Journal of Geophysical Research-Oceans最新文献

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°.

A tidal model based on altimeter observations reveals that first-mode diurnal internal tides (DITs) propagate approximately 2,100 km eastward from the Luzon Strait (LS) into the Pacific Ocean. As they radiate over long distances, the DITs refract equatorward due to the beta effect. In this study, we utilize in situ round-trip acoustic echo time measurements between the seafloor and the sea surface, obtained from an array of 10 pressure-recording inverted echo sounders (PIES), to investigate the variability of DITs in the eastern Philippine Sea (EPS). The observations conducted over 1-year and 1.5-year periods during 2020–2021 reveal a clear weakening of DIT amplitudes in summer, in contrast to the barotropic diurnal tides, which show maximum spring tide amplitudes at the solstices and minimum amplitudes at the equinoxes. The observed seasonal variation in DIT energy flux shows a significant correlation with the relative vorticity averaged over regions of energetic warm eddies. Ray-tracing using HYCOM ocean model outputs indicates that the warm eddies in the upstream region of the ray path during summer (July to September) enhance the equatorward refraction of DITs. This study suggests that the superposition of the K₁ and P₁ constituents induces a pronounced semi-annual cycle in the DITs, even over considerable propagation distances. In addition, warm eddies exert a substantial influence on the DIT propagation path. Our results imply that the pronounced temporal variability of DITs should be considered to improve the parameterization of internal-wave-induced ocean mixing in oceanic and climate models.

Submesoscale processes in the ocean typically peak in winter, driven by mixed-layer instability and intensified atmospheric forcing. However, coastal upwelling regions can deviate from this paradigm due to region-specific dynamics. Based on validated high-resolution simulations, we investigate the seasonal and regional variability of submesoscale activity in the Arabian Sea. Our results reveal that the western Arabian Sea exhibits a pronounced summer peak in submesoscale activity, primarily associated with wind-driven upwelling, enhanced frontogenesis, and mixed-layer baroclinic instability. Although earlier studies have reported intensification of submesoscale processes in coastal upwelling regions, detailed dynamical interpretations remain limited. Our work advances this understanding by explicitly diagnosing the regional physical mechanisms driving submesoscale variability under monsoon-influenced upwelling system. This regional contrast becomes more evident when considering the broader basin. In the northern open ocean, submesoscale processes exhibit the canonical winter-intensified pattern, whereas in the eastern Arabian Sea near the Maldives, they display a distinct bimodal structure with both summer and winter peaks. These findings highlight the importance of adopting region-specific frameworks to interpret submesoscale seasonality, moving beyond the winter-intensified paradigm dominant in open-ocean settings. Our results provide novel insights into how coastal and open-ocean submesoscale dynamics coexist in the Arabian Sea, with implications for seasonally varying energy cascades, vertical heat and nutrient fluxes, and air-sea exchange in the upper ocean.

This study employs the coupled conditional nonlinear optimal perturbation (C-CNOP) method, which incorporates initial coupling uncertainties, to identify sensitive areas of targeted observations for positive Indian Ocean Dipole (IOD) events. Results show that the initial errors most likely to yield large prediction uncertainties of IOD events are mainly concentrated in sea temperatures near the thermocline in the eastern Indian Ocean (IO_Temp: 70–110 m depth, 5°S–5°N, 85°E−105°E) and western Pacific (PO_Temp: 120–160 m depth, 5°S–5°N, 130°E−150°E), as well as zonal winds (UWind), exhibiting an east–west dipole pattern over the tropical Indo-western Pacific. Through sensitivity experiments—designed to assess the impact of initial uncertainties in different areas on IOD predictions while bypassing the assimilation process and avoiding initial shock effects—we find that prediction uncertainties are more sensitive to initial errors in the UWind area than in the IO_Temp and PO_Temp areas, demonstrating a stronger impact on forecast skill, particularly in winter and summer. Further analysis demonstrated that the IO_Temp & PO_UWind coupled area involving the eastern Indian Ocean subsurface temperature and western Pacific zonal winds, exhibits greater sensitivity than the UWind area alone, emerging as the most sensitive area of positive IOD events. This key area highlights both the Pacific's remote influence and the crucial role of local ocean on IOD development. These results underscore the critical role of coupled initialization in IOD predictability, offering a theoretical basis for advancing coupled data assimilation.

This study evaluates the performance of dynamical downscaling in the Northern Adriatic Sea, focusing on eddy kinetic energy spectra and dense water formation. Using the perfect model framework, a high-resolution (2 km) reference simulation of the entire Adriatic Sea serves as the benchmark for a series of one-way nesting downscaling experiments reaching the same horizontal resolution in the Northern Adriatic. Results show that a downscaling ratio of 1:3 effectively reproduces the local energy budget and multiscale features. However, the absence of feedback from small to large scales limits the downscaling performance. This limitation is evident in dense water formation, which is controlled by the interplay between local and remote drivers in the Northern Adriatic Sea. When local drivers, such as buoyancy fluxes, dominate, the dense water formation process is well reproduced. In contrast, when remote influences, particularly the inflow of salty Levantine Intermediate Water through the Otranto Strait, are not properly resolved by the parent model, reproducibility of dense water formation deteriorates. Our experiments indicate that a 2 km horizontal resolution effectively captures cross-scale interactions at the strait, while a 6 km resolution is insufficient. These interactions, particularly feedback from small scales to large scales, lead to changes in thermohaline dynamics that propagate toward the Northern Adriatic Sea.