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The Global Coupled 3 (GC3) configuration of the Met Office Unified Model is presented. Among other applications, GC3 is the basis of the United Kingdom's submission to the Coupled Model Intercomparison Project 6 (CMIP6). This paper documents the model components that make up the configuration (although the scientific descriptions of these components are in companion papers) and details the coupling between them. The performance of GC3 is assessed in terms of mean biases and variability in long climate simulations using present‐day forcing. The suitability of the configuration for predictability on shorter time scales (weather and seasonal forecasting) is also briefly discussed. The performance of GC3 is compared against GC2, the previous Met Office coupled model configuration, and against an older configuration (HadGEM2‐AO) which was the submission to CMIP5. In many respects, the performance of GC3 is comparable with GC2, however, there is a notable improvement in the Southern Ocean warm sea surface temperature bias which has been reduced by 75%, and there are improvements in cloud amount and some aspects of tropical variability. Relative to HadGEM2‐AO, many aspects of the present‐day climate are improved in GC3 including tropospheric and stratospheric temperature structure, most aspects of tropical and extratropical variability and top‐of‐atmosphere and surface fluxes. A number of outstanding errors are identified including a residual asymmetric sea surface temperature bias (cool northern hemisphere, warm Southern Ocean), an overly strong global hydrological cycle and insufficient European blocking.

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Models of the North Atlantic thermohaline circulation (THC) show a range of responses to the high-latitude warming and freshening characteristic of global warming scenarios. Most simulate a weakening of the THC, with some suggesting possible interruption of the circulation, but others exhibit little change. The mechanisms of the THC response to climate change using the HadCM3 coupled ocean-atmosphere general circulation model, which gives a good simulation of the present-day THC and does not require flux adjustment, were studied. In a range of climate change simulations, the strength of the THC in HadCM3 is proportional to the meridional gradient of steric height (equivalent to column-integrated density) between 30ЊS and 60ЊN. During an integration in which CO 2 increases at 2% per year for 70 yr, the THC weakens by about 20%, and it stabilizes at this level if the CO 2 is subsequently held constant. Changes in surface heat and water fluxes are the cause of the reduction in the steric height gradient that derives the THC weakening, 60% being due to temperature change (greater warming at high latitudes) and 40% to salinity change (decreasing at high latitude, increasing at low latitude). The level at which the THC stabilizes is determined by advective feedbacks. As the circulation slows down, less heat is advected northward, which counteracts the in situ warming. At the same time, northward salinity advection increases because of a strong increase in salinity in the subtropical Atlantic, due to a greater atmospheric export of freshwater from the Atlantic to the Pacific. This change in interbasin transport means that salinity effects stabilize the circulation, in contrast to a single basin model of the THC, where salinity effects are destabilizing. These results suggest that the response of the Atlantic THC to anthropogenic forcing may be partly determined by events occurring outside the Atlantic basin.

The Southern Ocean is a pivotal component of the global climate system yet it is poorly represented in climate models, with significant biases in upper-ocean temperatures, clouds and winds. Combining Atmospheric and Coupled Model Inter-comparison Project (AMIP5/CMIP5) simulations, with observations and equilibrium heat budget theory, we show that across the CMIP5 ensemble variations in sea surface temperature biases in the 40–60°S Southern Ocean are primarily caused by AMIP5 atmospheric model net surface flux bias variations, linked to cloud-related short-wave errors. Equilibration of the biases involves local coupled sea surface temperature bias feedbacks onto the surface heat flux components. In combination with wind feedbacks, these biases adversely modify upper-ocean thermal structure. Most AMIP5 atmospheric models that exhibit small net heat flux biases appear to achieve this through compensating errors. We demonstrate that targeted developments to cloud-related parameterisations provide a route to better represent the Southern Ocean in climate models and projections.