The Atlantic Meridional Overturning Circulation (AMOC), which includes the Gulf Stream, is a massive system of ocean currents that distributes heat across the planet. This circulation transports warm, salty water from the tropics northward into the North Atlantic Ocean. The heat released significantly influences global and regional temperature patterns. This movement is particularly important for the climate of the Northern Hemisphere, providing a relatively mild climate to northwestern Europe.
The Mechanics of the Atlantic Meridional Overturning Circulation
The operation of this current system is driven by thermohaline circulation, a process combining temperature (“thermo”) and salinity (“haline”) to determine seawater density. Warm, salty water travels north along the surface of the Atlantic, including the Gulf Stream, carrying heat toward the poles. As this water reaches higher latitudes near Greenland and the Nordic Seas, it loses heat to the atmosphere, causing it to cool.
This cooling process makes the water denser, and its high salt content contributes to its weight. The sinking of this dense water in the North Atlantic, known as deep-water formation, acts as the primary engine pulling warm surface water from the south, sustaining the circulation. Once submerged, this cold, deep water flows southward along the ocean floor, eventually returning to the surface in a process called upwelling. This entire cycle is slow, taking an estimated 1,000 years for a single parcel of water to complete the journey.
How Freshwater Input Weakens the Current
Climate change interferes with the AMOC’s mechanism by disrupting the density balance that drives deep-water formation. The primary cause is a massive influx of freshwater into the North Atlantic, particularly from the accelerated melting of the Greenland ice sheet. This meltwater, discharged through surface runoff and calving icebergs, enters the critical sinking regions like the Labrador Sea and the Nordic Seas.
The infusion of freshwater reduces the salinity of the surface water, which lowers its overall density. Even though the water is still cold, the reduced density prevents it from sinking effectively to the seafloor, which is necessary to maintain the circulation’s momentum. Increased precipitation over the North Atlantic, a secondary effect of a changing climate, also contributes to this freshening and density reduction. This stratification, where less-dense freshwater sits atop the denser saltwater, suppresses the vertical mixing required for deep-water formation, impeding the circulation’s engine.
Regional Climate Shifts from a Slowdown
The consequences of a weakened AMOC extend beyond the ocean, leading to significant regional climate shifts across the globe. One counterintuitive effect is the projected cooling in northwestern Europe, occurring despite the overall trend of global warming. This cooling is caused by the reduced northward transport of tropical heat, which otherwise moderates the region’s temperatures. Climate models suggest this could lead to increased snowfall in winter and a dampening of the temperature rise in the region.
A weakening of the AMOC also influences atmospheric circulation patterns, potentially shifting the position of major rainfall and arid climate zones. For example, a slowdown is linked to increased heat and drought conditions in the Sahel region of Africa, where millions of people depend on seasonal rains for agriculture. Changes in ocean temperatures and circulation can also influence the jet stream, leading to shifts in precipitation patterns and more persistent, extreme weather events across North America.
Another major consequence is the accelerated localized sea level rise along the North American East Coast. The slowing of the AMOC reduces the northward pull of water away from the coast, causing a “piling up” of water that leads to a faster rate of sea level increase in this specific region. This rise can be significantly higher than the global average, compounding the risks of coastal flooding and extreme weather events. A short-term reduction in the AMOC, for instance, caused a 13-centimeter sea level increase along the New York coastline in 2009–2010.
Oceanic and Biological Ecosystem Impacts
A weakened AMOC introduces changes within the ocean itself, affecting marine biology and ocean chemistry. The circulation transports nutrients, oxygen, and the larvae of marine organisms between different regions. A slowdown disrupts these cycles, which can significantly alter marine habitats and shift the distribution of commercial fish stocks.
The North Atlantic, which absorbs a large fraction of the world’s anthropogenic carbon dioxide, may lose some capacity to sequester carbon as the AMOC slows. The circulation normally transports dense, carbon-rich surface water to the deep ocean, but a weakening reduces this downward mixing. This reduction in ocean carbon uptake constitutes a carbon cycle feedback, potentially leaving more carbon dioxide in the atmosphere and contributing to stronger overall global warming. A weaker flow can also lead to decreased oxygen levels in the deep ocean, threatening deep-sea ecosystems.