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An idealized ice–ocean model is used to study the time-dependent Atlantic meridional overturning circulation (AMOC) responses to a sudden uniform surface warming and/or an amplified evaporation minus precipitation (E−P) forcing. At transient time scales, the AMOC initially weakens in response to both types of forcing as a result of buoyancy gain in the North Atlantic, but the amplified E−P response is an order of magnitude smaller when its amplitude is chosen based on the Clausius–Clapeyron scaling, consistent with its weaker initial buoyancy flux anomaly. At equilibrium, the AMOC here weakens under warming, contrasting with previous idealized modeling studies. The difference is attributed to a larger role of North Atlantic warming (acting to weaken the AMOC) and a weaker role of reduced brine rejection around Antarctica (acting to deepen and strengthen the AMOC). When E−P forcing is amplified, the AMOC strengthens, qualitatively consistent with a previously proposed passive response that predicts an enhancement of the existing salinity pattern in equilibrium, although the amplification of the salinity contrast is significantly damped by a negative salt advection feedback. For a small-amplitude change in both temperature and E−P, the AMOC response can be approximated by the linear combination of the individual responses. However, large-amplitude warming and amplified E−P forcing can lead to a positive salt advection feedback that collapses the AMOC in our simulations. To understand why the sign of the salt advection feedback varies across different simulations, its multifaceted roles are further investigated using box model theories, and their relevance to comprehensive models is discussed. 

Abstract Building on previous work using single-basin models, we here explore the time-dependent response of the Atlantic meridional overturning circulation (AMOC) to a sudden global temperature change in a two-basin ocean–ice model. We find that the previously identified mechanisms remain qualitatively useful to explain the transient and the long-term time-mean responses of the AMOC in our simulations. Specifically, we find an initial weakening of the AMOC in response to warming (and vice versa for cooling), controlled by the mid-depth meridional temperature contrast across the Atlantic basin. The long-term mean response instead is controlled primarily by changes in the abyssal stratification within the basin. In contrast to previous studies we find that for small-amplitude surface temperature changes, the equilibrium AMOC is almost unchanged, as the abyssal stratification remains similar due to a substantial compensation between the effects of salinity and temperature changes. The temperature-driven stratification change results from the differential warming/cooling between North Atlantic Deep Water and Antarctic Bottom Water, while the salinity change is driven by changes in Antarctic sea ice formation. Another distinct feature of our simulations is the emergence of AMOC variability in the much colder and much warmer climates. We discuss how this variability is related to variations in deep-ocean heat content, surface salinity, and sea ice in the deep convective regions, both in the North Atlantic and in the Southern Ocean, and its potential relevance to past and future climates. 

Abstract Although the reconfiguration of the abyssal overturning circulation has been argued to be a salient feature of Earth’s past climate changes, our understanding of the physical mechanisms controlling its strength remains limited. In particular, existing scaling theories disagree on the relative importance of the dynamics in the Southern Ocean versus the dynamics in the basins to the north. In this study, we systematically investigate these theories and compare them with a set of numerical simulations generated from an ocean general circulation model with idealized geometry, designed to capture only the basic ingredients considered by the theories. It is shown that the disagreement between existing theories can be partially explained by the fact that the overturning strengths measured in the channel and in the basin scale distinctly with the external parameters, including surface buoyancy loss, diapycnal diffusivity, wind stress, and eddy diffusivity. The overturning in the reentrant channel, which represents the Southern Ocean, is found to be sensitive to all these parameters, in addition to a strong dependence on bottom topography. By contrast, the basin overturning varies with the integrated surface buoyancy loss rate and diapycnal diffusivity but is mostly unaffected by winds and channel topography. The simulated parameter dependence of the basin overturning can be described by a scaling theory that is based only on basin dynamics. 

Abstract This study investigates the parameter dependence of eddy heat flux in a homogeneous quasigeostrophic two-layer model on a β plane with imposed environmental vertical wind shear and quadratic frictional drag. We examine the extent to which the results can be explained by a recently proposed diffusivity theory for passive tracers in two-dimensional turbulence. To account for the differences between two-layer and two-dimensional models, we modify the two-dimensional theory according to our two-layer f -plane analyses reported in an earlier study. Specifically, we replace the classic Kolmogorovian spectral slope, −5/3, assumed to predict eddy kinetic energy spectrum in the former with a larger slope, −7/3, suggested by a heuristic argument and fit to the model results in the latter. It is found that the modified theory provides a reasonable estimate within the regime where both and the strength of the frictional drag, , are much smaller than unity (here, c D is the nondimensional drag coefficient divided by the depth of the layer, k d is the wavenumber of deformation radius, and U is the imposed background vertical wind shear). For values of and that are closer to one, the theory works only if the full spectrum shape of the eddy kinetic energy is given. Despite the qualitative, fitting nature of this approach and its failure to explain the full parameter range, we believe its documentation here remains useful as a reference for the future attempt in pursuing a better theory. 

Abstract The use of coarse resolution and strong grid‐scale dissipation has prevented global ocean models from simulating the correct kinetic energy level. Recently parameterizing energy backscatter has been proposed to energize the model simulations. Parameterizing backscatter reduces long‐standing North Atlantic sea surface temperature (SST) and associated surface current biases, but the underlying mechanism remains unclear. Here, we apply backscatter in different geographic regions to distinguish the different physical processes at play. We show that an improved Gulf Stream path is due to backscatter acting north of the Grand Banks to maintain a strong deep western boundary current. An improved North Atlantic Current path is due to backscatter acting around the Flemish Cap, with likely an improved nearby topography‐flow interactions. These results suggest that the SST improvement with backscatter is partly due to the resulted strengthening of resolved currents, whereas the role of improved eddy physics requires further research. 

Abstract. We describe an idealized primitive-equation model for studying mesoscale turbulence and leverage a hierarchy of grid resolutions to make eddy-resolving calculations on the finest grids more affordable.The model has intermediate complexity, incorporating basin-scale geometry with idealized Atlantic and Southern oceans and with non-uniform ocean depth to allow for mesoscale eddy interactions with topography.The model is perfectly adiabatic and spans the Equator and thus fills a gap between quasi-geostrophic models, which cannot span two hemispheres, and idealized general circulation models, which generally include diabatic processes and buoyancy forcing.We show that the model solution is approaching convergence in mean kinetic energy for the ocean mesoscale processes of interest and has a rich range of dynamics with circulation features that emerge only due to resolving mesoscale turbulence. 

Abstract An idealized aquaplanet moist global atmospheric model with realistic radiative transfer but no clouds and no convective parameterization is found to possess multiple climate equilibria. When forced symmetrically about the equator, in some cases the Inter Tropical Convergence Zone (ITCZ) migrates to an off‐equatorial equilibrium position. Mechanism denial experiments prescribing relative humidity imply that radiation‐circulation coupling is essential to this instability. The cross‐equatorial asymmetry occurs only when the underlying slab ocean is sufficiently deep and the atmosphere's spectral dynamical core is sufficiently coarse (∼T170 or less with our control parameters). At higher resolutions, initializing with an asymmetric state indicates metastability with very slow (thousands of days) return to hemispheric symmetry. There is some sensitivity to the model timestep, which affects the time required to transition to the asymmetric state, with little effect on the equilibrium climate. The instability is enhanced when the planetary boundary layer scheme favors deeper layers or by a prescribed meridional heat transport away from the equator within the slab. The instability is not present when the model is run with a convective parameterization scheme commonly utilized in idealized moist models. We argue that the instability occurs when the asymmetric heating associated with a spontaneous ITCZ shift drives a circulation that rises poleward of the perturbed ITCZ. These results serve as a warning of the potential for instability and non‐uniqueness of climate that may complicate studies with idealized models of the tropical response to perturbations in forcing. 

In idealized models of the extratropical troposphere, both β and surface friction can control the equilibrated scales of baroclinic eddies by stopping the inverse cascade. A scaling theory on how surface friction alone sets these scales was proposed by Held in 1999 in the case of a quadratic drag law. However, the theory breaks down when friction is modeled by linear damping, and there are other reasons to suspect that it is oversimplified. An ideal system to test the theory is the homogeneous two-layer quasigeostrophic model in the β = 0 limit with quadratic damping. This study investigates some numerical simulations of the model to analyze two causes of the theory’s breakdown. They are 1) the asymmetry between two layers due to confinement of friction to the lower layer and 2) deviation from a spectrally local inverse energy cascade due to the spread of wavenumbers over which energy is input into the barotropic mode. The former is studied by comparing the simulations with drag appearing asymmetrically or symmetrically between the two layers. The latter is addressed with a heuristic modification of the theory. A regime where eddies equilibrate without an inverse cascade is also examined. A comparison is then made between quadratic and linear drag simulations. The connection to a competing theory based on the dynamics of equivalent barotropic vortices with thermal signatures is further discussed. Finally, we present an example of an inhomogeneous statistically steady state to argue that the diffusivity obtained from the homogeneous model has relevance to more realistic configurations.