Melting ice sheets are slowing the Antarctic Circumpolar Current – the world’s strongest ocean current – with implications, say researchers, for global sea level rises, ocean warming, and viability of healthy marine ecosystems.

A new collaborative study between researchers from the University of Melbourne and NORCE Norway Research Centre suggest that in what they call a ‘high carbon emissions scenario’, the current could slow by as much as 20% by the year 2050.

The influx of fresh water into the Southern Ocean – delivered by melting ice sheets – is expected to change the properties of the ocean, including its density, as well as its circulation.

The study – carried out by University of Melbourne researchers, fluid mechanist, Associate Professor Bishakhdatta Gayen and climate scientist Dr Taimoor Sohail, alongside oceanographer Dr Andreas Klocker from the NORCE Norwegian Research Centre – has analysed a high-resolution ocean and sea ice simulation of ocean currents, heat transport, and other factors.

The aim of the study was to investigate and diagnose the impact of changing temperature, saltiness, and wind conditions.

Associate Professor Gayen said: “The ocean is extremely complex and finely balanced. If this current ‘engine’ breaks down, there could be severe consequences. These could include more climate variability, with greater extremes in certain regions, and accelerated global warming due to a reduction in the ocean’s capacity to act as a carbon sink.”

The Antarctic Circumpolar Current (ACC) works as a barrier to invasive species from other continents – those such as the rafts of southern bull kelp that ride the ocean currents, or marine-borne animals like shrimp or molluscs – reaching Antarctica.

As the ACC slows and weakens, there is a higher likelihood such species will make their way onto the fragile Antarctic continent. This, researchers state, comes with the potential for severe impacts on the food web, which may, for one example, change the available diet of Antarctic penguins.

The ACC is a crucial part of the world’s “ocean conveyor belt” and is more than four times stronger than the gulf stream. It moves water around the globe – linking the Atlantic, Pacific and Indian Oceans. It’s also the main mechanism for the exchange of heat, carbon dioxide, chemicals, and biology across these ocean basins.

To carry out the study, its authors used Australia’s fastest supercomputer and climate simulator, GADI, located at Access National Research Infrastructure in Canberra. The projections explored in this analysis were conducted by a research team based at UNSW, who found that the transport of ocean water from the surface to the deep may also slow in the future.

Dr Sohail said it is predicted the slow-down will be similar under the lower emissions scenario, provided ice melting accelerates as predicted in other studies.

“The 2015 Paris Agreement aimed to limit global warming to 1.5°C above pre-industrial levels. Many scientists agree we have already reached this 1.5 degree target, and it is likely to get hotter, with flow-on impacts on Antarctic ice melting,” Dr Sohail said.

“Concerted efforts to limit global warming (by reducing carbon emissions) will limit Antarctic ice melting, averting the projected ACC slowdown.”

Published in Environmental Research Letters today, the research reveals the impact of ice melting and ocean warming on the ACC is more complex than previously thought.

“The melting ice sheets dump vast quantities of fresh water into the salty ocean. This sudden change in ocean ‘salinity’ has a series of consequences. These include the weakening of the sinking of surface ocean water to the deep (called the Antarctic Bottom Water), and, based on this study, a weakening of the strong ocean jet that surrounds Antarctica,” Associate Professor Gayen said.

Associate Professor Gayen added that this new research contrasts with previous studies, which suggested the ACC may be accelerating.

“Ocean models have historically been unable to adequately resolve the small-scale processes that control current strength. This model resolves such processes, and shows a mechanism through which the ACC is projected to actually slow-down in the future. 

“However, further observational and modelling studies of this poorly-observed region are necessary to definitively discern the current’s response to climate change,” he said.

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