scholarly journals On deep convection events and Antarctic Bottom Water formation in ocean reanalysis products

Ocean Science ◽  
2017 ◽  
Vol 13 (6) ◽  
pp. 851-872 ◽  
Author(s):  
Wilton Aguiar ◽  
Mauricio M. Mata ◽  
Rodrigo Kerr
Keyword(s):  
Sea Ice ◽  
Bottom Water ◽  
Deep Convection ◽  
Open Ocean ◽  
Weddell Sea ◽  
Ocean Dynamics ◽  
Antarctic Bottom Water ◽  
Ocean Reanalysis ◽  
Reanalysis Product ◽  
Warm Deep Water

Abstract. Open ocean deep convection is a common source of error in the representation of Antarctic Bottom Water (AABW) formation in ocean general circulation models. Although those events are well described in non-assimilatory ocean simulations, the recent appearance of a massive open ocean polynya in the Estimating the Circulation and Climate of the Ocean Phase II reanalysis product (ECCO2) raises questions on which mechanisms are responsible for those spurious events and whether they are also present in other state-of-the-art assimilatory reanalysis products. To investigate this issue, we evaluate how three recently released high-resolution ocean reanalysis products form AABW in their simulations. We found that two of the products create AABW by open ocean deep convection events in the Weddell Sea that are triggered by the interaction of sea ice with the Warm Deep Water, which shows that the assimilation of sea ice is not enough to avoid the appearance of open ocean polynyas. The third reanalysis, My Ocean University Reading UR025.4, creates AABW using a rather dynamically accurate mechanism. The UR025.4 product depicts both continental shelf convection and the export of Dense Shelf Water to the open ocean. Although the accuracy of the AABW formation in this reanalysis product represents an advancement in the representation of the Southern Ocean dynamics, the differences between the real and simulated processes suggest that substantial improvements in the ocean reanalysis products are still needed to accurately represent AABW formation.

scholarly journals On the deep convection events and Antarctic Bottom Water formation in ocean reanalysis products

2017 ◽  
Author(s):  
Wilton Aguiar ◽  
Mauricio M. Mata ◽  
Rodrigo Kerr
Keyword(s):  
General Circulation ◽  
Bottom Water ◽  
Deep Convection ◽  
General Circulation Models ◽  
Open Ocean ◽  
Weddell Sea ◽  
Antarctic Bottom Water ◽  
Ocean Reanalysis ◽  
Ocean General Circulation Models ◽  
Bottom Water Formation

Abstract. Deep convection in open ocean polynyas are common sources of error on the representation of Antarctic Bottom Water (AABW) formation in Ocean General Circulation Models. Even though those events are well described in non-assimilatory ocean simulations, recent appearance of open ocean polynya in Estimating the Circulation and Climate of the Ocean Phase II reanalysis product raises a question if this spurious event is also found in state-of-art reanalysis products. In order to answer this question, we evaluate how three recently released high-resolution ocean reanalysis form AABW in their simulations. We found that two of them (ECCO2 and SoSE) create AABW by open ocean deep convection events in Weddell Sea, showing that assimilation of sea ice has not been enough to avoid open ocean polynya appearance. The third reanalysis – My Ocean University Reading – actually creates AABW by a rather dynamically accurate mechanism, depicting both continental shelf convection, and exporting of Dense Shelf Water to open ocean. Although the accuracy of the AABW formation in this reanalysis allows an advance in represent this process, the differences found between the real ocean and the simulated one suggests that ocean reanalysis still need substantial improvements to accurately represent AABW formation.

scholarly journals Internal Ocean Dynamics Control the Long-Term Evolution of Weddell Sea Polynya Activity

2021 ◽  
Vol 3 ◽  
Author(s):  
Jonathan W. Rheinlænder ◽  
Lars H. Smedsrud ◽  
Kerim H. Nisanciouglu
Keyword(s):  
Sea Ice ◽  
Deep Convection ◽  
Vertical Mixing ◽  
Open Ocean ◽  
Weddell Sea ◽  
Ocean Dynamics ◽  
Heat Reservoir ◽  
Ice Dynamics

Open-ocean polynyas effectively couple the ocean and atmosphere through large ice-free areas within the sea-ice cover, release vast quantities of oceanic heat, and impact deep ocean ventilation. Changes in polynya activity, particularly in the Weddell Sea, may be key to longer time-scale climate fluctuations, feedbacks and abrupt change. While changes in the occurrence of Weddell Sea polynyas are generally attributed to changes in the atmospheric surface forcing, the role of internal ocean dynamics for polynya variability is not well-resolved. In this study we employ a global coupled ocean-sea ice model with a repeating annual atmospheric cycle to explore changes in Weddell Sea water mass properties, stratification and ocean circulation driven by open-ocean polynyas. During the 1300-year long simulation, two large polynyas occur in the central Weddell Sea. Our results suggest that Weddell polynyas may be triggered without inter-annual changes in the atmospheric forcing. This highlights the role of ocean processes in preconditioning and triggering open-ocean polynyas on multi-centennial time-scales. The simulated polynyas form due to internal ocean-sea ice dynamics associated with a slow build-up and subsequent release of subsurface heat. A strong stratification and weak vertical mixing is necessary for building the subsurface heat reservoir. Once the water column turns unstable, enhanced vertical mixing of warm and saline waters into the surface layer causes efficient sea ice melt and the polynya appears. Subsequent, vigorous deep convection is maintained through upwelling of warm deep water leading to enhanced bottom water formation. We find a cessation of simulated deep convection and polynya activity due to long-term cooling and freshening of the subsurface heat reservoir. As subsurface waters in the Southern Ocean are now becoming warmer and saltier, we speculate that larger and more persistent Weddell polynyas could become more frequent in the future.

Antarctic Bottom Water Variability in a Coupled Climate Model

2008 ◽  
Vol 38 (9) ◽  
pp. 1870-1893 ◽  
Author(s):  
Agus Santoso ◽  
Matthew H. England
Keyword(s):  
Sea Ice ◽  
Deep Water ◽  
Bottom Water ◽  
Climate Model ◽  
Weddell Sea ◽  
Antarctic Bottom Water ◽  
Coupled Climate Model ◽  
Atlantic Sector ◽  
Free Region ◽  
Warm Deep Water

Abstract The natural variability of the Weddell Sea variety of Antarctic Bottom Water (AABW) is examined in a long-term integration of a coupled climate model. Examination of passive tracer concentrations suggests that the model AABW is predominantly sourced in the Weddell Sea. The maximum rate of the Atlantic sector Antarctic overturning (ψatl) is shown to effectively represent the outflow of Weddell Sea deep and bottom waters and the compensating inflow of Warm Deep Water (WDW). The variability of ψatl is found to be driven by surface density variability, which is in turn controlled by sea surface salinity (SSS). This suggests that SSS is a better proxy than SST for post-Holocene paleoclimate reconstructions of the AABW overturning rate. Heat–salt budget and composite analyses reveal that during years of high Weddell Sea salinity, there is an increased removal of summertime sea ice by enhanced wind-driven ice drift, resulting in increased solar radiation absorbed into the ocean. The larger ice-free region in summer then leads to enhanced air–sea heat loss, more rapid ice growth, and therefore greater brine rejection during winter. Together with a negative feedback mechanism involving anomalous WDW inflow and sea ice melting, this results in positively correlated θ–S anomalies that in turn drive anomalous convection, impacting AABW variability. Analysis of the propagation of θ–S anomalies is conducted along an isopycnal surface marking the separation boundary between AABW and the overlying Circumpolar Deep Water. Empirical orthogonal function analyses reveal propagation of θ–S anomalies from the Weddell Sea into the Atlantic interior with the dominant modes characterized by fluctuations on interannual to centennial time scales. Although salinity variability is dominated by along-isopycnal propagation, θ variability is dominated by isopycnal heaving, which implies propagation of density anomalies with the speed of baroclinic waves.

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.

Impacts of open-ocean deep convection in the Weddell Sea on coastal and bottom water temperature

2016 ◽  
Vol 48 (9-10) ◽  
pp. 2967-2981 ◽  
Author(s):  
Zhaomin Wang ◽  
Yang Wu ◽  
Xia Lin ◽  
Chengyan Liu ◽  
Zelin Xie
Keyword(s):  
Water Temperature ◽  
Bottom Water ◽  
Deep Convection ◽  
Open Ocean ◽  
Weddell Sea ◽  
Bottom Water Temperature

scholarly journals Increasing vertical mixing to reduce Southern Ocean deep convection in NEMO

2015 ◽  
Vol 8 (3) ◽  
pp. 2949-2972 ◽  
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 ◽  
Deep Convection ◽  
Mixed Layer Depth ◽  
Vertical Mixing ◽  
Weddell Sea ◽  
Cmip5 Models ◽  
Antarctic Bottom Water

Abstract. Most CMIP5 models unrealistically form Antarctic Bottom Water by open ocean deep convection in the Weddell and Ross Seas. To identify the triggering mechanisms leading to Southern Ocean deep convection in models, we perform sensitivity experiments on the ocean model NEMO 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 are warmer, open larger polynyas 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.

The Relationship of Weddell Polynya and Open-Ocean Deep Convection to the Southern Hemisphere Westerlies

2014 ◽  
Vol 44 (2) ◽  
pp. 694-713 ◽  
Author(s):  
Woo Geun Cheon ◽  
Young-Gyu Park ◽  
J. R. Toggweiler ◽  
Sang-Ki Lee
Keyword(s):  
Sea Ice ◽  
Southern Hemisphere ◽  
General Circulation ◽  
Deep Convection ◽  
Circulation Model ◽  
Open Ocean ◽  
Weddell Sea ◽  
Warm Water ◽  
Balance Model ◽  
Zonal Winds

Abstract The Weddell Polynya of the mid-1970s is simulated in an energy balance model (EBM) sea ice–ocean coupled general circulation model (GCM) with an abrupt 20% increase in the intensity of Southern Hemisphere (SH) westerlies. This small upshift of applied wind stress is viewed as a stand in for the stronger zonal winds that developed in the mid-1970s following a long interval of relatively weak zonal winds between 1954 and 1972. Following the strengthening of the westerlies in this model, the cyclonic Weddell gyre intensifies, raising relatively warm Weddell Sea Deep Water to the surface. The raised warm water then melts sea ice or prevents it from forming to produce the Weddell Polynya. Within the polynya, large heat loss to the air causes surface water to become cold and sink to the bottom via open-ocean deep convection. Thus, the underlying layers cool down, the warm water supply to the surface eventually stops, and the polynya cannot be maintained anymore. During the 100-yr-long model simulation, two Weddell Polynya events are observed. The second one occurs a few years after the first one disappears; it is much weaker and persists for less time than the first one because the underlying layer is cooler. Based on these model simulations, the authors hypothesize that the Weddell Polynya and open-ocean deep convection were responses to the stronger SH westerlies that followed a prolonged weak phase of the southern annular mode.

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.

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