ALBERT

Description: As a contribution towards improving the climate mean states of the atmosphere and the ocean in Earth System Models (ESMs), we compare several coupled simulations conducted with the Max Planck Institute for Meteorology Earth System Model (MPI-ESM) following the HighResMIP protocol. Our simulations allow to analyse the separate effects of increasing the horizontal resolution of the ocean (0.4° to 0.1°) and atmosphere (T127 to T255) submodels, and the effects of substituting the Pacanowski and Philander (PP) vertical ocean mixing scheme with the K-Profile Parameterization (KPP). The results show clearly distinguishable effects from all three factors. The eddy-resolving ocean removes biases in the ocean interior and in the atmosphere. This leads to an important conclusion that ocean eddies have a major impact on the large-scale temperature distribution in the atmosphere, and on temperature and salinity distributions in the ocean. The near-surface wind forcing reduces with a T255 atmosphere and improves ocean mixed layer depths in both hemisphere. The reduced wind forcing further slows the Antarctic Circumpolar Current (ACC) and reduces the transport through Drake Passage to observed values. In the North Atlantic, however, it causes a slow down of the Atlantic Meridional Overturning Circulation (AMOC) due to a slower subpolar gyre, when the PP scheme is used. The KPP scheme causes stronger open-ocean convection that spins up the gyres and leads to a stronger and stable AMOC, when coupled to the T255 atmosphere, maintaining all the positive effects of a higher resolved atmosphere.

Description: As a contribution towards improving the climate mean state of the atmosphere and the ocean in Earth system models (ESMs), we compare several coupled simulations conducted with the Max Planck Institute for Meteorology Earth System Model (MPI-ESM1.2) following the HighResMIP protocol. Our simulations allow to analyse the separate effects of increasing the horizontal resolution of the ocean (0.4 to 0.1∘) and atmosphere (T127 to T255) submodels, and the effects of substituting the Pacanowski and Philander (PP) vertical ocean mixing scheme with the K-profile parameterization (KPP). The results show clearly distinguishable effects from all three factors. The high resolution in the ocean removes biases in the ocean interior and in the atmosphere. This leads to the important conclusion that a high-resolution ocean has a major impact on the mean state of the ocean and the atmosphere. The T255 atmosphere reduces the surface wind stress and improves ocean mixed layer depths in both hemispheres. The reduced wind forcing, in turn, slows the Antarctic Circumpolar Current (ACC), reducing it to observed values. In the North Atlantic, however, the reduced surface wind causes a weakening of the subpolar gyre and thus a slowing down of the Atlantic meridional overturning circulation (AMOC), when the PP scheme is used. The KPP scheme, on the other hand, causes stronger open-ocean convection which spins up the subpolar gyres, ultimately leading to a stronger and stable AMOC, even when coupled to the T255 atmosphere, thus retaining all the positive effects of a higher-resolved atmosphere.

Description: In this study, the authors discuss two different parameterizations for the effect of mixed layer eddies, one based on ageostrophic linear stability analysis (ALS) and the other one based on a scaling of the potential energy release by eddies (PER). Both parameterizations contradict each other in two aspects. First, they predict different functional relationships between the magnitude of the eddy fluxes and the Richardson number (Ri) related to the background state. Second, they also predict different vertical structure functions for the horizontal eddy fluxes. Numerical simulations for two different configurations and for a large range of different background conditions are used to evaluate the parameterizations. It turns out that PER is better suited to capture the Ri dependency of the magnitude of the eddy fluxes. On the other hand, the vertical structure of the meridional eddy fluxes predicted by ALS is more accurate than that of PER, while the vertical structure of the vertical eddy fluxes is well predicted by both parameterizations. Therefore, this study suggests the use of the magnitude of PER and the vertical structure functions of ALS for an improved parameterization of mixed layer eddy fluxes.

Description: This thesis aims to provide a better understanding of the role of ageostrophic processes in ocean dynamics by analyzing three different case studies - the large-scale circulation, the mixing of eddies in the upper ocean and the ability of ageostrophic dynamics to feature a direct route to dissipation. Furthermore, it examines to which extent parameterizations can yield adequate simplifications of the more complex ageostrophic phenomena. The first case study concerns zonally averaged models of the large-scale meridional overturning circulation. Ageostrophic processes need to be considered here to correctly describe the dynamics in western boundary currents, while the interior ocean can be described by a geostrophic balance. Both, interior geostrophic and ageostrophic dynamics in the western boundary current need to be considered for the zonally averaged flow. It is illustrated that many zonally averaged models which do not consider both regimes show dynamical inconsistencies in comparison with zonally resolved models. A new parameterization for the zonally averaged flow is developed, in which both dynamical regimes are directly represented and which does not suffer from those inconsistencies. Zonally resolved models show good agreement with the new zonally averaged model, demonstrating that the new parameterization is dynamically consistent. The second case study deals with the mixing of eddies in the upper ocean. Since the stratification is often weak within the mixed layer, ageostrophic processes are likely to occur here. Two parameterizations for the eddy mixing are compared, which especially take ageostrophic dynamics into account. The first is based on linear stability analysis while the second is based on a scaling of the potential energy release. Numerical simulations for a wide range of dynamical conditions are used to diagnose the ability of these parameterizations to predict the mixing effect of the eddies. It turns out that the mean difference between both parameterizations and the diagnosed eddy fluxes is less than a factor of two. While the parameterization based on linear stability analysis performs slightly better in an equilibrated forced-dissipative flow scenario, the parameterization based on the scaling of the potential energy release performs better in a scenario of a re-stratifying density front. In addition it is found that the vertical structure of the eddy fluxes is better described by the former in both scenarios. The third case study investigates the role of ageostrophic dynamics for kinetic energy dissipation. Numerical simulations for a wide range of different dynamical conditions characterized by their Richardson number are used to diagnose the energy flux in wavenumber space. It is found that quasi-geostrophic dynamics feature an upscale kinetic energy flux while kinetic energy is transferred towards smaller scales for ageostrophic dynamics. Horizontal divergent velocities evolving under ageostrophic conditions can be identified to be responsible for the downscale flux. An important consequence is that the small-scale dissipation is larger in the presence of ageostrophic dynamics. To quantify the effect of ageostrophic dynamics on the small-scale dissipation, its dependency on the Richardson number is investigated and a power law relating the energy dissipation with the Richardson number is estimated.

Description: Simple idealized layered models and primitive equation models show that the meridional gradient of the zonally averaged pressure has no direct relation with the meridional flow. This demonstrates a contradiction in an often-used parameterization in zonally averaged models. The failure of this parameterization reflects the inconsistency between the model of Stommel and Arons and the box model of Stommel, as previously pointed out by Straub. A new closure is proposed. The ocean is divided in two dynamically different regimes: a narrow western boundary layer and an interior ocean; zonally averaged quantities over these regions are considered. In the averaged equations three unknowns appear: the interior zonal pressure difference Delta p(i), the zonal pressure difference Delta p(b) of the boundary layer, and the zonal velocity us at the interface between the two regions. Here Delta p(i) is parameterized using a frictionless vorticity balance, Delta p(b), by the difference of the mean pressure in the interior and western boundary, and u(delta) by the mean zonal velocity of the western boundary layer. Zonally resolved models, a layer model, and a primitive equation model validate the new parameterization by comparing with the respective zonally averaged counterparts. It turns out that the zonally averaged models reproduce well the buoyancy distribution and the meridional flow in the zonally resolved model versions with respect to the mean and time changes.

Description: In this study, an idealized eddy-resolving model is employed to examine the interplay between the downwelling, ocean convection and mesoscale eddies in the Labrador Sea and the spreading of dense water masses. The model output demonstrates a good agreement with observations with regard to the eddy field and convection characteristics. It also displays a basin mean net downwelling of 3.0 Sv. Our analysis confirms that the downwelling occurs near the west Greenland coast and that the eddies spawned from the boundary current play a major role in controlling the dynamics of the downwelling. The magnitude of the downwelling is positively correlated to the magnitude of the applied surface heat loss. However, we argue that this connection is indirect: the heat fluxes affect the convection properties as well as the eddy field, while the latter governs the Eulerian downwelling. With a passive tracer analysis we show that dense water is transported from the interior towards the boundary, predominantly towards the Labrador coast in shallow layers and towards the Greenland coast in deeper layers. The latter transport is steered by the presence of the eddy field. The outcome that the characteristics of the downwelling in a marginal sea like the Labrador Sea depend crucially on the properties of the eddy field emphasizes that it is essential to resolve the eddies to properly represent the downwelling and overturning in the North Atlantic Ocean, and its response to changing environmental conditions.

Description: State-of-the-art Earth system models typically employ grid spacings of O(100 km), which is too coarse to explicitly resolve main drivers of the flow of energy and matter across the Earth system. In this paper, we present the new ICON-Sapphire model configuration, which targets a representation of the components of the Earth system and their interactions with a grid spacing of 10 km and finer. Through the use of selected simulation examples, we demonstrate that ICON-Sapphire can (i) be run coupled globally on seasonal timescales with a grid spacing of 5 km, on monthly timescales with a grid spacing of 2.5 km, and on daily timescales with a grid spacing of 1.25 km; (ii) resolve large eddies in the atmosphere using hectometer grid spacings on limited-area domains in atmosphere-only simulations; (iii) resolve submesoscale ocean eddies by using a global uniform grid of 1.25 km or a telescoping grid with the finest grid spacing at 530 m, the latter coupled to a uniform atmosphere; and (iv) simulate biogeochemistry in an ocean-only simulation integrated for 4 years at 10 km. Comparison of basic features of the climate system to observations reveals no obvious pitfalls, even though some observed aspects remain difficult to capture. The throughput of the coupled 5 km global simulation is 126 simulated days per day employing 21 % of the latest machine of the German Climate Computing Center. Extrapolating from these results, multi-decadal global simulations including interactive carbon are now possible, and short global simulations resolving large eddies in the atmosphere and submesoscale eddies in the ocean are within reach.

Description: This paper quantifies spurious dissipation and mixing of various advection schemes in idealised experiments of lateral shear and baroclinic instabilities in numerical simulations of a re-entrant Eady channel for configurations with large and small Rossby numbers. In addition, a two-dimensional barotropic shear instability test case is used to examine numerical dissipation of momentum advection in isolation, without any baroclinic effects. Effects of advection schemes on the evolution of background potential energy and the dynamics of the restratification process are analysed. The advection schemes for momentum and tracers are considered using several different methods including a recently developed local dissipation analysis. As highly accurate but computationally demanding schemes we apply WENO and MP5, and as more efficient lower-order total variation diminishing (TVD) schemes we use among others the SPL-max-View the MathML source13 and a third-order-upwind scheme. The analysis shows that the MP5 and SPL-max-View the MathML source13 schemes provide the most accurate results. Following our comprehensive analysis of computational costs, the MP5 scheme is approximately 2.3 times more expensive in our implementation. In contrast to the configuration with a small Rossby number, in which significant differences between schemes are apparent, the different advection schemes behave similarly for a larger Rossby number. Regions of high numerical dissipation are shown to be associated with low grid Reynolds numbers. The major outcome of the present study is that generally positive global numerical dissipation and positive background potential energy evolution delay the restratification process.