Effect of horizontal resolution in North Atlantic mixing and ocean circulation in the EC-Earth3P HighResMIP simulations

Abstract. We investigate the impact of increasing horizontal model resolution on the oceanic mixing processes in the North Atlantic, their drivers, their link with the Atlantic Meridional Overturning Circulation (AMOC), and the propagation of newly generated dense waters through the deep western boundary current (DWBC). We use three versions of the EC-Earth Earth System Model, one of standard resolution (SR, ~ 1° in the ocean), one of high resolution (HR, ~ 0.25° in the ocean) and one of very high resolution (VHR, ~ 1/12° in the ocean). The higher resolutions allow for the explicit simulation of mesoscale processes that are parametrized at the coarse resolution, with additional improvements in ocean topography, boundary currents and air-sea interactions.

We find that the North Atlantic Oscillation plays a critical role in driving the mixed layer depth (MLD) in the Labrador Sea at HR and VHR. The three resolutions also show the influence of surface salinity signals in the mixing, with the VHR configuration showing a distinct slow propagation of these signals from the eastern subpolar gyre into the Labrador Sea. Furthermore, March MLD shows a strong positive bias in HR, which is reduced in VHR. In terms of the AMOC, resolution plays a pivotal role in shaping its response to the mixing. At the higher resolutions, the signal of the newly formed dense waters propagates faster along the better-resolved boundary current, indicating a shift from advective propagation to wave propagation of the signals. Additionally, the persistence of the AMOC responses to MLD is much shorter in VHR (less than 2 years) than for SR and HR, which exhibit longer-lived changes. These differences highlight how resolution affects both the timing and spatial reach of the AMOC changes.

Our study underscores the importance of model resolution in accurately simulating the North Atlantic's oceanic processes and their implications for the AMOC. While the VHR configuration offers a more realistic climatology of the Labrador Sea MLD, the results also demonstrate significant differences in variability and persistence across resolutions. These findings stress the need for high-resolution simulations to improve the understanding of deep ocean processes and their connection to larger climate systems, although they also highlight challenges in comparing simulated and observed data, particularly given the sparse historical observations and the lack of decadal variability in the model simulations.