Contrasting sources of variability in subtropical and subpolar Atlantic overturning

<p><strong>The Atlantic meridional overturning circulation (AMOC) is pivotal for regional and global climate due to its key role in the uptake and redistribution of heat, carbon and other tracers. Establishing the causes of historical variability in the AMOC can tell us how the circulation responds to natural and anthropogenic changes at the ocean surface. However, attributing observed AMOC variability and inferring causal relationships is challenging because the circulation is influenced by multiple factors which co-vary and whose overlapping impacts can persist for years.&#160; Here we reconstruct and unambiguously attribute variability in the AMOC at the latitudes of two observational arrays to the recent history of surface wind stress, temperature and salinity. We use a state-of-the-art technique that computes space- and time-varying sensitivity patterns of the AMOC strength with respect to multiple surface properties from a numerical ocean circulation model constrained by observations. While on inter-annual timescales, AMOC variability at 26&#176;N is overwhelmingly dominated by a linear response to local wind stress, in contrast, AMOC variability at subpolar latitudes is generated by both wind stress and surface temperature and salinity anomalies. Our analysis allows us to obtain the first-ever reconstruction of subpolar AMOC from forcing anomalies at the ocean surface.</strong></p>

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The Dependence of Southern Ocean Meridional Overturning on Wind Stress

Journal of Physical Oceanography ◽

10.1175/jpo-d-11-023.1 ◽

2011 ◽

Vol 41 (12) ◽

pp. 2261-2278 ◽

Cited By ~ 102

Author(s):

Ryan Abernathey ◽

John Marshall ◽

David Ferreira

Keyword(s):

Heat Flux ◽

Wind Stress ◽

Mechanical Energy ◽

Surface Wind ◽

Zonal Flow ◽

Eddy Diffusivity ◽

Meridional Overturning Circulation ◽

Surface Boundary ◽

Surface Wind Stress ◽

Meridional Overturning

Abstract An eddy-resolving numerical model of a zonal flow, meant to resemble the Antarctic Circumpolar Current, is described and analyzed using the framework of J. Marshall and T. Radko. In addition to wind and buoyancy forcing at the surface, the model contains a sponge layer at the northern boundary that permits a residual meridional overturning circulation (MOC) to exist at depth. The strength of the residual MOC is diagnosed for different strengths of surface wind stress. It is found that the eddy circulation largely compensates for the changes in Ekman circulation. The extent of the compensation and thus the sensitivity of the MOC to the winds depend on the surface boundary condition. A fixed-heat-flux surface boundary severely limits the ability of the MOC to change. An interactive heat flux leads to greater sensitivity. To explain the MOC sensitivity to the wind strength under the interactive heat flux, transformed Eulerian-mean theory is applied, in which the eddy diffusivity plays a central role in determining the eddy response. A scaling theory for the eddy diffusivity, based on the mechanical energy balance, is developed and tested; the average magnitude of the diffusivity is found to be proportional to the square root of the wind stress. The MOC sensitivity to the winds based on this scaling is compared with the true sensitivity diagnosed from the experiments.

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Reduced Stability of the Atlantic Meridional Overturning Circulation due to Wind Stress Feedback during Glacial Times

Journal of Climate ◽

10.1175/2008jcli2291.1 ◽

2008 ◽

Vol 21 (23) ◽

pp. 6260-6282 ◽

Cited By ~ 14

Author(s):

Olivier Arzel ◽

Matthew H. England ◽

Willem P. Sijp

Keyword(s):

Sea Ice ◽

Wind Stress ◽

Surface Wind ◽

Meridional Overturning Circulation ◽

Nonlinear Dependence ◽

Upper Ocean ◽

Overturning Circulation ◽

Climate Conditions ◽

Meridional Overturning ◽

The Impact

Abstract A previous study by Mikolajewicz suggested that the wind stress feedback stabilizes the Atlantic thermohaline circulation. This result was obtained under modern climate conditions, for which the presence of the massive continental ice sheets characteristic of glacial times is missing. Here a coupled ocean–atmosphere–sea ice model of intermediate complexity, set up in an idealized spherical sector geometry of the Atlantic basin, is used to show that, under glacial climate conditions, wind stress feedback actually reduces the stability of the meridional overturning circulation (MOC). The analysis reveals that the influence of the wind stress feedback on the glacial MOC response to an external source of freshwater applied at high northern latitudes is controlled by the following two distinct processes: 1) the interactions between the wind field and the sea ice export in the Northern Hemisphere (NH), and 2) the northward Ekman transport in the tropics and upward Ekman pumping in the core of the NH subpolar gyre. The former dominates the response of the coupled system; it delays the recovery of the MOC, and in some cases even stabilizes collapsed MOC states achieved during the hosing period. The latter plays a minor role and mitigates the impact of the former process by reducing the upper-ocean freshening in deep-water formation regions. Hence, the wind stress feedback delays the recovery of the glacial MOC, which is the opposite of what occurs under modern climate conditions. Close to the critical transition threshold beyond which the circulation collapses, the glacial MOC appears to be very sensitive to changes in surface wind stress forcing and exhibits, in the aftermath of the freshwater pulse, a nonlinear dependence upon the wind stress feedback magnitude: a complete and irreversible MOC shutdown occurs only for intermediate wind stress feedback magnitudes. This behavior results from the competitive effects of processes 1 and 2 on the midlatitude upper-ocean salinity during the shutdown phase of the MOC. The mechanisms presented here may be relevant to the large meltwater pulses that punctuated the last glacial period.

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An Adjoint Analysis of the Meridional Overturning Circulation in a Hybrid Coupled Model

Journal of Climate ◽

10.1175/jcli3821.1 ◽

2006 ◽

Vol 19 (15) ◽

pp. 3751-3767 ◽

Cited By ~ 19

Author(s):

Véronique Bugnion ◽

Chris Hill ◽

Peter H. Stone

Keyword(s):

Boundary Conditions ◽

Wind Stress ◽

Ocean Circulation ◽

General Circulation ◽

Meridional Overturning Circulation ◽

Global Ocean ◽

Model Parameters ◽

Overturning Circulation ◽

Near Surface ◽

Meridional Overturning

Abstract Multicentury sensitivities in a realistic geometry global ocean general circulation model are analyzed using an adjoint technique. This paper takes advantage of the adjoint model’s ability to generate maps of the sensitivity of a diagnostic (i.e., the meridional overturning’s strength) to all model parameters. This property of adjoints is used to review several theories, which have been elaborated to explain the strength of the North Atlantic’s meridional overturning. This paper demonstrates the profound impact of boundary conditions in permitting or suppressing mechanisms within a realistic model of the contemporary ocean circulation. For example, the so-called Drake Passage Effect in which wind stress in the Southern Ocean acts as the main driver of the overturning’s strength, is shown to be an artifact of boundary conditions that restore the ocean’s surface temperature and salinity toward prescribed climatologies. Advective transports from the Indian and Pacific basins play an important role in setting the strength of the overturning circulation under “mixed” boundary conditions, in which a flux of freshwater is specified at the ocean’s surface. The most “realistic” regime couples an atmospheric energy and moisture balance model to the ocean. In this configuration, inspection of the global maps of sensitivity to wind stress and diapycnal mixing suggests a significant role for near-surface Ekman processes in the Tropics. Buoyancy also plays an important role in setting the overturning’s strength, through direct thermal forcing near the sites of convection, or through the advection of salinity anomalies in the Atlantic basin.

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Southern Ocean Circulation and Eddy Compensation in CMIP5 Models

Journal of Climate ◽

10.1175/jcli-d-12-00504.1 ◽

2013 ◽

Vol 26 (18) ◽

pp. 7198-7220 ◽

Cited By ~ 44

Author(s):

Stephanie M. Downes ◽

Andrew McC. Hogg

Keyword(s):

Southern Ocean ◽

Wind Stress ◽

Surface Wind ◽

Surface Heat ◽

Meridional Overturning Circulation ◽

Cmip5 Models ◽

Strong Increase ◽

Coupled Models ◽

Overturning Circulation ◽

Meridional Overturning

Abstract Thirteen state-of-the-art climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are used to evaluate the response of the Antarctic Circumpolar Current (ACC) transport and Southern Ocean meridional overturning circulation to surface wind stress and buoyancy changes. Understanding how these flows—fundamental players in the global distribution of heat, gases, and nutrients—respond to climate change is currently a widely debated issue among oceanographers. Here, the authors analyze the circulation responses of these coarse-resolution coupled models to surface fluxes. Under a future CMIP5 climate pathway where the equivalent atmospheric CO2 reaches 1370 ppm by 2100, the models robustly project reduced Southern Ocean density in the upper 2000 m accompanied by strengthened stratification. Despite an overall increase in overlying wind stress (~20%), the projected ACC transports lie within ±15% of their historical state, and no significant relationship with changes in the magnitude or position of the wind stress is identified. The models indicate that a weakening of ACC transport at the end of the twenty-first century is correlated with a strong increase in the surface heat and freshwater fluxes in the ACC region. In contrast, the surface heat gain across the ACC region and the wind-driven surface transports are significantly correlated with an increased upper and decreased lower Eulerian-mean meridional overturning circulation. The change in the eddy-induced overturning in both the depth and density spaces is quantified, and it is found that the CMIP5 models project partial eddy compensation of the upper and lower overturning cells.

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Larger Role for Shallow Intermediate Waters in Ocean Circulation

Eos ◽

10.1029/2020eo141674 ◽

2020 ◽

Vol 101 ◽

Author(s):

Sara Pratt

Keyword(s):

Ocean Circulation ◽

Global Climate ◽

Atlantic Meridional Overturning Circulation ◽

Water Masses ◽

Meridional Overturning Circulation ◽

Overturning Circulation ◽

Meridional Overturning ◽

Intermediate Waters

Water masses formed off southeastern Greenland may contribute more than previously thought to the variability of the Atlantic Meridional Overturning Circulation, which strongly influences global climate.

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The impact of topography and eddy parameterization on the simulated Southern Ocean circulation response to changes in surface wind stress

Journal of Physical Oceanography ◽

10.1175/jpo-d-20-0142.1 ◽

2020 ◽

Author(s):

Hailu Kong ◽

Malte F. Jansen

Keyword(s):

Southern Ocean ◽

Wind Stress ◽

Ocean Circulation ◽

Surface Wind ◽

Flat Bottom ◽

Coarse Resolution ◽

Surface Wind Stress ◽

Stress Changes ◽

The Impact ◽

Meso Scale

AbstractIt remains uncertain how the Southern Ocean circulation responds to changes in surface wind stress, and whether coarse resolution simulations, where meso-scale eddy fluxes are parameterized, can adequately capture the response. We address this problem using two idealized model setups mimicking the Southern Ocean: a flat bottom channel, and a channel with moderately complex topography. Under each topographic configuration and varying wind stress, we compare several coarse resolution simulations, configured with different eddy parameterizations, against an eddy-resolving simulation. We find that: (1) without topography, sensitivity of the Antarctic Circumpolar Current (ACC) to wind stress is overestimated by coarse resolution simulations, due to an underestimate of the sensitivity of the eddy diffusivity; (2) in the presence of topography, stationary eddies dominate over transient eddies in counteracting the direct response of the ACC and overturning circulation to wind stress changes; (3) coarse resolution simulations with parameterized eddies capture this counteracting effect reasonably well, largely due to their ability to resolve stationary eddies. Our results highlight the importance of topography in modulating the response of the Southern Ocean circulation to changes in surface wind stress. The interaction between meso-scale eddies and stationary meanders induced by topography requires more attention in future development and testing of eddy parameterizations.

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Implementation of a Geometrically Informed and Energetically Constrained Mesoscale Eddy Parameterization in an Ocean Circulation Model

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0017.1 ◽

2018 ◽

Vol 48 (10) ◽

pp. 2363-2382 ◽

Cited By ~ 11

Author(s):

J. Mak ◽

J. R. Maddison ◽

D. P. Marshall ◽

D. R. Munday

Keyword(s):

Ocean Circulation ◽

Surface Wind ◽

Mesoscale Eddy ◽

Circulation Model ◽

Mesoscale Eddies ◽

Meridional Overturning Circulation ◽

Ocean Circulation Model ◽

Geometric Framework ◽

Model Calculations ◽

Coarse Resolution

AbstractThe global stratification and circulation, as well as their sensitivities to changes in forcing, depend crucially on the representation of the mesoscale eddy field in a numerical ocean circulation model. Here, a geometrically informed and energetically constrained parameterization framework for mesoscale eddies—termed Geometry and Energetics of Ocean Mesoscale Eddies and Their Rectified Impact on Climate (GEOMETRIC)—is proposed and implemented in three-dimensional channel and sector models. The GEOMETRIC framework closes eddy buoyancy fluxes according to the standard Gent–McWilliams scheme but with the eddy transfer coefficient constrained by the depth-integrated eddy energy field, provided through a prognostic eddy energy budget evolving with the mean state. It is found that coarse-resolution models employing GEOMETRIC display broad agreement in the sensitivity of the circumpolar transport, meridional overturning circulation, and depth-integrated eddy energy pattern to surface wind stress as compared with analogous reference calculations at eddy-permitting resolutions. Notably, eddy saturation—the insensitivity of the time-mean circumpolar transport to changes in wind forcing—is found in the coarse-resolution sector model. In contrast, differences in the sensitivity of the depth-integrated eddy energy are found in model calculations in the channel experiments that vary the eddy energy dissipation, attributed to the simple prognostic eddy energy equation employed. Further improvements to the GEOMETRIC framework require a shift in focus from how to close for eddy buoyancy fluxes to the representation of eddy energetics.

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Decadal-Mean Impact of Including Ocean Surface Currents in Bulk Formulas on Surface Air–Sea Fluxes and Ocean General Circulation

Journal of Climate ◽

10.1175/jcli-d-17-0001.1 ◽

2017 ◽

Vol 30 (23) ◽

pp. 9511-9525 ◽

Cited By ~ 6

Author(s):

Yang Wu ◽

Xiaoming Zhai ◽

Zhaomin Wang

Keyword(s):

Wind Stress ◽

General Circulation ◽

Surface Wind ◽

Ocean Surface ◽

Meridional Overturning Circulation ◽

Global Ocean ◽

Turbulent Heat ◽

Surface Currents ◽

Ocean Surface Currents ◽

Ocean General Circulation

The decadal-mean impact of including ocean surface currents in the bulk formulas on surface air–sea fluxes and the ocean general circulation is investigated for the first time using a global eddy-permitting coupled ocean–sea ice model. Although including ocean surface currents in air–sea flux calculations only weakens the surface wind stress by a few percent, it significantly reduces wind power input to both geostrophic and ageostrophic motions, and damps the eddy and mean kinetic energy throughout the water column. Furthermore, the strength of the horizontal gyre circulations and the Atlantic meridional overturning circulation are found to decrease considerably (by 10%–15% and ~13%, respectively). As a result of the weakened ocean general circulation, the maximum northward global ocean heat transport decreases by about 0.2 PW, resulting in a lower sea surface temperature and reduced surface heat loss in the northern North Atlantic. Additional sensitivity model experiments further demonstrate that it is including ocean surface currents in the wind stress calculation that dominates this decadal impact, with including ocean surface currents in the turbulent heat flux calculations making only a minor contribution. These results highlight the importance of properly accounting for ocean surface currents in surface air–sea fluxes in modeling the ocean circulation and climate.

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Test and evaluation of a simple parameterization to enhance air-sea coupling in a global coupled model

Satellite Oceanography and Meteorology ◽

10.18063/som.v3i3.739 ◽

2018 ◽

Vol 3 (3) ◽

Author(s):

Fanghua Xu 1

Keyword(s):

Wind Stress ◽

Ocean Circulation ◽

Southern Oscillation ◽

Current System ◽

Surface Wind ◽

Coupled Model ◽

Global Ocean ◽

Western Boundary ◽

Surface Wind Stress ◽

Gyre Circulation

A simple temperature-dependent wind stress scheme is implemented in National Center for Atmospheric Research (NCAR) Community Earth System Model (CESM), aiming to enhance positive wind stress and sea surface temperature (SST) correlation in SST-frontal regions. A series of three-year coupled experiments are conducted to determine a proper coupling coefficient for the scheme based on the agreement of surface wind stress and SST at oceanic mesoscale between model simulations and observations. Afterwards, 80-year simulations with/without the scheme are conducted to explore its effects on simulated ocean states and variability. The results show that the new scheme indeed improves the positive correlation between SST and wind stress magnitude near the large oceanic fronts. With more realistic surface heat flux and wind stress, the global SST biases are reduced. The global ocean circulation represented by barotropic stream function exhibits a weakened gyre circulation close to the western boundary separation, in agreement with previous studies. The simulation of equatorial Pacific current system is improved as well. The overestimated El Niño Southern Oscillation (ENSO) magnitude in original CESM is reduced by ~30% after using the new scheme with an improved period.

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