Northern High-Latitude Heat Budget Decomposition and Transient Warming

Abstract Climate models simulate a wide range of climate changes at high northern latitudes in response to increased CO2. They also have substantial disagreement on projected changes of the Atlantic meridional overturning circulation (AMOC). Here, two pairs of closely related climate models are used, with each containing members with large and small AMOC declines to explore the influence of AMOC decline on the high-latitude response to increased CO2. The models with larger AMOC decline have less high-latitude warming and sea ice decline than their small AMOC decline counterpart. By examining differences in the perturbation heat budget of the 40°–90°N region, it is shown that AMOC decline diminishes the warming by weakening poleward ocean heat transport and increasing the ocean heat uptake. The cooling impact of this AMOC-forced surface heat flux perturbation difference is enhanced by shortwave feedback and diminished by longwave feedback and atmospheric heat transport differences. The magnitude of the AMOC decline within model pairs is positively related to the magnitudes of control climate AMOC and Labrador and Nordic Seas convection. Because the 40°–90°N region accounts for up to 40% of the simulated global ocean heat uptake over 100 yr, the process described here influences the global heat uptake efficiency.

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Southern Ocean Heat Uptake, Redistribution, and Storage in a Warming Climate: The Role of Meridional Overturning Circulation

Journal of Climate ◽

10.1175/jcli-d-17-0761.1 ◽

2018 ◽

Vol 31 (12) ◽

pp. 4727-4743 ◽

Cited By ~ 24

Author(s):

Wei Liu ◽

Jian Lu ◽

Shang-Ping Xie ◽

Alexey Fedorov

Keyword(s):

Southern Ocean ◽

Heat Transport ◽

Climate Models ◽

Climate Model ◽

Meridional Overturning Circulation ◽

The Other ◽

Anthropogenic Heat ◽

Overturning Circulation ◽

Ocean Heat Uptake ◽

Meridional Overturning

Climate models show that most of the anthropogenic heat resulting from increased atmospheric CO2 enters the Southern Ocean near 60°S and is stored around 45°S. This heat is transported to the ocean interior by the meridional overturning circulation (MOC) with wind changes playing an important role in the process. To isolate and quantify the latter effect, we apply an overriding technique to a climate model and decompose the total ocean response to CO2 increase into two major components: one due to wind changes and the other due to direct CO2 effect. We find that the poleward-intensified zonal surface winds tend to shift and strengthen the ocean Deacon cell and hence the residual MOC, leading to anomalous divergence of ocean meridional heat transport around 60°S coupled to a surface heat flux increase. In contrast, at 45°S we see anomalous convergence of ocean heat transport and heat loss at the surface. As a result, the wind-induced ocean heat storage (OHS) peaks at 46°S at a rate of 0.07 ZJ yr−1 (° lat)−1 (1 ZJ = 1021 J), contributing 20% to the total OHS maximum. The direct CO2 effect, on the other hand, very slightly alters the residual MOC but primarily warms the ocean. It induces a small but nonnegligible change in eddy heat transport and causes OHS to peak at 42°S at a rate of 0.30 ZJ yr−1 (° lat)−1, accounting for 80% of the OHS maximum. We also find that the eddy-induced MOC weakens, primarily caused by a buoyancy flux change as a result of the direct CO2 effect, and does not compensate the intensified Deacon cell.

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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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Transient Climate Sensitivity Depends on Base Climate Ocean Circulation

Journal of Climate ◽

10.1175/jcli-d-16-0581.1 ◽

2017 ◽

Vol 30 (4) ◽

pp. 1493-1504 ◽

Cited By ~ 21

Author(s):

Jie He ◽

Michael Winton ◽

Gabriel Vecchi ◽

Liwei Jia ◽

Maria Rugenstein

Keyword(s):

Climate Change ◽

Ocean Circulation ◽

Climate Sensitivity ◽

High Latitude ◽

Climate Models ◽

Meridional Overturning Circulation ◽

Future Climate Change ◽

Tropical Ocean ◽

Ocean Heat Uptake ◽

Starting Point

Abstract There is large uncertainty in the simulation of transient climate sensitivity. This study aims to understand how such uncertainty is related to the simulation of the base climate by comparing two simulations with the same model but in which CO2 is increased from either a preindustrial (1860) or a present-day (1990) control simulation. This allows different base climate ocean circulations that are representative of those in current climate models to be imposed upon a single model. As a result, the model projects different transient climate sensitivities that are comparable to the multimodel spread. The greater warming in the 1990-start run occurs primarily at high latitudes and particularly over regions of oceanic convection. In the 1990-start run, ocean overturning circulations are initially weaker and weaken less from CO2 forcing. As a consequence, there are smaller reductions in the poleward ocean heat transport, leading to less tropical ocean heat storage and less moderated high-latitude surface warming. This process is evident in both hemispheres, with changes in the Atlantic meridional overturning circulation and the Antarctic Bottom Water formation dominating the warming differences in each hemisphere. The high-latitude warming in the 1990-start run is enhanced through albedo and cloud feedbacks, resulting in a smaller ocean heat uptake efficacy. The results highlight the importance of improving the base climate ocean circulation in order to provide a reasonable starting point for assessments of past climate change and the projection of future climate change.

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Mechanisms of Southern Ocean Heat Uptake and Transport in a Global Eddying Climate Model

Journal of Climate ◽

10.1175/jcli-d-15-0579.1 ◽

2016 ◽

Vol 29 (6) ◽

pp. 2059-2075 ◽

Cited By ~ 23

Author(s):

Adele K. Morrison ◽

Stephen M. Griffies ◽

Michael Winton ◽

Whit G. Anderson ◽

Jorge L. Sarmiento

Keyword(s):

Southern Ocean ◽

Heat Transport ◽

Climate Model ◽

Heat Budget ◽

Global Climate Model ◽

Heat Content ◽

Ocean Heat Uptake ◽

The North ◽

Heat Uptake ◽

Mixed Layers

Abstract The Southern Ocean plays a dominant role in anthropogenic oceanic heat uptake. Strong northward transport of the heat content anomaly limits warming of the sea surface temperature in the uptake region and allows the heat uptake to be sustained. Using an eddy-rich global climate model, the processes controlling the northward transport and convergence of the heat anomaly in the midlatitude Southern Ocean are investigated in an idealized 1% yr−1 increasing CO2 simulation. Heat budget analyses reveal that different processes dominate to the north and south of the main convergence region. The heat transport northward from the uptake region in the south is driven primarily by passive advection of the heat content anomaly by the existing time mean circulation, with a smaller 20% contribution from enhanced upwelling. The heat anomaly converges in the midlatitude deep mixed layers because there is not a corresponding increase in the mean heat transport out of the deep mixed layers northward into the mode waters. To the north of the deep mixed layers, eddy processes drive the warming and account for nearly 80% of the northward heat transport anomaly. The eddy transport mechanism results from a reduction in both the diffusive and advective southward eddy heat transports, driven by decreasing isopycnal slopes and decreasing along-isopycnal temperature gradients on the northern edge of the peak warming.

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What is the heat uptake potential of Antarctic Bottom Water?

10.5194/egusphere-egu21-3825 ◽

2021 ◽

Author(s):

Jan Zika ◽

Abhishek Savita ◽

Ryan Holmes ◽

Taimoor Sohail

Keyword(s):

Heat Transport ◽

Upper Bound ◽

Bottom Water ◽

Climate Models ◽

Deep Ocean ◽

Meridional Overturning Circulation ◽

Antarctic Bottom Water ◽

Heat Uptake ◽

Vertical Heat ◽

Highly Correlated

Antarctic Bottom Water (AABW) is a cold dense water mass which sinks around Antarctica keeping the abyssal ocean relatively cool. Recent observations have suggested a component of recent deep ocean warming is linked to AABW. Here we explore how much changes in AABW could affect changes in vertical ocean heat transport in a warming climate. If the AABW circulation were to be completely extinguished, for example due to increases in upper ocean thermal stratification, AABW would cease to cool the deep ocean and hence lead to an effective warming of the abyss. Therefore, we propose that long term mean vertical heat transport of the AABW circulation is an effective upper bound on the change in heat transport that can be affected by changes in AABW. We call this upper bound the &#8216;heat uptake potential&#8217;. We analyse AABW circulations in an ensemble of numerical climate models. We find that the AABW circulation contributes between 0.05Wm-2 and 0.15Wm-2 to the global vertical heat balance in the model&#8217;s pre-industrial states. Indeed, under abrupt CO2 forcing changes, AABW heat transport systematically reduces (in some cases completely), with the largest reductions occurring in models with the largest pre-industrial mean heat transports. The AABW circulation vertical heat transport is found to be highly correlated with the minimum of the Meridional Overturning Circulation at 50oS in the models, suggesting there may be observable constraints on the heat uptake potential of AABW.

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Predicting Climate Change through Response Operators in a Coupled General Circulation Model

10.5194/egusphere-egu2020-9174 ◽

2020 ◽

Cited By ~ 1

Author(s):

Valerio Lembo ◽

Valerio Lucarini ◽

Francesco Ragone

Keyword(s):

Climate Change ◽

General Circulation ◽

Climate Models ◽

Circulation Model ◽

Global Climate Models ◽

Meridional Overturning Circulation ◽

Climate System ◽

Global Ocean ◽

Climate Response ◽

Ocean Heat Uptake

Global Climate Models are key tools for predicting the future response of the climate system to a variety of natural and anthropogenic forcings. Typically, an ensemble of simulations is performed considering a scenario of forcing, in order to analyse the response of the climate system to the specific forcing signal. Given that the the climate response spans a very large range of timescales, such a strategy often requires a dramatic amount of computational resources. In this paper we show how to use statistical mechanics to construct operators able to flexibly predict climate change for a variety of climatic variables of interest, going beyond the limitation of having to consider specific time patterns of forcing. We perform our study on a fully coupled GCM - MPI-ESM v.1.2 - and for the first time we prove the effectiveness of response theory in predicting future climate response to CO2&#160;increase on a vast range of temporal scales. We specifically treat atmospheric &#160;(surface temperature) and oceanic variables (strength of the Atlantic Meridional Overturning Circulation and of the Antarctic Circumpolar Current), as well as the global ocean heat uptake.

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Impacts on Ocean Heat from Transient Mesoscale Eddies in a Hierarchy of Climate Models

Journal of Climate ◽

10.1175/jcli-d-14-00353.1 ◽

2015 ◽

Vol 28 (3) ◽

pp. 952-977 ◽

Cited By ~ 175

Author(s):

Stephen M. Griffies ◽

Michael Winton ◽

Whit G. Anderson ◽

Rusty Benson ◽

Thomas L. Delworth ◽

...

Keyword(s):

Heat Transport ◽

Climate Models ◽

Climate Model ◽

Heat Budget ◽

Mesoscale Eddy ◽

Mesoscale Eddies ◽

Temperature Drift ◽

Coupled Models ◽

Ocean Heat Uptake ◽

Ocean Climate

Abstract The authors characterize impacts on heat in the ocean climate system from transient ocean mesoscale eddies. Their tool is a suite of centennial-scale 1990 radiatively forced numerical climate simulations from three GFDL coupled models comprising the Climate Model, version 2.0–Ocean (CM2-O), model suite. CM2-O models differ in their ocean resolution: CM2.6 uses a 0.1° ocean grid, CM2.5 uses an intermediate grid with 0.25° spacing, and CM2-1deg uses a nominal 1.0° grid. Analysis of the ocean heat budget reveals that mesoscale eddies act to transport heat upward in a manner that partially compensates (or offsets) for the downward heat transport from the time-mean currents. Stronger vertical eddy heat transport in CM2.6 relative to CM2.5 accounts for the significantly smaller temperature drift in CM2.6. The mesoscale eddy parameterization used in CM2-1deg also imparts an upward heat transport, yet it differs systematically from that found in CM2.6. This analysis points to the fundamental role that ocean mesoscale features play in transient ocean heat uptake. In general, the more accurate simulation found in CM2.6 provides an argument for either including a rich representation of the ocean mesoscale in model simulations of the mean and transient climate or for employing parameterizations that faithfully reflect the role of eddies in both lateral and vertical heat transport.

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Estimating Global Ocean Heat Content Changes in the Upper 1800 m since 1950 and the Influence of Climatology Choice*

Journal of Climate ◽

10.1175/jcli-d-12-00752.1 ◽

2014 ◽

Vol 27 (5) ◽

pp. 1945-1957 ◽

Cited By ~ 55

Author(s):

John M. Lyman ◽

Gregory C. Johnson

Keyword(s):

Heat Content ◽

Ocean Heat Content ◽

Global Ocean ◽

Ocean Temperature ◽

Vertical Separation ◽

Ocean Heat Uptake ◽

Expendable Bathythermograph ◽

Heat Uptake ◽

The Mean

Abstract Ocean heat content anomalies are analyzed from 1950 to 2011 in five distinct depth layers (0–100, 100–300, 300–700, 700–900, and 900–1800 m). These layers correspond to historic increases in common maximum sampling depths of ocean temperature measurements with time, as different instruments—mechanical bathythermograph (MBT), shallow expendable bathythermograph (XBT), deep XBT, early sometimes shallower Argo profiling floats, and recent Argo floats capable of worldwide sampling to 2000 m—have come into widespread use. This vertical separation of maps allows computation of annual ocean heat content anomalies and their sampling uncertainties back to 1950 while taking account of in situ sampling advances and changing sampling patterns. The 0–100-m layer is measured over 50% of the globe annually starting in 1956, the 100–300-m layer starting in 1967, the 300–700-m layer starting in 1983, and the deepest two layers considered here starting in 2003 and 2004, during the implementation of Argo. Furthermore, global ocean heat uptake estimates since 1950 depend strongly on assumptions made concerning changes in undersampled or unsampled ocean regions. If unsampled areas are assumed to have zero anomalies and are included in the global integrals, the choice of climatological reference from which anomalies are estimated can strongly influence the global integral values and their trend: the sparser the sampling and the bigger the mean difference between climatological and actual values, the larger the influence.

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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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Climate Change Detection and Attribution: Beyond Mean Temperature Signals

Journal of Climate ◽

10.1175/jcli3900.1 ◽

2006 ◽

Vol 19 (20) ◽

pp. 5058-5077 ◽

Cited By ~ 58

Author(s):

Gabriele C. Hegerl ◽

Thomas R. Karl ◽

Myles Allen ◽

Nathaniel L. Bindoff ◽

Nathan Gillett ◽

...

Keyword(s):

Climate Change ◽

Large Scale ◽

Climate Models ◽

Climate Extremes ◽

Global Ocean ◽

Sampled Data ◽

Free Atmosphere ◽

Ocean Heat Uptake ◽

Continental Scale ◽

Detection And Attribution

Abstract A significant influence of anthropogenic forcing has been detected in global- and continental-scale surface temperature, temperature of the free atmosphere, and global ocean heat uptake. This paper reviews outstanding issues in the detection of climate change and attribution to causes. The detection of changes in variables other than temperature, on regional scales and in climate extremes, is important for evaluating model simulations of changes in societally relevant scales and variables. For example, sea level pressure changes are detectable but are significantly stronger in observations than the changes simulated in climate models, raising questions about simulated changes in climate dynamics. Application of detection and attribution methods to ocean data focusing not only on heat storage but also on the penetration of the anthropogenic signal into the ocean interior, and its effect on global water masses, helps to increase confidence in simulated large-scale changes in the ocean. To evaluate climate change signals with smaller spatial and temporal scales, improved and more densely sampled data are needed in both the atmosphere and ocean. Also, the problem of how model-simulated climate extremes can be compared to station-based observations needs to be addressed.

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