Simulation of historical ocean heat and salt content changes in the Atlantic basin in CMIP6 models

The Atlantic Multidecadal variability (AMV) is a multivariate climate phenomenon with wide societal impacts in the North Atlantic region and beyond. In order to gain insight into the circulation dynamics controlling the AMV, we calculate Atlantic upper ocean heat and salt budgets at the basin and sub basin scale, focussing on multi-year to multidecadal timescales, for the upper ocean using output from a subset of CMIP6 models which have the same ocean component (the NEMO model) at nominal horizontal resolutions of 1 degree, ¼ degree and 1/12 degree and corresponding atmosphere-forced ocean-only models. We decompose the advection term into geostrophic and ageostrophic components and further use a Reynolds type decomposition to understand contributions from time-mean versus transient components of the flow. We use a novel decomposition of the large-area heat budget which highlights contributions due to spatial covariance between the large scale circulation and temperature/salinity gradients. Finally we relate the spatial pattern of the heat and salt advection to the meridional overturning (zonal) and horizontal gyre (azonal) components of the flow.