The highly salty Southern Indian Ocean is getting fresher due to global warming: Study

The Southern Indian Ocean off the southwest coast of Australia is one of the saltiest parts of the global ocean. However, climate change is altering the very nature of this section, freshening it, with implications for ocean biodiversity, according to a new study by Colorado University Boulder.

The area of salty seawater has decreased by 30 per cent over the past six decades, representing the most rapid increase in fresh water observed anywhere in the Southern Hemisphere, according to the study by Weiqing Han, professor in the Department of Atmospheric and Oceanic Sciences in the College of Arts and Sciences at CU Boulder.

“This freshening is equivalent to adding about 60 per cent of Lake Tahoe’s worth of freshwater to the region every year,” a statement by the university quoted first author Gengxin Chen as saying.

“To put that into perspective, the amount of freshwater flowing into this ocean area is enough to supply the entire U.S. population with drinking water for more than 380 years,” added the visiting scholar in the Department of Atmospheric and Oceanic Sciences and senior scientist at the Chinese Academy of Sciences’ South China Sea Institute of Oceanology.

The area in question

The Southern Indian Ocean off southwest Australia is known as the Indo-Pacific freshwater pool. It spreads out across an expansive region stretching from the eastern Indian Ocean into the western Pacific Ocean in the Northern Hemisphere tropics. The region plays a key role in global ocean circulation.

‘Thermohaline circulation’ is a giant “conveyor belt” of ocean circulation that redistributes heat, salt and freshwater around the planet. It channels warm, fresh surface waters from the Indo-Pacific flow toward the Atlantic Ocean, contributing to the mild climate in western Europe.

“In the Northern Atlantic Ocean, the water cools, becomes saltier and denser, and eventually sinks before flowing southward in the deep ocean back to the Indian and Pacific Oceans,” noted the statement.

The area off southwest Australia is typically dry and evaporation largely exceeds precipitation there. As a result, the seawater in the region has historically been salty.

Nature of change

So why is this hitherto salty area now freshening up? The answer is climate change.

According to the study, the freshening is not a result of local precipitation changes. The team used a combination of observations and computer simulations and found that global warming is altering surface winds over the Indian and tropical Pacific Oceans. These wind shifts are pushing ocean currents to channel more water from the Indo-Pacific freshwater pool to the Southern Indian Ocean.

“As seawater becomes less salty, its density decreases. Because fresher water usually sits on top of saltier, denser water, the surface water and deep ocean water become more separated into layers. These stronger contrasts in salinity between layers reduce vertical mixing, an important process that normally allows surface waters to sink and deeper waters to rise, redistributing nutrients and heat throughout the ocean,” the statement reasoned.

The implications are grim. For one, it could transport fresher water into the Atlantic.

Even more importantly, reduced mixing could impact marine ecosystems. “When nutrients from deeper waters fail to reach the sunlit surface, organisms living in shallow waters have less food. Weaker mixing also prevents excess heat in the surface waters from dissipating into deeper layers, making shallow waters even hotter for organisms already under stress from rising temperatures,” according to the statement.

The study has been published on February 3 in Nature Climate Change.