Previous studies on future scenarios identified two key effects of increasing CO2 on the African summer monsoon (ASM): Rising CO2 leads to an enhancement in moisture supply, favoring an increase in ASM precipitation (the thermodynamic effect). However, it also results in a weakening in mean atmospheric flow, thus facilitating a dryness across the ASM region (the dynamic effect). Therefore, the ultimate change in ASM precipitation stems from the balance of both the thermodynamic and dynamic effects. This study further examines the impact of rising CO2 on ASM rainfall, by taking into account various climate states. Our results suggest that an increase in CO2 during warm interglacial periods has a stronger influence from thermodynamic factors than from dynamic factors, resulting in an enhancement in ASM rainfall. In contrast, if CO2 increases under cold glacial climate backgrounds, its dynamic impact dominates a reduction of rainfall in the ASM region.

The last glacial inception (LGI) marks the transition from the interglacial warm climate to the glacial period with extensive Northern Hemisphere ice sheets and colder climate. This transition is initiated by decreasing summer insolation but requires positive feedbacks to stimulate the appearance of perennial snow. We perform simulations of LGI with climate model AWI-ESM-2.1, forced by the radiative and greenhouse gas forcing of 115,000 years before present. To compare with the preindustrial (PI) simulation, we use a consistent definition of the seasons during the LGI and the PI and evaluating model output on an angular astronomical calendar. Our study reveals a prominent role of sea ice in the albedo feedback to amplify the delayed climate siregnal at polar latitudes. Through a radiative budget analysis, we examine that the ice-albedo feedback exceeds the shortwave radiative forcing, contributing to the cooling and high latitude snow built-up during LGI.

The relationship between total solar irradiance (TSI) forcing and summer extreme precipitation and flood frequency over western Europe is investigated from a synoptic-scale perspective, with a focus on the role of Rossby Wave Packets (RWPs). Utilizing observational, model, and proxy data, we reveal a significant increase in RWP frequency along a zonal band centered around 50°N, extending from North America to western Europe, during periods of low TSI. This anomaly in RWP frequency is consistent with a significant increase in the frequency of extreme precipitation events recorded over western Europe. Sensitivity experiments conducted with a state-of-the-art chemistry-climate model corroborate our findings based on observational data. Additionally, a flood record from western Europe demonstrates a significant increase in flood frequency during low TSI years, a relationship that persists across timescales. We argue that the frequency patterns associated with TSI forcing presented in this study are robust and, therefore, valuable for estimating the frequency of extreme precipitation events over western Europe under various solar irradiance scenarios. Moreover, our findings indicate that the North Atlantic sector is more responsive to changes in solar forcing during the boreal summer than previously thought, with this effect manifesting primarily on synoptic timescales rather than the long-term climatological mean.

The Atlantic Meridional Overturning Circulation (AMOC) is a key feature of the North Atlantic with global ocean impacts. The AMOC’s response to past changes in forcings during the Holocene provides important context for the coming centuries. Here, we investigate AMOC trends using an emerging set of transient simulations using multiple global climate models for the past 6,000 years. We find no consistent changes in the overall AMOC strength across the simulations, which conforms with reconstructions assimilating proxy records. Similarly, the decadal variability of the AMOC does not change during the mid- and late-Holocene. There are interesting AMOC changes seen in the early Holocene, but their nature depends a lot on which inputs are used to drive the experiment.

Oceanic mesoscale eddies play an important role in preconditioning and restratifying the water column before and after mixing events, thereby affecting deep water formation variability. In the Labrador Sea, where deep convection occurs regularly, observations and models indicate a complex interplay of turbulence and associated tracer fluxes. Results from a realistic eddy-resolving (~5 km local horizontal resolution) ocean model in quasi-equilibrium (~300 years integration) suggest that small-scale temperature fluxes due to turbulent potential to kinetic energy conversion are the main driver of mixed layer restratification during deep convection triggered through atmospheric forcing. In addition to these baroclinic instabilities, buoyant water masses must be provided by the boundary current, where barotropic turbulence is equally important. Only acting together, the destabilizing forcing can be balanced. In a low-resolution control simulation (~20 km) the modeled turbulence is strongly reduced and the associated modeled and parameterized heat fluxes too weak to increase stratification.

The Miocene epoch, spanning 23.03-5.33Ma, was a dynamic climate of sustained, polar amplified warmth. Miocene atmospheric CO2 concentrations are typically reconstructed between 300-600ppm and were potentially higher during the Miocene Climatic Optimum (16.75-14.5Ma). 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 modelling 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℃, 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 datasets 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 towards simulating Miocene warmth while isolating remaining challenges that may be well served by community-led efforts to coordinate modelling and data activities within a common analysis framework.

In this paper we assessed the representation of Arctic sea surface salinity (SSS) and liquid freshwater content (FWC) in the historical simulation of 31 CMIP6 models with comparison to 39 CMIP5 models, and investigated the projected changes in Arctic liquid FWC and freshwater budget in two scenarios (SSP245 and SSP585) of the CMIP6 models. While CMIP6 multi-model mean (MMM) shows an amelioration in representing Arctic SSS compared to CMIP5, no significant reduction is found in the overestimation of FWC and overall model spreads of future changes of Arctic freshwater budget. CMIP6 MMM projects a SSS decrease in most parts of the Arctic Ocean, a slight SSS increase in the Eurasian Basin, and the strongest increase in FWC along the periphery of the Arctic Basin. In the historical simulation, the MMM river runoff, net precipitation, Bering Strait and Barents Sea Opening freshwater transports are 93±34 mSv, 58±109 mSv, 80±32 mSv, and -20±17 mSv, respectively. In the last decade of the 21st century, these budget terms will increase to 138±47 mSv, 123±93 mSv, 83±35 mSv, and 33±47 mSv in the SSP585 scenario. Sea ice meltwater flux will decrease to about zero in the mid-21st century in both SSP245 and SSP585. Freshwater exports through Fram and Davis straits will be higher in the future, and the Fram Strait export will remain larger. The Arctic Ocean is projected to hold a total of 160,300±62,330 km3 freshwater in the SSP585 scenario by 2100, about 60% more than its historical climatology.

The Atlantic Meridional Overturning Circulation (AMOC) is one of the most essential mechanisms influencing our climate system. By comparing constant depth (z-AMOC) and density (ρ-AMOC) frameworks under pre-industrial, historical and abrupt 4xCO2 scenarios we analyze how the circulation mean state and variability differ amongst them. Water mass transformations are also assessed as a matter of analyzing surface-induced and interior-mixing-induced transformations. As expected, both location and strength of AMOC maxima are deeply affected by the framework choice, with the AMOC reaching a maximum transport of 21 Sv at around 35°N under constant depth coordinates, as opposed to ∼25 Sv at 55°N when diagnosed from density surfaces for both pre-industrial and historical climate. When quadrupling the CO2, both frameworks exhibit an abrupt AMOC weakening followed by a steady recovery to maximum values of 10-15 Sv. The z-AMOC maxima timeseries correlates more with those at 26°N (r ∼0.7) than with the ρ-AMOC maxima (r ∼-0.3), due to the flatter isopycnals in the z framework even in the subpolar North Atlantic, where isopycnals are, in fact, steeper. Based on this discrepancy, we argue that the density framework is more coherent to the physics of this circulation by directly incorporating water mass transformations and their density structure. We suggest that more analysis across timescales and under different conditions must be performed with density surface outputs being provided by as many models as possible, to enable a more comprehensive analysis of these two frameworks and their applications.