scholarly journals Simulation of Subantarctic Mode and Antarctic Intermediate Waters in Climate Models

2007 ◽  
Vol 20 (20) ◽  
pp. 5061-5080 ◽  
Author(s):  
Bernadette M. Sloyan ◽  
Igor V. Kamenkovich
Keyword(s):  
Southern Ocean ◽  
Climate Models ◽  
Intermediate Water ◽  
Mixing Processes ◽  
Intergovernmental Panel ◽  
Mode Water ◽  
Buoyancy Forcing ◽  
The North ◽  
Circumpolar Current ◽  
Ipcc Ar4

Abstract The Southern Ocean’s Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) are two globally significant upper-ocean water masses that circulate in all Southern Hemisphere subtropical gyres and cross the equator to enter the North Pacific and North Atlantic Oceans. Simulations of SAMW and AAIW for the twentieth century in eight climate models [GFDL-CM2.1, CCSM3, CNRM-CM3, MIROC3.2(medres), MIROC3.2(hires), MRI-CGCM2.3.2, CSIRO-Mk3.0, and UKMO-HadCM3] that provided their output in support of the Intergovernmental Panel on Climate Change’s Fourth Assessment Report (IPCC AR4) have been compared to the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Atlas of Regional Seas. The climate models, except for UKMO-HadCM3, CSIRO-Mk3.0, and MRI-CGCM2.3.2, provide a reasonable simulation of SAMW and AAIW isopycnal temperature and salinity in the Southern Ocean. Many models simulate the potential vorticity minimum layer and salinity minimum layer of SAMW and AAIW, respectively. However, the simulated SAMW layer is generally thinner and at lighter densities than observed. All climate models display a limited equatorward extension of SAMW and AAIW north of the Antarctic Circumpolar Current. Errors in the simulation of SAMW and AAIW property characteristics are likely to be due to a combination of many errors in the climate models, including simulation of wind and buoyancy forcing, inadequate representation of subgrid-scale mixing processes in the Southern Ocean, and midlatitude diapycnal mixing parameterizations.

scholarly journals Changes in the Subduction of Southern Ocean Water Masses at the End of the Twenty-First Century in Eight IPCC Models

2010 ◽  
Vol 23 (24) ◽  
pp. 6526-6541 ◽  
Author(s):  
Stephanie M. Downes ◽  
Nathaniel L. Bindoff ◽  
Stephen R. Rintoul
Keyword(s):  
Climate Change ◽  
Southern Ocean ◽  
Ocean Circulation ◽  
Comparison Method ◽  
Water Masses ◽  
Intermediate Water ◽  
Intergovernmental Panel ◽  
Mode Water ◽  
Surface Warming ◽  
Freshwater Fluxes

Abstract A multimodel comparison method is used to assess the sensitivity of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) formation to climate change. For the Intergovernmental Panel on Climate Change A2 emissions scenario (where atmospheric CO2 is 860 ppm at 2100), the models show cooling and freshening on density surfaces less than about 27.4 kg m−3, a pattern that has been observed in the late twentieth century. SAMW (defined by the low potential vorticity layer) and AAIW (defined by the salinity minimum layer) warm and freshen as they shift to lighter density classes. Heat and freshwater fluxes at the ocean surface dominate the projected buoyancy gain at outcrop regions of SAMW and AAIW, whereas the net increase in the Ekman flux of heat and freshwater contributes to a lesser extent. This buoyancy gain, combined with shoaling of the winter mixed layer, reduces the volume of SAMW subducted into the ocean interior by a mean of 8 Sv (12%), and the subduction of AAIW decreases by a mean of 14 Sv (23%; 1 Sv ≡ 106 m3 s−1). Decreases in the projected subduction of the key Southern Ocean upper-water masses imply a slow down in the Southern Ocean circulation in the future, driven by surface warming and freshening. A reduction in the subduction of intermediate waters implies a likely future decrease in the capacity of the Southern Ocean to sequester CO2.

The Sensitivity of a Coupled Climate Model to Its Ocean Component

2010 ◽  
Vol 23 (19) ◽  
pp. 5126-5150 ◽  
Author(s):  
A. P. Megann ◽  
A. L. New ◽  
A. T. Blaker ◽  
B. Sinha
Keyword(s):  
North Atlantic ◽  
Climate Models ◽  
Climate Model ◽  
Ocean Model ◽  
Intermediate Water ◽  
Mode Water ◽  
Vertical Coordinate ◽  
The North ◽  
Almost Everywhere ◽  
Subantarctic Mode Water

Abstract The control climates of two coupled climate models are intercompared. The first is the third climate configuration of the Met Office Unified Model (HadCM3), while the second, the Coupled Hadley–Isopycnic Model Experiment (CHIME), is identical to the first except for the replacement of its ocean component by the Hybrid-Coordinate Ocean Model (HYCOM). Both models possess realistic and similar ocean heat transports and overturning circulation. However, substantial differences in the vertical structure of the two ocean components are observed, some of which are directly attributed to their different vertical coordinate systems. In particular, the sea surface temperature (SST) in CHIME is biased warm almost everywhere, particularly in the North Atlantic subpolar gyre, in contrast to HadCM3, which is biased cold except in the Southern Ocean. Whereas the HadCM3 ocean warms from just below the surface down to 1000-m depth, a similar warming in CHIME is more pronounced but shallower and confined to the upper 400 m, with cooling below this. This is particularly apparent in the subtropical thermoclines, which become more diffuse in HadCM3, but sharper in CHIME. This is interpreted as resulting from a more rigorously controlled diapycnal mixing in the interior isopycnic ocean in CHIME. Lower interior mixing is also apparent in the better representation and maintenance of key water masses in CHIME, such as Subantarctic Mode Water, Antarctic Intermediate Water, and North Atlantic Deep Water. Finally, the North Pacific SST cold error in HadCM3 is absent in CHIME, and may be related to a difference in the separation position of the Kuroshio. Disadvantages of CHIME include a nonconservation of heat equivalent to 0.5 W m−2 globally, and a warming and salinification of the northwestern Atlantic.

scholarly journals The Southern Ocean and Its Climate in CCSM4

2012 ◽  
Vol 25 (8) ◽  
pp. 2652-2675 ◽  
Author(s):  
Wilbert Weijer ◽  
Bernadette M. Sloyan ◽  
Mathew E. Maltrud ◽  
Nicole Jeffery ◽  
Matthew W. Hecht ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Climate System ◽  
Intermediate Water ◽  
Mode Water ◽  
Ice Dynamics ◽  
Climate System Model ◽  
Interbasin Exchange ◽  
Circumpolar Current ◽  
Earth’S Climate ◽  
New Community

Abstract The new Community Climate System Model, version 4 (CCSM4), provides a powerful tool to understand and predict the earth’s climate system. Several aspects of the Southern Ocean in the CCSM4 are explored, including the surface climatology and interannual variability, simulation of key climate water masses (Antarctic Bottom Water, Subantarctic Mode Water, and Antarctic Intermediate Water), the transport and structure of the Antarctic Circumpolar Current, and interbasin exchange via the Agulhas and Tasman leakages and at the Brazil–Malvinas Confluence. It is found that the CCSM4 has varying degrees of accuracy in the simulation of the climate of the Southern Ocean when compared with observations. This study has identified aspects of the model that warrant further analysis that will result in a more comprehensive understanding of ocean–atmosphere–ice dynamics and interactions that control the earth’s climate and its variability.

scholarly journals A Linear Decomposition of the Southern Ocean Thermohaline Structure

2017 ◽  
Vol 47 (1) ◽  
pp. 29-47 ◽  
Author(s):  
Etienne Pauthenet ◽  
Fabien Roquet ◽  
Gurvan Madec ◽  
David Nerini
Keyword(s):  
Southern Ocean ◽  
World Ocean ◽  
Principal Component ◽  
Functional Principal Component Analysis ◽  
Intermediate Water ◽  
Thermohaline Structure ◽  
The North ◽  
Functional Principal Component ◽  
Tracer Distribution ◽  
Circumpolar Current

AbstractThe thermohaline structure of the Southern Ocean is deeply influenced by the presence of the Antarctic Circumpolar Current (ACC), where water masses of the World Ocean are advected, transformed, and redistributed to the other basins. It remains a challenge to describe and visualize the complex 3D pattern of this circulation and its associated tracer distribution. Here, a simple framework is presented to analyze the Southern Ocean thermohaline structure. A functional principal component analysis (PCA) is applied to temperature θ and salinity S profiles to determine the main spatial patterns of their variations. Using the Southern Ocean State Estimate (SOSE), this study determines the vertical modes describing the Southern Ocean thermohaline structure between 5 and 2000 m. The first two modes explain 92% of the combined θ–S variance, thus providing a surprisingly good approximation of the thermohaline properties in the Southern Ocean. The first mode (72% of total variance) accurately describes the north–south property gradients. The second mode (20%) mostly describes salinity at 500 m in the region of Antarctic Intermediate Water formation. These two modes present circumpolar patterns that can be closely related with standard frontal definitions. By projecting any given hydrographic profile onto the SOSE-based modes, it is possible to determine its position relative to the fronts. The projection is successfully applied on the hydrographic profiles of the WOCE SR3 section. The Southern Ocean thermohaline decomposition provides an objective way to define water mass boundaries and their spatial variability and has useful application for comparing model output with observations.

scholarly journals Campbell Plateau: A major control on the SW Pacific sector of the Southern Ocean circulation

2017 ◽  
Author(s):  
Aitana Forcén-Vázquez ◽  
Michael J. M. Williams ◽  
Melissa Bowen ◽  
Lionel Carter ◽  
Helen Bostock
Keyword(s):  
Southern Ocean ◽  
Ocean Circulation ◽  
Intermediate Water ◽  
The South ◽  
Mode Water ◽  
The North ◽  
Sw Pacific ◽  
Consistent Picture ◽  
Southward Extension ◽  
Subantarctic Region

Abstract. New Zealand’s subantarctic region is a dynamic oceanographic zone with the Subtropical Front (STF) to the north and the Subantarctic Front (SAF) to the south. Both the fronts and their associated currents are strongly influenced by topography: the South Island of New Zealand and the Chatham Rise for the STF, and Macquarie Ridge and Campbell Plateau for the SAF. Here for the first time we present a consistent picture across the subantarctic region of the relationships between front positions, bathymetry and water mass structure using eight high resolution oceanographic sections that span the region. Our results show that the northwest side of Campbell Plateau is comparatively warm due to a southward extension of the STF over the plateau. The SAF is steered south and east by Macquarie Ridge and Campbell Plateau, with waters originating in the SAF also found north of the plateau in the Bounty Trough. Subantarctic Mode Water (SAMW) formation is confirmed to exist south of the plateau on the northern side of the SAF in winter, while on Campbell Plateau a deep reservoir persists into the following autumn. Antarctic Intermediate Water (AAIW) is observed in the deeper regions around the edges of the plateau, but not on the plateau, confirming that the waters on the plateau are effectively isolated from AAIW and deeper water masses that typify the open Southern Ocean waters.

scholarly journals Imprint of Southern Ocean mesoscale eddies on chlorophyll

2018 ◽  
Vol 15 (15) ◽  
pp. 4781-4798 ◽  
Author(s):  
Ivy Frenger ◽  
Matthias Münnich ◽  
Nicolas Gruber
Keyword(s):  
Southern Ocean ◽  
Light Exposure ◽  
Mixed Layer Depth ◽  
Mesoscale Eddies ◽  
Mode Water ◽  
Upper Echelon ◽  
Nutrient Contents ◽  
Mesoscale Ocean Eddies ◽  
Circumpolar Current ◽  
Lateral Advection

Abstract. Although mesoscale ocean eddies are ubiquitous in the Southern Ocean, their average regional and seasonal association with phytoplankton has not been quantified systematically yet. To this end, we identify over 100 000 mesoscale eddies with diameters of 50 km and more in the Southern Ocean and determine the associated phytoplankton biomass anomalies using satellite-based chlorophyll-a (Chl) as a proxy. The mean Chl anomalies, δChl, associated with these eddies, comprising the upper echelon of the oceanic mesoscale, exceed ±10 % over wide regions. The structure of these anomalies is largely zonal, with cyclonic, thermocline lifted, eddies having positive anomalies in the subtropical waters north of the Antarctic Circumpolar Current (ACC) and negative anomalies along its main flow path. The pattern is similar, but reversed for anticyclonic, thermocline deepened eddies. The seasonality of δChl is weak in subtropical waters, but pronounced along the ACC, featuring a seasonal sign switch. The spatial structure and seasonality of the mesoscale δChl can be explained largely by lateral advection, especially local eddy-stirring. A prominent exception is the ACC region in winter, where δChl is consistent with a modulation of phytoplankton light exposure caused by an eddy-induced modification of the mixed layer depth. The clear impact of mesoscale eddies on phytoplankton may implicate a downstream effect on Southern Ocean biogeochemical properties, such as mode water nutrient contents.

scholarly journals Eddy-Resolving Model Estimate of the Cabbeling Effect on the Water Mass Transformation in the Southern Ocean

2012 ◽  
Vol 42 (8) ◽  
pp. 1288-1302 ◽  
Author(s):  
L. Shogo Urakawa ◽  
Hiroyasu Hasumi
Keyword(s):  
Southern Ocean ◽  
Deep Water ◽  
Water Mass ◽  
Formation Rate ◽  
Ocean Model ◽  
Intermediate Water ◽  
Mode Water ◽  
Model Estimate ◽  
Circumpolar Deep Water ◽  
Water Mass Transformation

Abstract Cabbeling effect on the water mass transformation in the Southern Ocean is investigated with the use of an eddy-resolving Southern Ocean model. A significant amount of water is densified by cabbeling: water mass transformation rates are about 4 Sv (1 Sv ≡ 106 m3 s−1) for transformation from surface/thermocline water to Subantarctic Mode Water (SAMW), about 7 Sv for transformation from SAMW to Antarctic Intermediate Water (AAIW), and about 5 Sv for transformation from AAIW to Upper Circumpolar Deep Water. These diapycnal volume transports occur around the Antarctic Circumpolar Current (ACC), where mesoscale eddies are active. The water mass transformation by cabbeling in this study is also characterized by a large amount of densification of Lower Circumpolar Deep Water (LCDW) into Antarctic Bottom Water (AABW) (about 9 Sv). Large diapycnal velocity is found not only along the ACC but also along the coast of Antarctica at the boundary between LCDW and AABW. It is found that about 3 Sv of LCDW is densified into AABW by cabbeling on the continental slopes of Antarctica in this study. This densification is not small compared with observational and numerical estimates on the AABW formation rate, which ranges from 10 to 20 Sv.

Nonlinear Climate Responses to Changes in Antarctic Intermediate Water

2013 ◽  
Vol 26 (22) ◽  
pp. 9175-9193 ◽  
Author(s):  
Jennifer A. Graham ◽  
David P. Stevens ◽  
Karen J. Heywood
Keyword(s):  
Climate Model ◽  
Potential Temperature ◽  
Heat Fluxes ◽  
Meridional Overturning Circulation ◽  
Intermediate Water ◽  
Antarctic Intermediate Water ◽  
Surface Ocean ◽  
The North ◽  
Meridional Overturning ◽  
Circumpolar Current

Abstract The global impact of changes in Antarctic Intermediate Water (AAIW) properties is demonstrated using idealized perturbation experiments in a coupled climate model. Properties of AAIW were altered between 10° and 20°S in the Atlantic, Pacific, and Indian Oceans separately. Potential temperature was changed by ±1°C, along with density-compensating changes in salinity. For each of the experiments, sea surface temperature responds to changes in AAIW when anomalies surface at higher latitudes (>30°). Anomalous sea-to-air heat fluxes leave density anomalies in the ocean, resulting in nonlinear responses to opposite-sign perturbations. In the Southern Ocean, these affect the meridional density gradient, leading to changes in Antarctic Circumpolar Current transport. The response to cooler, fresher AAIW is both greater in magnitude and significant over a larger area than that for warmer, saltier AAIW. The North Atlantic is particularly sensitive to cool, fresh perturbations, with density anomalies causing reductions in the meridional overturning circulation of up to 1 Sv (1 Sv ≡ 106 m3 s−1). Resultant changes in meridional ocean heat transport, along with surfacing anomalies, cause basinwide changes in the surface ocean and overlying atmosphere on multidecadal time scales.

scholarly journals Carbon uptake and biogeochemical change in the Southern Ocean, south of Tasmania

2017 ◽  
Author(s):  
Paula C. Pardo ◽  
Bronte Tilbrook ◽  
Clothilde Langlais ◽  
Tom W. Trull ◽  
Steve R. Rintoul
Keyword(s):  
Southern Ocean ◽  
Water Mass ◽  
Dissolved Inorganic Carbon ◽  
Water Masses ◽  
Atmospheric Co2 ◽  
Positive Trend ◽  
Intermediate Water ◽  
Mode Water ◽  
Aragonite Saturation ◽  
Deep Waters

Abstract. Biogeochemical change in the water masses of the Southern Ocean, south of Tasmania, was assessed for the 16-year period between 1995 and 2011 using data from 4 summer repeats of the WOCE/JGOFS/CLIVAR/GO-SHIP SR03 hydrographic section (at ~ 140° E). Changes in temperature, salinity, oxygen, and nutrients were used to disentangle the effect of solubility, biology, circulation and anthropogenic carbon (CANT) uptake on the variability of dissolved inorganic carbon (DIC) for 8 water mass layers defined by neutral surfaces (ϒn). CANT was estimated using an improved back-calculation method. Warming (~ 0.0352 ± 0.0170 °C yr−1) of Subtropical Central Water (STCW) and Antarctic Surface Water (AASW) layers decreased their gas solubility, and accordingly DIC concentrations increased less rapidly than expected from equilibration with rising atmospheric CO2 (~ 0.86 ± 0.16 μmol kg−1 yr−1 versus ~ 1 ± 0.12 μmol kg−1 yr−1). An increase in apparent oxygen utilisation (AOU) occurred in these layers due to either remineralization of organic matter or intensification of upwelling. The range of estimates for the increases of CANT were 0.71 ± 0.08 to 0.93 ± 0.08 μmol kg−1 yr−1 for STCW and 0.35 ± 0.14 to 0.65 ± 0.21 μmol kg−1 yr−1 for AASW, with the lower values in each water mass obtained by assigning all the AOU change to remineralization. DIC increases in the Sub-Antarctic Mode Water (SAMW, 1.10 ± 0.14 μmol kg−1 yr−1) and Antarctic Intermediate Water (AAIW, 0.40 ± 0.15 μmol kg−1 yr−1) layers were similar to the calculated CANT trends. For SAMW, the CANT increase tracked rising atmospheric CO2. As a consequence of the general DIC increase, decreases in total pH (pHT) and aragonite saturation (ΩAr) were found in most water masses, with the upper ocean and the SAMW layer presenting the largest trends for pHT decrease (~ −0.0031 ± 0.0004 yr−1). DIC increases in deep and bottom layers (~ 0.24 ± 0.04 μmol kg−1 yr−1) resulted from the advection of old deep waters to resupply increased upwelling, as corroborated by increasing silicate (~ 0.21 ± 0.07 μmol kg−1 yr−1), which also reached the upper layers near the Antarctic Divergence (~ 0.36 ± 0.06 μmol kg−1 yr−1) and was accompanied by an increase in salinity. The observed changes in DIC over the 16-year span caused a shoaling (~ 340 m) of the aragonite saturation depth (ASD, ΩAr = 1) within Upper Circumpolar Deep Water that followed the upwelling path of this layer. From all our results, we conclude a scenario of increased transport of deep waters into the section and enhanced upwelling at high latitudes for the period between 1995 and 2011, probably linked to a positive trend in the Southern Annular Mode. Although enhanced upwelling lowered the capacity of the AASW layer to uptake atmospheric CO2, it did not limit that of the newly forming SAMW and AAIW, which exhibited CANT storage rates (~ 0.41 ± 0.20 mol m−2 yr−1) twice that of the upper layers.

scholarly journals Circulation and Water Mass Modification in the Brazil–Malvinas Confluence

2010 ◽  
Vol 40 (5) ◽  
pp. 845-864 ◽  
Author(s):  
Loic Jullion ◽  
Karen J. Heywood ◽  
Alberto C. Naveira Garabato ◽  
David P. Stevens
Keyword(s):  
Deep Water ◽  
Subtropical Gyre ◽  
Strong Interactions ◽  
Intermediate Water ◽  
Mode Water ◽  
Brazil Current ◽  
Subantarctic Mode Water ◽  
Circumpolar Current ◽  
The Antarctic ◽  
Argentine Basin

Abstract The confluence between the Brazil Current and the Malvinas Current [the Brazil–Malvinas Confluence (BMC)] in the Argentine Basin is characterized by a complicated thermohaline structure favoring the exchanges of mass, heat, and salt between the subtropical gyre and the Antarctic Circumpolar Current (ACC). Analysis of thermohaline properties of hydrographic sections in the BMC reveals strong interactions between the ACC and subtropical fronts. In the Subantarctic Front, Subantarctic Mode Water (SAMW), Antarctic Intermediate Water (AAIW), and Circumpolar Deep Water (CDW) warm (become saltier) by 0.4° (0.08), 0.3° (0.02), and 0.6°C (0.1), respectively. In the subtropical gyre, AAIW and North Atlantic Deep Water have cooled (freshened) by 0.4° (0.07) and 0.7°C (0.11), respectively. To quantify those ACC–subtropical gyre interactions, a box inverse model surrounding the confluence is built. The model diagnoses a subduction of 16 ± 4 Sv (1 Sv ≡ 106 m3 s−1) of newly formed SAMW and AAIW under the subtropical gyre corresponding to about half of the total subduction rate of the South Atlantic found in previous studies. Cross-frontal heat (0.06 PW) and salt (2.4 × 1012 kg s−1) gains by the ACC in the BMC contribute to the meridional poleward heat and salt fluxes across the ACC. These estimates correspond to perhaps half of the total cross-ACC poleward heat flux. The authors’ results highlight the BMC as a key region in the subtropical–ACC exchanges.

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