❄️🌊 Mysterious ‘cold blob’ in the Atlantic suggests the AMOC is weakening

A patch of ocean south-east of Greenland is the only place on Earth that is cooling, and it could be a sign that the warm water "conveyor belt" in the Atlantic is slowing down

Over the past 150 years, Earth’s entire surface has been warming, except for one patch of the north Atlantic. Located south-east of Greenland, this area has cooled by as much as 1°C and is known as the “warming hole” or the “cold blob”.

Scientists have been split over why this cold blob exists, but the latest evidence backs up the idea that it is caused by a weakening of the Atlantic Meridional Overturning Circulation (AMOC), the system of currents that transports warmth from the tropics to Europe.

The AMOC carries warm, salty water from the Gulf of Mexico towards the north Atlantic, where it cools and sinks, flowing back south along the ocean floor. Scientists are concerned that the surge of freshwater from Greenland’s melting ice is making this salty water less dense, so it sinks more slowly, weakening the circulation.

Some research suggests the AMOC could cross a tipping point within decades, locking in a future collapse that would freeze Europe and disrupt monsoon rains crucial for agriculture in Africa and Asia. But we only have 22 years of direct observation of AMOC strength, not enough to tease out a clear trend.

Climate modelling has suggested that a slowing AMOC is carrying less warm water to the north Atlantic, resulting in the cold blob. However, other modelling has placed most of the blame on the atmosphere.

Stefan Rahmstorf at the Potsdam Institute for Climate Impact Research in Germany and his colleagues have now investigated the cold blob with climate reanalyses, which are based on direct weather observations from satellites, buoys and ships, rather than climate modelling.

They found that heat loss from the ocean surface has decreased in the cold blob since 1955. In addition, the ocean has been cooling not just near the surface, but also 1000 metres down. That means that the AMOC is transporting less heat, not that winds are taking more heat away, they argue.

The finding reveals that Atlantic Ocean circulation has already been changing for decades, he adds, raising concerns about a collapse not only of the AMOC, but also of the subpolar gyre, a massive swirl of currents around the cold blob. The subpolar gyre helps bring in salty surface water to feed the sinking of cold, dense water that drives the AMOC. If it shuts down, it could reduce temperatures in the UK and nearby countries more rapidly than a full AMOC collapse.

“The subpolar gyre passing this tipping point could already lead to serious climate impacts in western Europe as early as in the 2040s,” says Rahmstorf.

🔗 https://www.newscientist.com/article/2529078-mysterious-cold-blob-in-the-atlantic-suggests-the-amoc-is-weakening/