Hydrographic Observations Research Articles

Transformation of Atlantic Water in the north-eastern Barents Sea in winter

Hydrographic observations, carried out in March-May, 2019 during “Transarktika-2019” expedition onboard R/V “Akademik Tryoshnikov” allowed studying mechanisms of Atlantic Water (AW) transformation in the Barents Sea. Although this research topic is rather traditional for oceanographic studies, there are still a number of questions, which require clarification. Among these is a deeper understanding of the AW transformation in specific regions in cold season, when the coverage by observations is scarce. In this study we performed temperature and salinity (TS) analysis of conductivity — temperature — depth (CTD) data, collected in the north-eastern “corner” of the Barents Sea — this is the area with difficult access in winter due to high concentration of pack ice. The results allowed identification of areas along the pathways of AW branches, where various types of open sea convection and cascading acted as dominant processes of AW properties change. We distinguish several driving mechanisms controlling modification of the waters of Atlantic origin. An advantage of winter measurements is that the active stage of AW transformation mechanisms is explicitly observed at the consecutive CTD sections.

Reversal of freshening trend of Antarctic Bottom Water in the Australian-Antarctic Basin during 2010s

The Antarctic continental margin supplies the densest bottom water to the global abyss. From the late twentieth century, an acceleration in the long-term freshening of Antarctic Bottom Waters (AABW) has been detected in the Australian-Antarctic Basin. Our latest hydrographic observations reveal that, in the late 2010s, the freshening trend has reversed broadly over the continental slope. Near-bottom salinities in 2018–2019 were higher than during 2011–2015. Along 170° E, the salinity increase between 2011 and 2018 was greater than that observed in the west. The layer thickness of the densest AABW increased during the 2010s, suggesting that the Ross Sea Bottom Water intensification was a major source of the salinity increase. Freshwater content on the continental slope decreased at a rate of 58 ± 37 Gt/a in the near-bottom layer. The decadal change is very likely due to changes in Ross Sea shelf water attributable to a decrease in meltwater from West Antarctic ice shelves for the corresponding period.

The Deep Western Boundary Current and Adjacent Interior Circulation at 24°–30°N: Mean Structure and Mesoscale Variability

AbstractThe mean North Atlantic Deep Water (NADW, 1000 < z < 5000 m) circulation and deep western boundary current (DWBC) variability offshore of Abaco, Bahamas, at 26.5°N are investigated from nearly two decades of velocity and hydrographic observations, and outputs from a 30-yr-long eddy-resolving global simulation. Observations at 26.5°N and Argo-derived geostrophic velocities show the presence of a mean Abaco Gyre spanning the NADW layer, consisting of a closed cyclonic circulation between approximately 24° and 30°N and 72° and 77°W. The southward-flowing portion of this gyre (the DWBC) is constrained to within ~150 km of the western boundary with a mean transport of ~30 Sv (1 Sv ≡ 106 m3 s−1). Offshore of the DWBC, the data show a consistent northward recirculation with net transports varying from 6.5 to 16 Sv. Current meter records spanning 2008–17 supported by the numerical simulation indicate that the DWBC transport variability is dominated by two distinct types of fluctuations: 1) periods of 250–280 days that occur regularly throughout the time series and 2) energetic oscillations with periods between 400 and 700 days that occur sporadically every 5–6 years and force the DWBC to meander far offshore for several months. The shorter-period variations are related to DWBC meandering caused by eddies propagating southward along the continental slope at 24°–30°N, while the longer-period oscillations appear to be related to large anticyclonic eddies that slowly propagate northwestward counter to the DWBC flow between ~20° and 26.5°N. Observational and theoretical evidence suggest that these two types of variability might be generated, respectively, by DWBC instability processes and Rossby waves reflecting from the western boundary.

Origin of the springtime South China Sea Warm Current in the southwestern Taiwan Strait: Evidence from seawater oxygen isotope

Oxygen isotope (δ18O) of seawater is an excellent proxy for tracing the origins of water masses and their mixing processes. Combining with hydrographic observation, hybrid coordinate ocean model (HYCOM) analysis data, and seawater oxygen isotope, we investigated the source of the South China Sea Warm Current (SCSWC) in the southwestern Taiwan Strait and its underlying mechanism. Results show that the Kuroshio subsurface water (KSSW) can intrude the continental slope in the southwestern Taiwan Strait, and thereby climb up the continental slope coupled with upwelling. The δ18O-salinity relationship further indicates that in spring, the SCSWC in the southwestern Taiwan Strait originates from the upslope deflection of the slope current formed by the KSSW intrusion into the South China Sea, rather than from the west segment of the SCSWC formed to the east of Hainan Island. In addition, the southward flowing Zhe-Min Coastal Current (ZMCC) can reach as far as the Taiwan Bank (TB) and deflects offshore over the western TB at approximately 23.5°N, to some extent affecting the SCSWC. Moreover, this study reveals that seawater δ18O is exquisitely sensitive to the determination of the origin and transport of water masses as compared with traditional potential temperature-salinity plot (θ-S) and HYCOM analysis data. In addition, their coupling can more reliably interpret the mixing processes of shelf water masses.

Freshening of Antarctic Bottom Water Off Cape Darnley, East Antarctica

AbstractRecently, a source of Antarctic Bottom Water (AABW) was identified off Cape Darnley at the eastern end of the Weddell‐Enderby Basin. However, the behavior and long‐term variability of Cape Darnley Bottom Water (CDBW) are not clearly understood. Hydrographic observations from 1974 to 2016 were compared, and a decade‐long bottom temperature record was analyzed to clarify multidecadal changes in the CDBW in this region and its downstream influences. In the Cooperation Sea, CDBW spread northwestward with its deepest part reaching to approximately 4,900 dbar. CDBW freshening of 0.001–0.003 decade−1 was revealed. In the Cosmonaut Sea, long‐term AABW warming of approximately 0.01–0.03°C decade−1 was prominent in the deep basin, while freshening was detected on the upper continental slope. Spatial patterns suggest that an interbasin deep transport of excess freshwater is carried by CDBW and fed into the Weddell Gyre, which might act as an abyssal freshwater buffer.

Along-Stream Evolution of Gulf Stream Volume Transport

AbstractThe Gulf Stream affects global climate by transporting water and heat poleward. The current’s volume transport increases markedly along the U.S. East Coast. An extensive observing program using autonomous underwater gliders provides finescale, subsurface observations of hydrography and velocity spanning more than 15° of latitude along the path of the Gulf Stream, thereby filling a 1500-km-long gap between long-term transport measurements in the Florida Strait and downstream of Cape Hatteras. Here, the glider-based observations are combined with shipboard measurements along Line W near 68°W to provide a detailed picture of the along-stream transport increase. To account for the influences of Gulf Stream curvature and adjacent circulation (e.g., corotating eddies) on transport estimates, upper- and lower-bound transports are constructed for each cross–Gulf Stream transect. The upper-bound estimate for time-averaged volume transport above 1000 m is 32.9 ± 1.2 Sv (1 Sv ≡ 106 m3 s−1) in the Florida Strait, 57.3 ± 1.9 Sv at Cape Hatteras, and 75.6 ± 4.7 Sv at Line W. Corresponding lower-bound estimates are 32.3 ± 1.1 Sv in the Florida Strait, 54.5 ± 1.7 Sv at Cape Hatteras, and 69.9 ± 4.2 Sv at Line W. Using the temperature and salinity observations from gliders and Line W, waters are divided into seven classes to investigate the properties of waters that are transported by and entrained into the Gulf Stream. Most of the increase in overall Gulf Stream volume transport above 1000 m stems from the entrainment of subthermocline waters, including upper Labrador Sea Water and Eighteen Degree Water.

On the seasonality of waters below the seasonal thermocline in the Gulf of Cádiz

This work examines the seasonal thermohaline variability in the Gulf of Cádiz (SW Iberian Peninsula) based on 2009–2020 repeated hydrographic observations. Subsurface water types are assorted within the mixing triangle formed by Mediterranean and Eastern North Atlantic Central Waters (ENACW). A sharp interface between 400-500 m depth separates the saline Mediterranean Overflow Waters (MOW) from the ENACW salinity minimum siting atop. The water column is warmer and more saline in winter (cooler, fresher in summer). Maximum differences of up to 0.6 ∘C and 0.15 emanate from the ENACW/MOW interface. Changes appear related to the wind-driven seasonal alternation of vertical displacement of isopycnals and poleward-equatorward transports. Upwelling-favorable winds in summer steer positive Ekman pumping velocities, which seem responsible for cooling over the ENACW salinity minimum. Below, the warm, saline signal of subtropical waters from the Azores current is attenuated by the summer approach of cooler, fresher waters from the Portugal Current system. The change of sign of Ekman pumping in winter suggests subsidence of isopycnals and warming/salinification under the seasonal thermocline. Seasonal thermohaline changes of waters leaving the Mediterranean Sea are insufficient to explain the variations under the ENACW/MOW interface. Rather, variability of Atlantic waters entrained by the overflow seem to dictate these differences.

Observational Analysis of Extratropical Cyclone Interactions with Northeast Pacific Sea Surface Temperature Anomalies

AbstractThis study examines the interaction between a northeast Pacific upper-ocean thermal anomaly and individual fall storm events between 2013 and 2016. In 2013, a large upper-ocean thermal anomaly formed in the Gulf of Alaska (GOA) with sea surface temperatures (SST) warmer than 4°C above the climatological norm. Formation of the anomaly was associated with a persistent atmospheric ridge in the GOA that produced a lull in storm activity in the boreal winter of 2013/14. While reduced storm activity was the apparent cause of this SST anomaly, we present cases where extratropical cyclones significantly eroded its mixed layer heat content on synoptic time scales. Case studies during the 4-yr period 2013–16 using satellite and Argo hydrographic observations show that early fall storms produced the largest surface heat fluxes and the greatest cooling of SST. The magnitude of thermal energy transfer from the ocean to the atmosphere during individual storm events was then determined using vertically integrated heat budgets based on Argo temperature profiles and reanalysis surface heat fluxes. Storm-induced surface heat flux anomalies accounted for approximately 50% of the warm anomaly cooling observed by Argo profiles. This rapid heat loss occurred over time scales of approximately 3–5 days. The decay of the warm SST anomaly (SSTa) occurred much more quickly than expected from classic thermal damping by SST-induced turbulent heat fluxes, which may be attributed here at least partly to much shallower mixed layers during early fall. Analysis of the individual surface flux terms indicated that the latent heat flux was the dominant contributor to storm-induced heat exchange across the air–sea interface.

Variability in Meridional Transport of the Subtropical Circulation in the South Indian Ocean for the Period From 2006 to 2017

AbstractThis study estimates variability in meridional velocity and transport in the interior of the south Indian Ocean using in situ hydrographic observations, satellite altimetry, and two reanalysis products for the period from 2006 to 2017. Previous studies used the zonal difference of satellite sea surface height (SSH) between the western and eastern parts of the basin as an index to variability in basinwide meridional geostrophic transport. This study estimates meridional geostrophic velocity in the upper 1,800 m from in situ observations and examines its vertical structure. Results show that zonal SSH difference represents a surface trapped variability in meridional velocity, the amplitude of which is large in the upper 250 m and decreases to zero at about 1,250 m depth. Zonal SSH difference is significantly correlated with zonally integrated meridional transport relative to 1,000 m depth. It is likely that wind variability both in the south Indian Ocean and tropical Pacific Ocean is responsible for this surface trapped variability, as is suggested by past studies. Results of this study also show meridional velocity variability at subsurface, which peaks in magnitude at about 400 to 800 m depths. This variability is not correlated with zonal SSH difference and represents decadal weakening of northward transport of the subtropical circulation for the analysis period. This subsurface variability can contribute to interannual variability in mode water transport, but its detailed forcing mechanism is not known and warrants a further study.

Bidirectional bedform fields at the head of a submarine canyon (NE Atlantic)

Submarine canyons are known to force ocean mesoscale circulation and local hydrodynamics. Alternate up- and down-canyon near-bottom flows have been widely documented along the upper reaches, connecting the canyon heads with the contiguous outer shelves and vice versa. Nonetheless, we still miss clear evidence of bedform fields expressing these complex patterns. In this study, through a multi-scale analysis in both space and time, we document rare asymmetric bedforms, up to 880 m long and 10 m high, developing within a depth range of 168-220 m at the head of the Whittard Canyon (NE Atlantic). One field of well-developed sandwaves has an atypical up-slope asymmetry, with the steeper slope facing the shallower regions of the shelf, and contrasting with surrounding down-slope sandwaves facing the canyon. The bedforms are interpreted to represent both up-slope and down-slope bottom currents connecting the upper reaches of the canyon to the outer shelf on the southern Celtic Margin, in the Bay of Biscay. The sandwaves were surveyed with shipboard Multibeam bathymetry (5 m grid cell resolution), AUV sidescan sonar (0.15 m grid cell resolution) and ROV footage, and sampled with three ROV-mounted vibro-cores and two box-cores. Sidescan sonar mosaics groundtruthed by sediment samples and ROV footage show with unprecedented detail spectacular trains of fresh overprinting megaripples, previously undocumented sand peaks and bowl-shaped depressions on the crests of the tallest sandwaves. Differences in sedimentary settings and benthic habitats indicate that these features are currently active in particularly dynamic areas, allowing for very slow migration of sandwaves. Numerical modelling together with concurrent hydrographic observations suggest large-amplitude semi-diurnal internal tides, possibly transitioning to asymmetric internal bores, as the main mechanism maintaining the mapped up-slope sandwaves. This work highlights the importance of uncommon sediment dynamics in canyon head environments and adds insight to the traditional notions of gravity-driven processes, being dominant in these environments, envisaging implications for improving geo-hazard assessment of mobile substrates and quantification of offshore sediment and carbon fluxes.

Reduced efficiency of the Barents Sea cooling machine

Dense water masses from the Barents Sea are an important part of the Arctic thermohaline system. Here, using hydrographic observations from 1971 to 2018, we show that the Barents Sea climate system has reached a point where ‘the Barents Sea cooling machine’—warmer Atlantic inflow, less sea ice, more regional ocean heat loss—has changed towards less-efficient cooling. Present change is dominated by reduced ocean heat loss over the southern Barents Sea as a result of anomalous southerly winds. The outflows have accordingly become warmer. Outflow densities have nevertheless remained relatively unperturbed as increasing salinity appears to have compensated the warming inflow. However, as the upstream Atlantic Water is now observed to freshen while still relatively warm, we speculate that the Barents Sea within a few years may export water masses of record-low density to the adjacent basins and deep ocean circulation. The Barents Sea cools the ocean, and dense water masses form that flow into the global overturning circulation. Hydrographic observations from 1971 to 2018 show reduced cooling efficiency with warmer Atlantic inflow, reduced sea ice and reduced wind-driven heat loss.

A comparison of five surface mixed layer models with a year of observations in the North Atlantic

Five upper ocean mixed layer models driven by ERA-Interim surface forcing are compared with a year of hydrographic observations of the upper 1000 m, taken at the Porcupine Abyssal Plain observatory site using profiling gliders. All the models reproduce sea surface temperature (SST) fairly well, with annual mean warm biases of 0.11 °C (PWP model), 0.24 °C (GLS), 0.31 °C (TKE), 0.91 °C (KPP) and 0.36 °C (OSMOSIS). The main exception is that the KPP model has summer SSTs which are higher than the observations by nearly 3°. Mixed layer salinity (MLS) is not reproduced well by the models and the biases are large enough to produce a non-trivial density bias in the Eastern North Atlantic Central Water which forms in this region in winter.All the models develop mixed layers which are too deep in winter, with average winter mixed layer depth (MLD) biases between 160 and 228 m. The high variability in winter MLD is reproduced more successfully by model estimates of the depth of active mixing and/or boundary layer depth than by model MLD based on water column properties. After the spring restratification event, biases in MLD are small and do not appear to be related to the preceding winter biases.There is a very clear relationship between MLD and local wind stress in all models and in the observations during spring and summer, with increased wind speeds leading to deepening mixed layers, but this relationship is not present during autumn and winter. We hypothesize that the deepening of the MLD in autumn is so strongly driven by the annual cycle in surface heat flux that the winds are less significant in the autumn. The surface heat flux drives a diurnal cycle in MLD and SST from March onwards, though this effect is much more significant in the models than in the observations.We are unable to identify one model as definitely better than the others. The only clear differences between the models are KPP’s inability to accurately reproduce summer SSTs, and the OSMOSIS model’s more accurate reproduction of MLS.

Improved Statistical Method for Quality Control of Hydrographic Observations

AbstractRealistic ocean state prediction and its validation rely on the availability of high quality in situ observations. To detect data errors, adequate quality check procedures must be designed. This paper presents procedures that take advantage of the ever-growing observation databases that provide climatological knowledge of the ocean variability in the neighborhood of an observation location. Local validity intervals are used to estimate binarily whether the observed values are considered as good or erroneous. Whereas a classical approach estimates validity bounds from first- and second-order moments of the climatological parameter distribution, that is, mean and variance, this work proposes to infer them directly from minimum and maximum observed values. Such an approach avoids any assumption of the parameter distribution such as unimodality, symmetry around the mean, peakedness, or homogeneous distribution tail height relative to distribution peak. To reach adequate statistical robustness, an extensive manual quality control of the reference dataset is critical. Once the data have been quality checked, the local minima and maxima reference fields are derived and the method is compared with the classical mean/variance-based approach. Performance is assessed in terms of statistics of good and bad detections. It is shown that the present size of the reference datasets allows the parameter estimates to reach a satisfactory robustness level to always make the method more efficient than the classical one. As expected, insufficient robustness persists in areas with an especially low number of samples and high variability.

An Updated View on Water Masses on the pan‐West Greenland Continental Shelf and Their Link to Proglacial Fjords

AbstractThe accelerated melt of the Greenland Ice Sheet has been linked to a sudden increase in the presence of warm subsurface coastal water in west Greenland. Yet pathways of warm coastal water along the entire west Greenland coast have remained largely unstudied. Here we present the first, near‐synoptic hydrographic observations at both the continental slope and fjord entrances of the west Greenland coastal system from Cape Farewell (59°N) to Melville Bay (75°N) in summer 2016. We observed a distinct north‐south division in the water mass distribution in west Greenland, approximately partitioned by the northern part of Davis Strait, and a division between the continental slope and fjord entrances. Waters from the regional southern freshwater source with origin in the East Greenland Current that rounds Cape Farewell are not observed to enter Baffin Bay. The regional heat source transported by the West Greenland Current is blocked by Southwest Greenland Coastal Water in the south but the deep connections in the north allow warm deep Subpolar Mode Water to enter fjords. Furthermore, we observed cold and relative saline Baffin Bay Polar Water over the inner part of the banks, periodically reaching as far south as 64°N, suggesting the presence of an undescribed southward current at the Southwest Greenland continental shelf.

Observations of the warm-tongue circulation in the northern East China Sea

A subsurface thermohaline front semi-permanently formed in association with near-bottom cyclonic circulation in the northern East China Sea was newly found from detailed hydrographic data collected during two cruises in February 2017 (winter) and April 2018 (spring) along with supplementary satellite remote sensing and historical hydrographic data. An alternate intruding frontal structure in water properties was observed across the cyclonic circulation in both seasons as formed by two contrasting water masses—low-temperature and low-salinity (i.e., low spiciness) water transported by the East China Sea Current and high-temperature and high-salinity (i.e., high spiciness) water transported by the Tsushima Warm Current. Consistent structures were confirmed from current observations during the two cruises, historical hydrographic observations, and satellite altimetry-derived sea surface height and surface frontal structure, indicative of retroflection of the Cheju Warm Current as deemed by the seasonal development of thermal stratification in spring. Our results reveal significant heat and material exchanges between the open Pacific and the broad shelf, particularly via diapycnal mixing and cross-front transports associated with across-front flow and cyclonic circulation, in the northern East China Sea.

Near-inertial internal waves and multiple-inertial oscillations trapped by negative vorticity anomaly in the central Sea of Japan

This study aims to reveal the interaction between wind-induced internal waves and mesoscale features in a frontal region of the central Sea of Japan. A mooring system with an upward-looking acoustic Doppler current profiler was deployed over a meandering pathway of the Tsushima Warm Current. The annual record of horizontal current showed an extraordinary event of near-inertial internal waves (NIWs) that were trapped in a mesoscale warm-core eddy, characterized by negative vorticity anomaly. Near the critical depth inside the eddy, where background vorticity vanishes, the amplitude of downward-traveling NIW reached 50–70 cm s−1 in root-mean-square current magnitude. The amplified NIWs were concurrent with the emergence of superinertial oscillations, peaking at multiple inertial (MI) frequencies, i.e., double, triple, and quadruple inertial. The MI oscillations are regarded as arising through nonlinear resonance of NIW. Examining local surface winds suggest that a fast-moving cyclone passing over the site initially generated the NIW event. The trapping and amplification processes of wind-generated NIW are examined by a series of ray-tracing simulations, where background vorticity is reconstructed from in situ hydrographic observations. Based on the results above, a specific picture model of kinetic energy circulation around the Sea of Japan can be presented: the atmospheric event provides momentum energy into the ocean surface, resulting in the generation of NIWs and trapping within anticyclonic vortices. A part of the near-inertial kinetic energy is then transferred into the MI oscillations if NIW is vigorously amplified near the critical depth; then it likely dissipates into turbulence.

Spatiotemporal changes of ocean carbon species in the western North Pacific using parameterization technique

We constructed parameterizations for the estimation of dissolved inorganic carbon (DIC) and pH in the western North Pacific, including Japanese coastal regions. Parameterizations, determined as a function of potential temperature (θ) and dissolved oxygen (DO), provided strong correlations with direct measurements for DIC [the coefficient of determination (R2) = 0.99; the root mean square error (RMSE) = 8.49 µmol kg−1] and pH (R2 = 0.98, RMSE = 0.030). Predicted DIC and pH values were consistent with previous independent hydrographic observations, generally within 3 µmol kg−1 for DIC and 0.010 for pH. By applying these parameterizations to climatological seasonal θ and DO data and time-series θ and DO data from autonomous profiling floats (Argo) in the western North Pacific, large spatiotemporal variations in DIC and pH were demonstrated. Results indicated that the rate of increase in anthropogenic DIC above a depth of 400 m along 30° N in the western North Pacific was + 0.86 ± 0.11 µmol kg−1 year−1 between 1994 and 2004. This decreased by + 0.58 ± 0.05 µmol kg−1 year−1 after 2004 partly due to increasing anthropogenic nitrogen input, suggesting that the decrease has induced the slowdown of ocean acidification in the western North Pacific. The declining trend in anthropogenically induced ocean pH was found to be 0.0021 ± 0.0016 during 1994–2013.

On the Wind Contribution to the Variability of Ocean Currents Over Wide Continental Shelves: A Case Study on the Northern Argentine Continental Shelf

AbstractThe southwestern Atlantic Ocean has one of the largest and most productive continental shelves of the southern hemisphere. Despite its relevance, its circulation patterns have been largely inferred from hydrographic observations and numerical models. Here we describe the variability of the shelf circulation based on the analysis of 11 months of multilevel currents measured by two bottom‐mounted acoustic Doppler current profilers deployed over the continental shelf at 39°S. The record‐length mean is 12 and 13 cm/s in the upper layer and decreases to 6 and 8 cm/s near the bottom, at the deployment nearer and further from the coast, respectively. The mean flow direction is toward the NE, following the orientation of the isobaths. Measurements at both sites show that the alongshore barotropic component accounts for 83% of the variability observed and are well correlated (0.86), suggesting a relatively uniform flow, which is presumably driven by large‐scale forcing. Indeed, large‐scale wind stress patterns dominate the temporal variability of the in situ currents and the passage of atmospheric fronts induces significant changes in the observed currents at all depths. We found that for 12% of the measurements the currents reverse the direction to the SW in response to these atmospheric patterns. Furthermore, the analysis of sea surface height reconstructed from bottom pressure measurements at both sites and from a coastal tide gauge reveals that the variability of the alongshore currents is driven by the cross‐shore pressure gradients generated by the alongshore wind stress.

A Barotropic Vorticity Budget for the Subtropical North Atlantic Based on Observations

AbstractTo ground truth the large-scale dynamical balance of the North Atlantic subtropical gyre with observations, a barotropic vorticity budget is constructed in the ECCO state estimate and compared with hydrographic observations and wind stress data products. The hydrographic dataset at the center of this work is the A22 WOCE section, which lies along 66°W and creates a closed volume with the North and South American coasts to its west. The planetary vorticity flux across A22 is quantified, providing a metric for the net meridional flow in the western subtropical gyre. The wind stress forcing over the subtropical gyre to the west and east of the A22 section is calculated from several wind stress data products. These observational budget terms are found to be consistent with an approximate barotropic Sverdrup balance in the eastern subtropical gyre and are on the same order as budget terms in the ECCO state estimate. The ECCO vorticity budget is closed by bottom pressure torques in the western subtropical gyre, which is consistent with previous studies. In sum, the analysis provides observational ground truth for the North Atlantic subtropical vorticity balance and explores the seasonal variability of this balance for the first time using the ECCO state estimate. This balance is found to hold on monthly time scales in ECCO, suggesting that the integrated subtropical gyre responds to forcing through fast barotropic adjustment.

Upper Ocean Distribution of Glacial Meltwater in the Amundsen Sea, Antarctica

AbstractPine Island Ice Shelf, in the Amundsen Sea, is losing mass due to increased heat transport by warm ocean water penetrating beneath the ice shelf and causing basal melt. Tracing this warm deep water and the resulting glacial meltwater can identify changes in melt rate and the regions most affected by the increased input of this freshwater. Here, optimum multiparameter analysis is used to deduce glacial meltwater fractions from independent water mass characteristics (standard hydrographic observations, noble gases, and oxygen isotopes), collected during a ship‐based campaign in the eastern Amundsen Sea in February–March 2014. Noble gases (neon, argon, krypton, and xenon) and oxygen isotopes are used to trace the glacial melt and meteoric water found in seawater, and we demonstrate how their signatures can be used to rectify the hydrographic trace of glacial meltwater, which provides a much higher‐resolution picture. The presence of glacial meltwater is shown to mask the Winter Water properties, resulting in differences between the water mass analyses of up to 4‐g/kg glacial meltwater content. This discrepancy can be accounted for by redefining the “pure” Winter Water endpoint in the hydrographic glacial meltwater calculation. The corrected glacial meltwater content values show a persistent signature between 150 and 400 m of the water column across all of the sample locations (up to 535 km from Pine Island Ice Shelf), with increased concentration toward the west along the coastline. It also shows, for the first time, the signature of glacial meltwater flowing off‐shelf in the eastern channel.