Subantarctic Mode Water Formation Research Articles

Southern Ocean circulation’s impact on atmospheric CO2 concentration

In the context of past and present climate change, the Southern Ocean (SO) has been identified as a crucial region modulating the concentration of atmospheric CO2. The sustained upwelling of carbon-rich deep waters and inefficient nutrient utilization at the surface of the SO leads to an outgassing of natural CO2, while anthropogenic CO2 is entrained to depth during the formation of Antarctic Bottom water (AABW), Antarctic intermediate water (AAIW) and sub-Antarctic mode water (SAMW). Changes to the SO circulation resulting from both dynamic and buoyancy forcing can alter the rate of upwelling as well as formation and subsequent transport of AABW, AAIW and SAMW, thus impacting the air-sea CO2 exchange in the SO. Models of all complexity robustly show that stronger southern hemispheric (SH) westerlies enhance SO upwelling, thus leading to stronger natural CO2 outgassing, with a sensitivity of 0.13 GtC/yr for a 10% increase in SH westerly windstress. While the impact of changes in the position of the SH westerly winds was previously unclear, recent simulations with high-resolution ocean/sea-ice/carbon cycle models show that a poleward shift of the SH westerlies also enhances natural CO2 outgassing with a sensitivity of 0.08GtC/yr for a 5° poleward shift. While enhanced AABW transport reduces deep ocean natural DIC concentration and increases surface natural DIC concentration, it acts on a multi-decadal timescale. Future work should better constrain both the natural and anthropogenic carbon cycle response to changes in AABW and the compound impacts of dynamic and buoyancy changes on the SO marine carbon cycle.

Sea Ice‐Driven Variability in the Pacific Subantarctic Mode Water Formation Regions

AbstractSubantarctic Mode Water (SAMW) forms north of the Subantarctic Front, in regions of deep winter mixed layers, and is important to the absorption and storage of anthropogenic CO2 and heat. Two SAMW pools exist in the Pacific, a lighter Central mode (CPSAMW), and a denser Southeast mode (SEPSAMW). Both have experienced significant interannual variability in thickness and properties in recent years. We compute mixed layer temperature and salinity budgets for the two SAMW formation regions, to determine the relative contribution of processes driving variability in the properties of mixed layers that subduct to form SAMW. The dominant drivers of temperature and salinity variability are shown to be surface fluxes, horizontal advection, and entrainment of deeper water. Salt advection into each SAMW formation region is found to be strongly correlated with changes in sea ice area in the northern Ross Sea, with lags of up to 2 years. Further correlation is found between meridional salt advection in the southeast Pacific formation regions, and sea ice area in the northern Amundsen/Bellingshausen seas, suggesting that freshwater derived from sea ice melt reaches the SEPSAMW formation region within 6 months. In 2016, strong advective freshening of the SEPSAMW formation region, linked to increased winter sea ice in the Amundsen/Bellingshausen seas, led to anomalously fresh mixed layers. However, a regime change in Antarctic sea ice in 2016 resulted in a subsequent lack of the usual advective freshening in the SEPSAMW formation region, driving increased salinity of the mixed layer the following year.

Changes of upper-ocean temperature in the Southeast Indian Subantarctic Mode Water formation region since the 1950s

Changes of upper-ocean temperature in the Southeast Indian Subantarctic Mode Water formation region since the 1950s

Ocean Ventilation Controls the Contrasting Anthropogenic CO2 Uptake Rates Between the Western and Eastern South Atlantic Ocean Basins

AbstractThe South Atlantic Ocean is an important region for anthropogenic CO2 (Canth) uptake and storage in the world ocean, yet is less studied. Here, after an extensive sensitivity test and method comparison, we applied an extended multiple linear regression method with six characteristic water masses to estimate Canth change or increase (ΔCanth) between 1980s and 2010s in the South Atlantic Ocean using two meridional transects (A16S and A13.5) and one zonal transect (A10). Over a period of about 25 years, the basin‐wide ΔCanth was 3.86 ± 0.14 Pg C decade−1. The two basins flanking the Mid‐Atlantic Ridge had different meridional patterns of ΔCanth, yielding an average depth‐integrated ΔCanth in the top 2000 m of 0.91 ± 0.25 mol m−2 yr−1 along A16S on the west and 0.57 ± 0.22 mol m−2 yr−1 along A13.5 on the east. The west‐east basin ΔCanth contrasts were most prominent in the tropical region (0–20°S) in the Surface Water (SW), approximately from equator to 35°S in the Subantarctic Mode Water (Subantarctic Mode Water (SAMW)), and all latitudes in the Antarctic Intermediate Water (AAIW). Less ΔCanth in the eastern basin than the western basin was caused by weaker ventilation driven by SAMW and AAIW formation and subduction and stronger Antarctic Bottom Water (AABW) formation in the former than the latter. In addition to the spatial heterogeneity, Canth increase rates accelerated from the 1990s to the 2000s, consistent with the overall increase in air‐sea CO2 exchange in the South Atlantic Ocean.

Changes in the Subantarctic Mode Water Properties and Spiciness in the Southern Indian Ocean based on Argo Observations

AbstractThe Subantarctic Mode Water (SAMW) plays an essential role in the global heat, freshwater, carbon, and nutrient budgets. In this study, decadal changes in the SAMW properties in the Southern Indian Ocean (SIO) and associated thermodynamic and dynamic processes are investigated during the Argo era. Both temperature and salinity of the SAMW in the SIO show increasing trends during 2004-2018. A two-layer structure of the SAMW trend, with more warm and salty light SAMW but less cool and fresh dense SAMW, is identified. The heaving and spiciness processes are important but have opposite contributions to the temperature and salinity trends of the SAMW. A significant deepening of isopycnals (heaving), peaking atσθ=26.7-26.8 kg m−3in the middle layer of the SAMW, expands the warm and salty light SAMW and compresses the cool and fresh dense SAMW corresponding to the change in subduction rate during 2004-2018. The change in the SAMW subduction rate is dominated by the change in the mixed layer depth, controlled by the changes in wind stress curl and surface buoyancy loss. An increase in the mixed-layer temperature due to weakening northward Ekman transport of cool water leads to a lighter surface density in the SAMW formation region. Consequently, density outcropping lines in the SAMW formation region shift southward and favor the intrusion and entrainment of the cooler and fresher Antarctic surface water from the south, contributing to the cooling/freshening trend of isopycnals (spiciness). Subsequently, the cooler and fresher SAMW spiciness anomalies spread in the SIO via the subtropical gyre.

Strong quasi-stationary wintertime atmospheric surface pressure anomalies drive a dipole pattern in the Subantarctic ModeWater formation

AbstractThe deepest wintertime (Jul-Sep) mixed layers associated with Subantarctic Mode Water (SAMW) formation develop in the Indian and Pacific sectors of the Southern Ocean. In these two sectors the dominant interannual variability of both deep wintertime mixed layers and SAMW volume is a east-west dipole pattern in each basin. The variability of these dipoles are strongly correlated with the interannual variability of overlying winter quasi-stationary mean sea level pressure (MSLP) anomalies. Anomalously strong positive MSLP anomalies are found to result in the deepening of the wintertime mixed layers and an increase in the SAMW formation in the eastern parts of the dipoles in the Pacific and Indian sectors. These effects are due to enhanced cold southerly meridional winds, strengthened zonal winds and increased surface ocean heat loss. The opposite occurs in the western parts of the dipoles in these sectors. Conversely, strong negative MSLP anomalies result in shoaling (deepening) of the wintertime mixed layers and a decrease (increase) in SAMW formation in the eastern (western) regions. The MSLP variability of the Pacific and Indian basin anomalies are not always in phase, especially in years with a strong El Niño, resulting in different patterns of SAMW formation in the western vs. eastern parts of the Indian and Pacific sectors. Strong isopycnal depth and thickness anomalies develop in the SAMW density range in years with strong MSLP anomalies. When advected eastward, they act to precondition downstream SAMW formation in the subsequent winter.

Fast and slow responses of the Subantarctic Mode Water in the South Indian Ocean to global warming in CMIP5 extended RCP4.5 simulations

The present study examines the long-term evolution of the Subantarctic Mode Water (SAMW) in the south Indian Ocean (SIO) based on the Coupled Model Intercomparison Project Phase 5 (CMIP5) extended simulations through 2300 under Representative Concentration Pathways 4.5 (RCP4.5). The SAMW in the SIO shrinks and thins rapidly during the fast radiative forcing increasing period between 2000 and 2100. After 2100, when the radiative forcing stabilizes, the SAMW expands and thickens slowly. The response of SAMW is dependent both on the evolution of surface buoyancy forcing and overlying westerly wind stress. During year 2000–2100, the surface heat flux dominates the net buoyancy gain over the SAMW formation region, whereas the enhanced westerly wind contributes to a lesser extent due to its poleward shift. As the radiative forcing increases rapidly from 2000 to 2100, the buoyancy gain over the SAMW formation region results in a fast shoaling mixed layer and a reduction in the SAMW subduction rate. The surface intensified warming enhances stratification and reduces the SAMW that is quantified by the volume of low potential vorticity (PV) water. Consequently, the SAMW shrinks and thins fastly. When the radiative forcing stabilizes after 2100, the warming is greater in the subsurface than the surface, and the associated slow destratification gradually increases the low PV water volume. Hence, the SAMW expands and thickens slowly. The distinct fast and slow responses of SAMW have implications for the heat uptake and circulation of the SIO.

Frontal dynamics and water mass variability on the Campbell Plateau

ABSTRACT The Campbell Plateau is a dominating bathymetric feature of New Zealand’s subantarctic region, strongly influencing the dynamics of both the Subtropical Front (STF) to the north and the Subantarctic Front (SAF) to the south. We present a consistent survey across New Zealand’s subantarctic of the relationships between front positions, bathymetry and water masses. The northwest side of the Campbell Plateau is comparatively warm due to a southward extension of the STF over the plateau. The SAF is steered south and east by the combined effect of the plateau and Macquarie Ridge, with waters originating in the SAF also found north of the plateau. Formation of Subantarctic Mode Water (SAMW) is confirmed, with a deep reservoir of SAMW persisting nearly a year after formation. Antarctic Intermediate Water (AAIW) is observed around the edges of the plateau, but not present on the plateau, confirming that the waters on the plateau are isolated from deeper Southern Ocean.

On the control of subantarctic stratification by the ocean circulation

A shallow mixed layer depth bias in Austral winter in the Subantarctic Zone is a common feature of Coupled Model Intercomparison Project (CMIP5) models, including the Community Earth System Model (CESM). The bias is related to other deficiencies in the model solution, including too-weak Subantarctic Mode water formation and excessive leakage of Agulhas waters into the Atlantic instead of into the Indian Ocean and Subantarctic Frontal Zone. This work investigates the hypothesis that the shallow bias is due to errors in the simulated ocean circulation. Results from a model with low resolution ocean component (1° grid) are compared against: (i) an experiment with an eddy resolving (0.1°) grid and (ii) experiments using the 1° grid and employing an adiabatic nudging of the ocean pressure field to observations that is referred to as a semi-prognostic method. For both the higher resolution and semi-prognostic experiments, improved horizontal advection of warm and salty water near the surface leads to a more realistic sea surface temperature (SST) and salinity front associated with the Agulhas Return Current and Antarctic Circumpolar Current. The warmer surface waters in the high-resolution model and semi-prognostic model lead to stronger air–sea heat loss, with a response of about 40 Wm−2 °C−1 in winter in the area of deep mixed layers. Budgets of the temperature and salinity stratification show that the deeper mixed layers in the high-resolution experiment are primarily a result of the increased surface heat loss, whereas in the semi-prognostic case salinity advection is the main factor leading to destabilization of the water column. Both results indicate that ocean circulation is a key factor in wintertime deep Southern Ocean mixing, associated with advection of water masses and air–sea feedbacks.

Variability of the Sub‐Antarctic Mode Water Subduction Rate During the Argo Period

AbstractBoth a quasi‐biennial variability and an overall linearly increasing trend are identified in the Sub‐Antarctic Mode Water (SAMW) subduction rate across the Southern Hemisphere ocean, using the Argo data during 2005–2019. The quasi‐biennial variability is mainly due to variability of the mixed layer depth. Variability of wind stress curl in the SAMW formation regions associated with the Southern Annular Mode plays a critical role in generating the quasi‐biennial variability of the mixed layer depth and consequently the SAMW subduction rates. The SAMW subduction rate across the Southern Hemisphere ocean, long‐term mean totaling 56 Sv, has increased at 0.73 ± 0.65 Sv year−1 over the past 15 years. The increase has directly contributed to the observed increase in the total SAMW volume. Much of this increasing trend can be explained by the deepening mixed layers, which in turn are primarily forced by the strengthening westerly winds under an increasing Southern Annular Mode.

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.

Mooring Observations of Air–Sea Heat Fluxes in Two Subantarctic Mode Water Formation Regions

AbstractWintertime surface ocean heat loss is the key process driving the formation of Subantarctic Mode Water (SAMW), but there are few direct observations of heat fluxes, particularly during winter. The Ocean Observatories Initiative (OOI) Southern Ocean mooring in the southeast Pacific Ocean and the Southern Ocean Flux Station (SOFS) in the southeast Indian Ocean provide the first concurrent, multiyear time series of air–sea fluxes in the Southern Ocean from two key SAMW formation regions. In this work we compare drivers of wintertime heat loss and SAMW formation by comparing air–sea fluxes and mixed layers at these two mooring locations. A gridded Argo product and the ERA5 reanalysis product provide temporal and spatial context for the mooring observations. Turbulent ocean heat loss is on average 1.5 times larger in the southeast Indian (SOFS) than in the southeast Pacific (OOI), with stronger extreme heat flux events in the southeast Indian leading to larger cumulative winter ocean heat loss. Turbulent heat loss events in the southeast Indian (SOFS) occur in two atmospheric regimes (cold air from the south or dry air circulating via the north), while heat loss events in the southeast Pacific (OOI) occur in a single atmospheric regime (cold air from the south). On interannual time scales, wintertime anomalies in net heat flux and mixed layer depth (MLD) are often correlated at the two sites, particularly when wintertime MLDs are anomalously deep. This relationship is part of a larger basin-scale zonal dipole in heat flux and MLD anomalies present in both the Indian and Pacific basins, associated with anomalous meridional atmospheric circulation.

A See‐Saw in Pacific Subantarctic Mode Water Formation Driven by Atmospheric Modes

AbstractSubantarctic Mode Water (SAMW) in the Pacific forms in two distinct pools in the south central and southeast Pacific, which subduct into the ocean interior and impact global storage of heat and carbon. Wintertime thickness of the central and eastern SAMW pools vary predominantly out of phase with each other, by up to ±150 m between years, resulting in an interannual thickness see‐saw. The thickness in the eastern (central) pool is found to be strongly positively (negatively) correlated with both the Southern Annular Mode (SAM) and El Niño–Southern Oscillation (ENSO). The relative phases of the SAM and ENSO set the SAMW thickness, with in phase reinforcing modes in 2005–2008 and 2012–2017 driving strong differences between the pools. Between 2008 and 2012 out of phase atmospheric modes result in less coherent SAMW patterns. SAMW thickness is dominated by local formation driven by SAM and ENSO modulated wind stress and turbulent heat fluxes.

Episodic Southern Ocean Heat Loss and Its Mixed Layer Impacts Revealed by the Farthest South Multiyear Surface Flux Mooring

AbstractThe Ocean Observatories Initiative air‐sea flux mooring deployed at 54.08°S, 89.67°W, in the southeast Pacific sector of the Southern Ocean, is the farthest south long‐term open ocean flux mooring ever deployed. Mooring observations (February 2015 to August 2017) provide the first in situ quantification of annual net air‐sea heat exchange from one of the prime Subantarctic Mode Water formation regions. Episodic turbulent heat loss events (reaching a daily mean net flux of −294 W/m2) generally occur when northeastward winds bring relatively cold, dry air to the mooring location, leading to large air‐sea temperature and humidity differences. Wintertime heat loss events promote deep mixed layer formation that lead to Subantarctic Mode Water formation. However, these processes have strong interannual variability; a higher frequency of 2 σ and 3 σ turbulent heat loss events in winter 2015 led to deep mixed layers (>300 m), which were nonexistent in winter 2016.

Regional Impacts of the Westerly Winds on Southern Ocean Mode and Intermediate Water Subduction

AbstractSubduction processes in the Southern Ocean transfer oxygen, heat, and anthropogenic carbon into the ocean interior. The future response of upper-ocean subduction, in the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) classes, is dependent on the evolution of the combined surface buoyancy forcing and overlying westerly wind stress. Here, the recently observed pattern of a poleward intensification of the westerly winds is divided into its shift and increase components. SAMW and AAIW formation occurs in regional “hot spots” in deep mixed layer zones, primarily in the southeast Indian and Pacific. It is found that the mixed layer depth responds differently to wind stress perturbations across these regional formation zones. An increase only in the westerly winds in the Indian sector steepens isopycnals and increases the local circulation, driving deeper mixed layers and increased subduction. Conversely, in the same region, a poleward shift and poleward intensification of the westerly winds reduces heat loss and increases freshwater input, thus decreasing the mixed layer depth and consequently the associated SAMW and AAIW subduction. In the Pacific sector, all wind stress perturbations lead to increases in heat loss and decreases in freshwater input, resulting in a net increase in SAMW and AAIW subduction. Overall, the poleward shift in the westerly wind stress dominates the SAMW subduction changes, rather than the increase in wind stress. The net decrease in SAMW subduction across all basins would likely decrease anthropogenic carbon sequestration; however, the net AAIW subduction changes across the Southern Ocean are overall minor.

Decadal changes in Southern Ocean ventilation inferred from deconvolutions of repeat hydrographies

AbstractChanges in Southern Ocean ventilation imprint on dissolved gases, nutrients, radiocarbon, temperature, and salinity. We deconvolve tracer measurements for the distribution, , of times and locations of last ventilation using a maximum‐entropy approach. Decadal changes of are quantified by deconvolving hydrographies measured during the early 1990s and again some 15 years later. Our analysis reveals coherent changes across the five meridional sections analyzed: The fraction of water younger than 30 years decreased by ∼20% per decade south of 40°S in circumpolar deep water (CDW) and increased by ∼10% per decade north of ∼40°S in subantarctic mode water (SAMW). Ventilation locations shifted, with more water south of 40°S being ventilated north of the subantarctic front. These ventilation changes impacted CFC uptake, with concentrations south of 40°S less than (and north of 40°S higher than) expected for steady flow. The inferred changes imply increased SAMW formation and CDW upwelling consistent with strengthened westerly winds.

The Spatiotemporal Structure of Diabatic Processes Governing the Evolution of Subantarctic Mode Water in the Southern Ocean

AbstractA coupled ice–ocean eddy-permitting Southern Ocean State Estimate (SOSE) for 2008–10 is used to describe and quantify the processes forming and destroying water in the Subantarctic Mode Water (SAMW) density range (σθ = 26.7–27.2 kg m−3). All the terms in the temperature and salinity equations have been diagnosed to obtain a three-dimensional and time-varying volume budget for individual isopycnal layers. This study finds that air–sea buoyancy fluxes, diapycnal mixing, advection, and storage are all important to the SAMW volume budget. The formation and destruction of water in the SAMW density range occurs over a large latitude range because of the seasonal migration of the outcrop window. The strongest formation is by wintertime surface ocean heat loss occurring equatorward of the Subantarctic Front. Spring and summertime formation occur in the polar gyres through the freshening of water with σθ > 27.2 kg m−3, with an important contribution from sea ice melt. Further buoyancy gain by heating is accomplished only after these waters have already been transformed into the SAMW density range. The spatially integrated and time-averaged SAMW formation rate in the ocean surface layer is 7.9 Sverdrups (Sv; 1 Sv ≡ 106 m3 s−1) by air–sea buoyancy fluxes and 8.8 Sv by diapycnal mixing, and it is balanced by advective export into the interior ocean. Maps show that these average rates are the result of highly variable processes with strong cancellation in both space and time, revealing the complexity of water mass transformation in the three-dimensional Southern Ocean overturning circulation.

Nitrate isotope distributions on the US GEOTRACES North Atlantic cross-basin section: Signals of polar nitrate sources and low latitude nitrogen cycling

We report and interpret the nitrate δ15N and δ18O distributions in the cross-basin transect of the US GEOTRACES North Atlantic expedition. Low nitrate δ15N observed in the shallow thermocline of the central western portion of the basin (reaching as low as 2.5‰ vs. air; ~2.3‰ lower than deep Atlantic nitrate) reflects the remineralization of nitrogen with a low δ15N, presumably mostly from nitrogen fixation. A strong correlation between the δ15N and concentration of nitrate argues that the focusing of the low δ15N signal in the upper thermocline of the North Atlantic subtropical gyre is at least partly due to the low concentration of nitrate in those waters, such that the east-to-west δ15N decrease in shallow thermocline nitrate does not require an east-to-west increase in nitrogen fixation rate. In underlying intermediate-depth water, which enters the section largely from the South, the nitrate δ15N is ~1‰ higher than in deep water while the δ18O is similar to that in deep water. The δ15N elevation derives from the partial assimilation of nitrate in Southern Ocean surface waters prior to Antarctic Intermediate and Subantarctic Mode Water formation. The δ18O elevation of preformed nitrate in southern-sourced intermediate waters has been diluted by the addition of nitrate from regeneration/nitrification in the low latitudes, which does not similarly weaken the δ15N elevation. In deep water, the nitrate δ15N in samples with a high relative contribution of North Atlantic Deep Water is almost indistinguishable from that of samples with a higher contribution of Antarctic Bottom Water (4.8–4.9‰ vs. air). Thus, the high nitrate δ15N in northward flowing intermediate waters is almost entirely erased by the input of low-δ15N newly fixed nitrogen to the Atlantic (including that from the Mediterranean) prior to the incorporation of those intermediate depth waters into southward flowing North Atlantic Deep Water. The data appear consistent with a nitrogen isotopic balance between northward and southward nitrate transports across the GEOTRACES section, suggesting that most Atlantic nitrogen fixation occurs south of the section.

Southern Ocean contributions to the Eastern Equatorial Pacific heat content during the Holocene

Temperature reconstructions from a shallow core (375 m) from the Peru Margin are used to test the influence of Subantarctic Mode Water (SAMW) on the eastern equatorial Pacific (EEP) thermostad and thus the effect of southern high latitude climate on interior ocean heat content (OHC). Temperature estimates, based on Mg/Ca measurements of planktonic and benthic foraminifera (Neogloboquadrina dutertrei and Uvigerina spp., respectively) show higher temperatures in the early Holocene, a cooling of ∼2° by 8 kyr B.P. and after relatively stable temperatures to the present. The temperature signal is similar in direction and timing to a rather robust Holocene climate signal from the southern high latitudes suggesting it originated there and was advected to the core site in the EEP. Based on the N. dutertrei and Uvigerina Mg/Ca temperature and δ13C records we conclude that SAMW acted as a conduit transporting the southern high latitude climate to the interior of the equatorial Pacific. We propose that the early Holocene warmth is related to a southward migration of the Subtropical Front, which enhanced the influence of warm subtropical water in the region of SAMW formation and was then transported to the EEP thermostad. The early Holocene warmth recorded in the EEP thermostad has a muted sea surface temperature expression indicating this mechanism is important for sequestering heat in the ocean interior.

Dissolved cadmium in the Southern Ocean: Distribution, speciation, and relation to phosphate

We report isotope dilution analyses of dissolved cadmium (Cd) and electrochemical Cd speciation measurements in the Atlantic sector of the Southern Ocean. Bioavailable inorganic Cd is > 100 times higher in near‐surface waters south of the Polar Front compared to the Subantarctic Zone because of upwelling and reduced complexation by organic Cd ligands. To trace local changes in the relation between Cd and P, we examine the deviations from a linear deep‐water Cd vs. P relation (Cd*), and find that changes in Cd* coincide with the position of frontal systems and covary with primary productivity and total dissolved Mn and Fe concentrations. These covariations agree with potential local changes in phytoplankton Cd uptake rates, resulting from differences in the availability of Cd, Zn, Mn, and Fe. A band of negative Cd* values is associated with formation of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). In contrast to SAMW, which may export low Cd : P ratios from the Southern Ocean, the Cd : P ratios in AAIW increase by mixing with underlying Upper Circumpolar Deep Water before being exported from the Southern Ocean. Deep waters show constant Cd : P ratios, and both elements behave conservatively with end‐member mixing between deep waters of the Weddell Gyre, the Antarctic Circumpolar Current, and inflowing North Atlantic Deep Water. Overall, our results support the hypothesis that the kink in the global Cd vs. P relation is largely caused by high Cd : P uptake ratios in the trace‐nutrient—limited Southern Ocean.