How ocean eddies can exert a collective, large-scale effect on the atmosphere at decadal time scales

Atmosphere and ocean are coupled via air--sea interactions. Atmospheric conditions fuel the ocean circulation and its variability, but the extent to which ocean processes can affect the atmosphere at decadal time scales remains unclear. We assess how the ocean's intrinsic variability leads to patterns of upper-ocean heat content that vary at decadal time scales. These patterns have the potential to feed back on the atmosphere and thereby affect climate modes of variability, such as El Niño or the Interdecadal Pacific Oscillation. We use the output from a global ocean--sea ice circulation model at three different horizontal resolutions, each driven by the same atmospheric reanalysis. Different atmospheric forcing schemes allow us to disentangle the variability of the ocean's direct response to atmospheric forcing from the variability due to intrinsic ocean dynamics. Models with coarse resolution that rely on eddy parameterizations, show (i) significantly reduced variance of the upper-ocean heat content at decadal time scales and (ii) differences in the spatial patterns of low-frequency variability compared with higher resolution simulations. These biases are prominent in eddy-rich regions like the ocean's mid-latitudes, western boundary currents, regions and the Southern Ocean. Climate projections are typically done with general circulation models with coarse-resolution ocean components. Therefore, these biases affect our ability to predict decadal climate modes of variability and, in turn, hinder climate projections. Our results suggest that the community should move towards coupled climate models with higher oceanic resolution.