Meridional heat transport in the North Atlantic region: Effects of ocean and atmosphere grid resolutions

The consequences of grid resolution refinement of both ocean and atmosphere components in and over the North Atlantic Ocean are examined in this study. The Flexible Ocean and Climate Infrastructure (FOCI) model is used to run three simulations carried out under constant 1950 forcing. Ocean and sea ice models run on the ORCA05.L46 grid; in two simulations, regional AGRIF grid refinement from 1/2˚ to 1/10˚ (VIKING10) is applied to the North Atlantic between 30˚ and ~80˚N. We also use two different atmospheric resolutions, Tco95.L91 (100km) and Tco319L137 (31 km) applied globally. The three combinations studied are: (1) coarse ocean and atmosphere, (2) refined ocean but coarse atmosphere, and (3) both components in the high-resolution configuration. Ocean grid refinement in regions of complex dynamics, such as the Gulf Stream and North Atlantic Current, is essential to capture mesoscale variabilities. Air-sea interaction and in particular surface heat fluxes only benefit from the explicit representation of mesoscale eddies, when also using the high-resolution atmosphere capturing a wider range of, for instance, the temperature distributions. 

We examine the ocean and atmospheric mean state changes over the North Atlantic and find significant changes: The Atlantic Meridional Overturning Circulation strengthens as we move to higher ocean and atmospheric resolution. Most parts of the North Atlantic Ocean surface become warmer and saltier in the high-resolution ocean configuration, but with the finer atmospheric grid, this warming reduces and transforms into a colder and fresher mean state. The surface cooling with increasing atmospheric resolution is a result of a reduced TOA imbalance. The mid-depth (500-1200m) ocean experiences strong cooling of more than 2˚C especially in the subtropics with the grid refinement in the ocean, a difference that is considerably weaker when also refining the atmosphere. The cooling in the subtropics has two reasons, a stronger gyre-gyre interaction mixing more subpolar water into the subtropical gyre and intensified deep mixing in the Labrador Sea (associated with a stronger overturning) in the high-resolution ocean compared to the coarser one.  Meridional volume and heat transports in the subtropical North Atlantic exhibit significant differences of up to 2-2.5 Sv with the change in resolution, which also suggests an influence by model resolution in the ocean and the air-sea interaction processes. With a higher ocean resolution, the oceanic heat transport into the Arctic increases by 0.15 PW at 62°N, causing a reduction of Arctic sea ice. Increasing the atmospheric resolution causes an expansion of Arctic sea ice, consistent with the overall surface cooling, but also changes the distribution of sea-ice thickness with thicker ice north of Greenland and thinner ice toward Russia, consistent with observations. Overall, the changes in the mean state of the ocean and atmosphere, as well as the feedback processes involved, will be discussed in this presentation, which we hope also benefits other modelling groups.