Simulations of the Eocene climate using state-of-the-art Earth system models provide a reference state for the future climate, as the Eocene was one of the warmest geological epoch with high atmospheric carbon dioxide (CO₂) concentration and global temperatures comparable to projections for the coming centuries. However, the paleogeographic configuration of the Eocene impact's distinct climate features. Here, we decompose the response of low-level monsoon dynamics over the Indian Ocean to the early Eocene hothouse using five climate model simulations from the Deep-time Model Intercomparison Project (DeepMIP). We see a circulation pattern resembling the paleo-monsoon across all models over the Indian Ocean. Surprisingly, we find low-level jet (LLJ) forming along the topographic barriers of the Eastern African Rift and the Deccan Plateau, which we refer to as the “Proto-LLJ.” Based on the analysis of the DeepMIP results, we find a reduction in the Proto-LLJ strength with elevated CO₂. Under present-day conditions, the northward shift of monsoonal LLJ is attributable to the increased land-sea contrast under global warming. Even though land-ocean temperature contrasts increased during the Eocene hothouse, Proto-LLJ weakened due to tropical atmospheric stabilization. This stabilization reduced vertical temperature gradients, suppressed convection, and weakened atmospheric overturning, limiting the upward motion needed to drive strong monsoonal winds under CO₂-induced warming.

Using outputs from eight Coupled Model Intercomparison Project Phase 6/Paleoclimate Model Intercomparison Project phase 4 models, we compare the hydrography and surface ocean circulation in the Arctic and sub-Arctic regions across three climate states: the pre-industrial (PI; 1850 CE), the Last Interglacial (LIG; 127 ka BP), driven by strong summer insolation in the high latitudes of the Northern Hemisphere, and an idealized future scenario forced by increasing atmospheric (Formula presented.) concentration. Insolation forcing at 127 ka causes an anomalous cyclonic circulation over Greenland and surrounding seas, which enhances the Baffin and Labrador currents, while slightly weakening the East Greenland Current relative to PI. These changes affect sea-ice and water export on both sides of Greenland. Most models also simulate a strengthened North Atlantic Subpolar Gyre (SPG) and increased volume transports through Fram Strait and Barents Sea Opening. However, this does not always correlate with a larger heat transport into the Arctic. In contrast, under increased (Formula presented.) forcing, the SPG weakens, but more heat is carried toward the Arctic compared with the PI and LIG simulations. Consequently, temperatures of the surface and subsurface waters are higher in the Eurasian Basin and sea-ice decline in the Barents Sea is more pronounced in the idealized future experiment. These changes closely resemble the ongoing Arctic Atlantification, whereas evidence for a similar process in the LIG simulation is less clear.

The global overturning circulation (GOC) is the largest scale component of the ocean circulation, associated with a global redistribution of key tracers such as heat and carbon. The GOC generates decadal to millennial climate variability, and will determine much of the long-term response to anthropogenic climate perturbations. This review aims at providing an overview of the main controls of the GOC. By controls, we mean processes affecting the overturning structure and variability. We distinguish three main controls: mechanical mixing, convection, and wind pumping. Geography provides an additional control on geological timescales. An important emphasis of this review is to present how the different controls interact with each other to produce an overturning flow, making this review relevant to the study of past, present and future climates as well as to exoplanets’ oceans.

We evaluate five commonly-applied criteria to validate that a climate model is in so-called “quasi-equilibrium,” using a suite of five simulations with CO₂ concentrations between 1× and 16× Pre-Industrial values. We find that major changes in ocean circulation can occur after common thermal equilibrium criteria are reached, such as a small Top of Atmosphere radiative flux imbalance, or weak trends in surface air temperature, sea surface temperature, and deep ocean temperature. Ocean circulation change, in turn, impact high-latitude SAT, sea ice, and the Inter-tropical Convergence Zone position. For future modeling studies and intercomparison projects aiming for an ocean in quasi-equilibrium, we suggest that time series of key meridional overturning circulation (MOC) metrics in the Atlantic, Pacific, and Southern Ocean are saved, and that MOC trends are less than 1 Sv/1000 years, and DOT trends less than 0.1°C/century for the final 1000 years of the simulations.

The Last Interglacial (LIG) period, approximately 130 000 to 115 000 years ago, represents one of the warmest intervals of the past 800 000 years. Here, we simulate water isotopes in precipitation over Antarctica and the Arctic during the LIG, using three isotope-enabled atmosphere–ocean coupled climate models: HadCM3, MPI-ESM-wiso, and GISS-E2.1. These models were run following the Paleoclimate Modelling Intercomparison Project phase 4 (PMIP4) protocol for the LIG at 127 ka (kiloyears ago), supplemented by a 3000-year Heinrich Stadial 11 (H11) experiment using HadCM3. The long H11 simulation applies Northern Hemisphere meltwater to the North Atlantic, causing large-scale changes in ocean circulation – including cooling in the North Atlantic and Arctic and warming in the Southern Ocean and Global Ocean. While the standard 127 ka simulations do not capture the observed Antarctic warming and sea ice reduction in the Southern Ocean and Antarctic regions, they do capture around half of the warming in the Arctic. The H11 simulations align more closely with observations than the 127 ka simulations. H11 captures more than 80 % of the warming, sea ice loss, and δ18O changes for both Greenland and Antarctica. Decomposition of seasonal δ18O drivers highlights the dominant role of sea ice retreat and associated changes in precipitation seasonality in influencing isotopic values across all simulations, alongside a smaller common response to orbital forcing. We use the H11 and multi-model 127 ka simulations together to infer LIG surface air temperature (SAT) changes based on ice core measurements. The peak inferred LIG Greenland SAT increase is +2.89 ± 1.32 K at the NEEM ice core site – less than half the previously inferred warming. Peak inferred LIG Antarctic SAT increases are +4.39 ± 1.45 K at EDC, dropping to +1.67 ± 3.67 K at TALDICE. These calculated warming values reflect climate effects alone and do not account for any ice-flow- or site-elevation-related impacts. Coastal sites in Greenland and Antarctica appear to have experienced less warming compared with higher central regions.

The Miocene (∼23–5 Ma) experienced substantial paleogeographic changes, including the shoaling of the Panama Seaway and closure of the Tethys Seaway, which altered exchange pathways between the Pacific and Atlantic Oceans. Changes in continental configuration and topography likely also influenced global wind patterns. Here, we investigate how these changes affected surface wind-driven gyre circulation and interbasin volume transport using 14 fully coupled climate model simulations of the early and middle Miocene. The North and South Atlantic gyres, along with the South Pacific gyre, are weaker in the Miocene simulations compared to pre-industrial (PI), while the North Pacific gyres are stronger. These changes largely follow the wind stress curl and basin width changes. Westward flow through the Panama Seaway occurs only in early Miocene simulations when the Tethys Seaway is open and transports are strongly westward. As the Tethys transport declines, flow across the Panama Seaway gradually reverses from westward (into the Pacific) to eastward (into the Atlantic). In simulations with a closed Tethys Seaway, the Panama transport is consistently eastward. The Southern Hemisphere westerlies are weaker than PI in all simulations, contributing to a reduced Antarctic Circumpolar Current (ACC) in 11 of the 14 cases. In the remaining three, a stronger ACC is simulated, likely due to a combination of enhanced meridional density gradients and model-dependent sensitivities. These findings highlight how changes in Miocene seaways and wind patterns reshaped ocean circulation, influencing interbasin exchange, thermohaline properties, and global climate.

The Miocene (∼23–5 Ma) is a past warm epoch when global surface temperatures varied between ∼5 and 8°C warmer than today, and CO₂ concentration was ∼400–800 ppm. The narrowing/closing of the tropical ocean gateways and widening of high-latitude gateways throughout the Miocene is likely responsible for the evolution of the ocean's overturning circulation to its modern structure, though the mechanisms remain unclear. Here, we investigate early and middle Miocene ocean circulation in an opportunistic climate model intercomparison (MioMIP1), using 14 simulations with different paleogeography, CO₂, and vegetation. The strength of the Southern Ocean-driven Meridional Overturning Circulation (SOMOC) bottom cell is similar in the Miocene and Pre-Industrial (PI) but dominates the Miocene global MOC due to weaker Northern Hemisphere overturning. The Miocene Atlantic MOC (AMOC) is weaker than PI in all the simulations (by 2–21 Sv), possibly due to its connection with an Arctic that is considerably fresher than today. Deep overturning in the North Pacific (PMOC) is present in three simulations (∼5–10 Sv), of which two have a weaker AMOC, and one has a stronger AMOC (compared to its PMOC). Surface freshwater fluxes control northern overturning such that the basin with the least freshwater gain has stronger overturning. While the orography, which impacts runoff direction (Pacific vs. Atlantic), has an inconsistent impact on northern overturning across simulations, overall, features associated with the early Miocene—such as a lower Tibetan Plateau, the Rocky Mountains, and a deeper Panama Seaway—seem to favor PMOC over AMOC.

The large-scale oceanic gyre circulation in the Pacific regulates its temperature, salinity and nutrient flow, profoundly influencing the biological environment and climate. However, how gyres evolved during the warm Eocene, with its different coastline configurations, remains unknown. Here, we investigate the response of Pacific gyre circulation during the warm early Eocene using eight models from the Deep-Time Model Intercomparison Project (DeepMIP). Our DeepMIP results suggest a northward expansion of the North Pacific subtropical gyre by up to 10 degrees latitude during the Eocene, while maintaining a strength comparable to that of the present day. This simulated poleward expansion of the North Pacific gyre circulation is corroborated by sedimentary evidence, including poleward shifts in the pattern of clay sediments and low sedimentation rate during the Eocene. In the southern Pacific, the super subtropical gyre was much stronger during the Eocene than at present, due to the southward position of Australia, which created a wide-open Indonesian gateway. The poleward shift in the boundary between the subtropical and subpolar gyre in in the North Pacific is attributed to the northward migration of the westerly winds maxima, as confirmed by an analysis of Sverdrup transport. The Sverdrup-balanced upper circulation in the Pacific extends further poleward than in the modern day, mainly due to differences in the position of the continents. Specifically, the circulation corresponds to Sverdrup transport up to ∼53°N in the North Pacific, slightly further north than modern of limit of 50°N, and up to ∼55°S in the South Pacific, further south than in the modern limit of ∼45°S.

The Miocene (23.03-5.33 Ma) is recognized as a period with close to modern-day paleogeography, yet a much warmer climate. With large uncertainties in future hydroclimate projections, Miocene conditions illustrate a potential future analog for the Earth system. A recent opportunistic Miocene Model Intercomparison Project 1 (MioMIP1) focused on synthesizing published Miocene climate simulations and comparing them with available temperature reconstructions. Here, we build on this effort by analyzing the hydrological cycle response to Miocene forcings across early-to-middle (E2MMIO; 20.03-11.6 Ma) and middle-to-late Miocene (M2LMIO; 11.5-5.33 Ma) simulations with CO2 concentrations ranging from 200 to 850 ppm and providing a model-data comparison against available precipitation reconstructions. We find global precipitation increases by similar to 2.1 and 2.3% per degree of warming for E2MMIO and M2LMIO simulations, respectively. Models generally agree on a wetter than modern-day tropics; mid and high-latitude, however, do not agree on the sign of subtropical precipitation changes with warming. Global monsoon analysis suggests most monsoon regions, except the North American Monsoon, experience higher precipitation rates under warmer conditions. Model-data comparison shows that mean annual precipitation is underestimated by the models regardless of CO2 concentration, particularly in the mid- to high-latitudes. This suggests that the models may not be (a) resolving key processes driving the hydrological cycle response to Miocene boundary conditions and/or (b) other boundary conditions or processes not considered here are critical to reproducing Miocene hydroclimate. This study highlights the challenges in modeling and reconstructing the Miocene hydrological cycle and serves as a baseline for future coordinated MioMIP efforts. This study looks at Earth's hydrological cycle during the Miocene (23-5 million years ago). During this period, the Earth's climate was 3-7 degrees C warmer than today, with carbon dioxide (CO2) estimates ranging between 400 and 850 ppm. Understanding how the hydrological cycle responded during warmer climate conditions can give us insight into what might happen as the Earth gets warmer. We analyzed a suite of Miocene paleoclimate simulations with different CO2 concentrations in the atmosphere and compared them against fossil plant data, which gives an estimate of the average annual rainfall during the period. We found that during the Miocene global rainfall increased by about 2.1%-2.3% for each degree of warming. The models agree that the tropics, mid- and high-latitude, became wetter than they are today but have lower agreement on whether subtropical areas got wetter or drier as they warmed. Compared to proxies, models consistently underestimated how much rain fell in a year, especially in the mid- to high-latitude. This illustrates the challenges in reconstructing the Miocene's hydrological cycle and suggests that the models might not fully capture the range of uncertainties associated with changes in the hydrological cycle due to warming or other factors that differentiated the Miocene. A multi-model comparison of the hydrological cycle in early-to-middle and middle-to-late Miocene simulations is conductedModels generally agree on wetter than modern tropics, middle and high latitudes, but not on the sign of subtropical precipitation changesModel-data comparison shows mean annual precipitation is underestimated by the models, particularly in the mid- to high-latitudes

Paleoclimate model simulations provide reference data to help interpret the geological record and offer a unique opportunity to evaluate the performance of current models under diverse boundary conditions. Here, we present a dataset of 35 climate model simulations of the warm early Eocene Climatic Optimum (EECO; ~ 50 million years ago) and corresponding preindustrial reference experiments. To streamline the use of the data, we apply standardised naming conventions and quality checks across eight modelling groups that have carried out coordinated simulations as part of the Deep-Time Model Intercomparison Project (DeepMIP). Gridded model fields can be downloaded from an online repository or accessed through a new web application that provides interactive data exploration. Local model data can be extracted in CSV format or visualised online for streamlined model-data comparisons. Additionally, processing and visualisation code templates may serve as a starting point for advanced analysis. The dataset and online platform aim to simplify accessing and handling complex data, prevent common processing issues, and facilitate the sharing of climate model data across disciplines.