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Return rates of Atlantic salmon (Salmo salar) from the sea to European rivers have declined in recent decades. The first months at sea are critical for growth and survival; recent evidence suggests that reduced food availability may be a contributory factor to the observed declines. Here, zooplankton abundance data are used to derive a measure of prey energy available to forage fish prey of salmon during early marine migration. This zooplankton prey energy has significantly and dramatically declined over much of the northeast Atlantic, and specifically within key salmon migration domains, over the past 60 years. Marine return rates from a set of southern European populations are found to exhibit clustering not entirely predictable from geographical proximity. Variability in grouped return rates from these populations is correlated with zooplankton energy on a range of scales, demonstrating the potential use of zooplankton energy as an indicator of salmon marine survival. Comparison with environmental variables derived from ocean model reanalysis data suggests zooplankton energy is regulated by a combination of climate change impacts on ecosystem productivity and multi-decadal variability in water mass influence along the migration routes. [ABSTRACT FROM AUTHOR]

Retention of surface water parcels can benefit coastal ecosystems by increasing the residence times of nutrient‐rich waters and marine larvae but can also negatively impact marine life and human health by concentrating oil and other pollutants. To investigate the spatial patterns, temporal variability, and drivers of retention, surface water parcel retention is quantified using particle simulations forced by high‐frequency radar surface current observations in the Santa Barbara Channel (SBC), California, USA. Retention is defined here as the time a particle remains within 20 km of its starting location and typical retention times are ∼4 days. However, the mechanisms driving retention differ across the SBC. In the central SBC, high retention times are driven by a persistent cyclonic eddy, while in the eastern SBC, high retention times are due to weak oscillatory flow. Areas in the western SBC outside of the cyclonic eddy exhibit the shortest mean retention times (<2 days) due to sustained horizontal advection. Stepwise regression is used to assess the drivers of interannual retention anomalies at sub‐SBC scales and is able to predict the greatest (least) amount of retention variability in the western (eastern) SBC. The presence of coherent eddies, both cyclonic and anticyclonic, is a dominant driver of high retention channel‐wide, while some factors such as wind stress and along‐channel flow have counteracting effects on retention at sub‐channel scales. Plain Language Summary: Surface water retention, the amount of time water stays within a certain region, is an important physical property which has significant consequences in the ocean. On one hand, the retention of water masses can positively affect coastal ecosystems by retaining marine larvae and high‐nutrient waters. On the other, it can have negative impacts on marine life and human health by concentrating oil and other pollutants. Through the analysis of surface current measurements, we observe distinct spatial patterns of retention in the Santa Barbara Channel, California, USA. We find that high retention occurs in areas where surface waters are recirculated often, and areas where surface transport is slow and variable. Low retention occurs in areas where the export of surface waters is quick and consistent in direction. We find that retention varies from year to year, and that the forces which drive this variability have contradicting effects from one part of the channel to the next. Key Points: Retention is quantified using particle simulations forced by high‐frequency radar surface current observationsBoth cyclonic and anticyclonic mesoscale eddies increase retention times across the Santa Barbara ChannelWind stress, along‐channel flow, and eddy circulation have counteracting effects on retention at sub‐channel scales [ABSTRACT FROM AUTHOR]

This study investigates the biogeochemical drivers of aragonite saturation state (ΩAr) in Baffin Bay, with a focus on the relatively undersampled west Greenland shelf. Our findings reveal two main depth‐dependant processes controlling the spatial distribution of ΩAr in Baffin Bay; within the upper 200 m, lower ΩAr coincides with increasing fractions of Arctic‐outflow waters, while below 200 m organic matter respiration decreases ΩAr. A temporal analysis comparing historical measurements from 1997 and 2004 with our 2019 data set reveals a significant decrease in the ΩAr of Arctic‐outflow waters, coinciding with reduced total alkalinity (TA). However, no discernible anthropogenic ocean acidification signal is identified. Significant Arctic water fractions (20%–40%) are found to be present on the west Greenland shelf, associated with reduced TA and ΩAr. A numerical modeling simulation incorporating a passive tracer demonstrates that periodic changes in wind direction lead to a switch from onshore to offshore Ekman transport along the Baffin Island current, transporting Arctic waters toward the west Greenland shelf. This challenges the conventional understanding of Baffin Bay's circulation and underscores the need for further research on the region's physical oceanography. Based on salinity‐TA relationships, surface waters on the west Greenland shelf have a significantly lower meteoric TA end‐member compared to waters of the Baffin Island Current in western Baffin Bay. The low eastern TA freshwater end‐member agrees well with recent glacial meltwater TA measurements, suggesting that glacial meltwater is the main freshwater source to surface waters on the west Greenland shelf. Plain Language Summary: Baffin Bay, with its complex interplay of Atlantic and Arctic water masses, is particularly susceptible to ongoing ocean acidification, mainly due to the presence of relatively fresh and low‐alkalinity Arctic waters. To date measurements of the inorganic carbon system in Baffin Bay have primarily been captured at key entrance and exit gateways, and on the Canadian side of the bay, leaving the west Greenland shelf relatively undersampled. This study provides a bay‐wide perspective of the main factors influencing aragonite saturation state (ΩAr; an indicator of ocean acidification impacts on marine calcifying organisms) across Baffin Bay, including the west Greenland shelf. We found that within the upper 200 m, low ΩAr coincides with increased presence of Arctic waters, while below 200 m the breakdown of organic matter decreases ΩAr. Historical data from 1997 and 2004 compared to our 2019 measurements show a significant drop in ΩAr of Arctic waters, but no clear signal of anthropogenic ocean acidification was observed. Surprisingly, the west Greenland shelf has significant fractions (20%–40%) of Arctic water. A numerical modeling simulation suggests that periodic changes in wind direction can transport Arctic waters from across Baffin Bay, challenging our traditional understanding of the bay's circulation. Key Points: Increased fractions of Arctic waters decrease aragonite saturation state (ΩAr) in the upper 200 m, while respiration does so below 200 mDecreased ΩAr of Arctic waters over the last two decades is associated with decreased alkalinity, not additional CO2Significant fractions of Arctic‐outflow waters on the west Greenland shelf challenges current knowledge of circulation in Baffin Bay [ABSTRACT FROM AUTHOR]

Sub-mesoscale and mesoscale (i.e., 1–10 km and 10–200 km, respectively) ocean processes are highly relevant for the understanding of global circulation, mixing of water masses and energy exchange between ocean layers. However, the processes happening at these scales are hard to be characterized using direct measurements of temperature and salinity. Direct measurements are obtained from vertical probes and/or autonomous vehicles, which, despite their high vertical resolution, are sparsely located in space and therefore unable to capture spatial details at these scales. Seismic oceanography (SO) data have been successfully used to imaging and characterize the ocean at these spatial scales. These data represent indirect measurements of the ocean temperature and salinity along kilometric transects with high horizontal resolution (i.e., a near-synaptic view of the system under investigation), but lower vertical resolution when compared with direct observations. Despite its complex oceanographic setting, the Madeira Abyssal Plain is still largely uncharacterized due to the lack of direct observations. We show for the first time a comprehensive processing, modelling and interpretation of three 2-D seismic oceanography sections from this region. The data show coherent seismic reflection in space, depth and time and shed light into this oceanographic setting with an unprecedent horizontal resolution. The SO modelling and interpretation are combined with existing direct measurements and a quantitative method to correlate thermohaline staircases interpreted from conductivity-temperature-depth casts and seismic reflections is proposed. The results show the relatively stable presence of thermohaline staircases in simultaneously time and space between 1200 and 2000 m of water depth and their spatial variability and contribute to the generalization of SO in physical oceanography. [ABSTRACT FROM AUTHOR]

Radiocarbon (14C) in corals can be used as a relatively high-sensitivity indicator of vertical and horizontal advection of water masses, which contributes to the understanding of ocean circulation. In this study, we reconstruct Kuroshio and Ryukyu current transport with a seasonal resolution Δ14C record spanning 1947-2009. This record covers the beginning of the atomic era and was obtained from a coral on Kikai Island in the south of Japan. The Kikai Δ14C curve features a newly discovered Δ14C spike in July 1955, a rapid increase after 1962, and a steady decrease after 1980. The spike in 1955 may directly reflect ocean current transport. The lack of periodicity in the Δ14C record suggests the existence of mesoscale eddies and the complexity of Kuroshio and Ryukyu current transport. In addition, comparing the high-resolution Δ14C of Kikai and Ishigaki islands, both situated along the path of the Kuroshio, reveals the influence of Pacific Decadal Oscillation and El Niño-Southern Oscillation on the Kuroshio and Ryukyu currents. This suggests that seasonally resolved Δ14C in corals along an ocean current can produce a long-term record of ocean mixing that responds to climate variability. [ABSTRACT FROM AUTHOR]

The Greenland Sea is a key region for open ocean convection and ventilation, which exhibit a large variability with periods of strong convection and shutdowns. After a long period of weak winter convection (from the 1970s to the early 1990s), a recovery has been reported, beginning in the 1990s and intensifying in the early 2000s until 2013. Using ISAS, an optimal interpolation product based on Argo observations, we document a recent significant weakening of deep convection between 2014 and 2020, accompanied by a continuous warming of the mixed layer but also a freshening after 2014. These hydrographic changes likely increase the ocean stratification and precondition the shutdown of winter convection. We suggest that these property changes result from a shift of the large scale atmospheric circulation, affecting the source of Atlantic Water to the Nordic seas, causing a freshening of about −0.1 g kg−1 that spreads into the Greenland Sea. Plain Language Summary: The Greenland Sea is a key region for the climate system. There, during winter, the ocean loses heat to the atmosphere in a process called "deep convection," that results in the formation of dense water masses that ventilate and fill the deeper layers of the ocean. The Greenland Sea exhibits a high variability in the intensity of winter deep convection, but the scientific community is still debating on which processes trigger or stop convection. After a recovery period of convection, mainly in the beginning of the 21st century, convection has stopped again after 2014 and at least until 2020. Here, we suggest that this new shutdown is mainly caused by changes in the ocean upper layer temperature and salinity that increase the upper ocean buoyancy, making it more difficult to trigger deep convection during winter. We further propose that the main mechanism driving the ocean properties changes is a shift of the large scale atmospheric circulation, which affects the amount and properties of Atlantic Water transported to the Greenland Sea. Key Points: After a period of intense deep convection since 2002, convection in the Greenland Sea has weakened from 2014 to 2020The upper salinity and temperature in the months prior of convection are driving the deep convection variabilityThe upper salinity changes are resulting from changes in large scale pattern of advection [ABSTRACT FROM AUTHOR]

Background: The coastal ocean's environment has changed owing to human activity, with eutrophication becoming a global concern. However, oligotrophication occurs locally and decreases fish production. Historically, the Secchi depth has been used as an index of primary productivity. We analyzed the results of over-a-half-century routine observations conducted in Sagami Bay and Tokyo Bay to verify the eutrophication/oligotrophication trend based on Secchi depth observations in a temperate coastal region near the Greater Tokyo area, which is highly affected by human activities. Methods: Data recorded in the Kanagawa Prefecture from 1963 to 2018 were used in this study. After quality control, the observation area was divided into Tokyo Bay, the Uraga Channel (outer part of Tokyo Bay), Sagami Bay (northern part), and Sagami Nada (southern part of Sagami Bay) based on temperature and salinity at a depth of 10 m. Because the environmental parameters showed autocorrelation, time-series and correlation analyses were conducted using generalized least squares (GLS) models with a Prais-Winsten estimator. Results: The Secchi depth was the shallowest in Tokyo Bay, followed by the Uraga Channel, Sagami Bay, and Sagami Nada, and was deep in winter (December and January), and shallow in summer (July) in all regions. The correlated analyses using the GLS model indicated that the shallowing of Secchi depth was significantly associated with decreases in temperature, salinity, and phosphate concentration. However, time-series analyses using GLS models indicated that the Secchi depth was significantly shallower, except in Tokyo Bay, where the surface temperature was significantly warming and the surface phosphate and nitrite concentrations decreased everywhere. A significant shallowing trend of the Secchi depth was mostly observed during the light-limiting season (January-March). Discussion: Correlation analyses suggested the importance of horizontal advective transport, particularly from Tokyo Bay, which has cold and less saline eutrophic water. However, long-term shallowing of the Secchi depth was associated with warming, and changes in salinity were not significant in most months when the Secchi depth trend was significant. Thus, horizontal advection is not the primary cause of long-term eutrophication. Because the eutrophication trend was primarily observed in winter, when light is the major limiting factor of primary production, we concluded that warming provides a better photoenvironment for phytoplankton growth and induces eutrophication. As a decline in anthropogenic nutrient input after 1990s was reported in the investigated area, the long-term eutrophication trend was most likely caused due to global warming, which is another alarming impact resulting from human activities. [ABSTRACT FROM AUTHOR]

The South Atlantic Ocean plays a key role in the heat exchange of the climate system, as it hosts the returning flow of the Atlantic Meridional Overturning Circulation (AMOC). To gain insights on this role, using data from three hydrographic cruises conducted in the South Atlantic Subtropical gyre at 34.5°S, 24°S, and 10°W, we identify water masses and compute absolute geostrophic circulation using inverse modeling. In the upper layers, the currents describe the South Atlantic anticyclonic gyre with the northwest flowing Benguela Current (26.3 ± 2.0 Sv at 34.5°S, and 21.2 ± 1.8 Sv at 24°S) flowing above the Mid‐Atlantic Ridge (MAR) between 22.4°S and 28.4°S (−19.2 ± 1.4 Sv), and the southward flowing Brazil Current (−16.5 ± 1.3 Sv at 34.5°S, and −7.3 ± 0.9 Sv at 24°S); the deep layers feature the southward transports of Deep Western Boundary Current (−13.9 ± 3.0 Sv at 34.5°S, and −8.7 ± 3.8 Sv at 24°S) and Deep Eastern Boundary Current (−15.1 ± 3.5 Sv at 34.5°S, and −16.3 ± 4.7 Sv at 24°S), with the interbasin west‐to‐east flow close to 24°S (7.5 ± 4.4 Sv); the abyssal waters present northward mass transports through the Argentina Basin (5.6 ± 1.1 Sv at 34.5°S, and 5.8 ± 1.5 Sv at 24°S) and Cape Basin (8.6 ± 3.5 Sv at 34.5°S–3.0 ± 0.8 Sv at 24°S) before returning southward (−2.2 ± 0.7 Sv at 24°S to −7.9 ± 3.6 Sv at 34.5°S), without any interbasin exchange across the MAR. In addition, we compute the upper AMOC strength (14.8 ± 1.0 and 17.5 ± 0.9 Sv), the equatorward heat transport (0.30 ± 0.05 and 0.80 ± 0.05 PW), and the freshwater flux (0.18 ± 0.02 and −0.07 ± 0.02 Sv) at 34.5°S and 24°S, respectively. Plain Language Summary: The location of the South Atlantic subtropical gyre plays a critical role in the Atlantic Meridional Overturning Circulation, which controls the Earth's climate system. We have examined the South Atlantic subtropical gyre's circulation between 34.5°S and 24°S and above the Mid‐Atlantic Ridge using data from three hydrographic cruises (34.5°S, 24°S, and 10°W). From these cruises, we have identified the water masses present on this region and computed their transport. Thus, we describe the path of the anticyclonic gyre and the northeast route of the Benguela Current in the upper water masses (<1,440 m depth); the southward flowing western and eastern boundary currents and an eastward interbasin flow close to 24°S above the Mid‐Atlantic Ridge in the deep water masses (those on the depth range from 1,440 to 3,800 m depth); and the northward flow of the abyssal water masses (>3,800 m depth). We have also reported the characteristic heat transport flowing northward across the subtropical South Atlantic Ocean, where evaporation dominates over precipitation. Key Points: The upper layers present the course of the anticyclonic South Atlantic subtropical gyre and the northeast route of the Benguela CurrentThe deep layers present the southward flowing boundary currents and an eastward interbasin flow close to 24°S above the Mid‐Atlantic RidgeThe heat transport flows northward across the subtropical South Atlantic Ocean, where evaporation dominates over precipitation [ABSTRACT FROM AUTHOR]

Abstract: Wintertime 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.

To investigate the diurnal variation in phytoplankton biomass and its regulating factors during the diurnal cycle, we conducted in situ observations in June 2018 at three buoy stations, including Douwei Buoy Station, Minjiang Estuary Buoy Station, and Huangqi Buoy Station on the western side of the Taiwan Strait. The calibration of buoy sensor data, including temperature, salinity, dissolved oxygen, pH, chlorophyll, and phycoerythrin, was conducted simultaneously. In addition, water sampling was conducted to measure chlorophyll a and phycoerythrin concentrations at hourly time intervals. The results showed that the 24 h cumulative chlorophyll a concentration order for the buoys was Minjiang Estuary (10.280 μg/L) > Huangqi (7.411 μg/L) > Douwei (4.124 μg/L). The Minjiang Estuary had a lower nighttime biomass proportion than Douwei and Huangqi. The diurnal variation in phytoplankton was jointly regulated by water masses, tides, and light. There were three response patterns, including the “light trumps tidal influences” pattern at Douwei, the “Low-tide, High-biomass” pattern at Minjiang Estuary, and the “High-tide, High-biomass” pattern at Huangqi. The prediction of algal blooms and hypoxia using buoy monitoring needs to be based on seasonal water mass background and tidal influence.

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

The southern Red Sea is a key region for investigating the effect of climate forces on a semi-closed basin. Detailed micropaleontological (planktic foraminifera) and isotopic (δ18Ο, δ13C) analyses along with reconstructions of sea surface temperature and salinity on a sediment core from the Farasan banks revealed the imprints of sea level changes and the South Asian Monsoon on the area. Comparison with surrounding records provided insights on the Red Sea-Gulf of Aden (GoA)-Northwestern Arabian Sea (NWArS) water exchange pattern over the last 30 ka. During glacial sea-level lowstand, flow of water from the GoA prevented hypersalinity in the southern Red Sea. Deglacial sea level rise improved water mass exchange between the NWArS, GoA and the entire Red Sea, resulting in relatively similar surface water conditions during the early Holocene when sea-level rise slowed. Thus, sea level change is the major driver of Red Sea δ18O variability. Southwest Monsoon (SWM), which was dominant during the late glacial and Early-Middle Holocene, enhanced surface productivity in the southern Red Sea. Northeast Monsoon (NEM) dominated around Heinrich stadial 1, as indicated by a nearly aplanktonic zone that was probably caused by restricted GoA inflow and low productivity. [ABSTRACT FROM AUTHOR]

Demographic connectivity is the process by which population persistence and growth are affected by the dispersal of individuals. In sessile marine species that disperse by planktonic larvae and form metapopulations, the interaction between oceanographic forcing and larval biology drives the spatial extent and magnitude of larval delivery to suitable habitat patches. The present study investigates the supply of Mytilus galloprovincialis larvae to rocky intertidal habitats located within and around two marine protected areas (MPA) in central Portugal, one located on the exposed coast and the other one in a sheltered bay, using time series of settlement intensity and oceanographic variables. The results indicate that larval delivery at short time scales (days) is strongly dependent on the interaction of upwelling circulation with coastline topography. In the exposed MPA, located on a coastal island, the magnitude of mussel larval delivery is limited by the dynamics of an upwelling front and filament that intercepts the island as upwelling increases and decreases. In the sheltered bay, under an upwelling shadow, the upwelling front promotes recirculation of the water mass, intensifying larval retention and settlement. The results are also consistent with larval transport distances of tens of kilometres and ongoing connectivity between the two MPA. This study highlights the relevance of coastal topography and its interaction with mesoscale hydrodynamics in the modulation of larval delivery, crucial for the persistence of populations and communities and in systematic conservation planning. • Topography regulates the formation and movement of upwelling fronts, jets and shadows, which determine settlement patterns. • Peniche is highly influenced by the upwelling jet from the north and the typical filament associated with pronounced capes. • This jet forms a front, which might act as a barrier for larval delivery to the RNB archipelago, which might receive larvae only under particular conditions. • At PNA, settlement is related to the formation of a front in that bay that is characterised as an upwelling shadow. [ABSTRACT FROM AUTHOR]

The Arabian Gulf is a very significant ocean body, which hosts more than 55% of the oil reserves of the world and produces about 30% of the total production, and thus, it is likely to face high risk and adverse problems by the intensified environmental stressors and severe climatic changes. Therefore, understanding the hydrography of the Gulf is very essential to identify various marine environmental issues and subsequently, developing marine protection and management plans. In this study, hydrography data collected at 11 stations along 3 linear transects in the early summer of 2016 were analyzed. The physicochemical parameters exhibited apparent variations along each transect, both laterally and vertically, connected to stratification, formation of different water masses and excessive heating. The temperature and salinity decreased laterally from nearshore to offshore, while layered density structures were identified in the offshore regions. The pH, dissolved oxygen (DO) and chlorophyll fluorescence (Fo) exhibited distinct horizontal and vertical variations. The observed pH is within the normal ranges, indicating that seawater acidification may not be a threat. The highest DO (6.13–8.37 mg/l) was observed in a layer of 24—36 m water depth in the deeper regions of the central transect.

The overturning streamfunction as measured at the OSNAP (Overturning in the Subpolar North Atlantic Program) mooring array represents the transformation of warm/salty Atlantic Water into cold/fresh North Atlantic Deep Water (NADW). The magnitude of the overturning at the OSNAP mooring array can therefore be linked to the water mass transformation by air--sea buoyancy fluxes and mixing in the region to the north of the OSNAP array. Here, we estimate these water mass transformations using a combination of observational-based, reanalysis-based and model-based datasets. Our results highlight the complementary roles of air--sea buoyancy fluxes and mixing in setting the time-mean magnitude of the overturning at OSNAP. A cooling by air--sea heat fluxes and a mixing-driven freshening in the Nordics Seas, Iceland Basin and Irminger Sea, precondition the warm/salty Atlantic Water, forming subpolar mode water classes. Mixing in the interior of the Nordic Seas, over the Greenland-Scotland ridge and along the boundaries of the Irminger Sea and Iceland Basin drive a water mass transformation that leads to the convergence of volume in the water mass classes associated with NADW. Air--sea buoyancy fluxes and mixing therefore play key and complementary roles in setting the magnitude of the overturning within the subpolar North Atlantic and Nordic Seas. This study highlights that for climate models to realistically simulate the overturning circulation in the North Atlantic, the small scale processes that lead to the mixing-driven formation of NADW must be adequately represented within the model's parameterisation scheme. [ABSTRACT FROM AUTHOR]

We investigated the recent warming of summer Atlantic Water in relation to the century-long trends in maximum temperature in three west Spitsbergen fjords: Isfjorden, Grønfjorden and Billefjorden. On the basis of repeated along-fjord transects in late summer 2003–2019, we found that the warming has been pronounced not only in the outer but also in the inner domain of Isfjorden, where the presence of waters of Atlantic origin was registered more frequently after 2011 compared to early 2000s. Furthermore, Atlantic waters occurred more frequently in the bottom layers in the inner part of the fjord. In all the investigated fjords, the year 2014 was the warmest and saltiest during the period 2003–2019, which is consistent with previous reports for other west Spitsbergen fjords. In 2014, the mean temperature and salinity in Isfjorden and Grønfjorden exceeded 4.9 °C and 34.7 (in Billefjorden, 4.0 °C and 34.67, respectively). With the new data for 2010–19, we extended the time-series of maximum Atlantic Water temperature in Isfjorden and Grønfjorden, covering 1912–2009, reported previously by Pavlov et al. 2013. For the period 1912–2019, the average long-term trend of Atlantic Water maximum temperature is 0.25 °C/decade and 0.22 °C/decade in the outer part of Isfjorden and Grønfjorden, respectively. In the first two decades of the 21st century, the warming trend is steeper compared to the 20th century, 0.78 °C/decade in Isfjorden and 0.56 °C/decade in Grønfjorden, highlighting the strength of the ongoing ‘Atlantification’ of west Spitsbergen fjords.

Seamounts affect the surrounding physical oceanography and form unique dynamic processes. The influences of these processes on biological and sedimentary distributions are quite different in seamount areas at different depths. The Y3 seamount is located in the Yap Arc of the tropical Western Pacific Ocean. The water depth of its summit is ∼280 m. Based on field data obtained in December 2014 and other open-access data, the physical oceanography around the Y3 seamount was preliminarily analyzed. The results show that the upper layer (0–150 m) was under the influence of the westward-flowing North Equatorial Current (NEC), while the eastward-flowing North Equatorial Undercurrent (NEUC) controlled the water between 200–800 m. The NEC was strong and steady, but the NEUC was disturbed by the Y3 seamount. The cold dome above the Y3 seamount was not caused by a Taylor cap or tidal rectification but probably by upwelling during the survey time. Tidal currents were squeezed against topography and greatly amplified in the Y3 seamount. The thicknesses of the surface turbulent layers were greatly influenced by the spring-neap tidal cycle. The turbulent diffusivities in the sea surface layer above the Y3 seamount were much larger than those in the open ocean. Calculations showed that the surface wind stress greatly affected the turbulent mixing in the surface layer of the Y3 seamount. The reciprocal action between the amplified tidal currents and topography was the most likely cause of the turbulent mixing near the bottom of the Y3 seamount. This study can provide a scientific basis for further study of biological and depositional characteristics at the Y3 seamount. [ABSTRACT FROM AUTHOR]

Mediterranean eddies (meddies) play an essential role in transferring heat, salinity and momentum into the Atlantic Ocean. The rate of heat (and salt) flux from the meddy and its ultimate lifetime are key proxies to understanding how meddies impact the redistribution of heat and salt in the ocean system. A Mediterranean eddy was observed in the Gulf of Caidz in 2007 using seismic and hydrographic data. The spatial distribution of turbulent dissipation rates around the meddy is estimated from the seismically derived internal wave spectra subrange using fine-scale parameterization. Turbulent dissipated rates are lowest ( 10 − 11   W / k g ) within the core of the meddy but rise by nearly two orders of magnitude at the upper and lower boundaries, where signs of double diffusive convection are observed. Along the left flank of the meddy, thermohaline intrusions and interleaving of water masses are found in inverted temperature and salinity profiles, transporting heat laterally from the warm core to the Atlantic water with a flux of around 470   W m − 2 . The meddy presented in this study is shown to decay in 2 years, primarily due to the heat loss associated with thermohaline intrusions. For the first time, heat fluxes around the meddy and its lifetime are quantified using seismic oceanography data, and the methods proposed here can be applied to more seismic datasets in the global oceans. [ABSTRACT FROM AUTHOR]

• The poleward shift of SB was concurrent with AABW volume reduction. • Reduction in westward ASC (>10 Sv) accompanied by a partial offshore shift of the 0 °C isotherm at 400 dbar. • CDW temperature increased near the shelf break (∼0.4 °C) beneath partial subsurface freshening. • Dense shelf water was found to the east of Mertz Polynya. Full-depth hydrographic sections of the BROKE experiment in 1996 (across the Antarctic margin from 80 to 150°E; Bindoff et al., 2000) were revisited for the first time during the 2018/2019 austral summer. We describe the subsurface physical oceanography in 2019 and the hydrographic changes between 1996 and 2019 not documented in earlier studies. The survey captured decadal changes in ocean structure from the southern flank of the Antarctic Circumpolar Current (ACC) to the continental shelves. In five cross-slope meridional sections, where 1996 and 2019 measurements are comparable (112, 120, 128, 140, and 150°E), the poleward shift of the southern boundary of the ACC (50–120 km) prevailed near the continental rise. The simultaneous displacement of barotropic ACC fronts and poleward migration of deep water contributed to full-depth warming (0.1–1.6 °C) and potentially to a reduction in the bottom water volume. Freshening was widely observed from the deep to bottom layers (∼0.02 g/kg), with the signal extending from the upper continental slope. Bottom-intensified freshening was accompanied by an oxygenation of 10–20 μmol/kg, indicating that freshening-driven oxygenation of bottom layers counteracted the deoxygenation effect of the poleward barotropic frontal shift. Westward transport of the Antarctic Slope Current decreased by more than 10 Sv from 1996 to 2019 in the five cross-slope sections; its frontal features and current axis shifted offshore by more than 20 km in 112–140°E. Additionally, subsurface warming along modified Circumpolar Deep Water by up to 0.4 °C was commonly detected across the upper continental slope. For the 2019 hydrography, shelf water sufficiently dense to form bottom water (>28.35 kg/m3) was found to the east of Mertz Polynya (142–148°E), implying a pathway for dense shelf water export from the eastern margin of Mertz Polynya. Our findings underscore the importance of sustained efforts for in-situ observations that widely cover the East Antarctic margin. [ABSTRACT FROM AUTHOR]

We provide a detailed description of the spatial distribution, seasonality and transformation of the main water masses within MacKenzie Polynya (MP) in Prydz Bay, East Antarctica, using data from instrumented southern elephant seals. Dense Shelf Water (DSW) formation in MP shows large spatial variability that is related to the (a) local bathymetry, (b) water column preconditioning from the presence/absence of different water masses, and (c) proximity to the Amery Ice Shelf meltwater outflow. MP exhibits sustained sea ice production and brine rejection (thus, salinity increase) from April to October. However, new DSW is only formed from June onward, when the mixed layer deepens and convection is strong enough to break the stratification set by Antarctic Surface Water above and Ice Shelf Water below. We found no evidence of DSW export from MP to Darnley polynya, as previously suggested. Rather, our observations suggest some DSW formed in Darnley Polynya may drain toward the western Prydz Bay. Then, DSW is exported offshore from Prydz Bay through the Prydz Channel. The interplay between sea ice formation, meltwater input, and sea floor topography is likely to explain why some coastal polynyas form more DSW than others, as well as the temporal variability in DSW formation within a particular polynya. Plain Language Summary: Coastal polynyas are regions of open water surrounded by sea ice. They form when strong winds from the Antarctic continent push newly formed sea ice away from the coast, as rapidly as it forms. Polynyas are therefore important sea ice factories. When sea ice forms, salt is released into the water below, increasing its salinity and density. The densest water in the World Ocean can be traced back to a few coastal polynyas along the Antarctic continent. This dense water formed in polynyas supplies the deep limb of a network of ocean currents that influences climate on global scales. Despite their importance, coastal polynyas remain poorly understood as they are difficult to reach and observe. Using data collected by instrumented elephant seals, we investigated seasonal changes in the MacKenzie Polynya in Prydz Bay, East Antarctica. Our study shows that dense water production is regulated by a complex interplay between three factors: strength of sea ice formation, the input of meltwater from ice shelves, and steering of the flow by sea floor topography. These new observations collected by seals contribute to a better understanding of dense water formation and the vulnerability of the global overturning circulation to future change. Key Points: High resolution sampling of seasonal water‐mass evolution in MacKenzie Polynya reveals processes regulating dense shelf water productionDense water formation is concentrated in the relatively shallow western polynya where topography blocks inflow of Ice Shelf WaterThe interplay between sea ice formation, meltwater input and topography regulates the amount of Dense Shelf Water formed in coastal polynyas [ABSTRACT FROM AUTHOR]

The present study investigates the characteristics and mechanism of intrathermocline eddy (ITE) formation in the northern Bay of Bengal (BoB) using observations from Research Moored Array for African–Asian–Australian Monsoon Analysis and Prediction (RAMA) buoys moored at 15°N, 12°N, and 8°N along 90°E and simulations from a high‐resolution numerical model. Among the RAMA locations, a relatively large number of anticyclonic ITEs are observed in the northern parts (15°N) compared to the other two (12°N and 8°N). The observed ITEs are characterized by a double‐convex shape of isotherms in the thermocline with a vertical extent of about 100–200 m and a positive sea level anomaly in the surface. Water mass properties show that Andaman Sea (AS) water is trapped inside the core of the ITEs. Trajectories of these ITEs (especially at 15°N) suggest that most of them originated in the vicinity of Preparis Channel, the northernmost passage between BoB and AS. Our study shows that vorticity developed downstream of Preparis Channel due to the strong subsurface flow (∼100 cm/s), which carries AS water into the BoB, results in the formation of observed anticyclonic ITE in the northern BoB. Further analysis revealed that this strong subsurface flow through the Preparis Channel is caused by the propagation of downwelling coastal‐trapped waves along the boundaries of BoB, which originates from the equatorial Indian Ocean. Our study suggests that ITEs could be an important pathway of water mass distribution in the intermediate layers in this region. Plain Language Summary: Intrathermocline eddies (ITEs) are subsurface‐intensified eddies that are often seen as double‐convex shapes in the thermocline. This study investigates the characteristics, variability, and formation of ITEs in the Bay of Bengal (BoB) using observations and numerical simulations. A relatively large number of ITEs in the BoB are observed in the northern part compared to the south. These eddies are characterized by the Andaman Sea (AS) water inside its core and tracks of these eddies suggest that they originate in the vicinity of the passage between BoB and AS, called Preparis Channel. It is found that most of these eddies are generated downstream of strong westward flow through the Preparis Channel caused by the propagation of coastal‐trapped waves from the equatorial Indian Ocean. This study suggests that the propagation of downwelling coastal‐trapped waves plays an important role in the formation of ITEs in the northern BoB and water mass exchange between AS and BoB. Key Points: A relatively large number of intrathermocline eddies in the Bay of Bengal are observed in the northern partObservations show that Andaman Sea water is trapped inside the core of the intrathermocline eddiesStrong flow through the Preparis Channel due to the propagation of coastal‐trapped waves causes the formation of intrathermocline eddies [ABSTRACT FROM AUTHOR]

Greenland's coastal zone encompasses a large number of fjords, many of which are impacted by glacial meltwater runoff from land‐terminating glaciers. This type of fjord has received limited research attention, yet may represent the future of other fjords currently impacted by marine‐terminating glaciers that are retreating. In this study we describe the seasonal hydrography of Ameralik, a fjord on the southwest coast of Greenland impacted by a land‐terminating glacier. To complement this analysis we compare our results with observations from the neighbouring Godthåbsfjord, which receives meltwater from both land‐ and marine‐terminating glaciers. We find that the absence of subglacial discharge and glacial ice in Ameralik has a strong impact on the inner fjord density profiles and on circulation. The mean temperature of the upper 50 m layer was lower in Ameralik than Godthåbsfjord in May, but by September was 2°C higher in Ameralik. Dense coastal inflows occur in the late winter months in Ameralik, flushing the fjord and contributing to the return to a weakly stratified state. During the runoff period the surface waters are subject to estuarine circulation and wind forcing, while at intermediate depths a density gradient between the inner and outer fjord regions produces an intermediate baroclinic circulation, resulting in the exchange of water in this layer and the deepening of isopycnals. During summer a large fraction of the meltwater runoff is retained within the fjord rather than being exported. A substantial export of this summer accumulated freshwater occurs in connection with coastal inflows during winter. Plain Language Summary: Marine‐terminating glaciers around Greenland are retreating, impacting the adjacent fjords with potential changes in the pathways taken by glacial meltwater. Few studies focus on fjords that receive meltwater only from land‐terminating glaciers, although knowledge of these systems can help us better understand how water from melting glaciers eventually ends up in the ocean in a future warmer climate. Here we present a full year of temperature and salinity observations from one such fjord on the southwest coast of Greenland and use these measurements to describe seasonal changes in water properties and circulation patterns. We compare the results with observations from a neighbouring fjord which receives meltwater from both land‐ and marine‐terminating glaciers. This comparison allows us to link differences observed between the fjords with the water movements associated with the type of glacier. While physical properties of the fjords, such as sill depth, play an important role in the circulation, the meltwater pathway also influences fjord hydrography, most prominently in the surface layers. This study will help us to understand the potential future state for many Greenland fjords with glaciers that are presently marine‐terminating. Key Points: We present seasonal hydrography from a fjord system (Ameralik) in southwest Greenland impacted by a land‐terminating glacierWe compare our observations with the neighbouring Godthåbsfjord, which receives meltwater from both land‐ and marine‐terminating glaciersA large fraction of the seasonal freshwater input is retained in the fjord in summer and autumn, and is exported primarily in winter [ABSTRACT FROM AUTHOR]

The identification of vortices in a fluid flow is a dynamically interesting problem that has practical applications in oceanography due to the outsized role eddies play in water mass, heat, and tracer transport. Here a new Eulerian scheme is developed to detect both vortices and strongly strained fronts, which are both ubiquitous in the World Ocean. The new scheme is conceptually linked to the well-known Okubo–Weiss parameter, but is extended to quasigeostrophic flows by recognizing the strong role played by vertical shear in ocean dynamics. Adapted from the λ 2 criterion for vortex identification, the scheme considers the curvature of the pressure field as the differentiator between vortical and strained flow structures, and it is shown that its underlying geometry also exhibits characteristics of quasigeostrophic flow. The uses and skill of the scheme are demonstrated using a high-resolution regional ocean simulation, and prospects for its use with observational products are discussed. [ABSTRACT FROM AUTHOR]

Because of the difficulties in tracking a water mass over time and conducting synoptic surveys over large spatial scales, measurements of biological variables in the ocean are often restricted to isolated Eulerian observations. Here a proof‐of‐concept of a pseudo‐Lagrangian transformation was applied to a time series of chlorophyll‐a profiles acquired at a single fixed station to diagnose the shape of a phytoplankton patch, and its physical‐biological rates of transformation. During August 2013, a 27‐hr time series of observations of horizontal currents, hydrographic properties, turbulent mixing and chlorophyll‐a was acquired at a central station of the Ría de Vigo (NW Iberian Peninsula). A chlorophyll‐a patch, tilted upward toward the inner part of the Ría, was observed moving back and forth past the sampling station. Its shape and position during the sampling period were modulated by the semi‐diurnal (M2) tidal currents and the vertically sheared upwelling circulation. The pseudo‐Lagrangian transformation allowed estimation of chlorophyll‐a net rates of change. These rates were positive between 12 and 14 m depth, and negative elsewhere, with a mean value of −0.001 ± 0.449 days−1 within the patch maximum. After accounting for the effects of diffusion and sinking, the mean net biological production rate in the upper 15 m (0.53 ± 0.25 days−1) was comparable with in vitro estimates of the balance between phytoplankton growth and mortality obtained from dilution experiments carried out under similar conditions (0.2 ± 0.7 days−1). This pseudo‐Lagrangian transformation is complementary to traditional analyses for the quantification of ecological processes and biogeochemical budgets. Plain Language Summary: Oceanography distinguishes two ways to make observations: Eulerian and Lagrangian. Eulerian approaches measure properties at fixed locations in space as a function of time as water flows past the instrument. In contrast, Lagrangian approaches quantify temporal changes in fluid parcels while following the flow. Isolated Eulerian observations are the most common approach to investigate biological properties. However, temporal variations in Eulerian observations result from combinations of physical and biological processes whose effects are often difficult to separate. We applied a pseudo‐Lagrangian methodology to reconstruct the spatial structure of a phytoplankton (microscopic algae) patch observed at a single station in a coastal embayment. Because the flow was dominated by tidal currents, some water parcels were sampled multiple times, allowing to compute the rate of change of properties in the phytoplankton patch. Biological production, or the rate of change due to biological processes, was comparable with traditional estimates from in vitro experiments performed in the region under similar hydrographic conditions. Our findings show that the pseudo‐Lagrangian transformation is a useful complement to data obtained from traditional analyses for the quantification of ecological processes and biogeochemical budgets. Key Points: A pseudo‐Lagrangian transformation was applied to diagnose the shape of a phytoplankton patchThe pseudo‐Lagrangian transformation allowed estimation of Chl‐a fluorescence net rates of changeNet biological production was comparable with in vitro estimates from dilution experiments under similar hydrographic conditions [ABSTRACT FROM AUTHOR]

Physical oceanography plays an important role in the formation of submarine sediments, and the distribution of nutriments and biocenoses in seamounts. The M4 seamount is located in the Caroline Island Ridge of the western Pacific Ocean. The physical properties around M4 seamount are preliminarily analyzed based on the in-situ data obtained in summer 2017 in Caroline M4 seamount and open-sourced data. We found that the water in the upper 200 m is controlled by the westward North Equatorial Current (NEC), while the water between 300–1 000 m is dominated by the eastward North Equatorial Undercurrent (NEUC). The current direction fluctuates significantly below 300 m at upstream stations. At the same depth of the lee sides, the current direction changes with the distance from seamount. These are likely caused by the obstacle of M4 seamount. The calculation results show that there is an anticyclonic cap above M4 seamount caused by tidal rectification. Tidal currents in M4 seamount are squeezed by the topography and amplified, and the amplified tidal currents play a dominant role in M4 seamount. First, the circulation system generated by the interaction of the amplified tidal current and M4 seamount drives the upward/downward movement of the isotherms. Secondly, the thickness of the surface turbulent layer is changed with the tidal phase. Thirdly, high turbulent diffusivities are found in the bottom of M4 seamount, and these are most likely attributed to the turbulent mixing induced by the mutual effect between semidiurnal tidal currents and steep bathymetry. This article of physical oceanography provides scientific basis for further analysis of the distribution of biological community and deposition mechanism in M4 seamount. [ABSTRACT FROM AUTHOR]

The North Atlantic ocean is key to climate through its role in heat transport and storage. Climate models suggest that the circulation is weakening but the physical drivers of this change are poorly constrained. Here, the root mechanisms are revealed with the explicitly transparent machine learning (ML) method Tracking global Heating with Ocean Regimes (THOR). Addressing the fundamental question of the existence of dynamical coherent regions, THOR identifies these and their link to distinct currents and mechanisms such as the formation regions of deep water masses, and the location of the Gulf Stream and North Atlantic Current. Beyond a black box approach, THOR is engineered to elucidate its source of predictive skill rooted in physical understanding. A labeled data set is engineered using an explicitly interpretable equation transform and k‐means application to model data, allowing theoretical inference. A multilayer perceptron is then trained, explaining its skill using a combination of layerwise relevance propagation and theory. With abrupt CO2 quadrupling, the circulation weakens due to a shift in deep water formation regions, a northward shift of the Gulf Stream and an eastward shift in the North Atlantic Current. If CO2 is increased 1% yearly, similar but weaker patterns emerge influenced by natural variability. THOR is scalable and applicable to a range of models using only the ocean depth, dynamic sea level and wind stress, and could accelerate the analysis and dissemination of climate model data. THOR constitutes a step toward trustworthy ML called for within oceanography and beyond, as its predictions are physically tractable. Plain Language Summary: The North Atlantic circulation is key to climate through heat transport and storage, and is projected to weaken under global heating. The mechanisms of change remain obscure, but are addressed here using a transparent machine learning (ML) method, engineered combining interpretable and explainable methods to reveal its source of predictive skill. Tackling the fundamental question of identifying dynamically coherent regimes governing the circulation, the Tracking global Heating with Ocean Regimes (THOR) method reveals a weakened circulation under abrupt CO2 quadrupling, seeing a shift in deep water formation, the Gulf Stream and North Atlantic Current. If CO2 is increased 1% yearly, similar but weaker patterns emerge. THOR is readily applicable to other models needing only depth, wind stress and sea surface height fields as input, and could accelerate discovery and analysis. THOR is a step toward trustworthy ML called for within oceanography and beyond because its predictions are physically tractable. Key Points: The transparent machine learning method Tracking global Heating with Ocean Regimes (THOR) is presented, applied to the North Atlantic oceanGlobal heating shifts regimes of the Gulf Stream north, the North Atlantic Current east and deep water mass formation toward lighter watersWidely applicable, THOR could accelerate analysis and dissemination of climate model data needing only depth, sea level and wind stress [ABSTRACT FROM AUTHOR]

There is strong evidence of an increase in primary production (PP) in the Arctic Ocean (AO) over the last two decades. Further increases will depend on the interplay between decreasing light limitation for primary producers, as the sea ice extent and thickness decrease, and the availability of nutrients, which is controlled by, but not limited to, inputs from the Atlantic and the Pacific Oceans. While these inputs are the major nutrient sources to the AO, ocean vertical mixing is required to bring the nutrients into the photic zone. We analyze data collected in the Western Eurasian Basin (WEB) between 1980 and 2016 and characterize the nutrient climatology of the various water masses. We conclude that there were no significant trends in the concentrations of the two macronutrients that typically limit PP in the AO (nitrate and silicic acid, in the case of diatoms), except a decreasing trend for silicic acid in Polar Surface Water (PSW), which is consistent with the reported increase in PP in the AO. We suggest that the Whalers Bay polynya, located in the northwestern corner of Svalbard, may act as a mixing hotspot, creating patches of nutrient replenished PSW. These patches may then be advected to higher latitudes under the ice pack, later boosting PP upon release from light limitation or else, keeping a nutrient reservoir that may be used in a subsequent growth season. It is likely that this remaining nutrient reservoir will decrease as sea ice cover retreats and light limitation alleviates. [ABSTRACT FROM AUTHOR]

Hydrographic and velocity data from a 2018 winter survey of the western Iceland and Greenland Seas are used to investigate the ventilation of overflow water feeding Denmark Strait. We focus on the two general classes of overflow water: warm, saline Atlantic-origin Overflow Water (AtOW) and cold, fresh Arctic-origin Overflow Water (ArOW). The former is found predominantly within the East Greenland Current (EGC), while the latter resides in the interior of the Iceland and Greenland Seas. Progressing north to south, the properties of AtOW in the EGC are modified diapycnally during the winter, in contrast to summer when along-isopycnal mixing dominates. The water column response to a 10-days cold-air outbreak was documented using repeat observations. During the event, the northerly winds pushed the freshwater cap of the EGC onshore, and convection modified the water at the seaward edge of the current. Lateral transfer of heat and salt from the core of AtOW in the EGC appears to have influenced some of this water mass transformation. The long-term evolution of the mixed layers in the interior was investigated using a 1-D mixing model. This suggests that, under strong atmospheric forcing, the densest component of ArOW can be ventilated in this region. Numerous anti-cyclonic eddies spawned from the EGC were observed during the winter survey, revealing that these features can play differing roles in modifying/prohibiting the open-ocean convection. [ABSTRACT FROM AUTHOR]

Vertical mixing of upper water masses in Arctic fjords is important for circulation and transport of nutrients and heat. However, the distribution of turbulent mixing is poorly known. Here we present hydrographic and microscale turbulence measurements from six fjords in Northwestern (69°N-75°N) Greenland. Water mass distributions showed the presence of warm bottom water of Atlantic origin in all fjord transects and a significant modification of upper water masses in fjords with tidewater outlet glaciers. Spatial and temporal distribution of turbulence in the fjords showed, in general, low rates of turbulent kinetic energy, ε (~10-9 W kg-1), and strong stratification implies that vertical exchange in the upper ~200 m was relatively weak. However, measurements within 2 km of the terminus of a tidewater outlet glacier showed values of ε and inferred turbulent diffusion coefficients >10-4 m² s-1 that were about two orders of magnitude larger than elsewhere in the deep fjords. This elevated mixing corresponded to comparatively large heat and salt fluxes. These observations suggest that vertical mixing near tidewater outlet glaciers results in localized mixing hot spots, likely due to subglacial discharge and near-glacial current shear, that contribute significantly to vertical exchange in the deep fjords. [ABSTRACT FROM AUTHOR]

We investigated the transport of Arctic shelf water and its impacts on the downstream hydrographic structures in the Amerasian Basin by combining data collected in the summer of 2016 from two international cruises in the Pacific sector. The East Siberian Sea (ESS), in which cold shelf water with salinity of 31.0-32.8 is produced, was identified to be a major source of the near-freezing temperature water for the Makarov Basin. This relatively low-salinity shelf water was transported into the Chukchi Abyssal Plain and occupied the subsurface layer (30-60 m), which led to the formation of a subsurface oceanic front stretching along the Chukchi Plateau and northern side of the Mendeleev Ridge. The offshore transport of waters from the ESS caused an intrusion of the Atlantic Water onto the shelf and promoted the formation of the diapycnally mixed Lower Halocline Water (D-LHW) at the bottom of the outer shelf. Our findings indicate that the source of the D-LHW in the Arctic Pacific sector might extend from the Chukchi Plateau to as far as the Makarov Basin. We hypothesize that advection of waters from the ESS into the Makarov Basin promotes development of the subsurface halocline in the Makarov Basin. The volume transport of surface waters (salinity less than 30) on the ESS shelf was estimated to be about 0.6 Sv (106 m³ s-1) eastward to the Canada Basin in 2016, consistent with the cyclonic Arctic circulation regime in 2016. [ABSTRACT FROM AUTHOR]

The East Sea Intermediate Water (ESIW) is the subsurface salinity minimum layer in the depth range 200–400 m in the East/Japan Sea. The origin, variability, and pathways of the ESIW were investigated using a 4‐year‐long (2015–2018) high‐resolution (1/24°) ocean reanalysis and Lagrangian particle modeling. A backward particle tracking analysis revealed that the ESIW was formed south of Vladivostok and shows strong interannual variability. In 2018, owing to strong northwesterly winds, heat loss to the air intensified, and freshwater transport from the northern East/Japan Sea was simultaneously decreased by about 18% compared with the previous year. Furthermore, there was increased formation of water masses heavier than that of the typical ESIW, and the volume of the ESIW was reduced. A forward particle tracking analysis showed that the contribution of the particles originated from the northern end of the East/Japan Sea was about two times greater than those from the southern end. The major portion of the ESIW's freshwater came from the northern East/Japan Sea, where sea ice melted and the Amur River discharged. The remaining portion was from the Changjiang River, which came through the Korea Strait. The ESIW reached the Ulleung Basin over the course of a year mainly through the deep channel, the Ulleung Trough, located to the north of the basin. The diapycnal diffusivity of 1.0 × 10−4 m2 s−1 could erode away the low salinity core in about 27 years. Plain Language Summary: In the southwestern part of the East/Japan Sea (EJS), low salinity water called the East Sea Intermediate Water (ESIW) is found between depths of 200 and 400 m. We studied the origin, variability, and pathways of the ESIW through high‐resolution ocean reanalysis and Lagrangian particle tracking modeling. It was found that that the major source of the ESIW is the Amur River freshwater and the sea ice meltwater from the northern end of the EJS. The ESIW forms southward of Vladivostok and extends southward below the mixed layer. It moves southward, mostly through deep channels until it reaches the Ulleung Basin, which is located in the southwestern part of the EJS. The ESIW characteristics show interannual variation. In 2018, the salinity of the ESIW was higher than that in the other years, and it corresponded to strong atmospheric forcing and reduced the low salinity water transport from the northern end. The amount of the ESIW, thus, was reduced by about 18% compared with that for the previous year. Owing to vertical mixing, the low salinity core of the ESIW could be eroded away in about 27 years. Key Points: The main source of the East Sea Intermediate Water is the low salinity water at the northern end of the East/Japan SeaThe formation of the water mass was significantly decreased in 2018 due to enhanced cooling and reduced freshwater transportA major portion of the water mass reaches the Ulleung Basin through the Ulleung Trough over a period of one year [ABSTRACT FROM AUTHOR]

Metal/calcium ratios in two long‐lived deep‐sea gorgonian corals (Lepidisis and Corallium spp.) in the Southwest Pacific evidence periodic decadal variability at depths that correspond to Antarctic Intermediate Water (AAIW) and shallow Upper Circumpolar Deep Water, and a shift in the mid‐1930s to late‐1930s in mean ambient temperatures, barium/silicate concentrations and possibly pH, the rate at which these properties change over time, and the relationship between temperatures at fixed depth and the Southern Annular Mode (SAM). The decadal periodicity, which is evident in other biological indices in the study area, can be accounted for by water mass heave on the order of 100–150 m, which is consistent with observed scales of variability in the AAIW. The proximate and ultimate causes of the midcentury shifts are unclear, but could be related to suggested mid‐20th century changes in climate parameters globally and, more specifically, in the subpolar SW Pacific. Plain Language Summary: The dynamics of the Southern Ocean play a critical role in global climate and oceanography, but sparse historical instrumental data make it difficult to assess the significance of recently observed changes. We help fill this information gap by analyzing apparent proxies for ambient temperature, barium/silicate concentrations, and possibly pH in the carbonate skeletons of a pair of very long‐lived deep‐sea corals. The data strongly suggest a previously unreported ∼23‐year periodicity in two Southern Ocean water masses (Antarctic Intermediate Water and Upper Circumpolar Deep Water), which is likely to be the result of vertical shifts in water mass distributions. The long‐term data also suggest the water masses cooled, became less acidic and had declining barium/silicate concentrations from at least the early 1700s until the early to mid‐1900s, after which the sign of the long‐term trends reversed. The causes of the long‐term trends and their reversal are not clear, but may be related to changes to climate parameters globally, but more specifically to those in the SE Pacific, where Antarctic Intermediate Water is formed. Key Points: Three environmental proxies were examined in skeletal composition of long‐lived deep‐water gorgonian corals in the SW PacificThe time series indicate coherent ca. 23‐year periodicity at depths corresponding to the AAIW and UCDWLong‐term trends in all three proxies reversed sign in the early to mid‐1900s, along with their relationship with the SAM [ABSTRACT FROM AUTHOR]

• The scientific knowledge on the southern Okhotsk Sea biogeochemistry and its relationship with its rich primary productivity was actualized. • The formation mechanisms of different water masses are starting to be revealed using chemical tracers, especially radionuclides. • The high seeding potential of Thalassiosira by ice–melting has been repeatedly featured, and it was placed as reason for its highest abundance in spring. • The total carbon mass flux is highest in April – June. Its maximum (up to 1165 mg C m−2 day−1) occurs several days after the sea ice retreatment • Further research is needed on the formation of the Intermediate Cold Water, the Cold Water Belt and the solubilization process of particulate Fe. The southern Okhotsk Sea is one of the best fishing grounds in Japan, and the biogeochemistry, primary productivity, and physical aspects of this region have been heavily researched. However, a comprehensive review of the literature has not been written since that by doctor Takatoshi Takizawa more than 40 years ago (1982). This review actualizes the share–ground knowledge of the influence of oceanographic conditions on the primary productivity of the area. This review includes topics from physical/chemical oceanography and hydrobiology, focusing on the complex oceanological structures that support the high primary productivity of the southern Okhotsk Sea. It is known that the formation of all water masses is ultimately a consequence of the seasonal interchange of the Soya Warm Current and the East Sakhalin Current. However, it was revealed that there may be over- or under-estimations in different material and heat fluxes among water masses, especially regarding the dense Soya Warm Current. Recent studies have investigated the formation mechanisms of different water masses using chemical tracers, particularly radionuclides. Although promising if properly combined with traditional techniques, more isotopes or compounds are required to derive further conclusions. In terms of biological aspects, the high seeding potential of Thalassiosira after sea ice melting has been featured by many authors, and it was considered as the reason for its highest abundance in spring. The total carbon mass flux, which is essential for scallop farming, is highest in April – June, normally 120 – 160 mg C m−2 day−1. The total carbon mass flux is maximum (up to 1165 mg C m−2 day−1) after the spring bloom peak, and the spring bloom is strongly linked to sea ice retreatment. It was identified that the routes of 134Cs towards the Intermediate Cold Water, the formation mechanism of the Cold Water Belt, and the solubilization process of sea ice-carried particulate Fe require further research. Three major potential threats were identified, including the appearance of alien toxic species, a decrease in sea ice production in the northern parts of the Okhotsk Sea, and offshore gas and oil drilling northeastern of the Sakhalin Island. Publicly available scientific knowledge about the biogeochemistry of the southern Okhotsk Sea and its relationship with its rich primary productivity was actualized to 2022, anticipating that this work will become the starting line for many early career scientists endeavoring in interdisciplinary studies. [ABSTRACT FROM AUTHOR]

Picoeukaryotes are numerous in the summer in the High-Arctic fjord Kongsfjorden, in western Spitsbergen, Svalbard. However, little research has been conducted on the community structure and diversity of picoeukaryotes. We conducted a detailed investigation of the distribution and environmental correlations of picoeukaryotes in Kongsfjorden in July 2012, using 454-pyrosequencing of 18S rDNA and redundant analysis. Eight classes were classified with proportions larger than 1%. These were Mamiellophyceae, Chrysophyceae, Spirotrichea, Telonemea, Cryptophyceae, Bolidophyceae, Picomonadea and Dictyochophyceae. Five genera were classified, with Micromonas (55.6%) and Bathycoccus (7.8%) as the dominant genera. The diversity and composition of the picoeukaryote community were very distinct in different water masses sampled in the water column (i.e., vertically), but were not distinct from station to station (i.e., horizontally). Biodiversity was greater in the Atlantic waters than in glacial waters. Mamiellophyceae, Bolidophyceae, Picomonadea and Dictyochophyceae had significantly different distributions (p < 0.01) in the three water masses (surface water, intermediate water and transformed Atlantic Water). Nitrogen, salinity and temperature were the first three primary environmental factors shaping the community structure of picoeukaryotes.

Physical oceanography of New Zealand/Aotearoa shelf seas - a review. Keywords: Oceanography; shelf seas; climate change; open data; continental shelf EN Oceanography shelf seas climate change open data continental shelf 1 5 5 02/20/21 20210301 NES 210301 Aotearoa New Zealand is a marine nation. The motivation for this special issue on New Zealand's Shelf Seas emerged from the 2017 Physical Oceanography Workshop held in Wellington organised by Dr. Natalie Robinson of NIWA. At the meeting it was clear that there was a critical mass of mature regionally-specific research to form the basis of a special issue on physical oceanography of New Zealand's shelf seas. [Extracted from the article]

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. [ABSTRACT FROM AUTHOR]

The shelf seas surrounding New Zealand/Aotearoa, a region nominally extending out to the 250 m depth contour, are complex and varied as they sit above the submerged continent of Zealandia. These seas surround the two main islands that span 1,500 km in latitude from 34°S to 47°S and include a series of plateaus that extend far beyond. This review summarises the present state of knowledge of the physical processes that control ocean transport and mixing in this domain. The focus is on documenting recent regional advances in understanding the balance between off-shelf drivers, terrestrially sourced freshwater, winds, tides, stratification and the underlying topography. The review considers outstanding themes such as connectivity, climate and stratification and closes with an assessment of the potential future for understanding the physical marine environment of the shelf seas of the New Zealand region. [ABSTRACT FROM AUTHOR]

The variability of water masses has been analyzed using temperature and salinity or density and various forms of spicity/spiciness. The objective way of comparing these spicity/spiciness functions is not the difference between these definitions; instead, it is through the application of these functions in tracing water masses and climate signal analysis. The major advantage of the spicity function introduced by Huang et al. Journal of Geophysical Research: Oceans, is that this function in combination with density gives rise to an orthogonal coordinate, in which the signals are separated into two independent components. In addition, a new quantity, distance, is introduced, that can be used to trace water masses both along and across isopycnal surfaces. Spicity can serve as an appropriate tool to extract globally varying water mass properties as a supplement to variations of density alone. Plain Language Summary Spicity in combination with density leads to a new mean of tracing water masses and climate signals. In particular, the difference between two water parcels induced by different temperature and salinity can be quantified in terms of a single quantity, distance, defined in terms of spicity and density. Through concrete examples, we demonstrate how to use spicity and distance in the study of water mass and double diffusive instability. Most importantly, these new tools can provide a clear separation of climate signals through two independent components without signal contamination. [ABSTRACT FROM AUTHOR]

The physical characteristics of water masses in the Cilician Basin were analyzed based on recent, comprehensive in-situ data. Nine seasonal cruises from June 2015 to November 2017 were carried out in an area located in the north-eastern Levantine Basin between the coasts of Turkey and Cyprus. In this little studied area, the structure of the water column, its seasonal variability, and water formation events are investigated. The extensive data set reveals that in summer Modified Atlantic Water (MAW) is observed at about 50 m depth. Saltier and warmer Levantine Intermediate Water (LIW) lies below the MAW between ~100 and 250 m. During winter, the water column becomes mixed due to surface cooling and vertical convection, and displays uniform properties from the surface to about 200-300 m depths. The yearly cycle of a vertically uniform water column during winter and the appearance of LIW in the following period suggests that LIW is being formed in this region, which is confirmed through heat and buoyancy flux calculations. It is the first time LIW formation is reported in this area of the north-eastern Levantine Basin and adds to the areas of LIW formation outside Rhodes Gyre. In the 2.5 years of the study, LIW salinity increased by 0.11 psu in 2017, while surface water salinity increased by 0.5 psu over the same period. The presence of meso-scale eddies in the study area is observed throughout the year. The eddies are short-lived, having a time span of a few months, during which they either change location or disappear. Plain Language Summary Variability of water mass properties in the Cilician Basin, located in the north-eastern part of the Mediterranean Sea, was investigated during nine research cruises spanning 2.5 years. This was done measuring temperature, salinity, and oxygen of the water column at several locations in the region. The study describes the seasonal cycle of the water masses and it is shown for the first time that in winter cold, dense water is formed in this region that sinks to depths of 100-250 m. This is because water increases its density due to cooling and forms a water mass known as Levantine Intermediate Water. It is of importance to know where and how much of this water mass is formed, because it travels throughout the Mediterranean Sea and exits through the Strait of Gibraltar, where it influences the water masses and circulation in the Atlantic Ocean. The study also found that the salinity of that water mass increased by 0.11 psu in 2017, while at the same time surface water salinity increased by 0.5 psu. This is a large increase over a short time compared to the long term 0.0007 ± 0.0003 psu/yr average increase calculated for the Mediterranean Sea in previous studies. [ABSTRACT FROM AUTHOR]

There have been a variety of definitions of a "spice" type variable in physical oceanography. These variables have been used to measure water-mass variations and to examine how the stability ratio varies vertically on a water column. Some authors have chosen to derive such an oceanographic variable, called potential spicity, so that its isolines are orthogonal to isolines of potential density on a suitably scaled salinity-temperature diagram. Here it is shown that isopycnal differences of potential spicity are more than a factor of 100 different to the corresponding isopycnal differences of other measures of watermass variations such as spiciness, whose isopycnal differences are proportional to the compensating contributions of salinity and temperature to potential density. This behavior of potential spicity is caused by the imposition of the orthogonal constraint which lacks a physical oceanographic motivation. Such orthogonal potential spicity variables also depend on the arbitrarily chosen scaling factor that relates the scales of the salinity and temperature axes on the salinity-temperature diagram. A preferred alternative to using either of these two spice variables is (1) to plot variations of Absolute Salinity in approximately neutral surfaces and (2) to calculate and plot the stability ratio on vertical water columns. The anomaly of Absolute Salinity is best made with respect to a reference relationship between Absolute Salinity and Conservative Temperature that is typical of vertical casts in the oceanographic region of interest. Plain Language Summary The constraint of orthogonality (on the salinity-temperature diagram), which is the basis of a previously published potential spicity variable, causes the isopycnal gradient of potential spicity to be more than a factor of 100 different to the isopycnal gradient of physically based measures of water-mass properties, such as the compensating contributions of salinity and temperature to density along isopycnals. Hence such potential spicity variables, based as they are on the orthogonality principle, are not viable measures of water-mass variations along isopycnals. [ABSTRACT FROM AUTHOR]

A series of results of data analysis is presented in this paper, carried for the 2020-2021 marine hydrological cycle. The method included the collection, processing, analysis and archiving of data sets (and information) recorded in the field and obtained from remote sensing (satellite data). The data was required to evaluate the hydrodynamic processes in the coastal/marine area, including the mouths of the Danube, the natural promontories, and marine obstacles such as the ports jetties. Data were collected during the oceanographic expeditions carried over June 2020 and July 2021, on the spatial extension of the Romanian coastal monitoring network. Measurements of physical, chemical, and hydrological parameters were performed in the areas of Sulina, Sf. Gheorghe, Sahalin in order to observe and evaluate the mixing phenomenon of water masses in the area of Danube discharge. The analysis of the physical and chemical parameters, associated with the hydrological parameters, were emphasizing the influence of the coastal water masses dynamics on the mixing processes afferent to the Danube Delta's offshore areas, due to the physiographic and hydro-morphologic characteristics of an asymmetric delta. [ABSTRACT FROM AUTHOR]

The dense waters formed by wintertime convection in the Labrador Sea play a key role in setting the properties of the deep Atlantic Ocean. To understand how variability in their production might affect the Atlantic Meridional Overturning Circulation (AMOC) variability, it is essential to determine pathways and associated timescales of their export. In this study, we analyze the trajectories of Argo floats and of Lagrangian particles launched at 53°N in the boundary current and traced backward in time in a high-resolution model, to identify and quantify the importance of upstream pathways. We find that 85% of the transport carried by the particles at 53°N originates from Cape Farewell, and it is split between a direct route that follows the boundary current and an indirect route involving boundary-interior exchanges. Although both routes contribute roughly equally to the maximum overturning, the indirect route governs its signal in denser layers. This indirect route has two branches: part of the convected water is exported rapidly on the Labrador side of the basin and part follows a longer route toward Greenland and is then carried with the boundary current. Export timescales of these two branches typically differ by 2.5 years. This study thus shows that boundary-interior exchanges are important for the pathways and the properties of water masses arriving at 53°N. It reveals a complex three-dimensional view of the convected water export, with implications for the arrival time of signals of variability therein at 53°N and thus for our understanding of the AMOC. [ABSTRACT FROM AUTHOR]

The aticle discusses the Atlantic Meridional Overturning Circulation (AMOC) has important to global climate. Topics include AMOC consists primarily of a northward-flowing upper limb that has warm and saline and a southward-flowing lower limb; and in contrast, profound hydrographic and oxygen changes, in the subpolar North Atlantic, have observed the same period.

Summer upwelling off Hainan Island (northern South China Sea) is influenced by fresher coastal water masses. To assess the ecological effects of this complicated oceanography, a cruise was conducted in August 2016. We investigated the spatial patterns of phytoplankton size structure, diversity and quantity of small eukaryotes (0.2–20 μm) across the upwelling system. In the inner-shelf waters, nanophytoplankton contributed a higher proportion to phytoplankton biomass than microphytoplankton and picophytoplankton. High-throughput sequencing of 18S rRNA genes and community analysis indicated that the horizontal variation in community structure of small eukaryotes was larger than the vertical. The relative sequence proportions of dinoflagellates, diatoms, ciliates and Micromonas in the surface layer gradually decreased from the inner shelf to the slope, whereas the Rhizaria, Bathycoccus and Ostreococcus exhibited an opposite trend. The spatial patterns of taxon-specific rDNA copy numbers were similar to the patterns based on the relative proportions. Redundancy analysis revealed that turbidity, salinity and |${\mathrm{NO}}_3^{-}$| were the most important environmental variables in structuring the community of small eukaryotes. The cell abundance of photosynthetic picoeukaryotes was significantly correlated with the rDNA abundance of Mamiellophyceae and the cell abundance of nanoeukaryotes was significantly correlated with the rDNA abundance of dinoflagellates plus diatoms. [ABSTRACT FROM AUTHOR]

Fine-scale thermohaline structure within ocean column can be mapped seismically in the Kuroshio Current, off the Muroto Peninsula of Shikoku Island, Japan. In this paper, we present the application of automatic sound speed picking analysis to the multi-channel seismic reflection data acquired in a different period to estimate time-lapse sound speed distribution across the Kuroshio Current. This method is based on an optimal velocity trajectory solving by the eikonal equation with a finite-difference algorithm. In contrast to the seismic inversion technique, this automatic analysis enables us to obtain contrast sound speed profiles without heavy dependency on sound speed or temperature data directly measured at discrete locations. As a result, this method can visualise sound speed profiles of fine-scale thermohaline structure developed at interleaving or diapycnal mixing processes of different water masses in the Kuroshio Current. The images of all profiles mapped from automatic sound speed analysis distinguish water masses and their fine-scale internal structure such as cold and warm water eddies, thermohaline staircases and internal waves revealing acoustic contrasts at interfaces across where sound speed and temperature change. Applying our approach for individual seismic line acquired in different time-steps for 3D seismic data can provide time–space variant images of fine-scale thermohaline structure for studies of oceanographic processes as well as large-scale ocean current and climate systems. [ABSTRACT FROM AUTHOR]

The temperature-salinity (T-S) diagram is widely used in water mass analysis, but the boundaries between water masses are vaguely distinguished by conventional T-S-based methods. Herein, we propose a new method based on the potential density-potential spicity (sigma-pi) diagram. The new method has been applied to the conductivity-temperature-depth data collected in the northern South China Sea during a spring cruise in 2011. The water masses in the study region are classified into 13 types according to both the standard deviation of potential spicity in each potential density layer and the water volumetric distribution in the sigma-pi space. The results suggest that this new method is reasonable and robust for classifying water masses in the sigma-pi space as compared to previous methods based on the traditional T-S space. In addition, the westward intrusion of the West Pacific Ocean water to the northern South China Sea can be clearly detected by the tongue-like potential spicity structure and relatively high potential spicity patches on potential density layers, further verifying the robustness and efficiency of our method in the water mass analysis. [ABSTRACT FROM AUTHOR]

Siphonophores are commonly considered to be useful indicators of water masses and water-mass movement, but their employment as such across the wider Southern Ocean has not so far been attempted. We redress this here using archived samples, collected during January–February 1993 along a transect from Cape Town to the South African National Antarctic Expedition (SANAE) base in Antarctica, and compare the patterns generated with those determined from a prior analysis of whole assemblages at lower taxonomic resolution. Twenty-one species were identified from 18 of the original 53 samples collected, and two distinct assemblages were confirmed as separated by the Sub-Antarctic Front. That to the south was characterised by low diversity and high abundance and was dominated by cold-water specialists, whereas that to the north comprised a larger number of subtropical and temperate species at low abundance. Assemblage structure was strongly influenced by the mixed layer depth, sea surface salinity and chlorophyll a concentration, as well as mesozooplankton biomass. Congruence with the whole-assemblage study was high, indicating that this taxon can be suitably employed as a proxy in studies such as this. The study emphasises the value of archived plankton samples and makes a plea for better curation. [ABSTRACT FROM AUTHOR]