365 Days of Climate Awareness 43 - Ocean Currents 1: Wind-Driven Surface Currents

The fluid of the ocean is constantly in motion, under several different influences. Wind, by friction, drives most surface (horizontal) currents. The tidal influence of the sun and moon create standing waves all around the world. Density differences drive vertical currents, part of the thermohaline circulation. (The term is derived from Greek: "thermos" meaning heat, "hals" meaning salt, those being the primary determinants of density). These all combine into a complicated systems of motion which operate within and between ocean basins.

Ocean surface currents.

Right now we're focusing on the surface currents generated by the wind. Wind blowing across the surface of the ocean generates waves, and also larger-scale motion of the water: currents. In the North Atlantic, the combined influence of tropical trade winds (easterlies) and midlatitude westerlies creates a pattern of wind stress which brings Caribbean-latitude water northward, known as the Gulf Stream. This narrow, rapid current is known also as a western boundary current.

Western boundary currents occur around the world and are all driven primarily by the wind. The Gulf Stream is best known to us, moving northward from the Straits of Florida (between Florida and Cuba) to roughly Cape Hatteras in North Carolina. From here the current diverges from North America and becomes the North Atlantic Drift. Along the eastern coast of Japan runs the Kuroshio, another western boundary current. Similarly, southward along the east coast of Australia runs the East Australia Current, and along the east coast of Africa runs the Agulhas current.

There are many other wind-driven surface currents around the world. To north and south of the Equator are the westward North and South Equatorial currents, with the eastward Equatorial Countercurrent running between them. Along the eastern boundaries of most ocean basins there are currents which move toward the equator. These are wider and slower than their western counterparts, but help complete the basin-wide cyclical gyres which occur around the world, running generally clockwise in the northern hemisphere, counterclockwise in the south.

Tomorrow: ocean currents 2: density (thermohaline) currents.

Be well!