Meridional Overturning Circulation Dynamics in Oceanic Systems

Meridional overturning circulation (MOC) encompasses the large-scale movement of ocean waters driven by gradients in density, which arise from variations in temperature and salinity. In the Atlantic sector, the Atlantic Meridional Overturning Circulation (AMOC) transports warm surface waters northwards, where cooling and buoyancy loss induce deep convection and the formation of dense water masses. These waters return southwards at depth, closing a global conveyer belt that redistributes heat, carbon and nutrients. MOC variability is governed by seasonal cycles of convection, wind-driven Ekman transport, freshwater inputs from ice melt and river runoff, and interactions with mesoscale eddies and boundary currents. Advances in observing systems and high-resolution models have refined our understanding of overturning strength, pathways and response to anthropogenic warming. The stability of MOC influences regional climates, sea-level patterns and marine ecosystems, making its dynamics central to projections of future climate change and to the design of effective mitigation strategies.

Research from Nature Portfolio

Recent studies have used coupled climate models to show that, unlike the broader North Atlantic, the Nordic Seas overturning circulation may strengthen throughout the twenty-first century. Enhanced horizontal circulation and a greater zonal density gradient in a warmer climate appear to stabilise deep overflow waters, reinforcing the deep limb of the AMOC. Observational time series from an array in the subpolar North Atlantic reveal a strong seasonal cycle, with overturning peaks in late spring and minima in early winter. This seasonality explains over half of the variance in meridional freshwater transport and highlights the role of winter water-mass transformation modulated by variable Ekman transport. Trans-basin mooring data in the subpolar region have prompted a revision of how interior basin convection influences boundary density changes, demonstrating that winter deep-water formation does not always translate into commensurate shifts in western boundary current densities and overturning variability.

Meridional Overturning Circulation Dynamics in Oceanic Systems publication trend

The graph below shows the total number of articles in meridional overturning circulation dynamics in oceanic systems across all publications each year (not limited to Nature Index journals).

Technical terms

Meridional Overturning Circulation (MOC): A global system of surface and deep currents driven by density contrasts, transporting heat and materials between low and high latitudes.

Atlantic Meridional Overturning Circulation (AMOC): The component of the MOC in the Atlantic Ocean, characterised by northward flow of warm surface waters and southward return of cold deep waters.

Thermohaline Circulation: Ocean circulation driven by temperature (thermo) and salinity (haline) differences that affect water density.

Ekman Transport: The net motion of fluid resulting from a balance between wind stress and the Coriolis effect, influencing surface currents and vertical water movement.

Deep Convection: The process by which surface waters sink to depth due to cooling or increased salinity, forming dense water masses.

Freshwater Flux: The input of freshwater from sources such as ice melt or river discharge, which can alter surface salinity and stratification.

References

1. Future strengthening of the Nordic Seas overturning circulation. Nature Communications (2023).
2. Seasonality of the Meridional Overturning Circulation in the subpolar North Atlantic. Communications Earth & Environment (2023).
3. Subpolar North Atlantic western boundary density anomalies and the Meridional Overturning Circulation. Nature Communications (2021).
4. A plausible emergence of new convection sites in the Arctic Ocean in a warming climate. Environmental Research Letters (2024).
5. Pending recovery in the strength of the meridional overturning circulation at 26∘ N. Ocean Science (2020).
6. The North Atlantic Ocean Is in a State of Reduced Overturning. Geophysical Research Letters (2018).