Surface Currents

Oceanography Fundamentals

Course 1 · Chapter 6 · Surface Currents: Gyres and Upwelling

In the 1760s Benjamin Franklin, then deputy postmaster for the American colonies, was puzzled that mail packets from England took about two weeks longer to reach Rhode Island than merchant ships did. A Nantucket whaling captain, Timothy Folger, told him why: the merchant captains knew of a strong current in the Atlantic and avoided sailing against it. Franklin had Folger sketch its course, and the chart was printed around 1770 (check the dates). It is the first well-known map of what we now call the Gulf Stream. This chapter explains what drives the great surface currents, why they circle the ocean basins, and what happens where they meet the coast.

What Drives Surface Currents

The winds that blow steadily over the ocean (the trade winds in the tropics, the westerlies in mid-latitudes) drag on the surface and set the top few hundred metres moving. Three other things shape the result:

The continents, which block the water and turn it.
The Coriolis effect, from Earth's rotation.
Differences in density, which add slower movements below the surface (Chapter 7).

The Coriolis effect, briefly Earth turns, so a moving object is seen from the rotating surface to be deflected: to the right in the northern hemisphere and to the left in the southern (check). The effect is zero at the equator and strongest at the poles. It is far too weak to matter in a bath or a sink, but over the distances and times of ocean currents it is decisive. It also bends the tide wave around the amphidromic points of Chapter 5.

Ekman Transport: Why Water Does Not Follow the Wind

The Norwegian explorer Fridtjof Nansen noticed, during his drift in the ship Fram in the Arctic in the 1890s, that the pack ice moved not with the wind but at an angle of 20 to 40 degrees to the right of it (check). He asked a young Swedish physicist, Vagn Walfrid Ekman, to explain it, and Ekman worked out the answer in 1902 (check).

The wind drags the top layer, which is deflected a little to the right (northern hemisphere).
That layer drags the one below, which is deflected further, and so on.
Each layer moves more slowly and further to the right than the one above, making a spiral with depth (the Ekman spiral).
Adding up all the layers, the net movement of the water is about 90 degrees to the right of the wind in the northern hemisphere, and 90 degrees to the left in the southern.

Real oceans only approximate the ideal spiral, but the 90-degree net transport is a good rule of thumb for the upper layer (check).

The Gyres

Put these ingredients together and the water in each major ocean basin circulates in a great loop called a gyre. The subtropical gyres are the largest.

Northern hemisphere

Gyres turn clockwise. In the North Atlantic: the Gulf Stream and North Atlantic Drift on the west and north, the Canary Current on the east, and the North Equatorial Current on the south.

Southern hemisphere

Gyres turn anticlockwise. Examples are the South Pacific gyre, with the Humboldt (Peru) Current on its east side, and the South Atlantic gyre with the Benguela Current.

The Southern Ocean

The Antarctic Circumpolar Current flows eastward all round Antarctica, with no land in the way. It is the largest current in the world, carrying over a hundred times the flow of all the rivers on Earth (check).

Current	Ocean	Type	Note
Gulf Stream	North Atlantic	Warm, western boundary	Narrow, fast, up to roughly 100 km wide; flows north-east from Florida (check)
North Atlantic Drift	North Atlantic	Warm	The slower, broader extension toward north-west Europe
Canary Current	North Atlantic	Cool, eastern boundary	Flows south along north-west Africa; strong upwelling
Kuroshio	North Pacific	Warm, western boundary	The Pacific equivalent of the Gulf Stream
Humboldt (Peru)	South Pacific	Cool, eastern boundary	Feeds one of the world's richest fisheries
Antarctic Circumpolar	Southern Ocean	Wind and density driven	Eastward all the way round

Fast on the West, Slow on the East

The gyres are lopsided. The currents on the western edge of each ocean (the Gulf Stream, the Kuroshio) are narrow, deep, fast and warm. The ones on the eastern edge (the Canary, the California, the Humboldt) are wide, shallow, slow and cool. This western intensification was explained by the American oceanographer Henry Stommel in 1948, as a result of how the Coriolis effect changes with latitude (check). Oceanographers measure the flow of water in sverdrups (Sv), where 1 Sv is a million cubic metres per second. The Gulf Stream carries about 30 Sv through the Straits of Florida and more after it is joined by other water, while all the world's rivers together carry only about 1 Sv (check).

The Gulf Stream and Scotland: how much credit? It is often said that the Gulf Stream makes Britain several degrees warmer than Labrador at the same latitude. The Gulf Stream and North Atlantic Drift do carry heat north, but studies suggest that the prevailing westerly winds, which carry the Atlantic's mild air over Europe, and the shape of the continents also play a big part (check). The honest answer is that the ocean as a whole, not one current, is responsible. Be wary of statements that the Gulf Stream "will switch off" and bring an Arctic winter; the real concern is a possible slowdown of the deeper circulation, covered in Chapter 7 and Chapter 8.

Upwelling: Bringing Up the Deep

Surface water can be moved away by the wind, and deeper water rises to replace it. This is upwelling. The deep water is cold and rich in nutrients (nitrate, phosphate), which sinking material has carried down; when it reaches sunlit water, it feeds plankton in large numbers, and so the whole food web above it.

Coastal upwelling. Wind blowing along a coast with the coast on its left in the northern hemisphere (equatorward on a western continental coast, such as California) gives Ekman transport away from the shore, and cold water rises to fill the gap. The same happens off Peru, north-west Africa, Namibia and Iberia.
Equatorial upwelling. The trade winds on each side of the equator drive surface water poleward in both hemispheres, leaving a gap on the equator that is filled from below.
Southern Ocean upwelling. Strong westerlies drive a transport that lifts deep water near Antarctica.

Upwelling regions cover a small part of the ocean but are credited with a very large share of the world's fish catch, with figures of around a fifth often quoted (check). The Peruvian anchoveta fishery, supported by the Humboldt upwelling, is the largest single-species fishery in the world (check) and collapses when the upwelling fails in El Niño years (Chapter 8).

Currents carry things The gyres also collect floating objects. In 1992 a container ship lost a cargo of plastic bath toys in the Pacific, and some were found years later on distant shores, helping oceanographers test their models of surface drift (check the details). Floating plastic gathers in the centres of the subtropical gyres, in areas popularly called garbage patches. They are not solid islands but dilute zones of fragments, and the reality is less visible than the name suggests.

Exercises

Exercise 1

A river flows at 20,000 cubic metres per second. Express this in sverdrups, then work out how many such rivers it would take to equal a current of 30 Sv.

→ Solution

Exercise 2

In the northern hemisphere, a steady wind blows from the south toward the north across the open ocean. In which compass direction is the net Ekman transport of the surface layer? Repeat for the southern hemisphere.

→ Solution

Exercise 3

Explain, in a short paragraph, why a northerly wind blowing south along the California coast brings cold, nutrient-rich water to the surface and boosts the plankton.

→ Solution

Quick Reference

Drivers: wind, Coriolis effect, continents
Coriolis: deflection to the right in the north and the left in the south; zero at the equator
Ekman transport: net surface movement about 90 degrees to the right (north) or left (south) of the wind
Gyres: clockwise in the north, anticlockwise in the south
Boundaries: western currents narrow, fast, warm; eastern currents wide, slow, cool
Units: 1 Sverdrup = 1 million cubic metres per second
Upwelling: deep, cold, nutrient-rich water rises where the wind moves surface water away
Check: Franklin chart date, Gulf Stream width and flow, share of fish catch

Coming next Chapter 7 descends below the wind-driven layer to the slow circulation of the deep ocean: how cold, salty water sinks near the poles and spreads through the basins over centuries, and what that means for climate.