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We explore the representation of the Atlantic Meridional Overturning Circulation (AMOC) in 27 models from the CMIP6 multimodel ensemble. Comparison with RAPID and SAMBA observations suggests that the ensemble mean represents the AMOC strength and vertical profile reasonably well. Linear trends over the entire historical period (1850-2014) are generally neutral, but many models exhibit an AMOC peak around the 1980s. Ensemble mean AMOC decline in future (SSP) scenarios is stronger in CMIP6 than CMIP5 models. In fact, AMOC decline in CMIP6 is surprisingly insensitive to the scenario at least up to 2060. We find an emergent relationship among a majority of models between AMOC strength and 21st century AMOC decline. Constraining this relationship with RAPID observations suggests that the AMOC might decline between 6 and 8 Sv (34-45%) by 2100. A smaller group of models projects much less AMOC weakening of only up to 30%. Plain Language Summary The Atlantic Meridional Overturning Circulation (AMOC) is a circulation pattern in the Atlantic Ocean that is an important component of the climate system, due to its ability to redistribute and sequester heat and carbon. An accurate representation of the AMOC is a critical test for climate models and essential for building confidence in their projections. Here we investigate the AMOC in 27 climate models that contributed simulations to the Coupled Model Intercomparison Project Phase 6 (CMIP6). We find that many models reproduce the observed AMOC quite well, but there are still several models in which the AMOC is too weak or too strong. Most models suggest a slight upward trend in the AMOC from 1850 to the 1980s. Simulations representing different scenarios for future socioeconomic development suggest a stronger AMOC decline compared to previous assessments. Using direct measurements of the AMOC since 2004 and an emerging across-model relationship between AMOC decline in the 21st century and their present-day mean state, we find that the majority of CMIP6 models point to an end of century AMOC weakening of 34-45% of its present-day strength. A smaller group of models projects much less weakening of only up to 30% of its present state.

Upwelling of global deep waters to the sea surface in the Southern Ocean closes the global overturning circulation and is fundamentally important for oceanic uptake of carbon and heat, nutrient resupply for sustaining oceanic biological production, and the melt rate of ice shelves. However, the exact pathways and role of topography in Southern Ocean upwelling remain largely unknown. Here we show detailed upwelling pathways in three dimensions, using hydrographic observations and particle tracking in high-resolution models. The analysis reveals that the northern-sourced deep waters enter the Antarctic Circumpolar Current via southward flow along the boundaries of the three ocean basins, before spiraling southeastward and upward through the Antarctic Circumpolar Current. Upwelling is greatly enhanced at five major topographic features, associated with vigorous mesoscale eddy activity. Deep water reaches the upper ocean predominantly south of the Antarctic Circumpolar Current, with a spatially nonuniform distribution. The timescale for half of the deep water to upwell from 30° S to the mixed layer is ~60–90 years.

1 AbstractThe heat and salt input from the Indian to Atlantic Oceans by Agulhas Leakage is found to influence the Atlantic overturning circulation in a low-resolution Ocean General Circulation Model. The model used is the Hamburg Large-Scale Geostrophic (LSG) model, which is forced by mixed boundary conditions. Agulhas Leakage is parameterized by sources of heat and salt in the upper South Atlantic Ocean, that extend well into the intermediate layers.It is shown that the model's overturning circulation is sensitive to the applied sources of heat and salt. The response of the overturning strength to changes in the source amplitudes is mainly linear, interrupted once by a stepwise change.The South Atlantic buoyancy sources influence the Atlantic overturning strength by modifying the basin-scale meridional density and pressure gradients. The nonlinear, stepwise response is caused by abrupt changes in the convective activity in the northern North Atlantic.

The thermohaline exchange between the Atlantic and the Southern Ocean is analyzed, using a data set based on WOCE hydrographic data. It is shown that the salt and heat transports brought about by the South Atlantic subtropical gyre play an essential role in the Atlantic heat and salt budgets. It is found that on average the exported North Atlantic Deep Water (NADW) is fresher than the return flows (basically composed of thermocline and intermediate water), indicating that the overturning circulation (OC) exports freshwater from the Atlantic.The sensitivity of the OC to interbasin fluxes of heat and salt is studied in a 2D model, representing the Atlantic between 60 • N and 30 • S. The model is forced by mixed boundary conditions at the surface, and by realistic fluxes of heat and salt at its 30 • S boundary. The model circulation turns out to be very sensitive to net buoyancy fluxes through the surface. Both net surface cooling and net surface saltening are sources of potential energy and impact positively on the circulation strength. The vertical distributions of the lateral fluxes tend to stabilize the stratification, and, as they extract potential energy from the system, tend to weaken the flow. These results imply that a change in the composition of the NADW return transports, whether by a change in the ratio thermocline/intermediate water, or by a change in their thermohaline characteristics, might influence the Atlantic OC considerably.It is also shown that the circulation is much more sensitive to changes in the shape of the lateral buoyancy flux than to changes in the shape of the surface buoyancy flux, as the latter does not explicitly impact on the potential energy of the system. It is concluded that interocean fluxes of heat and salt are important for the strength and operation of the Atlantic thermohaline circulation, and should be correctly represented in models that are used for climate sensitivity studies.