21 research outputs found
The Southern Ocean Freshwater Input from Antarctica (SOFIA) Initiative: scientific objectives and experimental design
Abstract. As the climate warms, the grounded ice sheet and floating ice shelves surrounding Antarctica are melting and releasing additional freshwater into the Southern Ocean. Nonetheless, almost all existing coupled climate models have fixed ice sheets and lack the physics required to represent the dominant sources of Antarctic melt. These missing ice dynamics represent a key uncertainty that is typically unaccounted for in current global climate change projections. Previous modelling studies that have imposed additional Antarctic meltwater have demonstrated regional impacts on Southern Ocean stratification, circulation, and sea ice, as well as remote changes in atmospheric circulation, tropical precipitation, and global temperature. However, these previous studies have used widely varying rates of freshwater forcing, have been conducted using different climate models and configurations, and have reached differing conclusions on the magnitude of meltwater–climate feedbacks. The Southern Ocean Freshwater Input from Antarctica (SOFIA) initiative brings together a team of scientists to quantify the climate system response to Antarctic meltwater input along with key aspects of the uncertainty. In this paper, we summarize the state of knowledge on meltwater discharge from the Antarctic ice sheet and ice shelves to the Southern Ocean and explain the scientific objectives of our initiative. We propose a series of coupled and ocean–sea ice model experiments, including idealized meltwater experiments, historical experiments with observationally consistent meltwater input, and future scenarios driven by meltwater inputs derived from stand-alone ice sheet models. Through coordinating a multi-model ensemble of simulations using a common experimental design, open data archiving, and facilitating scientific collaboration, SOFIA aims to move the community toward better constraining our understanding of the climate system response to Antarctic melt.
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Resolving and parameterising the ocean mesoscale in earth system models
Purpose of Review. Assessment of the impact of ocean resolution in Earth System models on the mean state, variability, and
future projections and discussion of prospects for improved parameterisations to represent the ocean mesoscale.
Recent Findings. The majority of centres participating in CMIP6 employ ocean components with resolutions of about 1 degree in
their full Earth Systemmodels (eddy-parameterising models). In contrast, there are alsomodels submitted toCMIP6 (both DECK
and HighResMIP) that employ ocean components of approximately 1/4 degree and 1/10 degree (eddy-present and eddy-rich
models). Evidence to date suggests that whether the ocean mesoscale is explicitly represented or parameterised affects not only
the mean state of the ocean but also the climate variability and the future climate response, particularly in terms of the Atlantic
meridional overturning circulation (AMOC) and the Southern Ocean. Recent developments in scale-aware parameterisations of
the mesoscale are being developed and will be included in future Earth System models.
Summary. Although the choice of ocean resolution in Earth System models will always be limited by computational considerations,
for the foreseeable future, this choice is likely to affect projections of climate variability and change as well as other
aspects of the Earth System. Future Earth System models will be able to choose increased ocean resolution and/or improved
parameterisation of processes to capture physical processes with greater fidelity
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Representation of Large-Scale Ocean Circulation in the Atlantic and Southern Ocean in Climate Model Simulations and Projected Changes under Increased Warming
The global ocean acts as a mediator of Earth’s climate due to its role in the storage of heat and carbon. Presently, the ocean accounts for the storage of approximately 93% of the anthropogenic heat on our planet and ~27% of the anthropogenic CO2. Two regions in particular, the Southern Ocean and North Atlantic Ocean (SO, NA), act as gateways for the exchange of CO2 and heat between the atmosphere and the interior ocean. This is due to the unique water mass transformation processes that occur in these regions. Despite their disproportionate role in the climate system, large uncertainty exists with respect to understanding how the ocean circulation patterns and properties are projected to change in these regions throughout the 21st century. One pathway toward reducing projection uncertainty in these regions is to use modern observations and observational products to comprehensively diagnose, quantify, and improve upon mean state biases that exist in the climate simulations used to produce future climate projections. The work presented in this dissertation is a comprehensive analysis of the large-scale ocean circulation and properties in historical and 21st century simulations of large-ensembles
of fully-coupled climate and Earth System Models contributed to multiple generations of the Coupled Model Intercomparison Project (CMIP).
In the subtropical NA, a key region through which properties from the tropics are advected to the subpolar latitudes, the volume transports of the major flow regimes are reasonably represented in many CMIP5 models relative that observed by the Rapid Climate Change (RAPID) instrumental array at 26.5ºN. As the climate warms, all components of the total flow through the subtropical NA, with the exception of the wind-driven surface Ekman transport, are projected to weaken. Particularly, by applying the dynamical theory of Sverdrup balance, this work highlights the fact that the wind-driven NA subtropical gyre itself is projected to spin-down in response to a reduced wind stress curl over the subtropical latitudes. This spin-down, in conjunction with the reduced overturning at high-latitudes, acts as a source of significant additional weakening to the northward western boundary current flow in the upper ocean.
In the SO, despite its dominant role in the oceanic uptake of anthropogenic carbon and heat relative to other basins, the large-scale circulation and properties have been poorly represented in climate models, resulting in low confidence ascribed to 21st century projections of the state of the SO. A comprehensive analysis of the simulation of the large-scale circulation and properties is presented for
the Southern Ocean (SO) across thirty-one CMIP5 models. The main focus lies in building a framework to understand the major contributors to a model’s ability to represent the Antarctic Circumpolar Current (ACC) transport. Across the CMIP5 ensemble, the models fall into five different categories: 1) models that produce a reasonable ACC transport for approximately the right reasons, 2) models that accurately simulate key metrics, yet produce a too weak ACC, 3) models that simulate the wind stress forcing at the ocean surface accurately, but have errors in the density gradient, 4) models that simulate an accurate density gradient, but exhibit errors in the wind stress forcing, and 5) models that produce errors in all the metrics.
Building on the framework presented in the CMIP5 study, a comprehensive assessment of the large-scale circulation and properties as simulated in the SO is performed across ensembles of models contributed to the past three CMIP generations (CMIP3-CMIP6). The CMIP6 models show improved representation of key observable-metrics in the SO including surface wind stress and wind stress curl, strength of the ACC, and meridional density gradients in the region of the ACC. However, some persistent biases have carried over into CMIP6 including an upper ocean that remains too fresh and too warm, significant warm biases at depth in several simulations, and a poor representation of Antarctic sea ice extent (SIE). These biases in observable metrics need to be considered when interpreting projected trends or biogeochemical properties in this region.Release after 11/05/202
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Impact of the Melting of the Greenland Ice Sheet on the Atlantic Meridional Overturning Circulation in 21st Century Model Projections
Contemporary observations show an increase in the melting of the Greenland Ice Sheet (GrIS) since the early 21st century. Located near the critical sites of oceanic deep convection and deep water formation, the melting of the GrIS has the potential to directly impact the Atlantic Meridional Overturning Circulation (AMOC) by freshening ocean surface waters in these regions. The majority of the Coupled Model Intercomparison Project Phase 5 (CMIP5) models project a decline in AMOC strength by 10-50% during the 21st century, in response to the increase in atmospheric greenhouse gas (GHG) concentrations. However, due to the simple treatment of polar ice sheets and the lack of a dynamical ice sheet component in these models, these projections likely underestimated the impacts of the GrIS melt, leading to uncertainty in projecting future AMOC evolution and climate change around Greenland. To better understand the impact of the GrIS melt on the AMOC, we perform a series of 21st century projection runs with a state-of-the-art Earth System Model-GFDL ESM2Mb. We consider a medium and a high Representative Concentration Pathway (RCP) scenario (RCP4.5 and RCP8.5, respectively). Unlike the CMIP5-standard RCP runs which included only radiative forcing, the new model experiments are also forced with additional and potentially more realistic meltwater discharge from the GrIS. This meltwater discharge is estimated based on a model-based relationship between the GrIS surface melt and the 500hPa atmospheric temperature anomalies over Greenland. The model simulations indicate that compared to the RCP4.5-only and RCP8.5-only projections, the additional melt water from the GrIS can further weaken the AMOC, but with a relatively small magnitude. The reason is that radiative forcing already weakens the deep convection and deep water formation in the North Atlantic, therefore limiting the magnitude of further weakening of AMOC due to the additional meltwater. The modeling results suggest that the AMOC's sensitivity to freshwater forcing due to the GrIS melt is highly dependent on the location and strength of oceanic deep convection sites in ESM2Mb as well as the pathways of the meltwater towards these regions. The additional meltwater contributes to the minimum surface warming (so-called "warming hole") south of Greenland. These simulations with ESM2Mb contribute to the Atlantic Meridional Overturning Circulation Model Intercomparison Project (AMOCMIP), a community effort between international modeling centers to investigate the impacts of the melting of the GrIS on the AMOC and quantify the associated uncertainty
Supporting GFDL data for Southern Ocean Freshwater release model experiments Initiative (SOFIA)
Note: This data collection is hosted at the Geophysical Fluid Dynamics Laboratory. Data DOI capability is provided by PUL. Please refer to the README for a detailed description of the dataset. For questions, please contact [email protected], with the subject line including the title of the dataset.See "how_to_access_data.txt" to access data files from GDFL servers.This output was produced in coordination with the Southern Ocean Freshwater release model experiments Initiative (SOFIA) and is the Tier 1 experiment where freshwater is delivered in a spatially and temporally uniform pattern at the surface of the ocean at sea surface temperature in a 1-degree latitude band extending from Antarctica’s coastline. The total additional freshwater flux imposed as a monthly freshwater flux entering the ocean is 0.1 Sv. Users are referred to the methods section of Beadling et al. (2022) for additional details on the meltwater implementation in CM4 and ESM4. The datasets in this collection contain model output from the coupled global climate model, CM4, and Earth System Model, ESM4, both developed at the Geophysical Fluid Dynamics Laboratory (GFDL) of the National Oceanic and Atmospheric Administration (NOAA). The ocean_monthly_z and ocean_annual_z output are provided as z depth levels in meters as opposed to the models native hybrid vertical ocean coordinate which consists of z* (quasi-geopotential) coordinates in the upper ocean through the mixed layer, transitioning to isopycnal (referenced to 2000 dbar) in the ocean interior. Please see README for further details.File list: doc/README
data/ cm4_tier1_antwater.agessc.ocean_annual_z.tar.gz
cm4_tier1_antwater.bsnk.ice_monthly.tar.gz
cm4_tier1_antwater.cld_amt.atmos_level_monthly.tar.gz
cm4_tier1_antwater.evap.atmos_level_monthly.tar.gz
cm4_tier1_antwater.evs.ocean_monthly.tar.gz
cm4_tier1_antwater.ficeberg.ocean_monthly.tar.gz
cm4_tier1_antwater.frazil.ice_monthly.tar.gz
cm4_tier1_antwater.friver.ocean_monthly.tar.gz
cm4_tier1_antwater.heat_content_surfwater.ocean_monthly.tar.gz
cm4_tier1_antwater.hfds.ocean_monthly.tar.gz
cm4_tier1_antwater.hflso.ocean_monthly.tar.gz
cm4_tier1_antwater.hfsifrazil.ocean_monthly.tar.gz
cm4_tier1_antwater.hfsso.ocean_monthly.tar.gz
cm4_tier1_antwater.lsrc.ice_monthly.tar.gz
cm4_tier1_antwater.mlotst.ocean_monthly.tar.gz
cm4_tier1_antwater.precip.atmos_level_monthly.tar.gz
cm4_tier1_antwater.prlq.ocean_monthly.tar.gz
cm4_tier1_antwater.prsn.ocean_monthly.tar.gz
cm4_tier1_antwater.rlntds.ocean_monthly.tar.gz
cm4_tier1_antwater.rsntds.ocean_monthly.tar.gz
cm4_tier1_antwater.sfdsi.ocean_monthly.tar.gz
cm4_tier1_antwater.siconc.ice_monthly.tar.gz
cm4_tier1_antwater.sithick.ice_monthly.tar.gz
cm4_tier1_antwater.siu.ice_monthly.tar.gz
cm4_tier1_antwater.siv.ice_monthly.tar.gz
cm4_tier1_antwater.slp.atmos_level_monthly.tar.gz
cm4_tier1_antwater.snowfl.ice_monthly.tar.gz
cm4_tier1_antwater.so.ocean_annual_z.tar.gz
cm4_tier1_antwater.so.ocean_monthly_z_complete.tar.gz
cm4_tier1_antwater.static_fields.tar.gz
cm4_tier1_antwater.tauuo.ocean_monthly.tar.gz
cm4_tier1_antwater.tauvo.ocean_monthly.tar.gz
cm4_tier1_antwater.temp.atmos_level_monthly.tar.gz
cm4_tier1_antwater.thetao.ocean_annual_z.tar.gz
cm4_tier1_antwater.thetao.ocean_monthly_z.tar.gz
cm4_tier1_antwater.t_ref.atmos_level_monthly.tar.gz
cm4_tier1_antwater.ucomp.atmos_level_monthly.tar.gz
cm4_tier1_antwater.umo.ocean_annual_z.tar.gz
cm4_tier1_antwater.umo.ocean_monthly_z.tar.gz
cm4_tier1_antwater.uo.ocean_annual_z.tar.gz
cm4_tier1_antwater.uo.ocean_monthly_z.tar.gz
cm4_tier1_antwater.u_ref.atmos_level_monthly.tar.gz
cm4_tier1_antwater.vcomp.atmos_level_monthly.tar.gz
cm4_tier1_antwater.vmo.ocean_annual_z.tar.gz
cm4_tier1_antwater.vmo.ocean_monthly_z.tar.gz
cm4_tier1_antwater.volcello.ocean_annual_z.tar.gz
cm4_tier1_antwater.volcello.ocean_monthly_z.tar.gz
cm4_tier1_antwater.vo.ocean_annual_z.tar.gz
cm4_tier1_antwater.vo.ocean_monthly_z.tar.gz
cm4_tier1_antwater.v_ref.atmos_level_monthly.tar.gz
cm4_tier1_antwater.wfo.ocean_monthly.tar.gz
cm4_tier1_antwater.zos.ocean_monthly.tar.gz
esm4_tier1_antwater.agessc.ocean_annual_z.tar.gz
esm4_tier1_antwater.bsnk.ice_monthly.tar.gz
esm4_tier1_antwater.cld_amt.atmos_level_monthly.tar.gz
esm4_tier1_antwater.evap.atmos_level_monthly.tar.gz
esm4_tier1_antwater.evs.ocean_monthly.tar.gz
esm4_tier1_antwater.ficeberg.ocean_monthly.tar.gz
esm4_tier1_antwater.frazil.ice_monthly.tar.gz
esm4_tier1_antwater.friver.ocean_monthly.tar.gz
esm4_tier1_antwater.heat_content_surfwater.ocean_monthly.tar.gz
esm4_tier1_antwater.hfds.ocean_monthly.tar.gz
esm4_tier1_antwater.hflso.ocean_monthly.tar.gz
esm4_tier1_antwater.hfsifrazil.ocean_monthly.tar.gz
esm4_tier1_antwater.hfsso.ocean_monthly.tar.gz
esm4_tier1_antwater.lsrc.ice_monthly.tar.gz
esm4_tier1_antwater.mlotst.ocean_monthly.tar.gz
esm4_tier1_antwater.precip.atmos_level_monthly.tar.gz
esm4_tier1_antwater.prlq.ocean_monthly.tar.gz
esm4_tier1_antwater.prsn.ocean_monthly.tar.gz
esm4_tier1_antwater.rlntds.ocean_monthly.tar.gz
esm4_tier1_antwater.rsntds.ocean_monthly.tar.gz
esm4_tier1_antwater.sfdsi.ocean_monthly.tar.gz
esm4_tier1_antwater.siconc.ice_monthly.tar.gz
esm4_tier1_antwater.sithick.ice_monthly.tar.gz
esm4_tier1_antwater.siu.ice_monthly.tar.gz
esm4_tier1_antwater.siv.ice_monthly.tar.gz
esm4_tier1_antwater.sivol.ice_monthly.tar.gz
esm4_tier1_antwater.slp.atmos_level_monthly.tar.gz
esm4_tier1_antwater.snowfl.ice_monthly.tar.gz
esm4_tier1_antwater.so.ocean_monthly_z.tar.gz
esm4_tier1_antwater.static_fields.tar.gz
esm4_tier1_antwater.tauuo.ocean_monthly.tar.gz
esm4_tier1_antwater.tauvo.ocean_monthly.tar.gz
esm4_tier1_antwater.temp.atmos_level_monthly.tar.gz
esm4_tier1_antwater.thetao.ocean_monthly_z.tar.gz
esm4_tier1_antwater.t_ref.atmos_level_monthly.tar.gz
esm4_tier1_antwater.ucomp.atmos_level_monthly.tar.gz
esm4_tier1_antwater.umo.ocean_monthly_z.tar.gz
esm4_tier1_antwater.uo.ocean_monthly_z.tar.gz
esm4_tier1_antwater.u_ref.atmos_level_monthly.tar.gz
esm4_tier1_antwater.vcomp.atmos_level_monthly.tar.gz
esm4_tier1_antwater.vmo.ocean_monthly_z.tar.gz
esm4_tier1_antwater.volcello.ocean_monthly_z.tar.gz
esm4_tier1_antwater.vo.ocean_monthly_z.tar.gz
esm4_tier1_antwater.v_ref.atmos_level_monthly.tar.gz
esm4_tier1_antwater.wfo.ocean_monthly_complete.tar.gz
esm4_tier1_antwater.zos.ocean_monthly.tar.g
Representation of Southern Ocean Properties across Coupled Model Intercomparison Project Generations: CMIP3 to CMIP6
International audienceThe air–sea exchange of heat and carbon in the Southern Ocean (SO) plays an important role in mediating the climate state. The dominant role the SO plays in storing anthropogenic heat and carbon is a direct consequence of the unique and complex ocean circulation that exists there. Previous generations of climate models have struggled to accurately represent key SO properties and processes that influence the large-scale ocean circulation. This has resulted in low confidence ascribed to twenty-first-century projections of the state of the SO from previous generations of models. This analysis provides a detailed assessment of the ability of models contributed to the sixth phase of the Coupled Model Intercomparison Project (CMIP6) to represent important observationally based SO properties. Additionally, a comprehensive overview of CMIP6 performance relative to CMIP3 and CMIP5 is presented. CMIP6 models show improved performance in the surface wind stress forcing, simulating stronger and less equatorward-biased wind fields, translating into an improved representation of the Ekman upwelling over the Drake Passage latitudes. An increased number of models simulate an Antarctic Circumpolar Current (ACC) transport within observational uncertainty relative to previous generations; however, several models exhibit extremely weak transports. Generally, the upper SO remains biased warm and fresh relative to observations, and Antarctic sea ice extent remains poorly represented. While generational improvement is found in many metrics, persistent systematic biases are highlighted that should be a priority during model development. These biases need to be considered when interpreting projected trends or biogeochemical properties in this region
Reduced Deep Convection and Bottom Water Formation Due To Antarctic Meltwater in a Multi‐Model Ensemble
The additional water from the Antarctic ice sheet and ice shelves due to climate‐induced melt can impact ocean circulation and global climate. However, the major processes driving melt are not adequately represented in Coupled Model Intercomparison Project phase 6 (CMIP6) models. Here, we analyze a novel multi‐model ensemble of CMIP6 models with consistent meltwater addition to examine the robustness of the modeled response to meltwater, which has not been possible in previous single‐model studies. Antarctic meltwater addition induces a substantial weakening of open‐ocean deep convection. Additionally, Antarctic Bottom Water warms, its volume contracts, and the sea surface cools. However, the magnitude of the reduction varies greatly across models, with differing anomalies correlated with their respective mean‐state climatology, indicating the state‐dependency of the climate response to meltwater. A better representation of the Southern Ocean mean state is necessary for narrowing the inter‐model spread of response to Antarctic meltwater.
Plain Language Summary
The melting of the Antarctic ice sheet and ice shelves can have significant impacts on ocean circulation and thermal structure, but current climate models do not fully capture these effects. In this study, we analyze seven climate models to understand how they respond to the addition of meltwater from Antarctica. We find that the presence of Antarctic meltwater leads to a significant weakening of deep convection in the open ocean. The meltwater also causes Antarctic Bottom Water to warm and its volume to decrease, while the sea surface cools and sea ice expands. However, the magnitude of the response to meltwater varies across models, suggesting that the mean‐state conditions of the Southern Ocean play a role. A better representation of the mean state and the inclusion of Antarctic meltwater in climate models will help reduce uncertainties and improve our understanding of the impact of Antarctic meltwater on climate.
Key Points
Antarctic meltwater substantially reduces the strength of simulated Southern Ocean deep convection in climate models
The additional meltwater induces Antarctic Bottom Water warming and contraction, with dense water classes converting to lighter ones
Differences in the magnitude of these responses between models can be partly attributed to their different base state