Structure and Performance of GFDL's CM4.0 Climate Model

Abstract. This work analyzes overshoot simulations from 13 models, including the SSP5-3.4OS and SSP1-1.9 experiments from the Coupled Model Intercomparison Project Phase 6 (CMIP6), to assess how regional temperatures after overshoot differ from those of before, and how these differences may be impacted by changes in sea ice, the Atlantic Ocean Heat Transport (OHT) and the Atlantic Meridional Overturning Circulation (AMOC). The overshoot scenarios, characterized by a peak in radiative forcing levels followed by a decline, show that changes during the CO2 increasing phase are not necessarily compensated during the CO2 decreasing phase, particularly at the regional level. Even if the global mean temperature may recover after the overshoot, regional conditions post-overshoot may still differ from those pre-overshoot, with spatial patterns characterized by large-scale temperature asymmetries. These asymmetries are found between Northern (NH) and Southern Hemisphere (SH), between high and mid-latitudes of the NH, and between western and eastern areas of the Southern Ocean. Changes in sea ice and ocean circulation are identified as potential sources of hysteresis, highlighting the impact of oceanic changes in the behavior of atmospheric variables in case of overshoot. The analyses from this work show that the relative contribution of each mechanism strongly depends on the model. Inter-model differences in the contributions of the meridional overturning can be associated with different climatologies of Mixed Layer Depth (MLD) in the northern North Atlantic (NNA) and in certain areas of the Southern Ocean. Despite these differences across models, both ocean circulation and sea ice changes contribute to shaping the regional temperatures after overshoot, with the temperature asymmetries between NH and SH mainly explained by changes in the AMOC, those between high and mid-latitudes of the NH by sea ice changes, and those between western and eastern areas of the Southern Ocean by the Southern Meridional Overturning Circulation (SMOC). These results highlight the importance of model intercomparison and analysis of ocean dynamics to understand the regional impacts of an overshoot, and more generally the responses to forcing changes.

Climate models are a valuable tool to study the interaction between ocean and atmosphere. Nevertheless, they are known to suffer from various biases and uncertainties. In the subpolar North Atlantic typical biases among models from the Coupled Model Intercomparison Project phase 6 (CMIP6) are found in the mean surface temperature and salinity, and in the mean sea ice concentration. These biases will affect the air-sea interaction.In this study, we are investigating the diversity of CMIP6 models with respect to their response of the Atlantic Meridional Overturning Circulation (AMOC) to the North Atlantic Oscillation (NAO) in pre-industrial control experiments. This response is sensitive to the mean spiciness of the North Atlantic. Thus, we focus on two categories of models: Models that are spicy (warm-salty) and models that are minty (cold-fresh) within the subpolar gyre of the North Atlantic. Spicy models tend to have a lower sea ice cover in the Labrador Sea (LS) and larger LS heat loss during a positive NAO, compared to minty models. Also, spicy models have a weaker stratification in the LS. Sub-surface density changes 1 to 3 years after the NAO are larger in the spicy models and establish a zonal density gradient that can cause a stronger delayed AMOC response that is also more coherent across latitudes.Although some metrics seem to be more realistic in the spicy models, other characteristics seem less realistic compared to the minty models, like the mixed layer depth relative importance between the eastern and the western subpolar North Atlantic. This could be a sign for how some mean states or processes might be right for the wrong reasons and stresses the need for model improvement.

Abstract. Deep water formation in climate models is indicative of their ability to simulate future ocean circulation, carbon and heat uptake, and sea level rise. Present-day temperature, salinity, sea ice concentration and ocean transport in the North Atlantic subpolar gyre and Nordic Seas from 23 CMIP5 (Climate Model Intercomparison Project, phase 5) models are compared with observations to assess the biases, causes and consequences of North Atlantic deep convection in models. The majority of models convect too deep, over too large an area, too often and too far south. Deep convection occurs at the sea ice edge and is most realistic in models with accurate sea ice extent, mostly those using the CICE model. Half of the models convect in response to local cooling or salinification of the surface waters; only a third have a dynamic relationship between freshwater coming from the Arctic and deep convection. The models with the most intense deep convection have the warmest deep waters, due to a redistribution of heat through the water column. For the majority of models, the variability of the Atlantic Meridional Overturning Circulation (AMOC) is explained by the volumes of deep water produced in the subpolar gyre and Nordic Seas up to 2 years before. In turn, models with the strongest AMOC have the largest heat export to the Arctic. Understanding the dynamical drivers of deep convection and AMOC in models is hence key to realistically forecasting Arctic oceanic warming and its consequences for the global ocean circulation, cryosphere and marine life.

An assessment is made of the modes of interannual variability in the seasonal mean summer and winter Southern Hemisphere (SH) 500-hPa geopotential height in the twentieth century in models from the Coupled Model Intercomparison Project (CMIP) phase 5 (CMIP5) dataset. Modes of variability of both the slow (signal) and intraseasonal (noise) components in the CMIP5 models are evaluated against those estimated from reanalysis data. There is general improvement in the leading modes of the slow (signal) component in CMIP5 models compared with the CMIP phase 3 (CMIP3) dataset. The largest improvement is in the spatial structures of the modes related to El Niño–Southern Oscillation variability in SH summer. An overall score metric is significantly higher for CMIP5 over CMIP3 in both seasons. The leading modes in the intraseasonal noise component are generally well reproduced in CMIP5 models, and there are few differences from CMIP3. A new total overall score metric is used to rank the CMIP5 models over both seasons. Weighting the seasons by the relative spread of overall scores is shown to be suitable for generating multimodel ensembles for further analysis of interannual variability. In multimodel ensembles, it is found that an ensemble of size 5 or 6 is sufficient in SH summer to reproduce well the dominant modes. In contrast, about 13 models are typically are required in SH winter. It is shown that it is necessary that the selected models individually reproduce well the leading modes of the slow component.

In the present paper, we study the climatological annual mean Hadley circulation simulated by 47 coupled atmospheric and ocean general circulation models (AOGCMs) of the phase 5 of the Coupled Model Intercomparison Project (CMIP5). Our results show that the climatological annual mean states of the Hadley circulation, i.e., strength, central line, and poleward boundaries of the Hadley cells, have a large inter-model spread. The strength of the Hadley cells, i.e., the maximum value of mean meridional streamfunction (MMS), varies from 7.0 × 1010 to 11.1 × 1010 kg s− 1. The variation range is about half of the multi-model ensemble mean strength of about 8.7 × 1010 kg s− 1. The central line location between the two Hadley cells varies from 3.7° S to 7.8° N, and the poleward boundary latitudes have a variation range of 5.5° in latitude. In contrast, inter-model spreads of the Hadley circulation are much smaller in reanalyses. Using a method of inter-model empirical orthogonal function (EOF) analysis of annual-mean MMS, we show that the first principal component (PC1) is closely correlated with the central line latitudes of the Hadley cells, and that the second principal component (PC2) has a high correlation with the poleward boundaries. Both PC1 and PC2 are significantly correlated with the strength of the Hadley cells. Regressions of sea surface temperatures (SSTs) and precipitation on PC1 and PC2 show that EOF1 is associated with the well-known double intertropical convergence zone (ITCZ) problem in AOGCMs, and that EOF2 is related to the cold tongue bias. The results suggest that the large inter-model spread of the climatological mean Hadley circulation is largely due to the double ITCZ and cold tongue biases, and that the key for improving the simulation performance of the Hadley circulation is to reduce the double ITCZ and cold tongue bias in AOGCMs.

Variability in the Atlantic Meridional Overturning Circulation (AMOC) has been analysed using a 600-year pre-industrial control simulation with the Bergen Climate Model. The typical AMOC variability has amplitudes of 1 Sverdrup (1 Sv = 106 m3 s−1) and time scales of 40–70 years. The model is reproducing the observed dense water formation regions and has very realistic ocean transports and water mass distributions. The dense water produced in the Labrador Sea (1/3) and in the Nordic Seas, including the water entrained into the dense overflows across the Greenland-Scotland Ridge (GSR; 2/3), are the sources of North Atlantic Deep Water (NADW) forming the lower limb of the AMOC’s northern overturning. The variability in the Labrador Sea and the Nordic Seas convection is driven by decadal scale air-sea fluxes in the convective region that can be related to opposite phases of the North Atlantic Oscillation. The Labrador Sea convection is directly linked to the variability in AMOC. Linkages between convection and water mass transformation in the Nordic Seas are more indirect. The Scandinavian Pattern, the third mode of atmospheric variability in the North Atlantic, is a driver of the ocean’s poleward heat transport (PHT), the overall constraint on northern water mass transformation. Increased PHT is both associated with an increased water mass exchange across the GSR, and a stronger AMOC.

Most open ocean and climate models assume a constant background mixing; however, vertical mixing should be reduced under the sea ice in polar region because the sea-ice cover acts as an insulator against the momentum transfer between the atmosphere and ocean. Using a global Ocean General Circulation Model (OGCM), we show that the Atlantic meridional overturning circulation (AMOC) can be substantially affected by reduced vertical mixing under the sea ice. When the background diffusivity under the sea ice is reduced by 1 order less than that in ice-free regions, the volume transport of the AMOC in the upper 3000 m is increased by up to 14% accordingly. The numerical experiment suggests that the reduced background diffusivity makes waters denser in the Arctic Ocean and the denser water is transported into the Nordic Seas to push up the isopycnal surfaces over the Greenland- Iceland-Scotland Ridge. Consequently, the AMOC is enhanced by overflows of the denser water crossing the Denmark Strait.

Climate models vary widely in projections of 21st century global warming and projections of the Atlantic Meridional Overturning Circulation (AMOC). However, the extent to which this uncertainty in AMOC contributes to uncertainty in warming has not yet been quantified. To investigate this, we perform climate model experiments that increase CO2 concentrations while imposing the range of AMOC declines found in the Coupled Model Intercomparison Project Phase 6 (CMIP6). We find that intermodel spread in the AMOC decline, imposed within a single model, reproduces 20% of the total intermodel spread in global warming. An idealized energy-balance model indicates that changes in ocean heat uptake and climate feedbacks contribute approximately equally to enhanced warming in our experiment with a smaller AMOC decline. A smaller AMOC decline produces greater ocean-to-atmosphere heating in the North Atlantic, which increases near-surface warming and reduces the lower-tropospheric stability to produce less-negative lapse-rate and shortwave cloud feedbacks. In the northern tropics, surface warming is enhanced by the wind-evaporation-SST feedback and by more-positive longwave cloud and water vapor feedbacks due to a northward shift of the Intertropical Convergence Zone. A Green’s function analysis confirms that a less-negative global feedback with a smaller AMOC decline is predominantly driven by sea-surface warming in the extratropical North Atlantic. Spread in the AMOC decline is correlated with similar differences in ocean heat uptake and climate feedbacks across CMIP6 models as in our experiments. These results suggest that model uncertainty in global warming may be substantially reduced by constraining projections of AMOC.

The Arctic has experienced rapid sea ice loss and a substantial surface albedo decline, altering its radiation budget. CMIP6 (Coupled Model Intercomparison Project Phase 6) models capture these trends but show considerable inter‐model spread in the magnitude, distribution, and seasonality of Arctic surface albedo. Over land, spread in AMIP (Atmospheric Model Intercomparison Project) and CMIP6 is associated with snow cover variations, while over the ocean, where sea ice dominates, the sources are less clear. We compare CMIP6 simulations with observations from the Clouds and the Earth's Radiant Energy System (CERES) and develop a decomposition method to quantify contributions from sea ice albedo, concentration, and extent. Over the Arctic Ocean, all three factors contribute to inter‐model spread in CMIP6. In AMIP, despite prescribed sea ice concentrations, we were surprised to find an inter‐model spread in Arctic Ocean albedo similar to that in CMIP6, driven solely by sea ice albedo. Applying the decomposition to future projections shows that the largest decline occurs in the Central Arctic, driven primarily by reductions in sea ice extent. After 2045, sea ice extent emerges as the dominant driver, highlighting ice edge retreat as key to future albedo decline. These findings pinpoint key sources of inter‐model spread in Arctic surface albedo and offer insights into quantifying shortwave (SW) radiative effect associated with sea ice responses in future projections.

Recently established North Atlantic ocean observing arrays, such as RAPID/MOCHA, have revealed a large degree of high-frequency variability in the Atlantic Meridional Overturning Circulation (AMOC). Climate modeling studies of the AMOC, however, have traditionally focused on the low-frequency variability of the annual mean AMOC, with an emphasis on multi-decadal and longer time-scale variability. Thus, little is known about the sources of interannual wintertime, wind-driven AMOC variability. Analyzing the Community Earth System Model, we here show the existence of a robust leading mode of interannual variability in the wintertime AMOC that is distinct from the leading mode of the annual mean. We further show that this mode of variability is significantly linked to the El Niño-Southern Oscillation via the North Atlantic Oscillation.

The Atlantic meridional overturning circulation (AMOC) is a system of ocean currents that has an essential role in Earth's climate, redistributing heat and influencing the carbon cycle1, 2. The AMOC has been shown to be weakening in recent years 1 ; this decline may reflect decadal-scale variability in convection in the Labrador Sea, but short observational datasets preclude a longer-term perspective on the modern state and variability of Labrador Sea convection and the AMOC1, 3-5. Here we provide several lines of palaeo-oceanographic evidence that Labrador Sea deep convection and the AMOC have been anomalously weak over the past 150 years or so (since the end of the Little Ice Age, LIA, approximately AD1850) compared with the preceding 1,500 years. Our palaeoclimate reconstructions indicate that the transition occurred either as a predominantly abrupt shift towards the end of the LIA, or as a more gradual, continued decline over the past 150 years; this ambiguity probably arises from non-AMOC influences on the various proxies or from the different sensitivities of these proxies to individual components of the AMOC. We suggest that enhanced freshwater fluxes from the Arctic and Nordic seas towards the end of the LIA-sourced from melting glaciers and thickened sea ice that developed earlier in the LIA-weakened Labrador Sea convection and the AMOC. The lack of a subsequent recovery may have resulted from hysteresis or from twentieth-century melting of the Greenland Ice Sheet 6 . Our results suggest that recent decadal variability in Labrador Sea convection and the AMOC has occurred during an atypical, weak background state. Future work should aim to constrain the roles of internal climate variability and early anthropogenic forcing in the AMOC weakening described here.

The Atlantic Meridional Overturning Circulation (AMOC), a key mechanism in the climate system, delivers warm and salty waters from the subtropical gyre to the subpolar gyre and Nordic Seas, where they are transformed into denser waters flowing southward in the lower AMOC limb. The prevailing hypothesis is that dense waters formed in the Labrador and Nordic Seas are the sources for the AMOC lower limb. However, recent observations reveal that convection in the Labrador Sea contributes minimally to the total overturning of the subpolar gyre. In this study, we show that the AMOC is instead primarily composed of waters formed in the Nordic Seas and Irminger and Iceland basins. A first direct estimate of heat and freshwater fluxes over these basins demonstrates that buoyancy forcing during the winter months can almost wholly account for the dense waters of the subpolar North Atlantic that are exported as part of the AMOC.

Abstract. To better understand the processes in the Nordic Seas and their connection to large-scale climate changes during Dansgaard-Oeschger (D-O) events, we reconstruct sea ice extent (SIE) and subsurface temperatures (SubSTs) in the eastern Fram Strait between 40 and 33.5 ka b2k. Our new proxy data from MD99-2304 reveal pronounced fluctuations in SIE and SubSTs both between and within each investigated Greenland Stadial (GS) and Greenland Interstadials (GIs). Consequently, variations in SIE and SubSTs in the eastern Fram Strait show a weaker connection to climate oscillations in Greenland ice cores, in comparison to changes observed in the southeastern Nordic Seas and the North Atlantic. Integrating our results with Atlantic Meridional Overturning Circulation (AMOC) strength reconstruction and sea ice records from the southeastern Nordic Seas, we identify different sea ice regimes between the eastern Fram Strait and the southeastern Nordic Seas. These findings suggest that fluctuations in the eastern Fram Strait were primarily driven by shifts in northward oceanic heat transport, which were regulated by changes in the strength of the AMOC.

The Atlantic Meridional Overturning Circulation (AMOC) is widely anticipated to weaken by the end of the 21st century, although there is considerable disparity among state-of-the-art Earth System Models (ESM) from the Coupled Model Intercomparison Project Phase 6 (CMIP6) regarding the magnitude of this weakening. Notably, the weakening of AMOC has been shown to influence future precipitation patterns over different regions, including across West Africa. Uncertainty in projections of Sahel precipitation has persisted across generations of ESM and remains a significant challenge for stakeholders. Towards the end of the 21st century, many climate models predict a dipole pattern, characterized by a drought across western Sahel and an increased rainfall across the central Sahel.Recent work has shown that models displaying this zonal dipole pattern tend to predict a southward shift in the position of the Atlantic Intertropical Convergence Zone (ITCZ), while those that do not show this dipole tend to predict a northward shift in the Atlantic ITCZ. Furthermore, for the models that do produce this dipole, variability in the longitude where drying transitions to increased precipitation remains difficult to explain. In this study, we seek to elucidate the relationship between changes in AMOC and changes in Sahel precipitation, particularly exploring the potential influence of AMOC on the Atlantic ITCZ and its connection with zonal variability across the Sahel. We use an ensemble of CMIP6 models and statistical analysis to connect the weakening of the AMOC to structural changes in Sahel precipitation across models. Finally, we explore potential mechanisms, including changes to the subtropical high, that may connect these critical climate signals.

The risk of crossing critical thresholds in the Earth system is continuously increasing due to anthropogenic climate change, potentially leading to accelerated responses. As one of the major tipping elements, the Atlantic Meridional Overturning Circulation (AMOC) can exhibit abrupt, nonlinear shifts between distinct regimes. Evidence of such tipping behavior is found in paleo-climate records, most prominently as Dansgaard-Oeschger (DO) events. Using the Bern3D fast Earth System Model, we investigated DO events driven by AMOC variability under Marine Isotope Stage 3 (MIS3) conditions. The model exhibits unforced, self-sustained oscillations resembling DO events within a narrow parameter space defined by CO2 concentration, wind stress forcing, and diapycnal diffusivity. We systematically explored this parameter space and its boundaries. Beyond these parameter space boundaries, the AMOC either remains in a weak regime or undergoes an abrupt transition to a stronger state. Within the parameter space, oscillations are stable, with the periodicity being strongly controlled by CO2. The mechanism underlying DO-like oscillations is primarily oceanic and involves heat accumulation and sea ice changes in the eastern North Atlantic. Sea ice acts as an insulating barrier, allowing subsurface heat to build up until it is rapidly redistributed through the water column, melts the sea ice, is released and triggers deep convection, producing an abrupt strengthening of the AMOC. Freshwater input from sea ice melt, in turn, weakens the circulation. These results indicate that abrupt shifts in the AMOC are an inherent feature of the climate system, although the implications for the AMOC’s future evolution remain unclear due to the vastly different boundary conditions.