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The Canadian Earth System Model version 5 (CanESM5) is a global model developed to simulate historical climate change and variability, to make centennial-scale projections of future climate, and to produce initialized seasonal and decadal predictions. This paper describes the model components and their coupling, as well as various aspects of model development, including tuning, optimization, and a reproducibility strategy. We also document the stability of the model using a long control simulation, quantify the model's ability to reproduce large-scale features of the historical climate, and evaluate the response of the model to external forcing. CanESM5 is comprised of three-dimensional atmosphere (T63 spectral resolution equivalent roughly to 2.8∘) and ocean (nominally 1∘) general circulation models, a sea-ice model, a land surface scheme, and explicit land and ocean carbon cycle models. The model features relatively coarse resolution and high throughput, which facilitates the production of large ensembles. CanESM5 has a notably higher equilibrium climate sensitivity (5.6 K) than its predecessor, CanESM2 (3.7 K), which we briefly discuss, along with simulated changes over the historical period. CanESM5 simulations contribute to the Coupled Model Intercomparison Project phase 6 (CMIP6) and will be employed for climate science and service applications in Canada.

The observed decline of sea ice in the Arctic, if it persists into the future, can create more favorable conditions for shipping activity in the region. To estimate possible changes in key sea ice characteristics over the next two decades, we use high‐resolution climate models. The focus is on two shipping routes: the Northwest Passage and the Northeast Passage. In addition to more traditionally analyzed ice concentration and thickness, we present projected changes in ice strength and pressure, which are especially relevant for shipping hazards. Along both routes, the mean September values of ice strength and pressure, projected for the period 2041–2050, decrease by an order of magnitude relative to the period 2015–2024. The decrease is largely driven by changes in ice concentration, rather than thickness or velocity. Increasing ocean resolution from eddy‐present to eddy‐rich leads to less reduction of sea ice area, volume and strength with global warming.

Using a set of experiments with an eddy-parameterizing ocean model, it is found that the strength of the Atlantic meridional overturning circulation (AMOC) intensifies with the decrease of the density-dependent mesoscale eddy transfer. However, the intensification is weaker than that suggested by simple scaling relationships previously applied. Perturbing the model control sea surface temperature (SST) to mimic its change in response to doubling of CO2, it is shown that the associated ocean heat uptake (OHU) increases and penetrates deeper with the decrease of the mesoscale eddy transfer. It is shown that the OHU correlates with the AMOC strength, and both these quantities are affected by the mesoscale eddy transfer. Passive tracer experiments in the ocean model provide a possible explanation for the finding in coupled-model climate simulations that the ocean heat uptake efficiency (OHUE) increases with the AMOC strength and decreases with the eddy energy generated from the mean state. It is also found that the OHU in the SST-perturbation experiments scales with the net downward advection of heat. The contribution of the AMOC to the downward heat flux is illustrated using a streamfunction in depth–temperature space.

The meridional ocean heat transport (MOHT), its seasonal variability, and projected changes simulated by the second generation Canadian Earth System Model (CanESM2) are presented. The global mean MOHT is within the uncertainty of the observational estimates. However, a correct simulation of the MOHT for individual ocean basins is more challenging, and the Atlantic MOHT south of 30°N is underestimated. The partitioning of the MOHT into the overturning and gyre components is generally consistent with such partitioning in an observationally optimized ocean model. At low latitudes, the time-mean MOHT is dominated by its overturning component, whereas in the Southern Ocean and, especially, in the subpolar North Atlantic, it is the gyre component that plays a more important role. In the projected warmer climates, CanESM2 simulates a weakening of the poleward MOHT essentially in both hemispheres. The projected MOHT changes are largely determined by the overturning component, except in the subpolar Atlantic where it is dominated by the gyre component. Consistent with (the limited number of) previous studies, the seasonal variability of the MOHT is large and is mostly driven by the seasonal variability of the meridional Ekman transport. In the simulated warmer climates, the seasonal cycle of the MOHT is projected to change, mostly in the tropics and also in the Southern Hemisphere midlatitudes. The eddy contribution to the MOHT is broadly consistent with that in the observationally optimized eddy-permitting model. However, in the tropics a significant fraction of the eddy energy is converted back to the mean circulation, and the heat transports due to the parameterized and permitted eddies differ.

Sea levels of different atmosphere–ocean general circulation models (AOGCMs) respond to climate change forcing in different ways, representing a crucial uncertainty in climate change research. We isolate the role of the ocean dynamics in setting the spatial pattern of dynamic sea-level ( ζ ) change by forcing several AOGCMs with prescribed identical heat, momentum (wind) and freshwater flux perturbations. This method produces a ζ projection spread comparable in magnitude to the spread that results from greenhouse gas forcing, indicating that the differences in ocean model formulation are the cause, rather than diversity in surface flux change. The heat flux change drives most of the global pattern of ζ change, while the momentum and water flux changes cause locally confined features. North Atlantic heat uptake causes large temperature and salinity driven density changes, altering local ocean transport and ζ . The spread between AOGCMs here is caused largely by differences in their regional transport adjustment, which redistributes heat that was already in the ocean prior to perturbation. The geographic details of the ζ change in the North Atlantic are diverse across models, but the underlying dynamic change is similar. In contrast, the heat absorbed by the Southern Ocean does not strongly alter the vertically coherent circulation. The Arctic ζ change is dissimilar across models, owing to differences in passive heat uptake and circulation change. Only the Arctic is strongly affected by nonlinear interactions between the three air-sea flux changes, and these are model specific.

We formulate a new conceptual model, named “ MT 2”, to describe global ocean heat uptake, as simulated by atmosphere–ocean general circulation models (AOGCMs) forced by increasing atmospheric CO 2 , as a function of global-mean surface temperature change T and the strength of the Atlantic meridional overturning circulation (AMOC, M ). MT 2 has two routes whereby heat reaches the deep ocean. On the basis of circumstantial evidence, we hypothetically identify these routes as low- and high-latitude. In low latitudes, which dominate the global-mean energy balance, heat uptake is temperature-driven and described by the two-layer model, with global-mean T as the temperature change of the upper layer. In high latitudes, a proportion p (about 14%) of the forcing is taken up along isopycnals, mostly in the Southern Ocean, nearly like a passive tracer, and unrelated to T . Because the proportion p depends linearly on the AMOC strength in the unperturbed climate, we hypothesise that high-latitude heat uptake and the AMOC are both affected by some characteristic of the unperturbed global ocean state, possibly related to stratification. MT 2 can explain several relationships among AOGCM projections, some found in this work, others previously reported: ∙  Ocean heat uptake efficiency correlates strongly with the AMOC. ∙  Global ocean heat uptake is not correlated with the AMOC. ∙  Transient climate response (TCR) is anticorrelated with the AMOC. ∙   T projected for the late twenty-first century under high-forcing scenarios correlates more strongly with the effective climate sensitivity than with the TCR.

The ocean’s meridional overturning circulation is closed by the upwelling of dense, carbon-rich waters to the surface of the Southern Ocean. It has been proposed that upwelling in this region is driven by strong westerly winds, implying that the intensification of Southern Ocean winds in recent decades may have enhanced the rate of upwelling, potentially affecting the global overturning circulation. However, there is no consensus on the sensitivity of upwelling to winds or on the nature of the connection between Southern Ocean processes and the global overturning circulation. In this study, the sensitivity of the overturning circulation to changes in Southern Ocean westerly wind stress is investigated using an eddy-permitting ocean–sea ice model. In addition to a suite of standard circulation metrics, an energy analysis is used to aid dynamical interpretation of the model response. Increased Southern Ocean wind stress enhances the upper cell of the overturning circulation through creation of available potential energy in the Southern Hemisphere, associated with stronger upwelling of deep water. Poleward shifts in the Southern Ocean westerlies lead to a complicated transient response, with the formation of bottom water induced by increased polynya activity in the Weddell Sea and a weakening of the upper overturning cell in the Northern Hemisphere. The energetic consequences of the upper overturning cell response indicate an interhemispheric connection to the input of available potential energy in the Northern Hemisphere.

The effect of anthropogenic climate change in the ocean is challenging to project because atmosphere-ocean general circulation models (AOGCMs) respond differently to forcing. This study focuses on changes in the Atlantic Meridional Overturning Circulation (AMOC), ocean heat content ( Δ OHC), and the spatial pattern of ocean dynamic sea level ( Δ ζ ). We analyse experiments following the FAFMIP protocol, in which AOGCMs are forced at the ocean surface with standardised heat, freshwater and momentum flux perturbations, typical of those produced by doubling CO 2 . Using two new heat-flux-forced experiments, we find that the AMOC weakening is mainly caused by and linearly related to the North Atlantic heat flux perturbation, and further weakened by a positive coupled heat flux feedback. The quantitative relationships are model-dependent, but few models show significant AMOC change due to freshwater or momentum forcing, or to heat flux forcing outside the North Atlantic. AMOC decline causes warming at the South Atlantic-Southern Ocean interface. It does not strongly affect the global-mean vertical distribution of Δ OHC, which is dominated by the Southern Ocean. AMOC decline strongly affects Δ ζ in the North Atlantic, with smaller effects in the Southern Ocean and North Pacific. The ensemble-mean Δ ζ and Δ OHC patterns are mostly attributable to the heat added by the flux perturbation, with smaller effects from ocean heat and salinity redistribution. The ensemble spread, on the other hand, is largely due to redistribution, with pronounced disagreement among the AOGCMs.

Using an ensemble of atmosphere–ocean general circulation models (AOGCMs) in an idealized climate change experiment, this study quantifies the contributions to ocean heat uptake (OHU) from ocean physical parameterizations and resolved dynamical processes operating at different scales. Analysis of heat budget diagnostics reveals a leading-order global heat balance in the subsurface upper ocean in a steady state between the large-scale circulation warming it and mesoscale processes cooling it, and shows that there are positive contributions from processes on all scales to the subsurface OHU during climate change. There is better agreement among the AOGCMs in the net OHU than in the individual scales/processes contributing to it. In the upper ocean and at high latitudes, OHU is dominated by small-scale diapycnal processes. Below 400 m, OHU is dominated by the superresidual transport, representing large-scale ocean dynamics combined with all parameterized mesoscale and submesoscale eddy effects. Weakening of the AMOC leads to less heat convergence in the subpolar North Atlantic and less heat divergence at lower latitudes, with a small overall effect on the net Atlantic heat content. At low latitudes, the dominance of advective heat redistribution is contrary to the diffusive OHU mechanism assumed by the commonly used upwelling-diffusion model. Using a density water-mass framework, it is found that most of the OHU occurs along isopycnal directions. This feature of OHU is used to accurately reconstruct the global vertical ocean warming profile from the surface heat flux anomalies, supporting advective (rather than diffusive) models of OHU and sea level rise.

The most prominent feature of the time-mean global meridional overturning circulation (MOC) is the Atlantic MOC (AMOC). However, interannual variability of the global MOC is shown here to be dominated by Pacific MOC (PMOC) variability over the full depth of the ocean at most latitudes. This dominance of interannual PMOC variability is robust across modern climate models and an observational state estimate. PMOC interannual variability has large-scale organization, its most prominent feature being a cross-equatorial cell spanning the tropics. Idealized experiments show that this variability is almost entirely wind driven. Interannual anomalies of zonal mean zonal wind stress produce zonally integrated Ekman transport anomalies that are larger in the Pacific Ocean than in the Atlantic Ocean, simply because the Pacific is wider than the Atlantic at most latitudes. This contrast in Ekman transport variability implies greater variability in the near-surface branch of the PMOC when compared with the near-surface branch of the AMOC. These near-surface variations in turn drive compensating flow anomalies below the Ekman layer. Because the baroclinic adjustment time is longer than a year at most latitudes, these compensating flow anomalies have baroclinic structure spanning the full depth of the ocean. Additional analysis reveals that interannual PMOC variations are the dominant contribution to interannual variations of the global meridional heat transport. There is also evidence of interaction between interannual PMOC variability and El Niño–Southern Oscillation.