The functioning of a complex ocean current system such as the AMOC in the North Atlantic could reshape the global climate. Between conflicting signals and model uncertainties, researchers and climatologists are questioning their natural variability and that caused by the disruption of certain balances, such as climate change. In this context, improving the effectiveness of digital forecasting tools is essential to better predict and anticipate the consequences of possible disruptions.
by Laurie Henry
Cover photo: The Gulf Stream, the main surface current in the North Atlantic, is part of the AMOC, a much larger oceanic loop. © NASA
Climate models are at the heart of political decisions in these times of environmental crisis. They make predictions about sea level rise, the intensity of heatwaves and the redistribution of precipitation. However, their ability to reproduce certain key phenomena in a system as vast and complex as the ocean is questionable.
This is the case for the AMOC (Atlantic Meridional Overturning Circulation), a system of Atlantic currents whose stability influences global climate balance. While the latest IPCC report considers its weakening ‘very likely’ in the 21st century, discrepancies remain between observed data and simulations, casting doubt on the reliability of forecasts. In their recent publication in Philosophical Transactions of the Royal Society A, G. D. McCarthy (Maynooth University) and L. Caesar (University of Bremen) analyse these discrepancies and highlight the urgent need to better understand the actual dynamics of the AMOC in order to improve the reliability of climate projections.
A highly complex and vital ocean system
A huge system of ocean currents, the AMOC acts as a global heat transporter. In the Atlantic, it is the main vehicle for transporting warm tropical surface waters to the high latitudes of the northern hemisphere, where they are cooled and sink into the depths due to density effects, before returning southwards.
This redistribution of heat by large ocean currents, from the surface to the bottom and from the North Pole to the South Pole, plays a central role in regulating the global climate and acts as an endless cycle. Without the AMOC, for example, regions such as Northern Europe would be much colder. ‘The maritime climates of Western Europe are on average 5°C milder than those of the Pacific at equivalent latitudes thanks to this circulation,’ the authors point out. It is thanks to the constant supply of heat from the AMOC that ports such as Murmansk, beyond the Arctic Circle, remain ice-free in winter.
This ‘central heating’ of the northern hemisphere is now being scrutinised with concern because climate models have predicted since the 1980s that warming due to CO₂ emissions could weaken this circulation. Ice melt in Greenland and increased freshwater input into the North Atlantic could disrupt the formation of deep waters, which drive the AMOC.