Beginner 9 minutes
Ocean currents: the invisible rivers of the sea
Wind, density, and tide moving water, heat, nutrients, and larvae across the whole planet
The ocean isn't a still body of water with waves on top. It's a system of riverbank-less rivers that distributes heat from the tropics to the poles, brings nutrients up from the bottom, carries larvae thousands of kilometers, and, in the process, decides which are the best dive sites on the planet.
In this article you'll see what forces move seawater, what types of currents exist (surface, deep, tidal, rip, and upwelling) what they distribute around the planet, and what each one means for someone who gets in the water.
Current is the variable that transforms a dive the most, and the one that best explains where life concentrates. This article covers where it comes from; the technique for diving in it is in the article on currents and drift diving.
What moves the water
Three distinct engines, and one force that deflects all of them.
The first engine is wind, which drags the surface layer along through friction and generates the large currents drawn on maps. It only affects the upper part of the water column, but that's where almost everything we see happens.
The second is density. Cold water is denser than warm water, and salty water denser than fresh water; when both conditions occur together, the water sinks and pulls behind it a slow, deep circulation that travels the entire planet.
The third is tide, which pushes enormous volumes of water into and out of coastal basins twice a day, driven by the astronomical mechanism covered in its own article.
Acting on all three is Earth's rotation, which deflects everything that moves: to the right in the northern hemisphere and to the left in the southern, as covered in the article on the atmosphere and the sea. That's why currents don't travel in straight lines and why large systems rotate.
Put together, these elements produce a layered system: a fast surface circulation driven by wind and a slow deep circulation driven by density, connected to each other in only a few places on the planet.
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