The deep western boundary current in an eddying ocean

Our understanding of the interaction between the large-scale ocean circulation and ocean mesoscale eddies is mainly based on those parts of the circulation and those eddies that lie in the upper ocean. In this dissertation, I use a 0.1 degree, eddy-resolving ocean model to reveal two so far unknown eddy phenomena that are distinct to eddies in the deep ocean and cannot be observed for the well-known eddies in the upper ocean. Firstly, eddies that are generated near the deep western boundary current (DWBC) in the Atlantic have a two-fold effect on mean density: Above the DWBC core (the level of maximum flow, ∼ 2000 m depth), they decrease the available potential energy of the mean flow by flat- tening isopycnals; this behavior is in agreement with our expectation from baroclinic instability theory and is furthermore the foundation of common eddy parameterizations. However, below the DWBC core, eddies systematically increase the available potential energy of the mean flow by steepening isopycnals. Two consequences arise from this anomalous eddy behavior: a so far unknown mean circulation normal to the DWBC evolves that balances the eddy effect on mean density; moreover, the steepening of isopycnals below the DWBC core can also be interpreted as a deepening of the DWBC. This eddy-induced deepening might serve as an explanation for a too shallow DWBC in coarse-resolution ocean models that do not resolve eddies and thus do not capture the effect. I think that the two-fold eddy effect might be a general property of eddies near deep mean currents and thus, it might be relevant in other regions of the world ocean that exhibit deep currents. Secondly, I observe different DWBC response behaviors in eddying and non-eddying ocean models that are subject to the same increase in surface wind stress: In the non-eddying model, the upper meridional overturning cell strengthens due to stronger winds over the southern ocean. For the DWBC, which closes both, the upper and bottom over- turning cell in the southward direction, this strengthening implies a speed-up; In the eddying model, the upper cell strengthens by roughly the same amount, however, the DWBC now slows down. This becomes possible if the bottom overturning weakens drastically so that the total DWBC transport can decrease. I show that the DWBC slow down is balanced by eddy fluxes of relative vorticity which are not present in the non-eddying model. Thereby, I can attribute the described response difference to whether eddies are resolved or not. For the real (eddying) ocean, this implies that the suggested link between the upper and the bottom overturning cell (’ocean seesaw’) might be weaker than previ- ously thought and that both cells are allowed to respond independently to forcing changes.Unser Verständnis von der Wechselwirkung zwischen der groß-skaligen Ozeanzirkulation und den meso-skaligen Ozeanwirbeln beruht im We- sentlichen auf den Teilen der Zirkulation und den Wirbeln, die im oberen Teil des Ozeans liegen. In dieser Dissertation verwende ich ein wirbelauflösendes Ozeanmodell mit 0,1 Grad Auflösung, um zwei bisher unbekannte Eigenschaften von Wirbeln im tiefen Ozean aufzu- decken. Diese Eigenschaften sind charakteristisch für tiefe Wirbel und können nicht für solche im oberen Ozean beobachtet werden. Erstens haben die tiefen Wirbel in der Nähe des tiefen westlichen Randstroms (TWR) im Atlantik zwei unterschiedliche Auswirkungen auf die mittlere Dichteverteilung: Oberhalb des TWR-Kerns (die Tiefe mit der maximalen Geschwindigkeit, ∼2000 m) verringern Wirbel die verfügbare potentielle Energie der mittleren Strömung durch Abfla- chung der Isopyknen; dieses Verhalten entspricht unserer Erwartung aus der Theorie der baroklinen Instabilität und ist darüber hinaus die Grundlage für gängige Wirbelparametrisierungen. Jedoch, unterhalb des TWR-Kerns erhöhen die Wirbel systematisch die verfügbare poten- tielle Energie der mittleren Strömung durch das Aufsteilen der Isopy- knen. Zwei Konsequenzen ergeben sich aus diesem ungewöhnlichen Verhalten der Wirbel: Es entsteht eine bisher unbekannte Zirkulation in der Ebene senkrecht zum TWR, die den Wirbel-Effekt auf die mittlere Dichte ausgleicht; darüber hinaus kann das Aufsteilen der Isopyknen unterhalb des TWR-Kerns auch als Vertiefung des TWRs interpretiert werden. Diese wirbelinduzierte Vertiefung könnte als Erklärung für einen zu flachen TWR in grob aufgelösten Ozeanmodellen dienen, die die Wirbel nicht auflösen und somit den genannten Effekt nicht erfas- sen. Möglicherweise ist die Tatsache, dass Wirbel zwei unterschiedliche Effekte auf die mittlere Dichte, abhängig von der Tiefe, haben, eine all- gemeine Eigenschaft von Wirbeln in der Nähe von tiefen Strömungen. Folglich könnte dies auch in anderen Regionen des Ozeans, in denen sich tiefe Strömungen befinden, relevant sein. Zweitens ist die Reaktion des TWRs auf eine Zunahme des Windes an der Meeresoberfläche unterschiedlich, abhängig davon, ob Wirbel in einem Modell aufgelöst werden oder nicht: Im nicht-wirbelauflösenden Modell verstärkt sich die obere Zelle der meridionalen Umwälzzirku- lation aufgrund der Zunahme der Winde über dem südlichen Ozean. Für den TWR, der sowohl die obere als auch die untere Zelle der Umwälzzirkulation in südlicher Richtung vervollständigt, bedeutet diese Verstärkung eine Beschleunigung; im wirbelauflösenden Modell verstärkt sich die obere Zelle um dasselbe Maß, jedoch wird der TWR nun langsamer. Dies ist möglich, wenn die untere Zelle der Umwälzzir- kulation deutlich schwächer wird, so dass der gesamte TWR-Transport abnehmen kann. Ich zeige, dass die Verlangsamung des TWR durch wirbelinduzierte Flüsse von relativer Vortizität ausgeglichen wird. Die- se Flüsse sind im nicht-wirbelauflösenden Modell nicht vorhanden. Somit können wir den beschriebenen Unterschied in der Reaktion des TWRs der beiden Modelle darauf zurückführen, ob Wirbel aufgelöst werden oder nicht. Für den realen Ozean (der Wirbel enthält) bedeutet dies, dass die angenommene Kopplung zwischen der oberen und der unteren Zelle der Umwälzzirkulation (’Ozeanwippe’) schwächer sein könnte als bisher vermutet und dass beide Zellen unabhängig von- einander eine Reaktion auf Änderungen im Strömungsantrieb zeigen können

Overturning response to a surface wind stress doubling in an eddying and a non-eddying ocean

In this paper, the overturning responses to wind stress changes of an eddying ocean and a non-eddying ocean are compared. Differences are found in the deep overturning cell in the low-latitude North Atlantic Ocean with substantial implications for the deep western boundary current (DWBC). In an ocean-only twin experiment with one eddying and one non-eddying configuration of the MPI ocean model, two different forcings are being applied: the standard NCEP forcing and the NCEP forcing with 2☓ surface wind stress. The response to the wind stress doubling in the Atlantic meridional overturning circulation is similar in the eddying and the non-eddying configuration, showing an increase by about 4 Sv (~25; 1 Sv = 106 m3 s-1). In contrast, the DWBC responds with a speedup in the non-eddying configuration and a slowdown in the eddying configuration. This paper demonstrates that the DWBC slowdown in the eddying configuration is largely balanced by eddy vorticity fluxes. Because those fluxes are not resolved and also not captured by an eddy parameterization in the non-eddying configuration, such a DWBC slowdown is likely not to occur in non-eddying ocean models, which therefore might not capture the whole range of overturning responses. Furthermore, evidence is provided that the balancing effect of the eddies is not a passive reaction to a remotely triggered DWBC slowdown. Instead, deep eddies that are sourced from the upper ocean provide an excess input of relative vorticity that then actively forces the DWBC mean flow to slow down. © 2021 American Meteorological Society

Nonlocal and local wind forcing dependence of the Atlantic meridional overturning circulation and its depth scale

We use wind sensitivity experiments to understand the wind forcing dependencies of the level of no motion and the e-folding pycnocline scale as well as their relationship to northward transport of the mid-depth Atlantic meridional overturning circulation (AMOC) south and north of the equator. In contrast to previous studies, we investigate the interplay of nonlocal and local wind effects on a decadal timescale. We use 30-year simulations with a high-resolution ocean general circulation model (OGCM) which is an eddy-resolving version of the Max Planck Institute Ocean Model (MPIOM). Our findings deviate from the common perspective that the AMOC is a nonlocal phenomenon only, because northward transport in the inter-hemispheric cell can only be understood by analyzing nonlocal Southern Ocean wind effects and local wind effects in the northern hemisphere downwelling region where Ekman pumping takes place. Southern Ocean wind forcing predominantly determines the magnitude of the pycnocline scale throughout the basin, whereas northern hemisphere winds additionally influence the level of no motion locally. In that respect, the level of no motion is a better proxy for northward transport and mid-depth velocity profiles despite the Ekman return flow which is found to be baroclinic. We compare our results inferred from the wind experiments and a 100-year global warming experiment in which the atmospheric CO2 concentration is quadrupled, using MPIOM coupled to an atmospheric model. We find that the evolution of the level of no motion in response to global warming represents changes in vertical velocity profiles or northward transport, whereas the changes of the pycnocline scale are opposite to the changes of the level of no motion over time. Using the level of no motion as depth scale, the analysis of the wind experiments and the warming experiment suggests a hemisphere-dependent scaling of the strength of AMOC. Furthermore, we put forward the idea that the ability of numerical models to capture the spatial and temporal variations of the level of no motion is crucial to reproduce the mid-depth cell in an appropriate wa

Diagnosing the influence of mesoscale eddy fluxes on the deep western boundary current in the 1/10° STORM / NCEP simulation

AbstractUsing a 0.1-degree ocean model, this paper establishes a consistent picture of the interaction of mesoscale eddy density fluxes with the geostrophic deep western boundary current (DWBC) in the Atlantic between 26°N and 20°S. Above the DWBC core (the level of maximum southward flow, ~2000 m depth), the eddies flatten isopycnals and hence decrease the potential energy of the mean flow, which agrees with their interpretation and parametrization in the Gent-McWilliams framework. Below the core, even though the eddy fluxes have a weaker magnitude, they systematically steepen isopycnals and thus feed potential energy to the mean flow, which contradicts common expectations. These two vertically separated eddy regimes are found through an analysis of the eddy density flux divergence in stream-following coordinates. In addition, pathways of potential energy in terms of the Lorenz energy cycle reveal this regime shift. The two-fold eddy effect on density is balanced by an overturning in the plane normal to the DWBC. Its direction is clockwise (with upwelling close to the shore and downwelling further offshore) north of the equator. In agreement with the sign change in the Coriolis parameter, the overturning changes direction to anti-clockwise south of the equator. Within the domain covered in this study, except in a narrow band around the equator, this scenario is robust along the DWBC