scholarly journals The Dependence of Southern Ocean Meridional Overturning on Wind Stress

2011 ◽  
Vol 41 (12) ◽  
pp. 2261-2278 ◽  
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.

Contrasting sources of variability in subtropical and subpolar Atlantic overturning

2020 ◽  
Author(s):  
Yavor Kostov ◽  
Helen L. Johnson ◽  
David P. Marshall ◽  
Gael Forget ◽  
Patrick Heimbach ◽  
...  
Keyword(s):  
Wind Stress ◽  
Ocean Circulation ◽  
Global Climate ◽  
Surface Wind ◽  
Circulation Model ◽  
Ocean Surface ◽  
Meridional Overturning Circulation ◽  
Surface Wind Stress ◽  
History Of ◽  
Meridional 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.  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°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>

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

2008 ◽  
Vol 21 (23) ◽  
pp. 6260-6282 ◽  
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.

scholarly journals An Idealized Model of Weddell Gyre Export Variability

2014 ◽  
Vol 44 (6) ◽  
pp. 1671-1688 ◽  
Author(s):  
Zhan Su ◽  
Andrew L. Stewart ◽  
Andrew F. Thompson
Keyword(s):  
Wind Stress ◽  
Deep Water ◽  
Surface Wind ◽  
Oscillation Amplitude ◽  
Mesoscale Eddy ◽  
Eddy Diffusivity ◽  
Weddell Sea ◽  
Alternative Mechanism ◽  
Weddell Gyre ◽  
Surface Wind Stress

Abstract Recent observations suggest that the export of Antarctic Bottom Water (AABW) from the Weddell Sea has a seasonal cycle in its temperature and salinity that is correlated with annual wind stress variations. This variability has been attributed to annual vertical excursions of the isopycnals in the Weddell Gyre, modifying the water properties at the depth of the Orkney Passage. Recent studies attribute these variations to locally wind-driven barotropic dynamics in the northern Weddell Sea boundary current. This paper explores an alternative mechanism in which the isopycnals respond directly to surface Ekman pumping, which is coupled to rapidly responding mesoscale eddy buoyancy fluxes near the gyre boundary. A conceptual model of the interface that separates Weddell Sea Deep Water from Circumpolar Deep Water is described in which the bounding isopycnal responds to a seasonal oscillation in the surface wind stress. Different parameterizations of the mesoscale eddy diffusivity are tested. The model accurately predicts the observed phases of the temperature and salinity variability in relationship to the surface wind stress. The model, despite its heavy idealization, also accounts for more than 50% of the observed oscillation amplitude, which depends on the strength of the seasonal wind variability and the parameterized eddy diffusivity. These results highlight the importance of mesoscale eddies in modulating the export of AABW in narrow boundary layers around the Antarctic margins.

Southern Ocean Circulation and Eddy Compensation in CMIP5 Models

2013 ◽  
Vol 26 (18) ◽  
pp. 7198-7220 ◽  
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.

scholarly journals Linking the Pacific Meridional Mode to ENSO: Utilization of a Noise Filter

2009 ◽  
Vol 22 (4) ◽  
pp. 905-922 ◽  
Author(s):  
Li Zhang ◽  
Ping Chang ◽  
Michael K. Tippett
Keyword(s):  
Heat Flux ◽  
Wind Stress ◽  
Surface Wind ◽  
Atmospheric Variability ◽  
Surface Wind Stress ◽  
Noise Filter ◽  
The Pacific ◽  
Meridional Mode ◽  
The Impact ◽  
Pacific Meridional Mode

Abstract A novel noise filter is used to effectively reduce internal atmospheric variability in the air–sea fluxes of a coupled model. This procedure allows for a test of the impact of the internal atmospheric variability on ENSO through its effect on the Pacific meridional mode (MM). Three 100-yr coupled experiments are conducted, where the filter is utilized to suppress internal atmospheric variability in 1) both the surface wind stress and the heat flux (fully filtered run), 2) only the surface heat flux (filtered-flux run), and 3) only the surface wind stress (filtered-wind run). The fully filtered run indicates that suppressing internal atmospheric variability weakens the MM, which in turn results in substantially reduced ENSO variability. ENSO is no longer phase locked to the boreal winter. The filtered-flux and filtered-wind experiments reveal that different types of noise affect ENSO in different ways. The noise in the wind stress does not have a significant impact on the MM and its relationship to ENSO. This type of noise, however, tends to broaden the spectral peak of ENSO while shifting it toward lower frequencies. The noise in the heat flux, on the other hand, has a direct impact on the strength of the MM and consequently its ability to influence ENSO. Reducing the effect of heat flux noise yields substantially weakened MM activity and a weakened relationship to ENSO, which leads to altered seasonal phase-locking characteristics.

scholarly journals Roles of Sea Ice–Surface Wind Feedback in Maintaining the Glacial Atlantic Meridional Overturning Circulation and Climate

2020 ◽  
Vol 33 (8) ◽  
pp. 3001-3018 ◽  
Author(s):  
Sam Sherriff-Tadano ◽  
Ayako Abe-Ouchi
Keyword(s):  
Heat Flux ◽  
Sea Ice ◽  
North Atlantic ◽  
Surface Wind ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Buoyancy Flux ◽  
Ice Surface ◽  
Oceanic Heat Flux ◽  
Meridional Overturning

AbstractDuring glacial periods, climate varies between two contrasting modes, the interstadials and stadials. These climate changes are often associated with drastic reorganizations of the Atlantic meridional overturning circulation (AMOC). Previous studies highlight the important role of sea ice in maintaining contrasting modes of the AMOC through its insulating effect on the oceanic heat flux and the buoyancy flux (sea ice–buoyancy flux feedback); however, the effect of feedback from the atmosphere remains unclear. Here, the effect of sea ice–surface wind interactions over the North Atlantic Ocean on the AMOC is explored. For this purpose, results from comprehensive atmosphere–ocean coupled general circulation models (AOGCMs) are analyzed. Then, sensitivity experiments are conducted with the atmospheric component of the AOGCM. Last, to explore the impact of modifications in surface winds induced by sea ice on the maintenance of the AMOC, partially coupled experiments are conducted with the AOGCMs. Experiments show that the expansion of sea ice associated with a weakening of the AMOC reduces surface winds by suppressing the oceanic heat flux and increasing the atmospheric static stability. This wind anomaly then causes a weakening of the wind-driven ocean salt transport to the northern North Atlantic and maintains the weak AMOC, therefore working as a positive feedback. It is shown that, together with the sea ice–buoyancy flux and sea ice–surface wind feedback, changes in sea ice and oceanic heat flux sustain the contrasting modes of the AMOC. These results may provide useful information for interpreting the differences in the self-sustained internal oscillations of the AMOC produced by recent AOGCMs.

scholarly journals Overturning response to a surface wind stress doubling in an eddying and a non-eddying ocean

2021 ◽  
Author(s):  
Veit Lüschow ◽  
Jochem Marotzke ◽  
Jin-Song von Storch
Keyword(s):  
Wind Stress ◽  
Surface Wind ◽  
Relative Vorticity ◽  
Ocean Model ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Boundary Current ◽  
Mean Flow ◽  
Surface Wind Stress ◽  
Stress Changes

AbstractIn this paper, the overturning responses to wind stress changes of an eddying and a non-eddying ocean are compared. Differences are found in the deep overturning cell in the low-latitude North Atlantic with substantial implications for the deep western boundary current (DWBC). In an ocean-only twin experiment with one eddying and one non-eddying configuration of the MPI ocean model, two different forcings are being applied: the standard NCEP forcing and the NCEP forcing with 2x surface wind stress. The response to the wind stress doubling in the Atlantic meridional overturning circulation is similar in the eddying and the non-eddying configuration, showing an increase by about 4 Sv (~25%, 1 Sv = 106 m3s−1). In contrast, the DWBC responds with a speedup in the non-eddying and a slowdown in the eddying configuration. This paper demonstrates that the DWBC slowdown in the eddying configuration is largely balanced by eddy vorticity fluxes. Because those fluxes are not resolved and also not captured by an eddy parameterization in the non-eddying configuration, such a DWBC slowdown is likely not to occur in non-eddying ocean models which therefore might not capture the whole range of overturning responses. Furthermore, evidence is provided that the balancing effect of the eddies is not a passive reaction to a remotely triggered DWBC slowdown. Instead, deep eddies which are sourced from the upper ocean provide an excess input of relative vorticity which then actively forces the DWBC mean flow to slow down.

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