Subpolar Southern Ocean response to changes in the surface momentum, heat and freshwater fluxes under 2xCO2

2021 ◽  
Author(s):  
Fabio Boeira Dias ◽  
Catia Domingues ◽  
Simon Marsland ◽  
Stephen Rintoul ◽  
Petteri Uotila ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Climate Models ◽  
Deep Convection ◽  
Open Ocean ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Surface Warming ◽  
Freshwater Fluxes ◽  
Poleward Shift

<ul><li>The subpolar Southern Ocean (sSO) around Antarctica has fundamental climate importance. The densest water mass in the global ocean, Antarctica Bottom Water (AABW), originates in the sSO and supplies the lower limb of the Meridional Overturning Circulation (MOC), occupying about 36% of the ocean’s volume. However, climate models struggle to represent the processes involved in formation of AABW on the continental shelf, resulting in large differences between models and observations and a wide spread in projections of sea level and other properties. We explore the source of these persistent model biases by examining the response of the sSO to perturbations in surface forcing. Using an ocean-sea ice model (ACCESS-OM2) that forms AABW both on the shelf and in open-ocean (similar to other coarse resolution models), we investigate the sSO response to individual and combined perturbations of surface heat, freshwater and momentum fluxes following the  FAFMIP-protocol. The wind perturbation (i.e. a poleward shift and intensification of the Southern Ocean Westerlies) has the dominant effect, enhancing AABW formation and accelerating the MOC. This occurs through upwelling of warm waters and inhibition of sea-ice growth during winter, which triggers large open-ocean polynya events with associated deep convection. These events occur in the Weddell and Ross Seas and their variability is associated with the heat available at mid-depth; open-ocean polynyas cease when the heat reservoir is depleted. The effects of surface warming and freshening only partially compensate the changes due to wind by increasing the ocean stratification and reducing AABW formation. These results are relevant for the interpretation of climate change projections, suggesting that other coarse  models might respond in similar way and present an opposite trend than those seen from observations.</li> </ul>

scholarly journals Subpolar Southern Ocean Response to Changes in the Surface Momentum, Heat, and Freshwater Fluxes under 2xCO2

2021 ◽  
Vol 34 (21) ◽  
pp. 8755-8775
Author(s):  
Fabio Boeira Dias ◽  
Catia M. Domingues ◽  
Simon J. Marsland ◽  
Stephen R. Rintoul ◽  
Petteri Uotila ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Climate Models ◽  
Deep Convection ◽  
Open Water ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Surface Warming ◽  
Freshwater Fluxes ◽  
Poleward Shift

AbstractThe Antarctic subpolar Southern Ocean (sSO) has fundamental climate importance. Antarctic Bottom Water (AABW) originates in the sSO and supplies the lower limb of the meridional overturning circulation (MOC), occupying 36% of ocean volume. Climate models struggle to represent continental shelf processes that form AABW. We explore sources of persistent model biases by examining response of the sSO to perturbations in surface forcing in a global ocean–sea ice model (ACCESS-OM2) that forms AABW both on shelf and in open ocean. The sSO response to individual and combined perturbations of surface heat, freshwater, and momentum fluxes follows the WCRP CMIP6 FAFMIP-protocol. Wind perturbation (i.e., a poleward shift and intensification of the westerlies) is dominant, enhancing AABW formation and accelerating the global MOC. This occurs through upwelling of warm waters and inhibition of sea ice growth during winter, which triggers large open water polynya (OWP) events with associated deep convection. These events occur in the Weddell and Ross Seas and their variability is associated with availability of heat at midocean depths. These OWPs cease when the heat reservoir is depleted. Effects of surface warming and freshening only partially compensate changes from increasing winds on ocean stratification and depletion of AABW formation. These results indicate that overly convective models, such ACCESS-OM2, can respond to CO2-perturbed scenarios by forming too much AABW in OWP, which might not hold in models without OWPs. This might contribute to the large intermodel spread thermosteric sea level projections, being relevant to the interpretation of future projections by current climate models.

scholarly journals Transient Overturning Compensation between Atlantic and Indo-Pacific Basins

2020 ◽  
Vol 50 (8) ◽  
pp. 2151-2172 ◽  
Author(s):  
Shantong Sun ◽  
Andrew F. Thompson ◽  
Ian Eisenman
Keyword(s):  
Southern Ocean ◽  
Time Scales ◽  
North Atlantic ◽  
Climate Models ◽  
Diffusion Processes ◽  
Formation Rate ◽  
Deep Ocean ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Overturning Circulation

AbstractClimate models consistently project (i) a decline in the formation of North Atlantic Deep Water (NADW) and (ii) a strengthening of the Southern Hemisphere westerly winds in response to anthropogenic greenhouse gas forcing. These two processes suggest potentially conflicting tendencies of the Atlantic meridional overturning circulation (AMOC): a weakening AMOC due to changes in the North Atlantic but a strengthening AMOC due to changes in the Southern Ocean. Here we focus on the transient evolution of the global ocean overturning circulation in response to a perturbation to the NADW formation rate. We propose that the adjustment of the Indo-Pacific overturning circulation is a critical component in mediating AMOC changes. Using a hierarchy of ocean and climate models, we show that the Indo-Pacific overturning circulation provides the first response to AMOC changes through wave processes, whereas the Southern Ocean overturning circulation responds on longer (centennial to millennial) time scales that are determined by eddy diffusion processes. Changes in the Indo-Pacific overturning circulation compensate AMOC changes, which allows the Southern Ocean overturning circulation to evolve independently of the AMOC, at least over time scales up to many decades. In a warming climate, the Indo-Pacific develops an overturning circulation anomaly associated with the weakening AMOC that is characterized by a northward transport close to the surface and a southward transport in the deep ocean, which could effectively redistribute heat between the basins. Our results highlight the importance of interbasin exchange in the response of the global ocean overturning circulation to a changing climate.

scholarly journals Sea-ice Induced Southern Ocean Subsurface Warming and Surface Cooling in a Warming Climate

2020 ◽  
Author(s):  
F. Alexander Haumann ◽  
Nicolas Gruber ◽  
Matthias Münnich
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Heat Content ◽  
Ocean Surface ◽  
Open Ocean ◽  
Global Ocean ◽  
Content Increase ◽  
Surface Cooling ◽  
Cold Surface

<p>Much of the Southern Ocean surface south of 55° S cooled and freshened between at least the early 1980s and the early 2010s. Many processes have been proposed to explain the unexpected cooling, including increased winds or increased surface freshwater fluxes from either the atmosphere or glacial meltwater. However, these mechanisms so far failed to fully explain the surface trends and the concurrently observed warming of the subsurface (100 to 500 m). Here, we argue that these trends are predominantly caused by an increased wind-driven northward transport of sea ice, enhancing the extraction of freshwater near Antarctica and releasing it in the open ocean. This conclusion is based on factorial experiments with a regional ocean model. In all experiments with an enhanced northward transport of sea ice, the open-ocean surface between the Subantarctic Front and the sea-ice edge is cooled by strengthening the salinity dominated oceanic stratification. The strengthened stratification reduces the downward mixing of cold surface water and the upward heat loss of the warmer waters below, thus warming the subsurface. This sea-ice induced subsurface warming mostly occurs around West Antarctica, where it likely enhances ice-shelf melting. Moreover, it could account for about 8±2% of the global ocean heat content increase between 1982 and 2011. Antarctic sea-ice changes thereby may have contributed to the slowdown of global surface warming over this period. The important role of sea-ice in driving changes in the high-latitude Southern Ocean are robust across all considered sensitivity cases, although their magnitude is sensitive to the forcing and the role of salinity in controlling the vertical stratification in the mean state. It remains yet unclear whether these sea-ice induced changes are associated with natural variability or a response to anthropogenic forcing.</p>

scholarly journals Can we reconstruct the formation of large open ocean polynyas in the Southern Ocean using ice core records?

2020 ◽  
Author(s):  
Hugues Goosse ◽  
Quentin Dalaiden ◽  
Marie G. P. Cavitte ◽  
Liping Zhang
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Climate Models ◽  
Ice Core ◽  
Ice Cores ◽  
Open Ocean ◽  
Weddell Sea ◽  
High Accumulation ◽  
Surface Mass ◽  
The Impact

Abstract. Large open-ocean polynyas, defined as ice-free areas within the sea ice pack, have been observed only rarely over the past decades in the Southern Ocean. In addition to smaller recent events, an impressive sequence occurred in the Weddell Sea in 1974, 1975 and 1976 with openings of more than 300,000 km2 that lasted the full winter. Those big events have a huge impact on the sea ice cover, deep-water formation and more generally on the Southern Ocean and the Antarctic climate. However, we have no estimate of the frequency of the occurrence of such large open-ocean polynyas before the 1970s. Our goal here is to test if polynya activity could be reconstructed using continental records, and specifically, observations derived from ice cores. The fingerprint of big open-ocean polynyas is first described in reconstructions based on data from weather stations, in ice cores for the 1970s and in climate models. It shows a clear signal, characterized by a surface air warming and increased precipitation in coastal regions adjacent to the eastern part of the Weddell Sea where several high-resolution ice cores have been collected. The signal of isotopic composition of precipitation is more ambiguous and we thus base our reconstructions on surface mass balance records only. A first reconstruction is obtained by performing a simple average of standardized records. Given the similarity between the observed signal and the one simulated in models, we also use data assimilation to reconstruct past polynya activity. The impact of open ocean polynyas on the continent is not large enough compared to the changes due, for instance, to atmospheric variability to detect without ambiguity the polynya signal and additional observations would be required to discriminate clearly the years with and without open ocean polynya. It is thus reasonable to consider that, in these preliminary reconstructions, some high accumulation events may be wrongly interpreted as the consequence of polynya formation while some years with polynya formation may be missed. Nevertheless, our reconstructions suggest that big open ocean polynyas, such as the ones that were observed in the 1970s, are rare events, occurring at most a few times per century. Century-scale changes in polynya activity are also likely but our reconstructions are unable to assess precisely this aspect at this stage.

scholarly journals Can we reconstruct the formation of large open-ocean polynyas in the Southern Ocean using ice core records?

2021 ◽  
Vol 17 (1) ◽  
pp. 111-131
Author(s):  
Hugues Goosse ◽  
Quentin Dalaiden ◽  
Marie G. P. Cavitte ◽  
Liping Zhang
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Climate Models ◽  
Ice Core ◽  
Ice Cores ◽  
Snow Accumulation ◽  
Open Ocean ◽  
Weddell Sea ◽  
Surface Mass ◽  
The Impact

Abstract. Large open-ocean polynyas, defined as ice-free areas within the sea ice pack, have only rarely been observed in the Southern Ocean over the past decades. In addition to smaller recent events, an impressive sequence occurred in the Weddell Sea in 1974, 1975 and 1976 with openings of more than 300 000 km2 that lasted the full winter. These big events have a huge impact on the sea ice cover, deep-water formation, and, more generally, on the Southern Ocean and the Antarctic climate. However, we have no estimate of the frequency of the occurrence of such large open-ocean polynyas before the 1970s. Our goal here is to test if polynya activity could be reconstructed using continental records and, specifically, observations derived from ice cores. The fingerprint of big open-ocean polynyas is first described in reconstructions based on data from weather stations, in ice cores for the 1970s and in climate models. It shows a signal characterized by a surface air warming and increased precipitation in coastal regions adjacent to the eastern part of the Weddell Sea, where several high-resolution ice cores have been collected. The signal of the isotopic composition of precipitation is more ambiguous; thus, we base our reconstructions on surface mass balance records alone. A first reconstruction is obtained by performing a simple average of standardized records. Given the similarity between the observed signal and the one simulated in models, we also use data assimilation to reconstruct past polynya activity. The impact of open-ocean polynyas on the continent is not large enough, compared with the changes due to factors such as atmospheric variability, to detect the polynya signal without ambiguity, and additional observations would be required to clearly discriminate the years with and without open-ocean polynya. Thus, it is reasonable to consider that, in these preliminary reconstructions, some high snow accumulation events may be wrongly interpreted as the consequence of polynya formation and some years with polynya formation may be missed. Nevertheless, our reconstructions suggest that big open-ocean polynyas, such as those observed in the 1970s, are rare events, occurring at most a few times per century. Century-scale changes in polynya activity are also likely, but our reconstructions are unable to precisely assess this aspect at this stage.

scholarly journals Identifying the Mechanisms of DO-scale Oscillations in a Gcm: a Salt Oscillator Triggered by the Laurentide Ice Sheet

2021 ◽  
Author(s):  
Edward Armstrong ◽  
Kenji Izumi ◽  
Paul Valdes
Keyword(s):  
Sea Ice ◽  
Ocean Circulation ◽  
North Atlantic ◽  
Climate Models ◽  
Salinity Gradient ◽  
Ice Sheet ◽  
Feedback Mechanism ◽  
Meridional Overturning Circulation ◽  
Laurentide Ice Sheet ◽  
Freshwater Fluxes

Abstract The driver mechanisms of Dansgaard-Oeschger (DO) events remain uncertain, in part because many climate models do not show similar oscillatory behaviour. Here we present results from glacial simulations of the HadCM3B coupled atmosphere-ocean-vegetation model that show stochastic, quasi-periodical DO-scale variability. This variability is driven by variations in the strength of the Atlantic Meridional Overturning Circulation in response to North Atlantic salinity fluctuations. The mechanism represents a salt oscillator driven by the salinity gradient between the subtropical gyre and the Northern North Atlantic. Utilising a full set of model salinity diagnostics, we identify a complex ocean-atmosphere-sea-ice feedback mechanism that maintains this oscillator, driven by the interplay between surface freshwater fluxes (tropical P-E balance and sea-ice), advection, and convection. The key trigger is the extent of the Laurentide ice sheet, which alters atmospheric and ocean circulation patterns, highlighting the sensitivity of the climate system to land-ice extent. This, in addition to the background climate state, pushes the climate beyond a tipping point and into an oscillatory mode on a timescale comparable to the DO events.

Planetary-wave induced strengthening of the AMOC forced by poleward intensified Southern Hemisphere westerly winds

2021 ◽  
pp. 1-45
Author(s):  
D. J. Webb ◽  
P. Spence ◽  
R. M. Holmes ◽  
M. H. England
Keyword(s):  
Southern Ocean ◽  
North Atlantic ◽  
Climate Models ◽  
Horizontal Resolution ◽  
Meridional Overturning Circulation ◽  
Global Ocean ◽  
Baroclinic Waves ◽  
The North ◽  
Shelf Slope ◽  
The North Atlantic

AbstractThe Atlantic meridional overturning circulation (AMOC) plays a key role in determining the distribution of heat and nutrients in the global ocean. Climate models suggest that Southern Ocean winds will strengthen and shift poleward in the future, which could have implications for future AMOC trends. Using a coupled global-ocean sea-ice model at 1/4°horizontal resolution, we study the response of the North Atlantic overturning to two anomalous Southern Ocean wind-forcing (τ+15%), and a poleward intensification(). In both scenarios a strengthening in the North Atlantic overturning develops within a decade, with a much stronger response in the case. In , we find that the primary link between the North Atlantic response and the Southern Ocean forcing is via the propagation of baroclinic waves. In fact, due to the rapid northward propagation of these waves, changes in the AMOC in the case appear to originate in the North Atlantic and propagate southward, whereas in the τ+15% case AMOC anomalies propagate northward from the Southern Ocean. We find the difference to be predominately caused by the sign of the baroclinic waves propagating from the forcing region into the North Atlantic; downwelling in the τ+15% case, versus upwelling in the case. In the case, upwelling waves propagating into the NADW formation regions along shelf-slope topography bringing dense water to the surface. This reduces vertical density gradients leading to deeper wintertime convective overturn of surface waters, and an intensification of the AMOC.

On the impact of sea ice in a global ocean circulation model

1997 ◽  
Vol 25 ◽  
pp. 111-115 ◽  
Author(s):  
Achim Stössel
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Ocean Circulation ◽  
General Circulation ◽  
Thermohaline Circulation ◽  
Circulation Model ◽  
Deep Ocean ◽  
Global Ocean ◽  
Convective Activity ◽  
The Impact

This paper investigates the long-term impact of sea ice on global climate using a global sea-ice–ocean general circulation model (OGCM). The sea-ice component involves state-of-the-art dynamics; the ocean component consists of a 3.5° × 3.5° × 11 layer primitive-equation model. Depending on the physical description of sea ice, significant changes are detected in the convective activity, in the hydrographic properties and in the thermohaline circulation of the ocean model. Most of these changes originate in the Southern Ocean, emphasizing the crucial role of sea ice in this marginally stably stratified region of the world's oceans. Specifically, if the effect of brine release is neglected, the deep layers of the Southern Ocean warm up considerably; this is associated with a weakening of the Southern Hemisphere overturning cell. The removal of the commonly used “salinity enhancement” leads to a similar effect. The deep-ocean salinity is almost unaffected in both experiments. Introducing explicit new-ice thickness growth in partially ice-covered gridcells leads to a substantial increase in convective activity, especially in the Southern Ocean, with a concomitant significant cooling and salinification of the deep ocean. Possible mechanisms for the resulting interactions between sea-ice processes and deep-ocean characteristics are suggested.

scholarly journals Ventilation of the abyss in the Atlantic sector of the Southern Ocean

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Camille Hayatte Akhoudas ◽  
Jean-Baptiste Sallée ◽  
F. Alexander Haumann ◽  
Michael P. Meredith ◽  
Alberto Naveira Garabato ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Bottom Water ◽  
Climate Models ◽  
Deep Ocean ◽  
Analytical Framework ◽  
Weddell Sea ◽  
Global Ocean ◽  
Shelf Water ◽  
Antarctic Bottom Water ◽  
Atlantic Sector

AbstractThe Atlantic sector of the Southern Ocean is the world’s main production site of Antarctic Bottom Water, a water-mass that is ventilated at the ocean surface before sinking and entraining older water-masses—ultimately replenishing the abyssal global ocean. In recent decades, numerous attempts at estimating the rates of ventilation and overturning of Antarctic Bottom Water in this region have led to a strikingly broad range of results, with water transport-based calculations (8.4–9.7 Sv) yielding larger rates than tracer-based estimates (3.7–4.9 Sv). Here, we reconcile these conflicting views by integrating transport- and tracer-based estimates within a common analytical framework, in which bottom water formation processes are explicitly quantified. We show that the layer of Antarctic Bottom Water denser than 28.36 kg m$$^{-3}$$ - 3 $$\gamma _{n}$$ γ n is exported northward at a rate of 8.4 ± 0.7 Sv, composed of 4.5 ± 0.3 Sv of well-ventilated Dense Shelf Water, and 3.9 ± 0.5 Sv of old Circumpolar Deep Water entrained into cascading plumes. The majority, but not all, of the Dense Shelf Water (3.4 ± 0.6 Sv) is generated on the continental shelves of the Weddell Sea. Only 55% of AABW exported from the region is well ventilated and thus draws down heat and carbon into the deep ocean. Our findings unify traditionally contrasting views of Antarctic Bottom Water production in the Atlantic sector, and define a baseline, process-discerning target for its realistic representation in climate models.

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.

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