Magazine article Oceanus

The Current That Feeds the Galapagos: GLIDERS REVEAL THE PATH OF A MAMMOTH 'RIVER' IN THE PACIFIC OCEAN

Magazine article Oceanus

The Current That Feeds the Galapagos: GLIDERS REVEAL THE PATH OF A MAMMOTH 'RIVER' IN THE PACIFIC OCEAN

Article excerpt

Just below the surface of the vast Pacific Ocean, a powerful current streams rapidly along the equator from west to east. Thousands of miles after this oceanic superhighway begins its journey near Indonesia, it slams squarely into a tight-knit group of rocky spires that jut up from the seafloor and pierce the water's surface off the coast of Ecuador.

In a rush, the current finds a new path around the rocky islands. As the water changes speed and direction, it balances the spinning of the Earth and the rough touch of the wind. Much of the water diverges rapidly 50 or so miles to the north or the south of the islands, continuing its journey eastward. The remaining water is forced upward by the obstacles and Hows up toward the sunny, warm, glittery surface. As the water upwells, it carries nutrients from the deeper ocean up with it.

Those nutrients sustain tiny plankton at the base of the food web of a unique ecosystem. The jutting spires in the current's way are known as the Galapagos Islands--home to giant tortoises, marine iguanas, finches famously studied by Darwin, and other species that are not found anywhere else on the planet. Scientists have long studied the animals that live on the Galapagos Islands. But they haven't been able to examine the current that supplies the essential nutrients that nourish life as the water reaches the Galapagos.

This immense river within the ocean is called the Pacific Equatorial Undercurrent, or EUC, and it's one of the most vigorous and focused currents in the world's ocean. It flows across the Pacific along the equator, extending about 100 miles wide and the length of a football field deep.

In 2009, Kris Karnauskas, a former scientist at Woods Hole Oceanographic Institution, began knocking on the doors of his WHOI colleagues in search of ways to directly observe the path of the EUC near the Galapagos. Karnauskas uses computer models to simulate the ocean. His models showed that the EUC' upwells nutrient-rich water when it reached the Galapagos Islands and that the islands play an important role in determining the path of the EUC, but he needed evidence from the real ocean.

It isn't an easy task to chase down how the EUC forks, races, or meanders its way around the Galapagos Islands, or to capture how it changes over days, seasons, and years as it encounters the islands. How could it be accomplished? What tools could best reveal the details of what is going on beneath the surface and beyond the purview of satellites?

Karnaukas found Breck Owens, a WHOI physical oceanographer with long experience making observations at sea. Owens had a solution--a set of tools he had used before in other regions of the ocean: Spray gliders. These untethered ocean-bound robots can dive below the ocean surface to measure seawater properties that scientists use to paint a picture of subsurface current systems.

Karnauskas and Owens teamed up with Daniel Rudnick, a scientist at Scripps Institution of Oceanography who also had experience with gliders. Their plan was to use the distinctive capabilities of Spray gliders to understand what was happening to the EUC as it approached the Galapagos Islands.

The glider fleet

Gliders descend below the surface, sinking to 1,000 meters (3,280 feet) deep before ascending six hours later through the watet, measuring salinity, temperature, and current velocities as they go. Returning to the surface, the gliders relay their data via satellite to scientists.

The gliders are 6.5 feet long, with two wings extending from their cylindrical hulls. Without propellers or motors, they move by changing their buoyancy. Each glider has a pump that moves oil from a bladder outside its hull to one inside the hull, and vice versa. Moving oil into the internal bladder increases the glider's density until it's greater than that of the seawater around it, and the glider descends. Moving oil out of the hull and into the external bladder allows it to ascend. …

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