Researchers have discovered that the Antarctic Circumpolar Current (ACC), the strongest ocean current in the world, is slowing down due to melting ice.

Sea level rise, ocean warming, and the sustainability of marine ecosystems are some of the indicators of the global climate that are impacted by this melting.

The study, which was published in Environmental Research Letters, shows that the ACC is more complicatedly affected by ocean warming and ice melting than previously believed.

In a scenario with high carbon emissions, the researchers from the University of Melbourne and NORCE Norway Research Centre have demonstrated that the current slowing will be about 20% by 2050.

The Antarctic Circumpolar Current (ACC) is an ocean current that flows clockwise (as seen from the South Pole) from west to east around Antarctica. An alternative name for the ACC is the West Wind Drift. The ACC is the dominant circulation feature of the Southern Ocean and has a mean transport estimated at 100–150 Sverdrups, making it the largest ocean current.

On potential impacts

It is anticipated that freshwater inflow from ice shelves melting, into the Southern Ocean, will alter the ocean’s characteristics, including its density (salinity) and circulation patterns.

The study was conducted by oceanographer Andreas Klocker of the NORCE Norwegian Research Centre, fluid mechanist Associate Professor Bishakhdatta Gayen of the University of Melbourne, and climate scientist Taimoor Sohail. To identify the effects of shifting temperature, salinity, and wind conditions, they examined a high-resolution ocean and sea ice simulation of ocean currents, heat transport, and other elements.

Invasive species, such as rafts of southern bull kelp that follow the currents or marine-borne creatures like mollusks or shrimp, are prevented from entering Antarctica by the Antarctic Circumpolar Current.

Such species have a greater chance of entering the vulnerable Antarctic continent as the ACC slows and weakens. The food web may be severely impacted by this, which could alter the Antarctic penguins’ available diet, for instance.

The ACC is more than four times stronger than the gulf stream and is an essential component of the global “ocean conveyor belt.” It connects the Atlantic, Pacific, and Indian Oceans and transports water all over the world. The primary mechanism for the movement of chemicals, biology, heat, and carbon dioxide between these ocean basins is the Antarctic Circumpolar Current.

Read also Antarctic ice melt slows Deep Ocean Current with potential Impact on World’s Climate for Centuries

The outlook

“The 2015 Paris Agreement aimed to limit global warming to 1.5 degrees Celsius 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,” Taimoor Sohail said.

Taimoor Sohail added, concerted efforts to limit global warming (by reducing carbon emissions) will limit Antarctic ice melting, averting the projected ACC slowdown. However, a similar current slow-down is expected if ice melting is accelerating.

“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.

“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.

Decline of Antarctic Circumpolar Current due to polar ocean freshening

Study

Abstract

The Antarctic Circumpolar Current (ACC) is the world’s strongest ocean current and plays a disproportionate role in the climate system due to its role as a conduit for major ocean basins. This current system is linked to the ocean’s vertical overturning circulation, and is thus pivotal to the uptake of heat and CO2 in the ocean.

The strength of the ACC has varied substantially across warm and cold climates in Earth’s past, but the exact dynamical drivers of this change remain elusive. This is in part because ocean models have historically been unable to adequately resolve the small-scale processes that control current strength.

Here, we assess a global ocean model simulation which resolves such processes to diagnose the impact of changing thermal, haline and wind conditions on the strength of the ACC. Our results show that, by 2050, the strength of the ACC declines by ∼20% for a high-emissions scenario.

This decline is driven by meltwater from ice shelves around Antarctica, which is exported to lower latitudes via the Antarctic Intermediate Water. This process weakens the zonal density stratification historically supported by surface temperature gradients, resulting in a slowdown of sub-surface zonal currents. Such a decline in transport, if realised, would have major implications on the global ocean circulation.