NEWS BRIEFING: Is the Southern Ocean about to have its own 'Day After Tomorrow' moment?

This landmark study addresses the slowing down of the vertical overturning circulation in the Southern Hemisphere, and the associated warming at the sea floor of the Southern Ocean.  There have been many studies on the corresponding slowing of the vertical overturning circulation throughout the depth of the North Atlantic Ocean, but this study is the first to shed light on the causes of the warming at the bottom of the Southern Ocean, and importantly, the mechanisms that are causing this warming.  They find that the melting of the Antarctic ice sheets is the main driver of the slowing of the overturning circulation, with the changing winds and the surface warming playing only minor roles.  The extra meltwater decreases the ocean salinity and the ocean density, and so affects the strength of the vertical overturning circulation.  The study warns that the slowing of the overturning circulation is only just beginning, with much more to come in the decades ahead, and the authors point to the likely implications of these changes to the health of ocean ecosystems in the global ocean for the next century and beyond. 

The global ocean overturning circulation where cold dense water sinks into the ocean depths at high latitudes and then returns to the upper ocean at lower latitudes is a key part of the global ocean circulation and the climate system. It transports and stores vast amounts of heat and carbon around the globe, influencing global climate, and brings nutrient rich water back to the upper ocean where it can be used by ocean ecosystems. Any significant change to the overturning circulation would have profound effects.  

Previous studies of the overturning circulation focussed most on the north Atlantic Meridional Overturning Circulation (AMOC). In contrast, the equivalent Southern Ocean overturning and particularly the impact on it of loss of Antarctic Ice (leading to sea level rise) has been much less studied.

Li et al. consider focus on the southern hemisphere overturning. Their study uses, for the first time, a high resolution ocean/sea ice model allowing simulation of important features of the circulation and includes consideration of the loss of ice from Antarctica. 

In a high emission scenario, they find that by 2050 the Southern Ocean overturning circulation decays by 40%, about twice the decay of the AMOC. The input of Antarctic melt water is the most important factor causing this decay. The decay result is a warming of abyssal water in the southern hemisphere oceans, particularly south of Australia.

Many uncertainties remain about the impact beyond 2050, including the potentially larger loss of mass from Antarctica postulated in some studies. But it seems almost certain that continuing on a high greenhouse gas emission pathway will lead to even more profound effects on the ocean and the climate system. The world urgently needs to drastically reduce our emissions to get off the high emission pathway we are currently following.

As the climate warms, the Antarctic ice sheet is melting at an accelerated rate. In this new study, a high-resolution ocean-sea ice model finds that this extra meltwater is changing the ocean circulation, with important implications.

Specifically, the meltwater causes a strong freshening around the Antarctic margins and this, in turn, is slowing down the deep ocean circulation. In the model, the meltwater is causing a strong warming of the abyssal Southern Ocean, consistent with observations. This study highlights that in addition to raising global sea levels, accelerated melting of the Antarctic ice sheet has important implications to global climate.