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Any type of non-buoyant material in the ocean is transported horizontally by currents during its sinking journey. This lateral transport can be far from negligible for small sinking velocities. To estimate its magnitude and direction, the material is often modelled as a set of Lagrangian particles advected by current velocities that are obtained from Ocean General Circulation Models (OGCMs). State-of-the-art OGCMs are strongly eddying, similar to the real ocean, providing results with a spatial resolution on the order of 10 km on a daily frequency. While the importance of eddies in OGCMs is well-appreciated in the physical oceanographic community, other marine research communities may not. Further, many long term climate modelling simulations (e.g. in paleoclimate) rely on lower spatial resolution models that do not capture mesoscale features. To demonstrate how much the absence of mesoscale features in low-resolution models influences the Lagrangian particle transport, we simulate the transport of sinking Lagrangian particles using low- and high-resolution global OGCMs, and assess the lateral transport differences resulting from the difference in spatial and temporal model resolution. We find major differences between the transport in the non-eddying OGCM and in the eddying OGCM. Addition of stochastic noise to the particle trajectories in the non-eddying OGCM parameterises the effect of eddies well in some cases (e.g. in the North Pacific gyre). The effect of a coarser temporal resolution (once every 5 days versus monthly) is smaller compared to a coarser spatial resolution (0.1° versus 1° horizontally). We recommend to use sinking Lagrangian particles, representing e.g. marine snow, microplankton or sinking plastic, only with velocity fields from eddying Eulerian OGCMs, requiring high-resolution models in e.g. paleoceanographic studies. To increase the accessibility of our particle trace simulations, we launch planktondrift.science.uu.nl, an online tool to reconstruct the surface origin of sedimentary particles in a specific location.

In modern environmental and climate science it is necessary to assimilate observational datasets collected over decades with outputs from numerical models, to enable a full understanding of natural systems and their sensitivities. During the twentieth and twenty-first centuries, numerical modelling became central to many areas of science from the Bohr model of the atom to the Lorenz model of the atmosphere. In modern science, a great deal of time and effort is devoted to developing, evaluating, comparing and modifying numerical models that help us synthesise our understanding of complex natural systems. Here we provide an assessment of the contribution of past (palaeo) climate modelling to multidisciplinary science and to society by answering the following question: What can palaeoclimate modelling do for you? We provide an assessment of how palaeoclimate modelling can develop in the future to further enhance multidisciplinary research that aims to understand Earth’s evolution, and what this may tell us about the resilience of natural and social systems as we enter the Anthropocene.

Microfossils from plankton are used for paleoceanographic reconstructions. An often‐made assumption in quantitative microplankton‐based paleoceanographic reconstructions is that sedimentary assemblages represent conditions of the directly overlying surface water. However, any immobile particle sinking down the water column is subjected to transport by three‐dimensional currents, which results in a lateral relocation along transport. We model dinoflagellate cyst (dinocyst) transport in a high‐resolution (0.1° horizontally) global model of the present‐day ocean and compare ocean conditions in the simulated origin of sedimentary particles to that in the directly overlying water. We find that the assumption that sedimentary particles represent the overlying surface waters is in most regions not valid. The bias induced by dinocyst transport depends on ocean current strength and direction, aggregation of particles which could increase the sinking speed, and the sediment sample depth. By using realistic sinking speeds of dinocysts and aggregates, extreme biases up to approximately ±16 °C warmer or ±4 PSU saltier are found, while other regions show lower bias from particle transport. Our model results provide a way to mechanistically and statistically explain the unexpected occurrences of some dinocyst species outside of their “normal” occurrence region, such as the northerly occurrence of the allegedly sea‐ice‐affiliated dinocyst Selenopemphix antarctica. Exclusion of such outlier occurrences will yield better constrained ecological affinites for dinocyst species, which has implications for microfossil‐based quantitative and qualitative proxies for paleoceanographic conditions. We recommend paleoceanographers to a priori evaluate the (paleo)water depth, oceanographic setting, current strength, and particle aggregation probability for their sedimentary microplankton assemblages.

We present a study of a delay differential equation (DDE) model for the Mid-Pleistocene Transition (MPT). We investigate the behavior of the model when subjected to periodic forcing. The unforced model has a bistable region consisting of a stable equilibrium along with a large amplitude stable periodic orbit. We study how forcing affects solutions in this region. Forcing based on astronomical data causes a sudden transition in time and under increase of the forcing amplitude, moving the model response from a non-MPT regime to an MPT regime. Similar transition behavior is found for periodic forcing. A bifurcation analysis shows that the transition is not due to a bifurcation but instead to a shifting basin of attraction. While determining the basin boundary we demonstrate how one can accurately compute the intersection of a stable manifold of a saddle with a slow manifold in a DDE by embedding the algorithm for planar maps proposed by England et al. (SIADS 2004(3)) into the equation-free framework by Kevrekidis et al. (Rev. Phys. Chem. 2009 (60))., Comment: 18 pages, supplementary material available on https://doi.org/10.6084/m9.figshare.7048292.v1

Climate sensitivity represents the global mean temperature change caused by changes in the radiative balance of climate; it is studied for both present/future (actuo) and past (paleo) climate variations, with the former based on instrumental records and/or various types of model simulations. Paleo-estimates are often considered informative for assessments of actuo-climate change caused by anthropogenic greenhouse forcing, but this utility remains debated because of concerns about the impacts of uncertainties, assumptions, and incomplete knowledge about controlling mechanisms in the dynamic climate system, with its multiple interacting feedbacks and their potential dependence on the climate background state. This is exacerbated by the need to assess actuo- and paleoclimate sensitivity over different timescales, with different drivers, and with different (data and/or model) limitations. Here, we visualize these impacts with idealized representations that graphically illustrate the nature of time-dependent actuo- and paleoclimate sensitivity estimates, evaluating the strengths, weaknesses, agreements, and differences of the two approaches. We also highlight priorities for future research to improve the use of paleo-estimates in evaluations of current climate change.

It is well known that periodic forcing of a nonlinear system, even of a two-dimensional autonomous system, can produce chaotic responses with sensitive dependence on initial conditions if the forcing induces sufficient stretching and folding of the phase space. Quasiperiodic forcing can similarly produce chaotic responses, where the transition to chaos on changing a parameter can bring the system into regions of strange non-chaotic behaviour. Although it is generally acknowledged that the timings of Pleistocene ice ages are at least partly due to Milankovitch forcing (which may be approximated as quasiperiodic, with energy concentrated near a small number of frequencies), the precise details of what can be inferred about the timings of glaciations and deglaciations from the forcing is still unclear. In this paper, we perform a quantitative comparison of the response of several low-order nonlinear conceptual models for these ice ages to various types of quasiperiodic forcing. By computing largest Lyapunov exponents and mean periods, we demonstrate that many models can have a chaotic response to quasiperiodic forcing for a range of forcing amplitudes, even though some of the simplest conceptual models do not. These results suggest that pacing of ice ages to forcing may have only limited determinism.

Centennial climate variability appears in several long records of climate observables. Understanding the processes responsible for this internally generated variability can be achieved by a combination of more observational data and the definition of falsifiable criteria for specific physical mechanisms

The evidence from both data and models indicates that specific equilibrium climate sensitivity S[X] — the global annual mean surface temperature change (DTg) as a response to a change in radiative forcing X (DR[X]) — is state-dependent. Such a state dependency implies that the best fit in the scatter plot of (DTg versus DR[X] is not a linear regression, but can be some non-linear or even non-smooth function. While for the conventional linear case the slope (gradient) of the regression is correctly interpreted as the specific equilibrium climate sensitivity S[X], the interpretation is not straightforward in the non-linear case. We here explain how such a state-dependent scatter plot needs to be interpreted, and provide a theoretical understanding — or generalization — how to quantify S[X] in the non-linear case. Finally, from data covering the last 2.1 Myr we show that — due to state dependency — the specific equilibrium climate sensitivity which considers radiative forcing of CO2 and land ice sheet (LI) albedo, S[CO2;LI], is larger during interglacial states than during glacial conditions by more than a factor two.

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.

A multichannel singular spectrum analysis (MSSA) applied simultaneously to tropical sea surface temperature (SST), zonal wind, and burstiness (zonal wind variability) reveals three significant oscillatory modes. They all show a strong ENSO signal in the eastern Pacific Ocean (PO) but also a substantial SST signal in the western Indian Ocean (IO). A correlation-based analysis shows that the western IO signal contains linearly independent information on ENSO. Of the three Indo-Pacific ENSO modes of the MSSA, one resembles a central Pacific (CP) El Niño, while the others represent eastern Pacific (EP) El Niños, which either start in the central Pacific and grow eastward (EPe) or start near Peru and grow westward (EPw). A composite analysis shows that EPw El Niños are preceded by cooling in the western IO about 15 months earlier. Two mechanisms are discussed by which the western IO might influence ENSO. In the atmospheric bridge mechanism, subsidence over the cool western IO in autumn (year 0) leads to enhanced convection above Indonesia, strengthening easterlies over the western PO, and the creation of a large warm water volume. This is essential for the creation of (EP) El Niños in the following spring–summer. In the state-dependent noise mechanism, a cool western IO favors a strong intraseasonal zonal wind variability over the western PO in early spring (year 1), which can partly be attributed to the Madden–Julian oscillation. This intraseasonal variability induces Kelvin waves, which in early spring lead to a strong warming of the eastern PO and can initiate EPw El Niños.

It is still an open question how equilibrium warming in response to increasing radiative forcing – the specific equilibrium climate sensitivity S – depends on background climate. We here present palaeodata-based evidence on the state dependency of S, by using CO2 proxy data together with a 3-D ice-sheet-model-based reconstruction of land ice albedo over the last 5 million years (Myr). We find that the land ice albedo forcing depends non-linearly on the background climate, while any non-linearity of CO2 radiative forcing depends on the CO2 data set used. This non-linearity has not, so far, been accounted for in similar approaches due to previously more simplistic approximations, in which land ice albedo radiative forcing was a linear function of sea level change. The latitudinal dependency of ice-sheet area changes is important for the non-linearity between land ice albedo and sea level. In our set-up, in which the radiative forcing of CO2 and of the land ice albedo (LI) is combined, we find a state dependence in the calculated specific equilibrium climate sensitivity, S[CO2,LI], for most of the Pleistocene (last 2.1 Myr). During Pleistocene intermediate glaciated climates and interglacial periods, S[CO2,LI] is on average ~ 45 % larger than during Pleistocene full glacial conditions. In the Pliocene part of our analysis (2.6–5 Myr BP) the CO2 data uncertainties prevent a well-supported calculation for S[CO2,LI], but our analysis suggests that during times without a large land ice area in the Northern Hemisphere (e.g. before 2.82 Myr BP), the specific equilibrium climate sensitivity, S[CO2,LI], was smaller than during interglacials of the Pleistocene. We thus find support for a previously proposed state change in the climate system with the widespread appearance of northern hemispheric ice sheets. This study points for the first time to a so far overlooked non-linearity in the land ice albedo radiative forcing, which is important for similar palaeodata-based approaches to calculate climate sensitivity. However, the implications of this study for a suggested warming under CO2 doubling are not yet entirely clear since the details of necessary corrections for other slow feedbacks are not fully known and the uncertainties that exist in the ice-sheet simulations and global temperature reconstructions are large.

From an analysis using almost 20 years of satellite altimetry data and in situ observations, we have shown thatat the southern tip of Madagascar the SEMC breaks up in a series of nearly symmetric dipolar vortex pairs of approximately zero net relative vorticity. They appear at a frequency of about 4–6 per year. Most vortex pairs break up shortly after their formation and interact with previously formed cyclones and anticyclones. The velocity shear across the SEMC is so large that the dynamics of the evolution of the dipole is dominantly nonlinear. A surprising consequence is that there is no retroflection of the SEMC nor does it continue as an unstable westward jet. Instead the lobes develop symmetrically and the dynamics appear more similar to that of modon formation [Flierl et al., 1983]. The EKE south of Madagascar was shown to correlate with the strength of the SEMC prior to separation. Such a relationship could not be found by Palastanga et al. [2006] probably because they used a more upstream satellite transect to determine the surface velocity of the SEMC. Within the SEMC, there is a short decorrelation length scale (also observed in the Gulf Stream [Sturges and Hong, 2001]), and the velocity at our transect does not significantly correlate with the velocity at locations further than 450 km upstream. The very early retroflection of the AC in 2001 was an exceptional one. Only one more appeared in the 18 year period (in 2008). In both cases, it needed a concerted action of cyclonic meanders of the AC and the ARC to shortcut the retroflection loop with an active role of the Madagascar dipoles in generating the Agulhas cyclones. Interannual variability in the separation region appeared related with the ENSO cycle, but not significantly with the IOD. Significant correlations were found between the NINO3.4 index and both the strength of the SEMC and the EKE southwest of Madagascar. Positive and negative lags are between 250 and 500 days. Preceding a positive (negative) ENSO phase, stronger (weaker) velocities in the SEMC were observed. Following a positive (negative) phase, weaker (stronger) velocities in the SEMC were observed. We are presently investigating the possible explanations of these relations and their impact.

The present-day global meridional overturning circulation (MOC) with formation of North Atlantic Deep Water (NADW) and the absence of a deep-water formation in the North Pacific is often considered to be caused by the fact that the North Pacific basin is a net precipitative, while the North Atlantic is a net evaporative basin. In this paper, the authors study the effect of asymmetries in continent geometry and freshwater fluxes on the MOC both in an idealized two-dimensional model and in a global ocean model. This study approaches the problem from a multiple equilibria perspective, where asymmetries in external factors constrain the existence of steady MOC patterns. Both this multiple equilibria perspective and the fact that a realistic global geometry is used add new aspects to the problem. In the global model, it is shown that the Atlantic forced by net precipitation can have a meridional overturning circulation with northern sinking and a sea surface salinity that resembles the present-day salinity field. The model results are suggestive of the importance of factors other than the freshwater flux asymmetries, in particular continental asymmetries, in producing the meridional overturning asymmetry.

The most marked step in the global climate transition from “Greenhouse” to “Icehouse” Earth occurred at the Eocene-Oligocene (E-O) boundary, 33.7 Ma. Evidence for climatic changes comes from many sources, including the marine benthic 18O record, showing an increase by 1.2– 1.5‰ at this time. This positive excursion is characterised by two steps, separated by a plateau. The increase in 18O values has been attributed to rapid glaciation of the Antarctic continent, previously ice-free. Simultaneous changes in the 13C record are suggestive of a greenhouse gas control on climate. Previous modelling studies show that a decline in pCO2 beyond a certain threshold value may have initiated the growth of a Southern Hemispheric ice sheet. These studies were not able to conclusively explain the remarkable two-step profile in 18O. Furthermore, they considered changes in the ocean circulation only regionally, or indirectly through the oceanic heat transport. The potential role of global changes in ocean circulation in the E-O transition has not been addressed yet. Here a new interpretation of the 18O signal is presented, based on model simulations using a simple coupled 8-box-ocean, 4-box-atmosphere model with an added land ice component. The model was forced with a slowly decreasing atmospheric carbon dioxide concentration. It is argued that the first step in the 18O record reflects a shift in meridional overturning circulation from a Southern Ocean to a bipolar source of deep-water formation, which is associated with a cooling of the deep sea. The second step in the 18O profile occurs due to a rapid glaciation of the Antarctic continent. This new mechanism is a robust outcome of our model and is qualitatively in close agreement with proxy data.

Whether interannual variability in the Pacific Ocean was a feature of the warm Pliocene climate is debated. Variance in reconstructed eastern tropical Pacific surface temperatures provides strong support for persistent El Niño activity at this time.

It has been suggested that a “permanent” El Ni˜no climate state has existed in the warm Pliocene. One of the main pieces of evidence of such conditions is the small eastwest sea surface temperature (SST) difference that is found in proxy temperature records of the equatorial Pacific. Using a coupled version of the Zebiak-Cane model of intermediate complexity for the tropical Pacific, we study the sensitivity of the time-mean Pacific background state and El Ni˜no/Southern Oscillation (ENSO) variability to Pliocene climate changes. The parameters varied in this sensitivity study include changes in the trade wind strength due to a reduced equator-to-pole temperature gradient, higher global mean temperatures and an open Panama gateway. All these changes lead to a westward shift of the position of the cold tongue along the equator by up to 2000 km. This result is consistent with data from the PRISM3D Pliocene SST reconstruction. Our model further suggests that ENSO variability is present in the Pliocene climate with only slight changes as compared to today. A background climate that would resemble a “permanent” El Ni˜no with weak to no east-west temperature difference along the equator is only found for very weak trade winds which seem unrealistic for the Pliocene climate.

The most marked step in the global climate transition from "Greenhouse" to "Icehouse" Earth occurred at the Eocene-Oligocene (E–O) boundary, 33.7 Ma. Evidence for climatic changes comes from many sources, including the marine benthic δ18O record, showing an increase by 1.2–1.5‰ at this time. This positive excursion is characterised by two steps, separated by a plateau. The increase in δ18O values has been attributed to rapid glaciation of the Antarctic continent, previously ice-free. Simultaneous changes in the δ13C record are indicative of a greenhouse gas control on climate. Previous studies show that a decline in pCO2 beyond a certain threshold value may have initiated the growth of a Southern Hemispheric ice sheet. These studies were not able to conclusively explain the remarkable two-step profile in δ18O. Furthermore, they did not address the potential role of changes in ocean circulation in the E–O transition. Here a new interpretation of the δ18O signal is presented, based on model simulations using a simple coupled 8-box-ocean, 4-box-atmosphere model with an added land ice component. The model was forced with a slowly decreasing atmospheric carbon dioxide concentration. It is argued that the first step in the δ18O represents a shift in meridional overturning circulation from a Southern Ocean to a bipolar source of deep-water formation, which is associated with a cooling of the deep sea. This shift can be initiated by a small density perturbation in the model, although there is also a parameter regime for which the shift occurs spontaneously. The second step in the δ18O profile occurs due to a rapid glaciation of the Antarctic continent. This new interpretation is a robust outcome of our model and is in good agreement with proxy data.

The issue of multidecadal variability in the North Atlantic has been an important topic of late. It is clear that there are multidecadal variations in several climate variables in the North Atlantic, such as sea surface temperature and sea level height. The details of this variability, in particular the dominant patterns and time scales, are confusing from both an observational as well as a theoretical point of view. After analyzing results from observational datasets and a 500-yr simulation of an Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) climate model, two dominant time scales (20–30 and 50–70 yr) of multidecadal variability in the North Atlantic are proposed. The 20–30-yr variability is characterized by the westward propagation of subsurface temperature anomalies. The hypothesis is that the 20–30-yr variability is caused by internal variability of the Atlantic Meridional Overturning Circulation (MOC) while the 50–70-yr variability is related to atmospheric forcing over the Atlantic Ocean and exchange processes between the Atlantic and Arctic Oceans

In this paper it is proposed that the stochastic excitation of a multidecadal internal ocean mode is at the origin of the multidecadal sea surface temperature variability in the North Atlantic. The excitation processes of the spatial sea surface temperature pattern associated with this multidecadal mode within an idealized three-dimensional model are studied by adding noise to the surface heat flux forcing. In the regime where the internal mode is damped, the amplitude of its sea surface temperature pattern depends on the type of noise forcing applied. While the mode is weakly excited by white noise, only the introduction of spatial and temporal coherence in the forcing, with characteristics of the North Atlantic Oscillation in particular, causes the amplitude of the variability to increase to levels comparable to those observed. Within this idealized model the physical mechanism of the excitation can be determined: the presence of the noise rectifies the background state and consequently changes the growth factor of the internal mode

In an idealized Atlantic-Pacific ocean model we study the steady state solutions versus freshwater input in the northern North Atlantic. We find that four different states, the Conveyor (C), the Southern Sinking (SS), the Northern Sinking (NS) and the Inverse Conveyor (IC), appear as two disconnected branches of solutions, where the C is connected with the SS and the NS with the IC. We argue that the latter has the intriguing consequence that the parameter volume for which multiple steady states exist is greatly increased.

We provide a dynamical systems framework to understand the Atlantic Multidecadal Oscillation and show that this framework is in many ways similar to that of the El Niño/Southern Oscillation. A so-called minimal primitive equation model is used to represent the Atlantic Ocean circulation. Within this minimal model, we identify a normal mode of multidecadal variability that can destabilize the background climate state through a Hopf bifurcation. Next, we argue that noise is setting the amplitude of the sea surface temperature variability associated with this normal mode. The results provide support that a stochastic Hopf bifurcation is involved in the multidecadal variability as observed in the North Atlantic.

Both geological data and climate model studies indicate that substantially different patterns of the global ocean circulation have existed throughout the Cenozoic. In a climate model study of the late Oligocene [von der Heydt, A., Dijkstra, H.A. (2006). Effect of ocean gateways on the global ocean circulation in the late Oligocene and early Miocene. Paleoceanography, 21, PA1011] a “northern sinking” type of circulation was found, with (shallow) deep water formation in both the North Pacific Ocean and the North Atlantic Ocean. This is in contrast to the present-day “conveyor” circulation, where there is deep water formation in the North Atlantic but not in the North Pacific. In order to explain these differences, we use an ocean general circulation model for idealized two-basin flows and study the effect of asymmetries in the continental geometry on the circulation patterns. Two types of asymmetry are considered: (i) the relative northward extent of the Pacific and the Atlantic basin, and (ii) the existence of a circum-global gateway at low latitudes. The more northward extent of the Pacific basin in the Oligocene makes the Conveyor solution less likely and facilitates deep water formation in the North Pacific compared to the North Atlantic. The low-latitude gateway on the other hand, allows salinity and heat exchange between the two main ocean basins and therefore leads to deep water formation in both the North Atlantic and the North Pacific.

Multidecadal SST variability is studied in idealized one- and two-ocean-basin configurations, using simulations with the Modular Ocean Model. The authors demonstrate that the multidecadal variability on the global “conveyor type” circulation is localized in the North Atlantic Ocean. Interbasin exchange processes determine the locations where regions of deep-water formation occur and induce a localization of SST multidecadal anomalies in the Atlantic. The physics of this localization is subsequently investigated by considering more equatorially symmetric background flows in two-basin and one-basin configurations. A cross-equatorial flow in the Atlantic induces the localization of the multidecadal variability in the North Atlantic. By using the mechanism of multidecadal variability as proposed in 2002 by Te Raa and Dijkstra in a single-hemispheric configuration, the physics of these localization processes can be explained.

In a companion paper, the authors have shown that in an idealized Atlantic–Pacific Ocean configuration with a conveyor-type overturning circulation, localized multidecadal variability occurs in the Atlantic. Results suggest that the multidecadal variability originates from the instability of the three-dimensional thermohaline circulation and that the physics of the spatial patterns of the SST anomalies can be understood from a study of an Atlantic-only configuration. Specific internal (multidecadal) modes, which obtain a positive growth factor depending on the background thermohaline flow, are associated with the instability. In this paper, the spectral origin of these internal modes is studied using eigensolution continuation techniques. As in the single-hemispheric case, multidecadal modes arise through mergers of so-called SST modes. In the double-hemispheric case studied here, there actually are two types of multidecadal modes that lead to oscillatory behavior. Depending on the background conditions, one of these oscillatory flows is preferred.

We investigate the effect of changes in the tectonic boundary conditions on global ocean circulation patterns. Using a fully coupled climate model in an idealized setup, we compare situations corresponding to the late Oligocene, the early Miocene, and present day. The model results show the existence of a flow reversal through the Panama Seaway between the Oligocene and Miocene. This flow reversal is induced by global tectonic changes related to the widening of the Southern Ocean passages and the closing of the Tethys Seaway. It mainly involves the wind-driven ocean circulation, in agreement with previous model studies. The global thermohaline circulation in the Oligocene and Miocene simulations is significantly different from the present-day conveyor circulation, as there is deepwater formation in both the North Atlantic and the North Pacific oceans. In particular, in the Oligocene simulation the salinity contrast between the Atlantic and Pacific oceans is reduced because of water mass exchange through the low-latitude connections between the two oceans.