An ensemble of experiments based on a ¼° global NEMO configuration is presented, including tidally forced and non-tidal simulations, and using both the default z* geopotential vertical coordinate and the z~ filtered Arbitrary Lagrangian-Eulerian coordinate, the latter being known to reduce numerical mixing. This is used to investigate the sensitivity of numerical mixing, and the resulting model drifts and biases, to both tidal forcing and the choice of vertical coordinate. The model is found to simulate an acceptably realistic external tide, and the first-mode internal tide has a spatial distribution consistent with estimates from observations and high-resolution tidal models, with vertical velocities in the internal tide of over 50 meters per day. Tidal forcing with the z* coordinate increases numerical mixing in the upper ocean between 30°S and 30°N where strong internal tides occur, while the z~ coordinate substantially reduces numerical mixing and biases in tidal simulations to levels below those in the z* non-tidal control. The implications for the next generation of climate models are discussed.

Recent studies, using data from the OSNAP observational campaign and from numerical ocean models, suggest that surface buoyancy losses over the Iceland Basin and the Irminger Sea may, in contradiction to the established consensus, be more significant than those over the Labrador Sea, and that these former regions are in fact the dominant sites for formation of upper North Atlantic Deep Water), with the Labrador Sea acting mainly as a region of further densification as the dense waters flow around the gyre. Here we present a set of hindcast integrations of a global 1/4° NEMO ocean configuration from 1958 until nearly the present day, forced with three standard surface forcing datasets. We use the surface-forced streamfunction, estimated from surface buoyancy fluxes, along with the overturning streamfunction, similarly defined in potential density space, to investigate the causal link between surface forcing and decadal variability in the strength of the Atlantic meridional overturning circulation (AMOC). A scalar metric based on the surface forced streamfunction, evaluated in critical density and latitude classes, and accumulated in time, is found to be a good predictor of changes in the overturning strength, and the surface heat loss from the Irminger Sea is confirmed to be the dominant mechanism for decadal AMOC variability. We use the streamfunctions to demonstrate that the watermasses in the simulations are transformed to higher densities as they propagate around the subpolar gyre from their formation locations in the north-east Atlantic and the Irminger Sea, consistent with the picture emerging from observations.

A recognized deficiency of ocean models with a constant-depth vertical coordinate is for truncation errors in the advection scheme to result in spurious numerical mixing of tracers, which can be substantial larger than that prescribed by the model’s mixing scheme. The z~ vertical coordinate allows vertical levels to displace in a lagrangian fashion on time scales shorter than a few days, but reverts to fixed levels on longer timescales, and is intended to reduce numerical mixing from transient vertical motions such as internal waves and tides. An assessment of z~ in a ¼° global implementation of the NEMO model is presented. It is shown that, in the presence of near-inertial gravity waves in the North Atlantic, z~ significantly reduces eulerian vertical velocities with respect to those in a control simulation with the default z* vertical coordinate; that the vertical coordinate approaches an isopycnal, or adiabatic, surface on short timescales; and that both tendences are enhanced when the z~ timescale parameters are lengthened with respect to the default settings. Evaluation of an effective diapycnal diffusivity, based on density transformation rates, shows that numerical mixing is consistently reduced as the z~ timescales are lengthened. The realism of the model simulation with different timescale parameters is assessed in the global domain, and it is shown that drifts in temperature and salinity, and the spindown in z*of the Antarctic Circumpolar Current, are reduced with z~, without incurring significant penalties in other metrics such as the strength of the overturning circulation or sea ice cover.