The surface waters of Earth’s oceans are relentlessly driven by winds. And there’s also plenty of action below the ocean surface. When polar winds chill the salty surface waters near Antarctica or Greenland, the water gets heavy and sinks. That cold water then begins a journey around the world that lasts a thousand years.
Cold currents creep along deep in the Atlantic and later loop through the Indian, Pacific and southern oceans — where they mix with warmer water — and get lighter and closer to the surface. Eventually the water circles back to its starting point as a warm, salty mass at the sea surface again, ready to repeat the cycle.
This is called thermohaline circulation — and it also pushes warm water around the ocean surface. According to Arnold Gordon of Columbia University, this watery loop can have an impact on the world’s climate.
Arnold Gordon: And the form of the thermohaline circulation and the vigor at which it moves, in many ways determines the nature of the climate. And some people for instance think that the shutdown of the thermohaline circulation that’s driven from the North Atlantic had a lot to do with the glacial-interglacial periods . . .
Oceanographers have two primary methods for tracking ocean currents
The simplest and most obvious is to use an instrument that measures how fast the water is flowing. These instruments can be mounted on buoys or used from ships. Oceanographers can also release drifters and track their motion as the currents push them around. But the oceans are huge. Buoys and ship pathways can only give us a hint of the many motions of the world’s ocean. A more comprehensive method — but one that only tells us about _surface_ currents is the TOPEX/POSEIDON satellite mission, and its successor JASON. These satellites map the surface topography of the ocean — the ups and downs of the ocean surface. Water surfaces do have topography — think of pond ripples, or the way the water’s surface in a bucket drops in the middle when you stir it with a stick. Wind patterns around the planet set up long-lasting features in ocean topography, piling up water in one place and moving it out of another. Oceanographers are able to convert these images of ocean topography into surface currents.
But wind doesn’t necessarily control the water layer topography below the surface
To get an idea of below-surface topography, oceanographers need to be able to compute the pressure profiles as you go down into the ocean — which means they need information about temperature, depth below the surface, and salinity. Deeper topography doesn’t necessarily have to look like the surface — and may not be affected by the wind. Imagine someone took a big dictionary and an sharp-edge and carved a hiding place into the middle pages between F to H. A tiny ant crawling along the title page of the dictionary would see only a flat surface with no topography. But if you crawl along the first page of the G section, you’d eventually come to a point where you could either climb down to the H pages or up to the F pages. Topography would definitely be an issue. The ocean isn’t full of huge bubbles of empty space — but it does have odd water masses moving through it. These water masses stand out from their surroundings because they have slightly different temperatures or saltiness. They can create bulges in layers, which then alters the topography of nearby layers.
How do oceanographers convert topography into currents?
Here’s a really short explanation — if you’d like more, see the links below. First, let’s pretend the Earth doesn’t rotate. If the winds managed to pile a big mound of water in the middle of the North Pacific, you could imagine that water at the top of the mound would try to flow down toward the lower elevations– in other words, water would be flowing out from the top of the bump in an exploding star sort of pattern. But because the Earth rotates, the water flowing down the mound gets pulled to the right (that’s due to the Coriolis Force) and the water ends up flowing AROUND the mound at the same elevation, instead of down to lower elevations. This means that if scientists know the shape of the topography– the mounds — they can get a sense for the shape of these big loops of current. The scientists get the relative speed from the slope of the topography — if the topography is really steep, the water flows faster. But flat regions of the ocean don’t move much. In order to get the absolute speed of the currents, the scientists need additional pieces of information — like ship observations of current, or how the earth’s uneven gravitational field is distorting the topography observed from the satellites. These complications mean there’s always uncertainty in knowing where the currents are and how fast they are moving.