DETOURS on the Oceanic Highway

By Le Bras, Isabela | Oceanus, Winter 2016 | Go to article overview

DETOURS on the Oceanic Highway


Le Bras, Isabela, Oceanus


Within the ocean there's a global highway system of currents. It transports heat across the planet, from the equator to the poles. If not for these currents, the equatorial ocean would be a scalding hot tub and the polar regions would freeze solid.

Imagine the Earth as a poorly insulated apartment-a railroad flat with three rooms. The kitchen in the middle is heated by a woodstove, but the two unheated outlying bedrooms can get cold. Earth's kitchen is the equatorial region, heated by sun; the far-flung poles are its chilly bedrooms.

But open the doors between rooms in the apartment, and warm air moves out from the middle room and heats up the bedrooms a little, while colder air circulates in and cools the kitchen. Our ocean works essentially the same way, but rather than air, great ocean currents circulate excess equatorial heat to the poles.

In the North Atlantic, for example, the Gulf Stream carries water warmed at the equator toward the North Pole. When the warm water meets frigid winter air in northern latitudes, it cools vigorously. Water becomes denser as it cools, and consequently sinks thousands of meters below the sea surface. This deep mass of cold water drifts southward in a current that hugs North America's continental slope on the western side of the Atlantic Ocean basin; the Deep Western Boundary Current.

These currents are major cogs in Earth's heating and ventilation system that regulate our planet's climate. To understand how our climate works and to unravel how it will change, we need to reveal the inner workings of these currents.

A mathematical ocean

As recently as 50 years ago, the Deep Western Boundary Current remained undiscovered. Oceanographers knew that water must be sinking at high latitudes, but they did not have the means to observe where it went. In the late 1950s, two scientists at Woods Hole Oceanographic Institution (WHOI), Henry Stommel and Arnold Arons, used mathematics to explore deep-sea circulation. Reducing the problem to its bare essentials, they conceived of an idealized ocean that looked more like a bathtub. It had straight walls, a flat bottom, and water sinking at the ends of the tub, which represented the real-world polar oceans.

In this theoretical model, they calculated the essential physical forces acting on the flow of water in bounded basins on a rotating sphere. Their model pointed to the existence of the Deep Western Boundary Current. A few years later, oceanographers John Swallow in England and Val Worthington at WHOI placed floats in the ocean. The floats were swept equatorward by the Deep Western Boundary Current, confirming what Stommel and Arons had predicted by mathematical equations alone.

Physical oceanographers have continually used this combination of observations and models to push forward our understanding of ocean dynamics. Models are mathematical approximations of reality that can distill the essence of the physics at work and help us explore how the world operates. Observations confirm the reality and unveil new complexities that require further explanation.

Tollbooths on the ocean highway

One important set of observations has come from Line W, where scientists at WHOI have been measuring the strength of the Deep Western Boundary Current in the North Atlantic since 2004. Line W consists of a series of six moorings that starts at the continental slope off Cape Cod, Mass., and heads southeast. Moorings are made of cables as tall as ten Empire State Buildings, which are anchored to the ocean floor and held upright by buoyant glass spheres. Instruments called moored profilers ride up and down along the cables, measuring the temperature, salinity, depth, and velocity of the water as it flows by.

The Line W moorings are like oceanic tollbooths. But instead of collecting a fee, they are measuring the strength and properties of the Deep Western Boundary Current as it flows past.

Line W is a way to observe and document changes in the current, which may be linked to changes in global heat transport. …

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