Antarctic circulation pump is already weakening alarmingly

Every second, gigantic quantities of water plunge into the depths of the Antarctic Ocean: the Antarctic overturning circulation, in which cold, oxygen-rich water shoots from the continent's shelf into the deep sea, is considered one of the great engines of global ocean currents. However, it already appears to be stuttering dangerously, as a team led by Kathryn Gunn from the University of Southampton writes in "Nature Climate Change". Measurements have shown that circulation in the region has already slowed by a third.

This confirms an earlier study from March 2023, which had modelled a 40 per cent weaker current by the middle of the millennium. If the data is confirmed, the decline would therefore become noticeable much earlier and perhaps even more strongly than previously feared. The consequences of this could be felt worldwide, as this overturning circulation redistributes thermal energy, carbon and nutrients on a global scale and supplies the deep sea with oxygen-rich water.

The team led by Gunn took a new approach to supplement the modelling study with real data. To do this, the group combined measurements taken by ships and stationary buoys between 1992 and 2017 in a numerical simulation. From this, they calculated the strength of the Antarctic bottom water current and the amount of oxygen that the waterfall transports into the deep sea. The study focussed on a deep-sea basin south of Australia that receives deep water from several sources, which in turn lie downstream of large meltwater inputs. The region can probably serve as a model region for how the deep sea is changing due to climate change.

And the results show that this process may already be taking place: According to the data, the overturning circulation has slowed down since 1992, which has also resulted in less oxygen being transported to the depths. The scientists identified the ice melt in the Antarctic, which has also increased in recent decades, as the trigger. As a result, the surface water around the continent is becoming less salty and less dense, which affects the sinking rate: the sweeter the water is, the "lighter" it becomes. As a result, the total volume of newly formed deep water decreases, which also flows more slowly.

The observed slowdown would have been even more pronounced had it not been for a short-term climate event in connection with a strong El Niño during the observation period, which led to a partial and temporary recovery in deep water formation from 2015 onwards. It led to a temporary increase in salinity in the Antarctic Ocean, which boosted circulation again. This illustrates how sensitive deep water formation is to changes in salt concentrations on the Antarctic continental shelf, Gunn and co. write on "The Conversation". With the end of El Niño in 2018, the increased salinity in the Antarctic waters also declined, meaning that the trend reversal is unlikely to be permanent, according to the researchers. Instead, the weakening should continue with increased ice melting.

Spectrum of Science

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