Journal of Geophysical Research-Oceans最新文献

Suspended sediment fluxes on continental shelves impact geomorphology, habitats, and biogeochemistry. In the coastal Arctic, the rate at which sediment is transported to locations where it can be sequestered also impacts the fate of carbon from thawing permafrost. This study used a numerical model to analyze the role of wave events on open water suspended sediment fluxes over hourly to monthly timescales. A coupled hydrodynamic—sediment transport model, the Regional Ocean Modeling System—Community Sediment Transport Modeling System, was implemented within the Coupled Ocean-Atmosphere-Wave-Sediment Transport (COAWST) Modeling System for the 2020 open water season on the Alaskan Beaufort Sea shelf. Results showed that wave- and current-induced bed shear stresses were frequently capable of resuspending sediment. Waves dominated bed shear stresses in depths shallower than 10 m and currents dominated in depths deeper than 20 m. Suspended sediment flux directions oscillated with the currents, which were eastward on average. However, since large waves tended to occur during westward currents, time-averaged suspended sediment fluxes on the inner shelf were westward. Sensitivity tests were performed where significant wave heights were (a) set to zero and (b) doubled, which showed that waves increased the fraction of time that sediment could be resuspended by up to 50% and increased westward suspended sediment fluxes on the inner shelf. Overall, the results improve our understanding of how waves impact sediment fluxes on the Beaufort Sea shelf during the open water season and suggest that terrestrially derived sediment may be transported westward along the inner shelf.

Stress–strain relationships are fundamental to understanding deformation mechanics in any material. In sea ice, stress–strain relationships are typically observed by measuring the strain resulting from known stress in samples wholly or partially isolated from the surrounding ice. Such observations show sea ice behaves elastically at short timescales, and the effective parameters describing this elastic behavior vary with temperature, salinity, and strain rate. However, these experiments often employ larger strain rates than are typical for intact, in situ ice, are labor intensive, and are typically limited to meter scale. Here we utilize a novel synthesis of existing observation techniques to quantify the effective elastic modulus and Poisson's ratio of a km-scale area of heterogeneous, drifting sea ice surrounding the Sea Ice Dynamics Experiment (SIDEx) drifting ice camp in the Beaufort Sea. In-ice point measurements of two-dimensional horizontal stress from an array of 31 vibrating wire stress gauges (VWSG), distributed over a ∼1.5 km radius area, allow us to observe natural forcing conditions. A ground-based interferometric radar provides contemporaneous one-dimensional surface strain measurements collocated with stress within 22.5 m resolution cells. We find an effective elastic modulus of 2.4 $��\pm �$ 0.6 GPa and effective Poisson's ratio of 0.5 $��\pm �$ 0.2 describe the heterogeneous, mixed multi- and first-year ice within the area of the VWSG array. These values are broadly consistent with prior laboratory and field experiments while offering a novel point of reference for real-world sea ice deformation behavior under natural conditions.

Tidal mixing in the Kara Gates Strait (KGS) modulates transport of warm water from the Barents Sea to the Kara Sea, playing a pivotal role in the transformation of Atlantic Water and the heat budget of the Arctic. Although previous studies have identified the presence of large-amplitude internal waves by the interaction of M₂ tidal currents with rough topography, understanding of the energetics and dynamics of M₂ internal tides (ITs) in the KGS remains limited. Using a high-resolution model, this study investigates the generation, propagation, and dissipation of M₂ ITs. The results reveal that, in addition to the previously identified source in the central KGS, the slope of Vaygach Island serves as another major source of ITs with energy intensity exceeding that of the central strait. Background circulation confines the ITs within ∼10 km off the slope forming atypical coastal trapped waves (CTWs). Theoretical solutions for these CTWs reveal a dominance of high vertical modes with energy dissipation exhibiting a “sandwich-like” structure. Applying the Lagrangian filtering, internal tides and lee waves along the KGS are separated. Dissipation associated with lee waves ranges from 10⁻⁷ to 10⁻⁶ W/kg comparable to those induced by ITs, yet their role in driving local mixing has been largely underestimated in previous studies. These results provide new insights into the dynamics around the KGS and their implications for Arctic tidal mixing processes.

The interannual variations of wintertime mixed layer depth (MLD) in the northern South China Sea (SCS), a region characterized by the deepest MLD within the basin and subduction process, are elucidated based on the Simple Ocean Data Assimilation (SODA, version 2.2.4) reanalysis data between 1950 and 2010. Our results reveal that the wintertime MLD possesses two predominant modes of 2–4-year and 7–8-year, involving different dynamical processes. Responses of thermal structure to local air-sea interface factors primarily explain the 2–4-year period MLD variability, with sea surface net heat flux playing a more crucial role than wind stress curl. Whereas for the 7–8-year period MLD variability, Luzon Strait Transport (LST), high-dense inflow from the Pacific Ocean into the northern SCS, is the dominant driver, while local air-sea interface factors play a minor role. The difference between surface and subsurface layers inflow ($��\Delta �\text{LST}�$), which is appropriate to represent the LST impact on vertical density gradient, can effectively modulate the upper layer stratification intensity in the northern SCS. Thus, a stronger (weaker) $��\Delta �\text{LST}�$ conduces to establish a more unstable (stable) state therein, favorable to a vigorous (stagnant) vertical mixing and MLD deepening. Further analyses demonstrate the importance of both horizontal heat and salt advections related to LST in the northern SCS. That is to say, different from the 2–4-year MLD variability, the salinity effect is of importance in driving this 7–8-year MLD variability.

The Southern Ocean plays a critical role in climate change while it remains incompletely understood due to lack of observations. This work utilizes the unique collection of temperature measurements from eXpendable BathyThermographs (XBTs) deployed along repeated transects together with the available Argo profiles across the Southern Ocean south of South Africa to examine long-term temperature changes in this region. Summertime temperature sections in the upper 800 m were constructed along the XBT transect using 2,833 XBT and 1,631 Argo profiles during the period 2004–2020. The results show different temperature changes north and south of the Polar Front (PF): (a) north of the PF: warming from 2004/2005 to 2011/2012 and cooling from 2011/2012 to 2019/2020 in the top 300 m, which are linked to the wind field changes associated with the Southern Annual Mode, and meridional shifts of the PF and the Subantarctic Front; (b) south of the PF: consistent warming in the Circumpolar Deep Water (CDW) below 200 m and cooling above in the surface layer during the whole study period. The warming trend in the CDW is small but significant compared with the interannual variability. These temperature changes south of the PF might be due to the reduced vertical mixing caused by the freshening of the surface water induced by the increased sea ice melting. These findings improve understanding of observed temperature changes in the south of South Africa and provide means to establish a connection across the Southern Ocean.

The grounding zone—where glaciers transition to floating ice shelves—critically influences ice sheet mass loss yet remains poorly observed. While tides and subglacial freshwater enhance basal melt, the role of sediment-laden subglacial water outflow on ice shelf–ocean dynamics is unexplored. Using idealized ice shelf/ocean simulations that, for the first time, couple sediment transport with evolving bathymetry, we show that subglacial discharge increases melt rates by $��\sim �$88%, but suspended sediment within the cavity suppresses them by $��\sim �$1.9% in the cavity but by as much as $��\sim �$10% near the grounding line. Sediment increases seawater bulk density, slowing buoyant fluxes, and cavity circulation, while deposition forms a seabed high at the grounding line that reduces melt there by limiting relatively warm modified Circumpolar Deep Water intrusions from reaching the ice base. Depositional events in the grounding zone may contribute to grounding zone wedge (GZW) formation and the potential development of pinning-points and grounding line advance that can impact ice sheet stability. New observations and multi-decadal simulations using a coupled system model are required to further investigate the impacts on ice sheet stability.

Lagrangian analysis using the velocity field of the Southern Ocean State Estimate (SOSE) at 1/6° resolution quantifies the origin of the lower branch of the abyssal circulation south of 30°S. Antarctic waters from 65°S contribute 62% of the northward bottom flow at 30°S. The remaining 38% comes from deep water recirculating from sources north of 30°S. Half of this deep water is North Atlantic Deep Water, with the remainder split between Indian and Pacific Deep Waters. Deep water densifies to bottom values following several circumpolar loops, which homogenize original properties to common characteristics of Lower Circumpolar Water. Transit times of deep water starting at 30°S, arriving as abyssal waters at 30°S, are about 150 years, twice as long as transit times of waters starting near Antarctica. Paths to 30°S originating from Antarctica avoid circumpolar loops and are steered by bottom topography.

The southwest Pacific Ocean (SWPO), located on the western side of the South Pacific subtropical gyre, has exhibited a notable trend of subsurface freshening over the past several decades. Using observation-based ocean salinity data sets, we find that SWPO subsurface freshening has slowed in the most recent three decades (1990–2020), to a rate less than one-fourth of that observed during the prior three decades (1960–1990). To understand the drivers of this subsurface salinity variability, we conduct surface forcing perturbation experiments using a global ocean-sea ice model, specifically targeting the distinct impacts of changes in surface heat, freshwater, and momentum (wind stress) fluxes. Our study reveals that the subsurface freshening slowdown in the SWPO since the 1990s is largely (75%) attributed to a weakened poleward shift of the westerly winds. This shift induces isopycnal deepening, enhanced poleward Ekman transport, and subsequent subduction of saltier subtropical surface waters into the ocean interior. Surface heat fluxes also contribute to these to a lesser extent (37%) via the spiciness component, as surface cooling at high latitudes shifts isopycnal outcrops equatorward, enabling saltier surface waters to be subducted; these effects are partly offset by freshwater fluxes (−12%). Further analysis suggests that these surface flux changes are at least partly linked to a multidecadal shift in the interdecadal Pacific oscillation around 1990. The mechanisms identified in this study strengthen our understanding of how ocean interior salinity responds to large-scale climate variability and provide insights for better detecting and quantifying the intensification of the global water cycle.

Satellite-derived sea ice products and ice model simulations are analyzed in the Bering Sea during the winters of 2018–2019, featuring historically low sea ice coverage on the Eastern Bering Sea shelf, and 2019–2020, when ice conditions were close to average. Hindcast simulations are conducted using the Community sea Ice CodE (CICE) as a standalone regional ice model forced with ERA5 atmospheric fields and oceanic fields derived from realizations of the Regional Ocean Modeling System (ROMS) internally coupled with its own ice model component. CICE is able to predict the observed seasonal variability in ice concentration and ice thickness provided that ocean mixed layer temperature and salinity are strongly nudged toward an accurate ocean state. Ice mass balance analysis shows that sustained periods of southerly wind during the 2018–19 season led to increased melting and a larger ice export into the Arctic when compared to 2019–20. More ice was produced in 2018–19, the season with lower sea ice areal extent, offset by larger melting and transport out of the domain. The volume of ice exported to the Russian shelf and basin is comparable to the net transport through the Bering Strait. The consistency between the model and satellite products demonstrates that the representation of sea ice thermodynamic and dynamic models present in CICE are adequate for representing key physical processes driving sea ice variability in this region during both low and normal ice coverage years.

Nares Strait is a critical gateway for the export of Arctic waters and nutrients toward the North Atlantic. Yet, the fine-scale structure, variability, and long-term evolution of its water masses and nutrient concentrations remain poorly characterized. Here, we combine almost two decades of high-resolution hydrographic and biogeochemical data (2006–2024) to resolve the spatial and temporal variability in nutrient availability and water mass (WM) properties across this key arctic gateway. Using unsupervised clustering and tracer-based diagnostics, we identify three dominant WM regimes shaped by Arctic and Atlantic influences, glacial inputs, and internal mixing. Our analysis reveals net freshening and nutrient shifts across all major water masses, including a doubling of nutrient concentrations in the arctic upper halocline as well as significant freshening of Polar Mode Water and Baffin Bay Polar Water properties. This multidecadal analysis provides crucial insights into the complex drivers of hydrographic and nutrient changes, enhancing our ability to predict how Arctic productivity will evolve.