A 250 year simulation of a strongly eddying global version of the Parallel Ocean Program (POP) model reveals a new mode of intrinsic multidecadal variability, the Southern Ocean Mode (SOM), with a period of 40–50 year. The peak‐to‐peak difference in the global ocean heat content within a multidecadal cycle is up to 60 ZJ. This change results from surface heat flux variations in the South Atlantic and propagation of temperature anomalies along the Antarctic Circumpolar Current and into the Weddell gyre around 30°E. The temperature anomalies propagate as deep as 5000 m along the isopycnals between 50°S and 30°S and induce multidecadal changes in the Atlantic Meridional Overturning Circulation. A positive feedback loop between the generation of eddies through baroclinic instability and the dynamics of the mean circulation is essential for the existence of the SOM. The dominant physics appears similar to that responsible for variability found in a three‐layer quasi‐geostrophic eddy‐resolving model. This combined with the fact that the SOM is not found in a noneddying version of the same global POP model further suggests that eddy processes are crucial for its existence and/or excitation.

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The role of standing eddies for the meridional overturning circulation (MOC) is discussed. The time-mean isopycnal meridional streamfunction is decomposed into a time- and zonal-mean part, a standing-eddy part, and a transient-eddy part. It turns out that the construction of an isopycnal MOC with an exactly vanishing standing-eddy part has to be performed by zonal integration along depth-dependent horizontal isolines of time-mean density. In contrast, zonal integration along time-mean geostrophic streamlines generally only leads to an isopycnal MOC with a reduced standing-eddy part. A generalized approach of constructing meridional transport streamfunctions by two tracer fields and the generalized way to neutralize the corresponding standing-eddy part is given to summarize the discussion. Using the results of an idealized Southern Ocean model, it is demonstrated that neglecting the depth dependence of the zonal integration paths by integrating along density contours or geostrophic streamlines of a fixed depth ("contour depth") may represent an acceptable approximation: although the standing-eddy part then exactly vanishes only at the contour depth (except for the ageostrophic surface layer using geostrophic streamlines), the overall standing-eddy part is significantly reduced for adequate contour depths. In the idealized Southern Ocean model, density contours at middepth and surface geostrophic streamlines represent the most adequate approximations. Moreover, it is found that the effect of changing the zonal integration paths from latitude circles to curvilinear paths on the zonally averaged density is of the same order as changing from Eulerian to isopycnal averaging

Over the last decade, our understanding of climate sensitivity has improved considerably. The climate system shows variability on many timescales, is subject to non-stationary forcing and it is most likely out of equilibrium with the changes in the radiative forcing. Slow and fast feedbacks complicate the interpretation of geological records as feedback strengths vary over time. In the geological past, the forcing timescales were different than at present, suggesting that the response may have behaved differently. Do these insights constrain the climate sensitivity relevant for the present day? In this paper, we review the progress made in theoretical understanding of climate sensitivity and on the estimation of climate sensitivity from proxy records. Particular focus lies on the background state dependence of feedback processes and on the impact of tipping points on the climate system. We suggest how to further use palaeo data to advance our understanding of the currently ongoing climate change

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A future collapse of the Atlantic Meridional Overturning Circulation (MOC) has been identified as one of the most dangerous tipping points in the climate system. It is therefore crucial to develop early warning indicators for such a potential collapse based on relatively short time series. So far, attempts to use indicators based on critical slowdown have been marginally successful. Based on complex climate network reconstruction, we here present a promising new indicator for the MOC collapse that efficiently monitors spatial changes in deep ocean circulation. Through our analysis of the performance of this indicator, we formulate optimal locations of measurement of the MOC to provide early warning signals of a collapse. Our results imply that an increase in spatial resolution of the Atlantic MOC observations (i.e., at more sections) can improve early detection, because the spatial coherence in the deep ocean arising near the transition is better captured.

Recently, multidecadal variability in the Southern Ocean has been found in a strongly eddying global ocean circulation model. In this paper, we study the Lorenz energy cycle of this so‐called Southern Ocean Mode (SOM). The Lorenz energy cycle analysis provides details on the energy pathways associated with the SOM. It shows that ocean eddies and the baroclinic energy pathway together with variations in the kinetic energy input by the wind are crucial aspects of the variability. It is also shown how convective mixing, which is induced by the SOM in particular in the Weddell Gyre, is responsible for the large‐scale multidecadal variability in Antarctic Bottom Water and Atlantic Meridional Overturning Circulation.

The climate impact of ocean gateway openings during the Eocene‐Oligocene transition is still under debate. Previous model studies employed grid resolutions at which the impact of mesoscale eddies has to be parameterized. We present results of a state‐of‐the‐art eddy‐resolving global ocean model with a closed Drake Passage and compare with results of the same model at noneddying resolution. An analysis of the pathways of heat by decomposing the meridional heat transport into eddy, horizontal, and overturning circulation components indicates that the model behavior on the large scale is qualitatively similar at both resolutions. Closing Drake Passage induces (i) sea surface warming around Antarctica due to equatorward expansion of the subpolar gyres, (ii) the collapse of the overturning circulation related to North Atlantic Deep Water formation leading to surface cooling in the North Atlantic, and (iii) significant equatorward eddy heat transport near Antarctica. However, quantitative details significantly depend on the chosen resolution. The warming around Antarctica is substantially larger for the noneddying configuration (∼5.5°C) than for the eddying configuration (∼2.5°C). This is a consequence of the subpolar mean flow which partitions differently into gyres and circumpolar current at different resolutions. We conclude that for a deciphering of the different mechanisms active in Eocene‐Oligocene climate change detailed analyses of the pathways of heat in the different climate subsystems are crucial in order to clearly identify the physical processes actually at work.