Mode Water Research Articles

A See‐Saw of Subduction Rate in the North Pacific Western and Eastern Subtropical Mode Water Formation Areas Induced by the Aleutian Low

AbstractSubduction of water masses serves as a critical linkage between the surface and subsurface ocean, playing a crucial role in redistributing heat, carbon, and nutrients in the ocean. This process significantly influences global climate and marine ecosystem dynamics. Analyzing five ocean reanalysis data sets, we reveal a distinctive see‐saw pattern in anomalous subduction rates between western and eastern Subtropical Mode Water (STMW) formation areas. This pattern is predominantly driven by variations in latent heat flux, which modulates the late‐winter mixed layer depth. We demonstrate that late‐winter Aleutian Low (AL) activity induces these anti‐phase variations through its impact on latent heat flux. Notably, the Pacific Decadal Oscillation (PDO) significantly influences this subduction see‐saw pattern. During PDO warm phases, AL southward shift intensifies the anti‐correlated variations. Conversely, during cold phases, AL northward shift weakens its influence on ESTMW area, resulting in a less stable anti‐phase relationship.

North Atlantic Subtropical Mode Water properties: intrinsic and atmospherically forced interannual variability

Abstract. This study investigates the contributions of the ocean's chaotic intrinsic variability (CIV) and atmospherically forced variability to the interannual fluctuations in the North Atlantic Subtropical Mode Water (STMW) properties. Utilizing a 0.25° regional 50-member ocean–sea-ice ensemble simulation driven by an original surface forcing method and perturbed initially, the forced variability in STMW properties is estimated from ensemble mean fluctuations, while CIV is determined from deviations around the ensemble mean within each member. The model successfully captures the main features of STMW, showing correct agreement with observation-based ARMOR3D data in terms of location, seasonality, mean temperature and volume, as well as interannual variance of STMW's main properties. CIV significantly impacts STMW, explaining 10 %–13 % and 28 %–44 % of the interannual variance of its geometric and thermohaline mean properties, respectively, with a maximum imprint on STMW temperature. Observation-based and simulated intrinsic-to-total variance ratios are mostly consistent, dispelling concerns about a signal-to-noise paradox. This study also illustrates the advantages of ensemble simulations over single simulations in understanding oceanic fluctuations and attributing them to external drivers, while also cautioning against overreliance on individual simulation assessments.

Long-term variation of the eddy kinetic energy in the Northeastern South China sea

The seasonal to interannual variability of eddy kinetic energy (EKE) in the Northeastern South China Sea (NE-SCS) has been widely studied and it is recognized that they are strongly related to the state of the Kuroshio pathway in the Luzon Strait. While, due to the lack of long-term observations and high-resolution simulations, the decadal change of EKE in NE-SCS remains unexplored. In this study, we show the EKE trend in the past ∼ 30 years in the NE-SCS by using satellite observation and global HYbrid Coordinate Ocean Model reanalysis with the Navy Coupled Ocean Data Assimilation. It is found that due to the weakening of the Kuroshio in the Luzon Strait since 1990 s, the Kuroshio shows an enhanced looping path in the NE-SCS, inducing stronger EKE in this region. Further analysis confirms that the energy transfer by baroclinic instability is dominant for the increasing of EKE, when the Kuroshio intrudes into the NE-SCS and brings more potential energy inside the circulation. The Kuroshio state along the Luzon Strait is the key for modulating the EKE in the NE-SCS. Furthermore, the long-term weakening of Kuroshio current along the Luzon strait during 1993–2020 is determined by the decreasing of subtropical mode water, corresponding to the positive phase of the Atlantic Multidecadal Oscillation. This study provides insight into the interaction between marginal sea (i.e., the SCS) and the open ocean (i.e., the western Pacific Ocean), finally linking to the global climate change.

Fixed-time fractional-order sliding mode controller with disturbance observer for U-tube steam generator

Water level control of a U-tube steam generator plays a critical and challenging role in ensuring safe and reliable operation for a pressurized-water reactor-type nuclear power plant, as both excessively high and low water level in the U-tube steam generator would affect the normal operation of the nuclear power plant and even lead to unplanned shutdown events. This work presents a fixed-time fractional-order sliding mode water level controller coupled with a fixed-time disturbance observer, aiming to further improve the water level control performance under full operating conditions, considering uncertainties and actuator saturation. The proposed scheme is formulatedbased on the popular Irving water level model with uncertainties and the pre-existing actual water level control framework, while at the same time incorporating the benefits of the state-of-the-art anti-windup techniques, disturbance observer-based feedforward compensation techniques, fractional-order calculus, sliding mode control techniques, and fixed-time stability theorem, such that the transient water level response and steady-state control accuracy can be further enhanced even in the presence of uncertainties and actuator saturation phenomenon. It is theoretically proved that under the proposed scheme, both the estimation error of the uncertainties and the water level control error can be driven to zero within a fixed time regardless of their initial values by Lyapunov’s theorem. Meanwhile, simulation studies, involving the comparisons with a real-world adopted water level controller and a pre-existing integer-order sliding mode water level controller coupled with an uncertainty estimator, reveal the feasibility and superiority of the proposed approach.

Spatiotemporal variations in vertical profiles of Fukushima-derived 137Cs in the Kuroshio-Oyashio confluence region from 2011 to 2018: Implications for local water mass dynamics and basin-scale circulations

Tracking the processes of the spread of Fukushima-derived 137Cs (137CsF) contributes to a better understanding of North Pacific water dynamics. In this study, the vertical distributions of 137Cs and 90Sr in the Kuroshio-Oyashio confluence region were investigated in May 2018, and 137CsF was separated from the background 137Cs by exploiting the constant global fallout 137Cs/90Sr ratio. To the north of 35°N, 137CsF peaked in the upper 100 m layer, whereas in and just south of the Kuroshio Extension (KE), 137CsF exhibited subsurface peaks at depths of 300–500 m. The T/S diagram indicated that the 137CsF maxima were distributed mainly within the range of lighter central mode water (L-CMW) during May 2018, even in and just south of the KE. We found that anticyclonic (cyclonic) eddies can promote (prevent) the intrusion of 137CsF into the ocean interior. In addition, the high activity of regional anticyclonic eddies in the upstream KE resulted in the modification of 137CsF-rich subtropical mode water (STMW) to L-CMW. Temporal changes in the 137CsF vertical profiles and inventories revealed that 137CsF in transitional and subarctic regions has increased since July 2014, implying the existence of additional sources of 137CsF after July 2014, whereas 137CsF in and just south of the KE has remained constant since July 2014, indicating that the 137CsF entrained by STMW has recirculated in the western subtropical gyre. The comparison between surface 137CsF concentrations in transitional and subarctic regions and those observed in Oyashio waters during 2018 did not support the return of 137CsF to our study area via the western or whole subarctic gyre by May 2018. In contrast, the sea surface height distributions from 2016 to 2017 provide clear evidence that the warm-core rings and quasistationary Isoguchi western jet generated from the Kuroshio Current and KE intruded into the transitional region and even into the subarctic region. Therefore, we concluded that a portion of the 137CsF that subducted into the subtropical western North Pacific during 2011–2012 have entered the transition zone and even the subarctic region since 2016. These results not only enhance our understanding of the protracted spread and fate of 137CsF in the North Pacific but also provide important insights into North Pacific water mass circulation and mixing patterns.

Transport and fate of Fukushima-derived 137Cs and 134Cs in the seawater of theNorthwest Pacific in 2015.

To understand the influence of the Fukushima accident on the Northwest Pacific, the distributions and transportations of 134Cs and 137Cs in the seawater in the Northwest Pacific in May and September 2015 were studied. The data showed that the Fukushima-derived 134Cs and 137Cs at some stations can still be distinguished from background level ~ 4years later. On the whole, the activities of 137Cs and 134Cs in seawater were decreasing from May to Sep 2015. But the increased inventories and the surface activities of 137Cs imply that there has ever been an extra 137Cs from offshore water transported to this study area (from 31° N to 27° N, 145° E to 152.5° E) in May 2015. The average activities of 137Cs in subtropical gyre area in south of KE were the highest and the least were to the east of Luzon Strait in 2015. In vertical direction, 137Cs in subtropical gyre area were mainly distributed at 100 ~ 500m layer and 137Cs only at 500m layer in this area showed an increasing trend from May to Sep 2015 which reflects more 137Cs were still penetrating to deeper layer of 500m from upper water. But they were almost not found below 1000m layer. It was associated with the subsurface transport of radiocesiums by Northwest Pacific Mode Water (NPMW) and the diffusion of mesoscale eddy. Different distribution characteristics of 137Cs existed between north of KE and south of KE. The low-temperature-low-salinity water mass likely to be the first Oyashio Intrusion was the main factor that resulted in higher 137Cs appearing at the upper 100m layers in north of KE.

Deeper and stronger North Atlantic Gyre during the Last Glacial Maximum

Subtropical gyre (STG) depth and strength are controlled by wind stress curl and surface buoyancy forcing1,2. Modern hydrographic data reveal that the STG extends to a depth of about 1 km in the Northwest Atlantic, with its maximum depth defined by the base of the subtropical thermocline. Despite the likelihood of greater wind stress curl and surface buoyancy loss during the Last Glacial Maximum (LGM)3, previous work suggests minimal change in the depth of the glacial STG4. Here we show a sharp glacial water mass boundary between 33° N and 36° N extending down to between 2.0 and 2.5 km—approximately 1 km deeper than today. Our findings arise from benthic foraminiferal δ18O profiles from sediment cores in two depth transects at Cape Hatteras (36–39° N) and Blake Outer Ridge (29–34° N) in the Northwest Atlantic. This result suggests that the STG, including the Gulf Stream, was deeper and stronger during the LGM than at present, which we attribute to increased glacial wind stress curl, as supported by climate model simulations, as well as greater glacial production of denser subtropical mode waters (STMWs). Our data suggest (1) that subtropical waters probably contributed to the geochemical signature of what is conventionally identified as Glacial North Atlantic Intermediate Water (GNAIW)5–7 and (2) the STG helped sustain continued buoyancy loss, water mass conversion and northwards meridional heat transport (MHT) in the glacial North Atlantic.

Variations in the Central Mode Water in the North Pacific as a manifestation of the Pacific Decadal Oscillation

The North Pacific Central Mode Water (CMW) is examined from the viewpoint of its volume variations. The volume of the CMW layers thicker than 150m\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$150 m$$\\end{document} is used as an index of CMW variations, which successfully represents the year-to-year and decadal variations in the CMW volume. The CMW index shows the variation close to that of the Pacific Decadal Oscillation (PDO). The CMW variation is strongly tied with large-scale, dominant variations in sea surface temperature (SST), surface dynamic height (SDH), and sea surface height (SSH) anomalies in the North Pacific, with a significant correlation with the PDO. Year-to-year and decadal variations of CMW volume in May to July are significantly correlated with the wintertime Aleutian Low and 500hPa\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$hPa$$\\end{document} geopotential height variations, which indicates that the Aleutian Low induces the eastward extension/retreat of the strong winter westerlies and resultant net surface heat flux anomaly over the CMW distribution region. Thus, the CMW volume variation can be regarded as a significant manifestation of the PDO. The SST, SDH, and SSH anomalies are associated with the surface cooling in the northern sector of the CMW distribution region. On the other hand, the SDH and SSH anomalies throughout a year and the SST anomaly in the cold season in the southeastern sector of the CMW region are formed due to the heaving of isopycnal surfaces in the subsurface layer above the CMW in response to its volume variations.

Summer Hydrography Conditions at Proglacial Fjord Entrances Along East Greenland

AbstractClimate change is causing widespread melting of land and sea ice in the Arctic, leading to an increase in freshwater input to the ocean. This may have an impact on ocean circulation, marine ecosystems, and biodiversity in the region. Our findings provide new insights into the summer hydrography conditions at proglacial fjord entrances along large parts of the east coast of Greenland. As most freshwater exits the Arctic Ocean through Fram Strait this study focuses on understanding the origin of spatial distribution of temperature and freshwater in the coastal waters in East Greenland. We find that Polar Water and the Atlantic Water variants (Eurasian Basin Atlantic Water/Subpolar Mode Water) dominate coastal waters north and south of Denmark Strait, respectively, and that coastal waters are influenced by advected meltwater from the Arctic Ocean, glacier meltwater and local precipitation. Combining hydrographic observations, water isotope data, and water source fraction analysis, we show that glacier meltwaters can be detected at fjord entrances and down to approximately 200 m. Integrated over the upper 200 m of the water column, glacial meltwater account for approximately 9%, precipitation for around 8% and Polar Water (including sea ice melt) for approximately 83% of the freshwater observed along the East Greenland coast. Below 200 m, Atlantic Water variants dominate the entire coastal section. The freshwater along East Greenland stays close to the coast and we could not detect any glacier meltwater at the slope.

Mesoscale Ocean–Atmosphere Coupling Effects on the North Pacific Subtropical Mode Water

Abstract Subtropical mode water (STMW) is a thick layer of water mass characterized by homogeneous properties within the main pycnocline, important for oceanic oxygen utilization, carbon sequestration, and climate regulation. North Pacific STMW is formed in the Kuroshio Extension region, where vigorous mesoscale eddies strongly interact with the atmosphere. However, it remains unknown how such mesoscale ocean–atmosphere (MOA) coupling affects the STMW formation. By conducting twin simulations with an eddy-resolving global climate model, we find that approximately 25% more STMW is formed with the MOA coupling than without it. This is attributable to a significant increase in ocean latent heat release primarily driven by higher wind speed over the STMW formation region, which is associated with the southward deflection of storm tracks in response to oceanic mesoscale imprints. Such enhanced surface latent heat loss overwhelms the stronger upper-ocean restratification induced by vertical eddy and turbulent heat transport, leading to the formation of colder and denser STMW in the presence of MOA coupling. Further investigation of a multimodel and multiresolution ensemble of global coupled models reveals that the agreement between the STMW simulation in eddy-present/rich coupled models and observations is superior to that of eddy-free ones, likely due to more realistic representation of MOA coupling. However, the ocean-alone model simulations show significant limitations in improving STMW production, even with refined model resolution. This indicates the importance of incorporating the MOA coupling into Earth system models to alleviate biases in STMW and associated climatic and biogeochemical impacts. Significance Statement North Pacific subtropical mode water (STMW) is a distinct pycnostad within the main thermocline located south of the Kuroshio Extension. As short-term heat and carbon silos, STMW is traditionally thought to be driven by the basin-scale atmospheric forcing. The role of air–sea interactions at mesoscales residing in the Kuroshio Extension region has been overlooked. Here, we demonstrate that the strong thermal feedback of mesoscale sea surface temperature anomalies is not negligible for the STMW formation. This is achieved by accelerating wind and consequently promoting ocean latent heat release. Our results pinpoint the significance of accounting for the role of oceanic mesoscale feedback in improving the simulation of STMW as well as its climatic and biogeochemical impacts in Earth system models.

Disentangling Carbon Concentration Changes Along Pathways of North Atlantic Subtropical Mode Water

AbstractNorth Atlantic subtropical mode water (NASTMW) serves as a major conduit for dissolved carbon to penetrate into the ocean interior by its wintertime outcropping events. Prior research on NASTMW has concentrated on its physical formation and destruction, as well as Lagrangian pathways and timescales of water into and out of NASTMW. In this study, we examine how dissolved inorganic carbon (DIC) concentrations are modified along Lagrangian pathways of NASTMW on subannual timescales. We introduce Lagrangian parcels into a physical‐biogeochemical model and release these parcels annually over two decades. For different pathways into, out of, and within NASTMW, we calculate changes in DIC concentrations along the path (ΔDIC), distinguishing contributions from vertical mixing and biogeochemical processes. The strongest ΔDIC is during subduction of water parcels (+101 μmol L−1 in 1 year), followed by transport out of NASTMW due to increases in density in water parcels (+10 μmol L−1). While the mean ΔDIC for parcels that persist within NASTMW in 1 year is relatively small at +6 μmol L−1, this masks underlying dynamics: individual parcels undergo interspersed DIC depletion and enrichment, spanning several timescales and magnitudes. Most DIC enrichment and depletion regimes span timescales of weeks, related to phytoplankton blooms. However, mixing and biogeochemical processes often oppose one another at short timescales, so the largest net DIC changes occur at timescales of more than 30 days. Our new Lagrangian approach complements bulk Eulerian approaches, which average out this underlying complexity, and is relevant to other biogeochemical studies, for example, on marine carbon dioxide removal.

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.

Anomalous North Pacific subtropical mode water volume and density decrease in a recent stable Kuroshio Extension period from Argo observations

Abstract. North Pacific subtropical mode water (NPSTMW) is formed as a low-stratification water mass in the wintertime mixed layer south of the Kuroshio Extension (KE). In a recent period of 2018–2021, the KE jet was in a persistent stable dynamic state. But based on analysis of Argo observation, the mean volume of NPSTMW in the ventilation region dropped anomalously by ∼ 21 % during 2018–2021 relative to 2012–2015, when the KE jet was likewise stable. Moreover, the NPSTMW volume in a denser density range (approximately σθ>25.2 kg m−3) has started to decrease since 2018. The decreases in the NPSTMW subduction and formation rate are associated with anomalously shallow wintertime mixed-layer depth (MLD) and weak heat loss in the NPSTMW formation region. The decrease in air–sea heat exchange acts to weaken the vertical mixing and decrease the MLD, resulting in the weakening of subduction. The interannual variations in the air–sea heat exchange and wintertime MLD reflect the variability in the overlying atmosphere, which is correlated with a Pacific Decadal Oscillation (PDO) shift in 2018–2021. When the PDO shifted from its positive phase to a negative phase in the analysis period, the effects of local wind stress anomalies seemed to play an evident role in driving the variability in NPSTMW on interannual timescales. The MLD and heat loss change during the cold season in 2018–2021 were strongly coupled with the poleward shift of the westerlies – which cause weaker wintertime wind and easterly wind anomalies over the NPSTMW formation region. The declines in heat loss and southward Ekman transport, owing to the wind stress anomalies, further prohibit upper-ocean convection and mixed-layer deepening and cooling. Additionally, the insufficient development of wintertime MLD in 2018–2021 may also be associated with the significantly intensified preconditioning of near-surface stratification (< 150 m depth) due to the persistent near-surface warming and the weak vertical entrainment process in winter.

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.

Characterizing the stable oxygen isotopic composition of the southeast Indian Ocean

New seawater stable oxygen isotope (δ18O) samples were collected from the southeast Indian Ocean as part of the Coring to Reconstruct Ocean Circulation and Carbon dioxide Across 2 Seas (CROCCA-2S) expedition in November – December of 2018. These data fill a gap in the δ18O sampling coverage of the southern Indian basin, providing new insights into the hydrologic and oceanographic processes influencing the δ18O distribution of the region and its relationship to salinity in the upper ocean. Our surface ocean data (<100 m)—in combination with decades of observations from the broader south Indian Ocean—show distinct δ18O – salinity characteristics on either side of ∼85°E. The balance between evaporation and precipitation yields a strong, robust δ18O – salinity relationship west of 85°E (δ18O = 0.50(±0.01) * S – 17.2(±0.22)). However, within the mesoscale eddy field initiated by the Leeuwin Current further east (∼85–120°E), our observations fall along a mixing line between the southwest Indian Ocean and data collected from the Australian coastal margin, illustrating for the first time how the unique eastern boundary system of the south Indian Ocean drives regional-scale variability in the δ18O – salinity relationship of the surface ocean. A comparison between our observations in the shallow subsurface (100–1000 m) and those from neighboring surveys reinforces this upper ocean connection across the Indo-Australian basin. Antarctic Intermediate Water from the Indian Ocean can be isotopically distinguished from the more regional Tasman Intermediate Water occupying the South Australian Bight, suggesting exchange between the two regions is most prevalent at surface and mode water depths. In deeper waters (> ∼1500 m), we observe a notable 0.87‰ spread in δ18O. This variability may represent interactions between distinct deep water masses in the region, although additional data are needed to confirm. Overall, our data provide a new look at the hydrography and isotopic chemistry of the southeast Indian Ocean, emphasizing the impact of the region's mesoscale eddy field and its interconnectivity with neighboring basins.

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.

On the Spatio‐Temporal Scales of the Subtropical Mode Water Formation in the South Atlantic

AbstractWe examined the changes in the ocean’s upper layer structure involved in the formation of the subtropical mode water in the South Atlantic. Here we present the results from a survey done in the region of formation between 37°W and 32°W and 34°S and 37°S from July–October 2018, using high‐resolution measurements obtained from an underwater glider. From its records, the mode water development was observed for the first time in the South Atlantic as localized chimney‐like patterns in the temperature, salinity, and potential vorticity (PV) measurements with a horizontal scale of (7 ± 2) km, associated with wintertime oceanic convective processes. Over time, these submesoscale structures tend to expand and dominate the region as a large volume of mode water. These vertical structures present highly homogeneous temperature and salinity values. The observed PV was on the order of 5.0 × 10−11 m−1 s−1, which is one order of magnitude smaller than the pre‐existent mode water at the lower layers; negative values were observed, indicating active convection. A 1‐D vertical model was used to predict the depth of the mixed layer conditioned by the surface fluxes and triggered by its changes in winter to late spring. Comparisons of the sea surface temperature measured from satellite collocated with the glider’s measurements showed a very good agreement. From this analysis, we were able to establish the decorrelation scales that characterize mode water formation. Further inspection of the glider data yielded temporal decorrelation scales indicative of convection modified by planetary rotation.

A study on the pathways and their interannual variability of the Fukushima-derived tracers in the northwestern Pacific

This study investigates that the subsurface pathways, travel time, and its interannual variability of Fukushima-derived tracers subducted with the North Pacific subtropical mode water (NPSTMW) using 22-year-long (1994–2015) eddy-resolving (1/12°) and eddy-permitting (1/4°) ocean reanalysis. The NPSTMW is a thick subsurface layer with low potential vorticity and relatively uniform potential density, making it a key indicator of the North Pacific oceanic conditions. A series of Lagrangian particle tracking simulations quantitatively revealed that the Fukushima-derived particles moved along the Kuroshio Extension (KE) and spread over the majority of the subtropical region in the northwestern Pacific within 4–5 years. Approximately 36% of the particles flowed eastward in the Kuroshio-Oyashio transition zone (KO) and thereafter re-emerged to the sea surface at the remote area (near dateline), and 30% of particles moved along the KE. The remaining 34% subducted into NPSTMW layer and then widely spread out to the subtropical region along the re-circulation gyre (RG), exhibiting a subsurface pathway during entire particle tracking. When the particles were released, their pathway was immediately determined, whether it flowed along the KO (&amp;gt;36°N), KE (30°–36°N), or RG (&amp;lt;30°N). Furthermore, the interannual variability of the pathways was significantly associated with the dynamic states of KE, such as the path length of the Kuroshio jet. This result implies that understanding the subsurface dynamics and its variability of the KE and NPSTMW is crucial for predicting the dispersion of radioactive materials in the subsurface layer and its potential impact.

Surface Heat Fluxes Drive a Two‐Phase Response in Southern Ocean Mode Water Stratification

AbstractSubantarctic mode waters have low stratification and are formed through subduction from thick winter mixed layers in the Southern Ocean. To investigate how surface forcing affects the stratification in mode water formation regions in the Southern Ocean, a set of adjoint sensitivity experiments are conducted. The objective function is the annual‐average stratification over the mode water formation region, which is evaluated from potential temperature and salinity adjoint sensitivity experiments. The analysis of impacts, from the product of sensitivities and forcing variability, identifies the separate effects of the wind stress, heat flux, and freshwater flux, revealing that the dominant control on stratification is from surface heat fluxes, as well as a smaller effect from zonal wind stress. The adjoint sensitivities of stratification to surface heat flux reveal a surprising change in sign over 2 years lead time: surface cooling leads to the expected initial local decrease in stratification, but there is a delayed response leading to an increase in stratification. This delayed response in stratification involves effective atmospheric damping of the surface thermal contribution, so that eventually the oppositely‐signed advective haline contribution dominates. This two‐phase response of stratification is found to hold over mode water formation regions in the South Indian and Southeast Pacific sectors of the Southern Ocean, where there are strong advective flows linked to the Antarctic Circumpolar Current.

Interannual Variability of Subpolar Mode Water in the Subpolar North Atlantic

AbstractSubpolar Mode Water (SPMW) is an important water mass originating in the eastern North Atlantic. Its formation, subject to modification through oceanic interior mixing, can directly influence the volume of water contributing to the Atlantic meridional overturning circulation. Utilizing observation‐based data sets spanning from 1993 to 2018, we estimated the formation rates and volume of SPMW within isopycnal layers and examined its temporal variability. Two complementary approaches were used to estimate the formation rate: a thermodynamic approach focusing on the air‐sea interactions and a kinematic approach involving volume transport from the mixed layer to the ocean's interior, including the entrainment/detrainment of the mixed layer itself. This is the first time that thermodynamic and kinematic approaches are applied to observation‐based data in the North Atlantic. Our results suggest a substantial role of diapycnal mixing in diluting the dense waters formed by air‐sea fluxes toward the range of SPMW densities. The study reveals a complex interplay of processes, with entrainment being the primary driver of subduction/obduction rates, while advection contributes to the overall small‐scale dynamics. Variations in the volume and location of SPMW formation are observed from year to year. Notably, when SPMW forms extensively in lighter isopycnal layers, the volume occupied by denser isopycnals decreases and vice versa. We attributed this compensation effect to a propagation signal, where formation in the lightest isopycnal bins influences the formation in denser isopycnal bins with a delay of a few years, emphasizing the circulation's role in shaping the SPMW distribution.