scholarly journals The depth scales of the AMOC on a decadal timescale

2021 ◽  
Author(s):  
Tim Rohrschneider ◽  
Johanna Baehr ◽  
Veit Lüschow ◽  
Dian Putrasahan ◽  
Jochem Marotzke
Keyword(s):  
Global Warming ◽  
Southern Ocean ◽  
Northern Hemisphere ◽  
Velocity Profiles ◽  
Meridional Overturning Circulation ◽  
Wind Forcing ◽  
Wind Effects ◽  
Local Wind ◽  
Decadal Timescale ◽  
Ocean Wind

Abstract. We use wind sensitivity experiments to understand the wind forcing dependencies of the level of no motion and the e-folding pycnocline scale as well as their relationship to northward transport of the mid-depth Atlantic meridional overturning circulation (AMOC) south and north of the equator. In contrast to previous studies, we investigate the interplay of nonlocal and local wind effects on a decadal timescale. We use 30-year simulations with a high-resolution ocean general circulation model (OGCM) which is an eddy-resolving version of the Max Planck Institute Ocean Model (MPIOM). Our findings deviate from the common perspective that the AMOC is a nonlocal phenomenon only, because northward transport in the inter-hemispheric cell can only be understood by analyzing nonlocal Southern Ocean wind effects and local wind effects in the northern hemisphere downwelling region where Ekman pumping takes place. Southern Ocean wind forcing predominantly determines the magnitude of the pycnocline scale throughout the basin, whereas northern hemisphere winds additionally influence the level of no motion locally. In that respect, the level of no motion is a better proxy for northward transport and mid-depth velocity profiles despite the Ekman return flow which is found to be baroclinic. We compare our results inferred from the wind experiments and a 100-year global warming experiment in which the atmospheric CO2 concentration is quadrupled, using MPIOM coupled to an atmospheric model. We find that the evolution of the level of no motion in response to global warming represents changes in vertical velocity profiles or northward transport, whereas the changes of the pycnocline scale are opposite to the changes of the level of no motion over time. Using the level of no motion as depth scale, the analysis of the wind experiments and the warming experiment suggests a hemisphere-dependent scaling of the strength of AMOC. Furthermore, we put forward the idea that the ability of numerical models to capture the spatial and temporal variations of the level of no motion is crucial to reproduce the mid-depth cell in an appropriate way.

scholarly journals Using Eulerian and Lagrangian Approaches to Investigate Wind-Driven Changes in the Southern Ocean Abyssal Circulation

2013 ◽  
Vol 44 (2) ◽  
pp. 662-675 ◽  
Author(s):  
Paul Spence ◽  
Erik van Sebille ◽  
Oleg A. Saenko ◽  
Matthew H. England
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
Climate Model ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Lagrangian Particle ◽  
Stress Changes ◽  
Abyssal Circulation ◽  
Flow Pathways ◽  
Ocean Wind

Abstract This study uses a global ocean eddy-permitting climate model to explore the export of abyssal water from the Southern Ocean and its sensitivity to projected twenty-first-century poleward-intensifying Southern Ocean wind stress. The abyssal flow pathways and transport are investigated using a combination of Lagrangian and Eulerian techniques. In an Eulerian format, the equator- and poleward flows within similar abyssal density classes are increased by the wind stress changes, making it difficult to explicitly diagnose changes in the abyssal export in a meridional overturning circulation framework. Lagrangian particle analyses are used to identify the major export pathways of Southern Ocean abyssal waters and reveal an increase in the number of particles exported to the subtropics from source regions around Antarctica in response to the wind forcing. Both the Lagrangian particle and Eulerian analyses identify transients as playing a key role in the abyssal export of water from the Southern Ocean. Wind-driven modifications to the potential energy component of the vorticity balance in the abyss are also found to impact the Southern Ocean barotropic circulation.

scholarly journals The Energetics of Southern Ocean Upwelling

2017 ◽  
Vol 47 (1) ◽  
pp. 135-153 ◽  
Author(s):  
Andrew McC. Hogg ◽  
Paul Spence ◽  
Oleg A. Saenko ◽  
Stephanie M. Downes
Keyword(s):  
Potential Energy ◽  
Southern Ocean ◽  
Northern Hemisphere ◽  
Wind Stress ◽  
Weddell Sea ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Available Potential Energy ◽  
Ocean Winds ◽  
Global Overturning Circulation

AbstractThe ocean’s meridional overturning circulation is closed by the upwelling of dense, carbon-rich waters to the surface of the Southern Ocean. It has been proposed that upwelling in this region is driven by strong westerly winds, implying that the intensification of Southern Ocean winds in recent decades may have enhanced the rate of upwelling, potentially affecting the global overturning circulation. However, there is no consensus on the sensitivity of upwelling to winds or on the nature of the connection between Southern Ocean processes and the global overturning circulation. In this study, the sensitivity of the overturning circulation to changes in Southern Ocean westerly wind stress is investigated using an eddy-permitting ocean–sea ice model. In addition to a suite of standard circulation metrics, an energy analysis is used to aid dynamical interpretation of the model response. Increased Southern Ocean wind stress enhances the upper cell of the overturning circulation through creation of available potential energy in the Southern Hemisphere, associated with stronger upwelling of deep water. Poleward shifts in the Southern Ocean westerlies lead to a complicated transient response, with the formation of bottom water induced by increased polynya activity in the Weddell Sea and a weakening of the upper overturning cell in the Northern Hemisphere. The energetic consequences of the upper overturning cell response indicate an interhemispheric connection to the input of available potential energy in the Northern Hemisphere.

Roles of meridional overturning in subpolar Southern Ocean SST trends: Insights from ensemble simulations

2021 ◽  
pp. 1-65
Keyword(s):  
Global Warming ◽  
Southern Ocean ◽  
Climate Models ◽  
Low Frequency ◽  
Internal Variability ◽  
Meridional Overturning Circulation ◽  
Surface Cooling ◽  
Recent Climate ◽  
Meridional Overturning ◽  
Cooling Trend

Abstract One of the most puzzling observed features of recent climate has been a multidecadal surface cooling trend over the subpolar Southern Ocean (SO). In this study we use large ensembles of simulations with multiple climate models to study the role of the SO meridional overturning circulation (MOC) in these sea surface temperature (SST) trends. We find that multiple competing processes play prominent roles, consistent with multiple mechanisms proposed in the literature for the observed cooling. Early in the simulations (20th century and early 21st century) internal variability of the MOC can have a large impact, in part due to substantial simulated multidecadal variability of the MOC. Ensemble members with initially strong convection (and related surface warming due to convective mixing of subsurface warmth to the surface) tend to subsequently cool at the surface as convection associated with internal variability weakens. A second process occurs in the late 20th and 21st centuries, as weakening of oceanic convection associated with global warming and high latitude freshening can contribute to the surface cooling trend by suppressing convection and associated vertical mixing of subsurface heat. As the simulations progress, the multidecadal SO variability is suppressed due to forced changes in the mean state and increased oceanic stratification. As a third process, the shallower mixed layers can then rapidly warm due to increasing forcing from greenhouse gas warming. Also, during this period the ensemble spread of SO SST trend partly arises from the spread of the wind-driven Deacon cell strength. Thus, different processes could conceivably have led to the observed cooling trend, consistent with the range of possibilities presented in the literature. To better understand the causes of the observed trend it is important to better understand the characteristics of internal low-frequency variability in the SO and the response of that variability to global warming.

scholarly journals The Effect of Northern Hemisphere Winds on the Meridional Overturning Circulation and Stratification

2018 ◽  
Vol 48 (10) ◽  
pp. 2495-2506 ◽  
Author(s):  
Paola Cessi
Keyword(s):  
Southern Ocean ◽  
Northern Hemisphere ◽  
Wind Stress ◽  
North Atlantic ◽  
Meridional Overturning Circulation ◽  
Ekman Transport ◽  
Subpolar Gyre ◽  
The North ◽  
Meridional Overturning ◽  
The North Atlantic

AbstractThe current paradigm for the meridional overturning cell and the associated middepth stratification is that the wind stress in the subpolar region of the Southern Ocean drives a northward Ekman flow, which, together with the global diapycnal mixing across the lower boundary of the middepth waters, feeds the upper branch of the interhemispheric overturning. The resulting mass transport proceeds to the Northern Hemisphere of the North Atlantic, where it sinks, to be eventually returned to the Southern Ocean at depth. Seemingly, the wind stress in the Atlantic basin plays no role. This asymmetry occurs because the Ekman transport in the Atlantic Ocean is assumed to return geostrophically at depths much shallower than those occupied by the interhemispheric overturning. However, this vertical separation fails in the North Atlantic subpolar gyre region. Using a conceptual model and an ocean general circulation model in an idealized geometry, we show that the westerly wind stress in the northern part of the Atlantic provides two opposing effects. Mechanically, the return of the Ekman transport in the North Atlantic opposes sinking in this region, reducing the total overturning and deepening the middepth stratification; thermodynamically, the subpolar gyre advects salt poleward, promoting Northern Hemisphere sinking. Depending on which mechanism prevails, increased westerly winds in the Northern Hemisphere can reduce or augment the overturning.

scholarly journals Eddy Saturation of Equilibrated Circumpolar Currents

2013 ◽  
Vol 43 (3) ◽  
pp. 507-532 ◽  
Author(s):  
David R. Munday ◽  
Helen L. Johnson ◽  
David P. Marshall
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
General Circulation ◽  
Circulation Model ◽  
Ocean General Circulation Model ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
Diapycnal Diffusivity ◽  
Ocean Wind

Abstract This study uses a sector configuration of an ocean general circulation model to examine the sensitivity of circumpolar transport and meridional overturning to changes in Southern Ocean wind stress and global diapycnal mixing. At eddy-permitting, and finer, resolution, the sensitivity of circumpolar transport to forcing magnitude is drastically reduced. At sufficiently high resolution, there is little or no sensitivity of circumpolar transport to wind stress, even in the limit of no wind. In contrast, the meridional overturning circulation continues to vary with Southern Ocean wind stress, but with reduced sensitivity in the limit of high wind stress. Both the circumpolar transport and meridional overturning continue to vary with diapycnal diffusivity at all model resolutions. The circumpolar transport becomes less sensitive to changes in diapycnal diffusivity at higher resolution, although sensitivity always remains. In contrast, the overturning circulation is more sensitive to change in diapycnal diffusivity when the resolution is high enough to permit mesoscale eddies.

scholarly journals Effects of Buoyancy and Wind Forcing on Southern Ocean Climate Change

2020 ◽  
Vol 33 (23) ◽  
pp. 10003-10020
Author(s):  
Jia-Rui Shi ◽  
Lynne D. Talley ◽  
Shang-Ping Xie ◽  
Wei Liu ◽  
Sarah T. Gille
Keyword(s):  
Southern Ocean ◽  
Hydrological Cycle ◽  
Meridional Overturning Circulation ◽  
Wind Forcing ◽  
Anthropogenic Heat ◽  
Subsurface Water ◽  
Induced Response ◽  
Wind Effect ◽  
Surface Salinity ◽  
Buoyancy Forcing

AbstractObservations show that since the 1950s, the Southern Ocean has stored a large amount of anthropogenic heat and has freshened at the surface. These patterns can be attributed to two components of surface forcing: poleward-intensified westerly winds and increased buoyancy flux from freshwater and heat. Here we separate the effects of these two forcing components by using a novel partial-coupling technique. We show that buoyancy forcing dominates the overall response in the temperature and salinity structure of the Southern Ocean. Wind stress change results in changes in subsurface temperature and salinity that are closely related to intensified residual meridional overturning circulation. As an important result, we show that buoyancy and wind forcing result in opposing changes in salinity: the wind-induced surface salinity increase due to upwelling of saltier subsurface water offsets surface freshening due to amplification of the global hydrological cycle. Buoyancy and wind forcing further lead to different vertical structures of Antarctic Circumpolar Current (ACC) transport change; buoyancy forcing causes an ACC transport increase (3.1 ± 1.6 Sv; 1 Sv ≡ 106 m3 s−1) by increasing the meridional density gradient across the ACC in the upper 2000 m, while the wind-induced response is more barotropic, with the whole column transport increased by 8.7 ± 2.3 Sv. While previous research focused on the wind effect on ACC intensity, we show that surface horizontal current acceleration within the ACC is dominated by buoyancy forcing. These results shed light on how the Southern Ocean might change under global warming, contributing to more reliable future projections.

scholarly journals A Conceptual Model of Ocean Heat Uptake under Climate Change

2014 ◽  
Vol 27 (22) ◽  
pp. 8444-8465 ◽  
Author(s):  
David P. Marshall ◽  
Laure Zanna
Keyword(s):  
Southern Ocean ◽  
Wind Forcing ◽  
System Model ◽  
Ekman Transport ◽  
Earth System ◽  
Diapycnal Mixing ◽  
Ocean Heat Uptake ◽  
Ocean Stratification ◽  
Heat Uptake ◽  
Ocean Wind

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.

scholarly journals Synergistic impacts of global warming and thermohaline circulation collapse on amphibians

2021 ◽  
Vol 4 (1) ◽  
Author(s):  
Julián A. Velasco ◽  
Francisco Estrada ◽  
Oscar Calderón-Bustamante ◽  
Didier Swingedouw ◽  
Carolina Ureta ◽  
...  
Keyword(s):  
Climate Change ◽  
Global Warming ◽  
Thermohaline Circulation ◽  
Climate Model ◽  
Global Climate ◽  
Extinction Risk ◽  
Meridional Overturning Circulation ◽  
Mitigation Strategies ◽  
Amphibian Species ◽  
Climate Conditions

AbstractImpacts on ecosystems and biodiversity are a prominent area of research in climate change. However, little is known about the effects of abrupt climate change and climate catastrophes on them. The probability of occurrence of such events is largely unknown but the associated risks could be large enough to influence global climate policy. Amphibians are indicators of ecosystems’ health and particularly sensitive to novel climate conditions. Using state-of-the-art climate model simulations, we present a global assessment of the effects of unabated global warming and a collapse of the Atlantic meridional overturning circulation (AMOC) on the distribution of 2509 amphibian species across six biogeographical realms and extinction risk categories. Global warming impacts are severe and strongly enhanced by additional and substantial AMOC weakening, showing tipping point behavior for many amphibian species. Further declines in climatically suitable areas are projected across multiple clades, and biogeographical regions. Species loss in regional assemblages is extensive across regions, with Neotropical, Nearctic and Palearctic regions being most affected. Results underline the need to expand existing knowledge about the consequences of climate catastrophes on human and natural systems to properly assess the risks of unabated warming and the benefits of active mitigation strategies.

The Southern Ocean benthic fauna and climate change: a historical perspective

1992 ◽  
Vol 338 (1285) ◽  
pp. 299-309 ◽  
Keyword(s):  
Global Warming ◽  
Southern Ocean ◽  
Environmental Change ◽  
Benthic Communities ◽  
Benthic Fauna ◽  
Marine Fauna ◽  
Short Term Change ◽  
Term Change ◽  
Continental Ice Sheets

Environmental change is the norm and it is likely that, particularly on the geological timescale, the temperature regime experienced by marine organisms has never been stable. These temperature changes vary in timescale from daily, through seasonal variations, to long-term environmental change over tens of millions of years. Whereas physiological work can give information on how individual organisms may react phenotypically to short-term change, the way benthic communities react to long-term change can only be studied from the fossil record. The present benthic marine fauna of the Southern Ocean is rich and diverse, consisting of a mixture of taxa with differing evolutionary histories and biogeographical affinities, suggesting that at no time in the Cenozoic did continental ice sheets extend sufficiently to eradicate all shallow-water faunas around Antarctica at the same time. Nevertheless, certain features do suggest the operation of vicariant processes, and climatic cycles affecting distributional ranges and ice-sheet extension may both have enhanced speciation processes. The overall cooling of southern high-latitude seas since the mid-Eocene has been neither smooth nor steady. Intermittent periods of global warming and the influence of Milankovitch cyclicity is likely to have led to regular pulses of migration in and out of Antarctica. The resultant diversity pump may explain in part the high species richness of some marine taxa in the Southern Ocean. It is difficult to suggest how the existing fauna will react to present global warming. Although it is certain the fauna will change, as all faunas have done throughout evolutionary time, we cannot predict with confidence how it will do so.

Production of Shear Profiles in a Wind Tunnel by Cylindrical Rods Placed Normal to the Stream

1966 ◽  
Vol 70 (667) ◽  
pp. 724-725 ◽  
Author(s):  
D. J. Cockrell ◽  
B. E. Lee
Keyword(s):  
Wind Tunnel ◽  
Velocity Profiles ◽  
Wind Effects ◽  
Turbulence Characteristics ◽  
Fluid Behaviour

The production of required velocity profiles in a duct or wind tunnel is a necessary part of much research aimed at understanding fluid behaviour. Perhaps the most obvious application is the simulation of wind gradients for the study of wind effects on structures, but equally important is the study of diffuser and duct behaviour when subjected to a variety of known and convenient velocity profiles. Furthermore, the effects of the variation in turbulence characteristics within a range of identical velocity profiles produced by different methods are not clearly understood.

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