Search

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.

While the Atlantic Meridional Overturning Circulation (AMOC) is expected to weaken under increasing GHGs, it is unclear how it would respond to stabilization of global warming of 1.5 or 2.0 °C, the Paris Agreement temperature targets, or 3.0 °C, the expected warming by 2100 under current emission reduction policies. On the basis of stabilized warming simulations with two Earth System Models, we find that, after temperature stabilization, the AMOC declines for 5–10 years followed by a 150-year recovery to a level that is approximately independent of the considered stabilization scenario. The AMOC recovery has important implications for North Atlantic steric sea-level rise, which by 2600 is simulated to be 25–31% less than the global mean, and for North Atlantic surface temperatures, which continue to increase despite global mean surface temperature stabilization. These results show that substantial ongoing climate trends are likely to occur after global mean temperature has stabilized. Warming is predicted to weaken the Atlantic Meridional Overturning Circulation. Simulated temperature stabilization at Paris Agreement targets shows recovery to a level independent of the target, with continued North Atlantic warming and North Atlantic sea-level rise lower than the global mean.

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.

A rise in global mean sea level is a robust feature of projected anthropogenic climate change using state-of-the-art atmosphere-ocean general circulation models (AOGCMs). However, there is considerable disagreement over the more policy-relevant regional patterns of sea level rise. The Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP) aims to improve our understanding of the mechanisms controlling regional and dynamic sea level change. In FAFMIP, identical air-sea buoyancy and momentum flux perturbations are applied to an ensemble of different AOGCMs, to sample the uncertainty associated with model structure and physical processes. Our novel implementation applies FAFMIP perturbations to an ensemble of OGCMs. This framework enables an estimate of the unknown atmosphere-ocean feedbacks, by comparing the coupled and ocean-only response to surface flux perturbations.Comparing the response to idealised FAFMIP forcing with more realistic, increasing CO2 forcing, much of the spread in regional sea level projections for the North Atlantic and Southern Ocean arises from ocean model structural differences. Ocean-only simulations indicate that only a small proportion of this spread is due to differences in the atmosphere-ocean feedback. Novel tendency diagnostics indicate the relative effect of resolved advection, parametrised eddies, and dianeutral mixing on regional and dynamic sea level change. This study helps to reduce uncertainty in regional sea level projections by refining our estimates of atmosphere-ocean feedbacks and developing our understanding of the physical processes controlling sea level change.

We estimate changes in the historical ocean carbon sink and their uncertainty using an ocean biogeochemical model driven with wind forcing from six different reanalyses and using two different eddy parameterization schemes. First, we quantify wind-induced changes over the extended period from 1871 to 2010 using the 20th Century Reanalysis winds. Consistent with previous shorter-term studies, we find that the wind changes act to reduce the ocean carbon sink, but the wind-induced trends are subject to large uncertainties. One major source of uncertainty is the parameterization of mesoscale eddies in our coarse resolution simulations. Trends in the Southern Ocean residual meridional overturning circulation and the globally integrated surface carbon flux over 1950 to 2010 are about 2.5 times smaller when using a variable eddy transfer coefficient than when using a constant coefficient in this parameterization. A second major source of uncertainty arises from disagreement on historical wind trends. By comparing six reanalyses over 1980 to 2010, we show that there are statistically significant differences in estimated historical wind trends, which vary in both sign and magnitude amongst the products. Through simulations forced with these reanalysis winds, we show that the influence of historical wind changes on ocean carbon uptake is highly uncertain, and the resulting trends depend on the choice of surface wind product.

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.

Meridional heat transport (MHT) in the Southern Ocean (SO) and its components are analyzed with two eddy-permitting climate models. The two models present a consistent picture of the MHT response to projected twenty-first-century changes in SO winds. In agreement with a recent analysis based on an ocean data synthesis product, much of the MHT in the SO is found to be due to the time-mean fields of meridional velocity and temperature. The change in the net MHT tends to be small relative to the interannual variability at most SO latitudes. However, both models exhibit significant changes at most latitudes south of 30°S in individual components of MHT. A simple framework wherein changes in the eddy and mean heat transports tend to compensate each other is not supported by the authors’ results. Instead, the MHT response is composed of sizeable contributions from essentially all of the MHT components, with the eddy and mean heat transports often having the same sign.

A climate model is used to study the climatic impact of the stress exerted on the ocean by the atmosphere. When this stress is set to zero everywhere, the climate becomes much colder, with global-mean near-surface air temperature dropping from 14.8° to 6.1°C. The largest temperature decrease occurs in high latitudes, where sea ice advances equatorward to 40° of latitude. Many of these changes are induced by the changes in the oceanic circulation. In particular, with momentum flux set to zero, the meridional transport of buoyancy in the ocean, including that fraction often associated with the buoyancy-driven circulation, essentially vanishes and, hence, so does much of the surface heat flux. Vertical transport of buoyancy in the ocean is also strongly affected. In addition, the model suggests that the flux of momentum to the ocean has a profound indirect influence on the transport of latent heat. However, the total radiative flux entering the planet at low and midlatitudes does not change much. Instead, the net energy transport across 40°S increases, whereas that across 40°N decreases. The poleward energy transport in the atmosphere increases at midlatitudes in both hemispheres, whereas the oceanic heat transport decreases most strongly in the Northern Hemisphere. The climate becomes colder in both hemispheres, which is not easy to infer from the meridional transport of energy either by the climate system as a whole or by its individual components. Furthermore, the model suggests that it is the wind stress driving the midlatitude oceans—that is, where the oceanic heat transport accounts for only a very tiny fraction of the total poleward energy transport by the climate system, which is of more importance for maintaining the mean position of sea ice edge and, hence, much of the global climate.

Recent analyses of the latest series of climate model simulations suggest that increasing CO2 emissions in the atmosphere are partly responsible for (i) the observed poleward shifting and strengthening of the Southern Hemisphere subpolar westerlies (in association with shifting of the southern annular mode toward a higher index state), and (ii) the observed warming of the subsurface Southern Ocean. Here the role that poleward-intensifying westerlies play in subsurface Southern Ocean warming is explored. To this end a climate model of intermediate complexity was driven separately, and in combination with, time-varying CO2 emissions and time-varying surface winds (derived from the fully coupled climate model simulations mentioned above). Experiments suggest that the combination of the direct radiative effect of CO2 emissions and poleward-intensified winds sets the overall magnitude of Southern Ocean warming, and that the poleward-intensified winds are key in terms of determining its latitudinal structure. In particular, changes in wind stress curl associated with poleward-intensified winds significantly enhance pure CO2-induced subsurface warming around 45°S (through increased downwelling of warm surface water), reduces it at higher latitudes (through increased upwelling of cold deep water), and reduces it at lower latitudes (through decreased downwelling of warm surface water). Experiments also support recent high-resolution ocean model experiments suggesting that enhanced mesoscale eddy activity associated with poleward-intensified winds influences subsurface (and surface) warming. In particular, it is found that increased poleward heat transport associated with increased mesoscale eddy activity enhances the warming south of the Antarctic Circumpolar Current. Finally, a mechanism involving offshore Ekman sea ice transport (modulated by enhanced mesoscale activity) that acts to significantly limit the human-induced high-latitude Southern Hemisphere surface temperature response is reported on.

We present several equilibrium runs under varying atmospheric CO2 concentrations using the University of Victoria Earth System Climate Model (UVic ESCM). The model shows two very different responses: for CO2 concentrations of 400 ppm or lower, the system evolves into an equilibrium state. For CO2 concentrations of 440 ppm or higher, the system starts oscillating between a state with vigorous deep water formation in the Southern Ocean and a state with no deep water formation in the Southern Ocean. The flushing events result in a rapid increase in atmospheric temperatures, degassing of CO2 and therefore an increase in atmospheric CO2 concentrations, and a reduction of sea ice cover in the Southern Ocean. They also cool the deep ocean worldwide. After the flush, the deep ocean warms slowly again and CO2 is taken up by the ocean until the stratification becomes unstable again at high latitudes thousands of years later. The existence of a threshold in CO2 concentration which places the UVic ESCM in either an oscillating or non-oscillating state makes our results intriguing. If the UVic ESCM captures a mechanism that is present and important in the real climate system, the consequences would comprise a rapid increase in atmospheric carbon dioxide concentrations of several tens of ppm, an increase in global surface temperature of the order of 1–2°C, local temperature changes of the order of 6°C and a profound change in ocean stratification, deep water temperature and sea ice cover.

Results from eight ocean–atmosphere general circulation models are used to evaluate the influence of the projected changes in the oceanic stratification on the first baroclinic Rossby radius of deformation in the ocean, associated with atmospheric CO2 increase. For each of the models, an oceanic state corresponding to the A1B stabilization experiment (with atmospheric CO2 concentration of 720 ppm) is compared to a state corresponding to the preindustrial control experiment (with atmospheric CO2 concentration of 280 ppm). In all of the models, the first baroclinic Rossby radius increases with increasing oceanic stratification in the warmer climate. There is, however, a considerable range among the models in the magnitude of the increase. At the latitudes of intense eddy activity associated with instability of western boundary currents (around 35°–40°), the increase reaches 4 km on average, or about 15% of the local baroclinic Rossby radius. Some of the models predict an increase of the baroclinic Rossby radius by more than 20% at these latitudes under the applied forcing. It is therefore suggested that in a plausible future warmer climate, the characteristic length scale of mesoscale eddies, as well as boundary currents and fronts, may increase. In addition, since the speed of long baroclinic Rossby waves is proportional to the squared baroclinic Rossby radius of deformation, the results suggest that the time scale for large-scale dynamical oceanic adjustment may decrease in the warmer climate, thereby increasing the frequency of long-term climate variability where the oceanic Rossby wave dynamics set the dominant period. Finally, the speed of equatorial Kelvin waves and Rossby waves, carrying signals along the equator, including those related to ENSO, is projected to increase.

Global climate models indicate that the poleward shift of the Antarctic Circumpolar Current observed over recent decades may have been significantly human induced. The poleward shift, along with a significant increase in the transport of water around Antarctica, is predicted to continue into the future. To appreciate the magnitude of the poleward shift it is noted that by century’s end the concomitant shrinking of the Southern Ocean is predicted to displace a volume of water close to that in the entire Arctic Ocean. A simple theory, balancing surface Ekman drift and ocean eddy mixing, explains these changes as the oceanic response to changing wind stress.

A global, flux-corrected climate model is em- ployed to predict the surface wind stress and associated wind-driven oceanic circulation for climate states corre- sponding to a doubling and quadrupling of the atmo- spheric CO2 concentration in a simple 1% per year CO2 increase scenario. The model indicates that in response to CO2 increase, the position of zero wind stress curl in the mid-latitudes of the Southern Hemisphere shifts pole- ward. In addition, the wind stress intensifies significantly in the mid-latitudes of the Southern Hemisphere. As a result, the rate of water circulation in the subpolar meridional overturning cell in the Southern Ocean in- creases by about 6 Sv (1 Sv=10 6 m 3 s � 1 ) for doubled CO2 and by 12 Sv for quadrupled CO2, implying an in- crease of deep water upwelling south of the circumpolar flow and an increase of Ekman pumping north of it. In addition, the changes in the wind stress and wind stress curl translate into changes in the horizontal mass trans- port, leading to a poleward expansion of the subtropical gyres in both hemispheres, and to strengthening of the Antarctic Circumpolar Current. Finally, the intensified near-surface winds over the Southern Ocean result in a substantial increase of mechanical energy supply to the ocean general circulation.

Different climate models simulate different behavior of the Atlantic meridional overturning circulation (MOC) under the same global warming scenario. We propose a plausible explanation for this and argue that a proper simulation of the present-day climate in the subpolar North Atlantic is important. This is illustrated using results from idealized global warming experiments, in which both the radiative forcing scenario and the model employed are the same, with the only major difference being the initial subpolar North Atlantic climate. The initial conditions are made progressively colder, with more extensive sea-ice cover in the northern North Atlantic.The key result is that starting from conditions which are too cold in the North Atlantic and with sea-ice that is too extensive leads to an MOC that is more stable to the radiative forcing. Furthermore, under considerably underestimated sea surface temperatures in subpolar regions, the MOC can even intensify. A reduction of freshwater flux associated with the reduction of sea-ice melt is shown to be important for such unusual behavior of the MOC. Other mechanisms are also considered, but not deemed as important in explaining published inter-model differences.

The most recent Intergovernmental Panel on Climate Change assessment report concludes that the Atlantic Meridional Overturning Circulation (AMOC) could weaken substantially but is very unlikely to collapse in the 21st century. However, the assessment largely neglected Greenland Ice Sheet (GrIS) mass loss, lacked a comprehensive uncertainty analysis, and was limited to the 21st century. Here in a community effort, improved estimates of GrIS mass loss are included in multicentennial projections using eight state-of-the-science climate models, and an AMOC emulator is used to provide a probabilistic uncertainty assessment. We find that GrIS melting affects AMOC projections, even though it is of secondary importance. By years 2090–2100, the AMOC weakens by 18% [−3%, −34%; 90% probability] in an intermediate greenhouse-gas mitigation scenario and by 37% [−15%, −65%] under continued high emissions. Afterward, it stabilizes in the former but continues to decline in the latter to −74% [+4%, −100%] by 2290–2300, with a 44% likelihood of an AMOC collapse. This result suggests that an AMOC collapse can be avoided by CO2 mitigation.

The influence of surface fluxes of heat, freshwater, and momentum on ocean circulation and transports are analyzed using a coupled atmosphere–ocean climate model. A control simulation is compared with experiments in which freshwater and wind stress forcing are suppressed separately and together. Thermal forcing is the dominant driving force of the Atlantic meridional overturning circulation, with freshwater and wind stress forcing both having a smaller positive influence. In the Pacific, on the other hand, freshwater forcing has a negative influence; eliminating it intensifies the meridional overturning, leading to the formation of a deep western boundary current. This difference in sign is consistent with the Atlantic being a net evaporative basin and the Pacific one with net precipitation. Deep outflow from the Atlantic at 30°S is more strongly dependent on freshwater than on wind stress forcing. The sensitivity to freshwater forcing is increased when the model is modified to include the eddy m...

A new earth system climate model of intermediate complexity has been developed and its climatology compared to observations. The UVic Earth System Climate Model consists of a three‐dimensional ocean general circulation model coupled to a thermodynamic/dynamic sea‐ice model, an energy‐moisture balance atmospheric model with dynamical feedbacks, and a thermomechanical land‐ice model. In order to keep the model computationally efficient a reduced complexity atmosphere model is used. Atmospheric heat and freshwater transports are parametrized through Fickian diffusion, and precipitation is assumed to occur when the relative humidity is greater than 85%. Moisture transport can also be accomplished through advection if desired. Precipitation over land is assumed to return instantaneously to the ocean via one of 33 observed river drainage basins. Ice and snow albedo feedbacks are included in the coupled model by locally increasing the prescribed latitudinal profile of the planetary albedo. The atmospher...

[1] An Earth System model is used to explore the response of the oceanic and terrestrial carbon sinks to strengthening and poleward shifting of the extratropical Southern Hemisphere winds, which is a robust feature of climate models' response to greenhouse gas forcing through the 20th and 21st centuries. We find that under time-varying CO2 emissions poleward intensifying Southern Hemisphere winds act on average to slightly enhance the efficacy of both the oceanic and terrestrial carbon sinks, thus providing a small negative feedback on the atmospheric CO2 concentration. Regionally, the effects of the changing winds on oceanic and terrestrial carbon uptake are more pronounced and partly of opposite sign. We further show that the magnitude and sign of global oceanic CO2 uptake is also controlled by changes in mesoscale eddy activity, which has been suggested to increase in response to intensifying Southern Hemisphere winds.

[1] We have analyzed a suite of state-of-the-art climate model simulations, and show that anthropogenic warming of the upper ocean produces a detectable speed-up of low-latitude North Pacific oceanic planetary waves by the end of the 20th century. The projected percent increase in propagation speed for the end of the 21st century is about 35% (relative to pre-industrial) following one of the standard emissions scenarios from the Intergovernmental Panel on Climate Change. This remarkable simulated effect of oceanic warming on planetary wave propagation speed portends important observed change in interannual climate variability.

[1] The dependence on the mean climate state of the response of the Atlantic meridional overturning circulation (AMOC) is investigated in 17 increasing greenhouse gas experiments with different initial conditions. The AMOC declines in all experiments by 15% to 31%, with typically the largest declines in those experiments with the strongest initial AMOC. In all cases, changes in surface heat fluxes, rather than changes in surface freshwater fluxes, are the dominant cause for the transient AMOC decrease. Surface freshwater fluxes actually switch from reducing the transient AMOC decrease, for low values of atmospheric CO2, to reinforcing the transient AMOC decrease, for higher values of atmospheric CO2. In addition, we find that due to changes in the strengths of feedbacks associated with water vapour and snow/sea ice, the climate sensitivity and transient climate response of the UVic model strongly depends on the mean climate state.

[1] We investigate the role of atmospheric moisture advection in determining Southern Ocean water-mass properties in a coupled climate model. Two sensitivity experiments are presented in which the winds advecting moisture are either reduced or enhanced south of 45°S. It is shown that a variation in meridional winds in the Southern Ocean can generate large anomalies of subsurface potential temperature (T) and salinity (S). A reduction (increase) in southward atmospheric moisture advection tends to be balanced by a corresponding reduction (increase) in northward freshwater transport in Antarctic Intermediate Water. The response of both T and S is larger in the case of reduced southward moisture advection, as this corresponds to an overall reduction of Southern Ocean stratification. The interior ocean T–S response is not always density-compensating, particularly in the Indian and Pacific Oceans. This is unlike zonal wind variations which have a tendency to change T–S in a density-conserving manner. Our study suggests that variability in meridional winds over the Southern Ocean can alter local water-mass formation rates significantly.