Ocean Currents - Physical Geography, UPSC, IAS. | Geography (Prelims) by Valor Academy PDF Download

Ocean currents

OCEANS CURRENTS

• when winds blow over oceans they set surface water in motion, driving the large scale surface currents in nearly constant patterns (differs from deep-sea circulation that is a result of changes in water's density)
• surface water is defined as water between the surface and thermocline in the deep ocean (~300-1000 m depth)
  • since the density of water is greater than that of air, once in motion, the mass of moving water is so great that its inertial force keeps it flowing
  • they are deflected by the Coriolis force in same way that moving air is deflected, but because water moves more slowly than air, time required for water to move same distance is much longer and earth will have rotated farther out from under the water than from the wind; thus the deflections will be greater (45° greater than the angle of the wind)
    • the wind's energy gets transmitted throughout the water column and sets each successive layer of water into motion; however each layer receives less and less wind energy so each has less and less velocity (note the arrows, which are vectors, decrease in size reflecting this velocity loss)
    • nonetheless, each layer is deflected with regard to the one immediately above it and this results in a spiral of current directions called the Ekman spiral
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• NOTE: in contrast to winds, a current is named for the direction in which it travels, not from which it comes
• each ocean gyre is made up of 3 separate currents:
  1. western boundary currents -- have a sharp boundary with coastal circulation; little or no coastal upwelling; waters are depleted in nutrients; unproductive; derived from trade winds.
  2. eastern boundary currents -- diffuse boundaries; coastal upwelling; waters derived from mid-latitudes; flow toward the equator; weak currents
  3. equatorial currents -- complex at edges of ocean basins; seasonally variable and move northward in the summer

• North Pacific Currents: less affected by continents here than in the Atlantic because the Pacific is larger
  • in North Pacific, northeast trade winds push water toward the west and northwest; this is the North Equatorial Current (flows west towards the Philippines and north to ~12°N)
  • westerlies create the Pacific Current or North Pacific Drift, moving west to east (moves water away from Asia and pushes it towards west coast of North America)
  • because water that accumulates in one area must flow toward the area from which it has been moved, two other currents are formed: California Current (moving north to south) and Kuroshio Current (moving from south to north) along the east coast of Japan
  • the Equatorial, North Pacific Drift, California and Kuroshio currents complete a circular, clockwise motion centered around 30° N latitude -- the North Pacific Gyre
  • other Pacific currents include the Oyashio Current, driven by polar easteries, the Alaska Current fed by the North Pacific Current and moving in a counterclockwise gyre
• South Pacific Currents:
  • the southeast trade winds move water to left of the wind and westward, forming the South Equatorial Current
  • the westeries push water to the east and produce the West Wind Drift which moves water almost continuously around the earth, but is partially deflected by the southern tip of South America and Africa.
• continuity currents (currents replacing water that has been pushed away) form between the South Equatorial Current and the West Wind Drift: these are the Peru (orHumbolt) Current, moving water from south to north along the coast of South America, and the East Australian Current, moving from north to south on the west side of the ocean
• these four currents produce the counterclockwise South Pacific gyre
• the North and South Pacific gyres are formed not on either side of the equator, but on either side of 5°N, because the doldrum belt is displaced northward due to the unequal heating of the Northern and Southern Hemispheres; there is a current moving in the opposite direction, from west to east, known as the Equatorial Countercurrent, that helps to return accumulated surface water eastward across the Pacific
• North Atlantic Currents:
  • the North Atlantic westerly winds move water eastward in the North Atlantic Current or North Atlantic Drift
  • the northeast trade winds push water to the west, forming the North Equatorial Current
  • the north-south continuity currents are the Gulf Stream, flowing northward along the coast of North America, and the Canary Current, moving to the south on the eastern side of the North Atlantic (the Gulf Stream is fed by the Florida Current and the North Equatorial Current)
  • these four currents produce the North Atlantic Gyre, which rotates clockwise
  • the polar easteries provide the driving force the Labrador and East Greenland currents that balance water flow into the Arctic Ocean from the Norwegian Current
• South Atlantic Currents:
  • the westeries continue the West Wind Drift
  • the southeast trade winds move water to the east and forms the South Equatorial Current, which is deflected somewhat by the bulge of Brazil and flows into the Caribbean Sea, the Gulf of Mexico, as well as forming the Florida Current
  • a portion of the South Equatorial Current is deflected south to become the Brazil Current
  • the Benguela Current moves northward up the African coast
  • these four currents form the South Atlantic Gyre which rotates counterclockwise
• Indian Ocean Currents:
  • mainly a Southern Hemisphere ocean, so the southeast trade winds push water to the west, creating the South Equatorial Current
  • westeries move the water eastward in the West Wind Drift
  • the counterclockwise gyre is completed by the West Australia Current, moving northward, and the Agulhas Current moving southward along the coast of Africa
  • northeast trade winds drive the North Equatorial Current to the west and the Equatorial Countercurrent returns water eastward toward Australia