Ocean circulation

Cromwell, D. (2000) Ocean circulation. New Scientist, 166 (2239, Inside Sc), 4pp.

Abstract

"How inappropriate," wrote Arthur C. Clarke, "to call this planet Earth when clearly it's Ocean. " It's hard to argue with that. Viewed from space, our planet looks predominantly blue because the oceans cover 71 per cent of the surface. We wouldn't be here if it were otherwise.
The world's seas act as enormous storage heaters smoothing out weather extremes and giving the Earth an equable climate for fostering life. Until future space missions reveal otherwise, our world appears to be the most hospitable place in the Solar System for life as we know it.
Climate is constantly on the change, probably nudged along by industrial activity. In 1996, the UN Intergovernmental Panel on Climate Change (IPCC) concluded that "the balance of evidence suggests a discernible human influence on global climate". Global climate includes effects such as monsoons and El Nino and La Nina (see Inside Science No. 120). The IPCC highlighted ocean circulation as a major component of the climate system that needed further study. This circulation is made up of a network of currents transporting huge quantities of heat, salt and other properties around the world. The volume of water moving in any one of the world's five largest surface currents, such as the Gulf Stream, is some 50 times the combined flow of all the Earth's freshwater rivers.
Ocean circulation influences the amount of carbon dioxide in the atmosphere. Water at the surface absorbs CO₂ from the air, locking it away safely as the water sinks into the ocean depths. So too do tiny marine plants called phytoplankton. If global warming disrupts the pattern of ocean circulation, as some climate modellers suggest, it could seriously limit the ocean's ability to soak up CO₂. This means that we can only fully understand climate change if we get to grips with the role of the oceans.
3D-CIRCULATION Driving forces.What is the most obvious thing about seawater apart from the fact that it is wet? It is salty of course. Surprisingly, salinity varies throughout the world's oceans. Typical seawater contains 35 grams of salt per 1000 grams water with, for example, lower values in the Baltic Sea and Arctic Ocean, and higher values in the Mediterranean Sea and Red Sea. However, to anyone who's taken a dip in the sea in hot and cold climates, it is obvious that water temperature can also vary greatly. Both temperature and salinity are important in determining the density of seawater. Density too varies from place to place, and as a consequence helps to drive ocean circulation.Winds at the surface of the ocean are also important. It's the difference in solar heating of the atmosphere at the tropics and at the poles that drives atmospheric winds, such as the mid-latitude trade winds (see Inside Science No. 44). These prevailing winds drive surface ocean currents.
Another effect comes into play, the Coriolis force, caused by the Earth's rotation. The force gives a sideways kick to ocean currents, just as it does to winds (see Inside Science No. 120). In the northern hemisphere, the Coriolis force deflects ocean currents to the right and in the southern hemisphere to the left. The basic pattern of circulation at the surface of the oceans ends up as a number of enclosed flow patterns called gyres. Because the Coriolis force varies with latitude, increasing from the equator to the poles, it turns out that the gyres have an asymmetrical shape. In the western margins of ocean basins, there are strong, narrow and fast gyre currents with speeds up to 2 metres per second. These are called western boundary currents, such as the Gulf Stream in the North Atlantic. In the east, gyre circulation consists of weak, broad and slow flows at less than 0.1 metres per second.
All the major ocean basins have subtropical and subpolar gyres (see Figure 1). Subtropical gyres are large closed patterns of circulation found between about 10 degrees and 40 degrees latitude. Subpolar gyres occur between about 50 degrees and 70 degrees . The wind piles up the water within these gyres. This means sea level is not flat (see Figure 3). For example, in the North Atlantic subtropical gyre, the Sargasso Sea - between the West Indies and the Azores - sea level is about a metre higher than the sea around Bermuda. The major exception to these gyre patterns is the Antarctic Circumpolar Current, a strong current that flows around the Southern Ocean from west to east. There are also complex patterns of currents driven by winds along the equator, where the Coriolis force vanishes.
But that's not the end of the story because the fate of ocean water depends in large part on an important interplay between its temperature, its density and its salinity. The warmer that ocean water gets, the less dense and lighter it becomes, causing it to rise towards the surface. The converse is true: the colder that water becomes the heavier it gets causing it to sink. Finally, the more salt there is in the water the denser it becomes. The greatest contrast in density therefore comes between cold, heavy saltwater and warm, light freshwater.
In winter in polar regions the ocean surface loses heat to the atmosphere, eventually leading to the formation of sea ice and increasingly salt-rich water. The now heavy, cold, salty polar water sinks to great depths causing surface water to be dragged from tropical regions towards the polar seas to replace the sinking water. Meanwhile, in the depths of the polar seas close to the ocean bottom, heavy water piles up and is pushed towards the equator en route to which the water becomes warmer and lighter. Thus a huge oceanic loop is completed. The whole process of turning warm, fresh surface water into cold, salty deep water is called deep convection and it acts as a kind of heat engine driving ocean circulation (see Figure 5).
Deep convection occurs in the polar waters of the North Atlantic and the Weddell Sea, near Antarctica. Because the pattern of flow is driven by temperature and salt differences in the water, it is called the thermohaline circulation, in contrast to the surface wind-driven circulation. The combined effect produces a three-dimensional circulation known simply as the oceanic conveyor belt (see Figure 2). The circulation of water in the Atlantic, Indian and Pacific ocean basins is linked by the Southern Ocean. The climate of Western Europe relies heavily on the conveyor belt. Thanks to the North Atlantic Drift, an extension of the Gulf Stream, Western Europe actually gets "free" heat - equivalent to the output of about a million power stations.
Seawater can store a great deal of heat - the top 3 metres of the world's oceans has the same thermal capacity as the entire atmosphere. The Sun heats the ocean surface directly, so surface waters are usually the warmest and lightest, while water near the ocean bottom is the coldest and most dense. This gradual layering or stratification of ocean waters is important because water can move more freely along layers of constant density than it can across them. But water can still drop from one depth to another in regions where there is deep convection or, as will become clear, where ocean layers tilt into the depths. CIRCULATION DYNAMICS
Waters old and new Surface winds churn up the water in a mixed layer, stretching down about 100 metres, until temperature and density are roughly constant. Beneath this mixed layer, the temperature and density vary according to depth. Most of the ocean contains cold water that has sunk at high latitudes in winter. Because warm tropical waters are lighter, they float on top of the colder polar waters. The relatively sharp divide between them, where the temperature drops rapidly with depth, is known as the thermocline (see Figure 4).The existence of the thermocline depends crucially on the thermohaline circulation. Water sinks relatively rapidly in a few sites of deep convection, but rises, or upwells, much more gradually around 1 centimetre a day, on average - across a huge area in the warmer temperate and tropical regions, and slowly returns towards the pole near the surface, to complete the cycle. This continual diffuse upwelling of deep water keeps the thermocline in place. If the upwelling was to stop, downward movement of heat from the surface would erase the sharp temperature contrast at the thermocline. In effect, the upwelling water from the ocean depths props up the thermocline above it.
Because surface polar waters are colder - and hence denser - than surface tropical waters, ocean density layers don't usually run parallel with the sea surface. Instead, layers of constant density - called isopycnals - generally slope from the ocean's surface into the interior. This is why, outside the local areas of deep convection, water from the surface can still penetrate into the ocean. The water carries with it some of the heat, salt, oxygen, CO₂ and other characteristics it had at the surface. This ferrying of water properties into the ocean interior is known as ventilation. Carbon dioxide which dissolves in surface seawater can then become isolated from the atmosphere. So changes in ocean circulation patterns affect ventilation and, thus, the ocean's ability to absorb atmospheric CO₂.
Oceanographers say the age of ocean water is the time since that water was last at the surface of the ocean. Studying the age of water helps them to piece together a picture of where water has come from, and where it is going - a map of the pathways of ocean circulation. Just as carbon dating can reveal the age of archaeological finds, so the age of ocean water can be deduced by examining its chemical content. The isotopic trace substances, or tracers, which oceanographers use for this purpose include natural radiocarbon (carbon-14), dissolved oxygen and silica. They can also use pollution from human activity, such as the chlorofluorocarbons that damage the ozone layer, and even tritium (the radioactive isotope of hydrogen) left over from the nuclear bomb tests of the 1950s and 1960s. Water near the surface may only be a few years old or less, while deeper water will be much older. For example, water in the North Atlantic colder than 2 degrees C was probably last in contact with the atmosphere 200 years ago, while, water deeper than 1500 metres in the Pacific Ocean is probably more than 500 years old.
The vertical circulation of ocean water means that even very old water must inevitably return to the surface, before starting to sink again. Water can travel great distances, taking hundreds or even thousands of years to complete a single complete path of circulation. It may take many twists and turns. Just as winds on land are channelled through valleys and deflected by hills, so are deep ocean currents affected by the shape of the ocean floor - its bathymetry. Narrow channels, where the flow is constrained by bathymetry, connect various parts of the world's oceans.
At Gibraltar, for example, Atlantic water that has a relatively low salinity flows into the Mediterranean Sea, while salty Mediterranean water flows out deep down. Regions where natural flow is restricted, such as the Strait of Gibraltar may be crucial for regulating the exchange of waters with different properties in the global ocean circulation. Someone once suggested, rather zealously, that there would be an advantage in building a massive dam at Gibraltar to regulate artificially the outflow of salty Mediterranean water into the North Atlantic. The idea was to maintain the thermohaline circulation that keeps Western Europe warm. Because there are so many uncertainties in such a technical fix, nobody has yet taken it up!
The picture, then, might seem to be one of a steady, slow-moving circulation. However, the fine detail is missing. Just as the atmosphere has its "highs" (anticyclones) and "lows" (cyclones), so the ocean has its eddies - ocean "weather systems" that move at speeds of a few centimetres a second carrying heat and salt from one part of the ocean to another. These eddies, though, are around 100 kilometres across - much smaller than their atmospheric counterparts which can stretch to 1000 kilometres (see Inside Science No. 120).
In active ocean regions such as the Gulf Stream, eddies may last only a couple of months before they are swallowed up again. By contrast, eddies travelling in less dynamic regions may survive for at least two years. Eddies form as the result of unstable flow, such as where a strong current meanders, or where there is a large difference in speed between neighbouring parts of a current, or where there is a significant change of velocity with depth.
Eddies carry a lot of kinetic energy and heat and can play an important part in the energy and heat budgets of the oceans. Eddies may also be important in transferring water characteristics between oceans. The Agulhas Current - a western boundary current off the east coast of Africa - regularly generates eddies which carry warm and salty water from the Indian Ocean into the South Atlantic.
Oceanographers have succeeded in building up a consistent picture of ocean circulation as a result of painstaking work, much of it done under arduous conditions at sea. Researchers must often endure weeks on ships just to collect enough measurements to confirm, or disprove, their theories. Over the years, ocean surveys have gradually built up our knowledge of the temperature and salinity characteristics with depth around the world's oceans. This allows researchers to calculate the water density and therefore ocean currents (see "The Large-scale Circulation").
To evaluate water density, oceanographers use a conductivity-temperature-depth probe. As it is lowered through the water, the CTD probe relays continuous profiles of temperature and conductivity (from which salinity can be estimated) back to the research ship. Mounted on the same frame along-side the CTD probe are bottles for collecting samples to calibrate, or cross-check, the salinity and other water properties the CTD probe has measured.
Current recording meters measure the speed and direction of a current at a fixed position under the surface. Most meters use a system of rotating vanes to measure current speed and a compass to measure direction. The meters are usually mounted on moorings throughout the sequence of ocean layers - or water column - and build up a picture of how currents vary with depth. The data are recorded on solid-state logging devices. A research cruise will set up the meters and when they are collected later they give a valuable record of how the currents have been changing with time.
Argo is an exciting international development creating a planet-wide network of 3000 free-drifting buoys. The aim is to observe the oceans in real time. The Argo array is part of the Global Climate Observing System and participating nations include Australia, Britain, Canada, France, Japan and the US. The buoys will measure the temperature and salinity of the upper 2000 metres of the ocean. All data will be relayed by satellite and made publicly available within hours of collection. Global ocean forecasting should be possible for the first time.
Satellites in a variety of orbits have now become part of the standard armoury of oceanographers. Satellite-borne radiometers take global measurements of sea surface temperature in cloud-free regions. Radar altimeters orbiting about 1000 kilometres above the Earth can measure the height of the sea surface with an accuracy and precision of just a few centimetres. From the differences in the height of the sea surface from place to place, oceanographers can calculate the surface currents (see "The Large-scale Circulation"). Examples of altimeters include the American/ French Topex/Poseidon system, launched in 1992, and the European Space Agency's satellites, ERS-1 (1991) and ERS-2 (1995). Follow-up satellite missions are due for launch soon. Satellite observations have revealed just how active the ocean circulation is, with more eddies and changeability from year to year than oceanographers expected.
MODELLING CIRCULATION
Future climate. Just as weather forecasters rely on powerful computers to predict the weather, so oceanographers need, and are developing, accurate computer models of ocean circulation. But ocean forecasting is far more difficult than weather forecasting. One reason is that in the ocean the crucial length scales are typically only a tenth of those in the atmosphere. Ocean eddies are around 100 kilometres in diameter, while weather systems are around 1000 kilometres. A computer model must take into account eddies to produce a realistic picture of ocean circulation. But the smaller the dimensions, the more grid points in a given area that the computer model needs, and thus the longer the model takes to run.Powerful supercomputers can now run ocean circulation models at resolutions as fine as 1/4 degrees (c. 25 kilometres) or less, so that they can represent small-scale features such as eddies, frontal regions and narrow currents for a realistic picture of ocean circulation. Models may also include up to 40 depth levels from the ocean surface to the bottom. Computer runs are conducted for hundreds of "model years" to reach a steady state and then tested by observations such as those from the World Ocean Circulation Experiment (see "Ocean Snapshot").
Another difficulty with forecasting is the sparseness of ocean observations. In weather forecasting, we can add in observations from the real world to improve the predictions but most of the ocean is simply not readily accessible.
Satellites routinely observe the ocean surface, but if we are ever to predict climate accurately we shall also need to monitor the ocean interior. We shall need to be able to couple computer models of the atmosphere with models of the ocean. Both the atmosphere and the ocean clearly interact in the real world, as in the case of El Nino and La Nina, so computer models must reflect this. Ultimately, realistic climate models will need to include the five major components of the Earth's climate system: atmosphere (air), hydrosphere (water), cryosphere (ice), lithosphere (geology) and biosphere (life). The impact of human activity, volcanic eruptions, solar variability, the cyclic effect of the periodic wobbles in the Earth's orbit (the so-called Milankovitch cycles) and possibly other phenomena will all need to be added to complete the description.
We have come a very long way from the first chart of the Gulf Stream that Benjamin Franklin drew up in 1769. Franklin's map was based on knowledge acquired by US trading merchants and whalers who exploited the strong current on their outbound leg and avoided it on their return home. For a while, British merchants scorned the apparently longer route but marvelled that American ships took less time to cross the Atlantic than theirs did.
Today, unravelling the mysteries of ocean circulation is a huge task involving scientists from many countries. If we are successful, then we might just have a chance to predict the future of the Earth's climate.
1: The Large-scale CirculationDIFFERENCES in pressure at the same depth in the ocean occur because the distributions of water temperature and salinity - and, therefore, water density - are not uniform. Water will start to flow from high to low pressure. But it will be deflected by the Coriolis force which is caused by the Earth's rotation. In the Northern hemisphere, the current will turn to the right, and in the Southern hemisphere, to the left. A balance is set up, called geostrophy, in which the forces of the Coriolis effect and pressure are of equal magnitude but in opposite directions, with the resultant water velocity, known as the geostrophic velocity, perpendicular to both forces. The velocity is along lines of constant pressure - just as the wind blows along the isobars in weather maps.
2: Ocean snapshotTHE World Ocean Circulation Experiment (WOCE) is a global ocean experiment designed to take a snapshot of ocean circulation which can be used to put computer models through their paces. This snapshot actually comprises observations between 1990 and 1997 using ships, moorings, satellites and other instruments. Data analysis will continue beyond 2000. The central element of the observational programme is a grid of sections across the major ocean basins. These sections consist of profiles of conductivity, temperature and depth from the sea surface to the seabed taken every 50 kilometres. Other measurements include the chemical content of water samples collected at various depths to determine the age of the water.
WOCE observations have been compared to previous datasets collected earlier, such as during the 1957 International Geophysical Year. For example, at 24 degrees North in the North Atlantic subtropical gyre - a slab of water 1 kilometre thick across the width of the ocean has warmed by 0.2 to 0.3 degrees C. Meanwhile, there has been cooling of the subpolar gyre at 60 degrees North. Although these changes appear to be related, the precise reasons are not yet known.

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