Understanding mesoscale eddies and their interaction with the basin scale mean flow remains an important problem in physical oceanography. Several different approaches to parameterization of the effects of mesoscale eddies have been examined in the literature. In quasigeostrophic potential vorticity (PV) transfer theory, mesoscale eddies are assumed on average to transfer PV downgradient and the main free parameter is the PV diffusivity coefficient which is assumed to depend on the mean flow. Here we adopt a new, complementary approach which aims to develop strong constraints on the possible magnitude of the PV diffusivity due to parameters independent of the flow such as the wind stress and bottom topography. Combining results from an eddy resolving quasigeostrophic model and a corresponding analytic model with parameterised eddies, in a barotropic channel configuration, it is demonstrated that the PV diffusivity strongly varies for different types of bottom topography and for different wind stress with important consequences for the strength of the mean circulation. For monoscale (sinusoidal) topography an algebraic equation is developed linking the PV diffusivity coefficient with the transport, wind stress, bottom topography and geophysical and geometrical parameters. We present the result of statistical analysis of solutions of this equation with prescribed zonal transport, obtained from a number of the eddy resolving model simulations and propose a new equation linking the PV diffusivity coefficient with wind stress and a parameter related to topographic roughness. We anticipate that similar relationships will hold for more realistic flow configurations and other types of mesoscale eddy closures.

The causes of decadal variations in global warming are poorly understood, however it is widely understood that variations in ocean heat content are linked with variations in surface warming. To investigate the forced response of ocean heat content (OHC) to anthropogenic aerosols (AA), we use an ensemble of historical simulations, which were carried out using a range of anthropogenic aerosol forcing magnitudes in a CMIP6-era global circulation model. We find that the centennial scale linear trends in historical ocean heat content are significantly sensitive to AA forcing magnitude ($-3.0\pm0.1$ x10$^{5}$ (J m$^{-3}$ century$^{-1}$)/(W m$^{-2}$), R$^2$=0.99), but interannual to multi-decadal variability in global ocean heat content appear largely independent of AA forcing magnitude. Comparison with observations find consistencies in different depth ranges and at different time scales with all but the strongest aerosol forcing magnitude, at least partly due to limited observational accuracy. We find broad negative sensitivity of ocean heat content to increased aerosol forcing magnitude across much of the tropics and sub-tropics. The polar regions and North Atlantic show the strongest heat content trends, and also show the strongest dependence on aerosol forcing magnitude. However, the ocean heat content response to increasing aerosol forcing magnitude in the North Atlantic and Southern Ocean is either dominated by internal variability, or strongly state dependent, showing different behaviour in different time periods. Our results suggest the response to aerosols in these regions is a complex combination of influences from ocean transport, atmospheric forcings, and sea ice responses.

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.