Southern Ocean State Estimate Research Articles

Freshwater Displacement Effect on the Weddell Gyre Carbon Budget

AbstractThe Weddell Gyre mediates carbon exchange between the abyssal ocean and atmosphere, which is critical to global climate. This region also features large and highly variable freshwater fluxes due to seasonal sea ice, net precipitation, and glacial melt; however, the impact of these freshwater fluxes on the regional carbon cycle has not been fully appreciated. Using a novel budget analysis of dissolved inorganic carbon (DIC) mass in the Biogeochemical Southern Ocean State Estimate, we highlight two freshwater‐driven transports. Where freshwater with minimal DIC enters the ocean, it displaces DIC‐rich seawater outwards, driving a lateral transport of 75 ± 5 Tg DIC/year. Additionally, sea ice export requires a compensating import of seawater, which carries 48 ± 11 Tg DIC/year into the gyre. Though often overlooked, these freshwater displacement effects are of leading order in the Weddell Gyre carbon budget in the state estimate and in regrouped box‐inversion estimates, with implications for evaluating basin‐scale carbon transport.

Using Existing Argo Trajectories to Statistically Predict Future Float Positions with a Transition Matrix

Abstract The Argo array provides nearly 4000 temperature and salinity profiles of the top 2000 m of the ocean every 10 days. Still, Argo floats will never be able to measure the ocean at all times, everywhere. Optimized Argo float distributions should match the spatial and temporal variability of the many societally important ocean features that they observe. Determining these distributions is challenging because float advection is difficult to predict. Using no external models, transition matrices based on existing Argo trajectories provide statistical inferences about Argo float motion. We use the 24 years of Argo locations to construct an optimal transition matrix that minimizes estimation bias and uncertainty. The optimal array is determined to have a 2° × 2° spatial resolution with a 90-day time step. We then use the transition matrix to predict the probability of future float locations of the core Argo array, the Global Biogeochemical Array, and the Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) array. A comparison of transition matrices derived from floats using Argos system and Iridium communication methods shows the impact of surface displacements, which is most apparent near the equator. Additionally, we demonstrate the utility of transition matrices for validating models by comparing the matrix derived from Argo floats with that derived from a particle release experiment in the Southern Ocean State Estimate (SOSE).

Interannual variations of heat budget in the lower layer of the eastern Ross Sea shelf and the forcing mechanisms in the Southern Ocean State Estimate

AbstractThe temporal variation of heat budget in the lower layer of the eastern Ross Sea shelf (ERSS) is crucial for understanding the stability of ice shelves in the Ross Sea. In this study, a 6‐year (2005–2010) simulation from the Southern Ocean State Estimate (SOSE) is employed to analyse the interannual variations of heat budget in the lower layer of the ERSS and the controlling mechanisms. The results reveal that the annual change in the heat content of the study region is dominated by the horizontal heat advection term, and only in 2015 the vertical advection and diffusion terms also play an important role. The horizontal advection term is affected by the intrusion of warm circumpolar deep water (CDW) onto the shelf across the northern boundary of the ERSS, the transport of cold dense shelf water (DSW) from the western Ross Sea shelf, and the transport of CDW across the western boundary of the ERSS. The contributions of these physical processes to the change in annual heat content vary between years. The interannual variation of the CDW intrusion is associated with the strength of eddy activity over the slope, and the interannual variation of heat transports associated with DSW is affected by both the extent of DSW on the western shelf and the coastal circulations that affect the eastward spread of DSW.

Water mass transformation variability in the Weddell Sea in ocean reanalyses

Abstract. This study investigates the variability of water mass transformation (WMT) within the Weddell Gyre (WG). The WG serves as a pivotal site for the Meridional Overturning Circulation (MOC) and ocean ventilation because it is the primary origin of the largest volume of water mass in the global ocean: Antarctic Bottom Water (AABW). Recent mooring data suggest substantial seasonal and interannual variability of AABW properties exiting the WG, and studies have linked the variability to the large-scale climate forcings affecting wind stress in the WG region. However, the specific thermodynamic mechanisms that link variability in surface forcings to variability in water mass transformations and AABW export remain unclear. This study explores how current state-of-the-art data-assimilating ocean reanalyses can help fill the gaps in our understanding of the thermodynamic drivers of AABW variability in the WG via WMT volume budgets derived from Walin's classic WMT framework. The three ocean reanalyses used are the following: Estimating the Circulation and Climate of the Ocean state estimate (ECCOv4), Southern Ocean State Estimate (SOSE) and Simple Ocean Data Assimilation (SODA). From the model outputs, we diagnose a closed form of the water mass budget for AABW that explicitly accounts for transport across the WG boundary, surface forcing, interior mixing and numerical mixing. We examine the annual mean climatology of the WMT budget terms, the seasonal climatology and finally the interannual variability. Our finding suggests that the relatively coarse resolution of these models did not realistically capture AABW formation, export and variability. In ECCO and SOSE, we see strong interannual variability in AABW volume budget. In SOSE, we find an accelerating loss of AABW during 2005–2010, driven largely by interior mixing and changes in surface salt fluxes. ECCO shows a similar trend during a 4-year time period starting in late 2007 but also reveals such trends to be part of interannual variability over a much longer time period. Overall, ECCO provides the most useful time series for understanding the processes and mechanisms that drive WMT and export variability in the WG. SODA, in contrast, displays unphysically large variability in AABW volume, which we attribute to its data assimilation scheme. We also examine correlations between the WMT budgets and large-scale climate indices, including El Niño–Southern Oscillation (ENSO) and Southern Annular Mode (SAM), and find no strong relationships.

Subantarctic Mode Water Biogeochemical Formation Properties and Interannual Variability

AbstractSubantarctic mode water (SAMW) is a key water mass for the transport of nutrients, oxygen, and anthropogenic carbon into the ocean interior. However, a lack of biogeochemical observations of SAMW properties during wintertime formation precluded their detailed characterization. Here we characterize for the first time SAMW properties across their entire wintertime formation regions based primarily on biogeochemical profiling floats. Observations show that the SAMW properties differ between the two main formation regions in the Pacific and Indian sectors of the Southern Ocean. SAMW formed in the Pacific is colder, fresher, and higher in oxygen, nitrate, and dissolved inorganic carbon (DIC) than its Indian Ocean counterpart. The relationship between potential density and biogeochemical water properties is nearly identical between the two formation regions; property differences thus predominantly reflect the difference in mean densities of SAMW formed in each region. SAMW is undersaturated in oxygen during formation, which will impact calculations of derived quantities that assume preformed oxygen saturation. SAMW is at or above atmospheric pCO2 during wintertime and therefore not a direct sink of contemporary carbon dioxide during the formation period. Results from the Biogeochemical Southern Ocean State Estimate suggest anti‐correlated interannual variability of DIC, nitrate, and oxygen between the central and southeastern Pacific formation regions similar to previously established patterns in mixed layer physical properties. This indicates that the mean properties of SAMW will vary depending on which sub‐region has a stronger formation rate, which is in turn linked to the Southern Annual Mode and the El‐Niño Southern Oscillation.

What Controls the Partition between the Cold and Warm Routes in the Meridional Overturning Circulation?

Abstract The origins of the upper limb of the Atlantic meridional overturning circulation and the partition among different routes has been quantified with models at eddy-permitting and one eddy-resolving model or with low-resolution models assimilating observations. Here, a step toward bridging this gap is taken by using the Southern Ocean State Estimate (SOSE) at the eddy-permitting 1/6° horizontal resolution to compute Lagrangian diagnostics from virtual particle trajectories advected between 6.7°S and two meridional sections: one at Drake Passage (cold route) and the other from South Africa to Antarctica (warm route). Our results agree with the prevailing concept attributing the largest transport contribution to the warm route with 12.3 Sv (88%) (1 Sv ≡ 106 m3 s−1) compared with 1.7 Sv (12%) for the cold route. These results are compared with a similar Lagrangian experiment performed with the lower-resolution state estimate from Estimating the Circulation and Climate of the Ocean. Eulerian and Lagrangian means highlight an overall increase in the transport of the major South Atlantic currents with finer resolution, resulting in a relatively larger contribution from the cold route. In particular, the Malvinas Current to Antarctic Circumpolar Current (MC/ACC) ratio plays a more important role on the routes partition than the increased Agulhas Leakage. The relative influence of the mean flow versus the eddy flow on the routes partition is investigated by computing the mean and eddy kinetic energies and the Lagrangian-based eddy diffusivity. Lagrangian diffusivity estimates are largest in the Agulhas and Malvinas regions but advection by the mean flow dominates everywhere.

Carbon to Nitrogen Uptake Ratios Observed Across the Southern Ocean by the SOCCOM Profiling Float Array

AbstractMeasurements of pH and nitrate from the Southern Ocean Carbon and Climate Observations and Modeling array of profiling floats were used to assess the ratios of dissolved inorganic carbon (DIC) and nitrate (NO3) uptake during the spring to summer bloom period throughout the Southern Ocean. Two hundred and forty‐three bloom periods were observed by 115 floats from 30°S to 70°S. Similar calculations were made using the Takahashi surface DIC and nitrate climatology. To separate the effects of atmospheric CO2 exchange and mixing from phytoplankton uptake, the ratios of changes in DIC to nitrate of surface waters (ΔDIC/ΔNO3) were computed in the Biogeochemical Southern Ocean State Estimate (B‐SOSE) model. Phytoplankton uptake of DIC and nitrate are fixed in B‐SOSE at the Redfield Ratio (RR; 6.6 mol C/mol N). Deviations in the B‐SOSE ΔDIC/ΔNO3 must be due to non‐biological effects of CO2 gas exchange and mixing. ΔDIC/ΔNO3 values observed by floats and in the Takahashi climatology were corrected for the non‐biological effects using B‐SOSE. The corrected, in situ biological uptake ratio (C:N) occurs at values similar to the RR, with two major exceptions. North of 40°S biological DIC uptake is observed with little or no change in nitrate giving high C:N. In the latitude band at 55°S, the Takahashi data give a low C:N value, while floats are high. This may be due to a change in CO2 air‐sea exchange in this region from uptake during the Takahashi reference year of 2005 to outgassing of CO2 during the years sampled by floats.

Why Mean Potential Vorticity Cannot Be Materially Conserved in the Eddying Southern Ocean

Abstract Downstream of Drake Passage, the Antarctic Circumpolar Current (ACC) veers abruptly northward along the continental slope of South America. This spins down the ACC, akin to the western boundary currents of ocean gyres. During this northward excursion, the mean potential vorticity (PV) increases dramatically (decreases in magnitude) by up to a factor of 2 along mean geostrophic streamlines on middepth buoyancy surfaces. This increase is driven by drag near the continental slope, or by breaking eddies further offshore, and is balanced by a remarkably steady, eddy-driven decrease of mean PV along these northern circumpolar streamlines in the open ocean. We show how two related eddy processes that are fundamental to ACC dynamics—poleward buoyancy fluxes and downward fluxes of eastward momentum—are also concomitant with materially forcing PV to increase on the northern flank of a jet at middepth, and decrease on the southern flank. For eddies to drive the required mean PV decrease along northern streamlines, the ACC merges with the subtropical gyres to the north, so these streamlines inhabit the southern flanks of the combined ACC–gyre jets. We support these ideas by analyzing the time-mean PV and its budget along time-mean geostrophic streamlines in the Southern Ocean State Estimate. Our averaging formalism is Eulerian, to match the model’s numerics. The thickness-weighted average is preferable, but its PV budget cannot be balanced using Eulerian 5-day averaged diagnostics, primarily because the z-level buoyancy and continuity equations’ delicate balances are destroyed upon transformation into the buoyancy-coordinate thickness equation. Significance Statement The Antarctic Circumpolar Current is the world’s largest ocean current and a key controller of Earth’s climate. As the westerly winds that drive this current shift poleward under global warming, it is vital to know whether the current will follow. To begin addressing this, we study the current’s fundamental dynamics, and constraints, under present-day conditions. By analyzing angular momentum and stratification together, we show that the current is weakened near boundaries and strengthened by eddies elsewhere. The strengthening effects of eddies are isolated to the current by merging the current with oceanic gyres to the north. This gives a new perspective on why the current travels so far northward alongside South America, and may provide dynamical constraints on future changes.

The Effect of Resolution on Vertical Heat and Carbon Transports in a Regional Ocean Circulation Model of the Argentine Basin

AbstractSimulations of the Argentine Basin have large uncertainties associated with quantities such as air‐sea exchanges of heat and carbon in current generation climate models and ocean reanalysis products. This is due to the complex topography, profound undersampling until recent years, and strong currents and mixing of subpolar and subtropical water masses in the basin. Because mixing of water masses is important here, model resolution is hypothesized to play an important role in estimating ocean quantities and determining overall budgets. We construct three regional ocean models with biogeochemistry at 1/3°, 1/6°, and 1/12° resolutions for the year 2017 to investigate heat and carbon dynamics in the region and determine the effect of model resolution on these dynamics. Initial conditions and boundary forcing from BSOSE (the Biogeochemical Southern Ocean State Estimate (Verdy & Mazloff, 2017), https://doi.org/10.1002/2016JC012650) and atmospheric forcing from ERA5 are used. The models are evaluated for accuracy by comparing output to Argo and BGC‐Argo float profiles, BSOSE, and other reanalyses and mapped products. We then quantify the effect of resolution on model upper ocean heat and carbon transport and the associated air‐sea exchanges. We determine that increasing the resolution from 1/3° to 1/12° enhances the upward vertical transport and surface exchanges of heat but causes no significant effect on surface carbon fluxes despite enhancing downward transport of anomalous DIC.

Evaluation of sea-ice thickness from four reanalyses in the Antarctic Weddell Sea

Abstract. Ocean–sea-ice coupled models constrained by various observations provide different ice thickness estimates in the Antarctic. We evaluate contemporary monthly ice thickness from four reanalyses in the Weddell Sea: the German contribution of the project Estimating the Circulation and Climate of the Ocean Version 2 (GECCO2), the Southern Ocean State Estimate (SOSE), the Ensemble Kalman Filter system based on the Nucleus for European Modelling of the Ocean (NEMO-EnKF) and the Global Ice–Ocean Modeling and Assimilation System (GIOMAS). The evaluation is performed against reference satellite and in situ observations from ICESat-1, Envisat, upward-looking sonars and visual ship-based sea-ice observations. Compared with ICESat-1, NEMO-EnKF has the highest correlation coefficient (CC) of 0.54 and lowest root mean square error (RMSE) of 0.44 m. Compared with in situ observations, SOSE has the highest CC of 0.77 and lowest RMSE of 0.72 m. All reanalyses underestimate ice thickness near the coast of the western Weddell Sea with respect to ICESat-1 and in situ observations even though these observational estimates may be biased low. GECCO2 and NEMO-EnKF reproduce the seasonal variation in first-year ice thickness reasonably well in the eastern Weddell Sea. In contrast, GIOMAS ice thickness performs best in the central Weddell Sea, while SOSE ice thickness agrees most with the observations from the southern coast of the Weddell Sea. In addition, only NEMO-EnKF can reproduce the seasonal evolution of the large-scale spatial distribution of ice thickness, characterized by the thick ice shifting from the southwestern and western Weddell Sea in summer to the western and northwestern Weddell Sea in spring. We infer that the thick ice distribution is correlated with its better simulation of northward ice motion in the western Weddell Sea. These results demonstrate the possibilities and limitations of using current sea-ice reanalysis for understanding the recent variability of sea-ice volume in the Antarctic.

Deep Eastern Boundary Currents: Realistic Simulations and Vorticity Budgets

AbstractConcentrated poleward flows near the eastern boundaries between 2- and 4-km depth have been observed repeatedly, particularly in the Southern Hemisphere. These deep eastern boundary currents (DEBCs) play an important role in setting the large-scale tracer distribution and have nonnegligible contribution to global transports of mass, heat, and tracers, but their dynamics are not well understood. In this paper, we first demonstrate the significant role of DEBCs in the southeastern Atlantic, Indian, and Pacific Oceans, using the Southern Ocean State Estimate (SOSE) data assimilating product, and using high-resolution regional general circulation model configurations. The vorticity balances of these DEBCs reveal that, over most of the width of such currents, they are in an interior-like vorticity budget, with the meridional advection of planetary vorticity βυ and vortex stretching fwz being the largest two terms, and with contributions of nonlinearity and friction that are of smaller spatial scale. The stretching is shown, using a temperature budget, to be largely forced by resolved or parameterized eddy temperature transport. Strongly decaying signals from the eastern boundary in friction and stretching form the dominant balance in a sublayer close to the eastern boundary. The temporal variability of DEBCs is then examined, to help to interpret observations that tend to be sporadic in both time and space. The probability distribution functions of northward velocity in DEBC regions are broad, implying that flow reversals are common. Although the regions of the simulated DEBCs are generally local minima of eddy kinetic energy, they are still constantly releasing westward-propagating Rossby waves.

Ertel Potential Vorticity versus Bernoulli Potential on Approximately Neutral Surfaces in the Antarctic Circumpolar Current

AbstractWe investigate the relationship between Ertel potential vorticity Q and Bernoulli potential B on orthobaric density surfaces in the Antarctic Circumpolar Current (ACC), using the Southern Ocean State Estimate. Similar to the extratropical atmospheres of Earth and Mars, Q and B correlate in the ACC in a function-like manner with modest scatter. Below the near-surface, the underlying function relating Q and B appears to be nearly linear. Nondimensionalizing its slope yields “Ma,” a “Mach” number for long Rossby waves, the ratio of the local flow speed to the intrinsic long Rossby wave speed. We empirically estimate the latter using established and novel techniques that yield qualitatively consistent results. Previous work related “Ma” to the degree of homogeneity of Q and to Arnol’d’s shear stability criteria. Estimates of “Ma” for the whole ACC are notably positive, implying inhomogeneous Q, on all circumpolar buoyancy surfaces studied. Upper layers generally exhibit “Ma” slightly less than unity, suggesting that shear instability may operate within these layers. Deep layers exhibit “Ma” greater than unity, implying stability. On surfaces shallower than 1000 m just north of the ACC, the Q versus B slope varies strongly on subannual and interannual time scales, but “Ma” hovers near unity. We also study spatial variability: the ACC is speckled with hundreds of small-scale features with “Ma” near unity, whereas away from the ACC “Ma” is more commonly negative or above unity, both corresponding to stability. Maps of the time-mean “Ma” show stable regions occupy most of the Southern Ocean, except for several topographically controlled hotspots where “Ma” is always near unity.

Spatial variations in silicate-to-nitrate ratios in Southern Ocean surface waters are controlled in the short term by physics rather than biology

Abstract. The nutrient composition (high in nitrate but low in silicate) of Subantarctic Mode Water (SAMW) forces diatom scarcity across much of the global surface ocean. This is because diatoms cannot grow without silicate. After formation and downwelling at the Southern Ocean's northern edge, SAMW re-emerges into the surface layers of the mid- and low-latitude oceans, providing a major nutrient source to primary producers in those regions. The distinctive nutrient composition of SAMW originates in the surface waters of the Southern Ocean, from which SAMW is formed. These waters are observed to transition from being rich in both silicate and nitrate in high-latitude areas of the Southern Ocean to being nitrate-rich but silicate-depleted at SAMW formation sites further north. Here we investigate the key controls of this change in nutrient composition with an idealised model, consisting of a chain of boxes linked by a residual (Ekman- and eddy-induced) overturning circulation. Biological processes are modelled on the basis of seasonal plankton bloom dynamics, and physical processes are modelled using a synthesis of outputs from the data-assimilative Southern Ocean State Estimate. Thus, as surface water flows northward across the Southern Ocean toward sites of SAMW formation, it is exposed in the model (as in reality) to seasonal cycles of both biology and physics. Our results challenge previous characterisations of the abrupt northward reduction in silicate-to-nitrate ratios in Southern Ocean surface waters as being predominantly driven by biological processes. Instead, our model indicates that, over shorter timescales (years to decades), physical processes connecting the deep and surface waters of the Southern Ocean (i.e. upwelling and entrainment) exert the primary control on the spatial distribution of surface nutrient ratios.

Physical Drivers of Phytoplankton Bloom Initiation in the Southern Ocean's Scotia Sea

AbstractThe Scotia Sea is the site of one of the largest spring phytoplankton blooms in the Southern Ocean. Past studies suggest that shelf‐iron inputs are responsible for the high productivity in this region, but the physical mechanisms that initiate and sustain the bloom are not well understood. Analysis of profiling float data from 2002 to 2017 shows that the Scotia Sea has an unusually shallow mixed‐layer depth during the transition from winter to spring, allowing the region to support a bloom earlier in the season than elsewhere in the Antarctic Circumpolar Current. We compare these results to the mixed‐layer depth in the 1/6° data‐assimilating Southern Ocean State Estimate and then use the model output to assess the physical balances governing mixed‐layer variability in the region. Results indicate the importance of lateral advection of Weddell Sea surface waters in setting the stratification. A Lagrangian particle release experiment run backward in time suggests that Weddell outflow constitutes 10% of the waters in the upper 200 m of the water column in the bloom region. This dense Weddell water subducts below the surface waters in the Scotia Sea, establishing a sharp subsurface density contrast that cannot be overcome by wintertime convection. Profiling float trajectories are consistent with the formation of Taylor columns over the region's complex bathymetry, which may also contribute to the unique stratification. Furthermore, biogeochemical measurements from 2016 and 2017 bloom events suggest that vertical exchange associated with this Taylor column enhances productivity by delivering nutrients to the euphotic zone.

Exchange of Water Between the Ross Gyre and ACC Assessed by Lagrangian Particle Tracking

AbstractTo reach upwelling and downwelling zones deep within the Southern Ocean seasonal sea ice cover, water masses must move across the Antarctic Circumpolar Current and through current systems including the Ross Gyre, Weddell Gyre, and Antarctic Slope Current. In this study we focus our attention on the Lagrangian exchange between the Ross Gyre and surrounding current systems. We conducted numerical experiments using five‐day 3‐D velocity fields from the Southern Ocean State Estimate with a particle tracking package to identify pathways by which waters move from near the Antarctic coastal margins or Antarctic Circumpolar Current into the interior of the Ross Gyre, and to identify the time scales of variability associated with these pathways. Waters from near the Antarctic margins enter the Ross Gyre along the western and northern boundaries of gyre until the gyre separates from the Pacific‐Antarctic Ridge near fracture zones. At this juncture, Antarctic Circumpolar Current‐derived inflow dominates the across‐gyre transport up to the Antarctic margin. Transport and exchange associated with different time‐average components of flow are calculated to determine the relative contributions of high‐ and low‐frequency and time‐mean components.

The Effects of Enhanced Sea Ice Export from the Ross Sea on Recent Cooling and Freshening of the Southeast Pacific

Abstract The top 2000 m of the Southern Ocean has freshened and warmed over recent decades. However, the high-latitude (south of 50°S) southeast Pacific was observed to be cooler and fresher in the years 2008–10 compared to 2005–07 over a wide depth range including surface, mode, and intermediate waters. The causes and impacts of this event are analyzed using the ocean–sea ice data-assimilating Southern Ocean State Estimate (SOSE) and observationally based products. In 2008–10, a strong positive southern annular mode coincided with a negative El Niño–Southern Oscillation and a deep Amundsen Sea low. Enhanced meridional winds drove strong sea ice export from the eastern Ross Sea, bringing large amounts of ice to the Amundsen Sea ice edge. In 2008, together with increased precipitation, this introduced a strong freshwater anomaly that was advected eastward by the Antarctic Circumpolar Current (ACC), mixing along the way. This anomaly entered the ocean interior not only as Antarctic Intermediate Water, but also as lighter Southeast Pacific Subantarctic Mode Water (SEPSAMW). A numerical particle release experiment carried out in SOSE showed that the Ross Sea sector was the dominant source of particles reaching the SEPSAMW formation region. This suggests that large-scale climate fluctuations can induce strong interannual variability of volume and properties of SEPSAMW. These fluctuations act at different time scales: instantaneously via direct forcing and also lagged over advective time scales of several years from upstream regions.

Barotropic Kelvin Wave‐Induced Bottom Boundary Layer Warming Along the West Antarctic Peninsula

AbstractIntrusions of warm circumpolar deep water onto the Antarctic continental shelf are thought to drive accelerated loss of Antarctic glacial ice mass by triggering melt at the ice shelf grounding line. However, the mechanisms responsible for driving on‐shelf circumpolar deep water intrusions are not well understood. Here we examine how sea surface height (SSH) anomalies propagating around the Antarctic coastline as coastal‐trapped waves can drive warm water intrusions through changes in bottom Ekman transport. A wind perturbation motivated by the recent intensification and poleward shift of the southern annular mode during its positive phase is applied over Eastern Antarctica between 20°E and 120°E in two global ocean sea‐ice models (1/4° and 1/10°) and a single‐layer shallow water model. The changes in winds generate a drop in coastal SSH that propagates around Antarctica as a barotropic Kelvin wave. The SSH drop is accompanied by a barotropic flow, which alters the bottom stress, generating an onshore transport of warm water wherever thermal gradients are favorable. We estimate the resulting anomalous bottom Ekman flow and use temperature gradients calculated from the Southern Ocean State Estimate, along with the 1/4° and 1/10° models, to evaluate the resultant heat advection. We find that this mechanism can drive warming of up to 0.7 °C along the West Antarctic Peninsula within a year, depending on the mean state of the cross‐shelf temperature gradient and the barotropic flow strength. Over longer time scales, warming eventually ceases due to saturation of the SSH field and arrest of the Ekman transport by buoyancy forces.

Southern Ocean Biogeochemical Float Deployment Strategy, With Example From the Greenwich Meridian Line (GO-SHIP A12).

Biogeochemical Argo floats, profiling to 2,000‐m depth, are being deployed throughout the Southern Ocean by the Southern Ocean Carbon and Climate Observations and Modeling program (SOCCOM). The goal is 200 floats by 2020, to provide the first full set of annual cycles of carbon, oxygen, nitrate, and optical properties across multiple oceanographic regimes. Building from no prior coverage to a sparse array, deployments are based on prior knowledge of water mass properties, mean frontal locations, mean circulation and eddy variability, winds, air‐sea heat/freshwater/carbon exchange, prior Argo trajectories, and float simulations in the Southern Ocean State Estimate and Hybrid Coordinate Ocean Model (HYCOM). Twelve floats deployed from the 2014–2015 Polarstern cruise from South Africa to Antarctica are used as a test case to evaluate the deployment strategy adopted for SOCCOM's 20 deployment cruises and 126 floats to date. After several years, these floats continue to represent the deployment zones targeted in advance: (1) Weddell Gyre sea ice zone, observing the Antarctic Slope Front, and a decadally‐rare polynya over Maud Rise; (2) Antarctic Circumpolar Current (ACC) including the topographically steered Southern Zone chimney where upwelling carbon/nutrient‐rich deep waters produce surprisingly large carbon dioxide outgassing; (3) Subantarctic and Subtropical zones between the ACC and Africa; and (4) Cape Basin. Argo floats and eddy‐resolving HYCOM simulations were the best predictors of individual SOCCOM float pathways, with uncertainty after 2 years of order 1,000 km in the sea ice zone and more than double that in and north of the ACC.

Subduction by Submesoscales

AbstractSeveral studies have demonstrated the relevance of submesoscales (SM) to the production of phytoplankton and the ocean absorption of anthropogenic CO2. One variable that thus far has not been fully quantified is the SM‐induced rates of subduction and obduction at the bottom of the mixed layer. In this work, we use an SM parameterization that had been previously assessed to estimate the rates of subduction‐obduction using a coarse resolution stand‐alone ocean code to compute the large‐scale fields. To separate the contributions of baroclinic instabilities from those due to wind stress, we derive an analytic expression for the SM subduction‐obduction rates. Since there are no numerical simulations and/or measurements to assess the results, we compare them to subduction rates due to mesoscales for which an eddy‐permitting southern ocean state estimate is available. An interesting result is that in the Antarctic Circumpolar Current the maximum SM subduction rates are not significantly smaller than those due to mesoscales, but the SM subduction rates exhibit a topology opposite to that of mesoscales: obduction is equatorward and subduction is poleward. Sensitivity studies are presented to explore the dependence of the results on the depth of the SM vertical extent and the SM kinetic energy. On the basis of these results, it is suggested that the SM‐induced subduction should not be neglected because topology and magnitude indicate that mesoscales and submesoscales may add to or subtract from one another.

Observing the Ice‐Covered Weddell Gyre With Profiling Floats: Position Uncertainties and Correlation Statistics

AbstractArgo‐type profiling floats do not receive satellite positioning while under sea ice. Common practice is to approximate unknown positions by linearly interpolating latitude‐longitude between known positions before and after ice cover, although it has been suggested that some improvement may be obtained by interpolating along contours of planetary‐geostrophic potential vorticity. Profiles with linearly interpolated positions represent 16% of the Southern Ocean Argo data set; consequences arising from this approximation have not been quantified. Using three distinct data sets from the Weddell Gyre—10‐day satellite‐tracked Argo floats, daily‐tracked RAFOS‐enabled floats, and a particle release simulation in the Southern Ocean State Estimate—we perform a data withholding experiment to assess position uncertainty in latitude‐longitude and potential vorticity coordinates as a function of time since last fix. A spatial correlation analysis using the float data provides temperature and salinity uncertainty estimates as a function of distance error. Combining the spatial correlation scales and the position uncertainty, we estimate uncertainty in temperature and salinity as a function of duration of position loss. Maximum position uncertainty for interpolation during 8 months without position data is 116 ± 148 km for latitude‐longitude and 92 ± 121 km for potential vorticity coordinates. The estimated maximum uncertainty in local temperature and salinity over the entire 2,000‐m profiles during 8 months without position data is 0.66 ∘C and 0.15 psu in the upper 300 m and 0.16 ∘C and 0.01 psu below 300 m.