New study eyes link to Southern Hemisphere
A slowdown of a key heat-carrying Atlantic current may not be due to melting Arctic ice, but to changes in the Southern Hemisphere, according to University of Washington scientists studying how climate change may affect global wind and ocean flows.
Their new study, published in the journal Geophysical Research Letters, analyzed data from satellites and ocean sensors off Miami that have tracked the Atlantic overturning circulation for more than a decade. Together they show a definite slowdown since 2004, confirming a trend suspected before then from spottier data.
Several recent studies have warned that a large influx of cold, fresh water from the melting Arctic might be impeding the current, which moves warmer water north along the ocean’s surface and sends cold water southward at depths. But the latest research found that changes in the Southern Hemisphere may be a bigger factor.
“It appears that this 10-year slowdown is not related to salinity,” said Kathryn Kelly, an oceanographer at the UW’s Applied Physics Laboratory. In fact, despite more ice melt, surface water in the Arctic is getting saltier and therefore denser, she said, because of less precipitation. “That means the slowdown could not possibly be due to salinity — it’s just backwards. The North Atlantic has actually been getting saltier.”
Kelly said the research found a connection between the slowdown and the Agulhas Current, which carries warm Indian Ocean water south along the east African coast around the continent’s tip toward the Atlantic, but then makes a sharp turn back to join the stormy southern circumpolar current.
Warm water that escapes into the Atlantic around the cape of South Africa is known as the Agulhas Leakage. The new research shows the amount of leakage changes with the quantity of heat transported northward by the overturning circulation.
“We’ve found that the two are connected, but I don’t think we’ve found that one causes the other,” Kelly said. “It’s more likely that whatever changed the Agulhas changed the whole system.”
She believes atmospheric changes may be affecting both currents simultaneously.
“Most people have thought this current should be driven by a salinity change, but maybe it’s the [Southern Ocean] winds,” Kelly said.
A slowdown of the Atlantic overturning current would decrease the amount of warm water being transported to the East Coast and Western Europe, but those effects are likely to be overwhelmed by the overall warming due to global climate change, the researchers said.
“So that whole concept in the movie of New York harbor freezing doesn’t make any sense,” Kelly said, referring to the climate disaster movie The Day After Tomorrow. “If the Gulf Stream doesn’t carry as much heat from the tropics, it just means that the North Atlantic is not going to warm up as fast as the rest of the ocean — it’s not going to cool down.”