Effect of Ocean Circulation on the Climate | Climatology | Geography

In this article we will discuss about the effects of ocean circulation on the climate.

Like the circulation of air, the circulation of the world’s oceans is important in the latitudinal redistribution of energy. Warm ocean currents are corridors of warm water moving from the tropics pole-ward where they release energy to the air. Cold ocean currents are corridors of cold water moving from higher latitudes toward the equator. They absorb energy received in the tropics thus cooling the air above. A distinct correlation between the pattern of ocean currents and the air circulation above them can be made.

The major ocean currents are wind-driven currents, though some ocean currents result from density and salinity variations of water. The subtropical high pressure cells are responsible for many of the Earth’s great ocean currents. Examine the location of the subtropical highs and then place their position on the map of world ocean currents. See any correlation? Notice how the position of the subtropical highs and the circulation around them coincide with the circulation of many of the world’s ocean currents.

Take the Gulf Stream for example. As air blows out of the western side of the subtropical high it flows over a warm pool of subtropical water dragging it northward creating a warm ocean current. Approaching the eastern seaboard of the United States it is deflected toward the northeast flowing towards the north Atlantic and Europe. After crossing the Atlantic it turns into the North Atlantic Current (Drift). The Gulf Stream enhances instability and the likelihood for precipitation as air passes over it. The warmth of the North Atlantic Drift moderates the climate of The British Isles.

As air circulates around the eastern sides of the subtropical highs it blows over cold pools of water dragging them equator-ward creating cold ocean currents. The Peru Current is a wind-driven cold current that flows along the coast of South America. Air moving over the cold current is stabilized, inhibiting uplift, the development of clouds and precipitation.

Cool coastal deserts form along some coasts that are bordered by cold ocean currents, for example, the Atacama Desert. These deserts derive their meager moisture from fogs that form when warm, moist air masses from further out in the ocean travel over the cold current causing condensation.

Surface Ocean Currents:

An ocean current can be defined as a horizontal movement of sea water in the ocean. Ocean currents are driven by the circulation of wind above surface waters, interacting with evaporation, sinking of water at high latitudes, and the Coriolis force generated by the earth’s rotation. Frictional stress at the interface between the ocean and the wind causes the water to move in the direction of the wind.

Large surface ocean currents are a response of the atmosphere and ocean to the flow of energy from the tropics to polar-regions. In some cases, currents are transient features and affect only a small area. Other ocean currents are essentially permanent and extend over large horizontal distances.

On a global scale, large ocean currents are constrained by the continental masses found bordering the three oceanic basins. Continental borders cause these currents to develop an almost closed circular pattern called a gyre. Each ocean basin has a large gyre located at approximately 30° North and one at 30° South latitude in the subtropical regions.

The currents in these gyres are driven by the atmospheric flow produced by the subtropical high pressure systems. Smaller gyres occur in the North Atlantic and Pacific Oceans centered at 50° north. Currents in these systems are propelled by the circulation produced by polar low pressure centers. In the Southern Hemisphere, these gyre systems do not develop because of the lack of constraining land masses.

A typical gyre displays four types of joined currents – two east-west aligned currents found respectively at the top and bottom ends of the gyre; and two boundary currents oriented north-south and flowing parallel to the continental margins.

Direction of flow within these currents is determined by the direction of the macro-scale wind circulation interacting with the Coriolis force. Boundary currents play a role in redistributing global heat latitudinally.

Surface Currents of the Subtropical Gyres:

On either side of the equator, in all ocean basins, there are two west-flowing currents – the North and South Equatorial (Figure 4.1). These currents flow between 3 and 6 kilometers per day and usually extend 100 to 200 meters in depth below the ocean surface. The Equatorial Counter Current, which flows towards the east, is a partial return of water carried westward by the North and South Equatorial currents. In El Niño years, this current intensifies in the Pacific Ocean.

Flowing from the equator to high latitudes are the western boundary currents. These warm water currents have specific names associated with their location – North Atlantic – Gulf Stream; North Pacific – Kuroshio; South Atlantic – Brazil; South Pacific–East Australia; and Indian Ocean – Agulhas. All of these currents are generally narrow, jet-like flows that travel at speeds between 40 and 120 kilometers per day. Western boundary currents are the deepest ocean surface flows, usually extending 1,000 meters below the ocean surface.

Flowing from high latitudes to the equator are the eastern boundary currents.

These cold water currents also have specific names associated with their location – North Atlantic – Canary; North Pacific – California; South Atlantic – Benguela; South Pacific – Peru; and Indian Ocean – West Australia. All of these currents are generally broad, shallow moving flows that travel at speeds between 3 and 7 kilometers per day.

In the Northern Hemisphere, the east-flowing North Pacific Current and North Atlantic Drift move the waters of western boundary currents to the starting points of the eastern boundary currents. The South Pacific Current, South Indian Current and South Atlantic Current provide the same function in the Southern Hemisphere.

These currents are associated with the Antarctic Circumpolar Current (West Wind Drift). Because of the absence of landmass at this latitude zone, the Antarctic Circumpolar flows in continuous fashion around Antarctica and only provides a partial return of water to the three Southern Hemispheric ocean basins.

Surface Currents of the Polar Gyres:

The polar gyres exist only in the Atlantic and Pacific basins in the Northern Hemisphere. They are propelled by the counter clockwise winds associated with the development of permanent low pressure centers at 50° of latitude over the ocean basins. Note that the west-flowing current forming the southern margin of the polar gyres is also the eastward-flowing flowing current forming the northern margin of the subtropical gyres. Other currents associated with these gyres are shown on Figure 4.1.

Subsurface Currents:

The world’s oceans also have significant currents that flow beneath the surface (Figure 4.2). Subsurface currents generally travel at a much slower speed when compared to surface flows. The subsurface currents are driven by differences in the density of sea water. The density of seawater deviates in the oceans because of variations in temperature and salinity. Near-surface seawater begins its travel deep into the ocean in the North Atlantic.

The down-welling of this water is caused by high levels of evaporation that cool and increase the salinity of the seawater as it flows pole-ward. The down-welling (sinking) of this cold, dense, saline water takes place between Northern Europe and Greenland and just north of Labrador, Canada. This seawater then moves south at depth along the coast of North and South America until it reaches Antarctica. At Antarctica, the cold and dense seawater then travels eastward joining another deep current that is created by evaporation and sinking occurring between Antarctica and the southern tip of South America.

Slightly into its eastward voyage, the deep cold flow splits off into two currents, one of which moves northward. In the North Pacific and in the northern Indian Ocean, these two currents are drawn up from the ocean floor to its surface by wind-induced upwelling. The water warms at the surface and forms a current that flows at the surface eventually back to the starting point in the North Atlantic, or creating a shallow flow that circles around Antarctica. One complete circuit of this flow of seawater is estimated to take about 1,000 years.

Thermohaline Circulation:

The term thermohaline circulation (THC) refers to the part of the large- scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective thermohaline derives from thermo – referring to temperature and haline referring to salt content, factors which together determine the density of sea water. Wind-driven surface currents (such as the Gulf Stream) head pole-wards from the equatorial Atlantic Ocean, cooling all the while and eventually sinking at high latitudes (forming North Atlantic Deep Water).

This dense water then flows into the ocean basins. While the bulk of it upwells in the Southern Ocean, the oldest waters (with a transit time of around 1600 years) up-well in the North Pacific. Extensive mixing therefore takes place between the ocean basins, reducing differences between them and making the Earth’s ocean a global system. On their journey, the water masses transport both energy (in the form of heat) and matter (solids, dissolved substances and gases) around the globe. As such, the state of the circulation has a large impact on the climate of the Earth.

The thermohaline circulation is sometimes called the ocean conveyor belt, the great ocean conveyor or the global conveyor belt. On occasion, it is used to refer to the meridional overturning circulation (often abbreviated as MOC). The term MOC, however, is more accurate and well defined, as it is difficult to separate the part of the circulation which is actually driven by temperature and salinity alone as opposed to other factors such as the wind. Temperature and salinity gradients can also lead to a circulation which does not add to the MOC itself.

The movement of surface currents pushed by the wind is fairly intuitive. For example, the wind easily produces ripples on the surface of a pond. Thus the deep ocean — devoid of wind — was assumed to be perfectly static by early oceanographers. However, modern instrumentation shows that current velocities in deep water masses can be significant (although much less than surface speeds).

In the deep ocean, the predominant driving force is differences in density, caused by salinity and temperature (the more saline the denser and the colder the denser). There is often confusion over the components of the circulation that are wind and density driven. Note that ocean currents due to tides are also significant in many places; most prominent in relatively shallow coastal areas, tidal currents can also be significant in the deep ocean.

The density of ocean water is not globally homogeneous, but varies significantly and discretely. Sharply defined boundaries exist between water masses which form at the surface, and subsequently maintain their own identity within the ocean. They position themselves one above or below each other according to their density, which depends on both temperature and salinity.

Warm sea water expands and is thus less dense than cooler seawater. Saltier water is denser than fresher water because the dissolved salts fill interstices between water molecules, resulting in more mass per unit volume. Lighter water masses float over denser ones (just as a piece of wood or ice will float on water, see buoyancy). This is known as “stable stratification”.

When dense water masses are first formed, they are not stably stratified. In order to take up their most stable positions, water masses of different densities must flow, providing a driving force for deep currents. The thermohaline circulation is mainly triggered by the formation of deep water masses in the North Atlantic and the Southern Ocean and Haline forcing caused by differences in temperature and salinity of the water.

Formation of Deep Water Masses:

The dense water masses that sink into the deep basins are formed in quite specific areas of the North Atlantic and the Southern Ocean. In these polar-regions, seawater at the surface of the ocean is intensely cooled by the wind. Wind moving over the water also produces a great deal of evaporation, leading to a decrease in temperature, called evaporative cooling. Evaporation removes only molecules of pure water, resulting in an increase in the salinity of the seawater left behind, and thus an increase in the density of the water mass.

In the Norwegian Sea evaporative cooling is predominant, and the sinking water mass, the North Atlantic Deep Water (NADW), fills the basin and spills southwards through crevasses in the submarine sills that connect Greenland, Iceland and Great Britain. It then flows very slowly into the deep abyssal plains of the Atlantic, always in a southerly direction. Flow from the Arctic Ocean Basin into the Pacific, however, is blocked by the narrow shallows of the Bering Strait.

The formation of sea ice also contributes to an increase in seawater salinity; saltier brine is left behind as the sea ice forms around it (pure water preferentially being frozen). Increasing salinity depresses the freezing temperature of seawater, so cold liquid brine is formed in inclusions within a honeycomb of ice. The brine progressively melts the ice just beneath it, eventually dripping out of the ice matrix and sinking. This process is known as brine exclusion. By contrast in the Weddell Sea off the coast of Antarctica near the edge of the ice pack, the effect of wind cooling is intensified by brine exclusion.

The resulting Antarctic Bottom Water (AABW) sinks and flows north into the Atlantic Basin, but is so dense it actually underflows the NADW. Again, flow into the Pacific is blocked, this time by the Drake Passage between the Antarctic Peninsula and the southernmost tip of South America.

The dense water masses formed by these processes flow downhill at the bottom of the ocean, like a stream within the surrounding less dense fluid, and fill up the basins of the polar seas. Just as river valleys direct streams and rivers on the continents, the bottom topography steers the deep and bottom water masses.

Note that, unlike fresh water, saline water does not have a density maximum at 4°C but gets denser as it cools all the way to its freezing point of approximately “1.8°C.

Movement of Thermohaline Circulation:

Formation and movement of the deep water masses at the North Atlantic Ocean, creates sinking water masses that fill the basin and flows very slowly into the deep abyssal plains of the Atlantic. This high latitude cooling and the low latitude heating drives the movement of the deep water in a polar southward flow. The deep water flows through the Antarctic Ocean Basin around South Africa where it is split into two routes – one into the Indian Ocean and one past Australia into the Pacific.

At the Indian Ocean, some of the cold and salty water from Atlantic — drawn by the flow of warmer and fresher upper ocean water from the tropical Pacific — causes a vertical exchange of dense, sinking water with lighter water above. It is known as overturning. In the Pacific Ocean, the rest of the cold and salty water from the Atlantic undergoes Haline forcing and slowly becomes warmer and fresher.

The out-flowing undersea of cold and salty water makes the sea level of the Atlantic slightly lower than the Pacific and salinity or halinity of water at the Atlantic higher than the Pacific. This generates a large but slow flow of warmer and fresher upper ocean water from the tropical Pacific to the Indian Ocean through the Indonesian Archipelago to replace the cold and salty Antarctic Bottom Water. This is also known as Haline forcing (net high latitude freshwater gain and low latitude evaporation).

This warmer, fresher water from the Pacific flows up through the South Atlantic to Greenland, where it cools off and undergoes evaporative cooling and sinks to the ocean floor, providing a continuous thermohaline circulation. Hence, a recent and popular name for the thermohaline circulation, emphasizing the vertical nature and pole-to-pole character of this kind of ocean circulation, is the meridional overturning circulation.

Gulf Stream:

The Gulf Stream, together with its northern extension towards Europe, the North Atlantic Drift, is a powerful, warm, and swift Atlantic Ocean current that originates in the Gulf of Mexico, exits through the Strait of Florida, and follows the eastern coastlines of the United States and Newfoundland before crossing the Atlantic Ocean. The process of western intensification causes the Gulf Stream to be a northward accelerating current offshore the east coast of North America. At about 30°W, 40°N, it splits in two, with the northern stream crossing to northern Europe and the southern stream recirculating off West Africa.

The Gulf Stream influences the climate of the east coast of North America from Florida to Newfoundland, and the west coast of Europe. Although there has been recent debate, there is consensus that the climate of Western Europe and Northern Europe is warmer than it would otherwise be due to the North Atlantic drift, one of the branches from the tail of the Gulf Stream. It is part of the North Atlantic Subtropical Gyre. Its presence has led to the development of strong cyclones of all types, both within the atmosphere and within the ocean. The Gulf Stream is also a significant potential source of renewable power generation.

Upwelling:

All these dense water masses sinking into the ocean basins displace the water below them, so that elsewhere water must be rising in order to maintain a balance. However, because this thermohaline upwelling is so widespread and diffuse, its speeds are very slow even compared to the movement of the bottom water masses. It is therefore difficult to measure where upwelling occurs using current speeds, given all the other wind-driven processes going on in the surface ocean. Deep waters do however have their own chemical signature, formed from the breakdown of particulate matter falling into them over the course of their long journey at depth; and a number of authors have tried to use these tracers to infer where the upwelling occurs.

Wallace Broecker, using box models, has asserted that the bulk of deep upwelling occurs in the North Pacific, using as evidence the high values of silicon found in these waters. However, other investigators have not found such clear evidence. Computer models of ocean circulation increasingly place most of the deep upwelling in the Southern Ocean, associated with the strong winds in the open latitudes between South America and Antarctica.

While this picture is consistent with the global observational synthesis of William Schmitz at Woods Hole and with low observed values of diffusion, not all observational syntheses agree. Recent papers by Lynne Talley at the Scripps Institution of Oceanography and Bernadette Sloyan and Stephen Rintoul in Australia suggest that a significant amount of dense deep water must be transformed to light water somewhere north of the Southern Ocean.