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The Antarctic Slope Current is guided by the topographic gradient of the Antarctic continental slope and creates a dynamical barrier between the continental shelf and the open ocean. The current's vertical structure varies around the continent affecting cross‐slope water mass exchange with consequences for Antarctic mass loss, ventilation of the deep ocean, and carbon uptake. The Antarctic Slope Current is surface‐intensified in many regions but bottom‐intensified in regions of dense overflows. This study investigates the role of dense overflows in modifying the dynamics of the bottom‐intensified flow using a 0.1° global ocean‐sea ice model. The occurrence of bottom‐intensification is tightly linked with dense overflows and bottom speeds correlate with dense overflows on interannual time scales. A lack of vertical connectivity between the bottom and surface flow, however, suggests that the along‐slope bottom water flows are coincidentally co‐located with the Antarctic Slope Current, rather than dynamically a part of the current. Plain Language Summary: The Antarctic Slope Current is a narrow ocean current that travels around Antarctica following the continental slope. It separates the shallow and cold continental shelf from much warmer waters in the open ocean. Intrusions of the relatively warm water across the continental slope impacts melting of Antarctic ice shelves and global sea level rise. Understanding what controls the strength and variability of the Antarctic Slope Current is therefore important. The Antarctic Slope Current usually has the largest velocities near the surface, but there are regions where there is also a strong bottom flow. We use a coupled ocean‐sea ice model and show that the bottom flow is controlled by the export of very dense shelf water that flows down the continental slope in a few locations around the continent. However, the bottom and surface flow does not vary together on interannual time scales which tells us that the two components of the Antarctic Slope Current are largely independent. The result is important as the formation of dense shelf water is expected to reduce in the future which will impact the deep flow, but not the surface component. Key Points: Simulations reveal a close spatial relationship between the bottom‐intensified Antarctic Slope Current and Dense Shelf Water exportInterannual variability of Dense Shelf Water is reflected in the bottom speed of the bottom‐intensified Antarctic Slope CurrentThe surface component varies independently from the bottom‐intensified flow implying two distinct, co‐located currents [ABSTRACT FROM AUTHOR]

The geodynamic response of continental margins to erosion may reveal current and past changes in the location of the continental slope. We model the three‐dimensional isostatic response to erosion along the retreating Beringian Margin to test the hypothesis that the margin and its canyons were dominantly eroded during the Pleistocene. Our results predict 900 m of flexural uplift, though evidence for this magnitude of uplift is not observed in Pleistocene and younger deposits, suggesting a substantial amount of margin erosion occurred pre‐Pleistocene. Uplift of this magnitude may be represented by a Late Miocene unconformity across the Bering Shelf and upwarped subsurface units at the base of the Beringian Margin. The distribution of uplift from varying canyon incision and margin retreat scenarios has implications for the extent of geodynamic response during erosional periods. Plain Language Summary: Reconstructing the balance between erosion and deposition along continental margins necessitates constraining multiple variables such as sedimentation rates, tectonic subsidence, isostatic compensation, and compaction. However, reconstructing this balance along erosion‐dominated margins is hindered by the inherent removal (from eroding areas) and access challenges (deep‐water, where eroded materials deposit) to geologic records of erosion. We use geodynamics to link erosion and the resulting rebound of the crust to constrain the magnitude and timing of continental margin retreat. We model the isostatic response to a range of submarine erosion scenarios along the Beringian Margin, which are hypothesized to have formed primarily during the Pleistocene. Our results show that the magnitude of uplift predicted by the mass of sediment removed along the Beringian is too substantial to have entirely occurred since the Pleistocene, suggesting the canyons are older than previously thought. Geophysical subsurface data may instead favor isostatic upwarping that began as early as the Miocene suggesting that margin retreat and canyon incision along the deeply incised Beringian Margin has been ongoing for several millions of years. Key Points: Flexural isostatic modeling of the Beringian margin canyons suggests their erosion predates the PleistoceneSlope retreat has a greater impact than canyon enlargement on geodynamic responses to continental margin erosion, even for very large canyonsCumulative erosion of multiple incising canyons predicts a flexural response beyond the flexural wavelength of an individual canyon [ABSTRACT FROM AUTHOR]

Recent progress in understanding Beaufort Gyre (BG) dynamics reveals an important role of ice‐ocean stress in stabilizing BG freshwater content (FWC) over seasonal to interannual timescales. But how the BG's stratification and FWC respond to surface forcing over decadal timescales has not been fully explored. Using a global ocean‐sea ice model, we partition the BG into upper, middle (halocline), and lower (thermocline) layers and perform a volume budget analysis over 1948–2017. We find that the BG's asymmetric geometry (with steep and tight isohalines over continental slopes relative to the deep basin) is key in determining the mean volume transport balance. We further find that a net Ekman suction during 1983–1995 causes the upper and middle layers to deflate isopycnally, while an enhanced Ekman pumping during 1996–2017 causes these layers to inflate both isopycnally and diapycnally, the latter via anomalous flux from the upper to the middle layer. Plain Language Summary: The Beaufort Gyre (BG) has increased its liquid freshwater content (FWC) by 40% in the past two decades. If released and transported downstream to the subpolar North Atlantic Ocean, the excess water might affect the ocean circulation via suppression of deep‐water formation. However, which layer is responsible for BG freshwater accumulations and releases over decadal timescales and the corresponding physical processes remain unclear, hampering our attempts to make future predictions. Here we use an ocean‐sea ice model to explore such changes in its three characteristic layers (upper mixed‐layer water, Pacific Water in the middle layer, and Atlantic Water in the lower layer). We find that the asymmetry of the BG, which has been simplified as a symmetric bowl shape in most previous studies, is important in determining the BG's layered mean state. Over decadal timescales, changes in BG volume are controlled by annual‐mean Ekman pumping/suction resulting from combined wind and ice‐ocean stresses. This study emphasizes the role of asymmetric geometry in determining the BG mean volume balance. It also explores the role of mean flow across the gyre's lateral boundaries in regulating BG's volume and FWC over decadal timescales. Key Points: The Beaufort Gyre's asymmetric geometry is the key to explain a net mean lateral outflow in the upper layer, despite Ekman convergenceIn the mean, the Gyre is fed by a northeast inflow into its middle layer, with outflow to its southwest from both upper and middle layersDeflation/inflation processes are asymmetric in response to anomalous Ekman suction/pumping on decadal timescales [ABSTRACT FROM AUTHOR]

The Gulf of Maine's lunar semidiurnal (M2) ocean tide exhibits spatially coherent amplitude changes of ∼1–3 cm on interannual time scales, though no causative mechanism has been identified. Here we show, using a specially designed numerical modeling framework, that stratification changes account for 32%–48% (Pearson coefficient 0.58–0.69) of the observed M2 variability at tide gauges from 1994 to 2019. Masking experiments and energy diagnoses reveal that the modeled variability is primarily driven by fluctuations in barotropic‐to‐baroclinic energy conversion on the continental slope south of the gulf's mouth, with a 1‐cm amplitude increase at Boston corresponding to a ∼7% (0.30 GW) drop in the area‐integrated conversion rate. Evidence is given for the same process to have caused the decade‐long M2 amplitude decrease in the Gulf of Maine beginning in 1980/81. The study has implications for nuisance flooding predictions and space geodetic analyses seeking highest accuracies. Plain Language Summary: The height of the twice‐daily tide at Boston is about 135 cm, but researchers have long noted that this value fluctuates by about 1–3 cm from year to year. Here we show that the annual tidal height changes—seen in fact throughout the Gulf of Maine—are closely linked to how seawater density is distributed three‐dimensionally in the region. In particular, as tidal currents enter the gulf over steep underwater topography, the vertical distribution of density determines how much of the incoming wave energy is scattered back as internal tides into the deeper Northwest Atlantic. In years where this conversion of wave energy drops by 7% from its nominal value of 4 Gigawatt, the surface tide at Boston typically increases by 1 cm. Climate‐induced changes in ocean temperature and density may strengthen or weaken the conversion effect and thus slightly alter the role of tides in coastal flood events. Key Points: We propagate the M2 tide through realistic, annually varying density structures (1993–2019) in a regional Gulf of Maine modelStratification changes explain 32%–48% of the observed, cm‐level M2 amplitude variability at coastal tide gauges from 1994 to 2019Modeled M2 changes mainly reflect fluctuations in the barotropic‐baroclinic energy conversion rate on the New England continental slope [ABSTRACT FROM AUTHOR]

The Gulf Stream is the only pathway in the subtropical North Atlantic by which warm water flows poleward. This transport of warm water and return of cold water at depth is called the Atlantic Meridional Overturning Circulation (AMOC). The dynamic method is applied to hydrocasts collected since the 1930s to estimate upper‐ocean transport (0–1,000 m) between the U.S. Continental Slope and Bermuda and separately to Africa with focus on the longest directly observable timescale. Calculating transport between the Slope and Bermuda eliminates the Gulf Stream's northern and southern recirculation gyres, while calculations between the Slope and Africa remove all other recirculating geostrophic flow. The net Slope‐Bermuda upper‐ocean transport is estimated to be 41.1 ± 0.4 Sv, decreasing by 2.0 ± 0.8 Sv between 1930 and 2020. The AMOC contribution is 18.4 ± 0.6 Sv, decreasing by 0.4 ± 0.6 Sv between 1930 and 2020. Plain Language Summary: The Gulf Stream is the only pathway by which the ocean transports warm water to high latitudes in the North Atlantic and Nordic Seas where it cools, sinks and flows back south at intermediate depths from the subpolar North Atlantic and at greater depths from the Nordic Seas. This circulation pattern, the Atlantic Meridional Overturning Circulation (AMOC), plays an important role in Earth's climate. But the Gulf Stream is also the wester boundary current to the horizontal circulation driven by the trade winds and westerlies at low and mid‐latitudes. The Gulf Stream also drives large recirculations to its south and north. In this study we use hydrographic data taken since the 1930s to calculate the poleward flow in the Gulf Stream, while canceling out the recirculation and wind‐driven transports to estimate the strength of the north‐flowing portion of the AMOC. There is evidence for a 2.0 Sv Gulf Stream slow‐down between 1930 and 2020. Whether and to what extent this reflects a slowdown of the AMOC or wind‐driven circulation cannot be established with certainty. Our estimate of a 0.4 Sv AMOC decrease is reported with low confidence. Key Points: Direct measurements along the Oleander line (∼37°N) show that the depth of the Atlantic Meridional Overturning Circulation (AMOC) maximum is close to 1,000 mNet northward transport between North America and Bermuda equals 41.1 ± 0.4 Sv of which the AMOC accounts for about 18.4 ± 0.6 SvNet northward transport NW of Bermuda slowed by 2 ± 0.8 Sv between 1930 and 2020 with 0.4 ± 0.6 Sv attributed to an AMOC decline [ABSTRACT FROM AUTHOR]

The global ocean mass has significantly changed during recent decades, which may induce changes in the abyssal ocean circulation. Due to the lack of in situ observations, the response of the abyssal South China Sea (SCS) circulation to ocean mass changes remains unclear. The ocean state estimate, Estimating the Circulation and Climate of the Ocean (ECCO), provides the long‐term ocean mass changes in terms of ocean bottom pressure (OBP). Here we use the ECCO OBP data to quantify decadal changes in the abyssal SCS circulation and reveal a weakening trend. The OBP gradient trend manifests in forms of an anomalous anticyclonic circulation that is intensified by topographic effects along the continental slopes to reduce the existing abyssal circulation. The weakening abyssal circulation corresponds to the weakening Luzon Strait overflow that yields a decreasing vorticity budget in the abyssal SCS. Plain Language Summary: Caused by global warming, ice melt and long‐term atmospheric forcing changes, ocean mass has been increased and redistributed over the past decades. The changes in ocean mass distribution may lead to changes in abyssal ocean circulation. The changes in the abyssal South China Sea (SCS) circulation remains unclear due to the lack of in situ observations. This study uses the Estimating the Circulation and Climate of the Ocean (ECCO) ocean state estimate to quantify decadal changes in the abyssal SCS circulation. Our analysis of the ECCO ocean bottom pressure data reveals a weakening trend in the abyssal SCS circulation that is consistent with the recently ascertained weakening Luzon Strait overflow. Key Points: Trends in the ECCO ocean bottom pressure (OBP) gradients indicate a decadal weakening abyssal South China Sea (SCS) circulationThe weakening circulation coincides with the weakening Luzon Strait overflow that yields a decreasing vorticity budget in the abyssal SCSThe anomalous anticyclonic circulation is dominated by OBP gradient trends and intensified by topographic effects along continental slopes [ABSTRACT FROM AUTHOR]

The 2022 volcanic eruption in Tonga caused an unusually large tsunami around the Pacific. It travels with a faster apparent velocity and has larger amplitudes at long distances than what would be expected from a conventional tsunami from the volcanic source. This tsunami was generated by the moving atmospheric Lamb wave and traveled at the speed of the Lamb wave (0.31 km/s). Japanese data showed the amplitude of this first tsunami becomes small when approaching the coast, due to the weaker air‐sea coupling at the shallow depth. This wave split when passing the continental slope, and traveled at the speed of the ocean gravity wave. Therefore, the tsunami observed at the coast is delayed by thousands of seconds from the passage of the Lamb wave. Tsunamis generated by this atmospheric mechanism have not been previously observed by modern digital recording systems and should be considered in the tsunami warning systems. Plain Language Summary: A tsunami is usually generated by the sudden changes of the water heights, and caused by offshore earthquakes, coastal landslides, and submarine volcanic eruptions. The generated vertical displacement of the water propagates as a very long wave, and the tsunami waves become compressed with increased heights as they approach the coast. The 2022 volcanic eruption in Tonga caused an unusually large tsunami, which cannot be explained by conventional sources. The speed and amplitude are very different from theoretical values: the speed is about 0.31 km/s, whereas the average tsunami speed in the Pacific is 0.2 km/s. These data suggest that the tsunami from the Tonga eruption was excited by a pulse of atmospheric pressure as it traveled from the volcano. This source of the tsunami in the atmosphere needs to be considered for the tsunami warning system in the future. Key Points: Lamb wave produced by the volcanic eruption excites the sea surface as it travels across the Pacific and generates a tsunami pulseThe effect of the air‐sea coupling becomes weaker as the Lamb wave approaches the coastThe tsunami wave splits when passing the continental slope of Japan causing more complex waves [ABSTRACT FROM AUTHOR]

A significant fraction of the Eulerian‐mean downwelling feeding the lower limb of the Atlantic Meridional Overturning Circulation (AMOC) occurs along the subpolar North Atlantic continental slopes and is maintained by along‐boundary densification and large‐scale geostrophic balance. We here use Argo and shipboard hydrography data to map the 2002–2015 long‐term mean density field along the boundary via a dedicated optimal interpolation tool. The overall downstream densification implies an Eulerian‐mean downwelling of 2.12 ± 0.43 Sv at 1100 m depth between Denmark Strait and Flemish Cap. A clear regional pattern emerges with downwelling in the Irminger Sea and western Labrador Sea and upwelling along Greenland western continental slope. Comparisons with independent cross‐basin estimates confirm that vertical overturning transport across the marginal seas of the subpolar North Atlantic mainly occurs along the continental slopes, and suggest the usefulness of hydrographic data in providing quantitative information about the sinking branch of the AMOC. Plain Language Summary: The Atlantic Meridional Overturning Circulation (AMOC), a critical component of the Earth's climate system due to its role in redistributing heat and freshwater between low and high latitudes, is anticipated to decline over the next century. The downwelling of surface waters in the subpolar North Atlantic that feeds the lower limb of AMOC is a vital yet vulnerable process. As revealed by previous theoretical and modeling work, the overall downstream densification along the boundary results in a significant boundary downwelling. Here, the density along the western boundary between Denmark Strait and Flemish Cap is reconstructed to provide a first observation‐based description of the regional and seasonal distribution of this boundary‐focused downwelling in the subpolar North Atlantic. This study not only provides valuable insights into how to improve existing ocean circulation theories of overturning but also contributes to a solid benchmark for evaluating how climate models simulate the sinking branch of the AMOC. Key Points: The long‐term mean full‐depth density field of the subpolar North Atlantic's boundary is reconstructed from hydrography dataThe along‐boundary densification results in a 2.12 ± 0.43 Sv Eulerian‐mean downwelling between Denmark Strait and Flemish CapA first observation‐based regional and seasonal distribution of near‐boundary Eulerian‐mean downwelling is provided [ABSTRACT FROM AUTHOR]

Fluid seepage sometimes impacts occurrences of submarine landslides, but the role of landslide deposits in fluid seepage is often neglected. Bathymetric data show that more than 1,000 seabed mounds are scattered over an elongated area at the shelf edge of the northwestern South China Sea. Vertical seismic pipes in the subsurface are spatially correlated with both surface mounds and buried landslide deposits. Detailed analyses within the three‐dimensional (3D) seismic survey show that at least 221 of 231 (∼96%) of the mounds have spatial correlation with the top edges of multiple stages of buried landslide scarps that are stepping seawards from deep to shallow, indicating a causal relationship between buried landslide deposits and fluid seepage. Fluids emitted via differential compaction and hydrofracturing in geological setting characterized by frequent tectonism, flowed upwards along shear surfaces and scarps of landslide deposits and pipes, and eventually provided hard substrates for mound building. Plain Language Summary: Submarine landslides are serious geohazards and the most common sedimentary units on the continental slope. Submarine landslides can be induced by sediment input and processes of becoming rocks, tectonism events such as earthquakes, seawater movement such as tsunamis. Among these factors, fluid seepage‐associated structures such as fluid conduits frequently coexist with landslide deposits; hence, fluid seepage can contribute significantly to occurrences of landslides. However, the impact of landslide deposits on fluid flow is of less concern. Using geophysical data in the northwestern South China Sea, a multitude of seabed mounds are observed in an elongated region along the shelf edge. Most mounds are underlain by pipe‐shaped fluid conduits that link downwards to buried landslide deposits. Mounds have a strong vertical correspondence with the top edges of buried landslide scarps. We conclude that different degrees of compaction between landslide deposits and surrounding sediment and generation of fractures driven by high hydraulic pressure, induced the release of overpressure fluids below bases of landslide deposits, which served as effective fluid conduits and perhaps as part of walls of overpressure fluid layers. The building and release of overpressure fluids were also impacted by ample sediment input and frequent tectonic movements like faulting. Key Points: We uncover more than 1,000 seabed cold‐water coral mounds in the NW South China SeaBuried landslide deposits induced fluid seepage with shear surfaces and scarps as conduits, providing hard substrates for mounds buildingTectonic faulting assisted buried landslide deposits to release overpressure fluids [ABSTRACT FROM AUTHOR]

Primary productivity in the Arctic Ocean is experiencing dramatic changes linked to the receding sea ice cover. The vertical transport of nutrients from deeper water layers is the limiting factor for primary production. Here, we compare coincident profiles of turbulence and nutrients from the Siberian Seas in 2007, 2008, and 2018. In all years, the water column structure in the upstream region of the Arctic Boundary Current promotes upward nutrient transport, in contrast to the regions further downstream, and there are first indications for an eastward progression of these conditions. In summer 2018, strongly enhanced vertical nitrate flux and primary production above the continental slope were observed, likely related to a remote storm. The estimated contribution of these elevated fluxes above the slope to the Pan‐Arctic vertical nitrate supply is comparable with the basin‐wide transport, and is predicted to increase with declining sea ice cover in the future. Plain Language Summary: Microscopic algae, growing in the sunlit surface layer of the ocean, provide food for other species and form the basis of the ecosystem. In the Arctic Ocean, their growth is limited by the availability of nutrients. The main source of these nutrients are waters entering from the Atlantic and Pacific Oceans. These nutrient‐rich waters reside far below the sunlit zone, and vertical mixing is required to bring them upwards to support algal growth. With rapidly declining summer sea ice and changes in the ocean layering, these mixing processes might substantially change. Changes are considered most likely in the region of the steep slopes in the Siberian Seas. To investigate this, we analyze nutrient and mixing measurements in this region from 2007, 2008, and 2018. In 2018, we observed strong mixing, which is connected to ice free conditions and a process that has only recently been described. This strong mixing only happens at the narrow, steep slope region, but might supply the same amount of nutrients to the surface zone as the weak mixing over the much larger area of the deep basins. Key Points: Nutrient fluxes are higher in the western compared with the eastern Laptev SeaEpisodic mixing above the slope might significantly contribute to Pan‐Arctic vertical nitrate supplyIn the central Laptev Sea, a thinning halocline and shoaling Atlantic Water indicate an emerging regime shift in line with Atlantification [ABSTRACT FROM AUTHOR]

Tropical marginal seas host important sedimentary archives that may be exploited to reveal past changes in continental erosion, chemical weathering, and ocean dynamics. However, these records can be challenging to interpret due to the complex interactions between climate and particulate transport across ocean margins. For the southern South China Sea over the last 90 Kyr, we observe a contrasting temporal relationship between the deposition of clay minerals and magnetic minerals, which were associated with two different hydrodynamic modes. Fine‐grained clay minerals can be carried in suspension by ocean currents, leading to a rapid response to regional climate‐driven inputs. In contrast, changes in magnetic mineralogy were linked to glacial‐interglacial sea level variability, from which we infer a control by bedload transport and resuspension. Overall, this study indicates that the transfer pathways and mechanisms imparted by varying hydrodynamic conditions exert a substantial influence on the distribution of terrigenous material in continental margin sediments. Plain Language Summary: Sediments that accumulate in the ocean along continental margins contain the minerals that were eroded and transported by rivers on the nearby land masses. Past changes in the composition of such sediments can be used to reconstruct the history of physical erosion and chemical weathering on the continents, but can also be affected by ocean dynamics over orbital and millennial timescales. Here, clay mineralogy, sediment grain size, and rock magnetic signatures are measured in a core from the continental slope of the southern South China Sea, spanning the last 90 thousand years. Differences between the clay mineral and magnetic records, which are linked to finer and coarser sediment fractions respectively, indicate that specific mechanisms and timescales influenced sediment transport to the core site. This depositional framework must be considered when interpreting the timing of changes in weathering and erosion proxies hosted in sediment records from marginal seas. Specifically, fine‐grained clay minerals may reveal a rapid response to continental weathering changes, whereas the magnetic records are influenced by the transport of denser coarser‐grained minerals under the influence of sea level change. In addition, this framework could potentially serve as an indicator of past sea level change on wide continental shelves. Key Points: In the southern South China Sea, clay mineralogy responds rapidly to climate‐driven changes in inputs on orbital timescalesMagnetic content within the coarser and denser fraction is influenced by sea level change, through bedload and resuspension transportThis depositional framework may serve as a guideline for evaluating sea level change on wide continental shelves over orbital timescales [ABSTRACT FROM AUTHOR]

The North Atlantic deep water (NADW), according to the classic ocean circulation theory, moves southward as a deep western boundary current (DWBC) even though it may veer into interior and then rejoin DWBC when encountering regional circulation features, such as eddy‐driven recirculation. In potential vorticity dynamics, the eastern side of the Mid‐Atlantic Ridge (MAR) may provide a similar topographic support as the continental slope off the western boundary for a southward transport of NADW. In this article, we quantify the mean meridional NADW transports on both sides of the MAR using a data‐assimilated product and find that the flow in the eastern basin contributes about 38±14% $38\pm 14\%$ of the net southward transport of NADW from 50° to 35°N. Our study points to the importance of observing NADW transport variations on the eastern side of the MAR in order to monitor the transport strength of Atlantic Meridional Overturning Circulation. Plain Language Summary: Classic theory suggested that the Deep Western Boundary Current (DWBC) carries the cold and fresh North Atlantic deep water (NADW) from the high latitude to the south. However, float and tracer observations, albeit sparse, have revealed a possible southward pathway in the eastern North Atlantic, that is, east of the Mid‐Atlantic Ridge (MAR). Using an ocean reanalysis product, the Estimating the Circulation and Climate of the Ocean (ECCOV4r4), we show that the steady‐state NADW meridional transport in the basin east of the MAR accounts for about 38±14% $38\pm 14\%$ of the meridional transport in the whole basin between 35° and 50°N. It is found that the NADW circulation pathways are strongly influenced by ocean bottom topography. For example, the eastern flank of the MAR acts as a "western boundary" in the eastern basin against which NADW flows southward. The topographic constraint from the MAR weakens south of 35°N due to changes in the MAR height and smaller planetary vorticity f $f$. The current study has improved our understanding of NADW transport structure at mid‐latitude by quantifying the equatorward transport east of the MAR. Key Points: The eastern Atlantic basin supports nearly 38±14% $38\pm 14\%$ of the southward North Atlantic deep water (NADW) transport in mid‐latitudesThe meridional transport of NADW in the eastern basin is due to the potential vorticity constraint imposed by the Mid‐Atlantic Ridge [ABSTRACT FROM AUTHOR]

Inflow of warm modified Circumpolar Deep Water (CDW) onto the Antarctic continental shelf and into ice shelf cavities is a key driver of Antarctic ice shelf mass loss. While recent research has advanced understanding of CDW heat transport onto the continental shelf, the fate of CDW on the shelf is less understood. Here, we use Lagrangian particle tracking in an ocean‐sea ice model without ice shelf cavities to map the residence time of CDW on the Antarctic continental shelf. Mean residence times vary from <1 month in the East Antarctic to >1 year in the West Antarctic. In regions of dense water formation, transformation of CDW on the shelf limits access of CDW to ice shelves, despite strong onshore CDW heat transport. Elsewhere transformation of CDW on the shelf is weak, implying that temperature on the shelf is limited by heat transport onto the shelf or the offshore heat reservoir. Plain Language Summary: Antarctic ice shelf melt is accelerating and is linked to the inflow of warm ocean waters that melt ice shelves from below. Multiple processes regulate the amount of warm water that reaches the Antarctic ice shelves, and the relative importance of these processes in different regions is not well known. Here, we conduct a virtual particle tracking experiment in a state‐of‐the‐art ocean model to trace the pathways and timescales of warm water on the Antarctic continental shelf. We find that there is a large regional variation in how long warm waters spend on the continental shelf from less than a month to well over a year. In the regions along the continental slope where very dense water formed near the coast flows off the shelf to the abyssal ocean, this is balanced by a strong inflow of warm water onto the continental shelf, which is rapidly converted to denser water, limiting the heat from reaching the ice shelves. Elsewhere warm water can remain on the continental shelf unmodified for a long time, so the heat available for ice shelf melt is instead limited by the strength of the flow of warm water onto the continental shelf. Key Points: Particle tracking in an ocean model shows Circumpolar Deep Water (CDW) residence times on the Antarctic shelf range from a month to several yearsWater mass transformation limits CDW access to ice shelves in Dense Shelf Water formation regions, despite strong onshore CDW transportIn warm shelf regions, long residence times of CDW are associated with weak water mass transformation [ABSTRACT FROM AUTHOR]

Pacific Summer Water (PSW) plays a critical role in the ecosystem of the western Arctic Ocean, impacting sea‐ice melt and providing freshwater to the basin. Most of the water exits the Chukchi Sea shelf through Barrow Canyon, but the manner in which this occurs and the ultimate fate of the water remain uncertain. Using an extensive collection of historical hydrographic and velocity data, we demonstrate how the PSW outflow depends on different wind conditions, dictating whether the warm water progresses eastward or westward away from the canyon. The current carrying the water westward along the continental slope splits into different branches, influenced by the strength and extent of the Beaufort Gyre, while the eastward penetration of PSW along the shelfbreak is limited. Our results provide the first broad‐scale view of how PSW is transferred from the shelf to the basin, highlighting the role of winds, boundary currents, and eddy exchange. Plain Language Summary: Warm Pacific water that flows northward through Bering Strait significantly influences the melting of sea‐ice, the distribution of freshwater, and the ecosystem of the Arctic Ocean. North of Bering Strait, the water progresses poleward across the shallow Chukchi Sea shelf, with most of it draining from the shelf through Barrow Canyon. Presently, the manner in which the flow exits the canyon, as well as its ultimate fate in the basin, remains unclear. We analyzed historical data sets of temperature, salinity, and velocity, and found that the local wind alters the manner in which the water exits Barrow Canyon to the east and west, as well as the corresponding distribution of Pacific Summer Water. The current carrying the water westward along the continental slope subsequently splits into different branches farther downstream. This is modulated by the strength and extent of the Beaufort Gyre, a large recirculating flow in the basin. In contrast, the signature of the warm water in the current flowing to the east is limited. Here, we provide the first observational broad‐scale view of the pathways and evolution of warm Pacific‐origin water from the shelf to the basin, and clarify the effects of wind, boundary currents, and small‐scale turbulent motions. Key Points: Outflow of Pacific Summer Water (PSW) through Barrow Canyon is dictated predominantly by the local windThe westward flow carrying the water away from the canyon bifurcates downstream, modulated by the strength and extent of the Beaufort GyreThe eastward flow emanating from the canyon progresses far downstream, but the signature of the PSW is limited [ABSTRACT FROM AUTHOR]

Climatic changes have decreased the stability of the Gulf Stream (GS), increasing the frequency at which its meanders interact with the Mid-Atlantic Bight (MAB) continental shelf and slope region. These intrusions are thought to suppress biological productivity by transporting low-nutrient water to the otherwise productive shelf edge region. Here we present evidence of widespread, anomalously intense subsurface diatom hotspots in the MAB slope sea that likely resulted from a GS intrusion in July 2019. The hotspots (at ~50 m) were associated with water mass properties characteristic of GS water (~100 m); it is probable that the hotspots resulted from the upwelling of GS water during its transport into the slope sea, likely by a GS meander directly intruding onto the continental slope east of where the hotspots were observed. Further work is required to unravel how increasingly frequent direct GS intrusions could influence MAB marine ecosystems. Plain Language Summary As the climate has warmed, the changing large-scale circulation of the northwest Atlantic has resulted in increasing western boundary current instability. As a consequence, onshore intrusions of Gulf Stream (GS) water into the Northeast U.S. continental shelf have become increasingly frequent. The impacts of this shift on marine ecosystems have yet to be resolved. While these intrusions of low-nutrient GS water have been thought to potentially diminish biological productivity, we present evidence of an unexpectedly productive subsurface diatom bloom resulting from the direct intrusion of a GS meander toward the continental shelf. These results suggest that changing large-scale circulation has consequences for regional productivity that are not detectable by satellites by virtue of their occurrence well below the surface. [ABSTRACT FROM AUTHOR]

Intermediate nepheloid layers (INLs) form important pathways for the cross‐slope transport and vertical export of particulate matter, including carbon. While intermediate maxima in particle settling fluxes have been reported in the Eurasian Basin of the Arctic Ocean, direct observations of turbid INLs above the continental slope are still lacking. In this study, we provide the first direct evidence of an INL, coinciding with enhanced mid‐water turbulent dissipation rates, over the Laptev Sea continental slope in summer 2018. Current velocity data show a period of enhanced downslope flow with depressed isopcynals, suggesting that the enhanced turbulent dissipation is probably the consequence of the presence of an unsteady lee wave. Similar events occur mostly during ice free periods, suggesting an increasing frequency of episodic cross‐slope particle transport in the future. The discovery of the INL and the episodic generation mechanism provide new insights into particle transport dynamics in this rapidly changing environment. Plain Language Summary: In the Arctic Ocean deep basins, only a tiny fraction of the algae that grows in the surface layer sinks down to the sea floor. Most of the particles reaching the sea floor originate from the shallower regions closer to the coast. These particles have already settled on the sea floor once, and originate from rivers or algae that grew, died and sank down in shallow regions. Later, these particles are lifted off the ground again by strong turbulent motions and transported toward deeper regions in the middle of the water column. These lift‐off and transport events happen only occasionally, and have not been directly observed in the Eurasian part of the Arctic Ocean yet. Also, we present a new mechanism for the creation of turbulence, which is necessary to lift particles off the sea floor. This mechanism happens mostly during the summer season, when less sea ice is present. Based on this seasonality, it is likely that sediment transport events will become more frequent in the future, when the Arctic sea ice is further declining. Key Points: First direct observation of an intermediate nepheloid layer in the Eurasian part of the Arctic OceanCoinciding strong midwater turbulence is likely caused by a down‐slope current displacing isopycnalsSimilar downslope flow events exhibit a strong seasonality toward the ice free season [ABSTRACT FROM AUTHOR]

The stability of the West Antarctic Ice Sheet (WAIS) depends on ocean heat transport toward its base and remains a source of uncertainty in sea level rise prediction. The Antarctic Slope Current (ASC), a major boundary current of the ocean's global circulation, serves as a dynamic gateway for heat transport toward Antarctica. Here, we use observations collected from the Bellingshausen Sea to propose a mechanistic explanation for the initiation of the westward‐flowing ASC. Waters modified throughout the Bellingshausen Sea by ocean‐sea‐ice and ocean‐ice‐shelf interactions are exported to the continental slope in a narrow, topographically steered western boundary current. This focused outflow produces a localized front at the shelf break that supports the emerging ASC. This mechanism emphasizes the importance of buoyancy forcing, integrated over the continental shelf, as opposed to local wind forcing, in the generation mechanism and suggests the potential for remote control of melt rates of WAIS' largest ice shelves. Plain Language Summary: The Antarctic Slope Current (ASC) is an ocean current that separates warmer, offshore water from colder waters over the continental shelf. The ASC helps to control the movement of warm water on to the continental shelf and ultimately under floating ice shelves. Warm ocean water has contributed to the melting and thinning of West Antarctic ice shelves in recent decades. This study describes where and how the ASC first forms in West Antarctica. Previous studies have suggested that the ASC responds primarily to atmospheric winds blowing over the current itself, whereas here it is argued that processes occurring over a much larger region of the west Antarctic continental shelf may determine the strength and structure of the ASC. These results contribute to our understanding of the long‐term, for example, decades or longer, evolution of the West Antarctic Ice Sheet. Key Points: The initiation of a westward Antarctic Slope Current (ASC) occurs in the western Bellingshausen SeaThe ASC arises from the export of waters, modified by ocean‐ice interactions, to the shelf break, rather than from local wind forcingThis mechanism suggests the potential for remote control of shelf properties as well as West Antarctic ice shelf melt rates [ABSTRACT FROM AUTHOR]

Simulating Arctic Ocean mesoscale eddies in ocean circulation models presents a great challenge because of their small size. This study employs an unstructured‐mesh ocean‐sea ice model to conduct a decadal‐scale global simulation with a 1‐km Arctic. It provides a basinwide overview of Arctic eddy energetics. Increasing model resolution from 4 to 1 km increases Arctic eddy kinetic energy (EKE) and total kinetic energy (TKE) by about 40% and 15%, respectively. EKE is the highest along main currents over topography slopes, where strong conversion from available potential energy to EKE takes place. It is high in halocline with a maximum typically centered in the depth range of 70–110 m, and in the Atlantic Water layer of the Eurasian Basin as well. The seasonal variability of EKE along the continental slopes of southern Canada and eastern Eurasian basins is similar, stronger in fall and weaker in spring. Plain Language Summary: Ocean mesoscale eddies play crucial roles in the ocean, climate, and ecosystem. While this is presumably also true for Arctic eddies, their dynamics and impacts are far less understood than for lower latitudes: It is a great challenge to resolve the very small Arctic eddies in numerical simulations. Just now, owing to the development of new‐generation models, it becomes possible to resolve Arctic eddies in realistic global ocean configurations. The study presents the results from a first‐ever decadal‐scale global simulation with the Arctic Ocean at 1‐km resolution. An overview of the eddy kinetic energy and its generation is provided for the Arctic deep basin, with a focus on both spatial and seasonal variability. The current results fill some knowledge gaps in Arctic eddy energetics, and also help to identify questions that we will be able to answer with such frontier simulations in the future. Key Points: A decadal‐scale global simulation with the whole Arctic Ocean at 1‐km resolution is carried out using FESOM2A basinwide overview of Arctic eddy energetics is provided with a focus on spatial patterns and seasonal changes of eddy kinetic energyContinental slopes in southern Canada and eastern Eurasian basins are the most energetic regions featuring similar seasonality [ABSTRACT FROM AUTHOR]

A bolus is a vortex formed by an internal wave propagating upslope with the shoaling wave. The amount of sediment and biota transported by boluses travelling up the world's continental slopes and onto the continental shelves has not been established but could be vital for maintaining nutrient‐rich coastal ecosystems. Previous laboratory experiments and simulations of shoaling boluses have considered a two‐layer density stratification or a linear density profile. We present experiments on bolus formation and propagation in a stratification with a density profile ρ(z) represented by a tanh‐profile, which describes many ocean pycnoclines. Our laboratory experiments examine pycnoclines with thicknesses varying from nearly zero (a two‐layer system) to large thicknesses where the density varies almost linearly with depth. The bolus volume, displacement upslope, and available potential energy are measured as a function of pycnocline thickness. Maximum upslope displacement is found to be twice that observed for a two‐layer stratification. Plain Language Summary: Interfacial waves can be observed along the surface of the ocean, and as they approach the coastline the waves steepen, break, and ultimately surge up the beach. These waves are analogous to internal waves, which occur within the ocean at the horizontal interface of more and less dense water. These internal waves can be an order of magnitude taller than the waves seen at the beach, and the surge (called a bolus) can move kilometers along the ocean bottom. Previous efforts to model these boluses have approximated the ocean's density as two layers of different density water. However, the ocean has a more gradual transition between the layers, which can be over 100 meters thick. The impact of this transition layer, called a pycnocline, on bolus transport is poorly understood. Here, we perform laboratory experiments to demonstrate that the presence of a thick pycnocline can double the distance the bolus travels upslope and increase the amount of energy for mixing. This demonstrates the need for a better model of bolus propagation to understand how these breaking internal waves move material along the bottom of the ocean. Key Points: Boluses propagate further for pycnoclines of finite thickness than for a discontinuous stratification with two layers of uniform densityThe experiments reveal the existence of a pycnocline thickness that results in a maximum bolus propagation displacement upslopeThin pycnoclines yield large bolus volumes with high available potential energy despite short bolus propagation displacement [ABSTRACT FROM AUTHOR]

Circumpolar Deep Water (CDW) transport across the Antarctic continental slope regulates the delivery of heat to the shelf and its availability to melt floating ice shelves. The cross‐slope density field, calculated from profiles collected by conductivity‐temperature‐depth‐tagged marine mammals on the East Antarctic slope (0–160°E, above 1,000‐ to 3,000‐m isobaths), indicates eddy‐driven overturning: onshore transport of CDW and offshore transport of shallower Antarctic Surface Water. Enhanced eddy activity, determined by a spice standard deviation threshold in the CDW layer, is present over about a third of the East Antarctic slope analyzed. Significantly stronger CDW transport in regions of elevated spice variability produces subsurface temperature anomalies of 0.2–0.25 °C relative to the East Antarctic average. Estimating eddy diffusivity from the hydrography yields about 0.8 m2/s of warm CDW transport to the shelf break in high‐variability regions. Variability of eddy‐induced CDW transport influences the reservoir of heat available for transport across the shelf break. Plain Language Summary: The transport of warm water from the open ocean to the Antarctic continental shelf has global significance, as it regulates the melt of ice shelves and dense water formation. We quantify the strength of eddy‐driven transport of relatively warm Circumpolar Deep Water (CDW) across the continental slope of East Antarctica from the observed cross‐slope density structure. We find that, in general, there is an eddy‐driven overturning: deeper CDW is transported onshore while shallower Antarctic Surface Water is transported offshore. Upon closer inspection, we find that regions with elevated temperature and salinity variability on density surfaces are associated with significantly stronger onshore transport of CDW by eddies. These hot spots of eddy transport produce warm and salty anomalies at the shelf break, impacting the reservoir of heat available to the shelf. Key Points: Cross‐slope eddy transport is inferred from observed density‐layer thickness gradients in East Antarctica between 0°E and 160°EEnhanced onshore transport (≈1.5 × East Antarctic mean) of warm, salty Circumpolar Deep Water occurs in regions of high isopycnal spice variabilityEddy‐induced overturning carries ≈0.8 m2/s of CDW poleward to produce warm, salty anomalies at the shelf break in eddying regions [ABSTRACT FROM AUTHOR]

Submarine groundwater discharge could impact the transport of critical solutes to the ocean. However, its driver(s), significance over geological time scales, and geographical coverage are poorly understood. We characterize a submarine groundwater seep from the continental slope off northern Norway where substantial amount of meteoric water was detected. We reconstruct the seepage history from textural relationships and U‐Th geochronology of authigenic minerals. We demonstrate how glacial‐interglacial dynamics have promoted submarine groundwater circulation more than 100 km offshore and result in high fluxes of critical solutes to the ocean. Such cryosphere‐hydrosphere coupling is likely common in the circum‐Arctic implying that future decay of glaciers and permafrost in a warming Arctic is expected to attenuate such a coupled process and thus decreases the export of critical solutes. Plain Language Summary: Occurrence of meteoric groundwater (freshwater originated from precipitation such as rain and snow) in the global ocean is an unexpected but seemingly common phenomenon. Here, we report evidence for meteoric groundwater flow at ~800‐m water depth from the coast of northern Norway. Dating of the chemically formed carbonate rocks on the seafloor, an oxidation product of methane, reveals that the groundwater flow was strongest when large ice sheets occupied the nearby shelf. Our results confirm the temporal and geographical scales of meteoric groundwater flow in the Arctic region and highlight its impact on carbon cycling and ocean chemistry. Key Points: We document the connection between glacial dynamics and the discharge of meteoric water from the eastern Norwegian SeaA large quantity of critical solutes discharge to the ocean as a result of water circulationOffshore presence/discharge of freshwater is a common phenomenon in the circum‐Arctic Ocean that have profound impact on ocean chemistry [ABSTRACT FROM AUTHOR]

The meridional coherence, connectivity, and regional inhomogeneity in long‐term sea surface temperature (SST) variability over the Northwest Atlantic continental shelf and slope from 1982–2018 are investigated using observational data sets. A meridionally concurrent large SST warming trend is identified as the dominant signal over the length of the continental shelf and slope between Cape Hatteras in North Carolina and Cape Chidley, Newfoundland and Labrador, Canada. The linear trends are 0.37 ± 0.06 and 0.39 ± 0.06 °C/decade for the shelf and slope regions, respectively. These meridionally averaged SST time series over the shelf and slope are consistent with each other and across multiple longer observational data sets with records dating back to 1900. The coherence between the long‐term meridionally averaged time series over the shelf and slope and basin‐wide averaged SST in the North Atlantic implies approximately two thirds of the warming trend during 1982–2018 may be attributed to natural climate variability and the rest to externally forced change including anthropogenic warming. Plain Language Summary: This study investigates long‐term changes in the sea surface temperature (SST) since 1982 over the Northwest Atlantic continental shelf and slope. In particular, we focus on the changes consistently found from Cape Hatteras, North Carolina, United States, to Cape Chidley, Newfoundland and Labrador, Canada. The SST warming rates concurrently found over this large latitudinal range are 0.37 ± 0.06 and 0.39 ± 0.06 °C/decade for the shelf and slope regions, respectively. Analysis indicates that approximately two thirds of the warming trend during 1982–2018 may be attributed to natural climate variability and the rest to externally forced change including anthropogenic warming. Our findings are further confirmed by comparing with multiple other observational data sets. Key Points: Meridionally coherent surface warming is observed over the Northwest Atlantic continental shelf and slopeThe linear trends in SST exhibit distinct spatial maxima in the slope region off Georges Bank and Cape HatterasLong‐term variations in the meridional means of SST are highly correlated with basin‐averaged values [ABSTRACT FROM AUTHOR]