Journal of Geophysical Research-Oceans最新文献

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.

This study provides a comprehensive assessment of the spatial and temporal variability of euphotic zone depth (Zeu) across the Arabian Sea from 1998 to 2023, focusing on three sub-regions: the Northern Arabian Sea (NAS), South Eastern Arabian Sea (SEAS), and South Western Arabian Sea (SWAS), using satellite-derived data sets. The analysis reveals that Zeu variability occurs on interannual, intra-annual, and decadal timescales. Annual mean Zeu values across the basin ranges from 6 to 80 m, reflecting considerable spatial heterogeneity in water clarity. Regionally, NAS recorded Zeu values ranging from ∼10 to 62 m, SEAS from ∼10 to 74 m, and SWAS from ∼12 to 72 m. Empirical Orthogonal Function (EOF) analysis confirmed that interannual variability accounts for 21.57% of the total variance. Interannual variability is primarily modulated by the Indian Ocean Dipole (IOD), followed by the El Niño–Southern Oscillation (ENSO), particularly in the SEAS and SWAS regions. Zeu in the NAS showed a significant lagged response, with Principal Component (PC1) lagging Dipole Mode Index (DMI) and ENSO by approximately 10 and 8 months, respectively. A statistically significant positive trend in Zeu was observed across the basin and within all three sub-regions, with the NAS exhibiting the strongest increase. Seasonal trend analysis showed increasing Zeu trends across all seasons, with the NAS during the ON (October–November) season demonstrating the highest trend (0.47 m/year). Overall, this work offers the first detailed analysis of Zeu variability in the Arabian Sea using multi-sensor satellite data.

Zooplanktonic organisms are considered a key link between different trophic levels. The ecosystem structure and dynamics are affected by changes in their population and phenology. In this work, the causes and timing of changes within the seasonal cycle of zooplankton biomass vertical distribution in oceanic waters of the Bay of Biscay were investigated. For this purpose, long-term oceanographic time-series from zooplankton nets and high-frequency Acoustic Doppler Current Profiler backscattering data, employed as a proxy for zooplankton biomass, were used. This combination enables the observation of the concurrence of a strong seasonality in zooplankton biomass and its daily oscillation tightly linked to phytoplankton stock and its vertical distribution in relation to water column stability and air–sea forcing. During the spring bloom, when food availability is guaranteed, an increase in zooplankton biomass is observed and it is concentrated at the ocean surface, with a reduction in its diel vertical migration. Later in the year, zooplankton biomass is concentrated above the deep chlorophyll maximum (DCM), and the depth range that it occupied is increased or reduced as the DCM deepens or shoals.

Free-drifting buoyant objects, including plastics, marine debris, and organisms, move with the wind, waves, and surface currents. These objects also surf on breaking waves; this process adds to the total transport of the objects and can control beaching. Observations of surfing transport are made using small free-drifting buoys called microSWIFTs. The drifters are deployed nearshore at the US Army Corps of Engineers Field Research Facility in Duck, NC, USA, as part of the During Nearshore Events Experiment in October 2021. Surfing events are observed in the drift trajectories of the buoys as “jumps” in the time series of cross-shore position. There are 3,172 surfing events observed, with a median jump amplitude of 8.3 m and a median duration of 2.5 s. These median values are 13$��\%�$ of a characteristic offshore wavelength and 32$��\%�$ of a characteristic offshore wave period, respectively. The median bulk jump speed (jump amplitude/jump duration) is 82$��\%�$ of the linear phase speed for waves in the corresponding jump depth. The buoys' trajectories are simulated using three models of increasing complexity: “Wind-Only,” “Wind and Waves,” and “Wind, Waves, and Surfing.” The surfing process is represented using a probabilistic parameterization. When surfing is included in the models, the terminal location of the modeled objects (on beach or offshore) is correctly predicted in 93$��\%�$ of cases compared to 76$��\%�$ and 84$��\%�$ for the “Wind-Only” and “Wind and Waves” models, respectively. Including surfing also significantly improves the accuracy of the time-to-beach and alongshore beaching location.

River plume-impacted shelf marginal seas exhibit strong carbon sequestration potential due to their high biological productivity. However, frequent coastal upwelling events complicate the carbon source-sink dynamics because of the competing effects on seawater partial pressure of carbon dioxide (pCO₂): the upwelling of dissolved inorganic carbon (DIC)-rich deep waters initially elevate surface pCO₂, while subsequent biological uptake lowers it. As a case study, we used a novel wave-driven profiler to obtain high-resolution vertical profiles in the Changjiang plume-impacted shelf area (CPS) and to investigate upwelling-induced variability in pCO₂ and carbon source-sink dynamics. The observations were conducted near Gouqi Island, where coastal upwelling frequently occurs. Based on a pCO₂ mass balance model, we found that biological processes (contributing 30.6% to pCO₂ increase) and physical transport (contributing 21.2% to pCO₂ decrease) jointly dominated hourly mixed layer pCO₂ variability in the study area. Importantly, we found that α_SBW (shelf bottom water fraction) served as a good quantitative proxy for upwelling intensity, with each 1% increase in α_SBW associated with a 6-μatm increase in mixed layer. Given the significantly higher mean α_SBW values during 20–22 August (34 ± 5%) than 28–30 August (11 ± 7%), we defined the former as the upwelling period and the latter as the post-upwelling period. The air-sea CO₂ flux ($��F_{{\text{CO}}_{�2�}}�$) during the upwelling period (24.04 ± 16.24 mmol m⁻² d⁻¹) was significantly higher than post-upwelling period (1.25 ± 0.98 mmol m⁻² d⁻¹). These findings provide new mechanistic insights into how coastal upwelling regulates carbon source-sink dynamics in large river-dominated shelf seas and highlight its importance for improving predictions of carbon sequestration potential in marginal seas.

Submarine groundwater discharge (SGD) is a significant source of nutrients to continental shelf waters. Increasing evidence suggests that most of this flow is saline (∼seawater salinity) and occurs across broad continental shelves, making it challenging to observe. Using new, cost-effective heat-tracer methods and geochemical analyses of subseafloor saline groundwater, we found that offshore saline SGD delivers repeated pulses of nutrients to the overlying South Atlantic Bight (SAB) water column in the summer. Thermal time-series measurements were collected below the sandy seafloor 10–15 km offshore of Charleston, SC, during three consecutive summers. Up to seven pulses of saline SGD occurred each summer. Discharge velocities ranged from 1 to 8 cm day⁻¹. Pulses of SGD coincided with upwelling-favorable winds related to annual shifts of the North Atlantic Subtropical High and with winds from storm activity. We observed spatial and temporal variability between SGD pulses from analogous sites 5–10 km apart. Cumulative summertime SGD in 2019 corresponded to 65%–70% of the radium-based estimates of SGD offshore of South Carolina and was similar in magnitude to the input from regional river discharge. Thermal and geochemical analyses indicate two-way seawater-groundwater exchange across the seafloor, reflecting infiltration of seawater into the seafloor and SGD to overlying seawater. Geochemical analyses confirmed high nutrient concentrations in SGD compared to the overlying water column and river water. The δ¹⁵N of total dissolved nitrogen and δ¹³C of dissolved inorganic carbon in groundwater suggest significant contributions from mineralized organic matter with a geochemical composition distinct from overlying seawater.

The Pacific South Equatorial Countercurrent (SECC) is a crucial but poorly characterized component of the equatorial current system. Here, we investigate its seasonal and interannual variability using Argo-derived absolute geostrophic currents. During boreal spring, the SECC attains its widest meridional (5°S–13°S) but narrowest zonal (150°E−170°W) extent, with deepest penetration of over 300 m and a maximum transport of 12.8 Sv. In contrast, the SECC in summer weakens substantially as it shoals to depths less than 150 m, resulting in a transport of no more than 6.5 Sv. By autumn, the SECC extends eastward, reaching a peak zonal range (150°E−140°W). Subsequently, its western branch strengthens and deepens, while the eastern part retreats. These seasonal shifts are closely linked to the first-mode baroclinic Rossby waves forced by remote wind stress curl anomalies, particularly over 180°–140°W. Contrasting responses are also observed during El Niño and La Niña events. The SECC expands horizontally but contracts vertically during El Niño, with positive velocity anomalies progressing from north to south; and vice versa during La Niña. Correspondingly, the SECC transport shows little difference between El Niño and La Niña during the developing phases, but by summer, during its decaying phase, the El Niño transport reaches nearly twice that of La Niña. Sensitivity experiments show that the wind stress curl anomalies east of 140°W primarily control the SECC on El Niño-Southern Oscillation (ENSO) timescales between 180° and 140°W, while anomalies over 180°–140°W govern the SECC west of 180°.