Earth’s oceans simmer with heat trapped by increasing amounts of greenhouse gases. But a patch of water in the North Atlantic stubbornly resists the trend and actually drops the temperature.
This ‘cold drop’ has been a topic of interest to climatologists since it was first seen in 2015. Unfortunately, the complexity of ocean circulation makes it difficult to explain easily.
A new study adds detail to the phenomenon, revealing that there is more than one cause at work.
A team of researchers from the Max Planck Institute for Meteorology in Germany applied long-term climate models to simulate various settings to find those that match the observed temperature drop.
One of the factors they identified is not a real surprise, as they support previous studies showing that a water current called the Atlantic Southern Dump Circulation (AMOC) has weakened significantly since the mid-20th century.
When running at full steam, the circulation draws warm, salty surface waters from the tropics near the Gulf of Mexico north to the European coast, exchanging it for cold, fresh water supplied by melted ice.
Exactly what could be causing this tropical waters highway to slow down is not entirely clear, although some models suggest that more Greenland meltwater coupled with rising global temperatures would fit what we are seeing.
With warmer temperatures that make ocean water more buoyant, it is less likely to fall as fast, decreasing the spiral. Meanwhile, a healthy dose of fresh water dripping from melted Arctic ice and increased precipitation would also prevent circulating currents by forming a layer of less salty water on the surface.
Still, the AMOC data is not of the highest quality before 2004, leaving open the small possibility that the slowdown could be a return to business as usual rather than something caused by a warming planet.
To discover the connections between Earth’s climate and the cold bubble, the researchers behind this latest study used a detailed planetary climate model to link the variations in energy, carbon dioxide, and water across the ocean, land, and atmosphere.
Simulations done through this model allowed them to see what could happen if they forced AMOC to shake at full speed, leaving the atmosphere to act as a major influencing factor on its own.
Sure enough, there was a small but noticeable effect. As the warm incoming waters cooled, they produced low clouds that would reflect incoming radiation, and in turn further cool the surface.
The team then ran another scenario that only analyzed AMOC’s heat transport, discovering that it not only carried less energy, but poured more into circulating Arctic water currents.
For complicated reasons, these subpolar circulations are increasing in speed, drawing heat from the AMOC and leaving the cold droplet even colder.
There is still much work to be done to develop these explanations and determine how much impact our insatiable desire to burn fossil fuels has had on what would otherwise be a natural cycle.
But the study shows a lot how important it is that we take various factors into account when evaluating local and global climate changes.
The researchers will undoubtedly pay even more attention to AMOC’s strength in the years to come. But knowing exactly how this cold bubble works in a changing climate will help us better understand what to expect in a future that is likely to be several degrees warmer.
This research was published in Climate change.
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