We saw in Lectures 1 and 2 that the atmospheric circulation is driven primarily by temperature differences across the globe, which sets up large convective systems, which are then modified by the Coriolis Force. The importance of convection in the atmosphere is because the atmosphere is a fluid heated from below. (Recall that the main sources of energy to the atmosphere are longwave radiation and latent heating, both of which are most important near the earth's surface). Fluids which expand when heated (like air) will convect when heated from below. So, the atmosphere is heated from below, cooled from above (as energy is lost to space), and the atmosphere convects and circulates. The world's oceans, however, differ from the atmosphere in important ways.
These two facts have important implications for ocean circulation. Because water becomes less dense (expands) when heated above 4°C, heating from the surface will form a warm, buoyant, and stable layer at the surface. There is no tendency towards deep convection and mixing. Conversely, if water is cooled below 4° C, it becomes less dense, so a cold, buoyant and stable layer forms on the surface. Water also undergoes a dramatic expansion on freezing, increasing in volume by 9%, so pack ice also forms a stable upper layer. Thus, in many circumstances, vertical motions of the oceans are suppressed. Only where ocean surface waters with temperatures over 4° C undergo cooling can surface temperature changes cause vertical motions, and these tend not to extend to very great depths.
However, ocean-water density is also influenced by salinity. The sea contains dissolved ions (charged molecules or atoms), mainly sodium and chlorine, and is consequently denser than fresh water (typically 1035 kilograms per cubic metre compared with 1000 kilograms per cubic metre). The salinity of sea-water is rather variable, however. Evaporation at the sea surface will increase the salinity (as water is evaporated off but dissolved ions are left behind), whereas inputs from rain or river run-off will decrease the salinity. Salinity will also be increased by the formation of sea ice. As ice forms, salt is rejected from the freezing water, so the remaining liquid water is enriched in salt.
The density of sea-water is thus a combination of its temperature and salinity. Saline waters at around 4° C will be dense, and will tend to sink, whereas warm, less saline water will be less dense, and thus will be buoyant. This has important implications for vertical motions in the sea. Deep convection, whereby surfce waters are transported to the deep oceans, is only possible where saline waters are cooled. This occurs only in a few locations in the North Atlantic off Greenland, and in the Weddell Sea, part of the Antarctic Ocean south of the Atlantic. In both of these locations, saline surface waters are cooled and sink, a process known as downwelling. The surface waters are made saline by evaporation nearer the equator in the case of the North Atlantic, and by the formation of sea-ice - which also expels salt -in the case of the Weddell Sea. Shallower downwelling, to intermediate depths in the oceans is also possible in areas of high evaporation.
Horizontal motions in the sea can be triggered by differences in surface evaporation, and associated variations in temperature and salinity. For example, deep downwelling in the polar oceans draws in surface waters to replace the sinking waters, while the downwelling water reaches the sea bed, then travels equatorwards. Thus, deep convection sets up horizontal motions. A similar convective cycle is set up in the Mediterranean and Atlantic. Evaporation from the Mediterrannean makes its waters more saline than in the neighbouring Atlantic. Mediterrannean waters therefore flow out through the Straits of Gibraltar, where they sink to intermediate depths in the Atlantic. Atlantic surface waters flow back into the Mediterrannean to replace the saline water and the water lost to evaporation.
Water evaporated from the oceans mostly returns to its ocean of origin in river flows. However, in some areas, atmospheric motions and drainage patterns mean that water evaporated from one ocean is returned to another. In particular, some of the water evaporated from the Atlantic is returned to the Pacific and Indian Oceans following transport in clouds across Central America and Asia. Thus, the Atlantic loses water and is left more saline, whereas the Indian and Pacific Oceans gain water and become less saline. These imbalances are redressed by large-scale transports of water and salt, which are linked to the deep downwelling discussed above. Because this circulation involves the transport of heat and salt, it is known as the thermo-haline circulation. A schematic diagram of the thermo-haline circulation is shown below. Deep waters are formed by downwelling in the North Atlantic and Weddell Sea. Evaporation removes water from the Atlantic, whereupon some is transported to the Pacific and Indian Oceans by weather systems and river runoff. Low salinity surface waters flow back to the Atlantic to redress the balance.
An important component of the thermo-haline circulation is the North Atlantic Conveyor, the part of the system that conveys saline surface waters to the North Atlantic, whereupon cooling causes downwelling and the formation of deep waters. It is thought to play a role in cliamte change, although reports that Global Warming will cause a "shut down" of the conveyor and a climatic cooling in North-west Europe have little foundation.
The main force driving the oceanic circulation is the wind. As wind blows across the sea-surface, part of the wind's momentum is transferred to the ocean, setting up currents. It is therefore clear that ocean currents should show a relationship with the global atmospheric circulation. The ocean currents do not simply flow in the same direction as the driving winds, however. Just as the winds are deflected by the Coriolis Force, so are ocean currents. Wind-driven currents thus flow to the right of the wind direction in the northern hemisphere, and to the left of the wind direction in the southern hemisphere. This effect does not apply at the Equator, where the Coriolis Force is zero.
This deflection of ocean currents was first noted by Fritjof Nansen, the great Norwegian polar explorer, during the voyage of the ship Fram in 1893-4. Fram was deliberately set fast in arctic sea-ice in the hope that drift would bring the vessel within striking distance of the North Pole, allowing an attempt to reach it to in relative comfort. Nansen noticed that the vessel and the enclosing ice floes consistently drifted at 20° - 40° to the right of the wind. On his return, he discussed the observations with V. W. Eckman, who formulated the equations to describe the motions. The deflection of wind-driven currents is now known as Eckman motion. Because the surface layer of the ocean transfers some of its momentum to deeper water, and because these deeper layers are subject to Coriolis deflection too, successively deeper layers of the ocean are deflected to the right of the upper layers. The wind-driven currents thus form a spiral - the Eckman Spiral - which gradually dies out at depth. The average motion of the Eckman layer is 90° to the right of the wind direction in the Northern Hemisphere (to the left in the Southern).
Wind-driven currents are also influenced by the topography of the oceans, both the form of the sea bed (bathymetry) and the position of the coasts. For example, currents driven onshore will be forced to flow along the coast. Thus, ocean currents in many parts of the world flow along the boundaries of the oceans (see map below). Currents driven offshore result in vertical motions. For example, a wind blowing parallel to a coast in the Northern Hemisphere, with the ocean on the right, will set up a surface current flowing to the right (i.e. offshore). This transports water away from the coast, which must be replaced somehow. Deeper waters are brought to the surface (upwelling), thus balancing the flow. This type of upwelling is important off the coast of Arabia during the summer monsoon, and off the coast of Peru. Upwelling brings nutrients from depth to the surface. This has important implications for biological productivity, as most marine organisms live near the surface where light can penetrate and fuel photosynthesis (the photic zone). Productivity is commonly limited by nutrient supply, so upwelling zones are typically very rich environments.
Wind-driven currents and Eckman motion can also result in verical motions in the oceans far from coasts. Cyclones (which rotate anticlockwise in the northern hemisphere) cause Eckman transport of surface waters (to the right in the NH) away from the centre of the cyclone. Outward transport in all directions from the centre results in a net export of water, which must be replenished by upwelling. Conversely, Eckman transport below high pressure, anticyclonic systems (which rotate clockwise in the NH) transports surface waters towards the centre of the system: a net import of water. To balance the flow, downwelling must occur. These upwellings and downwellings affect relatively shallow layers, unlike thermo-haline circulation.
In the above map, one of the most striking features is the presence of huge circulations, or Gyres in the sub-Tropical Pacific and Atlantic Oceans and to a lesser degree in the Indian Ocean. These gyres are clockwise in the northern hemisphere, and anticlockwise in the southern hemisphere, the same directions as winds in the Sub Tropical Highs in the atmosphere. Indeed, the oceanic gyres are driven by the atmospheric circulation, with due modification by the Coriolis effect (Eckman transport) and coastal configuration. To illustrate this point, consider the tropical Pacific. The atmospheric motion here is dominated by the trade winds, which are South Easterly in the SH and North Easterly in the NH (Lecture 2). The ocean currents are deflected to the right in the northern hemisphere, and to the left in the southern hemisphere. Thus, the ocean currents should flow from east to west in both hemispheres in the Tropical Pacific, which is what we see in the above map. Close to the Equator itself is an eastward-flowing current, the Equatorial Counter Current. This exists because the Trade Winds drive water to the western Pacific, where it is blocked by land masses. Sea surface rises westward (due to the wind stress), so in the area of light winds near the Equator (The Doldrums), water is able to flow back eastwards. This flow is very much accentuated during El Nino events (see below).
The Sub Tropical Gyres result in equatorward flowing currents along the eastern margins of the oceans, and polewards along the western margins. For example, at the eastern Pacific margin we find the northward-flowing Humboldt Current off South America, and the southward-flowing California Current off North America. Both these currents bring cool waters to low latitudes. Conversely, at the western margin of the Pacific, we have the warm northward-flowing Kuroshio Current off Japan, and at the north-western margin of the Atlantic we have the Gulf Stream.
Around the Southern Ocean, the major feature of the oceanic circulation is the Antarctic Circumpolar Current or West Wind Drift. As we saw in Lecture 2, winds in this region are westerlies. Although we might expect Eckman transport to produce currents to the left of these winds (i.e. flowing from south-west to north-east), for such large-scale motions a balance is achieved, in which the sea surface rises to the left (the direction of Eckman transport) and the surface currents flow in the same direction as the wind. This is called a geostrophic ("Earth-turning") current.
In the western Pacific, the Kuroshio Current has a similar warming effect on Japan, whereas in the eastern Pacific, the California and Humboldt Currents have a cooling influence.
Another way in which oceans can influence weather systems is in the formation of Tropical Cyclones. These immensely destructive rotating storms form only where the sea-surface temperature is above 27.5° C, because it is only here that enough energy can be supplied to fuel the intense convection required to trigger these storms.
This situation breaks down during El Nino conditions (shown above for December 1997). Warm waters piled up in the western Pacific "break out" eastwards, forming a tongue of warm water that shoots across the Pacific in an enlarged Equatorial Counter Current, warming the eastern Pacific and cooling the western Pacific. This rapid re-arrangement of ocean temperatures has a profound influence on the atmosphere. The normally fair weather of the eastern Pacific is replaced by frequent storms and flooding, whereas the usually wet summer monsoon in Northern Australia is replaced by drought. The disruption can spread further afield, affecting rainfall patterns throughout South America and in India, North America, Africa, and even Europe.
El Nino conditions recur about once every 5-10 years, although they have become more frequent and long-lasting since the 1970s. Whether this is an effect of global warming is still uncertain.
El Nino is an example of an atmospheric-oceanic oscillation, which brings about cyclic changes in weather patterns and ocean circulation. There are other large-scale oscillations, such as the Arctic Oscillation, which affects wind and weather patterns in the North Atlantic and Pacific Oceans, and may underlie patterns of flooding in Europe.