Regional to Interhemispheric Connectivity of the Atlantic Ocean Circulation

This thesis investigates the connectivity and interaction of remote regions in the Atlantic Ocean based on high-resolution model experiments. Connectivity between remote regions has important implications on a range of spatial and temporal scales. It can affect global climate variability, the coherence of circulation changes on regional scales and the spreading of marine organisms. Based on several advancements in modelling, it is demonstrated how interhemispheric connectivity contributes to changes of the Atlantic Meridional Overturning Circulation (AMOC) on climate timescales. At the same time, the effect of wind-forcing and the interaction of individual AMOC pathways with eddies on regional scales are shown to be highly important to understand AMOC variability on sub-decadal timescales, with further implications on interdisciplinary research questions

Robust estimates for the decadal evolution of Agulhas leakage from the 1960s to the 2010s

Agulhas leakage, the transport of warm and salty waters from the Indian Ocean into the South Atlantic, has been suggested to increase under anthropogenic climate change, due to strengthening Southern Hemisphere westerly winds. The resulting enhanced salt transport into the South Atlantic may counteract the projected weakening of the Atlantic overturning circulation through warming and ice melting. Here we combine existing and new observation- and model-based Agulhas leakage estimates to robustly quantify its decadal evolution since the 1960s. We find that Agulhas leakage very likely increased between the mid-1960s and mid-1980s, in agreement with strengthening winds. Our models further suggest that increased leakage was related to enhanced transport outside eddies and coincided with strengthened Atlantic overturning circulation. Yet, it appears unlikely that Agulhas leakage substantially increased since the 1990s, despite continuously strengthening winds. Our results stress the need to better understand decadal leakage variability to detect and predict anthropogenic trends

Regional to Interhemispheric Connectivity of the Atlantic Ocean Circulation

This thesis investigates the connectivity and interaction of remote regions in the Atlantic Ocean based on high-resolution model experiments. Connectivity between remote regions has important implications on a range of spatial and temporal scales. It can affect global climate variability, the coherence of circulation changes on regional scales and the spreading of marine organisms. Based on several advancements in modelling, it is demonstrated how interhemispheric connectivity contributes to changes of the Atlantic Meridional Overturning Circulation (AMOC) on climate timescales. At the same time, the effect of wind-forcing and the interaction of individual AMOC pathways with eddies on regional scales are shown to be highly important to understand AMOC variability on sub-decadal timescales, with further implications on interdisciplinary research questions

A glimpse on ParaView - visualisation of 4D gridded simulation output

Contribution to the interactive lecture series "Visualisation in Ocean Research" at GEOMAR Short introduction into the open-source visualization software ParaView with examples and informations on support

VIKING20X_SST_5day_2000_2009

Sea Surface Temperature (5-daily average) with shading from Sea Surface Height overlayed with Sea Ice coverage from the high-resolution VIKING20X simulation for the period 2000 to 2009 (GEOMAR Ocean modelling group; contact: Klaus Getzlaff

Simulierte Obeflächentemperatur im Nordatlantik (VIKING20X)

Animation der simulierten 5-tägigen Oberflächentemperatur (SST) im Atlantik projeziert auf die Oberflächenauslenkung (SSH), zusätzlich überlagert mit der Eisdicke. Die Daten basieren auf den simulierten Jahren 2012 bis 2018 der hochauflösenden VIKING20X-Konfiguration. Die Animation wurde mit der Visualisierungssoftware Paraview (www.paraview.org) im Rahmen eines Rechenzeitprojektes am Deutschen Klimarechenzentrum (www.dkrz.de) erstellt

Simulated near-surface speed combined with ice cover from VIKING20X simulation

Visualisation of 5-daily near-surface speed (100m depth) projected on surface elevation combined with sea ice cover from the high-resolution VIKING20X simulation for the period 2000 to 200

Simulated see surface temperature combined with ice cover with an overlay of total cloud cover and windspeed from FOCI simulation

Visualisation of daily sea surface temperature projected on surface elevation combined with sea ice cover with an overlay of total cloud cover and windspeed from FOCI simulation for one year displaying the Southern Ocean/Atlantic region. This simulation was generated with the open-source software Paraview (www.paraview.org)

Animation of pathways of the upper limb of the overturning circulation in the South Atlantic, realized by tracking virtual fluid particles in an eddy-rich ocean model

The animation visualizes pathways of the upper limb of the overturning circulation in the South Atlantic by means of virtual fluid particle trajectories. The animation has been created with ParaView. The trajectories present a subset of the trajectories evaluated in Rühs et al. (2019). They have been calculated by using the ARIANE tool (version 2.2.6; Blanke and Raynauld, 1997; Blanke et al., 1999) in qualitative mode and 5-day mean 3D velocity output from the eddy-rich ocean-sea-ice model configuration INALT20 (experiment KFS044; Schwarzkopf et al., 2019). Specifically, particles were released over the full depth and width of the northward flowing North Brazil current at 6S and were then advected backwards in time until they reach the Pacific Ocean through Drake Passage or the Indian Ocean through the Agulhas Current system. Along-track temperatures were recorded and are visualized by the color of the trajectories (red=warm, blue=cold). The animation shows that water particles flowing from the Indian Ocean to the tropical Atlantic are relative warm, while particles entering from the Pacific Ocean tend to colder – the reason for the frequently used terminology “warm water route” and “cold water route”

Toward ocean hindcasts in earth system models: AMOC variability in a partially coupled model at eddying resolution.

While forced ocean hindcast simulations are useful for a wide range of applications, a key limitation is their inability to simulate ocean-atmosphere feedbacks. As a consequence, they need to rely on artificial choices such as sea surface salinity restoring and other corrections affecting the surface freshwater fluxes. Fully coupled models overcome these limitations, but lack the correct timing of variability due to weaker observational constraints. This leads to a mismatch between forced and coupled models on interannual to decadal timescales. A possibility to combine the advantages of both modeling strategies is to apply a partial coupling (PCPL), that is, replacing the surface winds stress in the ocean component by wind stress derived from reanalysis. To identify the capabilities, limitations and possible use cases of partial coupling, we perform a fully coupled, two partially coupled and an ocean-only experiment using an all-Atlantic nested ocean configuration at eddying resolution in a global climate model. We show that the correct timing of Atlantic Meridional Overturning Circulation (AMOC) variability in PCPL experiments is robust on timescales below 5 years. Mid-latitude wind stress curl changes contribute to decadal AMOC variability, but North Atlantic buoyancy fluxes are not significantly altered by incorporating reanalyzed wind stress anomalies, limiting the success of PCPL on this timescale. Long term trends of the AMOC in PCPL mode are consistent with fully coupled model experiments under historic atmospheric boundary conditions, suggesting that a partially coupled model is still able to simulate the important ocean-atmosphere feedbacks necessary to maintain a stable AMOC