A Conceptual Model of Ocean Heat Uptake under Climate Change

Abstract A conceptual model of ocean heat uptake is developed as a multilayer generalization of Gnanadesikan. The roles of Southern Ocean Ekman and eddy transports, North Atlantic Deep Water (NADW) formation, and diapycnal mixing in controlling ocean stratification and transient heat uptake are investigated under climate change scenarios, including imposed surface warming, increased Southern Ocean wind forcing, with or without eddy compensation, and weakened meridional overturning circulation (MOC) induced by reduced NADW formation. With realistic profiles of diapycnal mixing, ocean heat uptake is dominated by Southern Ocean Ekman transport and its long-term adjustment controlled by the Southern Ocean eddy transport. The time scale of adjustment setting the rate of ocean heat uptake increases with depth. For scenarios with increased Southern Ocean wind forcing or weakened MOC, deepened stratification results in enhanced ocean heat uptake. In each of these experiments, the role of diapycnal mixing in setting ocean stratification and heat uptake is secondary. Conversely, in experiments with enhanced diapycnal mixing as employed in “upwelling diffusion” slab models, the contributions of diapycnal mixing and Southern Ocean Ekman transport to the net heat uptake are comparable, but the stratification extends unrealistically to the sea floor. The simple model is applied to interpret the output of an Earth system model, the Second Generation Canadian Earth System Model (CanESM2), in which the atmospheric CO2 concentration is increased by 1% yr−1 until quadrupling, where it is found that Southern Ocean Ekman transport is essential to reproduce the magnitude and vertical profile of ocean heat uptake.

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Emergent constraints on the Southern Ocean anthropogenic carbon and heat uptake

10.5194/egusphere-egu21-5336 ◽

2021 ◽

Author(s):

Thomas Frölicher ◽

Jens Terhaar ◽

Fortunat Joos

Keyword(s):

Southern Ocean ◽

Ocean Circulation ◽

Earth System Model ◽

System Model ◽

Carbon Uptake ◽

Earth System ◽

Excess Heat ◽

The Past ◽

Heat Uptake ◽

Anthropogenic Carbon

The Southern Ocean south of 30&#176;S, occupying about a third of global surface ocean area, accounts for approximately 40% of the past anthropogenic carbon uptake and about 75% of excess heat uptake by the ocean. However, Earth system models have large difficulties in reproducing the Southern Ocean circulation, and therefore its historical and future anthropogenic carbon and excess heat uptake. In the first part of the talk, we show that there exists a tight relation across two Earth system model ensembles (CMIP5 and CMIP6) between present-day sea surface salinity in the subtropical-polar frontal zone, the formation region of mode and intermediate waters, and the past and future anthropogenic carbon uptake in the Southern Ocean. By using observations and Earth system model results, we constrain the projected cumulative Southern Ocean anthropogenic carbon uptake over 1850-2100 by the CMIP6 model ensemble to 158 &#177; 6 Pg C under the low-emissions scenario SSP1-2.6 and to 279 &#177; 14 Pg C under the high emissions scenario SSP5-8.5. Our results suggest that the Southern Ocean anthropogenic carbon sink is 14-18% larger and 46-54% less uncertain than estimated by the unconstrained CMIP6 Earth system model results. The identified constraint demonstrated the importance of the freshwater cycle for the Southern Ocean circulation and carbon cycle. In the second part of the talk, potential emergent constraints for the Southern Ocean excess heat uptake will be discussed.

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Sensitivity of Global Warming to Carbon Emissions: Effects of Heat and Carbon Uptake in a Suite of Earth System Models

Journal of Climate ◽

10.1175/jcli-d-16-0468.1 ◽

2017 ◽

Vol 30 (23) ◽

pp. 9343-9363 ◽

Cited By ~ 17

Author(s):

Richard G. Williams ◽

Vassil Roussenov ◽

Philip Goodwin ◽

Laure Resplandy ◽

Laurent Bopp

Keyword(s):

Climate Sensitivity ◽

Carbon Emissions ◽

Radiative Forcing ◽

Carbon Uptake ◽

Earth System ◽

Ocean Heat Uptake ◽

Surface Warming ◽

System Models ◽

Heat Uptake ◽

Earth System Models

Climate projections reveal global-mean surface warming increasing nearly linearly with cumulative carbon emissions. The sensitivity of surface warming to carbon emissions is interpreted in terms of a product of three terms: the dependence of surface warming on radiative forcing, the fractional radiative forcing from CO2, and the dependence of radiative forcing from CO2 on carbon emissions. Mechanistically each term varies, respectively, with climate sensitivity and ocean heat uptake, radiative forcing contributions, and ocean and terrestrial carbon uptake. The sensitivity of surface warming to fossil-fuel carbon emissions is examined using an ensemble of Earth system models, forced either by an annual increase in atmospheric CO2 or by RCPs until year 2100. The sensitivity of surface warming to carbon emissions is controlled by a temporal decrease in the dependence of radiative forcing from CO2 on carbon emissions, which is partly offset by a temporal increase in the dependence of surface warming on radiative forcing. The decrease in the dependence of radiative forcing from CO2 is due to a decline in the ratio of the global ocean carbon undersaturation to carbon emissions, while the increase in the dependence of surface warming is due to a decline in the ratio of ocean heat uptake to radiative forcing. At the present time, there are large intermodel differences in the sensitivity in surface warming to carbon emissions, which are mainly due to uncertainties in the climate sensitivity and ocean heat uptake. These uncertainties undermine the ability to predict how much carbon may be emitted before reaching a warming target.

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Emergence of multiple ocean ecosystem drivers in a large ensemble suite with an Earth system model

Biogeosciences ◽

10.5194/bg-12-3301-2015 ◽

2015 ◽

Vol 12 (11) ◽

pp. 3301-3320 ◽

Cited By ~ 71

Author(s):

K. B. Rodgers ◽

J. Lin ◽

T. L. Frölicher

Keyword(s):

Southern Ocean ◽

Marine Ecosystem ◽

Marine Ecosystems ◽

Earth System Model ◽

System Model ◽

Global Ocean ◽

Earth System ◽

Biological Productivity ◽

The Tropics ◽

Ecosystem Drivers

Abstract. Marine ecosystems are increasingly stressed by human-induced changes. Marine ecosystem drivers that contribute to stressing ecosystems – including warming, acidification, deoxygenation and perturbations to biological productivity – can co-occur in space and time, but detecting their trends is complicated by the presence of noise associated with natural variability in the climate system. Here we use large initial-condition ensemble simulations with an Earth system model under a historical/RCP8.5 (representative concentration pathway 8.5) scenario over 1950–2100 to consider emergence characteristics for the four individual and combined drivers. Using a 1-standard-deviation (67% confidence) threshold of signal to noise to define emergence with a 30-year trend window, we show that ocean acidification emerges much earlier than other drivers, namely during the 20th century over most of the global ocean. For biological productivity, the anthropogenic signal does not emerge from the noise over most of the global ocean before the end of the 21st century. The early emergence pattern for sea surface temperature in low latitudes is reversed from that of subsurface oxygen inventories, where emergence occurs earlier in the Southern Ocean. For the combined multiple-driver field, 41% of the global ocean exhibits emergence for the 2005–2014 period, and 63% for the 2075–2084 period. The combined multiple-driver field reveals emergence patterns by the end of this century that are relatively high over much of the Southern Ocean, North Pacific, and Atlantic, but relatively low over the tropics and the South Pacific. For the case of two drivers, the tropics including habitats of coral reefs emerges earliest, with this driven by the joint effects of acidification and warming. It is precisely in the regions with pronounced emergence characteristics where marine ecosystems may be expected to be pushed outside of their comfort zone determined by the degree of natural background variability to which they are adapted. The results underscore the importance of sustained multi-decadal observing systems for monitoring multiple ecosystems drivers.

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Diagnosing CO2 emission-induced feedbacks between the Southern Ocean carbon cycle and the climate system: A multiple Earth System Model analysis using a water mass tracking approach

Journal of Climate ◽

10.1175/jcli-d-20-0889.1 ◽

2021 ◽

pp. 1-62

Author(s):

Tilla Roy ◽

Jean Baptiste Sallée ◽

Laurent Bopp ◽

Nicolas Metzl

Keyword(s):

Climate Change ◽

Southern Ocean ◽

Water Mass ◽

Buffering Capacity ◽

Climate Impacts ◽

System Model ◽

Earth System ◽

Local Climate ◽

Mode Waters ◽

Ocean Carbon

AbstractAnthropogenic CO2 emission-induced feedbacks between the carbon cycle and the climate system perturb the efficiency of atmospheric CO2 uptake by land and ocean carbon reservoirs. The Southern Ocean is a region where these feedbacks can be largest and differ most among Earth System Model projections of 21st century climate change. To improve our mechanistic understanding of these feedbacks, we develop an automated procedure that tracks changes in the positions of Southern Ocean water masses and their carbon uptake. In an idealised ensemble of climate change projections, we diagnose two carbon–concentration feedbacks driven by atmospheric CO2 (due to increasing air-sea CO2 partial pressure difference, dpCO2, and reducing carbonate buffering capacity) and two carbon–climate feedbacks driven by climate change (due to changes in the water mass surface outcrop areas and local climate impacts). Collectively these feedbacks increase the CO2 uptake by the Southern Ocean and account for one-fifth of the global uptake of CO2 emissions. The increase in CO2 uptake is primarily dpCO2-driven, with Antarctic intermediate waters making the largest contribution; the remaining three feedbacks partially offset this increase (by ~25%), with maximum reductions in Subantarctic mode waters. The process dominating the decrease in CO2 uptake is water mass-dependent: reduction in carbonate buffering capacity in Subtropical and Subantarctic mode waters, local climate impacts in Antarctic intermediate waters, and reduction in outcrop areas in circumpolar deep waters and Antarctic bottom waters. Intermodel variability in the feedbacks is predominately dpCO2–driven and should be a focus of efforts to constrain projection uncertainty.

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Evolving Relative Importance of the Southern Ocean and North Atlantic in Anthropogenic Ocean Heat Uptake

Journal of Climate ◽

10.1175/jcli-d-18-0170.1 ◽

2018 ◽

Vol 31 (18) ◽

pp. 7459-7479 ◽

Cited By ~ 21

Author(s):

Jia-Rui Shi ◽

Shang-Ping Xie ◽

Lynne D. Talley

Keyword(s):

Southern Ocean ◽

North Atlantic ◽

Radiative Forcing ◽

Meridional Overturning Circulation ◽

Anthropogenic Heat ◽

Anthropogenic Aerosols ◽

Ocean Heat Uptake ◽

The North ◽

Heat Uptake ◽

The North Atlantic

Ocean uptake of anthropogenic heat over the past 15 years has mostly occurred in the Southern Ocean, based on Argo float observations. This agrees with historical simulations from phase 5 of the Coupled Model Intercomparison Project (CMIP5), where the Southern Ocean (south of 30°S) accounts for 72% ± 28% of global heat uptake, while the contribution from the North Atlantic north of 30°N is only 6%. Aerosols preferentially cool the Northern Hemisphere, and the effect on surface heat flux over the subpolar North Atlantic opposes the greenhouse gas (GHG) effect in nearly equal magnitude. This heat uptake compensation is associated with weakening (strengthening) of the Atlantic meridional overturning circulation (AMOC) in response to GHG (aerosol) radiative forcing. Aerosols are projected to decline in the near future, reinforcing the greenhouse effect on the North Atlantic heat uptake. As a result, the Southern Ocean, which will continue to take up anthropogenic heat largely through the mean upwelling of water from depth, will be joined by increased relative contribution from the North Atlantic because of substantial AMOC slowdown in the twenty-first century. In the RCP8.5 scenario, the percentage contribution to global uptake is projected to decrease to 48% ± 8% in the Southern Ocean and increase to 26% ± 6% in the northern North Atlantic. Despite the large uncertainty in the magnitude of projected aerosol forcing, our results suggest that anthropogenic aerosols, given their geographic distributions and temporal trajectories, strongly influence the high-latitude ocean heat uptake and interhemispheric asymmetry through AMOC change.

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Dependence of regional ocean heat uptake on anthropogenic warming scenarios

Science Advances ◽

10.1126/sciadv.abc0303 ◽

2020 ◽

Vol 6 (45) ◽

pp. eabc0303

Author(s):

Xiaofan Ma ◽

Wei Liu ◽

Robert J. Allen ◽

Gang Huang ◽

Xichen Li

Keyword(s):

Southern Ocean ◽

North Atlantic ◽

Meridional Overturning Circulation ◽

Anthropogenic Aerosols ◽

Ocean Heat Uptake ◽

The North ◽

Heat Uptake ◽

The North Atlantic ◽

Aerosol Effects ◽

Business As Usual

The North Atlantic and Southern Ocean exhibit enhanced ocean heat uptake (OHU) during recent decades while their future OHU changes are subject to great uncertainty. Here, we show that regional OHU patterns in these two basins are highly dependent on the trajectories of aerosols and greenhouse gases (GHGs) in future scenarios. During the 21st century, North Atlantic and Southern Ocean OHU exhibit similarly positive trends under a business-as-usual scenario but respectively positive and negative trends under a mitigation scenario. The opposite centurial OHU trends in the Southern Ocean can be attributed partially to distinct GHG trajectories under the two scenarios while the common positive centurial OHU trends in the North Atlantic are mainly due to aerosol effects. Under both scenarios, projected decline of anthropogenic aerosols potentially induces a weakening of the Atlantic Meridional Overturning Circulation and a divergence of meridional oceanic heat transport, which leads to enhanced OHU in the subpolar North Atlantic.

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Historical ocean heat uptake in CMIP6 Earth System models: global and regional perspectives

10.5194/egusphere-egu2020-4696 ◽

2020 ◽

Author(s):

Till Kuhlbrodt ◽

Aurore Voldoire ◽

Matthew Palmer ◽

Rachel Killick ◽

Colin Jones

Keyword(s):

Time Series ◽

Climate Models ◽

Global Climate ◽

Heat Content ◽

Global Ocean ◽

Earth System ◽

Ocean Heat Uptake ◽

The Pacific ◽

Heat Uptake ◽

Earth System Models

Ocean heat content is arguably one of the most relevant metrics for tracking global climate change and in particular the current global heating. Because of its enormous heat capacity, the global ocean stores about 93 percent of the excess heat in the Earth System. Time series of global ocean heat content (OHC) closely track Earth&#8217;s energy imbalance as observed as the net radiative balance at the top of the atmosphere. For these reasons simulated OHC time series are a cornerstone for assessing the scientific performance of Earth System models (ESM) and global climate models. Here we present a detailed analysis of the OHC change in simulations of the historical climate (20th century up to 2014) performed with four of the current, state-of-the art generation of ESMs and climate models. These four models are UKESM1, HadGEM3-GC3.1-LL, CNRM-ESM2-1 and CNRM-CM6-1. All four share the same ocean component, NEMO3.6 in the shaconemo eORCA1 configuration, and they all take part in CMIP6, the current Phase 6 of the Coupled Model Intercomparison Project. Analysing a small number of models gives us the opportunity to analyse OHC change for the global ocean as well as for individual ocean basins. In addition to the ensemble means, we focus on some individual ensemble members for a more detailed process understanding. For the global ocean, the two CNRM models reproduce the observed OHC change since the 1960s closely, especially in the top 700 m of the ocean. The two UK models (UKESM1 and HadGEM3-GC3.1-LL) do not simulate the observed global ocean warming in the 1970s and 1980s, and they warm too fast after 1991. We analyse how this varied performance across the models relates to the simulated radiative forcing of the atmosphere. All four models show a smaller ocean heat uptake since 1971, and a larger transient climate response (TCR), than the CMIP5 ensemble mean. Close analysis of a few individual ensemble members indicates a dominant role of heat uptake and deep-water formation processes in the Southern Ocean for variability and change in global OHC. Evaluating OHC change in individual ocean basins reveals that the lack of warming in the UK models stems from the Pacific and Indian basins, while in the Atlantic the OHC change 1971-2014 is close to the observed value. Resolving the ocean warming in depth and time shows that regional ocean heat uptake in the North Atlantic plays a substantial role in compensating small warming rates elsewhere. An opposite picture emerges from the CNRM models. Here the simulated OHC change is close to observations in the Pacific and Indian basins, while tending to be too small in the Atlantic, indicating a markedly different role for the Atlantic meridional overturning circulation (AMOC) and cross-equatorial heat transport in these models.

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Connecting tropical climate change with Southern Ocean heat uptake

Geophysical Research Letters ◽

10.1002/2017gl074972 ◽

2017 ◽

Vol 44 (18) ◽

pp. 9449-9457 ◽

Cited By ~ 14

Author(s):

Yen-Ting Hwang ◽

Shang-Ping Xie ◽

Clara Deser ◽

Sarah M. Kang

Keyword(s):

Climate Change ◽

Southern Ocean ◽

Tropical Climate ◽

Ocean Heat Uptake ◽

Heat Uptake ◽

Tropical Climate Change

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Simulating transient ice-ocean Ekman transport in the Regional Arctic System Model and Community Earth System Model

Annals of Glaciology ◽

10.3189/2015aog69a760 ◽

2015 ◽

Vol 56 (69) ◽

pp. 211-228 ◽

Cited By ~ 21

Author(s):

Andrew Roberts ◽

Anthony Craig ◽

Wieslaw Maslowski ◽

Robert Osinski ◽

Alice Duvivier ◽

...

Keyword(s):

Sea Ice ◽

Arctic Sea Ice ◽

Earth System Model ◽

System Model ◽

Ekman Transport ◽

Earth System ◽

Inertial Oscillations ◽

Arctic Sea ◽

Community Earth System Model

AbstractThis work evaluates the fidelity of the polar marine Ekman layer in the Regional Arctic System Model (RASM) and Community Earth System Model (CESM) using sea-ice inertial oscillations as a proxy for ice-ocean Ekman transport. A case study is presented that demonstrates that RASM replicates inertial oscillations in close agreement with motion derived using the GPS. This result is obtained from a year-long case study pre-dating the recent decline in perennial Arctic sea ice, using RASM with sub-hourly coupling between the atmosphere, sea-ice and ocean components. To place this work in context, the RASM coupling method is applied to CESM, increasing the frequency of oceanic flux exchange from once per day in the standard CESM configuration, to every 30 min. For a single year simulation, this change causes a considerable increase in the median inertial ice speed across large areas of the Southern Ocean and parts of the Arctic sea-ice zone. The result suggests that processes associated with the passage of storms over sea ice (e.g. oceanic mixing, sea-ice deformation and surface energy exchange) are underestimated in Earth System Models that do not resolve inertial frequencies in their marine coupling cycle.

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Multidecadal preconditioning of the Maud Rise polynya region

Ocean Science ◽

10.5194/os-16-1443-2020 ◽

2020 ◽

Vol 16 (6) ◽

pp. 1443-1457

Author(s):

René M. van Westen ◽

Henk A. Dijkstra

Keyword(s):

High Resolution ◽

Southern Ocean ◽

Large Scale ◽

Heat Content ◽

Earth System Model ◽

System Model ◽

Earth System ◽

Model Resolution ◽

Community Earth System Model ◽

Maud Rise

Abstract. In this paper, we consider Maud Rise polynya formation in a long (250-year) high-resolution (ocean 0.1∘, atmosphere 0.5∘ horizontal model resolution) of the Community Earth System Model. We find a dominant multidecadal timescale in the occurrence of these Maud Rise polynyas. Analysis of the results leads us to the interpretation that a preferred timescale can be induced by the variability of the Weddell Gyre, previously identified as the Southern Ocean Mode. The large-scale pattern of heat content variability associated with the Southern Ocean Mode modifies the stratification in the Maud Rise region and leads to a preferred timescale in convection through preconditioning of the subsurface density and consequently to polynya formation.

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