arXiv subjects

Elian Vanderborght

Publications and source records attributed to Elian Vanderborght.

3 recordsLinked to original sources

arXiv2026

Multi-stability of Atlantic and Pacific overturning: The role of Freshwater Forcing Asymmetries and the Hydrological Cycle

A defining feature of the present-day global overturning circulation (GOC) is the absence of deep water formation in the Pacific, in contrast to the Atlantic. This asymmetry, associated with higher surface salinities in the North Atlantic, is reflected in the Atlantic Meridional Overturning Circulation (AMOC) and the lack of a Pacific overturning (PMOC). A commonly cited explanation is the asymmetry in surface freshwater fluxes, with the Pacific receiving more freshwater per unit area than the Atlantic. Here, we develop a two-basin conceptual ocean model, consisting of a wide and a narrow basin. The model admits three states: sinking confined to the narrow basin, sinking confined to the wide basin, and sinking in both basins. We analyze the (co-)existence of these states as a function of freshwater asymmetry and hydrological cycle strength, defined as the longitudinally symmetric freshwater flux. For a weak hydrological cycle, representative of warm Pliocene-like climate conditions, sinking occurs in both basins, with symmetry breaking only when one basin is sufficiently more evaporative. For intermediate conditions, representative of the present-day climate, the basin with slightly stronger evaporation tends to host sinking, with a stronger preference in the narrow basin. For a strong hydrological cycle, single-basin sinking states are preferred, although a large interbasin freshwater asymmetry is required to uniquely localize sinking. These results provide insight into GOC sinking configurations under past, present, and potential future climates, and show good agreement with a three-dimensional global circulation model.

physics.ao-ph↗

arXiv2025

A Reduced-Dimensional Model for the Interhemispheric Geostrophic Meridional Overturning Circulation

The Global Overturning Circulation (GOC) is a key component of the climate system, transporting heat, carbon, and salt throughout the global ocean. Previous reduced-dimensional models have sought to represent this three-dimensional circulation but often neglected three key observational features: (1) the meridional overturning circulation is in geostrophic balance below the Ekman layer, (2) diapycnal mixing is strongly enhanced near ocean boundaries, and (3) upwelling is partly driven by adiabatic dynamics in the Southern Ocean. Building on Callies and Marotzke (2012), we develop a reduced model that consistently incorporates all three by simulating temperature in latitude-depth space along the eastern and western boundaries of a semi-enclosed basin connected in the south to a zonally periodic re-entrant channel. The model clarifies how zonal temperature differences in the basin arise and are maintained through adiabatic and diffusive processes, giving rise to the geostrophic GOC. It also provides a transparent framework for understanding how geostrophic currents cross the equator to form the interhemispheric overturning, and how boundary-intensified mixing and Southern Ocean winds regulate polar downwelling rates. The reduced model shows good agreement with both a three-dimensional ocean model and theoretical scaling laws for stratification and overturning strength. Owing to its simplicity, it is well suited for long integrations exploring the GOC response under extreme forcing scenarios and offers a useful framework for testing eddy and mixing parameterizations.

physics.ao-ph↗

arXiv2024

Feedback Processes causing an AMOC Collapse in the Community Earth System Model

The Atlantic Meridional Overturning Circulation (AMOC) is recognized as a tipping element within the global climate system. Central to its tipping behavior is the salt-advection feedback mechanism, which has been extensively studied in box models and models of intermediate complexity. However, in contemporary, highly complex climate models, the importance and functioning of this feedback mechanism is less clear due to the intricate interplay of numerous ocean-atmosphere-sea ice feedbacks. In this study, we conduct a detailed mechanistic analysis of an AMOC collapse under quasi-equilibrium forcing conditions using the Community Earth System Model (CESM). By reconstructing the AMOC strength from the meridional density contrast across the Atlantic Ocean, we demonstrate that AMOC stability can be related to the Atlantic freshwater budget, revealing several important feedbacks. The dominant contribution is the destabilising salt-advection feedback, which is quantified through a negative sign of the overturning freshwater transport at 34$^{\circ}$S, indicated by $F_{\mathrm{ovS}}$. Other feedbacks are related to changes in North Atlantic sea-ice melt (destabilising), ocean-atmosphere freshwater fluxes (destabilising) and gyre transports (stabilising). Our study clarifies the role of $F_{\mathrm{ovS}}$ as an indicator of the background state stability of the AMOC. As many modern climate models have a positive $F_{\mathrm{ovS}}$ bias this implies that their AMOC is too stable which leads to an underestimation of the risk of an AMOC collapse under climate change.

physics.ao-ph↗