Abstract. Stratospheric ozone and water vapour are key components of the Earth system, and past and future changes to both have important impacts on global and regional climate. Here, we evaluate long-term changes in these species from the pre-industrial period (1850) to the end of the 21st century in Coupled Model Intercomparison Project phase 6 (CMIP6) models under a range of future emissions scenarios. There is good agreement between the CMIP multi-model mean and observations for total column ozone (TCO), although there is substantial variation between the individual CMIP6 models. For the CMIP6 multi-model mean, global mean TCO has increased from ∼ 300 DU in 1850 to ∼ 305 DU in 1960, before rapidly declining in the 1970s and 1980s following the use and emission of halogenated ozone-depleting substances (ODSs). TCO is projected to return to 1960s values by the middle of the 21st century under the SSP2-4.5, SSP3-7.0, SSP4-3.4, SSP4-6.0, and SSP5-8.5 scenarios, and under the SSP3-7.0 and SSP5-8.5 scenarios TCO values are projected to be ∼ 10 DU higher than the 1960s values by 2100. However, under the SSP1-1.9 and SSP1-1.6 scenarios, TCO is not projected to return to the 1960s values despite reductions in halogenated ODSs due to decreases in tropospheric ozone mixing ratios. This global pattern is similar to regional patterns, except in the tropics where TCO under most scenarios is not projected to return to 1960s values, either through reductions in tropospheric ozone under SSP1-1.9 and SSP1-2.6, or through reductions in lower stratospheric ozone resulting from an acceleration of the Brewer–Dobson circulation under other Shared Socioeconomic Pathways (SSPs). In contrast to TCO, there is poorer agreement between the CMIP6 multi-model mean and observed lower stratospheric water vapour mixing ratios, with the CMIP6 multi-model mean underestimating observed water vapour mixing ratios by ∼ 0.5 ppmv at 70 hPa. CMIP6 multi-model mean stratospheric water vapour mixing ratios in the tropical lower stratosphere have increased by ∼ 0.5 ppmv from the pre-industrial to the present-day period and are projected to increase further by the end of the 21st century. The largest increases (∼ 2 ppmv) are simulated under the future scenarios with the highest assumed forcing pathway (e.g. SSP5-8.5). Tropical lower stratospheric water vapour, and to a lesser extent TCO, shows large variations following explosive volcanic eruptions.

The influence of a high horizontal resolution (5–15 km) on the general circulation and hydrography in the North Atlantic is investigated using the Finite Element Sea Ice–Ocean Model (FESOM). We find a stronger shift of the upper-ocean circulation and water mass properties during the model spinup in the high-resolution model version compared to the low-resolution (~1°) control run. In quasi equilibrium, the high-resolution model is able to reduce typical low-resolution model biases. Especially, it exhibits a weaker salinification of the North Atlantic subpolar gyre and a reduced mixed layer depth in the Labrador Sea. However, during the spinup adjustment, we see that initially improved high-resolution features partially reduce over time: the strength of the Atlantic overturning and the path of the North Atlantic Current are not maintained, and hence hydrographic biases known from low-resolution ocean models return in the high-resolution quasi-equilibrium state. We identify long baroclinic Rossby waves as a potential cause for the strong upper-ocean adjustment of the high-resolution model and conclude that a high horizontal resolution improves the state of the modeled ocean but the model integration length should be chosen carefully.

We simulate the two Coupled Model Intercomparison Project scenarios RCP4.5 and RCP8.5, to assess the effects of melt‐induced fresh water on the Atlantic meridional overturning circulation (AMOC). We use a newly developed climate model with high resolution at the coasts, resolving the complex ocean dynamics. Our results show an AMOC recovery in simulations run with and without an included ice sheet model. We find that the ice sheet adds a strong decadal variability on the freshwater release, resulting in intervals in which it reduces the surface runoff by high accumulation rates. This compensating effect is missing in climate models without dynamic ice sheets. Therefore, we argue to assess those freshwater hosing experiments critically, which aim to parameterize Greenland's freshwater release. We assume the increasing net evaporation over the Atlantic and the resulting increase in ocean salinity, to be the main driver of the AMOC recovery.

Transient simulations of the global fully coupled climate model COSMOS under realistic varying orbital and greenhouse gas forcings are systematically compared to diatom oxygen isotope ( δ 18 O 0.1em diatom0.1em) records from Russian lakes with focus on Eurasian Holocene climate trends. The measured δ 18 O 0.1em diatom0.1em decrease and other temperature proxies are interpreted as large‐scale cooling throughout the Holocene while the model simulations are biased too warm, likely through missing radiative forcings. This large‐scale warm bias also dictates the modeled δ 18 O 0.1em precipitation. Hence, at locations where the signs of model and proxy temperature/precipitation trends agree, measured δ 18 O 0.1em diatom0.1em and modeled δ 18 O 0.1em precipitation0.1em trends show notable accordance. An increased temporal variability of modeled δ 18 O 0.1em precipitation0.1em is linked to persistent atmospheric circulation patterns. Applying the transient forcings in an accelerated way (every 10th year only) yields a similar, yet weaker or delayed model response, especially in the ocean.

The equilibrium climate sensitivity (ECS) and the transient climate response (TCR) are important metrics of how a climate model reacts to greenhouse gas forcing (Meehl et al., 2020). The ECS has been already computed from climate models as early as in the 1970s (Charney et al., 1979) and the TCR in the 1980s (Stouffer et al., 1989). Both metrics have been used over generations of Model Intercomparison Projects (MIPs) that serve as a basis for the assessment reports of the Intergovernmental Panel on Climate Change (IPCC). The ECS is commonly defined as the 2 m temperature response to a doubling of CO 2 after equilibration of the climate system. The TCR is the transient 2 m temperature response from a 1% per year CO 2 increase at the time of a doubling of CO 2 . The ECS is known to be mainly dependent on the atmosphere model while the TCR also critically depends on the pattern of ocean heat uptake (Meehl et al., 2020). The ocean with its high heat capacity plays an important role in delaying the response to a forcing resulting in a clearly lower TCR compared to the ECS (Meehl et al., 2020). Cloud feedbacks and cloud-aerosol interactions in models with prognostic aerosols have a large influence on ECS

No abstract

The Southern Ocean is a major region of ocean carbon uptake, but its future changes remain uncertain under climate warming. Here we show the projected shift in the Southern Ocean CO2 sink using a suite of Earth System Models, revealing changes in the mechanism, position and seasonality of the carbon uptake. Dominant CO2 uptake shifts from the Subtropical to the Antarctic region under the high-emission scenario by the end of the 21st century. The warming-driven sea-ice melt, increased ocean stratification, mixed layer shoaling, and a weaker vertical carbon gradient will together reduce the winter outgassing in the future, which will trigger the switch from mixing-driven outgassing to solubility-driven uptake in the Antarctic region during the winter season. The future Southern Ocean carbon sink will be poleward-shifted, operating in a hybrid mode between biologically-driven summertime and solubility-driven wintertime uptake with further amplification of biological uptake by the increasing Revelle Factor.

The equilibrium climate sensitivity (ECS) and the transient climate response (TCR) are important metrics of how a climate model reacts to greenhouse gas forcing (Meehl et al., 2020). The ECS has been already computed from climate models as early as in the 1970s (Charney et al., 1979) and the TCR in the 1980s (Stouffer et al., 1989). Both metrics have been used over generations of Model Intercomparison Projects (MIPs) that serve as a basis for the assessment reports of the Intergovernmental Panel on Climate Change (IPCC). The ECS is commonly defined as the 2 m temperature response to a doubling of CO 2 after equilibration of the climate system. The TCR is the transient 2 m temperature response from a 1% per year CO 2 increase at the time of a doubling of CO 2 . The ECS is known to be mainly dependent on the atmosphere model while the TCR also critically depends on the pattern of ocean heat uptake (Meehl et al., 2020). The ocean with its high heat capacity plays an important role in delaying the response to a forcing resulting in a clearly lower TCR compared to the ECS (Meehl et al., 2020). Cloud feedbacks and cloud-aerosol interactions in models with prognostic aerosols have a large influence on ECS