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a b s t r a c tSimulation characteristics from eighteen global ocean-sea-ice coupled models are presented with a focus on the mean Atlantic meridional overturning circulation (AMOC) and other related fields in the North Atlantic. These experiments use inter-annually varying atmospheric forcing data sets for the 60-year period from 1948 to 2007 and are performed as contributions to the second phase of the Coordinated Oceanice Reference Experiments (CORE-II). The protocol for conducting such CORE-II experiments is summarized. Despite using the same atmospheric forcing, the solutions show significant differences. As most models also differ from available observations, biases in the Labrador Sea region in upper-ocean potential temperature and salinity distributions, mixed layer depths, and sea-ice cover are identified as contributors to differences in AMOC. These differences in the solutions do not suggest an obvious grouping of the models based on their ocean model lineage, their vertical coordinate representations, or surface salinity restoring strengths. Thus, the solution differences among the models are attributed primarily to use of different subgrid scale parameterizations and parameter choices as well as to differences in vertical and horizontal grid resolutions in the ocean models. Use of a wide variety of sea-ice models with diverse snow and sea-ice albedo treatments also contributes to these differences. Based on the diagnostics considered, the majority of the models appear suitable for use in studies involving the North Atlantic, but some models require dedicated development effort.

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Analyses of sea surface height (SSH) records based on satellite altimeter data and hydrographic properties have suggested a considerable weakening of the North Atlantic subpolar gyre during the 1990s. Here we report hindcast simulations with high‐resolution ocean circulation models that demonstrate a close correspondence of the SSH changes with the volume transport of the boundary current system in the Labrador Sea. The 1990s‐decline, of about 15% of the long‐term mean, appears as part of a decadal variability of the gyre transport driven by changes in both heat flux and wind stress associated with the North Atlantic Oscillation (NAO). The changes in the subpolar gyre, as manifested in the deep western boundary current off Labrador, reverberate in the strength of the meridional overturning circulation (MOC) in the subtropical North Atlantic, suggesting the potential of a subpolar transport index as an element of a MOC monitoring system.

Large amounts of the greenhouse gas methane are released from the seabed to the water column 1 where it may be consumed by aerobic methanotrophic bacteria 2. This microbial filter is consequently the last marine sink for methane before its liberation to the atmosphere. The size and activity of methanotrophic communities, which determine the capacity of the water column methane filter, are thought to be mainly controlled by nutrient and redox dynamics 3-7 , but little is known about the effects of ocean currents. Here we show that cold bottom water at methane seeps west off Svalbard, containing a large number of aerobic methanotrophs, was rapidly displaced by warmer water with a considerably smaller methanotrophic community. This water mass exchange, caused by short-term variations of the West Spitsbergen Current, constitutes an oceanographic switch severely reducing methanotrophic activity in the water column. Strong and fluctuating currents are widespread oceanographic features common at many methane seep systems and are thus likely to globally affect methane oxidation in the ocean water column. Large amounts of methane are stored in the subsurface of continental margins as solid gas hydrates, gaseous reservoirs or dissolved in pore water 8. At cold seeps, various physical, chemical, and geological processes force subsurface methane to ascend along pathways of structural weakness to the sea floor where a portion of this methane is utilised by anaerobic and aerobic methanotrophic microbes 1,9. On a global scale, about 0.02 Gt yr-1 (3-3.5% of the atmospheric budget 10) of methane bypasses the benthic filter system and is liberated to the ocean water column 1 where some of it is oxidised aerobically (aerobic oxidation of methane-MOx) (ref 2), or less commonly where the water column is anoxic, anaerobically (anaerobic oxidation of methane-AOM) (refs 2, 11). MOx is performed by methanotrophic bacteria (MOB) typically belonging to the Gamma-(type I) or Alphaproteobacteria (type II) (refs 12, 13): CH 4 + 2 O 2 → CO 2 + 2 H 2 O Water column MOx is consequently the final sink for methane before its release to the atmosphere, where it acts as a potent greenhouse gas. The water column MOx filter could become more