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 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.

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 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.

Competition between distributed and localized buoyancy fluxes in a confined volume

1999 ◽  
Vol 391 ◽  
pp. 319-336 ◽  
Author(s):  
M. G. WELLS ◽  
R. W. GRIFFITHS ◽  
J. S. TURNER
Keyword(s):  
Heat Flux ◽  
Mixed Layer ◽  
Deep Convection ◽  
Mixed Layer Depth ◽  
Vertical Mixing ◽  
Flux Ratio ◽  
Density Stratification ◽  
Uniform Heat Flux ◽  
Uniform Heat ◽  
Boundary Flux

We investigate the convection and density stratification that form when buoyancy fluxes are simultaneously applied to a finite volume in both a turbulent buoyant plume from a small source and as a uniform heat flux from a horizontal boundary. The turbulent plume tends to produce a stable density stratification, whereas the distributed flux from a boundary tends to force vigorous overturning and vertical mixing. Experiments show that steady, partially mixed and partially stratified states can exist when the plume buoyancy flux is greater than the distributed flux.When the two fluxes originate from the same boundary, the steady state involves a balance between the rate at which the mixed layer deepens due to encroachment and vertical advection of the stratified water far from the plume due to the plume volume flux acquired by entrainment. There is a monotonic relationship between the normalized mixed layer depth and flux ratio R (boundary flux/plume flux) for 0<R<1, and the whole tank overturns for R>1. The stable density gradient in the stratified region is primarily due to the buoyancy from the plume but is strengthened by a stabilizing temperature gradient resulting from entrainment of heat into the plume from the mixed layer. This result may be relevant to the upper oceans of high latitude where there is commonly a destabilizing heat flux from the sea surface as well as more localized and intense deep convection from the surface.For the case of fluxes from a plume on one boundary and a uniform heat flux from the opposite boundary the shape of the density profile is that given by the Baines & Turner (1969) ‘filling-box’ mechanism, with the gradient reduced by a factor (1 + R) due to the heating. Thus, when R<−1 there is no stratified region and the whole water column overturns. When 0>R>−1, the constant depth of the convecting layer is determined by a balance between buoyancy and turbulent kinetic energy in the outflow layer from the plume.

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 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 Assessment of the representation of Antarctic Bottom Water properties in the ECCO2 reanalysis

Ocean Science ◽  
2014 ◽  
Vol 10 (6) ◽  
pp. 923-946 ◽  
Author(s):  
M. Azaneu ◽  
R. Kerr ◽  
M. M. Mata
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Bottom Water ◽  
Potential Temperature ◽  
World Ocean ◽  
Volume Transport ◽  
Weddell Sea ◽  
Antarctic Bottom Water ◽  
Water Properties ◽  
The Absolute

Abstract. We analyzed the ability of the Estimating the Circulation and Climate of the Ocean – Phase II (ECCO2) reanalysis to represent the hydrographic properties and variability of Antarctic Bottom Water (AABW) in the Southern Ocean. We used a 20-year (1992–2011) observational database to perform comparisons of hydrographic properties and reanalysis output for the same time period. Four case studies based on current meter data and the AABW volume transport estimates previously reported in the literature were also evaluated. The opening and maintenance of an oceanic polynya in the Weddell Sea sector is observed after 2004 in the reanalysis product. Moreover, intense deep water production due to deep convection occurs, which leads to a scenario in which the Weddell Sea is flooded with AABW. For this reason, our analyses focused on the period that was identified as more reliable (1992–2004). The main Southern Ocean oceanographic features, as well as the characteristic shape of the regional potential temperature–salinity (θ–S) diagrams, are coincident with observations. However, the reanalysis output produces surface waters that are generally denser than observations due to the reproduction of waters that are generally saltier than expected, which probably resulted from the strong seasonality of sea ice concentrations. Bottom waters are warmer and less dense, while intermediate waters are statistically closest to the observations. The differences in bottom water properties are partially due to the inability of the reanalysis to properly reproduce the formation and export of dense waters from the shelf and the consequent absence of the densest AABW variety for most of the analyzed period. Despite differences in the absolute values, the upper AABW limit (γn ≥ 28.27 kg m−3) and AABW occupied area estimates are coincident with the observations in the World Ocean Circulation Experiment (WOCE) repeat sections SR2 and SR4. Moreover, the AABW volume export and current velocity variability are correlated with the observed time series in the most important region of dense water export (i.e., the Weddell Sea). Despite the consistency in terms of variability, the absolute volume transport and velocity estimates are underrepresented in all cases.

scholarly journals Assessment of the ECCO2 reanalysis on the representation of Antarctic Bottom Water properties

2014 ◽  
Vol 11 (2) ◽  
pp. 1023-1091 ◽  
Author(s):  
M. Azaneu ◽  
R. Kerr ◽  
M. M. Mata
Keyword(s):  
Southern Ocean ◽  
Case Studies ◽  
Bottom Water ◽  
Potential Temperature ◽  
Volume Transport ◽  
Reanalysis Data ◽  
Weddell Sea ◽  
Good Representation ◽  
Antarctic Bottom Water ◽  
Water Properties

Abstract. We analyzed the ability of the Estimating the Circulation and Climate of the Ocean – Phase II (ECCO2) reanalysis to represent the hydrographic properties and variability of the Antarctic Bottom Water (AABW) in the Southern Ocean. We used a twenty-year observational database to perform comparisons of hydrographic properties and reanalysis data for the same time period (1992–2011). In addition, we evaluated four case studies based on current meter data and the AABW volume transport estimates previously reported in the literature. The main Southern Ocean oceanographic features, as well as the characteristic shape of the regional potential temperature–salinity (θ–S) diagrams, are adequately represented by the reanalysis. However, the opening of an oceanic polynya in the Weddell Sea Sector, which has been clearly visible since 2005, contributed to an unrealistic representation of the hydrographic properties of the Southern Ocean primarily after 2004. In this sense, our analyses focused on the period that was identified as more reliable (1992–2004). In general, the reanalysis data showed surface waters that were warmer, saltier, and denser than observations, which may have resulted from the absence of Ice Shelf Water and from the overestimation of sea ice concentrations that limit oceanic heat loss during austral winters. Intermediate waters were generally colder, fresher, and denser than observations, whereas deep waters were warmer and less dense. These differences in deep water properties were partially a result of the inability to reproduce the densest AABW variety by reanalysis for most of the analyzed period and also because of the model's relatively coarse vertical resolution. Despite differences in absolute values, the upper AABW limit (γn ≥ 28.27 kg m−3) and AABW occupied area were well represented in the WOCE repeat sections SR2 and SR4 for the studied periods. In section WOCE SR3, however, the estimates from the differences were not as well correlated, and the AABW layer thickness was underrepresented. The case studies showed a good representation of the AABW volume export and current velocity variability in the most important region of dense water export (i.e., the Weddell Sea). The exception is the AABW volume transport near the Kerguelen Plateau, in which the rugged local bathymetry and the relatively coarse model resolution hampered a fair representation of the transport variability by the reanalysis. Despite the consistency in terms of variability, absolute volume transport, and velocity, estimates were underrepresented in all cases. Moreover, the reanalysis was capable of reproducing the general variability pattern and trends of the AABW hydrographic properties reported by previous studies. Therefore, the ECCO2 data from the 1992–2004 period was considered adequate for investigating the circulation of the AABW and variability of the hydrographic properties, whereas data from the latter period (2005–2011) must be given careful attention.

scholarly journals Southern Ocean Sector Centennial Climate Variability and Recent Decadal Trends

2013 ◽  
Vol 26 (19) ◽  
pp. 7767-7782 ◽  
Author(s):  
Mojib Latif ◽  
Torge Martin ◽  
Wonsun Park
Keyword(s):  
Southern Ocean ◽  
Sea Ice ◽  
Atmospheric Circulation ◽  
Climate Model ◽  
Deep Convection ◽  
Surface Air Temperature ◽  
Weddell Sea ◽  
Sea Ice Extent ◽  
Low Level ◽  
Antarctic Sea Ice

Abstract Evidence is presented for the notion that some contribution to the recent decadal trends observed in the Southern Hemisphere, including the lack of a strong Southern Ocean surface warming, may have originated from longer-term internal centennial variability originating in the Southern Ocean. The existence of such centennial variability is supported by the instrumental sea surface temperatures (SSTs), a multimillennial reconstruction of Tasmanian summer temperatures from tree rings, and a millennial control integration of the Kiel Climate Model (KCM). The model variability was previously shown to be linked to changes in Weddell Sea deep convection. During phases of deep convection the surface Southern Ocean warms, the abyssal Southern Ocean cools, Antarctic sea ice extent retreats, and the low-level atmospheric circulation over the Southern Ocean weakens. After the halt of deep convection the surface Southern Ocean cools, the abyssal Southern Ocean warms, Antarctic sea ice expands, and the low-level atmospheric circulation over the Southern Ocean intensifies, consistent with what has been observed during the recent decades. A strong sensitivity of the time scale to model formulation is noted. In the KCM, the centennial variability is associated with global-average surface air temperature (SAT) changes of the order of a few tenths of a degree per century. The model results thus suggest that internal centennial variability originating in the Southern Ocean should be considered in addition to other internal variability and external forcing when discussing the climate of the twentieth century and projecting that of the twenty-first century.

scholarly journals Transient Response of the Southern Ocean to Changing Ozone: Regional Responses and Physical Mechanisms

2017 ◽  
Vol 30 (7) ◽  
pp. 2463-2480 ◽  
Author(s):  
William J. M. Seviour ◽  
Anand Gnanadesikan ◽  
Darryn Waugh ◽  
Marie-Aude Pradal
Keyword(s):  
Southern Ocean ◽  
Climate Model ◽  
Cold Water ◽  
Deep Convection ◽  
Weddell Sea ◽  
Freshwater Flux ◽  
Surface Salinity ◽  
Sea Ice Concentration ◽  
Climate Model Simulations ◽  
The Impact

The impact of changing ozone on the climate of the Southern Ocean is evaluated using an ensemble of coupled climate model simulations. By imposing a step change from 1860 to 2000 conditions, response functions associated with this change are estimated. The physical processes that drive this response are different across time periods and locations, as is the sign of the response itself. Initial cooling in the Pacific sector is driven not only by the increased winds pushing cold water northward, but also by the southward shift of storms associated with the jet stream. This shift drives both an increase in cloudiness (resulting in less absorption of solar radiation) and an increase in net freshwater flux to the ocean (resulting in a decrease in surface salinity that cuts off mixing of warm water from below). A subsurface increase in temperature associated with this reduction in mixing then upwells along the Antarctic coast, producing a subsequent warming. Similar changes in convective activity occur in the Weddell Sea but are offset in time. Changes in sea ice concentration also play a role in modulating solar heating of the ocean near the continent. The time scale for the initial cooling is much longer than that seen in NCAR CCSM3.5, possibly reflecting differences in natural convective variability between that model (which has essentially no Southern Ocean deep convection) and the one used here (which has a large and possibly unrealistically regular mode of convection) or to differences in cloud feedbacks or in the location of the anomalous winds.

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