Ocean Currents. Forces that Drive Currents. Two Kinds of Forces : Primary forces : start water moving and determine it’s velocity Ex: wind, gravity & temperature Secondary forces : influence the direction and nature of the flow. Ex: Coriolis, friction, shape of ocean basin.
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Forces that Drive Currents • Two Kinds of Forces: • Primary forces: start water moving and determine it’s velocity • Ex: wind, gravity & temperature • Secondary forces: influence the direction and nature of the flow. • Ex: Coriolis, friction, shape of ocean basin
Effect of secondary forces on surface currents like Coriolis, Gravity and Friction.
Ocean Currents: • Curve to right due to Coriolis • Deflect due to the shape of the ocean basin.
What is a Gyre? • Defined as the flow around the periphery (edge) of an ocean basin. • Redistribute heat in ocean: • Warm water from equator to poles; cold water from poles to equator • Gyre: • N. Hemi they rotate clockwise • S. Hemi they rotate c. clockwise
Equatorial Currents Equatorial currents are found in every ocean basin. They form the low-latitude leg of the subtropical gyres and are driven to the west by trade winds.
Equatorial Counter Currents Along the equator, between the northern and southern trades, winds become very light. Because there is no wind, the sea surface slope drives some water back to the east, resulting in an equatorial countercurrent
West Wind Drift In the S. hemisphere, no significant land mass lies between Antarctica and 40° S latitude. Consequently, persistent westerly winds from 30°S to 60°S drive an ocean current eastward all the way around Antarctica. Prevents warm water from reaching Antarctica.
Coastal Upwelling • Upwelling supports highly productive ecosystems. • During the summer, winds blow from the north to the south along the California coast. • Caused by wind blowing parallel to the shoreline. • Upwelling occurs when: • Coriolis deflects water to right (N. Hemi) • Friction further deflects water to right • Surface water is replaced by deeper water rising along shore. • Upwelling brings nutrient-rich (fertilizer) water to surface and also influences weather. 90 from wind direction
Coastal Upwelling along the California coastline. • Colors represent biological productivity: • Red is high productivity • Blue is low or no productivity • Most fisheries are located in areas of upwelling.
Thermohaline Circulation Density is the Primary force for these vertical currents: Cold, salty water sinks (more dense) Warm, less salty water floats (less dense) Drives the mixing of the surface water with the deep water. Crucial in moving heat & nutrients around the globe. Very dense water accumulates at surface when sea ice forms near the poles during winter.
Density Structure of Ocean Mixed Layer • Thermocline – a layer in the ocean where temperature decreases rapidly with depth. • Ocean is layered by density. • Thermoclines are absent near the poles because temperature is = from surface to depth. Deep Layer