Ocean Circulation, Thermohaline Currents, and Tides

Ocean circulation is the large-scale movement of seawater, driven at the surface by winds and in the deep ocean by differences in density; thermohaline circulation is the density-driven flow caused by differences in temperature (thermo) and salinity (haline); and tides are the regular rise and fall of sea level caused mainly by the gravitational pull of the Moon and the Sun. In brief: winds drive surface gyres, and the Coriolis effect turns the water to one side (Ekman transport). Cold, salty water sinks in the North Atlantic and near Antarctica, flows slowly through the deep ocean and rises elsewhere, forming a global “conveyor belt” that takes centuries. Most coasts have two high tides and two low tides each day, and the tidal range varies with the Moon’s phase.

These three systems carry heat and nutrients, shape climate and coasts and provide a source of energy and a hazard for navigation. This guide explains wind-driven circulation and upwelling, the thermohaline circulation, how tides work and vary, compares the systems in a table, works through calculations, outlines the history and corrects common mistakes.

How do surface currents and upwelling work?

Winds blowing over the ocean drag the surface water. The Coriolis effect, caused by Earth’s rotation, deflects the moving water to the right in the Northern Hemisphere and to the left in the Southern. The water layers below the surface are deflected progressively more, forming a spiral, and the net transport of the surface layer is about 90° to the wind direction. This is Ekman transport. In each ocean basin, the winds (trade winds and westerlies) and the shape of the continents produce large circular gyres, which turn clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere. Currents are fastest and narrowest along the western edges of oceans (the Gulf Stream, Kuroshio and Brazil Current), and broad and slow along the eastern edges.

Upwelling occurs where Ekman transport moves surface water away from a coast or from the equator, and cold, nutrient-rich water rises to replace it. This supports high productivity and rich fisheries off Peru, California, Namibia and northwest Africa. Downwelling is the opposite. In El Niño years, the weakening of the trade winds reduces upwelling off Peru.

What is the thermohaline circulation?

The density of seawater increases when it gets colder and saltier. In the North Atlantic (near Greenland and in the Labrador and Nordic Seas), warm surface water from the Gulf Stream cools, and the formation of sea ice leaves behind saltier water, so it becomes dense and sinks, forming North Atlantic Deep Water. Around Antarctica, very cold, salty water sinks to form Antarctic Bottom Water, the densest water mass. These deep waters spread through the Atlantic, Indian and Pacific basins at speeds of a few millimeters to centimeters per second, and rise slowly back to the surface elsewhere, mostly in the Indian and Pacific Oceans and the Southern Ocean, and flow back as warm surface water. Wallace Broecker called the system the “great ocean conveyor belt,” which takes roughly 1,000 years to complete a circuit. The Atlantic part is the Atlantic Meridional Overturning Circulation (AMOC), which carries heat northward and has a strength of about 17 sverdrups (17 million cubic meters per second), measured by instruments at 26° N. Winds and tides also drive deep mixing, so the “conveyor” is partly an idealization.

The circulation transports heat, carbon dioxide, oxygen and nutrients. Melting ice and increased rainfall can reduce the salinity of the surface water in the North Atlantic, and many models indicate that the AMOC may weaken in the 21st century, although the extent and the risk of a more abrupt change are still under study.

How do tides work?

Tides are caused by the gravitational attraction of the Moon (and, to a lesser extent, the Sun) and by the movement of Earth and the Moon around their common center. The pull is strongest on the side of Earth facing the Moon and weakest on the opposite side, and the difference creates two bulges of water: one facing the Moon, one opposite. As Earth rotates beneath the bulges, most places have two high tides and two low tides in about 24 hours and 50 minutes (a lunar day), so each tide is about 12 hours and 25 minutes after the previous one of the same type, and the times move about 50 minutes later each day. This pattern is semi-diurnal. Some places, such as the Gulf of Mexico, have one high and one low tide a day (diurnal), and others have a mixture.

• Spring tides: at new moon and full moon, when the Sun, Moon and Earth are in line, the pulls add up, and the tidal range is largest (high tides are highest and low tides lowest).
• Neap tides: at the first and third quarter, when the Sun and Moon are at right angles, the pulls partly cancel, and the tidal range is smallest.

The tidal range is the difference between high and low water. It is below 1 m in the open ocean and in enclosed seas such as the Mediterranean, and can be very large where the shape of the coast and the seabed amplify the tide: the Bay of Fundy in Canada has the world’s highest tides, with a range of up to about 16 m. Tidal currents move large volumes of water through estuaries and straits, and tidal bores can occur in some rivers, such as the Severn and the Qiantang. Atmospheric pressure and winds produce storm surges, which add to the astronomical tide.

Why do these systems matter?

Surface currents and the AMOC redistribute heat and affect climate, for example by keeping northwestern Europe mild. Upwelling supports fisheries, and ocean mixing moves carbon and oxygen. Tides shape the intertidal zone, with its habitats and species, influence navigation and port operations and affect the erosion and deposition of sediments in estuaries. Tidal energy can be harnessed: the La Rance barrage in France (1966) has a capacity of 240 MW, and tidal stream turbines work in places such as the Pentland Firth in Scotland, while the predictable nature of tides is an advantage for power planning.

The systems compared

Worked example: flows, tidal timing and tidal energy

1. Size of the AMOC: one sverdrup is 1,000,000 m³ per second. The AMOC has a strength of about 17 Sv, which is 17,000,000 m³ per second. The Amazon River discharges about 0.2 Sv, so the AMOC carries about 17 ÷ 0.2 = 85 times as much water.
2. Volume per year: 17,000,000 m³ per second × 31,536,000 s = 5.4 × 10¹⁴ m³ a year.
3. Lunar day and tides: the Moon rises about 50 minutes later each day, so a lunar day is 24 h 50 min. With two high tides in this period, they are 24 h 50 min ÷ 2 = 12 h 25 min apart. If high water is at 06:00 today, the next high water is at 18:25, and tomorrow’s morning high water is at 06:50.
4. Tidal range: high water is 7.4 m above chart datum and low water is 1.1 m. The range is 7.4 − 1.1 = 6.3 m. At neap tides, the range may be 3.5 m, so the spring range is 6.3 ÷ 3.5 = 1.8 times as large.
5. Tidal energy of a basin: the potential energy stored in a barrage basin filled to the height of the tide is E = ½ × ρ × g × A × R², where A is the basin area and R the tidal range. For A = 10 km² (10,000,000 m²) and R = 8 m: E = 0.5 × 1,025 × 9.81 × 10,000,000 × 64 = 3.2 × 10¹² J, which is 3.2 × 10¹² ÷ 3.6 × 10⁹ = about 895 MWh per tide, before losses.
6. Number of tides: with 705 tides a year (about 1.93 per day), the theoretical energy is 895 × 705 = 631,000 MWh, but a real plant would capture only a part of this, perhaps 30%, or about 190,000 MWh a year.

How did the understanding develop?

Sailors have long used the main currents and tides: Benjamin Franklin charted the Gulf Stream in the 1770s, and Matthew Fontaine Maury produced wind and current charts in the 1850s. Isaac Newton explained the tides in his Principia (1687) by gravitation, and Pierre-Simon Laplace developed the dynamic theory of tides in the 1770s. Vagn Walfrid Ekman, a Swedish oceanographer, described the Ekman spiral in 1902 from observations by Fridtjof Nansen, who had noticed that icebergs drifted at an angle to the wind. Henry Stommel explained the western intensification of currents in 1948. The deep circulation was recognized in the 20th century through measurements of temperature, salinity and, later, tracers, and Broecker popularized the conveyor belt concept in the 1980s. Satellites and the Argo network of floats now provide continuous data.

Which mistakes are common?

Mistake 1: Thinking that the ocean moves in a single “conveyor belt” loop

The thermohaline circulation is a complex three-dimensional pattern, mixed with wind-driven and tidal flows. The conveyor belt is a simplified picture.

Mistake 2: Saying that tides are caused by the Moon only

The Sun also has an effect, about 46% of the lunar effect, and the shape of the basins and coasts strongly changes the tides at each place.

Mistake 3: Believing that water moves with the wind direction

Because of the Coriolis effect, the net transport of the surface layer is about 90° to the right of the wind in the Northern Hemisphere (Ekman transport).

Mistake 4: Thinking that high tides occur when the Moon is overhead only

There are two bulges, so a high tide also occurs when the Moon is on the opposite side of Earth, giving two high tides a day at most places.