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

The “cold blob” appears in a data visualisation showing average temperatures in 2015, relative to the 1951-80 average NASA Scientific Visualization Studio/Goddard Space Flight Center 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 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. Shift in the Gulf Stream could signal ocean current collapse 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. In a 2022 study, Chengfei He at Northeastern University in Boston and his colleagues found that rapid warming of the Arctic has reduced the temperature difference between the pole and the tropics, shifting the jet stream northwards into the cold blob region. The arrival of these strong westerly winds has forced more evaporation and churned up the water, drawing heat out of the ocean. Greater evaporation has also led to more clouds, shading the cold blob from the sun’s warmth, another study suggested. The jet stream may be starting to shift in response to climate change 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.