OREGON STATE UNIVERSITY

Scientists unlock keys to global ocean circulation

06/14/2011

CORVALLIS, Ore. – Though the United Kingdom and the Aleutian Islands are at the same latitude, they have vastly different climates – due largely to the difference in salinity between the northern Atlantic and Pacific Oceans, and the system of currents those oceans produce.

Now researchers may have solved the mystery of why the Atlantic is saltier than the Pacific; the cause appears to be global mountain ranges and the Antarctic ice sheet.

When the cold, salty surface water of the North Atlantic Ocean sinks and begins its long journey toward Antarctica, it triggers a complex pattern of global ocean currents that brings enough warmer water back along the European shoreline to keep most of that continent’s climate temperate. The northern Pacific Ocean doesn’t have that same mechanism because its salinity is much lower, and scientists have long speculated as to why.

The new study pinpointing the role of mountains and ice sheets was published by researchers at Oregon State University and the University of Hamburg. Funded by the National Science Foundation’s Paleoclimate Program, it was just published online in the Journal of Climate.

The Rocky Mountains of North America and the Andes of South America block water vapor transport from the Pacific Ocean to the Atlantic, according to Andreas Schmittner, an Oregon State oceanographer and lead author on the study. Most of the water that evaporates from the Pacific is blocked by those mountains and falls as rain or snow, eventually returning to the Pacific Ocean and keeping it fresher.

“Without those mountains, much of the precipitation would fall in the middle of the continents and drain into the Atlantic instead of the Pacific,” said Schmittner, an associate professor in the College of Oceanic and Atmospheric Sciences at Oregon State.

Water vapor from the tropical Atlantic and Caribbean Sea, on the other hand, comes across Central America via tradewinds and dumps into the Pacific – creating the salinity disparity. The amount of fresh water this mechanism creates is significant, Schmittner said, about 200,000 cubic meters per second.

“That is roughly equivalent to the output of the Amazon River flowing into the Pacific,” he pointed out.

The mountains of East Africa keep water transport originating in the Indian Ocean from reaching the Atlantic.

Meanwhile, the massive Antarctic ice sheet also plays a major role, the researchers report in their study. This ice sheet intensifies the winds and shifts the Antarctic Circumpolar Current to the south. Without the sheet, the temperature contrasts between the land mass and the atmosphere at lower latitudes would lessen, decreasing winds, Schmittner said.

“Those winds push the Circumpolar Current, which is the most powerful ocean current in the world, to the south,” he said. “If the ice sheet disappeared and was replaced by air, the current would be pushed northward and block the flow of salty water from the Indian Ocean, around the tip of South Africa, into the Atlantic.”

Climate model simulations by the researchers found that removing the mountain ranges creates a fresh North Atlantic and a salty North Pacific.