Stability of the thermohaline circulation examined with a one-dimensional fluid loop

The Stommel box model elegantly demonstrates that the oceanic response to mixed boundary conditions, combining a temperature relaxation with a fixed salt flux forcing, is nonlinear owing to the so-called salt advection feedback. This nonlinearity produces a parameter range of bi-stability associated with hysteresis effects characterised by a fast thermally-driven mode and a slow salinity-driven mode. Here we investigate whether a similar dynamical behaviour can be found in the thermohaline loop model, a one-dimensional analogue of the box model. A semi-analytical method to compute possible steady states of the loop model is presented, followed by a linear stability analysis carried out for a large range of loop configurations. While the salt advection feedback is found as in the box model, a major difference is obtained for the fast mode: an oscillatory instability is observed near the turning point of the fast mode branch, such that the range of bi-stability is systematically reduced, or even removed, in some cases. The oscillatory instability originates from a salinity anomaly that grows exponentially as it turns around the loop, a situation that may occur only when the salinity torque is directed against the loop flow. Factors such as mixing intensity, the relative strength of thermal and haline forcings, the nonlinearity of the equation of state or the loop geometry can strongly affect the stability properties of the loop

Interacting internal waves explain global patterns of interior ocean mixing

Across the stable density stratification of the abyssal ocean, deep dense water is slowly propelled upward by sustained, though irregular, turbulent mixing. The resulting mean upwelling is key to setting large-scale oceanic circulation properties, such as heat and carbon transport. It is generally accepted that in the ocean interior, this turbulent mixing is caused mainly by breaking internal waves, which are predominantly generated by winds and tides, interact nonlinearly, thereby fluxing energy down to ever smaller scales, and finally become unstable, break and mix the water column. This paradigm forms the conceptual backbone of the widely used Finescale Parameterization. This formula estimates small-scale mixing from the readily observable internal wave activity at larger scales and theoretical scaling laws for the downscale nonlinear energy flux, but has never been fully explained theoretically. Here, we close this gap using wave-wave interaction theory with input from both localized high-resolution experiments and combined global observational datasets. We find near-ubiquitous agreement between our predictions, derived from first-principles alone, and the observed mixing patterns in the global ocean interior. Our findings lay the foundations for a new type of wave-driven mixing parameterization for ocean general circulation models that is entirely physics-based, which is key to reliably represent climate states that differ substantially from today's

Public Painting - Malerei und Handlung

Das Magazin "Public Painting – Malerei und Handlung" versammelt künstlerischen Projekte, die auf unterschiedliche Weise Verhältnisse von Malerei zum öffentlichen Raum reflektieren, und Verhältnisse thematisieren, die im öffentlichen Raum ausagiert werden. Die Beiträge entstanden als Resultat der Kooperation des kunstwissenschaftlichen Seminars "Malerei als Handlung" von Anne Röhl und des kunstpraktischen Seminars "public painting" von Sebastian Freytag während des Sommersemesters 2021 an der Universität Siegen. Unter dem Titel "public painting" wurden Schnittstellen von malerischer Praxis und öffentlichen Raum diskutiert. Die Studierenden des Seminars "Malerei als Handlung" haben sich wiederum damit beschäftigt, wie die Gattung Malerei im Verlauf des 20. Jahrhunderts durch die Abkehr von traditionellen Entstehungsprozessen und Malhandlungen befragt wurde und skulptural und/oder performativ wird. Daneben widmeten sich die Studierenden des kunsthistorischen Seminars der schriftlichen Auseinandersetzung mit den künstlerischen Resultaten des kunstpraktischen Seminars. Die Auseinandersetzung mit dem öffentlichen Raum stellte im Sommersemester 2021 eine besondere Herausforderung dar, da er pandemiebedingt nur eingeschränkt nutzbar war. Die künstlerischen Ergebnisse dokumentieren somit auch diese Zeit

Evaluating the Global Internal Wave Model IDEMIX Using Finestructure Methods

Small-scale turbulent mixing affects large-scale ocean processes such as the global overturning circulation but remains unresolvedin ocean models. Since the breaking of internal gravity waves is a major source of this mixing, consistent parameterizations take internal wave energetics into account. The model Internal Wave Dissipation,EnergyandMixing(IDEMIX)predictstheinternalwaveenergy,dissipationrates,anddiapycnal diffusivities based on a simplification of the spectral radiation balance of the wave field and can be used as a mixing module in global numerical simulations. In this study, it is evaluated against finestructure estimates of turbulent dissipation rates derived from Argo float observations. In addition, a novel method to compute internal gravity wave energy from finescale strain information alone is presented and applied. IDEMIX well reproducesthe magnitudeandthelarge-scalevariations ofthe Argo-derived dissipationrateandenergylevel estimates. Deficiencies arise with respect to the detailed vertical structure or the spatial extent of mixing hot spots. This points toward the need to improve the forcing functions in IDEMIX, both by implementing additionalphysicaldetailandbybetterconstrainingtheprocessesalreadyincludedinthemodel.Aprominent example is the energy transfer from the mesoscale eddies to the internal gravity waves, which is identified as an essential contributor to turbulent mixing in idealized simulations but needs to be better understood through the help of numerical, analytical, and observational studies in order to be represented realistically in ocean models

Effects of the Asymmetry between Surface and Interior Flow on the Dynamics of a Thermohaline Loop

International audienc

Parameterized Internal Wave Mixing in Three Ocean General Circulation Models

Abstract The non‐local model of mixing based on internal wave breaking, IDEMIX, is implemented as an enhancement of a turbulent kinetic energy closure model in three non‐eddy resolving general circulation ocean models that differ in the discretization and choice of computational grids. In IDEMIX internal wave energy is generated by an energy flux resulting from near‐inertial waves induced by wind forcing at the surface, and at the bottom, by an energy flux that parameterizes the transfer of energy between baroclinic and barotropic tides. In all model simulations with IDEMIX, the mixing work is increased compared to the reference solutions without IDEMIX, reaching values in better agreement with finestructure observations. Furthermore, the horizontal structure of the mixing work is more realistic as a consequence of the heterogeneous forcing functions. All models with IDEMIX simulate deeper thermocline depths related to stronger shallow overturning cells in the Indo‐Pacific. In the North Atlantic, deeper mixed layers in simulations with IDEMIX are associated with an increased Atlantic overturning circulation and an increase of northward heat transports toward more realistic values. The response of the deep Indo‐Pacific overturning circulation and the weak bottom cell of the Atlantic to the inclusion of IDEMIX is incoherent between the models, suggesting that additional unidentified processes and numerical mixing may confound the analysis. Applying different tidal forcing functions leads to simulation differences that are small compared to differences between the different models or between simulations with IDEMIX and without IDEMIX