The Atlantic meridional overturning circulation: dynamics, measurements, and implications for climate change

Carbon dioxide levels, sea levels, and temperature are rising – all indicators commonly associated with climate change. However, an indicator more complex and less well known is the movement of ocean water in the Atlantic, specifically the Atlantic Meridional Overturning Circulation (AMOC). 

The AMOC is the large-scale process of the movement of warm water on the surface of the ocean from the equator to the northern part of the Atlantic Ocean. As water moves closer to the North Pole, its temperature decreases. Evaporation also occurs when the water moves North. Since it is the freshwater component of the water that is evaporating, the salinity (saltiness) of the surrounding water increases. Cooler, saltier water has a greater density and therefore will fall to a great depth in the ocean. The cold water that sinks then moves along the bottom of the ocean and becomes warmer and less dense as it returns to the equator. Once the water reaches the equator, it rises to the top of the ocean via a phenomenon known as “upwelling.”

The AMOC is responsible for distributing heat and moisture around the globe and circulating water throughout the ocean. This shapes the long-term weather conditions on Earth in modern times. In addition, research has shown that major fluctuations in the AMOC’s strength trigger sudden climatological disruptions with worldwide significance. Research suggests that the AMOC may be weakening, meaning the current is becoming slower and less powerful. Implications for the AMOC weakening include a decrease in North Atlantic sea surface temperatures, which have dramatic climate-related impacts such as causing even lower temperatures and even more severe weather during the winter in the United States and other areas of North America. Precipitation may decrease in North America and Europe, and monsoons may be impacted in Africa and South America as a result of the AMOC weakening.

The AMOC is responsible for distributing heat and moisture around the globe and circulating water throughout the ocean. This shapes the long-term weather conditions on Earth in modern times. 

The AMOC has two modes of functioning: strong and weak. Experts still don’t know when the AMOC undergoes the transition from a strong to a weak current, but there are some indicators for when a transition will occur. Critical slowing down refers to environmental changes occurring, which in this case are signs that the current is transitioning from strong to weak. Critical slowing down can provide hints to the extent of climate change possible in the future and how to prevent it. 

Decreasing salinity and increasing sea surface temperature are examples of critical slowing down. Research has indicated that the AMOC is weakening. Recent measurements of the Atlantic Ocean reveal that sea surface temperatures have been increasing, which destabilizes the temperature-mediated density turnover. Measurements also reveal that salinity levels are decreasing in two key areas: the North and South Pole. If water is less salty in the Poles, the circulating water doesn’t sink as well.

It is important to note that salinity and sea surface temperature are indirect means of characterizing the AMOC, and the AMOC has only been directly studied for approximately 20 years. Thus, variations in the AMOC that have occurred could be due to random fluctuation. This means the AMOC may not truly be becoming weaker, although known indicators point to the AMOC weakening. 

While directly studying the AMOC for longer is key to confirming if it is truly weakening, reducing greenhouse gas emissions to target climate change is necessary in order to ensure human activity doesn’t further weaken the AMOC.

Humans contribute to the hypothesized weakening of the AMOC. For instance, the burning of fossil fuels releases greenhouse gasses into the atmosphere. This increases global temperatures because gasses trap heat in the troposphere of the atmosphere. Oceans absorb some of the emitted heat and greenhouse gasses, causing oceanic warming. When oceans warm, sea ice melts, diluting the salt concentration in the poles, which leads to the observed decrease in salinity and therefore density. While directly studying the AMOC for longer is key to confirming if it is truly weakening, reducing greenhouse gas emissions to target climate change is necessary in order to ensure human activity doesn’t further weaken the AMOC. Preserving the strength of the AMOC is essential for the planet and the survival and well-being of all of the species that inhabit it.

Nature Climate Change (2021). DOI: 10.1038/s41558-021-01097-4 

Nature (2018). DOI: 10.1038/s41586-018-0006-5 

npj Climate and Atmospheric Sciences (2021). DOI: 10.1038/s41612-021-00182-x