coastal circulation, oceanic fronts, upwelling, dense water plumes, and convection, as well as sea-ice thickness distribution, concentration, deformations (including leads and polynyas), drift, and export. In particular, high spatial resolution is needed in the Arctic Ocean, where the local Rossby radius of deformation (determining the size of the smallest eddies) is on the order of 10 km or less, and exchanges with other oceans occur via narrow and shallow straits. Bryan et al. (2010) showed improvements in simulating the mean state as well as variability with an ocean model at 10 km versus 20-50 km spatial resolution, suggesting a regime change in approaching the 10 km resolution.

Besides improving stationary features such as those associated with terrain, increasing spatial resolution also allows transient eddies such as synoptic-scale frontal systems and local convective systems to be better represented. These transient eddies, as well as small-scale phenomena in the ocean-atmosphere system such as tropical cyclones, play important roles in the energy, moisture, and momentum transports that determine the mean climate and its variability.

Most current climate models divide the atmospheric column into 20-30 vertical layers, but some models include more than 50 layers with the increased vertical levels mostly added near the surface (to better resolve boundary-layer processes) or near the tropopause (to better simulate atmospheric waves and moisture advection).

Typical vertical resolution of ocean models that are part of climate models is 30-60 vertical layers, which could be at fixed depths or vary according to density or topography. Ocean models whose vertical grids extend to the ocean bottom are better able to represent the abyssal circulation. As in the case of atmospheric models, increased vertical resolution is added near the surface in order to better resolve the surface mixed layer and upper ocean stratification, as well as shelf and slope bathymetry. In addition, high vertical resolution is often needed near the bottom, especially to improve representation of bottom boundary-layer, density-driven gravity flows (e.g., over the Arctic shelves) and dense water overflows (e.g., Denmark Strait or Strait of Gibraltar).

Finally, there is evidence of feedbacks that are strongly dependent on model resolution and that therefore influence a model’s response to perturbations, for example:

•  atmospheric blocking, which is dependent on the feedbacks between the large-scale atmospheric circulation and mesoscale eddies (Jung et al., 2011);

•  feedbacks between western boundary currents with sharp temperature gradients in the ocean and the overlying atmospheric circulation (Bryan et al., 2010; Minobe et al., 2008);



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