Geography · Ch 19 — Movements of Ocean Water
Major Ocean Currents
Major Ocean Currents
The map is a global one, stretching from 90°N down to 90°S and across the full 360° of longitude, so every ocean basin appears in full. Its purpose is to show the major surface currents of the Pacific, Atlantic, and Indian Oceans as a single interconnected system. Each current is drawn as an arrow tracing its path across the water, and the legend distinguishes two types: solid arrows mark warm currents, dashed arrows mark cold currents. A scale bar in the lower right corner (0–2000–6000 km) gives a sense of the vast distances these flows cover.
Look first at the Atlantic. The Gulf Stream runs north along the eastern coast of North America, then becomes the North Atlantic Drift as it crosses toward Europe. Cold water comes back down the other side as the Labrador Current off Canada and the Canaries Current off northwest Africa. In the South Atlantic, the pattern mirrors this: the warm Brazilian Current flows south along South America, the cold Falkland Current moves north along the same coast, and the Benguela Current carries cold water up the western coast of Africa. The North and South Equatorial Currents, driven by the trade winds, push water westward across the ocean, with the Equatorial Counter Current flowing back eastward between them.
The Pacific shows the same logic. The warm Kuroshio Current runs north past Japan, and the cold Oyashio Current comes down to meet it. Across the northern Pacific, the North Pacific Drift carries water eastward, then splits — part turns south as the cold California Current along the US coast, part turns north as the warm Alaska Current. In the South Pacific, the Humboldt (Peru) Current brings cold water up the western coast of South America, while the warm East Australian Current flows south along Australia. Again, the North and South Equatorial Currents and the Equatorial Counter Current complete the tropical circulation.
The Indian Ocean is simpler. The warm Agulhas Current runs down the eastern coast of Africa, and the West Australian Current carries cooler water up the western coast of Australia. The equatorial currents appear here too, though the map shows them less prominently than in the other two oceans. Finally, ringing the entire Southern Ocean is the West Wind Drift, a cold current that circles the globe continuously through all three ocean basins, driven by the relentless westerly winds of the high southern latitudes. …
The entire system of ocean currents is driven by two forces working together: the push of prevailing winds and the deflection caused by the Coriolis force. Because of this, the pattern of oceanic circulation mirrors the pattern of atmospheric circulation on Earth. Where the winds blow in a particular way, the currents tend to follow.
In the middle latitudes, the air circulation over the oceans is mainly anticyclonic — that is, it rotates around a high-pressure centre. This pattern is much more pronounced in the southern hemisphere than in the northern hemisphere, and the ocean currents in these latitudes follow the same anticyclonic rotation. At higher latitudes, where the wind flow is mostly cyclonic (rotating around a low-pressure centre), the oceanic circulation again matches that pattern. In regions with strong monsoonal flow, the monsoon winds directly influence how the currents move.
The Coriolis force also shapes the direction of currents. Warm currents originating in low latitudes are deflected to the right in the northern hemisphere and to the left in the southern hemisphere. This deflection is why the major warm currents do not flow straight north or south but curve as they travel.
The ocean circulation plays a vital role in transporting heat across latitude belts, much like the general circulation of the atmosphere does. Cold waters from the Arctic and Antarctic circles move toward the warmer tropical and equatorial regions, while warm waters from the lower latitudes move poleward. This exchange of warm and cold water is what keeps the planet's heat budget balanced.
Characteristics of Ocean Currents
Currents are referred to by their "drift," which is the term used for the speed of a current. Drift is measured in knots. The strength of a current simply means its speed — a fast current is considered a strong one.
- Currents are strongest near the surface and may attain speeds over five knots.
- At depth, currents are generally slow, with speeds less than 0.5 knots. …
Major ocean currents are greatly influenced by the stresses exerted by the prevailing winds and coriolis force. The oceanic circulation pattern roughly corresponds to the earth's atmospheric circulation pattern. The air circulation over the oceans in the middle latitudes is mainly anticyclonic (more pronounced in the southern hemisphere than in the northern hemisphere). The oceanic circulation pattern also corresponds with the same. At higher latitudes, where the wind flow is mostly cyclonic, the oceanic circulation follows this pattern. In regions of pronounced monsoonal flow, the monsoon winds influence the current movements. Due to the coriolis force, the warm currents from low latitudes tend to move to the right in the northern hemisphere and to their left in the southern hemisphere. …