OS13C-1199
Structure of the Antarctic Circumpolar Current in Drake Passage From Direct Velocity Observations
The structure of the Antarctic Circumpolar Current (ACC) in the upper 1000~m of Drake Passage is examined using nearly three years of shipboard Acoustic Doppler Current Profiler (SADCP) velocity data from a 38~kHz Ocean Surveyor mounted in the hull of the Antarctic supply vessel ARSV Laurence M. Gould. The principal fronts of the ACC are clearly visible, with the Subantarctic Front (SAF) and Polar Front (PF) jets having widths of about 100~km and 150~km, respectively. Depth-mean current speeds in the SAF and PF jets are ~40~cm~s-1, while the eddy kinetic energy (EKE) has a maximum of ~700~cm2s-2 between the PF and the SAF. Horizontal-wavenumber velocity spectra peak at ~350~km. These numbers are similar to surface-layer values found by Lenn et al. (J. Mar. Res., 2007). The transport estimated from the mean section between the surface and 1030~m is ~100~Sv, or about 70% of the canonical total transport. The extended depth range available from the 38~kHz instrument allows us to investigate the depth structure of the current. The mean current is largely barotropic, while EKE and shear variance exhibit strong depth dependence. In cross-sectional averages current shear is small and nearly constant to 600~m, below which depth the current speed drops off more quickly; mean jet speeds are around 50~cm~s-1 at 46~m (the first depth bin) and 20~cm~s-1 at 1030~m. Various possibilities for a vertical structure function are explored. EKE is intensified above 600~m between the SAF and PF. Shear variance is strongest in the surface layer. Vertical-wavenumber spectra of currents and current shear reveal negligible rotation. Through-passage currents have more energy at the lowest vertical wavenumbers (wavelengths of ~1000~m), while at scales smaller than ~100~m, energy in across-passage currents is greater.
OS13C-1200
Small-Scale Turbulent Mixing in Southern Drake Passage
In southern Drake Passage, the Shackleton Fracture Zone (SFZ) marks a transition, with consistently low chlorophyll-a (Chl-a) values to the west and high values in the Ona Basin to the east. Iron incubation experiments in the region suggest that the elevated Chl-a levels are the result of iron that is entrained into shelf waters along the Antarctic Peninsula and then advected offshore. However, the year-round persistence of the Chl-a gradient cannot be explained by a simple horizontal advection model, implying that additional processes, such as small-scale vertical mixing, may contribute to the process. We examine the spatial pattern of vertical mixing in the SFZ using shipboard observations of temperature and salinity profiles. Profiles were sampled with expendable CTD probes in winter 2006, and with rosette-mounted CTDs in winter 2006 and summer 2004. Diapycnal eddy diffusivities are computed from Thorpe-scale density overturns in the profiles, and vertical velocities are computed from the diffusivities to estimate the rate at which iron can be entrained into the mixed layer by small-scale mixing.
OS13C-1201
Mixed-Layer Depth Variability in Drake Passage
Long-term monitoring by repeat XBT and XCTD transects across Drake Passage is used to examine mixed- layer variability in the region. Two geographical regimes are noted, with the Polar Front serving as an approximate division point. South of the Polar Front, mixed-layer depth varies nearly uniformly in space. To the north, the mixed layer reaches its greatest depths and exhibits small-scale (O(50 km)) features. A strong seasonal cycle is evident in both regions, with minima in mean and variance of mixed-layer depth coincident with the start of austral summer. Preliminary results show substantial interannual variability. Mechanisms governing this variability will be discussed.
OS13C-1202
Isopycnal Diffusivities in the Antarctic Circumpolar Current of 1/10 degree POP Inferred From Subsurface Numerical Floats
In the ocean interior, mesoscale eddy motions are thought to be the main mechanism for meridional along- isopycnal transport of mass and tracers across the Antarctic Circumpolar Current. In coarse-resolution ocean models, the large-scale overturning circulation in the Southern Ocean is sensitive to the eddy diffusivity coefficients used to parameterize these eddy transports. However, the spatial distributions and even the magnitudes of the ocean diffusivities remain uncertain, and virtually no reliable observations exist for the subsurface. Here, Lagrangian interior isopycnal eddy diffusivities are measured by subsurface numerical floats, released in several regions of the Antarctic Circumpolar Current of the 0.1° POP model. These floats follow the model flow. Across-stream and along-stream diffusivities, obtained by projecting the float velocities locally along and across geostrophic streamlines, show elevated values in the core of the ACC and near topographic features. On average, diffusivities are 1000 ± 500 m2 s-1 across streamlines and 2-5 times larger along streamlines, with little depth dependence. Diffusivity distributions are discussed in the context of theories predicting specific depth and latitude dependence and are compared to recent diffusivity estimates from several indirect methods and models. The reasons for differences are explored.
OS13C-1203
Frontal Structure of the Southwest Indian Ocean and the Antarctic Circumpolar Current From CLIVAR I6S Repeat Hydrographic Data
The location and structure of the principal oceanic fronts across the Agulhas Current System (ACS) and Antarctic Circumpolar Current (ACC) are analyzed using hydrographic CTD and XBT data from the CLIVAR I6S Repeat Hydrography cruise near 30 E in February - March 2008. The Polar Front (PF), Sub-Antarctic Front (SAF), Subtropical Front (STF), and the Aghulas Front (AF) across two different sections are analyzed for their horizontal and vertical temperature, salinity, and potential vorticity structure and transport. The first section consists of the I6S line with relatively high resolution across the southeast African continental shelf and across the continental margin of Antarctica, along which CTD and Acoustic Doppler Current Profiler (ADCP, shipboard and lowered) data were taken. A second, higher-resolution XBT section was occupied on the return leg. Sea surface temperature and salinity relationships from underway sampling are also examined to support the findings of the data from greater depths. A discussion is given regarding the expected and observed findings of frontal widths, strengths, and locations from both sections, and a comparison among these findings is presented. The spatial variability and the short-term (days to months) temporal variability of frontal features are discussed, along with a speculation about the presence of semi-permanent features in this region. Bottom-referenced geostrophic velocities calculated from CTD data reveal a strong correlation with observed ADCP currents through the water column. The strongest currents outside of the ACS are associated with the PF, which exhibits meanders and a sub-surface bifurcation south of Africa. The influence of the AF on the width of the STF is clearly visible in both sections, especially in the energetic Aghulas Retroflection.
OS13C-1204
Quantitative Evaluations of Turbulent Mixing and Double-Diffusive Convection Around Adelie Depression
Adelie Land Bottom Water is thought to be mixture of Shelf Water from Adelie Depression and Circumpolar Deep Water (Gordon and Tchernia, 1972; Williams et al., 2008). To evaluate mixing strengths of turbulence and double-diffusive convection around the Adelie Depression, hydrographic observations using Sea-Bird Electronics SBE911plus with Lowered Acoustic Doppler Current Profiler (LADCP, RDI) were carried out off Adelie Land including Adelie Depression by TR/V Umitaka-Maru in austral summer of 2008. This observation was conducted as a part of CEAMARC project (Collaborative East Antarctic Marine Census, URL: http://www.caml.aq). Shelf Water (θ<-1.7°C, σ2>37.16) was only found in the depression. Temperature inversions due to intrusions of warmer offshore waters of Circumpolar Deep Water origin were found from 250 to 350 dbar at some stations in the depression. Velocity field around the depression obtained by LADCP indicated that the intrusions were related to inflow of southward current. Turbulent eddy diffusivity Kv were estimated from vertical overturning scale of density inversions. Relatively large values of Kv (10-4~10-3m2s-1) were found at continental shelf and slope regions, and in the Adelie Depression. On the other hand, Kv in the open ocean showed one or two order smaller than that in the boundary region and Adelie Depression. These results showed the possibility of boundary mixing in these regions. Next, activities of double-diffusive convection were examined from Turner angle Tu. Values of Tu in the upper part of the intrusions in the depression were up to about -80°, indicating active diffusive convection, and the values were about 60° in the lower part of the intrusions, indicating weak salt finger convection. While there were few active double-diffusive convection layers in the shelf and the depression, many active double-diffusive convection layers (75°<|Tu|<90°) distributed on the continental slope region. In this region, diffusivities of heat due to diffusive convection were O(10-5)m2s-1 above 500 dbar, whereas diffusivities of salt due to salt finger convection were up to about 10-4m2s-1 below the depth. Therefore double-diffusive convection may play an important role on the formation of bottom water which sinks along the continental slope.
OS13C-1205
A Numerical Study of Antarctic Convective Overturning
A high resolution non-hydrostatic ocean model is used to examine the impact of convection on high-latitude Southern Ocean winter stratification in an idealized domain of the size comparable to that of a single grid cell in current general circulation models (GCMs) typically used for IPCC-type of climate studies. Our numerical experiments consider a range of salt flux values, percentage of open water, strength of stratification, and duration of open water to determine under which of these conditions stratification is eroded to the point that deep warm water (Circumpolar Deep Water) is brought to the surface to affect sea ice. Details of the non- hydrostatic simulations areA Numerical Study of Antarctic Convective Overturning spatially averaged over the computational domain to produce profiles that are comparable to those of large-scale GCMs. It is shown that for typical salt fluxes, lead percentage, and stratification, non-hydrostatic plumes are unable to overturn stratification and produce surface melting on short time scales of leads. This suggests that the tendency of GCMs to over-predict overturning, and thus ice melting due to ocean heat flux, occurs mainly because the open-water fraction in those coarse-grid models is not partitioned into smaller ice-free patches. In our non- hydrostatic simulations, vertical overturning only emerges when a substantial salt flux is provided over a domain of several tens of square kilometers, and a time period of several days, i.e. conditions more pertinent to new-ice formation in coastal polynyas. Our results imply that the problem of excessive open-ocean convection in the Southern Ocean of large-scale ocean GCMs can be largely avoided by a subgrid-scale distribution of the simulated open-water fraction into small non-stationary patches that more closely resemble actual leads.
OS13C-1206
What Should a Subgridscale Parameterization Look Like?
The Southern Ocean is notable both for its important role within the global ocean circulation and for an abundance of mesoscale eddies. It can hence be expected that eddy parameterizations employed in large- scale climate models exert a significant influence on ocean dynamics in this region. This motivated us to investigate fundamental aspects of the subgridscale dynamics emerging from a two-layer wind-driven channel model, a simple approximation to the Antarctic Circumpolar Current (ACC). The problem of parameterizing small-scale mixing is often cast in terms of finding a mixing coefficient to drive viscous or diffusive mixing with a Fickian formulation. Yet in a bigger sense the role of subgridscale parameterization is really to characterize the total impact of the subgridscales on the resolved flow, effects which may not be characterized by downgradient transport of tracer or momentum. This is well established on global scales, where the necessity of including eddy-driven advective effects in coarse-resolution models of the ocean general circulation has been known for many years, and where eddies can pump energy into large-scale mean flows as well as extracting energy from such flows. We argue that in the quest for better parameterizations, we should not confine ourselves to one single mathematical model, and to that end present a flexible method that allows us to search among a much larger set of functional forms. The method involves running a coarse-resolution model in parallel with a fine- resolution model and computing the corrections to the coarse-resolution model required to make it evolve like the fine-resolution model. A temporal subset of the resulting forcing on momentum and layer thickness is then regressed against the coarse-resolution velocities and thicknesses to develop a single spatially averaged model for subgridscale forcing. The functional dependence which emerges in our ACC-like setting includes a bolus-velocity-like term dependent on the gradient of thickness, but also an new "eddy-enhanced stretching" term.
OS13C-1207
The Southern Ocean overturning: Parametrized Versus Permitted Eddies
Four versions of the same global climate model, with horizontal resolution ranging from 1.8 (degrees latitude) x 3.6 (degrees longitude) to 0.2x0.4, are employed to evaluate the resolution dependence of the Southern Ocean meridional overturning circulation. At coarse resolutions North Atlantic deepwater tends to upwell diabatically at low-latitudes, so that the Southern Ocean is weakly coupled with the rest of the ocean. As resolution increases and eddy effects become less parametrized the interior circulation becomes nearly adiabatic and deep water increasingly upwells by flowing along isopycnals in the Southern Ocean, despite each model having the same vertical diffusivity profile. Separating the overturning circulation into mean and eddy-induced components demonstrates that the circulation induced by the permitted eddies also tends to be nearly adiabatic in the interior. In contrast, the circulation induced by the eddy parametrization scheme can have a strong diapycnal component and consequently may not represent the circulation pathway induced by permitted eddies. Changes in the Southern Ocean overturning in response to poleward intensifying southern hemisphere winds concomitant with increasing atmospheric CO2 through the 21st century are also investigated. Results suggest that the circulation associated with the formation of Antarctic Intermediate Water is likely to strengthen, or stay essentially unchanged, rather than to slow down. It is also found that for some density classes the transformation rate derived from surface buoyancy fluxes can provide a proxy for the meridional transport in the upper Southern Ocean.
OS13C-1208
Tidal Impacts on Oceanographic and Sea-ice Processes in the Southern Ocean
We review recent field and modeling results that demonstrate the importance of tides in establishing the oceanographic and sea-ice conditions in the boundary regions of the Southern Ocean. The tidal component dominates the total oceanic kinetic energy throughout much of the circum-Antarctic seas. This domination is especially pronounced over the continental slope and shelf including the sub-ice-shelf cavities. Tides provide most of the energy that forces diapycnal mixing under ice shelves and thereby contributes to basal melting. The resulting Ice Shelf Water is a significant component of the Antarctic Bottom Water (AABW) filling much of the deep global ocean. Tides exert significant divergent forcing on sea ice along glacial ice fronts and coastal regions, contributing to creation and maintenance of the coastal polynyas where much of the High Salinity Shelf Water component of AABW is formed. Additional tidally forced ice divergence along the shelf break and upper slope significantly impacts area-averaged ice growth and upper-ocean salinity. Tidally forced cross- slope advection, and mixing by the benthic stress associated with tidal currents along the shelf break and upper slope, strongly influence the paths, volume fluxes and hydrographic properties of benthic outflows of dense water leaving the continental shelf. These outflows provide primary source waters for the AABW. These results confirm that general ocean circulation and coupled ocean/ice/atmosphere climate models must incorporate the impacts of tides.
OS13C-1209
The Representation of Thin Ice in the Southern Ocean of a Global Ocean GCM
The high-latitude Southern Ocean is a critical region for global climate models. In this region, freezing and melting have a direct impact on the formation of the densest major water mass of the world's ocean, the Antarctic Bottom Water (AABW), the abundance of which in turn affects the global meridional overturning circulation. Brine release due to freezing is mostly observed in conjunction with new-ice formation. The fraction of new ice (or nilas, thickness smaller than 10 cm) is thus an important variable to track in models used for long-term climate studies. This paper compares Southern Ocean thin ice derived from simulations using a global sea-ice - ocean general circulation model with such derived from satellite passive-microwave data. Thin ice not constituting a prognostic model variable, it is being diagnosed from the rate of new-ice formation. The resolution of the sea-ice component allows for a comparison with the satellite data on the same time and space scales (about 20 km). In fall and winter, the model- and satellite-derived products consistently reveal considerable thin-ice concentration along the ice edge. Along the coast, thin ice is generally more pronounced in the satellite data than in the simulations. Modifying the simulations in the way warm deep water affects the upper-layer temperature suggests that in order to capture nilas in conjunction with coastal polynyas, oceanic heat along the Antarctic coastline is critical in preventing new ice from readily becoming thick ice (i.e. larger than 10 cm). During spring and summer, thin ice reflects mostly melting ice, and can therefore not be diagnosed from the simulations in the manner described above. Also, the microwave retrievals of thin ice are less reliable during these seasons. The results imply that the simulated rate of new-ice formation can be used as a measure for the amount of thin ice in fall and winter, which in turn can enhance our understanding of the processes that determine the rate of AABW formation.
OS13C-1210
Numerical modeling of coastal polynyas in East Antarctica
Coastal polynyas are the area of open water or thin ice surrounded by coastline and thick ice in winter, ranging in the horizontal scale from a few to 100 km. In coastal polynyas, large heat loss occurs due to the direct contact of open water/thin ice with the colder air. As a result of the large heat loss, coastal polynyas exhibit high sea-ice production rates. The high sea-ice production leads to formation of dense shelf water. Around Antarctica, such dense shelf water formed in the coastal polynyas plays an important role in the formation and spreading of Antarctic Bottom Water. Using a sea ice - ocean coupled model with fine horizontal resolution around East Antarctica ( ~ 15 km), sea-ice production and dense shelf water formation in the coastal polynyas are investigated. The model well reproduces the locations of coastal polynyas and high sea-ice production there. In East Antarctica, the Cape Darnley polynya (CDP) is the highest sea-ice production area and the Mertz Glacier polynya (MGP) is the second highest one in the model. Water denser than 27.88 kg m-3 over the shelf is formed in coastal polynyas. Besides the CDP and the MGP, polynyas near Barrier Bay, Shackleton Ice Shelf, Vincenness Bay, Dibble Iceberg Tongue, and Ninnis Glacier also contribute to dense shelf water formation. Sea-ice production largely depends on the value of the minimum sea-ice thickness in a grid. To model the formation of frazil ice and high sea-ice production in coastal polynyas, a large value of the minimum sea-ice thickness ( 50 cm in this study) is required. Blocking effect of sea-ice transport by grounded icebergs is also tested by the model. From a series of numerical experiments, we found that the blocking effect has a large impact on both sea-ice production and dense shelf water formation in coastal polynyas.
OS13C-1211
Recent Precipitation Trends Over the Southern Ocean in Relation to Oceanic Freshening Near Antarctica
Quantitative assessments of large-scale precipitation over the world's oceanic regions are problematic, particularly for significant regions of the data-sparse Southern Hemisphere. Available data sets are based on the assimilation of land-based measurements, satellite radiance values, numerical weather forecast models, or some combination of the three. In this study we examine several products that cover most or all of the satellite era 1979-2007 over the Southern Ocean and surrounding mid-latitudes to 45°S. These include CMAP, the NCEP Reanalysis II, ERA-40, GPCP version 2, and the Japanese Re-analysis. Averaged fields from these data show large discrepancies in the mean spatial depiction and the annual cycle. Comparisons with unique in situ snowfall measurements and satellite-derived accumulation on sea ice are presented. The available record of oceanographic measurements in the Ross Sea indicates that salinity below 200 m in the Ross Sea has decreased by 0.03 per decade since 1958, with the highest (lowest) values in 1967 (2000). The fields examined here suggest that precipitation is likely not directly influencing the oceanic freshening observed in the Ross Sea, or in other coastal seas adjacent to Antarctica. The salinity anomaly is consistent with increasing attrition of continental ice, but places a heavy demand on the melt rate. Potential contributions to oceanic freshening from changes in sea ice extent, transport, and thickness are discussed.
OS13C-1212
A Synthesis Of In Situ Measured Export Production in the Southern Ocean
The southern ocean is a key region contributing to the regulation of both atmospheric CO2 concentration and deep ocean O2 levels in part through the "biological pump. Although numerous studies on the production and fate of biomass in this region have been carried out in the last two decades, a proper synthesis of the data has yet to be done. We have compiled and carried out a comprehensive analysis of the published POC flux data that were collected using three different techniques – moored sediment traps, surface tethered traps and 234Th based measurements. All the available POC fluxes have been normalized to 100m using the Martin curve, which predicts carbon attenuation as a function of depth. The dataset can be segregated into three geographical/spatial regions (Atlantic Sector, Pacific Sector and Indian Sector) and two broad temporal regimes -spring (October-December) and summer (January-March). The Pacific and Indian sectors show a large temporal variability in the POC export between spring (95 mg C m-2 d-1 and 157 mg C m-2 d-1 respectively) and summer (62 mg C m-2 d-1 and 43 mg C m-2 d-1 respectively) but no such temporal variability in POC export between spring (118 mg C m-2 d-1) and summer (113 mg C m-2 d-1) was observed for the Atlantic sector. Based on the available in situ primary production data, the export efficiency (POC flux: Primary Production) was computed for each of these regions. The average spring time export efficiency for Atlantic, Pacific and Indian sectors were 0.08, 0.37 and 0.13 respectively while during summer the average export ratios were estimated to be 0.27, 0.11 and 0.06 respectively. The overall low export ratio for the Indian sector is due to the fact that all the available data falls to the north of the Polar Front compared with the Atlantic and Pacific sector data, which fall to the south of the Polar Front. Similar spring and summer time POC fluxes in the Atlantic sector can be explained to some extent by the higher biological production and lower export efficiency in spring compared to relatively lower biological production and higher export efficiency in summer. The ultimate goal of this data synthesis is to refine the satellite-derived export flux algorithms with special emphasis on Southern Ocean, which in turn will be used to balance the mixed layer O2 budget and ocean climatologies of dissolved O2. Preliminary model run matchups between monthly satellite based new production and the monthly in situ measured POC flux data are not high but significant. Further analysis needs to be done to effectively assimilate the observational dataset into the model in order to derive a better estimation of export flux that can reproduce the large seasonal and spatial variability associated with this region.
OS13C-1213
Seismic Imaging Of The South Atlantic Ocean
Seismic oceanography is a powerful, new technique which utilises seismic reflection profiling to yield detailed
acoustic images of ocean finestructure. The method uses seismic reflections generated by acoustic
impedance changes which occur across water mass boundaries as a result of variations in temperature and
salinity.
Here we present a series of seismic images of the water column from a region in the southwest Atlantic using
legacy seismic reflection datasets shot in the 1990s. The seismic surveys follow the Antarctic Circumpolar
Current as it veers northward after exiting Drake Passage, looping around the Falkland Trough and finally
entering the Argentinian Basin, largely following the Sub Antarctic Front route.
Finestructure imaged in the data occurs dominantly in the boundary layer between Antarctic Intermediate
Water and underlying Upper Circumpolar Deep Water, and correlates well with hydrographic interpretations.
Structures such as dipping bands of heterogeneous water and eddies characterized by homogeneous cores
and strong reflective inter-leaving edges are seen in the vicinity of ocean fronts. Further to the north,
interesting thermohaline structures associated with the intrusion of North Atlantic Deep Water into the region
have been captured. Seasonal variations in the time of data acquisition and the subsequent differences in
the acoustic images provide interesting insights into the temporal variability of the water masses.
Turbulent mixing in this part of the Southern Ocean is known to be remarkably intense and widespread and
thought to contribute significantly to driving the upward transport of water closing the ocean's meridional
overturning circulation. The deformation of thermohaline finestructure by such mixing and the ambient
internal wavefield results in small undulations along seismic reflection horizons. Spectral analysis of these
sinusoidal displacements has been used to extract quantitative information on internal wave energy and
deduce tentative estimates of turbulent dissipation rates.
OS13C-1214
Formation and Transport of Subantarctic Mode Water and Antarctic Intermediate Water in the Southeast Pacific
Subantarctic Mode Water and Antarctic Intermediate Water are mid-depth (600-1500 m) water masses, which transport heat, freshwater and CO2 into the subtropical gyre and equatorial thermocline. A primary formation site in austral winter is the southeast Pacific, north of the Subantarctic and Polar Fronts. Formation rates of SAMW and AAIW are calculated using CFC-12 inventories. A significant portion of these waters circulate within the South Pacific subtropical gyre. In addition, the Antarctic Circumpolar Current transports some of this newly formed SAMW and AAIW from the southeast Pacific through the Drake Passage into the Atlantic Ocean. There it is modified and contributes to Atlantic SAMW and AAIW. Using inverse methods, transport estimates of SAMW and AAIW are calculated across the South Pacific in 5° bins north of the Polar Front. How much newly formed SAMW and AAIW are leaking from the southeast Pacific into the Atlantic has implications for quantifying present day inter-basin circulation and exchange of properties.
OS13C-1215
Sources and Fates of Southern Ocean Surface Water: A trajectory analysis
Eventually, about 85% of the anthropogenic carbon dioxide emitted to the atmosphere will dissolve in the ocean (e.g., Sarmiento et al., 1992), but the rate at which this happens is limited by the rate at which deep waters are exposed to the surface. The Southern Ocean is of special importance in this respect, accounting for about 40% of the total oceanic uptake (e.g., Watson et al., 2003). We will present the results of a time- dependent trajectory algorithm used to trace the sources of the Antarctic Bottom Water, Antarctic Intermediate Water and Subantarctic Mode Water produced in the modeled Southern Ocean. The trajectory algorithm also allows us to diagnose the sources of the deep water that upwells in the Southern Ocean in the different IPCC AR4 coupled climate models simulations of the 20th century. The biogeochemically significant source waters of the Southern Ocean upwelling and water mass formation regions are critically sensitive to the simulated (and evolving) climate, and these sources and sensitivities are revealed by this technique. We will quantify the rates of Southern Ocean ventilation associated with different circulations and compare them with observations, and discuss why these rates differ so greatly between models.
OS13C-1216
On the linkage between Southern Ocean winds and the global ocean circulation
The Southern Ocean (SO) winds affect the global ocean circulation via two processes: 1) the northward Ekman transport in association with the Southern Hemisphere (SH) westerlies, and 2) the buoyancy-fluxes in association with the SH atmosphere – sea-ice – ocean system. In this study, the effects of these processes are investigated in the framework of a global sea-ice – ocean general circulation model. Perturbations are applied in the form of poleward shifted SH westerlies (PSW) and an interactive momentum flux (IMF) forcing over the SO sea ice in three fundamentally different reference experiments. The PSW appears to consistently decrease the northward Ekman transport, but enhances the Antarctic Intermediate Water characteristics because colder and fresher water from further south is getting involved. This leads to a shoaling (descending) of the pycnocline (sea surface) at low latitudes. As a result, the north-south pressure gradient of the North Atlantic (NA) decreases, and so do the frictional currents along the deep western boundary and the formation rate of North Atlantic Deep Water (NADW). It directly weakens the NA overturning, and indirectly intensifies the SH overturning. On the other hand, the IMF enhances wind-driven divergence of sea ice, consistently increasing the fraction of leads and polynyas, which intensifies convection, and thus increases dense water formation in the SO. This, in turn, increases the formation rate of Antarctic Bottom Water (AABW), which directly intensifies the SH overturning and indirectly weakens the NA overturning. The anti-correlation between the two major overturning circulations in the PSW (IMF) experiments is attributed to 1) the decreased (increased) material volume of NADW (AABW) due to the decreased (increased) supply and 2) the vortex squeezing (stretching) of the material volume of NADW (AABW) due to the decelerated (accelerated) density-driven deep (bottom) flow along the deep western boundary. These inter-hemispheric changes in meridional overturning circulations modify the baroclinic part of the Antarctic Circumpolar Current.
OS13C-1217
Where goes the wind power input to the Southern Ocean?
The mechanical energy budget is a useful diagnostic for understanding the processes involved in the ocean general circulation, and especially the energy available to drive diapycnal mixing. A decade ago Carl Wunsch observed that most of the wind power input to the World Ocean occurred in the Southern Ocean. The results stood the test fo time and was confirmed with more reliable wind stress and ocean current data. If the ocean circulation is to reach equilibrium, the wind work at large scales must be dissipated through dissipation at molecular scales. We will show that in most of the ocean there appears to be a local balance between surface wind work and dissipation by quadratic drag in the bottom boundary layers. However the Southern Ocean is special in that the dissipation by quadratic bottom drag is not sufficient to dissipate the local wind power input. So not only is the Southern Ocean the largest source of mechanical energy, it also appears to be unique in the the way energy is eventually dissipated. Where does the wind power input to the Southern Ocean go? We explore this issue using large numbers of moored current meters and eddy resolving ocean circulation models.
OS13C-1218
Elements of the connection between the Southern Ocean and the global ocean in a scaling theory and numerical experiments
The Southern Ocean catalyzes the truly global character of the Meridional Overturning Circulation (MOC)
and its importance in the climate system. The water mass formation/conversion processes and the
overturning circulation in the Southern Ocean determines properties of the bulk of the global ocean domain
while the Antarctic Circumpolar Current (ACC) enables vigorous inter-basin exchange.
This study explores elements of inter-hemispheric and inter-basin ocean dynamics of the combined MOC-
ACC system. We formulate an extended scaling theory for the water mass conversion processes and link it
with a set of coarse-resolution ocean general circulation experiments based on Modular Ocean Model
version 4.1 (developed at NOAA/GFDL). Constructed theory and numerical experiments encompass a
hierarchy of ocean configurations (from single-basin to multiple-basin geometry). Our results provide further
understanding and a simple predictive framework of how changing surface forcing fields and/or
representation of ocean internal processes will affect the stratification, the MOC strength and the ACC
transport.
As an example of inter-hemispheric interaction, we confirm that an increase (decrease) of the amplitude of
the Southern Ocean westerlies will increase (decrease) the global ocean heat content. Also, we show that
surface warming (cooling) of the northern hemisphere high latitudes will intensify (weaken) the ACC transport
and the Southern Ocean overturning circulation. As an example of inter-basin interaction, facilitated by the
Southern Ocean, we show that changing the intensity of vertical eddy mixing or deep water production in one
basin can affect the stratification in the other(s).
These results constitute a new contribution to understanding mechanisms that control the global pycnocline,
the MOC and the ACC, and specifically the Southern Ocean role in climate. It is our hope that these findings
stimulate further investigations with more sophisticated models.
http://www.princeton.edu/~gkv/papers/Fuckar-VallisGRL07.pdf
OS13C-1219
Intercomparison of IPCC Coupled Climate Models' Antarctic Sea Ice Predictions
The suite of climate models included in the Intergovernmental Panel on Climate Change's Fourth Assessment Report shows a wide range of predictive variance for modeled diagnostics, including sea ice concentrations. Using the SRESA1B scenario, the model aggregate predicts a 35% reduction in sea ice around Antarctica by the time the 2°C warming benchmark is reached in the middle of this century (~2042, according to the ensemble). A subset of models is selected for their ability to accurately represent modern sea ice concentrations in the Southern Ocean. Comparing the SRES A1B and A2 scenarios through the next century as simulated by the ensemble, the future sea ice extent, seasonality and thickness in Antarctica's Ross and Weddell seas is assessed.