Subantarctic Mode Water Research Articles

Changes in Oceanic Radiocarbon and CFCs Since the 1990s

AbstractAnthropogenic perturbations from fossil fuel burning, nuclear bomb testing, and chlorofluorocarbon (CFC) use have created useful transient tracers of ocean circulation. The atmospheric 14C/C ratio (∆14C) peaked in the early 1960s and has decreased now to pre‐industrial levels, while atmospheric CFC‐11 and CFC‐12 concentrations peaked in the early 1990s and early 2000s, respectively, and have now decreased by 10%–20%. We present the first analysis of a decade of new observations (2007 to 2018–2019) and give a comprehensive overview of the changes in ocean ∆14C and CFC concentration since the WOCE surveys in the 1990s. Surface ocean ∆14C decreased at a nearly constant rate from the 1990–2010s (20‰/decade). In most of the surface ocean ∆14C is higher than in atmospheric CO2 while in the interior ocean, only a few places are found to have increases in ∆14C, indicating that globally, oceanic bomb 14C uptake has stopped and reversed. Decreases in surface ocean CFC‐11 started between the 1990 and 2000s, and CFC‐12 between the 2000–2010s. Strong coherence in model biases of decadal changes in all tracers in the Southern Ocean suggest ventilation of Antarctic Intermediate Water was enhanced from the 1990 to the 2000s, whereas ventilation of Subantarctic Mode Water was enhanced from the 2000 to the 2010s. The decrease in surface tracers globally between the 2000 and 2010s is consistently stronger in observations than in models, indicating a reduction in vertical transport and mixing due to stratification.

Changes in Spreading of Southeast Indian Subantarctic Mode Water During Argo Period

AbstractSubantarctic Mode Water (SAMW) is one of the most important water masses for the global ocean uptake and storage of heat and carbon. Based on Argo observations, this study focus on the Southeast Indian Subantarctic mode water (SEISAMW), and investigates the changes of SEISAMW spreading and associated mechanisms. SEISAMW is formed through air‐sea interaction as low potential vorticity water in late winter and subducts into permanent thermocline between outcrop lines of 26.6 σθ and 26.9 σθ in Southern Indian Ocean (SIO). After subduction, the SEISAMW spreads northwestward into subtropical gyre and southeastward into Antarctic Circumpolar Current (ACC) separately. During Argo period, the percentage of SEISAMW volume spreading into SIO subtropics decreases at light layer (26.6–26.7 σθ) while increases at medium (26.7–26.8 σθ) and dense (26.8–26.9 σθ) layer. These changes are attributed to the meridional shifts of outcrop lines. The outcrop lines of 26.6 σθ and 26.7 σθ in the central of SIO (75°–90°E), where light SEISAMW forms, shift poleward. The poleward shifts of outcrop lines result in less light SEISAMW spreading into subtropical gyre. In contrast, outcrop lines of 26.7 σθ and 26.8 σθ south of Australia shift equatorward and favor more medium and dense SEISAMWs spreading into subtropical gyre. The shifts of outcrop lines are closely related to the enhancement of Southern Annular Mode.

The Southern Ocean deep mixing band emerges from a competition between winter buoyancy loss and upper stratification strength

Abstract. The Southern Ocean hosts a winter deep mixing band (DMB) near the Antarctic Circumpolar Current's (ACC) northern boundary, playing a pivotal role in Subantarctic Mode Water formation. Here, we investigate what controls the presence and geographical extent of the DMB. Using observational data, we construct seasonal climatologies of surface buoyancy fluxes, Ekman buoyancy transport, and upper stratification. The strength of the upper-ocean stratification is determined using the columnar buoyancy index, defined as the buoyancy input necessary to produce a 250 m deep mixed layer. It is found that the DMB lies precisely where the autumn–winter buoyancy loss exceeds the columnar buoyancy found in late summer. The buoyancy loss decreases towards the south, while in the north the stratification is too strong to produce deep mixed layers. Although this threshold is also crossed in the Agulhas Current and East Australian Current regions, advection of buoyancy is able to stabilise the stratification. The Ekman buoyancy transport has a secondary impact on the DMB extent due to the compensating effects of temperature and salinity transports on buoyancy. Changes in surface temperature drive spatial variations in the thermal expansion coefficient (TEC). These TEC variations are necessary to explain the limited meridional extent of the DMB. We demonstrate this by comparing buoyancy budgets derived using varying TEC values with those derived using a constant TEC value. Reduced TEC in colder waters leads to decreased winter buoyancy loss south of the DMB, yet substantial heat loss persists. Lower TEC values also weaken the effect of temperature stratification, partially compensating for the effect of buoyancy loss damping. TEC modulation impacts both the DMB characteristics and its meridional extent.

Formation and Variability of Barrier Layer in the South Pacific

AbstractThe oceanic barrier layer (BL), dominated by salinity stratification, plays an important role in regulating heat exchange between the mixed layer and the interior ocean. However, the salinity contribution is ignored or underestimated when analyses relying only on temperature threshold methods. Here we introduce the Turner Angle method to quantify the salinity contribution and to obtain relatively accurate BL thickness (BLT), and further to investigate the spatial‐temporal distribution, thickness variation and formation mechanism of the BL in the South Pacific using the updated Argo data during 2005–2021. With the successful usage of this method, we eliminate 41% of the invalid BLs and therefore improve the assessment of the BLs in the South Pacific. Formation of the thick BLs in the South Pacific is attributed to the faster deepening rate of the isothermal layer than the mixed layer. The deepening rate of the isothermal layer is facilitated by subsurface subantarctic mode water while that of the mixed layer is constrained by the subsurface salinity maximum in the South Pacific. On the interannual scale, the BLT in the South Pacific manifests a quasi‐biennial variability, which is closely connected with the variability in the isothermal layer depth and is primarily caused by the anomalous winds driven by the Southern Annular Mode (SAM). The SAM‐related anomalous winds regulate the isothermal layer and further the BL by exerting influences air‐sea heat fluxes and the Ekman layer advection.

Unique transport paths of 137Cs from the Indian to Southern Oceans

To assess ocean-scale transport systems, we examined the latitudinal cross-sectional distribution of 137Cs activity concentrations in the Indian and Southern Oceans between December 2019 and January 2020 using low-background γ-spectrometry. At 0°–20°S, 137Cs concentrations exhibited a gradual decrease below the mixing layer (1–0.1 mBq/L). However, the concentrations steeply decreased toward the Southern Ocean along a transect of 30°–60°S (from 0.8 to 0.02 mBq/L) with minor vertical variation at each site. For the 137Cs inventories (0–800 m depth) from 15 to 600 Bq/m2, a maximum value was recorded at 30°S, indicating the downwelling of 137Cs as a reservoir for the Subantarctic Mode Water. The significantly low concentrations (0.02 mBq/L) at 60°S suggest minimal transport of 137Cs to the Southern Ocean. These findings assist in understanding 137Cs circulation patterns and provide valuable insights into the transport pathways of soluble contaminants.

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.

Pulses of South Atlantic water into the tropical North Atlantic since 1825 from coral isotopes.

Decadal and multidecadal changes in the meridional overturning circulation may originate from either the subpolar North Atlantic or the Southern Hemisphere. New records of carbon and oxygen isotopes from an eastern Martinique Island (Lesser Antilles) coral reveal irregular, decadal, double-step events of low ∆14C and enhanced vertical mixing, high δ18O and high δ13C values starting in 1885. Comparison of the new and published ∆14C records indicates that the last event (1956-1969) coincides with a widespread, double-step ∆14C low of South Atlantic origin from 32°N to 18°S, associated with a major slowdown of the Caribbean Current transport between 1963 and 1969. This event and the past Martinique ∆14C lows are attributed to pulses of northward advection of low ∆14C Sub-Antarctic Mode Waters into the tropical Atlantic. They are coeval with changes of the tropical freshwater budget and likely driven by meridional overturning circulation changes since ~1880.

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.

Biotic response of plankton communities to Middle to Late Miocene monsoon wind and nutrient flux changes in the Oman margin upwelling zone

Abstract. Understanding past dynamics of upwelling cells is an important aspect of assessing potential upwelling changes in future climate change scenarios. Our present understanding of nutrient fluxes throughout the world's oceans emphasizes the importance of intermediate waters transporting nutrients from the Antarctic divergence into the middle and lower latitudes. These nutrient-rich waters fuel productivity within wind-driven upwelling cells in all major oceans. One such upwelling system is located along the Oman margin in the western Arabian Sea (WAS). Driven by cross-hemispheric winds, the WAS upwelling zone's intense productivity led to the formation of one of the most extensive oxygen minimum zones known today. In this study covering the Middle to Late Miocene at Ocean Drilling Program (ODP) Site 722, we investigate the inception of upwelling-derived primary productivity. This study presents new plankton assemblage data in the context of existing model- and data-based evidence constraining the tectonic and atmospheric boundary conditions for upwelling in the WAS. With this research, we build upon the original planktonic foraminifer-based research by Dick Kroon in 1991 as part of his research based on the ODP LEG 117. We show that monsoonal winds likely sustained upwelling since the emergence of the Arabian Peninsula after the Miocene Climatic Optimum (MCO) ∼ 14.7 Ma, with fully monsoonal conditions occurring since the end of the Middle Miocene Climatic Transition (MMCT) at ∼ 13 Ma. However, changing nutrient fluxes through Antarctic Intermediate and sub-Antarctic Mode Waters (AAIW/SAMW) were only established after ∼ 12 Ma. Rare occurrences of diatom frustules correspond to the maximum abundances of Reticulofenestra haqii and Reticulofenestra antarctica, indicating higher upwelling-derived nutrient levels. By 11 Ma, diatom abundance increases significantly, leading to alternating diatom blooms and high-nutrient-adapted nannoplankton taxa. These changes in primary producers are also well reflected in geochemical proxies with increasing δ15Norg. values (> 6 ‰) and high organic carbon accumulation. These proxies provide further independent evidence for high productivity and the onset of denitrification simultaneously. Our multi-proxy-based evaluation of Site 722 primary producers provides evidence for a stepwise evolution of Middle to Late Miocene productivity in the western Arabian Sea for the first time. The absence of a clear correlation with existing deep marine climate records suggests that both local wind patterns and intermediate water nutrient changes likely modulated productivity in the western Arabian Sea during the Middle to Late Miocene. Finally, we show that using a multi-proxy record provides novel insights into how plankton responded to changing nutrient conditions through time in a monsoon-wind-driven upwelling zone.

Quantifying the contribution of temperature, salinity, and climate change to sea level rise in the Pacific Ocean: 2005-2019

In recent decades, Pacific Ocean’s steric sea level anomaly (SSLA) has shown prominent patterns among global sea level variations. With ongoing global warming, the frequency and intensity of climate and sea level changes have increased, particularly in the tropical Pacific region. Therefore, it is crucial to comprehend the overall trends and mechanisms governing volumetric sea level changes in the Pacific. To accurately quantify the spatiotemporal evolution characteristics of density-driven sea level change in the Pacific Ocean (PO) from 2005 to 2019, we decomposed temperature and salinity into linear trends, interannual variations, seasonal variations, and residual terms using the STL (seasonal-trend decomposition based on loess) method. To evaluate the influence of ocean temperature, salinity, and climate change on density-driven sea level change and its underlying mechanisms, we decompose temperature as well as salinity changes through into the Heaving (vertical displacements of isopycnal surfaces) and Spicing (density-compensated temperature and salinity change) modes. The findings reveal an average steric sea level rise rate of 0.34 ± 0.16 mm/yr in the PO from 2005 to 2019. Thermosteric sea-level accounts for 82% of this rise, primarily due to seawater temperature rise at depths of 0-700 m caused by Heaving mode changes. Accelerated SSLA increase via the thermosteric effect has been connected to interactions between greater Ekman downwelling from surface winds, radiation forcing linked to global greenhouse gases, and changes in the Pacific warm currents triggered by El Niño-Southern Oscillation (ENSO) episodes. Although salinity is affected by the Subantarctic Mode Water (SAMW) and the Antarctic Intermediate Water (AAIW) in the southern Indian Ocean, however the significance of salinity in sea level change is little compared to the role played by thermocline shift. This study offers a substantial contribution to the field, providing robust data and technical support, and facilitating a deeper understanding of the mechanisms underlying the effects of temperature and salinity on sea level changes during periods of rapid climate change, thus enhancing the accuracy of future predictions regarding sea level rise.

Quasi‐Decadal Temperature Variability in the Intermediate Layer of Subtropical South Indian Ocean During the Argo Period

AbstractIt has been reported that the subtropical South Indian Ocean (SIO) has been rapidly warming over the past two decades and can therefore be characterized as one of the major heat accumulators among the oceanic basins. However, this strong warming is not uniformly distributed in the vertical direction. In comparison to the decade‐long warming in the upper layer (0–300 m) in 2004–2013, the intermediate layer (300–1,000 m) displays a shorter warming during 2004–2009 and an intense cooling during 2010–2016. By decomposing temperature variations into heaving and spice components, and performing a heat budget analysis, we show that temperature variations in the intermediate layer during these two periods are primarily contributed by isopycnal migrations driven by local wind forcing. Local wind change in the subtropical SIO can be explained by the Indian Ocean Dipole and El Niño–Southern Oscillation during 2004–2016, while Southern Annular Mode (SAM) favors anomalous wind change in mid‐latitudes and the formation of basin‐wide wind change in the SIO. Additionally, wind forcing in the Subantarctic Mode Water (SAMW) formation region, which is closely linked to the SAM, modulates the anomalous spreading of SAMW into the interior of the subtropical SIO. This, therefore, leads to the SAMW intrusion being of secondary importance to the quasi‐decadal temperature variability. Our findings demonstrate the independence of wind‐driven temperature changes on the quasi‐decadal scale in the intermediate layer of the subtropical SIO under the overall warming background of SIO waters.

Decadal Reorganization of Subantarctic Mode Water

AbstractSubantarctic Mode Water (SAMW) is one of the most important water masses globally in taking up anthropogenic heat and carbon dioxide. However, its long‐term changes in response to varying climatic conditions are not well understood. We use an ocean state estimate to analyze SAMW volume budgets for the period 1992 to 2017. They reveal a decadal SAMW volume reorganization comparable to the long‐term trend in Indian Ocean, and a multi‐decadal volume reorganization exceeding the long‐term trend in the Pacific. In both sectors, the SAMW reorganization exhibits a two‐layer density structure, with compensating volume changes of lighter and denser SAMW, driven by heat flux changes in the Indian sector and central Pacific and freshwater flux changes in the southeast Pacific. This variability is governed by a cumulative effect of surface flux anomalies associated with the Interdecadal Pacific Oscillation. Shorter‐term trends observed during the Argo period are largely explained by this variability.

Surface and thermocline ocean circulation intensity changes in the western Arabian Sea during ∼172 kyr

The relationship between monsoon-induced productivity and ocean circulation over millennial timescales is crucial. However, a study regarding the circulation of the western Arabian Sea (WAS) needs to be conducted. Understanding the seasonal, surface, and thermocline ocean circulation throughout the glacial-Interglacial period in the WAS is also elusive and essential. The long distant Antarctic Intermediate Water (AAIW) flow, and Sub-antarctic Mode Water (SAMW) penetration to the north side of the equator in the Indian Ocean needs to be understood and underlined using the micropaleontological (planktonic foraminiferal assemblages) and geochemical analysis such as oxygen isotope of Globigerinoides ruber and Neogloboquadrina dutertrei (δ18OG.ruber and δ18ON.dutertrei) in this study. Furthermore, understanding the variations in paleoproductivity, upper oceanic water column stratification, surface mixing, and thermocline layer variability in the WAS is also helpful. According to the results, the variation in the upwelling intensity during Glacial-Interglacial and monsoon variation suggest the upper water stratification/vertical mixing in the WAS. Here, we provide new evidence of poorly ventilated water mass in our region. Our results indicate that enhanced northward expansion of AAIW coincides with the weakening of summer monsoon during glacial periods except mid-MIS4, reflected by isotopic study and assemblages. In this context, depleted δ18ON. dutertrei value and high concentration of Globorotalia menardii (G. menardii) were due to lack/absence of AAIW ventilation and SAMW; therefore, warming occurred in the mid-MIS4.

Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities.

Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities.

Rice husk as a potential source of silicate to oceanic phytoplankton

Global oceans are witnessing changes in the phytoplankton community composition due to various environmental stressors such as rising temperature, stratification, nutrient limitation, and ocean acidification. The Arabian Sea is undergoing changes in its phytoplankton community composition, especially during winter, with the diatoms being replaced by harmful algal blooms (HABs) of dinoflagellates. Recent studies have already highlighted dissolved silicate (DSi) limitation and change in Silicon (Si)/Nitrogen (N) ratios as the factors responsible for the observed changes in the phytoplankton community in the Arabian Sea. Our investigation also revealed Si/N < 1 in the northern Arabian Sea, indicating DSi limitation, especially during winter. Here, we demonstrate that rice husk with its phytoliths is an important source of bioavailable DSi for oceanic phytoplankton. Our experiment showed that a rice husk can release ∼12 μM of DSi in 15 days and can release DSi for ∼20 days. The DSi availability increased diatom abundance up to ∼9 times. The major benefitted diatom species from DSi enrichment were Nitzshia spp., Striatella spp., Navicula spp., Dactiliosolen spp., and Leptocylindrus spp. The increase in diatom abundance was accompanied by an increase in fucoxanthin and dimethyl sulphide (DMS), an anti-greenhouse gas. Thus, the rice husk with its buoyancy and slow DSi release has the potential to reduce HABs, and increase diatoms and fishery resources in addition to carbon dioxide (CO2) sequestration in DSi-limited oceanic regions such as the Arabian Sea. Rice husk if released at the formation site of the Subantarctic mode water in the Southern Ocean could supply DSi to the thermocline in the global oceans thereby increasing diatom blooms and consequently the biotic carbon sequestration potential of the entire ocean.

The Agulhas Current Transports Signals of Local and Remote Indian Ocean Nitrogen Cycling

AbstractThe greater Agulhas Current region is an important component of the climate system, yet its influence on carbon and nutrient cycling is poorly understood. Here, we use nitrate isotopes (δ15N, δ18O, Δ(15–18) = δ15N–δ18O) to trace regional water mass circulation and investigate nitrogen cycling in the Agulhas Current and adjacent recirculating waters. The deep and intermediate waters record processes occurring remotely, including partial nitrate assimilation in the Southern Ocean and denitrification in the Arabian Sea. In the thermocline and surface, tropically sourced waters are biogeochemically distinct from adjacent subtropically sourced waters, confirming inhibited lateral mixing across the current core. (Sub)tropical thermocline nitrate δ15N is lower (4.9–5.8‰) than the sub‐thermocline source, Subantarctic Mode Water (6.9‰); we attribute this difference to local N2 fixation. Using a one‐box model to simulate the newly fixed nitrate flux, we estimate a local N2 fixation rate of 7–25 Tg N.a−1, with the upper limit likely biased high. In the mixed layer, nitrate δ15N and δ18O rise in unison, indicating that phytoplankton nitrate assimilation dominates in surface waters, with nitrification restricted to deeper waters. Because nitrate assimilation and nitrification are vertically decoupled, the rate of nitrate assimilation plus N2 fixation can be used to approximate carbon export. Thermocline and mixed‐layer nitrate Δ(15–18) is low, due to both N2 fixation and coupled partial nitrate assimilation and nitrification. Similarly low‐Δ(15–18) nitrate in Agulhas rings indicates leakage of low‐δ15N nitrogen into the South Atlantic, which should be recorded in the organic matter sinking to the seafloor, providing a potential tracer of past Agulhas leakage.

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

Origin of surface water in the Southern Ocean: Implications of soluble radionuclide distributions

Concentration data of soluble radionuclides in the southern South Indian Ocean and Southern Ocean transition zone are rare or insufficient for the study of its current system. We examined the lateral surface variations in soluble natural (226Ra and 228Ra) and anthropogenic (134Cs and 137Cs) radionuclide activity concentrations in the surface waters in this area from November 2021 to March 2022. The surface distributions of 226Ra and 137Cs concentrations were classified into Subantarctic Mode Water-, Antarctic Intermediate Water-, and Upper Circumpolar Deep Water (UCDW)-dominated areas along latitudinal band (40°S–65°S, 110°E−120°E). Notably, the highest 226Ra concentrations occurred along the longitudinal band (60°S–65°S, 40°E−120°E). Significantly lower 137Cs concentrations in the Southern Ocean than those in surface waters in other global oceans were observed along with depletion of 228Ra. Additionally, 226Ra and 137Cs concentrations appeared to show small variations between eastern and western areas (2.5–3.0 mBq/L and 0.06–0.03 mBq/L, respectively). Lateral profiles in the Southern Ocean are governed by a large contribution from deep/old waters (e.g., UCDW), with a small effect from southward transport of Subantarctic Mode Water.

Physical and biogeochemical responses in the Southern Ocean to a simple parameterization of Langmuir circulation

Langmuir circulation (LC) plays an important role in deepening the mixed layer, especially when the ocean is weakly stratified. LC parameterization has been known to improve the accuracy of the mixed-layer depth, sea surface temperature, and ocean ventilation in the Southern Ocean. However, changes in ocean dynamics and biogeochemical processes owing to LC mixing have rarely been investigated. In this study, we implemented LC parameterization to a physical–biogeochemical coupled model and examined the changes in water properties and circulation and their effects on biogeochemical processes in the Southern Ocean. The LC effect enhances the turbulent mixing length scale and turbulent kinetic energy, especially in the subantarctic region, resulting in the deepening of the mixed layer. Increased vertical mixing causes deeper warm and saline water to reach the surface layer and weakens the surface meridional velocity by transferring momentum deeper. The weakening of the meridional velocity decreases equatorward freshwater transport, causing the surface water to become saltier and denser north of 50°S adjacent to the formation site of Subantarctic Mode Water, which enhances ocean ventilation eventually. Additionally, the weak equatorward velocity drives the retreat of sea-ice south of 60°S, leading to cooler and fresher water in the southern region of 60°S. Changes in the sea-ice distribution, mixed-layer depth, and dissolved inorganic carbon distribution alter the air-sea CO2 exchange, suggesting that the LC parameterization in the model can ameliorate 10% of the air-sea CO2 exchange. The LC effects on biogeochemical tracers, such as iron (Fe) and dissolved inorganic carbon, are mainly determined by changes in ocean circulation, and the modest improvement in primary productivity is mainly attributed to the changes in the Fe distribution in the high-nutrient, low-chlorophyll region, where Fe availability limits primary production. Although the direct LC effects occur near the surface, the altered meridional circulation spreads the effects to a deeper layer and improves the overall representation of physical, biogeochemical tracers and air-sea CO2 exchange in the Southern Ocean, suggesting that a simple LC parameterization can improve the ocean model.

The Origin and Fate of Antarctic Intermediate Water in the Southern Ocean

Abstract Using observationally based hydrographic and eddy diffusivity datasets, a volume budget analysis is performed to identify the main mechanisms governing the spatial and seasonal variability of Antarctic Intermediate Water (AAIW) within the density range γn = (27.25–27.7) kg m−3 in the Southern Ocean. The subduction rates and water mass transformation rates by mesoscale and small-scale turbulent mixing are estimated. First, Ekman pumping upwells the dense variety of AAIW into the mixed layer south of the Polar Front, which can be advected northward by Ekman transport into the subduction regions of lighter-variety AAIW and Subantarctic Mode Water (SAMW). The subduction of light AAIW occurs mainly by lateral advection in the southeast Pacific and Drake Passage as well as eddy-induced flow between the Subantarctic and Polar Fronts. The circumpolar-integrated total subduction yields from −5 to 19 Sv (1 Sv ≡ 106 m3 s−1) of AAIW volume loss. Second, the diapycnal transport from subducted SAMW into the AAIW layer is predominantly by mesoscale mixing (2–13 Sv) near the Subantarctic Front and vertical mixing in the South Pacific, while AAIW is further replenished by transformation from Upper Circumpolar Deep Water by vertical mixing (1–10 Sv). Last, 3–14 Sv of AAIW are exported out of the Southern Ocean. Our results suggest that the distribution of AAIW is set by its formation due to subduction and mixing, and its circulation eastward along the ACC and northward into the subtropical gyres. The volume budget analysis reveals strong seasonal variability in the rate of subduction, vertical mixing, and volume transport driving volume change within the AAIW layer. The nonzero volume budget residual suggests that more observations are needed to better constrain the estimate of geostrophic flow and mesoscale and small-scale mixing diffusivities.