Journal of Geophysical Research-Oceans最新文献

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.

Diurnal warming (DW) at the ocean surface occurs when there is a combination of solar heating in the absence of vertical mixing typically derived from wind stress. DW has been well described, mostly from satellite data, but also with some in situ observations. Evidence of DW has mostly been restricted to the subtropics, and there are very few reports of DW at northerly latitudes. We present here observations of a DW event of 1.5°C confined to the upper 2 m in the Labrador Sea at $��>�55�°�$N. These measurements were conducted with the Air-Sea Interaction Profiler (ASIP), an upwardly rising, ocean microstructure instrument. Cloud cover obscured the ocean surface to passive remote-sensing instruments and as a result no evidence of this particular DW event was available from the nine independent satellite products that were analyzed. Therefore, the event would have gone undetected without the deployment of ASIP at precisely this time and location. The ASIP observations were used to derive a heuristic set of criteria for potential occurrences of DW in the Labrador Sea region: (a) shortwave radiation above 600 W m⁻² and (b) 10-m wind speed below 4 m s⁻¹. These criteria were subsequently applied to $��\sim �$40 years of the ERA5 reanalysis product indicating that DW events in the Labrador Sea have the potential to occur more frequently than satellites observe. Attaching microstructure temperature sensors on Argo floats would provide a more accurate assessment of the occurrence of DW events globally as well as their effect on surface mixing rates.

The influence of a subsurface ridge on a subinertial coastal Kelvin wave is investigated using a numerical model and a simplified analytical framework. A first baroclinic mode Kelvin wave is made to impinge on progressively complex subsurface geometries. For an idealized submerged shelf that extends infinitely in both offshore and alongshore directions, the over-shelf response includes low vertical modes of the shallower domain and an upward-propagating beam from the shelf-top corner, inducing alongshore variability in the shelf flow. When the alongshore extent is truncated to form a ridge between two deep basins, the downstream response strongly depends on ridge width. Phase patterns reveal upward and downward energy propagation from the downstream ridge corner, along with horizontal propagation of multiple vertical modes. As the ridge narrows, beam amplitudes decrease, though vertical propagation remains visible in phase patterns. If the ridge width is less than approximately twice the Rossby radius of deformation of the ridge-top domain, evanescent modes along the ridge flanks begin to overlap and interact, promoting more horizontal energy transmission into the downstream basin. Thus, the Rossby radius of deformation sets a natural scale governing the balance between vertical scattering and horizontal transmission. In the most realistic configuration, a submerged ridge of finite offshore and alongshore extent, waves trapped to and propagating around its periphery affect the downstream basin through combination with the signals that propagate over the top of the ridge.

High-resolution mooring observations at 17.8°N, 89.5°E in the northern Bay of Bengal (BoB) are used to investigate the fraction of near-inertial (NI) wind power input ($��{\Pi }_W�$) at the ocean surface that propagates below the mixed layer (ML) and its role in modulating diapycnal heat flux-induced ML temperature (MLT) cooling during Tropical Cyclone (TC) Bulbul (5–9 November 2019). The $��{\Pi }_W�$ induces a sharp rise in ML NI Kinetic Energy (NIKE) (∼80 Jm⁻³), followed by its downward propagation, with subsurface NIKE (∼40 Jm⁻³) about half of the ML value and lagging by approximately 2 days. Though the ML NIKE budget indicates that approximately 10% of the $��{\Pi }_W�$ is radiated downward from the ML, the enhancement of NIKE below the ML is plausible due to stalling of NIWs at the base of the ML. Radiation of NIKE at the ML base increases vertical shear of currents, leading to a fourfold enhancement of diapycnal diffusivity to 1.25 × 10⁻³ m²s⁻¹ compared to the pre-cyclonic value (3.3 ± 0.04 × 10⁻⁴ m² s⁻¹). As a result, diapycnal heat flux at the ML base increased to −126.4 ± 11.4 Wm⁻² from the pre-cyclonic value of ∼2.7 ± 1.9 Wm⁻². During the peak cooling phase of ML, diapycnal heat flux (−200 Wm⁻²) shows a comparable contribution with respect to the net heat flux term. During the post-TC phase, the ML heat storage term has exhibited a pronounced high-frequency variability ranging between −4,000 and −2,000 Wm⁻² due to the NI current in the presence of cold wake.