Journal of Geophysical Research-Oceans最新文献

Marine heatwaves (MHWs) are intensifying globally, yet their impact on phytoplankton is typically assessed as a spatially uniform thermal stress, overlooking their internal structure. Here, we investigate the spatial coupling between MHW intensity and chlorophyll-a (Chl-a) in the South China Sea using satellite observations (2003–2020). By applying a segmented linear regression model, we statistically identified two robust critical radii ($��\sim �$150 km and $��\sim �$380 km) that demarcate distinct biophysical regimes. In warm seasons, Chl-a anomalies exhibit a distinct “increase-decrease-increase” radial pattern, with a significant suppression zone within the core (<150 km). Dynamical analysis reveals that this core suppression is driven by a “double-lock” mechanism: enhanced thermal stratification coupled with wind-driven Ekman downwelling. This synergistic barrier physically blocks vertical nutrient supply, creating an oligotrophic core. Conversely, a divergent regime emerges in winter, where Chl-a exhibits a sustained increase even within the core. We identify anomalous Ekman upwelling in the winter MHW core as the key driver that overrides thermal stratification, maintaining nutrient flux. These findings demonstrate that the biological impact of MHWs is not solely determined by temperature magnitude but by the spatially structured competition between thermal stratification and wind-driven mixing.

We investigate the response of shallow reef flow to tidal variability and wave exposure during a 4-month field campaign in southern Huvadhu Atoll, Maldives. Incident waves breaking on steep fore reefs and reef crests generate a setup proportional to offshore wave height that drives a cross-reef flow. We emphasize a critical threshold—where the depth on the reef flat equals the depth at wave breaking—that separates two distinct reef flow regimes: one dominated by strong, unidirectional flow from the ocean into the lagoon and another where wave breaking ceases, flow rates decrease, and occasionally reverse direction. Recognizing the relative importance of water depth and wave energy, we develop a framework for interpreting shallow reef hydrodynamics in a combined tide-wave parameter space. This framework allows us to project how rising sea levels may alter reef flows—potentially leading to prolonged and more frequent periods of limited wave breaking and a decline in wave-driven transport.

In-situ, metabolic flux measurements are increasingly being employed in shallow coastal settings to monitor ecosystem health, quantify carbon sequestration and understand biogeochemical cycling more broadly. However, the effectiveness of these techniques in wavy environments with high-relief roughness remains unclear, as waves and canopy dynamics may modify turbulent mixing. We present measurements of net community production (benthic oxygen flux) from three sites along a wave-exposed fore reef in Palau, using three different approaches: (a) aquatic eddy covariance, (b) scalar variance, and (c) gradient flux. Results are compared between methods and evaluated across a wide range of mean flow and wave conditions. Eddy covariance measurements were separated into wave- and turbulence-induced fluxes using the Benilov method, a spectral technique commonly used to correct wave bias in momentum flux measurements. Removing wave fluxes from eddy covariance measurements improved agreement between NCP rates at the different sites, suggesting that waves primarily contributed to measurement bias and not to real fluxes for our data sets. With this bias removed, NCP measurements were well correlated across the three methods, even when waves were large relative to mean flows. The primary difficulty in implementing scalar variance and gradient flux methods was identifying the appropriate mixing-length $��\left(z_c\right)�$ to parametrize eddy diffusivity. While $��z_c�$ should be equivalent to the measurement height above the coral substrate, this distance is hard to constrain in environments with nonuniform, canopy-like roughness such as coral reefs. We discuss challenges and provide recommendations for metabolic flux deployments in shallow coastal environments with waves.

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.