Unlocking the Secrets of the Atlantic Meridional Overturning Circulation (AMOC)

A halt in the decline of the AMOC since the early 2010s

A recent research conducted by experts at AOML revealed that significant weakening of the Atlantic Meridional Overturning Circulation (AMOC) happened in the 2000s, but has paused since the early 2010s due to a struggle between natural and human-induced signals. 

The AMOC represents the Atlantic component of the global ocean conveyor belt, a vast ocean circulation system that transports heat, salt, carbon, and other biogeochemical substances along its routes. Consequently, the AMOC is an essential factor in the global balance of heat, salt, nutrients, and carbon in the ocean, influencing regional climate, sea levels, and marine ecosystems.

The latest advanced climate models indicate that the human-induced weakening of the AMOC has been occurring since the mid-1980s. Nevertheless, direct observational data over the past two decades have revealed impressive resilience of the AMOC. 

To delve into this inconsistency, researchers examined various climate and ocean model simulations alongside direct observational data. They discovered that significant weakening of the AMOC transpired in the 2000s, primarily fueled by human activities (i.e. increased CO2 and diminished aerosol). However, starting from the early 2010s, the natural aspect of the AMOC has considerably strengthened due to the emergence of a strong positive North Atlantic Oscillation (NAO). The amplified natural AMOC signal counteracts the human-induced weakening signal, resulting in a near halt of the AMOC decline.

“Interdecadal fluctuations in the NAO are likely generated by atmospheric internal dynamics and are thus random and unpredictable. Consequently, the struggle between the natural and human signals may persist in the coming decades, or the NAO phase may reverse to strengthen the human-induced weakening of the AMOC. We should therefore investigate the two potential scenarios of the AMOC state for the next decades and their impacts,” stated Sang-Ki Lee, the lead author of the study and AOML Oceanographer.

Although not specifically examined in this research, the state-of-the-art climate models, including those analyzed in this study, do not predict a complete shutdown of the AMOC during the 21st century. However, potential future freshening of the North Atlantic due to melting land ice (i.e. the Greenland ice sheet and glaciers and ice caps in the Arctic) is not sufficiently represented in those models. Thus, the study emphasized the necessity of enhancing climate models by integrating realistic scenarios of the Greenland ice sheet melting to address this low likelihood-high impact possibility of AMOC collapse.

AOML’s investigations in 2024 unveiled crucial aspects of the AMOC

Throughout 2024, scientists at AOML conducted multiple studies to improve the understanding of the AMOC and its long-term variability. Variations in the strength of the AMOC could influence numerous global climate phenomena such as sea level, extreme weather, and precipitation patterns, making it significantly important to comprehend.

Moreover, the ocean’s surface waters absorb human-generated carbon dioxide. The AMOC routes then transport this carbon dioxide down into the deep ocean, often referred to as carbon sequestration. Thus, the AMOC-related carbon sinking in the North Atlantic constitutes a significant portion of the anthropogenic carbon inventory within the global ocean. Therefore, the degree of anthropogenic carbon sequestered by the ocean is closely linked to the strength of the AMOC. 

Researchers at AOML made noteworthy strides in understanding the AMOC in the past year. One study, led by a PhD student from the University of Miami in collaboration with scientists at AOML, is the first to evaluate the AMOC’s heat transport at 22 5°S in the South Atlantic, underscoring the importance of sustained in situ observations. Utilizing a novel mapping technique, the study offered insights into the seasonal and interannual variability of the AMOC in the South Atlantic.

Another investigation revealed that although there has been no evident decline in the overall AMOC transport, the abyssal (near the ocean floor) limb of the AMOC in the North Atlantic has experienced weakening over the last two decades, contributing to an escalation in deep-sea heat content and, consequently, sea level rise in the area.

Furthermore, researchers at AOML discovered through direct measurements that the Florida Current, one of the swiftest currents in the ocean and a crucial component of the AMOC, has remained exceptionally stable over the last 40 years. In this research, the scientists re-evaluated the general trend in the transport of the Florida Current inferred from submarine cable data and identified that an adjustment was essential for the gradual shifts in the Earth’s magnetic field over time. This adjustment nearly eliminated a previously observed negative trend in the records, indicating that the Florida Current has remained constant for the past four decades. This research illustrates the significance of ongoing observations for the AMOC, as current observational records are just beginning to uncover signals at the decadal scale that are pertinent to climate variability and changes.

These investigations underline the necessity of ongoing data collection across multiple latitudes, enhancing our comprehension of the pertinent physical phenomena, and refining climate models in order to advance our ability to monitor and forecast the future of the AMOC.