Reference:
How do ocean currents work?
Jennifer Verduin, TED-Ed, 2019

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00:03:33 In 1992, a cargo ship carrying bath toys got caught in a storm. Shipping containers washed overboard, and the waves swept 28,000 rubber ducks.So what do you think happened after?

00:03:58 So let’s think about driving forces of ocean currents. Ocean currents are driven by many forces. What forces do ocean currents drive?
00:04:53 Ocean currents fall into two main categories: surface currents and deep ocean currents.

00:05:46 Here, let’s make an equation for the momentum between the atmosphere and ocean surfaces. Now, you have the surface wind speed of 10 m/s, the atmospheric density, seawater density, duration, and momentum exchange coefficient that depends on the surface roughness.
The relation of each momentum would be like this, and you can estimate ocean surface current speed

00:06:13 If you zoom out to look at the patterns of surface currents all over the earth, you’ll see that they form big loops called gyres, which travel clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere. There are five major gyres of the Indian Ocean Gyre, North Pacific Gyre, South Pacific Gyre, North Atlantic Gyre, and South Atlantic Gyre.
00:07:50 The Coriolis force is an apparent force that changes the direction of flows and it increases with the higher latitude.
00:08:13 Unlike surface currents, deep ocean currents are driven primarily by changes in the density of seawater.

00:08:51 Thermohaline circulation of deep water and wind-driven surface currents combine to form a winding loop called the Global Conveyor Belt. As water moves from the depths of the ocean to the surface.

00:09:25 That’s why we need to observe their actual status. Then, how can we observe the ocean?

00:12:17 In this way, we did observational campaigns. Let me introduce some examples from the Pre-YMC in 2015 and YMC in 2017.

00:13:23 Looking at time-depth sections of the temperature in 2015 and 2017, they are entirely different. Please look at the mixed layer by the dashed line. The mixed layer depth in 2015 is very shallow less than 20 m but that in 2017 is more than 100 m.

00:15:10 these different atmospheric features and also reversely feed back to the atmosphere.

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