬ ‭produced the current on the surface (to the right in the northern hemisphere and to the left in the‬ ‭southern hemisphere).‬ ‭In both hemispheres, starting from the subtropical areas at high pressure, i.e. a latitude of 30°, there‬ ‭will be a movement of water towards areas at low pressure, i.e. the equator and 60° latitude, both in‬ ‭the northern hemisphere and in the southern hemisphere. The movement of water masses will be‬ ‭diverted eastward at latitudes greater than 30°, and westward at latitudes less than 30°. As a result,‬ ‭current vortices, gyres, are formed in areas between the equator and 50° north and south latitude.‬ ‭North of the equator the movement occurs clockwise, south of the equator the movement occurs‬ ‭counterclockwise. Gyres can be considered the main elements of global ocean circulation.‬ ‭This image shows the main currents divided into warm‬ ‭and cold depending on the origin of the water, from low to‬ ‭high latitudes and vice versa. The presence of circular‬ ‭movements can be detected, clockwise in the northern‬ ‭hemisphere and anti-clockwise in the south. Due to the‬ ‭combined rotation of the trade winds and the rotation of‬ ‭the earth, the movement of the water at the equator is‬ ‭parallel to the equator itself. This involves a net movement‬ ‭of ocean water from east to west with accumulation of‬ ‭water in the western part of the basins. Here the water‬ ‭meets continental landmasses (may encounter island‬ ‭chains) that veer north and south. The currents then skirt‬ ‭the edges of the continents going towards high latitudes,‬ ‭fall under the influence of western winds, which give the‬ ‭water masses a new‬ ‭energy, push them eastwards, and crossing the ocean basin, these waters return to its eastern part.‬ ‭Here, the presence of continental masses diverts the currents back towards the equator. This system‬ ‭of circular surface currents is found in all major ocean basins: the only difference is that in the Pacific‬ ‭Ocean and the Atlantic Ocean there are two circulations, one clockwise in the northern hemisphere‬ ‭and one anticlockwise in the 'southern hemisphere; in the Indian Ocean we have a single circulation‬ ‭which is determined by the monsoons. In the Pacific Ocean it can be seen how, under the pressure‬ ‭of the trade winds, a warm current starts from the American coasts and heads westwards and‬ ‭divides into two branches: the south Equatorial current and the north Equatorial current. The first‬ ‭deviates towards the south, the second towards the north.‬ ‭when they arrive at high latitudes, after having released their heat, they return along the coasts‬ ‭towards the equator, forming cold currents, which are precisely, along the California coasts of North‬ ‭America, the California current and, along the southern coasts America, the cold Yumbolt current.‬ ‭There is also an equatorial countercurrent, deeper and faster than the two equatorial currents, and it‬ ‭moves in the opposite direction, always along the equator but from west to east. In the Atlantic Ocean,‬ ‭due to the more irregular shape of its basin, the circulation is more complex than in the Pacific: from‬ ‭the African coasts, again due to the presence of the trade winds, we have the north and south‬ ‭Equatorial currents heading west. The North Equatorial Current is divided into two parts, one part‬ ‭forms the Gulf Stream which moves from the Gulf of Mexico northwards, until it touches the northern‬ ‭European coasts; a part forms a circular pattern and then returns cold to the equator as the Canary‬ ‭Current. The South Equatorial current, on the other hand, after reaching the coasts of South America,‬ ‭runs along the Brazilian coasts and then reaches the Antarctic waters where it flows, turns back‬ ‭towards the east, towards Africa, and returns towards the African coasts in the form of a current cold,‬ ‭the Benguela current.‬ ‭The horizontal currents just described transport enormous volumes of water across the oceans and‬ ‭over truly great distances.‬ ‭Currents distribute stored heat by transporting warm water to areas climatically colder and vice versa.‬ ‭In this way they influence the distribution of marine species, which lead to the identification and‬ ‭characterization of the different biogeographical areas. The currents essentially act like a large‬ ‭thermostat, mitigating temperatures in polar regions and cooling tropical areas. They thus regulate the‬ ‭ eneral climate of our planet.‬ g ‭The role of currents in‬ ‭transporting enormous amounts of heat is reflected in surface temperatures which are therefore higher‬ ‭on the eastern coasts of the continents,‬ ‭ here the warm waters arrive from the equator, compared to what happens on the western coasts of the‬ w ‭continents‬ ‭where cold waters flow towards the equator. As a result, on the eastern coasts tropical species such‬ ‭as coral tend to be distributed more towards high latitudes, whereas on the western coasts organisms‬ ‭adapted to the cold, such as many phaeophyta, spread more towards the equator. It should be‬ ‭remembered, however, that currents at any time can show differences compared to general circulation‬ ‭patterns due to the influence of meteorological conditions or the season of the year. At the continental‬ ‭shelf level, currents can be particularly influenced by the type of bottom, the characteristics of the‬ ‭coast and also by the tidal regime.‬