Oceans: currents, water masses and salinity

The first principle

The ocean moves because energy is put into it unevenly. The sun heats the tropics more than the poles, the winds that this heating drives drag the surface water, and cooling and evaporation make some water heavier than the water around it, so it sinks. Once water moves, the Earth's rotation deflects it and the continents block it, which organises the motion into giant loops at the surface and slow, deep flows below. Salinity matters because it changes density, and so decides which water sinks.

The core

Forces that drive ocean currents

• Primary forces start the motion. Solar heating makes equatorial water expand, so the sea surface near the equator stands about 8 cm higher than in the middle latitudes, and water tends to flow down this slight slope. Wind stress drags the top layer along, so the trades push water west and the westerlies push it east. Gravity pulls water down surface slopes and pulls dense water down. The Coriolis force, which arises from the Earth's rotation, deflects moving water to the right in the northern hemisphere and to the left in the southern.
• Secondary forces shape and steer it. Differences in temperature and salinity create differences in density, which drive the slow deep circulation. The shape of coasts and the sea floor splits, turns and steers currents, as when the South Equatorial Current divides at the bulge of Brazil.

Gyres, Ekman transport and upwelling

• Because wind pushes water and the Coriolis force deflects it, the net movement of the wind driven surface layer is at about 90° to the wind, to the right in the north. This is called Ekman transport. Under the trades and westerlies, water piles up in the middle of each ocean basin, and the flow around this mound forms a gyre, clockwise in the northern hemisphere and anticlockwise in the southern.
• On the western side of each basin the return flow is narrow, fast and warm, such as the Gulf Stream and the Kuroshio, because it carries tropical water poleward. On the eastern side the flow is broad, slow and cold, such as the Canary, California, Peru (Humboldt) and Benguela currents, because it brings polar water towards the equator.
• Where wind blows along a coast with the coast on its left in the southern hemisphere, as off Peru, Ekman transport moves surface water offshore and cold, nutrient rich water rises from below to replace it. This is coastal upwelling.

Water masses and the global conveyor

• A water mass is a large body of water with a near uniform temperature and salinity, which it gets at the surface in its source region before it sinks and spreads.
• North Atlantic Deep Water (NADW) forms when the salty Gulf Stream water is cooled in winter in the Greenland, Norwegian and Labrador seas, becomes dense enough to sink, and flows south at 2,000 to 4,000 m.
• Antarctic Bottom Water (AABW) forms when sea ice freezes in the Weddell and Ross seas; ice rejects its salt, so the water left behind is the coldest and densest in the ocean and hugs the sea floor.
• This sinking at high latitudes, and the slow return of deep water to the surface mainly in the Southern Ocean, makes the thermohaline circulation, often called the global conveyor belt. A parcel of water takes roughly a thousand years to complete the loop.

Dimensions

How currents and water masses differ in their effects on marine life and coasts

• Currents move heat and moisture sideways. The North Atlantic Drift keeps the ports of Norway free of ice and gives Britain mild winters at the latitude of Labrador. Cold currents chill the air above them, so it cannot rise and rain, which is why the Atacama, Namib and Baja California deserts lie beside the Peru, Benguela and California currents, and why fog forms where warm moist air passes over them, as over the Grand Banks and the Namib coast.
• Currents carry life. They carry plankton, eggs and larvae, so warm currents bring tropical species and coral far poleward, as the Kuroshio does to southern Japan, while cold currents carry cold water species towards the equator, as the Humboldt penguin off Peru shows.
• Water masses work at depth and slowly. Sinking water masses carry dissolved oxygen into the deep ocean, which lets animals live on the abyssal floor; without this ventilation the deep sea would become anoxic. Old deep water also gathers nutrients from sinking dead matter, and when it wells up, as around Antarctica, it feeds the krill and the whales and seals that eat them.
• Water masses also store carbon and heat. Sinking water takes carbon dioxide and heat down for centuries, which slows surface warming.

Role of currents in the world's fishing industry

• Meeting of warm and cold water. Where the warm Gulf Stream meets the cold Labrador Current over the shallow Grand Banks of Newfoundland, mixing stirs nutrients into sunlit water and plankton multiply; the same happens where the Kuroshio meets the Oyashio off Japan, one of the world's richest fishing areas.
• Upwelling coasts. The Peru, Benguela, Canary and California currents are upwelling systems; though they cover a tiny share of the ocean surface they supply a large share of the world's fish catch, with the Peruvian anchoveta among the largest single species fisheries on Earth.
• Shallow banks. The Dogger Bank in the North Sea, shallow and well mixed by tides and currents and fed by the North Atlantic Drift, has long sustained herring and cod.
• When currents fail. In an El Nino year the trade winds weaken, warm water spreads east, upwelling off Peru stops, and the anchoveta catch collapses, as it did in 1972 to 1973 and 1997 to 1998.

In practice

Salinity: how much, and why it varies

• Salinity is the weight of dissolved salts in a kilogram of sea water. The average is about 35 parts per thousand, mostly sodium chloride, with magnesium, sulphate, calcium and potassium.
• It rises with evaporation and with salt left behind when sea ice freezes, and it falls with rainfall, river inflow and melting ice. Mixing by currents evens out the differences.
• By latitude. Salinity is lower at the equator, about 34.5, despite the heat, because rain is heavy there; it is highest at about 20° to 30° north and south, about 36 to 37, under dry sinking air of the subtropical highs; and it falls towards the poles because of melting ice and low evaporation.
• In enclosed seas. The Red Sea reaches about 40 because it is hot, dry and receives no rivers. The Baltic Sea is below 10 in parts because many rivers enter it, it is cold, and its narrow entrance limits exchange with the Atlantic. The Dead Sea, an enclosed salt lake, is about ten times saltier than the ocean.

Multidimensional effects of salinity

• Density and circulation. Saltier water is denser, so salinity together with temperature drives the thermohaline circulation, which is why salty Mediterranean water sinks beneath the Atlantic at Gibraltar.
• Freezing point. Sea water of 35 parts per thousand freezes at about −1.9°C, which keeps the open ocean liquid in conditions where fresh water would freeze.
• Marine life. Most marine animals are adapted to a narrow range of salinity, so estuaries and lagoons such as Chilika, where fresh and salt water mix, host specialised species and serve as nurseries.
• Economy. Salt pans in Gujarat and Tamil Nadu depend on high evaporation; buoyancy and ship draught vary with salinity, as load lines on ships mark.
• Climate. If Greenland's melt freshens the North Atlantic, sinking could weaken. The IPCC Sixth Assessment Report (2021) judges that the Atlantic Meridional Overturning Circulation is very likely to weaken this century, though an abrupt collapse before 2100 is not expected.

Data and examples

• The Food and Agriculture Organization (FAO) reported total fisheries and aquaculture production of about 223 million tonnes in 2022, with aquaculture overtaking capture fisheries for aquatic animals for the first time.

Issues and way forward

• Overfishing. Rich fishing grounds attract too many boats; the Grand Banks cod fishery collapsed and Canada closed it in 1992. Catch limits based on stock science, as FAO recommends, and the World Trade Organization agreement on fisheries subsidies (2022) address the cause.
• Warming and acidification. Warmer surface water stratifies the ocean, which reduces upwelling of nutrients and mixing of oxygen down; the IPCC Special Report on the Ocean and Cryosphere (2019) records spreading low oxygen zones. Cutting emissions is the root remedy.
• Weakening overturning. Freshwater from melting ice may slow the conveyor and shift rainfall belts; long term monitoring arrays across the Atlantic and Argo floats need sustained funding.

Common confusions

• Current and water mass. A current is moving water; a water mass is a body of water defined by its temperature and salinity, and it may move slowly as part of a current.
• Gulf Stream and North Atlantic Drift. The Gulf Stream is the fast western boundary current off the United States; beyond the Grand Banks its broader extension towards Europe is the North Atlantic Drift.
• Salinity and density. Density depends on both temperature and salinity; cold fresh water can be denser than warm salty water.
• Upwelling and El Nino. Upwelling is the normal state off Peru; El Nino is the year when it stops.

For the exam

• Keyword bank: wind stress, Coriolis force, Ekman transport, gyre, western boundary current, upwelling, North Atlantic Deep Water, Antarctic Bottom Water, thermohaline circulation, global conveyor belt, salt rejection, parts per thousand, Grand Banks, anchoveta, Kuroshio and Oyashio.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction (data): The average salinity of the ocean is about 35 parts per thousand, yet it ranges from below 10 in the Baltic to about 40 in the Red Sea, and that range shapes density, circulation and life.
  • Introduction (judgement, quote or report): The IPCC Sixth Assessment Report judges that the Atlantic overturning circulation is very likely to weaken this century, which shows that ocean water masses are now a climate question.
  • Conclusion: Currents act quickly at the surface and shape coastal climates and fisheries, while water masses act slowly at depth and keep the ocean breathing; both depend on the same balance of heat, salt and rotation.
  • Conclusion: The world's richest fishing grounds are gifts of ocean circulation, and protecting them needs both catch limits and a stable climate.