Journal of Geophysical Research-Oceans最新文献

Oligotrophic ocean is generally characterized by lower primary productivity, which has traditionally been considered to result in reduced transport of particulate organic carbon (POC) to the deep ocean compared to high-productivity regions. However, our findings challenge this paradigm based on studying transparent exopolymer particles (TEP) in the western tropical North Pacific (WTNP). This paper systematically examines the formation and distribution of TEP across the 0–2,000 m depth range in the WTNP, analyzes the influence of hydrodynamic factors on TEP dynamics, and investigates the role of Pelagibacter, a dominant bacterium in oligotrophic waters, in facilitating in situ TEP production. Despite low TEP concentrations in the water column (7.53–34.22 μg Xeq/L), the proportion of TEP-C within POC remained stable with increasing depth. Furthermore, the vertical transport efficiency of POC in oligotrophic waters was significantly higher than in high-productivity regions, indicating that TEP plays a crucial yet overlooked role in facilitating the deep transport of POC in oligotrophic environments. Given this unique promotion mechanism, we proposed that the amount of POC transported to the deep oligotrophic ocean has been underestimated by at least fivefold.

This study explores the biases in westerly wind events (WWEs) and their induced oceanic Kelvin waves (OKWs) in seven CMIP6 models from three modeling centers that provide daily thermocline depth output. Among the WWEs and OKWs identified in historical simulations, WWEs are generally weaker than observed for a given OKW response, suggesting that OKWs in models respond too strongly to WWE forcing. The equatorial ocean mixed layers in the models are generally too thin and overly stratified compared to observations. These biases may help inhibit the turbulence-induced mixed layer deepening and trap the WWE-provided momentum within the ocean surface layers. Thus, they support an exaggerated sea surface height buildup and thermocline deepening in OKW initiation regions, which leads to an overly strong OKW response. These biases may be associated with a too-strong net heat flux between the atmosphere and the ocean.

This study evaluates the tsunami generation potential and scaling characteristics of rotational submarine landslides along the eastern margin of the Sea of Japan, where many submarine active faults and fine-grained sediments are distributed. These landslides pose significant tsunami hazards, but their locations and magnitudes are difficult to predict, making them a major source of uncertainty in tsunami hazard assessments. We identified landslide traces using high-resolution bathymetric data, estimated landslide volumes, and applied an established empirical formula to calculate initial tsunami amplitudes. The initial tsunami amplitudes calculated using the empirical formula were validated against numerical simulations using a two-layer model. Several potential tsunami sources were identified with initial amplitudes exceeding 10 m, all located in shallow waters on the continental shelf and characterized by thick landslide deposits. A clear power-law relationship was observed between landslide area and volume, consistent with previous studies of rotational slides. The volume–frequency relationship also followed a power-law, in contrast to the logarithmic trends typically associated with flow-type landslides. Spatial analysis revealed that landslide events with significant tsunami impact often clustered near submarine active faults, suggesting strong ground motion as a primary trigger. The consistency between empirical and numerical estimates supports the validity of the empirical formula for rotational landslide tsunamis. These findings demonstrate the value of failure mode-specific scaling analysis and source characterization for improving quantitative tsunami hazard assessments. The results may contribute to probabilistic hazard analysis, such as Monte Carlo simulations, and support coastal disaster prevention planning.

Nares Strait is an important export pathway of sea ice, where its transport is highly intermittent due to the formation and collapse of sea ice arches. The islands in the strait, especially Hans Island, contribute to heightened collision forces between distinct ice floes and the land. However, since even state-of-the-art large-scale models remain relatively coarse and use continuous sea ice rheology, the complexities of floe-scale sea ice interactions with small islands in the Nares Strait have not been much explored. Here, we use a novel discrete element model, SubZero, to identify the role of small islands in affecting intense summer-time sea ice transport in the Nares Strait. We show that SubZero can reproduce crucial sea ice characteristics, including observed area transport, intermittency of area fluxes, and floe size distribution (FSD) derived from satellite imagery. We find that the intermittency of sea ice fluxes relates to the power-law exponent of the simulated FSD, and matching it to observations implies that the floe strength for fracturing must be inversely proportional to the square root of its length scale. Conducting sensitivity simulations with modified coastlines, we identified several islands as crucial pinning points that suppress sea ice transport and cause jamming, especially during low-to moderate-wind conditions. The momentum budget reveals the islands slow down sea ice through direct normal contact with colliding floes and by increasing tangential drag forces from lateral coastal boundaries. Our study emphasizes floe-scale interactions with islands and other coastlines in large-scale sea ice transport through narrow straits.

Seasonal variability of the magnitude and structure of semidiurnal internal tides radiated from the Mariana Arc was examined based on year-long mooring observations. Internal tidal currents, energy density, and energy flux at the focal point of the Mariana Arc were weakest in summer, but typically comparable in the other three seasons. Mode-1 internal tides dominate over the observed period, except in summer when Mode-2 explains a higher proportion. The coherent variance accounts for about 60% of the total semidiurnal motions, with a reduced proportion in summer and winter, generally consistent with the seasonal trend of the barotropic tides. In contrast, the incoherent internal tides show a close temporal consistency with the low-frequency flows. During summer, the background currents are predominantly influenced by energetic mesoscale eddies, whereas the North Equatorial Current (NEC) system dominates in the other seasons. Internal tides influenced by eddies exhibit more standing wave characteristics, higher modal structures, and more incoherent properties compared to those modulated by the NEC. Internal tides are more progressive and propagate outward more easily when modulated by the NEC, whereas those modulated by eddies are more unstable and prone to nearby dissipation, resulting in a significant summer weakening at the Mariana Arc focus.

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.