Abyssal Ocean Warming and Salinification after Weddell Polynyas in the GFDL CM2G Coupled Climate Model

2015 ◽  
Vol 45 (11) ◽  
pp. 2755-2772 ◽  
Author(s):  
Hannah Zanowski ◽  
Robert Hallberg ◽  
Jorge L. Sarmiento
Keyword(s):  
Southern Ocean ◽  
Climate Model ◽  
Deep Ocean ◽  
Weddell Sea ◽  
Coupled Climate Model ◽  
Composite Event ◽  
Recent Warming ◽  
Time Period ◽  
Ocean Properties

AbstractThe role of Weddell Sea polynyas in establishing deep-ocean properties is explored in the NOAA Geophysical Fluid Dynamics Laboratory’s (GFDL) coupled climate model CM2G. Using statistical composite analysis of over 30 polynya events that occur in a 2000-yr-long preindustrial control run, the temperature, salinity, and water mass changes associated with the composite event are quantified. For the time period following the composite polynya cessation, termed the “recovery,” warming between 0.002° and 0.019°C decade−1 occurs below 4200 m in the Southern Ocean basins. Temperature and salinity changes are strongest in the Southern Ocean and the South Atlantic near the polynya formation region. Comparison of the model results with abyssal temperature observations reveals that the 1970s Weddell Polynya recovery could account for 10% ± 8% of the recent warming in the abyssal Southern Ocean. For individual Southern Ocean basins, this percentage is as little as 6% ± 11% or as much as 34% ± 13%.

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 Increasing vertical mixing to reduce Southern Ocean deep convection in NEMO3.4

2015 ◽  
Vol 8 (10) ◽  
pp. 3119-3130 ◽  
Author(s):  
C. Heuzé ◽  
J. K. Ridley ◽  
D. Calvert ◽  
D. P. Stevens ◽  
K. J. Heywood
Keyword(s):  
Southern Ocean ◽  
Mixed Layer ◽  
Surface Waters ◽  
Bottom Water ◽  
Climate Model ◽  
Deep Convection ◽  
Mixed Layer Depth ◽  
Vertical Mixing ◽  
Weddell Sea ◽  
Antarctic Bottom Water

Abstract. Most CMIP5 (Coupled Model Intercomparison Project Phase 5) models unrealistically form Antarctic Bottom Water by open ocean deep convection in the Weddell and Ross seas. To identify the mechanisms triggering Southern Ocean deep convection in models, we perform sensitivity experiments on the ocean model NEMO3.4 forced by prescribed atmospheric fluxes. We vary the vertical velocity scale of the Langmuir turbulence, the fraction of turbulent kinetic energy transferred below the mixed layer, and the background diffusivity and run short simulations from 1980. All experiments exhibit deep convection in the Riiser-Larsen Sea in 1987; the origin is a positive sea ice anomaly in 1985, causing a shallow anomaly in mixed layer depth, hence anomalously warm surface waters and subsequent polynya opening. Modifying the vertical mixing impacts both the climatological state and the associated surface anomalies. The experiments with enhanced mixing exhibit colder surface waters and reduced deep convection. The experiments with decreased mixing give warmer surface waters, open larger polynyas causing more saline surface waters and have deep convection across the Weddell Sea until the simulations end. Extended experiments reveal an increase in the Drake Passage transport of 4 Sv each year deep convection occurs, leading to an unrealistically large transport at the end of the simulation. North Atlantic deep convection is not significantly affected by the changes in mixing parameters. As new climate model overflow parameterisations are developed to form Antarctic Bottom Water more realistically, we argue that models would benefit from stopping Southern Ocean deep convection, for example by increasing their vertical mixing.

scholarly journals Relationship between Ocean Carbon and Heat Multidecadal Variability

2018 ◽  
Vol 31 (4) ◽  
pp. 1467-1482 ◽  
Author(s):  
Jordan Thomas ◽  
Darryn Waugh ◽  
Anand Gnanadesikan
Keyword(s):  
Southern Ocean ◽  
Carbon Content ◽  
Climate Model ◽  
Deep Convection ◽  
Heat Content ◽  
Weddell Sea ◽  
Global Ocean ◽  
Multidecadal Variability ◽  
Model Simulations ◽  
Ocean Carbon

The global ocean serves as a critical sink for anthropogenic carbon and heat. While significant effort has been dedicated to quantifying the oceanic uptake of these quantities, less research has been conducted on the mechanisms underlying decadal-to-centennial variability in oceanic heat and carbon. Therefore, little is understood about how much such variability may have obscured or reinforced anthropogenic change. Here the relationship between oceanic heat and carbon content is examined in a suite of coupled climate model simulations that use different parameterization settings for mesoscale mixing. The differences in mesoscale mixing result in very different multidecadal variability, especially in the Weddell Sea where the characteristics of deep convection are drastically changed. Although the magnitude and frequency of variability in global heat and carbon content is different across the model simulations, there is a robust anticorrelation between global heat and carbon content in all simulations. Global carbon content variability is primarily driven by Southern Ocean carbon variability. This contrasts with global heat content variability. Global heat content is primarily driven by variability in the southern midlatitudes and tropics, which opposes the Southern Ocean variability.

scholarly journals Impacts of Broad-Scale Surface Freshening of the Southern Ocean in a Coupled Climate Model

2018 ◽  
Vol 31 (7) ◽  
pp. 2613-2632 ◽  
Author(s):  
Ariaan Purich ◽  
Matthew H. England ◽  
Wenju Cai ◽  
Arnold Sullivan ◽  
Paul J. Durack
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Ocean Circulation ◽  
Climate Model ◽  
Cmip5 Models ◽  
Coupled Climate Model ◽  
Surface Cooling ◽  
Subsurface Waters ◽  
The Impact ◽  
Scale Surface

The Southern Ocean surface has freshened in recent decades, increasing water column stability and reducing upwelling of warmer subsurface waters. The majority of CMIP5 models underestimate or fail to capture this historical surface freshening, yet little is known about the impact of this model bias on regional ocean circulation and hydrography. Here experiments are performed using a global coupled climate model with additional freshwater applied to the Southern Ocean to assess the influence of recent surface freshening. The simulations explore the impact of persistent and long-term broad-scale freshening as a result of processes including precipitation minus evaporation changes. Thus, unlike previous studies, the freshening is applied as far north as 55°S, beyond the Antarctic ice margin. It is found that imposing a large-scale surface freshening causes a surface cooling and sea ice increase under preindustrial conditions, because of a reduction in ocean convection and weakened entrainment of warm subsurface waters into the surface ocean. This is consistent with intermodel relationships between CMIP5 models and the simulations, suggesting that models with larger surface freshening also exhibit stronger surface cooling and increased sea ice. Additional experiments are conducted with surface salinity restoration applied to capture observed regional salinity trends. Remarkably, without any mechanical wind trend forcing, these simulations accurately represent the spatial pattern of observed surface temperature and sea ice trends around Antarctica. This study highlights the importance of accurately simulating changes in Southern Ocean salinity to capture changes in ocean circulation, sea surface temperature, and sea ice.

The Role of Oscillating Southern Hemisphere Westerly Winds: Southern Ocean Coastal and Open-Ocean Polynyas

2018 ◽  
Vol 31 (3) ◽  
pp. 1053-1073 ◽  
Author(s):  
Woo Geun Cheon ◽  
Chang-Bong Cho ◽  
Arnold L. Gordon ◽  
Young Ho Kim ◽  
Young-Gyu Park
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Southern Hemisphere ◽  
Ross Sea ◽  
Deep Ocean ◽  
Open Ocean ◽  
Weddell Sea ◽  
Wind Forcing ◽  
Westerly Winds ◽  
Southern Hemisphere Westerly Winds

Abstract An oscillation in intensity of the Southern Hemisphere westerly winds is a major characteristic of the southern annular mode. Its impact upon the sea ice–ocean interactions in the Weddell and Ross Seas is investigated by a sea ice–ocean general circulation model coupled to an energy balance model for three temporal scales and two amplitudes of intensity. It is found that the oscillating wind forcing over the Southern Ocean plays a significant role both in regulating coastal polynyas along the Antarctic margins and in triggering open-ocean polynyas. The formation of coastal polynya in the western Weddell and Ross Seas is enhanced with the intensifying winds, resulting in an increase in the salt flux into the ocean via sea ice formation. Under intensifying winds, an instantaneous spinup within the Weddell and Ross Sea cyclonic gyres causes the warm deep water to upwell, triggering open-ocean polynyas with accompanying deep ocean convection. In contrast to coastal polynyas, open-ocean polynyas in the Weddell and Ross Seas respond differently to the wind forcing and are dependent on its period. That is, the Weddell Sea open-ocean polynya occurs earlier and more frequently than the Ross Sea open-ocean polynya and, more importantly, does not occur when the period of oscillation is sufficiently short. The strong stratification of the Ross Sea and the contraction of the Ross gyre due to the southward shift of Antarctic Circumpolar Current fronts provide unfavorable conditions for the Ross Sea open-ocean polynya. The recovery time of deep ocean heat controls the occurrence frequency of the Weddell Sea open-ocean polynya.

scholarly journals The Southern Hemisphere Westerlies in a Warming World: Propping Open the Door to the Deep Ocean

2006 ◽  
Vol 19 (24) ◽  
pp. 6382-6390 ◽  
Author(s):  
Joellen L. Russell ◽  
Keith W. Dixon ◽  
Anand Gnanadesikan ◽  
Ronald J. Stouffer ◽  
J. R. Toggweiler
Keyword(s):  
Carbon Dioxide ◽  
Southern Ocean ◽  
Climate Model ◽  
Deep Ocean ◽  
Global Ocean ◽  
Westerly Winds ◽  
Anthropogenic Carbon ◽  
Warming World ◽  
The Impact ◽  
Anthropogenic Carbon Dioxide

Abstract A coupled climate model with poleward-intensified westerly winds simulates significantly higher storage of heat and anthropogenic carbon dioxide by the Southern Ocean in the future when compared with the storage in a model with initially weaker, equatorward-biased westerlies. This difference results from the larger outcrop area of the dense waters around Antarctica and more vigorous divergence, which remains robust even as rising atmospheric greenhouse gas levels induce warming that reduces the density of surface waters in the Southern Ocean. These results imply that the impact of warming on the stratification of the global ocean may be reduced by the poleward intensification of the westerlies, allowing the ocean to remove additional heat and anthropogenic carbon dioxide from the atmosphere.

scholarly journals Small phytoplankton contribute greatly to CO2-fixation after the diatom bloom in the Southern Ocean

2021 ◽  
Author(s):  
Solène Irion ◽  
Urania Christaki ◽  
Hugo Berthelot ◽  
Stéphane L’Helguen ◽  
Ludwig Jardillier
Keyword(s):  
Southern Ocean ◽  
Co2 Fixation ◽  
Carbon Fixation ◽  
Deep Ocean ◽  
Small Cells ◽  
Cell Level ◽  
Carbon Pump ◽  
Small Phytoplankton ◽  
Important Pathway

AbstractPhytoplankton is composed of a broad-sized spectrum of phylogenetically diverse microorganisms. Assessing CO2-fixation intra- and inter-group variability is crucial in understanding how the carbon pump functions, as each group of phytoplankton may be characterized by diverse efficiencies in carbon fixation and export to the deep ocean. We measured the CO2-fixation of different groups of phytoplankton at the single-cell level around the naturally iron-fertilized Kerguelen plateau (Southern Ocean), known for intense diatoms blooms suspected to enhance CO2 sequestration. After the bloom, small cells (<20 µm) composed of phylogenetically distant taxa (prymnesiophytes, prasinophytes, and small diatoms) were growing faster (0.37 ± 0.13 and 0.22 ± 0.09 division d−1 on- and off-plateau, respectively) than larger diatoms (0.11 ± 0.14 and 0.09 ± 0.11 division d−1 on- and off-plateau, respectively), which showed heterogeneous growth and a large proportion of inactive cells (19 ± 13%). As a result, small phytoplankton contributed to a large proportion of the CO2 fixation (41–70%). The analysis of pigment vertical distribution indicated that grazing may be an important pathway of small phytoplankton export. Overall, this study highlights the need to further explore the role of small cells in CO2-fixation and export in the Southern Ocean.

scholarly journals Transient tracer applications in the Southern Ocean

2014 ◽  
Vol 11 (5) ◽  
pp. 2289-2335
Author(s):  
T. Stöven ◽  
T. Tanhua ◽  
M. Hoppema
Keyword(s):  
Southern Ocean ◽  
Sulfur Hexafluoride ◽  
Time Lag ◽  
Deep Ocean ◽  
Weddell Sea ◽  
Similar Data ◽  
Data Set ◽  
Western Atlantic ◽  
Application Limit ◽  
Transient Tracer

Abstract. Transient tracers can be used to constrain the Inverse-Gaussian transit time distribution (IG-TTD) and thus provide information about ocean ventilation. Individual transient tracers have different time and application ranges which are defined by their atmospheric history (chronological transient tracers) or their decay rate (radioactive transient tracers). The classification ranges from tracers for highly ventilated water masses, e.g. sulfur hexafluoride (SF6), the decay of Tritium (δ3H) and to some extent also dichlorodifluoromethane (CFC-12) to tracers for less ventilated deep ocean basins, e.g. CFC-12, Argon-39 (39Ar) and radiocarbon (14C). The IG-TTD can be empirically constrained by using transient tracer couples with sufficiently different input functions. Each tracer couple has specific characteristics which influence the application limit of the IG-TTD. Here we provide an overview of commonly used transient tracer couples and their validity areas within the IG-TTD by using the concept of tracer age differences (TAD). New measured CFC-12 and SF6 data from a section along 10° E in the Southern Ocean in 2012 are presented. These are combined with a similar data set of 1998 along 6° E in the Southern Ocean as well as with 39Ar data from the early 1980s in the western Atlantic Ocean and the Weddell Sea for investigating the application limit of the IG-TTD and to analyze changes in ventilation in the Southern Ocean. We found that the IG-TTD can be constrained south to 46° S which corresponds to the Subantarctic Front (SAF) denoting the application limit. The constrained IG-TTD north of the SAF shows a slight increase in mean ages between 1998 and 2012 in the upper 1200 m between 42–46° S. The absence of SF6 inhibits ventilation analyses below this depth. The time lag analysis between the 1998 and 2012 data shows an increase in ventilation down to 1000 m and a steady ventilation between 2000 m-bottom south of the SAF between 51–55° S.

scholarly journals Natural ocean carbon cycle sensitivity to parameterizations of the recycling in a climate model

2013 ◽  
Vol 10 (7) ◽  
pp. 11111-11153
Author(s):  
A. Romanou ◽  
J. Romanski ◽  
W. W. Gregg
Keyword(s):  
Southern Ocean ◽  
Climate Model ◽  
Deep Ocean ◽  
Ocean Model ◽  
Pump Efficiency ◽  
Biological Pump ◽  
Vertical Coordinate ◽  
Ocean Models ◽  
Ocean Biogeochemistry ◽  
Ocean Carbon

Abstract. Sensitivities of the oceanic biological pump within the GISS climate modeling system are explored here. Results are presented from twin control simulations of the air-sea CO2 gas exchange using two different ocean models coupled to the same atmosphere. The two ocean models (Russell ocean model and Hybrid Coordinate Ocean Model, HYCOM) use different vertical coordinate systems, and therefore different representations of column physics. Both variants of the GISS climate model are coupled to the same ocean biogeochemistry module (the NASA Ocean Biogeochemistry Model, NOBM) which computes prognostic distributions for biotic and abiotic fields that influence the air-sea flux of CO2 and the deep ocean carbon transport and storage. In particular, the model differences due to remineralization rate changes are compared to differences attributed to physical processes modeled differently in the two ocean models such as ventilation, mixing, eddy stirring and vertical advection. The Southern Ocean emerges as a key region where the CO2 flux is as sensitive to biological parameterizations as it is to physical parameterizations. Mixing in the Southern Ocean is shown to be a~good indicator of the magnitude of the biological pump efficiency regardless of physical model choice.

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