Transient versus Equilibrium Response of the Ocean’s Overturning Circulation to Warming

Much of the existing theory for the ocean’s overturning circulation considers steady-state equilibrium solutions. However, Earth’s climate is not in a steady state, and a better understanding of the ocean’s nonequilibrium response to changes in the surface climate is urgently needed. Here, the time-dependent response of the deep-ocean overturning circulation to atmospheric warming is examined using a hierarchy of idealized ocean models. The transient response to surface warming is characterized by a shoaling and weakening of the Atlantic meridional overturning circulation (AMOC)—consistent with results from coupled climate simulations. The initial shoaling and weakening of the AMOC occurs on decadal time scales and is attributed to a rapid warming of northern-sourced deep water. The equilibrium response to warming, in contrast, is associated with a deepening and strengthening of the AMOC. The eventual deepening of the AMOC is argued to be associated with abyssal density changes and driven by modified surface fluxes in the Southern Ocean, following a reduction of the Antarctic sea ice cover. Full equilibration of the AMOC requires a diffusive adjustment of the abyss and takes many millennia. The equilibration time scale is much longer than most coupled climate model simulations, highlighting the importance of considering integration time and initial conditions when interpreting the deep-ocean circulation in climate models. The results also show that past climates are unlikely to be an adequate analog for changes in the overturning circulation during the coming decades or centuries.

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Co-variability of salinity and temperature changes in the North Atlantic

10.5194/egusphere-egu21-5324 ◽

2021 ◽

Author(s):

Levke Caesar ◽

Gerard McCarthy

Keyword(s):

Ocean Circulation ◽

North Atlantic ◽

Climate Model ◽

Surface Fluxes ◽

Meridional Overturning Circulation ◽

Geophysical Research ◽

Overturning Circulation ◽

The North ◽

Cold Anomaly ◽

The North Atlantic

While there is increasing paleoclimatic evidence that the Atlantic Meridional Overturning Circulation (AMOC) has weakened over the last one to two hundred years (Caesar et al., 2018; Thornalley et al., 2018), this is not confirmed by climate model simulations. Instead, the new simulations from the 6th Coupled Model Intercomparison Project (CMIP6) show a slight strengthening of the multimodel mean AMOC from 1850 until about 1985 (Menary et al., 2020), attributed to anthropogenic aerosol forcing. Arguing for a recent weakening of the AMOC, some studies attribute the emergence of the North Atlantic warming hole as a sign of the reduced meridional heat transport associated with a weaker AMOC (e.g. Caesar et al., 2018), yet this cold anomaly has also been interpreted as being aerosol-forced (Booth et al., 2012) and therefore not necessarily a sign of a weakening AMOC but rather a possible driver of a strengthening of the AMOC.Looking beyond temperature, a fresh anomaly has recently emerged in the subpolar North Atlantic (Holliday et al., 2020). While a strengthening AMOC has been linked with an increase in salinity in the subpolar gyre region (Menary et al., 2013), an AMOC weakening would, due to the salt-advection feedback, likely lead to a reduction in salinity in the North Atlantic region. To shed some light on the question of whether the cold anomaly is internally (AMOC) or externally (aerosol-forced) driven we consider the co-variability of salinity and temperature in the North Atlantic in respect of changes in surface fluxes or alternate drivers.&#160;ReferencesBooth, B.B.B., Dunstone, N.J., Halloran, P.R., Andrews, T. and Bellouin, N., 2012. Aerosols implicated as a prime driver of twentieth-century North Atlantic climate variability. Nature, 484(7393): 228&#8211;232.Caesar, L., Rahmstorf, S., Robinson, A., Feulner, G. and Saba, V., 2018. Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature, 556(7700): 191-196.Holliday, N.P., Bersch, M., Berx, B., Chafik, L., Cunningham, S., Florindo-L&#243;pez, C., H&#225;t&#250;n, H., Johns, W., Josey, S.A., Larsen, K.M.H., Mulet, S., Oltmanns, M., Reverdin, G., Rossby, T., Thierry, V., Valdimarsson, H. and Yashayaev, I., 2020. Ocean circulation causes the largest freshening event for 120 years in eastern subpolar North Atlantic. Nature Communications, 11(1): 585.Menary, M.B., Roberts, C.D., Palmer, M.D., Halloran, P.R., Jackson, L., Wood, R.A., M&#252;ller, W.A., Matei, D. and Lee, S.-K., 2013. Mechanisms of aerosol-forced AMOC variability in a state of the art climate model. Journal of Geophysical Research: Oceans, 118(4): 2087-2096.Menary, M.B., Robson, J., Allan, R.P., Booth, B.B.B., Cassou, C., Gastineau, G., Gregory, J., Hodson, D., Jones, C., Mignot, J., Ringer, M., Sutton, R., Wilcox, L. and Zhang, R., 2020. Aerosol-Forced AMOC Changes in CMIP6 Historical Simulations. Geophysical Research Letters, 47(14): e2020GL088166.Thornalley, D.J.R., Oppo, D.W., Ortega, P., Robson, J.I., Brierley, C.M., Davis, R., Hall, I.R., Moffa-Sanchez, P., Rose, N.L., Spooner, P.T., Yashayaev, I. and Keigwin, L.D., 2018. Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years. Nature, 556(7700): 227-230.

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Fueling primary productivity: nutrient return pathways from the deep ocean and their dependence on the Meridional Overturning Circulation

Biogeosciences Discussions ◽

10.5194/bgd-7-4045-2010 ◽

2010 ◽

Vol 7 (3) ◽

pp. 4045-4088 ◽

Cited By ~ 3

Author(s):

J. B. Palter ◽

J. L. Sarmiento ◽

A. Gnanadesikan ◽

J. Simeon ◽

D. Slater

Keyword(s):

Southern Ocean ◽

Primary Productivity ◽

Deep Ocean ◽

Ocean Model ◽

Surface Fluxes ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Nutrient Return ◽

Mode Water ◽

Meridional Overturning

Abstract. In the Southern Ocean, mixing and upwelling in the presence of heat and freshwater surface fluxes transform subpycnocline water to lighter densities as part of the upward branch of the Meridional Overturning Circulation (MOC). One hypothesized impact of this transformation is the restoration of nutrients to the global pycnocline, without which biological productivity at low latitudes would be catastrophically reduced. Here we use a novel set of modeling experiments to explore the causes and consequences of the Southern Ocean nutrient return pathway. Specifically, we quantify the contribution to global productivity of nutrients that rise from the ocean interior in the Southern Ocean, the northern high latitudes, and by mixing across the low latitude pycnocline. In addition, we evaluate how the strength of the Southern Ocean winds and the parameterizations of subgridscale processes change the dominant nutrient return pathways in the ocean. Our results suggest that nutrients upwelled from the deep ocean in the Antarctic Circumpolar Current and subducted in Subantartic Mode Water support between 33 and 75% of global primary productivity between 30° S and 30° N. The high end of this range results from an ocean model in which the MOC is driven primarily by wind-induced Southern Ocean upwelling, a configuration favored due to its fidelity to tracer data, while the low end results from an MOC driven by high diapycnal diffusivity in the pycnocline. In all models, the high preformed nutrients subducted in the SAMW layer are converted rapidly (in less than 40 years) to remineralized nutrients, explaining previous modeling results that showed little influence of the drawdown of SAMW surface nutrients on atmospheric carbon concentrations.

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Chaotic variability of the meridional overturning circulation on subannual to interannual timescales

Ocean Science ◽

10.5194/os-9-805-2013 ◽

2013 ◽

Vol 9 (5) ◽

pp. 805-823 ◽

Cited By ~ 23

Author(s):

J. J.-M. Hirschi ◽

A. T. Blaker ◽

B. Sinha ◽

A. Coward ◽

B. de Cuevas ◽

...

Keyword(s):

Initial Conditions ◽

Large Fraction ◽

Mesoscale Eddy ◽

Surface Fluxes ◽

Meridional Overturning Circulation ◽

Global Ocean ◽

Overturning Circulation ◽

Ocean Eddies ◽

Mesoscale Ocean Eddies ◽

Meridional Overturning

Abstract. Observations and numerical simulations have shown that the meridional overturning circulation (MOC) exhibits substantial variability on sub- to interannual timescales. This variability is not fully understood. In particular it is not known what fraction of the MOC variability is caused by processes such as mesoscale ocean eddies and waves which are ubiquitous in the ocean. Here we analyse twin experiments performed with a global ocean model at eddying (1/4°) and non-eddying (1°) resolutions. The twin experiments are forced with the same surface fluxes for the 1958 to 2001 period but start from different initial conditions. Our results show that on subannual to interannual timescales a large fraction of MOC variability directly reflects variability in the surface forcing. Nevertheless, in the eddy-permitting case there is an initial-condition-dependent MOC variability (hereinafter referred to as "chaotic" variability) of several Sv (1Sv = 106 m3 s−1) in the Atlantic and the Indo-Pacific. In the Atlantic the chaotic MOC variability represents up to 30% of the total variability at the depths where the maximum MOC occurs. In comparison the chaotic MOC variability is only 5–10% in the non-eddying case. The surface forcing being almost identical in the twin experiments suggests that mesoscale ocean eddies are the most likely cause for the increased chaotic MOC variability in the eddying case. The exact formation time of eddies is determined by the initial conditions which are different in the two model passes, and as a consequence the mesoscale eddy field is decorrelated in the twin experiments. In regions where eddy activity is high in the eddy-permitting model, the correlation of sea surface height variability in the twin runs is close to zero. In the non-eddying case in contrast, we find high correlations (0.9 or higher) over most regions. Looking at the sub- and interannual MOC components separately reveals that most of the chaotic MOC variability is found on subannual timescales for the eddy-permitting model. On interannual timescales the amplitude of the chaotic MOC variability is much smaller and the amplitudes are comparable for both the eddy-permitting and non-eddy-permitting model resolutions. Whereas the chaotic MOC variability on interannual timescales only accounts for a small fraction of the total chaotic MOC variability in the eddy-permitting case, it is the main contributor to the chaotic variability in the non-eddying case away from the Equator.

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Fueling export production: nutrient return pathways from the deep ocean and their dependence on the Meridional Overturning Circulation

Biogeosciences ◽

10.5194/bg-7-3549-2010 ◽

2010 ◽

Vol 7 (11) ◽

pp. 3549-3568 ◽

Cited By ~ 47

Author(s):

J. B. Palter ◽

J. L. Sarmiento ◽

A. Gnanadesikan ◽

J. Simeon ◽

R. D. Slater

Keyword(s):

Southern Ocean ◽

Deep Ocean ◽

Ocean Model ◽

Surface Fluxes ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Nutrient Return ◽

Mode Water ◽

Export Production ◽

Meridional Overturning

Abstract. In the Southern Ocean, mixing and upwelling in the presence of heat and freshwater surface fluxes transform subpycnocline water to lighter densities as part of the upward branch of the Meridional Overturning Circulation (MOC). One hypothesized impact of this transformation is the restoration of nutrients to the global pycnocline, without which biological productivity at low latitudes would be significantly reduced. Here we use a novel set of modeling experiments to explore the causes and consequences of the Southern Ocean nutrient return pathway. Specifically, we quantify the contribution to global productivity of nutrients that rise from the ocean interior in the Southern Ocean, the northern high latitudes, and by mixing across the low latitude pycnocline. In addition, we evaluate how the strength of the Southern Ocean winds and the parameterizations of subgridscale processes change the dominant nutrient return pathways in the ocean. Our results suggest that nutrients upwelled from the deep ocean in the Antarctic Circumpolar Current and subducted in Subantartic Mode Water support between 33 and 75% of global export production between 30° S and 30° N. The high end of this range results from an ocean model in which the MOC is driven primarily by wind-induced Southern Ocean upwelling, a configuration favored due to its fidelity to tracer data, while the low end results from an MOC driven by high diapycnal diffusivity in the pycnocline. In all models, nutrients exported in the SAMW layer are utilized and converted rapidly (in less than 40 years) to remineralized nutrients, explaining previous modeling results that showed little influence of the drawdown of SAMW surface nutrients on atmospheric carbon concentrations.

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Relationship between Air–Sea Density Flux and Isopycnal Meridional Overturning Circulation in a Warming Climate

Journal of Climate ◽

10.1175/jcli-d-11-00682.1 ◽

2013 ◽

Vol 26 (8) ◽

pp. 2683-2699 ◽

Cited By ~ 2

Author(s):

MyeongHee Han ◽

Igor Kamenkovich ◽

Timour Radko ◽

William E. Johns

Keyword(s):

Climate Model ◽

Linear Trend ◽

Deep Ocean ◽

Surface Fluxes ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Intergovernmental Panel ◽

Pull Out ◽

Deep Layers ◽

Meridional Overturning

Abstract This study aims to explore the relationship between air–sea density flux and isopycnal meridional overturning circulation (MOC), using the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) model projections of the twenty-first-century climate. The focus is on the semiadiabatic component of MOC beneath the mixed layer; this component is described using the concept of the push–pull mode, which represents the combined effects of the adiabatic push into the deep ocean in the Northern Hemisphere and the pull out of the deep ocean in the Southern Hemisphere. The analysis based on the GFDL Climate Model version 2.1 (CM2.1) simulation demonstrates that the push–pull mode and the actual isopycnal MOC at the equator evolve similarly in the deep layers, with their maximum transports decreasing by 4–5 Sv (1 Sv ≡ 106 m3 s−1) during years 2001–2100. In particular, the push–pull mode and actual isopycnal MOC are within approximately 10% of each other at the density layers heavier than 27.55 kg m−3, where the reduction in the MOC strength is the strongest. The decrease in the push–pull mode is caused by the direct contribution of the anomalous heat, rather than freshwater, surface fluxes. The agreement between the deep push–pull mode and MOC in the values of linear trend and variability on time scales longer than a decade suggests a largely adiabatic pole-to-pole mechanism for these changes. The robustness of the main conclusions is further explored in additional model simulations.

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A Toy Model for the Response of the Residual Overturning Circulation to Surface Warming

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0187.1 ◽

2019 ◽

Vol 49 (5) ◽

pp. 1249-1268 ◽

Cited By ~ 4

Author(s):

Malte F. Jansen ◽

Louis-Philippe Nadeau

Keyword(s):

General Circulation ◽

Circulation Model ◽

Deep Ocean ◽

Surface Fluxes ◽

Meridional Overturning Circulation ◽

Partial Recovery ◽

Surface Mixed Layer ◽

Overturning Circulation ◽

Surface Warming ◽

Meridional Overturning

A simple model for the deep-ocean overturning circulation is presented and applied to study the ocean’s response to a sudden surface warming. The model combines one-dimensional predictive residual advection–diffusion equations for the buoyancy in the basin and Southern Ocean surface mixed layer with diagnostic relationships for the residual overturning circulation between these regions. Despite its simplicity, the model reproduces the results from idealized general circulation model simulations and provides theoretical insights into the mechanisms that govern the response of the overturning circulation to an abrupt surface warming. Specifically, the model reproduces a rapid shoaling and weakening of the Atlantic meridional overturning circulation (AMOC) in response to surface warming, followed by a partial recovery over the following decades to centuries, and a full recovery after multiple millennia. The rapid partial recovery is associated with adjustment of the lower thermocline, which itself is shown to be accelerated by the weakened AMOC. Full equilibration instead requires adjustment of the abyssal buoyancy, which is shown to be governed by diapycnal diffusion and surface fluxes around Antarctica.

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Chaotic variability of the meridional overturning circulation on subannual to interannual timescales

Ocean Science Discussions ◽

10.5194/osd-9-3191-2012 ◽

2012 ◽

Vol 9 (5) ◽

pp. 3191-3238 ◽

Cited By ~ 6

Author(s):

J. J.-M. Hirschi ◽

A. T. Blaker ◽

B. Sinha ◽

A. Coward ◽

B. de Cuevas ◽

...

Keyword(s):

Initial Conditions ◽

Large Fraction ◽

Mesoscale Eddy ◽

Surface Fluxes ◽

Meridional Overturning Circulation ◽

Global Ocean ◽

Overturning Circulation ◽

Ocean Eddies ◽

Mesoscale Ocean Eddies ◽

Meridional Overturning

Abstract. Observations and numerical simulations have shown that the meridional overturning circulation (MOC) exhibits substantial variability on sub- to interannual timescales. This variability is not fully understood. In particular it is not known what fraction of the MOC variability is caused by processes such as mesoscale ocean eddies and internal waves which are ubiquitous in the ocean. Here we analyse twin experiments performed with a global ocean model at eddying (1/4°) and non-eddying (1°) resolutions. The twin experiments are forced with the same surface fluxes for the 1958 to 2001 period but start from different initial conditions. Our results show that on subannual to interannual timescales a large fraction of MOC variability directly reflects variability in the surface forcing. Nevertheless, in the eddy-permitting case there is an initial condition dependent MOC variability (hereinafter referred to as "chaotic" variability) of several Sv (1 Sv = 106 m3 s−1) in the Atlantic and the Indo-Pacific. In the Atlantic the chaotic MOC variability represents up to 30% of the total variability at the depths where the maximum MOC occurs. In comparison the chaotic MOC variability is only 5–10% in the non-eddying case. The surface forcing being identical in the twin experiments suggests that mesoscale ocean eddies are the most likely cause for the increased chaotic MOC variability in the eddying case. The exact formation time of eddies is determined by the initial conditions which are different in the two accordance with and as a consequence the mesoscale eddy field is decorrelated in the twin experiments. In regions where eddy activity is high in the eddy-permitting model, the correlation of sea surface height variability in the twin runs is close to zero. In the non-eddying case in contrast, we find high correlations (0.9 or higher) over most regions. Looking at the sub- and interannual MOC components separately reveals that despite the amplitude of the chaotic variability being larger on subannual than on interannual timescales, the ratio of chaotic to total MOC variability is larger on interannual than on subannual timescales.

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Response of North Atlantic Ocean Circulation to Atmospheric Weather Regimes

Journal of Physical Oceanography ◽

10.1175/jpo-d-12-0217.1 ◽

2014 ◽

Vol 44 (1) ◽

pp. 179-201 ◽

Cited By ~ 31

Author(s):

Nicolas Barrier ◽

Christophe Cassou ◽

Julie Deshayes ◽

Anne-Marie Treguier

Keyword(s):

Atlantic Ocean ◽

Wind Stress ◽

Ocean Circulation ◽

North Atlantic ◽

North Atlantic Ocean ◽

Meridional Overturning Circulation ◽

Subpolar Gyre ◽

Wind Stress Curl ◽

Overturning Circulation ◽

Weather Regimes

Abstract A new framework is proposed for investigating the atmospheric forcing of North Atlantic Ocean circulation. Instead of using classical modes of variability, such as the North Atlantic Oscillation (NAO) or the east Atlantic pattern, the weather regimes paradigm was used. Using this framework helped avoid problems associated with the assumptions of orthogonality and symmetry that are particular to modal analysis and known to be unsuitable for the NAO. Using ocean-only historical and sensitivity experiments, the impacts of the four winter weather regimes on horizontal and overturning circulations were investigated. The results suggest that the Atlantic Ridge (AR), negative NAO (NAO−), and positive NAO (NAO+) regimes induce a fast (monthly-to-interannual time scales) adjustment of the gyres via topographic Sverdrup dynamics and of the meridional overturning circulation via anomalous Ekman transport. The wind anomalies associated with the Scandinavian blocking regime (SBL) are ineffective in driving a fast wind-driven oceanic adjustment. The response of both gyre and overturning circulations to persistent regime conditions was also estimated. AR causes a strong, wind-driven reduction in the strengths of the subtropical and subpolar gyres, while NAO+ causes a strengthening of the subtropical gyre via wind stress curl anomalies and of the subpolar gyre via heat flux anomalies. NAO− induces a southward shift of the gyres through the southward displacement of the wind stress curl. The SBL is found to impact the subpolar gyre only via anomalous heat fluxes. The overturning circulation is shown to spin up following persistent SBL and NAO+ and to spin down following persistent AR and NAO− conditions. These responses are driven by changes in deep water formation in the Labrador Sea.

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An improved parameterization of tidal mixing for ocean models

Geoscientific Model Development ◽

10.5194/gmd-7-211-2014 ◽

2014 ◽

Vol 7 (1) ◽

pp. 211-224 ◽

Cited By ~ 10

Author(s):

A. Schmittner ◽

G. D. Egbert

Keyword(s):

Climate Model ◽

Global Climate ◽

Global Climate Model ◽

Deep Ocean ◽

Meridional Overturning Circulation ◽

Tidal Mixing ◽

Overturning Circulation ◽

Microstructure Measurements ◽

High Resolution Bathymetry ◽

Diurnal Tides

Abstract. Two modifications to an existing scheme of tidal mixing are implemented in the coarse resolution ocean component of a global climate model. First, the vertical distribution of energy flux out of the barotropic tide is determined using high resolution bathymetry. This shifts the levels of mixing higher up in the water column and leads to a stronger mid-depth meridional overturning circulation in the model. Second, the local dissipation efficiency for diurnal tides is assumed to be larger than that for the semi-diurnal tides poleward of 30°. Both modifications are shown to improve agreement with observational estimates of diapycnal diffusivities based on microstructure measurements and circulation indices. We also assess impacts of different spatial distributions of the barotropic energy loss. Estimates based on satellite altimetry lead to larger diffusivities in the deep ocean and hence a stronger deep overturning circulation in our climate model that is in better agreement with observation based estimates compared to those based on a tidal model.

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Antarctic Sea Ice Control on the Depth of North Atlantic Deep Water

Journal of Climate ◽

10.1175/jcli-d-18-0519.1 ◽

2019 ◽

Vol 32 (9) ◽

pp. 2537-2551 ◽

Cited By ~ 6

Author(s):

Louis-Philippe Nadeau ◽

Raffaele Ferrari ◽

Malte F. Jansen

Keyword(s):

Sea Ice ◽

Deep Water ◽

Formation Rate ◽

Deep Ocean ◽

Meridional Overturning Circulation ◽

Ice Formation ◽

Overturning Circulation ◽

Antarctic Sea Ice ◽

Meridional Overturning

Abstract Changes in deep-ocean circulation and stratification have been argued to contribute to climatic shifts between glacial and interglacial climates by affecting the atmospheric carbon dioxide concentrations. It has been recently proposed that such changes are associated with variations in Antarctic sea ice through two possible mechanisms: an increased latitudinal extent of Antarctic sea ice and an increased rate of Antarctic sea ice formation. Both mechanisms lead to an upward shift of the Atlantic meridional overturning circulation (AMOC) above depths where diapycnal mixing is strong (above 2000 m), thus decoupling the AMOC from the abyssal overturning circulation. Here, these two hypotheses are tested using a series of idealized two-basin ocean simulations. To investigate independently the effect of an increased latitudinal ice extent from the effect of an increased ice formation rate, sea ice is parameterized as a latitude strip over which the buoyancy flux is negative. The results suggest that both mechanisms can effectively decouple the two cells of the meridional overturning circulation (MOC), and that their effects are additive. To illustrate the role of Antarctic sea ice in decoupling the AMOC and the abyssal overturning cell, the age of deep-water masses is estimated. An increase in both the sea ice extent and its formation rate yields a dramatic “aging” of deep-water masses if the sea ice is thick and acts as a lid, suppressing air–sea fluxes. The key role of vertical mixing is highlighted by comparing results using different profiles of vertical diffusivity. The implications of an increase in water mass ages for storing carbon in the deep ocean are discussed.

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