Warming in Deep Southern Ocean Linked to Sea-Level Rise

Warming waters in the deepest parts of the ocean surrounding Antarctica have contributed to sea-level rise over the past two decades, scientists report today (Sept. 20).

In an attempt to pinpoint all culprits for the rising oceans, scientists analzyed warming trends in the abyssal ocean — below about 3,300 feet (1,000 meters), said study team member and oceanographer Sarah Purkey of the University of Washington in Seattle.

The scientists found that the strongest deep warming occurred in the water around Antarctica, and the warming lessens as it spreads around the globe. The temperature increases are small — about 0.05 degrees Fahrenheit (0.03 degrees Celsius) per decade in the deep Southern Ocean, and less elsewhere. But the large volume of the ocean over which they are found and the high capacity of water to absorb heat means that this warming accounts for a huge amount of energy storage.

If this deep ocean heating were going into the atmosphere instead — a physical impossibility — it would be warming at a rate of just over 5 degrees F (3 degrees C) per decade.

This amount of energy would be the equivalent of giving every person on Earth five 1,400-Watt hair dryers, and running them constantly during the 20-year study period, said study team member and oceanographer Gregory Johnson of the National Oceanic and Atmospheric Administration (NOAA).

Sea level has been rising at around one-eighth of an inch (3 millimeters) per year on average since 1993, with about half of that caused by the ocean expanding as it's heated, and the other half due to additional water added to the ocean, mostly from melting continental ice.

The oceanographers note that deep warming of the Southern Ocean accounts for about one-twentieth of an inch (1.2 mm) per year of the sea-level rise around Antarctica in the past two decades.

The authors note that there are several possible causes for this deep warming: a shift in Southern Ocean winds; a change in the density of what is called Antarctic Bottom Water (which would change how much gets mixed with surface waters); or how quickly that bottom water is formed near the Antarctic, where it sinks to fill the deepest, coldest portions of the ocean around much of the globe.

The study draws on temperature trends between the 1990s and 2000s in the deep Southern Ocean. Though there are no continental boundaries there, and all oceans contribute water to the Southern Ocean, its distinct circulation makes the area a separate water body. This study shows that the deep ocean is taking up about 16 percent of the energy that the upper ocean is absorbing.

To study how much heat was building up in the Southern Oceans, the researchers divided the ocean into 28 sections, or basins, for which they computed warming rates. As a part of a large international research mission, researchers sailed across the ocean, stopping every 30 miles (48 kilometers) to lower their instruments into the deep ocean to take measurements for four hours.

"It was like crossing the ocean at a jogging pace," Johnson said.

The three southernmost basins showed a strong statistically significant abyssal warming trend, with that warming signal weakening to the north in the central Pacific, western Atlantic and eastern Indian Oceans, the researchers said. Eastern Atlantic and western Indian Ocean basins showed slight cooling trends, though the results weren't statistically significant, meaning they could be due to measurement or other errors.

The study is detailed in a recent online edition of the Journal of Climate.

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