Currents: water masses in motion Both the atmosphere and the oceans transport heat from low latitudes near the equator to high latitudes near the poles. Cool air and water currents make the return trip from high latitudes back towards the equator. For the atmosphere, this circulation that globally redistributes heat is accomplished by winds; for the ocean, it is accomplished by currents. In the most general sense, a current is a region of water that moves
more rapidly than its surroundings. There are surface ocean currents and deep ocean currents. We will learn about both here. In this lesson we will study these currents and will learn about the forces responsible for their movement.
Ben Franklin's map of the Gulf Stream, circa 1786 AVHRR imagery of the Gulf Stream, 01 January 1985 (click on the image to learn more) Currents are produced by physical forces Ocean surface currents are caused by:
Deep ocean currents (also known as Thermohaline Circulation) are caused by:
In general, wind-driven currents dominate the upper ocean, and thermohaline circulation drives the movement of the deep ocean. How ocean currents are identified in satellite imagery Ocean surface currents that are warmer than the surrounding water they may have a thermal signature, that can be seen in (Advanced Very High Resolution Radiometer (AVHRR) imagery. Depending on their thermal contrast (the temperature difference between the two water masses) ocean surface currents can have a strong or weak thermal signature. Ocean surface currents with a sufficiently strong thermal contrast, can be readily detected in sea surface temperature (SST) measurements, e.g. from AVHRR or Special Sensor Microwave/Imagery (SSM/I). Depending on wind conditions, warm currents can also be distinguished in Synthetic Aperture Radar( SAR) imagery, but this is a more subtle effect. Altimeters obtain extremely precise measurements of the distance from the satellite’s position in orbit to the earth’s surface. These precise distance measurements enable us to estimate sea surface height, which can be used to identify regions of warm and cold water (since warm water expands, it ‘sits’ slightly higher than cold water), as well as to locate topographic features. The image below shows data acquired by the TOPEX/Poseidon satellite. This image uses color to show inferred ocean topography with arrows indicating the speed and direction of ocean currents. Notice how the currents move clockwise around higher regions in the ocean in the northern hemisphere. This is due to the Coriolis effect.
Satellite image showing global ocean currents (click on the image to learn more)
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