scholarly journals Sensitivity of Antarctic Bottom Water to Changes in Surface Buoyancy Fluxes

2015 ◽  
Vol 29 (1) ◽  
pp. 313-330 ◽  
Author(s):  
Kate Snow ◽  
Andrew McC. Hogg ◽  
Bernadette M. Sloyan ◽  
Stephanie M. Downes
Keyword(s):  
Heat Flux ◽  
Bottom Water ◽  
Climate Models ◽  
Deep Ocean ◽  
Exchange Flow ◽  
Antarctic Bottom Water ◽  
Sector Model ◽  
Climate Change Responses ◽  
Decadal Scale ◽  
Surface Buoyancy

Abstract The influence of freshwater and heat flux changes on Antarctic Bottom Water (AABW) properties are investigated within a realistic bathymetry coupled ocean–ice sector model of the Atlantic Ocean. The model simulations are conducted at eddy-permitting resolution where dense shelf water production dominates over open ocean convection in forming AABW. Freshwater and heat flux perturbations are applied independently and have contradictory surface responses, with increased upper-ocean temperature and reduced ice formation under heating and the opposite under increased freshwater fluxes. AABW transport into the abyssal ocean reduces under both flux changes, with the reduction in transport being proportional to the net buoyancy flux anomaly south of 60°S. Through inclusion of shelf-sourced AABW, a process absent from most current generation climate models, cooling and freshening of dense source water is facilitated via reduced on-shelf/off-shelf exchange flow. Such cooling is propagated to the abyssal ocean, while compensating warming in the deep ocean under heating introduces a decadal-scale variability of the abyssal water masses. This study emphasizes the fundamental role buoyancy plays in controlling AABW, as well as the importance of the inclusion of shelf-sourced AABW within climate models in order to attain the complete spectrum of possible climate change responses.

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.

What is the heat uptake potential of Antarctic Bottom Water?

2021 ◽  
Author(s):  
Jan Zika ◽  
Abhishek Savita ◽  
Ryan Holmes ◽  
Taimoor Sohail
Keyword(s):  
Heat Transport ◽  
Upper Bound ◽  
Bottom Water ◽  
Climate Models ◽  
Deep Ocean ◽  
Meridional Overturning Circulation ◽  
Antarctic Bottom Water ◽  
Heat Uptake ◽  
Vertical Heat ◽  
Highly Correlated

<p>Antarctic Bottom Water (AABW) is a cold dense water mass which sinks around Antarctica keeping the abyssal ocean relatively cool. Recent observations have suggested a component of recent deep ocean warming is linked to AABW. Here we explore how much changes in AABW could affect changes in vertical ocean heat transport in a warming climate. If the AABW circulation were to be completely extinguished, for example due to increases in upper ocean thermal stratification, AABW would cease to cool the deep ocean and hence lead to an effective warming of the abyss. Therefore, we propose that long term mean vertical heat transport of the AABW circulation is an effective upper bound on the change in heat transport that can be affected by changes in AABW. We call this upper bound the ‘heat uptake potential’. We analyse AABW circulations in an ensemble of numerical climate models. We find that the AABW circulation contributes between 0.05Wm<sup>-2</sup> and 0.15Wm<sup>-2</sup> to the global vertical heat balance in the model’s pre-industrial states. Indeed, under abrupt CO<sub>2</sub> forcing changes, AABW heat transport systematically reduces (in some cases completely), with the largest reductions occurring in models with the largest pre-industrial mean heat transports. The AABW circulation vertical heat transport is found to be highly correlated with the minimum of the Meridional Overturning Circulation at 50<sup>o</sup>S in the models, suggesting there may be observable constraints on the heat uptake potential of AABW.</p>

scholarly journals Impact of sea-ice formation on the properties of Antarctic bottom water

1997 ◽  
Vol 25 ◽  
pp. 276-281 ◽  
Author(s):  
H. Goosse ◽  
J. M. Campin ◽  
T. Fichefet ◽  
E. Deleersnijder
Keyword(s):  
Fresh Water ◽  
Deep Water ◽  
Bottom Water ◽  
Formation Rate ◽  
Water Flux ◽  
Three Dimensional ◽  
Deep Ocean ◽  
Ocean Model ◽  
Antarctic Bottom Water ◽  
Water Fluxes

It is generally accepted that fresh-water fluxes due to ice accretion or melting profoundly influence the formation of Antarctic bottom water (AABW). This is investigated by means of a global, three-dimensional ice–ocean model. Two model runs were conducted. At the high southern latitudes, the control experiment exhibits positive (i.e. towards the ocean) fresh-water fluxes over the deep ocean, and large negative fluxes over the Antarctic continental shelf, because of the intense ice-production taking place in this region. The salinity of shelf water can increase in such a way that deep-water formation is facilitated. The simulated net fresh-water flux over the shelf has an annual mean value of −1 m a−1. This flux induces a transport of salt to bottom waters, which corresponds to an increase of their salinity estimated to be around 0.05 psu. In the second model run, the fresh-water fluxes due to ice melting or freezing are neglected, leading to a rearrangement of the water masses. In particular, the AABW-formation rate decreases, which allows the influence of North Atlantic deep water (NADW) to increase. As NADW is warmer and saltier than AABW, the bottom-water salinity and temperature become higher.

scholarly journals Impact of sea-ice formation on the properties of Antarctic bottom water

1997 ◽  
Vol 25 ◽  
pp. 276-281 ◽  
Author(s):  
H. Goosse ◽  
J. M. Campin ◽  
T. Fichefet ◽  
E. Deleersnijder
Keyword(s):  
Fresh Water ◽  
Deep Water ◽  
Bottom Water ◽  
Formation Rate ◽  
Water Flux ◽  
Three Dimensional ◽  
Deep Ocean ◽  
Ocean Model ◽  
Antarctic Bottom Water ◽  
Water Fluxes

It is generally accepted that fresh-water fluxes due to ice accretion or melting profoundly influence the formation of Antarctic bottom water (AABW). This is investigated by means of a global, three-dimensional ice-ocean model. Two model runs were conducted. At the high southern latitudes, the control experiment exhibits positive (i.e. towards the ocean) fresh-water fluxes over the deep ocean, and large negative fluxes over the Antarctic continental shelf, because of the intense ice-production taking place in this region. The salinity of shelf water can increase in such a way that deep-water formation is facilitated. The simulated net fresh-water flux over the shelf has an annual mean value of −1 m a−1. This flux induces a transport of salt to bottom waters, which corresponds to an increase of their salinity estimated to be around 0.05 psu. In the second model run, the fresh-water fluxes due to ice melting or freezing are neglected, leading to a rearrangement of the water masses. In particular, the AABW-formation rate decreases, which allows the influence of North Atlantic deep water (NADW) to increase. As NADW is warmer and saltier than AABW. the bottom-water salinity and temperature become higher.

scholarly journals Projected Slowdown of Antarctic Bottom Water Formation in Response to Amplified Meltwater Contributions

2019 ◽  
Vol 32 (19) ◽  
pp. 6319-6335 ◽  
Author(s):  
Véronique Lago ◽  
Matthew H. England
Keyword(s):  
Ocean Circulation ◽  
Bottom Water ◽  
Climate Model ◽  
Formation Rate ◽  
Vertical Mixing ◽  
Antarctic Bottom Water ◽  
Cold Temperatures ◽  
Ice Melt ◽  
Long Term Impact ◽  
Surface Buoyancy

Abstract The sinking and recirculation of Antarctic Bottom Water (AABW) are a major regulator of the storage of heat, carbon, and nutrients in the ocean. This sinking is sensitive to changes in surface buoyancy, in particular because of freshening since salinity plays a greater role in determining density at cold temperatures. Acceleration in Antarctic ice-shelf and land-ice melt could thus significantly impact the ventilation of the world’s oceans, yet future projections do not usually include this effect in models. Here we use an ocean–sea ice model to investigate the potential long-term impact of Antarctic meltwater on ocean circulation and heat storage. The freshwater forcing is derived from present-day estimates of meltwater input from drifting icebergs and basal melt, combined with RCP2.6, RCP4.5, and RCP8.5 scenarios of projected amplification of Antarctic meltwater. We find that the additional freshwater induces a substantial slowdown in the formation rate of AABW, reducing ventilation of the abyssal ocean. Under both the RCP4.5 and RCP8.5 meltwater scenarios, there is a near-complete shutdown of AABW formation within just 50 years, something that is not captured by climate model projections. The abyssal overturning at ~30°S also weakens, with an ~20-yr delay relative to the onset of AABW slowdown. After 200 years, up to ~50% of the original volume of AABW has disappeared as a result of abyssal warming, induced by vertical mixing in the absence of AABW ventilation. This result suggests that climate change could induce the disappearance of present-day abyssal water masses, with implications for the global distribution of heat, carbon, and nutrients.

scholarly journals Antarctic Bottom Water and North Atlantic Deep Water in CMIP6 models

2020 ◽  
Author(s):  
Céline Heuzé
Keyword(s):  
North Atlantic ◽  
Deep Water ◽  
Bottom Water ◽  
Climate Models ◽  
Deep Convection ◽  
North Atlantic Deep Water ◽  
Global Ocean ◽  
Antarctic Bottom Water ◽  
Deep Water Formation ◽  
Water Formation

Abstract. Deep water formation is the driver of the global ocean circulation, yet it was poorly represented in the previous generation of climate models. We here quantify biases in Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) formation, properties, transport and global extent in 35 climate models that participated in the latest Climate Model Intercomparison Project (CMIP6). Several CMIP6 models are correctly forming AABW via shelf processes, but in both hemispheres, the large majority of climate models form deep water via open ocean deep convection, too deep, too often, over too large an area. Models that convect the least form the most accurate AABW, but the least accurate NADW. The four CESM2 models with their pipe/overflow parameterisation are among the most accurate models. In the Atlantic, the colder AABW, the stronger the abyssal overturning at 30° S, and the further north the AABW layer extends. The saltier NADW, the stronger the Atlantic Meridional Overturning Circulation (AMOC), and the further south the NADW layer extends. In the Indian and Pacific oceans in contrast, the fresher models are the ones who extend the furthest regardless of the strength of their abyssal overturning, most likely because they also are the models with the weakest fronts in the Antarctic Circumpolar Currents. There are clear improvements since CMIP5: several CMIP6 models correctly represent or parameterise Antarctic shelf processes, fewer models exhibit Southern Ocean deep convection, more models convect at the right location in the Labrador Sea, bottom density biases are reduced, and abyssal overturning is more realistic. But more improvements are required, e.g. by generalising the use of overflow parameterisations or by coupling to interactive ice sheet models, before deep water formation, and hence heat and carbon storage, are represented accurately.

Global Contraction of Antarctic Bottom Water between the 1980s and 2000s*

2012 ◽  
Vol 25 (17) ◽  
pp. 5830-5844 ◽  
Author(s):  
Sarah G. Purkey ◽  
Gregory C. Johnson
Keyword(s):  
Southern Ocean ◽  
Bottom Water ◽  
Potential Temperature ◽  
Deep Ocean ◽  
Global Scale ◽  
Meridional Overturning Circulation ◽  
Antarctic Bottom Water ◽  
Ocean Heat Uptake ◽  
Near Surface ◽  
The North

Abstract A statistically significant reduction in Antarctic Bottom Water (AABW) volume is quantified between the 1980s and 2000s within the Southern Ocean and along the bottom-most, southern branches of the meridional overturning circulation (MOC). AABW has warmed globally during that time, contributing roughly 10% of the recent total ocean heat uptake. This warming implies a global-scale contraction of AABW. Rates of change in AABW-related circulation are estimated in most of the world’s deep-ocean basins by finding average rates of volume loss or gain below cold, deep potential temperature (θ) surfaces using all available repeated hydrographic sections. The Southern Ocean is losing water below θ = 0°C at a rate of −8.2 (±2.6) × 106 m3 s−1. This bottom water contraction causes a descent of potential isotherms throughout much of the water column until a near-surface recovery, apparently through a southward surge of Circumpolar Deep Water from the north. To the north, smaller losses of bottom waters are seen along three of the four main northward outflow routes of AABW. Volume and heat budgets below deep, cold θ surfaces within the Brazil and Pacific basins are not in steady state. The observed changes in volume and heat of the coldest waters within these basins could be accounted for by small decreases to the volume transport or small increases to θ of their inflows, or fractional increases in deep mixing. The budget calculations and global contraction pattern are consistent with a global-scale slowdown of the bottom, southern limb of the MOC.

scholarly journals Antarctic Bottom Water and North Atlantic Deep Water in CMIP6 models

Ocean Science ◽  
2021 ◽  
Vol 17 (1) ◽  
pp. 59-90
Author(s):  
Céline Heuzé
Keyword(s):  
Southern Ocean ◽  
North Atlantic ◽  
Deep Water ◽  
Bottom Water ◽  
Climate Models ◽  
Deep Convection ◽  
North Atlantic Deep Water ◽  
Antarctic Bottom Water ◽  
Water Formation ◽  
Bottom Water Formation

Abstract. Deep and bottom water formation are crucial components of the global ocean circulation, yet they were poorly represented in the previous generation of climate models. We here quantify biases in Antarctic Bottom Water (AABW) and North Atlantic Deep Water (NADW) formation, properties, transport, and global extent in 35 climate models that participated in the latest Climate Model Intercomparison Project (CMIP6). Several CMIP6 models are correctly forming AABW via shelf processes, but 28 models in the Southern Ocean and all 35 models in the North Atlantic form deep and bottom water via open-ocean deep convection too deeply, too often, and/or over too large an area. Models that convect the least form the most accurate AABW but the least accurate NADW. The four CESM2 models with their overflow parameterisation are among the most accurate models. In the Atlantic, the colder the AABW, the stronger the abyssal overturning at 30∘ S, and the further north the AABW layer extends. The saltier the NADW, the stronger the Atlantic Meridional Overturning Circulation (AMOC), and the further south the NADW layer extends. In the Indian and Pacific oceans in contrast, the fresher models are the ones which extend the furthest regardless of the strength of their abyssal overturning, most likely because they are also the models with the weakest fronts in the Antarctic Circumpolar Current. There are clear improvements since CMIP5: several CMIP6 models correctly represent or parameterise Antarctic shelf processes, fewer models exhibit Southern Ocean deep convection, more models convect at the right location in the Labrador Sea, bottom density biases are reduced, and abyssal overturning is more realistic. However, more improvements are required, e.g. by generalising the use of overflow parameterisations or by coupling to interactive ice sheet models, before deep and bottom water formation, and hence heat and carbon storage, are represented accurately.

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