A recent study published in the journal Nature Geoscience indicates that the Atlantic Meridional Overturning Circulation (AMOC) deep-water limb in the North Atlantic has weakened over the past two decades. The research, led by scientists at the University of Miami Rosenstiel School of Marine, Atmospheric, and Earth Science, and the National Oceanic and Atmospheric Administration’s Atlantic Oceanographic and Meteorological Laboratory, used mooring observations and hydrographic data to gain a better understanding of the Earth’s climate regulation system. This weakening of the deep-ocean branch, known as the abyssal limb, is a critical component of the AMOC, which acts as a “conveyor belt” to distribute heat, nutrients, and carbon dioxide across the world’s oceans.
The study highlighted the impact of human-induced environmental changes around Antarctica on sea level rise in the North Atlantic. The analysis of two decades of deep-sea oceanographic data collected by observational mooring programs revealed that the abyssal limb of the AMOC has weakened by approximately 12 percent over the past two decades. Despite being thousands of miles apart, the link between environmental changes in Antarctica and the weakening of ocean currents in the North Atlantic emphasizes the interconnectedness of the world’s oceans and the influence of human activity on remote marine environments.
As part of the NOAA-funded project DeepT, the research team analyzed data from various observational programs to study changes over time in a cold, dense, and deep-water mass located at depths exceeding 4,000 meters below the ocean surface. This deep-water mass originates from the Southern Ocean and flows northward, eventually upwelling to shallower depths in other parts of the global ocean, such as the North Atlantic. The shrinking of this deep-ocean branch, fueled by changes in the formation of Antarctic bottom water, has significant implications for global ocean circulation and climate regulation.
The formation of Antarctic bottom water, which plays a crucial role in the AMOC, is driven by processes such as brine rejection, where salty water released during sea ice formation increases the density of surrounding water, causing it to sink and spread northward across ocean basins. The researchers observed a slowdown in the flow of this Antarctic layer across 16°N latitude in the Atlantic during the 21st century, leading to a reduction in the inflow of cold waters to higher latitudes and resulting in the warming of deep-ocean waters. This warming has expanded over thousands of miles and depths between 4,000 and 6,000 meters, contributing to local sea level rise through thermal expansion of the water.
William Johns, a co-author of the study and professor of ocean sciences at the Rosenstiel School, explained that the observed increase in abyssal ocean heat content has implications for sea level rise and ocean circulation patterns. The study’s lead author, Tiago Biló, emphasized the importance of the interdisciplinary and collaborative efforts of multiple oceanographic institutions worldwide in enabling this observational analysis. By corroborating numerical model predictions with real-world data, the study underscores the potential for human activity to impact ocean circulation patterns on a global scale and highlights the interconnected nature of the Earth’s climate system.
