Warming in Antarctic deep waters raises the North Atlantic

April 19 () –

Human-induced environmental changes around Antarctica are contributing to sea level rise no less than in the North Atlantic.

A research team led by the University of Miami and NOAA analyzed two decades of deep-sea oceanographic data collected by mooring observing programs, to show that a critical part of Earth's global ocean current system in the North Atlantic has weakened by approximately 12% in the last two decades. These results are published in Nature Geosciences.

“Although these regions are tens of thousands of kilometers away from each other and the abyssal areas are a few kilometers below the ocean surface, our results reinforce the notion that even the most remote areas of the world's oceans they are not outside of human activity,” said the study's lead author, Tiago Biló, an assistant scientist at NOAA's Cooperative Institute for Marine and Atmospheric Studies' Resential School.

Scientists analyzed data from several observing programs to study changes over time in a cold, dense, deep mass of water located at depths greater than 4,000 meters below the ocean surface that flows northward from the Southern Ocean. and eventually rises to shallower waters in other parts of the global oceanlike the North Atlantic.

This shrinking branch of the deep ocean, which scientists call the abyssal branch, is part of the Atlantic Meridional Overturning Circulation (AMOC), a three-dimensional system of ocean currents that acts as a “conveyer belt” to distribute heat, nutrients and carbon dioxide to the world's oceans.

This near-bottom branch is composed of Antarctic bottom water, which forms from the cooling of seawater in the Southern Ocean around Antarctica during the winter months. Among the different mechanisms of formation of this bottom water, perhaps the most important is the so-called brine rejectiona process that occurs when salt water freezes.

As sea ice forms, it releases salt into the surrounding water, increasing its density. This dense water sinks to the ocean floor, creating a layer of cold, dense water that spreads north to fill the three ocean basins: the Indian, Pacific, and Atlantic oceans. During the 21st century, researchers observed that the flow of this Antarctic layer along latitude 16°N in the Atlantic had slowed, reducing the entry of cold waters to higher latitudes and causing warming of waters in the deep ocean.

“The areas affected by this warming span thousands of kilometers in north-south and east-west directions, between 4,000 and 6,000 meters deep,” said William Johns, co-author and professor of ocean sciences at the Rosenstiel School. “As a result, there is a significant increase in the heat content of the abyssal ocean, which contributes to local sea level rise due to thermal expansion of water.”

“Our observational analysis matches what numerical models have predicted: human activity could impose circulation changes across the ocean,” Biló said. “This analysis was only possible thanks to decades of collective planning and efforts by multiple oceanographic institutions around the world.”