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Lecture

What is an ocean current?

The term current describes a mass of ocean water that moves from one place to another. You can think of these as “rivers” of water within the ocean. As we will see, currents vary in their characteristics – some travel along the surface, others travel vertically through the ocean; some are small, others are large; some are made of cold water, some are made of warm water; some are narrow and fast, others are broad and slow. Through the next few sections, we will find that these characteristics are not random and specific processes determine them.From your reading, please be able to describe the fundamental difference between wind-driven and density-driven currents. Think about things such as (1) driving forces or processes that move water, (2) amount of water moved, (3) direction of water movement, (4) specfic examples of each.

In this module, we will investigate the two main types of currents – wind-driven currents and density-driven currents. Wind-driven currents are characterized by horizontal movement of water in the upper reaches of the water column of all of the Earth’s ocean basins. Collectively, we term wind-driven currents as surface circulation; the major driving force of surface currents is the major wind belts you were introduced to in the “Atmospheric Circulation” module. Density-driven currents are characterized by vertical movement of water throughout the water column (from surface to deep or vice versa). Collectively, we term density-driven currents as thermohaline circulation; the major driving force of thermohaline circulation is differences in temperature (“thermo”) and salinity (“haline”) characteristics of water masses.

How do we know about large scale movements in the ocean basins?

Before we can talk about surface and thermohaline circulation in detail, it is important that we understand how scientists determine the characteristics of water motion. Generally speaking, there are two groups of methods used by oceanographers to measure water movement – direct measurement and indirect measurement.

Direct measurement includes all methods used to measure the movement of water directly within the water. This is similar to putting your hand under a running faucet – you are directly sensing the motion of the water. Your book describes some tools used for both direct and indirect measurement of water movement. Please visit the following links for specific examples of how scientists are measuring currents use a variety of technology from drifters to satellites, and in some unconventional ways as well!
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