Journal of Geophysical Research-Oceans最新文献

To investigate the migration and transformation of black carbon (BC) under rapid Arctic changes, we systematically quantified its abundance, distribution and particulate (PBC) flux in the Chukchi Shelf and Borderland, western Arctic Ocean. The average sedimentary BC concentration was 1.29 mg g⁻¹, with spatial distributions controlled primarily by distance from the coast, water column stratification, and seafloor bathymetry. In the Chukchi Borderland, the mean PBC sinking fluxes reached 0.40 mg m⁻² d⁻¹, exhibiting pronounced seasonal variability, with fluxes during the melting season quadrupling those observed during periods of ice formation. Stable carbon isotopic signatures (δ¹³C_BC) revealed a tripartite BC source mixture throughout the study area: modern biomass combustion, pyrogenic fossil fuel combustion, and petrogenic (coastal erosion) inputs. Finally, we estimated annual BC fluxes of 61 and 655 Gg in the Chukchi Borderland and Central Arctic Ocean, respectively. These findings establish BC as an essential component of the Arctic carbon sink.

While the variability of the North Pacific Subtropical Countercurrent has been widely studied, its counterpart, the North Atlantic Subtropical Countercurrent, remains poorly understood. Using RG Argo and OISST data from 2004 to 2024, we investigate the mean state, seasonal and decadal-scale variability of the North Atlantic Subtropical Countercurrent and its associated Subtropical Front, and their dynamical linkage to the North Atlantic Subtropical Mode Water. The North Atlantic Subtropical Countercurrent/Front is located within 70°–46°W and 24°–34°N, from surface to 150 m depth. The meridional section of potential density across the North Atlantic Subtropical Front exhibits a wedge-shaped vertical pattern, characterized by northward deepening of the lower pycnocline and shoaling of the upper pycnocline, resulting from the North Atlantic Subtropical Mode Water intrusion. Through the thermal wind relation, the northward shoaling of the upper pycnocline induces eastward shear, giving rise to the North Atlantic Subtropical Countercurrent and Front. The North Atlantic Subtropical Mode Water peaks in April, while the North Atlantic Subtropical Front peaks in May, indicating a 1-month lag in the front's response to the mode water change. The North Atlantic Subtropical Mode Water and North Atlantic Subtropical Countercurrent/Front exhibit synchronous decadal-scale change, with the mode water further modulated by the North Atlantic Oscillation. This study systematically illustrates the dynamic linkage between the North Atlantic Subtropical Countercurrent/Front and North Atlantic Subtropical Mode Water, offering a new mechanistic framework for understanding how subsurface mode-water variability regulates upper-ocean circulation and air-sea coupling, thereby influencing climate variability in the North Atlantic.

Marginal seas significantly impact the global carbon cycle. However, current knowledge on the role of marginal seas is limited, and only a few in situ data sets on air-sea CO₂ exchange are available. This study presents the first direct measurements of CO₂ partial pressure (p$��{\text{CO}}_2�$) and derived air-sea $��{\text{CO}}_2�$ flux carried out in the central Mediterranean region. Measurements used in this study were made at the Lampedusa Oceanographic Observatory (35.49°N, 12.47°E) and cover the period from December 2021 to June 2023. The daily air-sea flux is calculated based on high-resolution measurements of sea temperature, salinity, p$��{\text{CO}}_2�$ in the ocean and in the atmosphere, and wind. The study takes advantage of the presence of two close observation sites contributing to the oceanic and atmospheric domains of the Integrated Carbon Observation (ICOS) network. The data show that the Central Mediterranean on the annual scale currently acts as a CO₂ sink, with an absorption phase in winter and an emission phase during summer. However, large differences exist between the two consecutive absorption periods included in the data set, with a 30% lower absorption rate during early 2023 compared to early 2022. An intense and prolonged marine heatwave occurred from May 2022 to April 2023, and the potential effects of this heatwave on $��{\text{CO}}_2�$ air-sea fluxes have been investigated. The reduced incidence of intense wind episodes during the heatwave period appears to be the main driver for the reduced air-sea CO₂ exchange taking place during winter 2023.

The upper-layer western boundary currents (WBCs) in the South China Sea (SCS) play a crucial role in regulating ocean dynamics, marine ecosystems, and regional climate. However, due to limited observations and coarse model resolution, changes in these currents on centennial timescales remain unclear. This study investigates trends in upper-layer circulation in the SCS from 1951 to 2050 using outputs from an ensemble of 16 high-resolution climate model members. Under global warming, the WBCs exhibit strengthening in both winter and summer, with a 7.7% increase (0.60 ± 0.47 Sv) in the winter Vietnam Coastal Current (VCC) and an 8.5% increase (1.10 ± 0.55 Sv) in the summer Vietnam Offshore Current (VOC) over the study period. Further analysis using a 1.5-layer reduced-gravity model suggests that enhanced stratification drives the intensification of the VCC in winter while the strengthening of the VOC in summer is primarily modulated by increased stratification and partially influenced by the intensified local wind stress curl. The results of this study may serve as a foundation for further research on the ecological effects caused by enhanced WBCs of the SCS.

Winter Antarctic sea ice extent has historically shown relatively low variability compared to warmer seasons, but recent winters have shown very extreme low ice cover. As a result, there is significant interest in understanding the physical constraints on winter Antarctic sea ice cover. In this work the ocean's role in constraining maximum extent is explored. Using basic physical principles, the concept of “Stability at Freezing Temperature” is introduced, and then applied to calculate the total ocean heat that must be lost to the atmosphere before sea ice can begin to form. This method is found to capture spatial and interannual variability of the winter sea ice edge, including during recent extreme sea ice cover states. This framework can be used to separate the effects of ocean thermal and haline anomalies on the ice edge. It is also used to quantify the contributions from summer preconditioning, atmospheric cooling, and subsurface ocean heat transport, all of which are shown to contribute significantly to winter ice extent variability. This method provides an invaluable tool for understanding recent sudden changes in Antarctic sea ice cover.

Polynyas are within the sea ice cover, typically formed by wind-driven sea ice divergence or upwelling of warm subsurface waters. They play a crucial role in ocean-atmosphere interactions, climate regulation and marine ecosystems by substantially enhancing primary production. Open-ocean polynyas in the Southern Ocean are rare and are typically associated with deep convection, which disrupts conventional circulation pathways and impacts regional heat and carbon budgets. The Cosmonauts Sea (30°E–60°E) is an exception, with open-ocean polynyas forming annually. Using satellite-derived sea ice observations, we examined the spatiotemporal variability of polynyas in this region over the past two decades. The Cosmonauts Sea polynya exhibited large spatial and interannual variability, with the largest event occurring in 2016 (139,000 km²). An Argo float near the polynya recorded deep mixed layers (>400 m) and near-complete erosion of stratification, and the presence of dense water. This event coincided with anomalously intense cyclonic wind stress curl due to synoptic scale storms and a prolonged positive Southern Annular Mode (SAM) phase (2014–2016), both generally associated with reduced sea ice concentrations. While the southward shift of the Antarctic Circumpolar Current (ACC) during 2015 acted as a preconditioning mechanism, bringing warmer water towards the polynya region and inducing upwelling by vortex stretching. Additionally, anomalously high shortwave radiative fluxes (∼+20 Wm⁻²) were observed in the summer preceding the 2016 event. The deep convective mixing observed during this event, together with the presence of dense water, indicates that the Cosmonauts Sea could be a potential dense water formation site.

While the importance of organic sulfur for marine bacteria is widely recognized, how it relates to bacterial community composition, particularly in coastal environments, remains poorly understood. In this study, we quantified dissolved organic sulfur (DOS) and particulate organic sulfur (POS) in the Bohai and Yellow Seas. DOS ranged from 1.7 to 6.8 μM in surface waters and 0.9–5.4 μM in bottom waters, while POS ranged from 0.3 to 1.7 μM in surface waters and 0.2–1.6 μM in bottom waters. In the Bohai Sea, particulate and dissolved C:S ratios averaged 57 ± 38 and 41.4 ± 7.5, respectively, with minimum sulfur inventories of 0.039 ± 0.025 (POS) and 0.300 ± 0.058 Tg S (DOS). In contrast, the Yellow Sea had a lower particulate C:S ratio (25 ± 17) but a comparable dissolved ratio (46.5 ± 12.5), alongside minimum inventories of 1.14 ± 0.69 (POS) and 1.91 ± 0.52 Tg S (DOS). In contrast to previously reported data from the open ocean, the Bohai and Yellow Seas are characterized by distinctly higher organic sulfur concentrations and lower molar C:S ratios. Bacterial communities differed significantly between surface and bottom waters in terms of diversity and network complexity. However, path analysis showed no significant association of organic sulfur with bacterial community composition in either layer. These findings suggest that despite high total organic sulfur concentrations in these coastal waters, its low concentration of bioavailable fractions and taxon-specific utilization may explain its limited association with bacterial community.

The poleward warm Atlantic Water and returning cold water in the Nordic Seas play a crucial role in regulating the Northern Hemisphere climate. While previous studies have recognized the importance of mesoscale dynamics, a quantitative assessment of the role of mesoscale eddies in poleward heat transport is lacking. Our study investigates the role of eddies in poleward oceanic heat transport in the Nordic Seas using an ocean model where eddies are well represented. Using a novel configuration of the MITgcm ocean-ice model, we analyze 21 years of simulation. We show that eddy heat flux divergence offsets more than 70% of heat flux convergence induced by the mean flow along the Norwegian Atlantic Slope Current. Eddy heat flux divergence peaks at a depth of about 400 m near the thermocline and reaches a maximum near the steep Lofoten Escarpment. A temporal decomposition reveals that eddy heat flux divergence is stable on interannual timescales, although there is strong seasonality. Our study emphasizes the significant role of eddies in reducing poleward heat transport by diverting heat out of the Norwegian Atlantic Current into the Nordic Seas.

This paper presents physics improvements to the cool skin parameterization in the Coupled Ocean-Atmosphere Response Experiment (COARE) bulk flux algorithm. The principal improvement is adopting a specification of the ocean side mixing profile that combines molecular and turbulent diffusivities via a form that allows turbulent dissipation to suppress turbulence near the interface. The turbulence is also scaled with the viscous friction velocity, since the stress input to waves is not realized continuously as turbulence at the interface but only intermittently at localized regions where the waves are breaking. Additional improvements include adopting a newer specification of the solar absorption profile in the ocean and incorporating the impacts of the rain sensible heat flux. The new parameterization is tuned to published observations of cool skin from a series of cruises and a recent publication of the turbo-molecular mixing term deduced for observations of gas fluxes. Data from three recent ship-based field programs, particularly the Propagation of Intraseasonal Oscillations in the Maritime Continent Region (PISTON) experiment, with radiometric sea surface and floating near-surface temperature sensors as well as high-quality air-sea flux measurements were analyzed to evaluate the model. The improvements led to modest decreases in the nonsolar cool skin (∼16%) and in the solar heating contribution, both principally in light winds. The new model better reproduced mean nighttime cool skin amplitudes and was somewhat better than the previous COARE v3.6 model at reproducing the mean diurnal cycle. Overall, cool skin predictions for a large cruise database were reduced by ∼0.01°C.