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The ocean takes up 93 % of the excess heat in the climate system and approximately a quarter of the anthropogenic carbon via air–sea fluxes. Ocean ventilation and subduction are key processes that regulate the transport of water from the surface mixed layer to the ocean's interior, which is isolated from the atmosphere for a timescale set by the large-scale circulation. Using numerical simulations (NEMO framework), we assess where the ocean subducts water and takes up properties from the atmosphere, and how ocean currents transport and redistribute these properties. This is achieved by adding a set of simulated seawater vintage dyes (passive tracers) that are released annually from different ocean surface “patches”, representing water masses’ source regions. The dyes’ distribution captures years of strong and weak convection at deep and mode water formation sites in both hemispheres, showing good agreement with observations in the subpolar North Atlantic. We show that interannual variability in subduction rates, driven by changes in surface forcing, is key in setting the different sizes of the long-term inventory of the dyes. The Northern and Southern Hemispheres are characterised by different ventilation pathways and timescales, but our analysis highlights a strong correlation between the strength of ventilation in recently subducted waters and the longer-term dye inventory in each hemisphere. This means that the conditions close to the time of dye injection are driving the amount of seawater being subducted, but also that this signal persists over time and the longer-term tracer inventory is strongly related to the initial surface conditions. The correlation still holds for the different source regions, where it is even stronger, but the slope of the correlation does vary. Export and isolation of subducted waters is shown to be faster in the Northern Hemisphere, defining a stronger ventilation “persistence” – represented by the slope of the correlation between subduction and the longer-term inventory. The highest ventilation persistence is found in the subpolar North Atlantic and specifically in the Labrador and Irminger Seas, which are the dominant regions in retaining tracer on multi-decadal time scales.

Deep convection from dense water formation in the Southern Ocean drives the lower limb of the global overturning circulation, sequesters anthropogenic heat and carbon from the atmosphere and ventilates the abyssal ocean. The rate and location of dense water formation and its trajectory to the deep ocean is determined by changes in ocean density and stratification and influenced by ocean-ice-atmosphere interactions such as polynya openings (both open-ocean and coastal), sea ice formation and ice shelf collapse.Signatures of deep convection are logistically difficult to measure. The highest-quality observations of water column density are currently provided by in-situ moorings and profiles from Argo floats or CTDs mounted on elephant seals (MEOP data[1]), but these data are spatially and temporally sparse. Satellite products providing complete coverage of high latitudes at regular repeat periods are becoming more readily available and offer an alternative method for capturing changes the extent and variability of deep-water formation in polar regions. We compute steric height anomalies in the Southern Ocean from 2002-2018 using a novel method combining satellite altimetry and gravimetry data. We use these to explore density changes, focussing on deep water formation regions including the Weddell and Ross seas, the Adelie coastline and Amery shelf region, and infer multi-decadal changes in deep convective processes. Long term changes in the steric height anomalies can be linked to recorded ocean-ice events, such as the 2010 collapse of the Mertz glacier, the 2017 Maud Rise polynya and recent recovery of Ross Sea Bottom Water. The satellite-derived steric height anomalies have been validated against in-situ Argo and MEOP profiles and show good agreement in regions with a high data density. [1]https://meop.net/meop-portal/

Understanding what sets the T--S relation within the thermocline, andhow long and what volume of ventilated waters in each T--S class stay in the sub-surfacethermocline is a key question for climate prediction. In particular the sparsity ofthe T--S distribution has been a puzzle since the days ofStommel. Here we use runs performed for the TICTOC project, in which water is labelled by itsyear of ventilation and its source region, to understand how thevolumetric T--S relation is laid down year on year, and evaluate theimportance of near-surface (mostly vertical) mixing in the first year of ventilationagainst longer term mixing (much of which is isopycnal) in specifying the T--S distribution.

Scientific research vessels are highly specialist resources in constant demand, often scheduled many years in advance of a research cruise and typically awarded to a narrow selection of permanently employed eligible investigators. The ability to access vessels is essential for a wide range of geoscientific research from paleoenvironmental sedimentary studies through to understanding modern day marine biodiversity. The high demand and low supply problem of accessing cruises means that participation can often be limited to small networks within the awarded discipline. This can present a barrier to marine scientists who wish to gain offshore experience, despite the fact that research vessels may have greater capacity than is required by any one scientific party. This is particularly true for early career scientists who usually work on timelines shorter than those required for cruise planning, and those from non-traditional academic backgrounds who may be less well connected to funded networks.FindAScienceBerth is a project aiming to match those who wish to partake in a scientific cruise with spare capacity through identifying available berths on scheduled cruises. The long term goal of FindAScienceBerth is to provide opportunities to those who would otherwise be excluded from conducting offshore research to gain experience essential for career development, and in turn better utilise available ship capacity. Here, we will present our background research, quantifying unused berths on UK research vessels, demonstrating the potential of our initiative. Additionally, we will introduce the interface of FindAScienceBerth, showing how we have adapted the existing pan-European Marine Facilities Planning tool for cruise scheduling in order to identify and advertise spare capacity. We will demonstrate how principal scientific officers can advertise spare berths and how prospective participants can identify and apply for these roles. Finally, we will give an overview of the process we have developed to ensure an EDI compliant recruitment practice for filling ship berth vacancies and our EDI monitoring work throughout the application process. Such a process could be applicable to similar schemes across the geosciences which seek to increase equality, diversity and inclusivity by creating opportunities for the development of practical skills.For further information please see our project site: findascienceberth.wordpress.com

Antarctic sea ice plays a critical role in the Earth system, influencing energy, heat and freshwater fluxes, air–sea gas exchange, ice shelf dynamics, ocean circulation, nutrient cycling, marine productivity and global carbon cycling. However, accurate simulation of recent sea-ice changes remains challenging and, therefore, projecting future sea-ice changes and their influence on the global climate system is uncertain. Reconstructing past changes in sea-ice cover can provide additional insights into climate feedbacks within the Earth system at different timescales. This paper is the first of two review papers from the Cycles of Sea Ice Dynamics in the Earth system (C-SIDE) working group. In this first paper, we review marine- and ice core-based sea-ice proxies and reconstructions of sea-ice changes throughout the last glacial–interglacial cycle. Antarctic sea-ice reconstructions rely mainly on diatom fossil assemblages and highly branched isoprenoid (HBI) alkenes in marine sediments, supported by chemical proxies in Antarctic ice cores. Most reconstructions for the Last Glacial Maximum (LGM) suggest that winter sea ice expanded all around Antarctica and covered almost twice its modern surface extent. In contrast, LGM summer sea ice expanded mainly in the regions off the Weddell and Ross seas. The difference between winter and summer sea ice during the LGM led to a larger seasonal cycle than today. More recent efforts have focused on reconstructing Antarctic sea ice during warm periods, such as the Holocene and the Last Interglacial (LIG), which may serve as an analogue for the future. Notwithstanding regional heterogeneities, existing reconstructions suggest that sea-ice cover increased from the warm mid-Holocene to the colder Late Holocene with pervasive decadal- to millennial-scale variability throughout the Holocene. Studies, supported by proxy modelling experiments, suggest that sea-ice cover was halved during the warmer LIG when global average temperatures were ∼2 ∘C above the pre-industrial (PI). There are limited marine (14) and ice core (4) sea-ice proxy records covering the complete 130 000 year (130 ka) last glacial cycle. The glacial–interglacial pattern of sea-ice advance and retreat appears relatively similar in each basin of the Southern Ocean. Rapid retreat of sea ice occurred during Terminations II and I while the expansion of sea ice during the last glaciation appears more gradual especially in ice core data sets. Marine records suggest that the first prominent expansion occurred during Marine Isotope Stage (MIS) 4 and that sea ice reached maximum extent during MIS 2. We, however, note that additional sea-ice records and transient model simulations are required to better identify the underlying drivers and feedbacks of Antarctic sea-ice changes over the last 130 ka. This understanding is critical to improve future predictions.

Palaeo-oceanographic reconstructions indicate that the distribution of global ocean water masses has undergone major glacial–interglacial rearrangements over the past ~2.5 million years. Given that the ocean is the largest carbon reservoir, such circulation changes were probably key in driving the variations in atmospheric CO2 concentrations observed in the ice-core record. However, we still lack a mechanistic understanding of the ocean’s role in regulating CO2 on these timescales. Here, we show that glacial ocean–sea ice numerical simulations with a single-basin general circulation model, forced solely by atmospheric cooling, can predict ocean circulation patterns associated with increased atmospheric carbon sequestration in the deep ocean. Under such conditions, Antarctic bottom water becomes more isolated from the sea surface as a result of two connected factors: reduced air–sea gas exchange under sea ice around Antarctica and weaker mixing with North Atlantic Deep Water due to a shallower interface between southern- and northern-sourced water masses. These physical changes alone are sufficient to explain ~40 ppm atmospheric CO2 drawdown—about half of the glacial–interglacial variation. Our results highlight that atmospheric cooling could have directly caused the reorganization of deep ocean water masses and, thus, glacial CO2 drawdown. This provides an important step towards a consistent picture of glacial climates.

The ocean takes up 93 % of the excess heat in the climate system and approximately a quarter of the anthropogenic carbon via air–sea fluxes. Ocean ventilation and subduction are key processes that regulate the transport of water (and associated properties) from the surface mixed layer, which is in contact with the atmosphere, to the ocean's interior, which is isolated from the atmosphere for a timescale set by the large-scale circulation. Utilising numerical simulations with an ocean–sea-ice model using the NEMO (Nucleus for European Modelling of the Ocean) framework, we assess where the ocean subducts water and, thus, takes up properties from the atmosphere; how ocean currents transport and redistribute these properties over time; and how, where, and when these properties are ventilated. Here, the strength and patterns of the net uptake of water and associated properties are analysed by including simulated seawater vintage dyes that are passive tracers released annually into the ocean surface layers between 1958 and 2017. The dyes' distribution is shown to capture years of strong and weak convection at deep and mode water formation sites in both hemispheres, especially when compared to observations in the North Atlantic subpolar gyre. Using this approach, relevant to any passive tracer in the ocean, we can evaluate the regional and depth distribution of the tracers, and determine their variability on interannual to multidecadal timescales. We highlight the key role of variations in the subduction rate driven by changes in surface atmospheric forcing in setting the different sizes of the long-term inventory of the dyes released in different years and the evolution of their distribution. This suggests forecasting potential for determining how the distribution of passive tracers will evolve, from having prior knowledge of mixed-layer properties, with implications for the uptake and storage of anthropogenic heat and carbon in the ocean.

The Miocene epoch, spanning 23.03–5.33 Ma, was a dynamic climate of sustained, polar amplified warmth. Miocene atmospheric CO2 concentrations are typically reconstructed between 300 and 600 ppm and were potentially higher during the Miocene Climatic Optimum (16.75–14.5 Ma). With surface temperature reconstructions pointing to substantial midlatitude and polar warmth, it is unclear what processes maintained the much weaker-than-modern equator-to-pole temperature difference. Here, we synthesize several Miocene climate modeling efforts together with available terrestrial and ocean surface temperature reconstructions. We evaluate the range of model-data agreement, highlight robust mechanisms operating across Miocene modeling efforts and regions where differences across experiments result in a large spread in warming responses. Prescribed CO2 is the primary factor controlling global warming across the ensemble. On average, elements other than CO2, such as Miocene paleogeography and ice sheets, raise global mean temperature by ∼2°C, with the spread in warming under a given CO2 concentration (due to a combination of the spread in imposed boundary conditions and climate feedback strengths) equivalent to ∼1.2 times a CO2 doubling. This study uses an ensemble of opportunity: models, boundary conditions, and reference data sets represent the state-of-art for the Miocene, but are inhomogeneous and not ideal for a formal intermodel comparison effort. Acknowledging this caveat, this study is nevertheless the first Miocene multi-model, multi-proxy comparison attempted so far. This study serves to take stock of the current progress toward simulating Miocene warmth while isolating remaining challenges that may be well served by community-led efforts to coordinate modeling and data activities within a common analytical framework.

Anthropogenic warming added to the climate system accumulates mostly in the ocean interior and discrepancies in how this is modelled contribute to uncertainties in predicting sea level rise. Temperature changes are partitioned between excess, due to perturbed surface heat fluxes, and redistribution, that arises from the changing circulation and perturbations to mixing. In a model (HadCM3) with realistic historical forcing (anthropogenic and natural) from 1960 to 2011, we firstly compare this excess-redistribution partitioning with the spice and heave decomposition, in which ocean interior temperature anomalies occur along or across isopycnals, respectively. This comparison reveals that in subtropical gyres (except in the North Atlantic) heave mostly captures excess warming in the top 2000 m, as expected from Ekman pumping, whereas spice captures redistributive cooling. At high-latitudes and in the subtropical Atlantic, however, spice predicts excess warming at the winter mixed layer whereas below this layer, spice represents redistributive warming in southern high latitudes. Secondly, we use Eulerian heat budgets of the ocean interior to identify the process responsible for excess and redistributive warming. In southern high latitudes, spice warming results from reduced convective cooling and increased warming by isopycnal diffusion, which account for the deep redistributive and shallow excess warming, respectively. In the North Atlantic, excess warming due to advection contains both cross-isopycnal warming (heave found in subtropical gyres) and along-isopycnal warming (spice). Finally, projections of heat budgets —coupled with salinity budgets— into thermohaline and spiciness-density coordinates inform us about how water mass formation occurs with varying T-S slopes. Such formation happens preferentially along isopycnal surfaces at high-latitudes and along isospiciness surfaces at mid-latitudes, and along both coordinates in the subtropical Atlantic. Because spice and heave depend only on temperature and salinity, our study suggests a method to detect excess warming in observations.

Over 90% of the buildup of additional heat in the Earth system over recent decades is contained in the ocean. Since 2006, new observational programs have revealed heterogeneous patterns of ocean heat content change. It is unclear how much of this heterogeneity is due to heat being added to and mixed within the ocean leading to material changes in water mass properties or is due to changes in circulation that redistribute existing water masses. Here we present a novel diagnosis of the “material” and “redistributed” contributions to regional heat content change between 2006 and 2017 that is based on a new “minimum transformation method” informed by both water mass transformation and optimal transportation theory. We show that material warming has large spatial coherence. The material change tends to be smaller than the redistributed change at any geographical location; however, it sums globally to the net warming of the ocean, whereas the redistributed component sums, by design, to zero. Material warming is robust over the time period of this analysis, whereas the redistributed signal only emerges from the variability in a few regions. In the North Atlantic Ocean, water mass changes indicate substantial material warming while redistribution cools the subpolar region as a result of a slowdown in the meridional overturning circulation. Warming in the Southern Ocean is explained by material warming and by anomalous southward heat transport of 118 ± 50 TW through redistribution. Our results suggest that near-term projections of ocean heat content change and therefore sea level change will hinge on understanding and predicting changes in ocean redistribution.

We present the first subprecessional record of seawater 87Sr/86Sr isotope ratios for a marginal Mediterranean subbasin. The sediments contained in this interval (three precessional cycles between 6.60 and 6.55 Ma) are important because they record conditions during the transition to the Messinian Salinity Crisis (MSC; 5.97 to 5.33 Ma), an event for which many details are still poorly understood. The record, derived from planktic foraminifera of the late Miocene Sorbas Basin (SE Spain), shows brief excursions with precessional cyclicity to 87Sr/86Sr ratios higher than coeval ocean 87Sr/86Sr. The hydrologic conditions required to generate the observed record are investigated using box modeling, constrained using a new paleodepth estimate (150 to 250 m) based on benthic foraminiferal assemblages. The box model results highlight the role of climate-driven interbasin density contrast as a significant driver of, or impediment to, exchange. The results are particularly significant in the context of the MSC, where 87Sr/86Sr excursions have been interpreted purely as a consequence of physical restriction. To replicate the observed temporal patterns of lithological variations and 87Sr/86Sr isotope excursions, the Sorbas Basin “box” must have a mainly positive hydrologic budget, in contrast with the Mediterranean's negative budget during the late Miocene. This result has implications for the assumption of synchronous deposition of specific sedimentary layers (sapropels) between marginal and open Mediterranean settings at subprecessional resolution. A net positive hydrologic budget in marginal Mediterranean subbasins may reconcile observations of freshwater inclusions in gypsum deposits.

Changes in regional ocean heat content are not only sensitive to anthropogenic and natural influences, but also substantially impacted by the redistribution of heat, which is in turn driven by changes in ocean circulation and air-sea fluxes. Using a set of numerical simulations with an ocean-sea-ice model of the NEMO framework, we assess where the ocean takes up heat from the atmosphere and how ocean currents transport and redistribute that heat. Here, the strength and patterns of the net uptake of heat by the ocean are treated like a passive tracer, by including simulated sea water vintage dyes, which are released annually between 1958 and 2017. An additional tracer released in year 1800 is also used to investigate longer-term variability. All dye tracers are released from 29 surface patches, representing different water mass production sites, allowing us to identify when and where water masses were last ventilated. The tracers’ distribution and fluxes are shown to capture years of strong and weak convection at deep and mode water formation sites in both hemispheres, when compared to the available observations. Using this approach, which can be applied to any passive tracer in the ocean, we can: (1) assess the relative role of each of the water mass production sites, (2) evaluate the regional and depth distribution of the tracers, and (3) determine their variability on interannual, multidecadal and centennial time scales.

The warming climate influences the ocean by changing its wind‐driven dynamics and by inputting extra heat. This study analyzes the warming where temperature anomalies penetrate the ocean interior, that is, by focusing on the winter mixed layer base. This allows to distinguish regions where ocean circulation contributes to warm anomalies from locations where density‐compensated temperature anomalies locally enter the ocean along isopycnals. Multidecadal (1980–2018) local temperature trends from a hydrographic data set are examined at the winter mixed layer base and partitioned into components relating to isopycnal movement (heave) and change along isopycnals (spice). Subtropical gyres and western boundary currents show warming larger than the global average that mostly projects onto heave. This is the result of the strengthening of the circulation in the Southern Hemisphere subtropical gyres and is related to both wind‐driven changes and Southern Ocean warming. Subtropical regions of surface salinity maxima are influenced by warm anomalies along isopycnals.

LongRunMIP is the first collection of millennial-length simulations of complex coupled climate models and enables investigations of how these models equilibrate in response to radiative perturbations.\ud \ud We present a model intercomparison project, LongRunMIP, the first collection of millennial-length (1000+ year) simulations of complex coupled climate models with a representation of ocean, atmosphere, sea ice, and land surface, and their interactions. Standard model simulations are generally only a few hundred years long. However, modeling the long-term equilibration in response to radiative forcing perturbation is important for understanding many climate phenomena, such as the evolution of ocean circulation, time-and temperature-dependent feedbacks, and the differentiation of forced signal and internal variability. The aim of LongRunMIP is to facilitate research into these questions by serving as an archive for simulations that capture as much of this equilibration as possible. The only requirement to participate in LongRunMIP is to contribute a simulation with elevated, constant CO2 forcing that lasts at least 1000 years. LongRunMIP is a MIP of opportunity in that the simulations were mostly performed prior to the conception of the archive without an agreed-upon set of experiments. For most models, the archive contains a preindustrial control simulation and simulations with an idealized (typically abrupt) CO2 forcing. We collect 2D surface and top-of-atmosphere fields, and 3D ocean temperature and salinity fields. Here, we document the collection of simulations and discuss initial results, including the evolution of surface and deep ocean temperature and cloud radiative effects. As of summer 2019, the collection includes 50 simulations of 15 models by 10 modeling centers. The data of LongRunMIP are publicly available. We encourage submission of more simulations in the future.

International audience; The Atlantic meridional overturning circulation (AMOC) represents the zonally integrated stream function of meridional volume transport in the Atlantic Basin. The AMOC plays an important role in transporting heat meridionally in the climate system. Observations suggest a heat transport by the AMOC of 1.3 PW at 26°N-a latitude which is close to where the Atlantic northward heat transport is thought to reach its maximum. This shapes the climate of the North Atlantic region as we know it today. In recent years there has been significant progress both in our ability to observe the AMOC in nature and to simulate it in numerical models. Most previous modeling investigations of the AMOC and its impact on climate have relied on models with horizontal resolution that does not resolve ocean mesoscale eddies and the dynamics of the Gulf Stream/North Atlantic Current system. As a result of recent increases in computing power, models are now being run that are able to represent mesoscale ocean dynamics and the circulation features that rely on them. The aim of this review is to describe new insights into the AMOC provided by high-resolution models. Furthermore, we will describe how high-resolution model simulations can help resolve outstanding challenges in our understanding of the AMOC.

The presence of contourite drifts in the southern Gulf of Cadiz (GoC) along the Moroccan margin raises questions about the (re)circulation of Mediterranean Outflow Water (MOW) in the GoC and the origin of the currents depositing them. Here, we compare two cores representative of Iberian and Moroccan contourite drifts, covering the last 22 kyr. Although the whole sequence is contouritic in character, it reflects the interaction of distinctive silty-contourite facies (high flow velocity periods) imbedded in muddy-contourite facies (low flow velocity periods). Evidence from benthic foraminifera d13C, sortable silt grain-size, oceanographic CTD profiles and numerical simulations, indicate the Mediterranean water mass as the source of the southern contourite deposits. Our data, therefore, suggests an additional branch of upper-MOW veering southwards off the Straits of Gibraltar along the Moroccan margin. During MIS-(Marine Isotope Stage) 2, upper-MOW was a sluggish current while in the Holocene upper-MOW dominated as a fast, semi-steady flow. Throughout the deglaciation, silty contourites associated with higher flow speeds were deposited in the northern and southern GoC during cold events such as Heinrich Stadial 1 (HS1) and the Younger Dryas, forced by global millennial-scale climate variability. Millennial variability also appears to drive the deposition of silty-contourites in the Holocene. We estimated an average duration of 1 ka for the process of depositing a fast contourite unit. The case of silty-contourite I6 (within HS1) allows us to illustrate with extremely high resolution a “rapid” sequential change in circulation, with gradual slow-down of dense Mediterranean water while surface was freshening (HS1), provoking injection of high-salinity intermediate waters (via contour-currents) into the GoC,and hence the North Atlantic. The subsequent brief collapse of dense water formation in the Mediterranean Sea triggered a major increase in sea surface temperatures (10 C/ka) in the GoC, developing into the next interstadial (Bølling/Allerød). The impact of Mediterranean intermediate waters is manifested here by triggering a substantial rearrangement of intermediate and deep circulation in the North Atlantic, which would have further impacted the Atlantic Meridional Overturning Circulation (AMOC)., Instituto Geológico y Minero de España, España, Unidad de Tres Cantos, Instituto Geológico y Minero de España, España, National Oceanography Centre, Reino Unido

Understanding natural and anthropogenic climate change processes involves using computational models that represent the main components of the Earth system: the atmosphere, ocean, sea ice, and land surface. These models have become increasingly computationally expensive as resolution is increased and more complex process representations are included. However, to gain robust insight into how climate may respond to a given forcing, and to meaningfully quantify the associated uncertainty, it is often required to use either or both ensemble approaches and very long integrations. For this reason, more computationally efficient models can be very valuable tools. Here we provide a comprehensive overview of the suite of climate models based around the HadCM3 coupled general circulation model. This model was developed at the UK Met Office and has been heavily used during the last 15 years for a range of future (and past) climate change studies, but has now been largely superseded for many scientific studies by more recently developed models. However, it continues to be extensively used by various institutions, including the BRIDGE (Bristol Research Initiative for the Dynamic Global Environment) research group at the University of Bristol, who have made modest adaptations to the base HadCM3 model over time. These adaptations mean that the original documentation is not entirely representative, and several other relatively undocumented configurations are in use. We therefore describe the key features of a number of configurations of the HadCM3 climate model family, which together make up HadCM3@Bristol version 1.0. In order to differentiate variants that have undergone development at BRIDGE, we have introduced the letter B into the model nomenclature. We include descriptions of the atmosphere-only model (HadAM3B), the coupled model with a low-resolution ocean (HadCM3BL), the high-resolution atmosphere-only model (HadAM3BH), and the regional model (HadRM3B). These also include three versions of the land surface scheme. By comparing with observational datasets, we show that these models produce a good representation of many aspects of the climate system, including the land and sea surface temperatures, precipitation, ocean circulation, and vegetation. This evaluation, combined with the relatively fast computational speed (up to 1000 times faster than some CMIP6 models), motivates continued development and scientific use of the HadCM3B family of coupled climate models, predominantly for quantifying uncertainty and for long multi-millennial-scale simulations.

Antarctic sea-ice formation plays a key role in shaping the abyssal overturning circulation and stratification in all ocean basins, by driving surface buoyancy loss through the associated brine rejection. Changes in Antarctic sea ice have therefore been suggested as drivers of major glacial-interglacial ocean circulation rearrangements. Here, the relationship between Antarctic sea ice, buoyancy loss, deep-ocean stratification, and overturning circulation is investigated in Last Glacial Maximum and preindustrial simulations from the Paleoclimate Modelling Intercomparison Project (PMIP). The simulations show substantial inter-model differences in their representation of the glacial deep-ocean state and circulation, which is often at odds with the geological evidence. We argue that these apparent inconsistencies can largely be attributed to differing (and likely insufficient) Antarctic sea-ice formation. Discrepancies can be further amplified by short integration times. Deep-ocean equilibration and sea-ice representation should, therefore, be carefully evaluated in the forthcoming PMIP4 simulations.

The high–low latitude climate interplay during the Plio–Pleistocene global cooling is not yet well understood. Insight on the Mediterranean region can provide some clues about past significant climate changes since the basin reflects the climate dynamics of both high-latitude and low-latitude regions, being connected to the North Atlantic and subjected to monsoon influence. Here we shade light on this connection problem by performing a spectral analysis on an Eastern Mediterranean stack of planktonic records spanning the last 5 Ma and by further comparing it to North Atlantic and Pacific deep- and surface-water records. Our main conclusion is that the Mediterranean detected the main global climate transitions over the last 5 Myr although sapropel depositions indicate that it remained influenced by the African summer monsoon during the whole interval. Our analysis reveals that until 2.2 Ma the Mediterranean planktonic record is driven by regional processes dominated by precession. The progressive emergence of the 41-kyr frequency in the Mediterranean records around 2.8 Ma suggests that, since this date, the Mediterranean was more and more affected by the high-latitude climate dynamics forcing than by the low-latitude one. Moreover, during the ongoing Plio–Pleistocene cooling, the 41-kyr frequency signal in the Mediterranean records anticipated high-latitude deep-water response to the intensification of the Northern Hemisphere Glaciations (NHG) and lagged the signal in tropical latitudes. Finally, toward 1.2 Ma the results suggest that the progressive shift from the 41-kyr to the 100-kyr frequency was led by the northern high latitudes. Overall, our results confirm that the Mediterranean is an ideal site to study the interplay between high and low latitude climates.

The subpolar North Atlantic represents a key region for global climate, but most numerical models still have well-described limitations in correctly simulating the local circulation patterns. Here, we present the analysis of a 30-year run with a global eddy-resolving (1/12°) version of the NEMO ocean model. Compared to the 1° and 1/4° equivalent versions, this simulation more realistically represents the shape of the Subpolar Gyre, the position of the North Atlantic Current, and the Gulf Stream separation. Other key improvements are found in the representation of boundary currents, multi-year variability of temperature and depth of winter mixing in the Labrador Sea, and the transport of overflows at the Greenland–Scotland Ridge. However, the salinity, stratification and mean depth of winter mixing in the Labrador Sea, and the density and depth of overflow water south of the sill, still present challenges to the model. This simulation also provides further insight into the spatio-temporal development of the warming event observed in the Subpolar Gyre in the mid 1990s, which appears to coincide with a phase of increased eddy activity in the southernmost part of the gyre. This may have provided a gateway through which heat would have propagated into the gyre's interior.

First of all we thank Hilgen and co-authors for their interest in our manuscript. Here we provide an answer to their comment in order to prove the robustness of our conclusions, despite the tuning process that we performed on the original MEDSTACK from Lourens et al. (2004) and Wang et al. (2010). The tuning of the MEDSTACK to LR04 (Lisiecki and Raymo, 2005) was explicitly asked by one of the reviewers during the review process. The exact request was: “It seems to me that in order to really be confident that the phase relationships of the kind reported here are accurate, each record should be initially correlated to a single master record (i.e. something like the LR04 stack), then at least all records are referenced to the same target”. We therefore followed the reviewer’s recommendation, which seemed scientifically valid. In contrast to what Hilgen and co-authors state, our work did not aim at proposing a revised age model and we never claimed it in the paper. On the other hand, we did tune the original MEDSTACK to LR04 only in order to support the phase analysis performed with the other records. In addition as a new age model of the MEDSTACK was not among the objectives of our work, we did not provide the tuned MEDSTACK as supplementary material since it is evident that tuning processes are method dependent. We would also like to clarify that the sedimentation rates presented in our Fig. S2 correspond to the original MEDSTACK sedimentation rates (not shown on logarithmic scale), and we apologise if this was not clearly stated in our Supplementary Information. However, in Section 2.1.1, we clearly wrote that the sedimentation rates fluctuations were not explored since this issue was beyond the scope of our work.