10.1: Main Features

In this chapter, we largely follow the terminology and definitions adopted by Talley et al. (2011). Additional references are cited where helpful.

The surface circulation crosses the ocean like a vast network of oceanic rivers. However, unlike rivers, the currents that make up the surface circulation in each basin form an ensemble of currents known as oceanic gyres, or simply gyres. The word comes from the Greek word gyros, translated as “circle or ring.” It describes a system of currents that rotate around a common center like spokes on a wheel. The centers of the oceanic gyres can be found in two general locations in the world ocean: the subtropical gyres, straddling the Tropic of Cancer and the Tropic of Capricorn; and the subpolar gyres, adjacent to the Arctic and Antarctic Circles. Their boundaries are fluid and may change seasonally, but these two gyre archetypes—subtropical and subpolar—help us simplify the complex patterns of the surface circulation and allow us to make some broad generalizations.

I should also note that oceanographers refer to currents that move along lines of latitude—east to west or west to east—as zonal currents. The equatorial currents and the west wind drift currents represent examples of zonal currents in a gyre. Currents that move along lines of longitude—toward the poles or the equator—are referred to as meridional currents. The California Current along the West Coast of the US and the Gulf Stream along the East Coast represent meridional currents in a gyre. Look for other examples in Figure 17.10.

Subtropical Gyres

As pointed out by Roemmich et al. (2016), most people in the world live under weather patterns influenced by the subtropical gyres. Five major subtropical gyres occupy the three major ocean basins: the North Pacific Gyre (NPG), South Pacific Gyre (SPG), North Atlantic Gyre (NAG), South Atlantic Gyre (SAG), and the South Indian Gyre (SIG)—one for each major ocean basin except the North Indian. Squeezed and shortened by the presence of the Indian subcontinent, the North Indian Ocean basin lacks a true gyre, but in its place lives a fascinating system of currents that change direction seasonally (see below). Note that the gyres in the Northern Hemisphere rotate clockwise, while the Southern Hemisphere gyres rotate counterclockwise. These rotations result, of course, from the Coriolis force (introduced in Chapter 15).

Polar and Subpolar Gyres

Polar gyres exist only in the Arctic Ocean whose waters reach the very highest latitudes of the world ocean. Here we find the Beaufort Gyre (BG) in the Beaufort Sea—named after Irish hydrographer Sir Francis Beaufort (1774–1857). This gyre spins clockwise in the Arctic Basin (when viewed from above).

Subpolar gyres occur at latitudes above 60°. Generally, landmasses restrict their size in the Northern Hemisphere, while in the Southern Hemisphere, the Antarctic Circumpolar Current (described below) sweeps such a broad path that its gyres are confined to a few locations nestled against Antarctica.

Like their subtropical counterparts, subpolar gyres are named and feature zonal and meridional currents that mark their boundaries. They don’t rotate consistently because at these smaller scales, the momentum of the currents that feed them and the shape of the land that surrounds them govern their behavior. Nevertheless, they exert an important influence on regional oceanographic and meteorological conditions.

Notable in the North Pacific are the Alaskan Gyre (AG), where we find the northward-flowing coastal-hugging Alaska Current; and the Western Subarctic Gyre (WSAG), fed by the East Kamchatka and Oyashio Currents. In the Southern Ocean, two subpolar gyres can be found. The Ross Gyre (RG) acts as a gatekeeper for warm-water exchanges between the Antarctic Circumpolar Current and the Ross Sea—a broad, shallow sea located at the “mouth” of the elephant head–shaped continent. It occupies a region roughly between East and West Antarctica (e.g., Dotto et al. 2018). Across the “trunk” of the West Antarctic peninsula lies the Weddell Sea and the Weddell Gyre (WG), a site of bottom water formation thought to be critically important to world ocean circulation and climate change (e.g., Vernet et al. 2019).

Equatorial Currents

In all three major basins—Atlantic, Pacific, and Indian—currents flow along the equator from east to west in a complex of currents known as the equatorial currents. Occupying the tropical regions of our planet, this set of currents receives the most intense solar radiation. As their waters move from east to west, they warm up. By the time they travel the full width of the basin, their waters reach their highest temperatures. Indeed, the warmest waters in the world ocean can generally be found at the western terminus of the equatorial currents, places such as the Caribbean Sea and coastal Brazil, the Philippine Sea and Indonesian Seas, and north of Madagascar off the coast of Somalia, where modern-day pirates skulk.

The principal equatorial currents include the North Equatorial Current (NEC), the South Equatorial Current (SEC), the North Equatorial Countercurrent (NECC), the South Equatorial Countercurrent (SECC), and the Equatorial Undercurrent (EUC) in the Pacific, Atlantic, and Indian Oceans. Note that in the Pacific Ocean, the Equatorial Undercurrent is also known as the Cromwell Current, named after the man who discovered it, Scripps oceanographer Townsend Cromwell (1922–1958). In the Pacific Ocean, the South Equatorial Current exists as a broad, westward-flowing current that flows along the equator in both hemispheres. The South Equatorial Countercurrent develops seasonally as an eastward flow in the western part of the basin (e.g., Chen and Qiu 2004). In the Atlantic Ocean, the South Equatorial Current develops two additional branches: the North South Equatorial Current (NSEC) and the Central South Equatorial Current (CSEC). In the Indian Ocean, the South Equatorial Current and Equatorial Undercurrent only develop seasonally (e.g., Phillips et al. 2021).

Equatorial currents move waters from the eastern sides of ocean basins to their western sides. Indeed, oceanographers use the terms eastern and western to distinguish the eastern and western halves of ocean basins. For example, the US West Coast lies along the eastern North Pacific while the US East Coast lies along the western North Atlantic. It can get a bit confusing, but it helps to remember that eastern refers to the right-hand side of the basin while western refers to the left-hand side. Just remember western US (left-hand side of the continent) and eastern US (right-hand side of the continent) and apply the same reasoning to figure out which side of an ocean basin you’re on.