Journal of Geophysical Research-Oceans最新文献

In the South Atlantic Bight (SAB), changes in the Gulf Stream (GS), particularly its strength and proximity to the coast, are thought to be primary factors determining the shelf-break upwelling rate. However, it is still not entirely clear if and to what extent those factors influence cross-shelf nutrient fluxes and shape the ocean biogeochemistry at interannual and longer timescales. Here, we use a high-resolution regional ocean-biogeochemical model and an ocean reanalysis product (1993–2022), along with a satellite-derived chlorophyll data set (1997–2022), to investigate the interannual ocean-biogeochemical variability in the SAB. Regional model outputs suggest that year-to-year changes in phytoplankton production are indeed largely driven by upwelling of cold and nutrient-rich water to the shelf-break. The upwelling variability, reflected in bottom temperature and vertically integrated production patterns, is strongly linked to surface velocity changes in the GS near the shelf break, but weakly related to the depth-integrated GS transport. The GS's velocity changes, and the temperature and production anomalies, are well correlated to the alongshore wind stress, suggesting that local wind is the leading driver of the shelf-break upwelling variability at interannual timescales. Those relationships are also supported by circulation patterns from ocean reanalysis and satellite chlorophyll anomalies. Finally, examining the simulated shelf-slope interchanges in the carbonate system, we find that shelf-break upwelling significantly increases bottom acidification, a pattern linked to the low carbonate concentration in the slope waters. This study thus provides new insight for understanding and predicting GS and winds impacts on biogeochemical patterns from the SAB.

Medicanes, a class of the most intense Mediterranean cyclones, are known to have a substantial influence on the physical and biogeochemical properties of the marine environment. Yet, our understanding of how this response under various precyclone sea conditions is still lacking. Here, we conducted a comprehensive analysis of 14 medicanes focusing on the two days before and after their observed maximum intensity. We analyzed the medicane's influence on surface and subsurface physical and biogeochemical properties and also their interactions with various ocean structures in the marine environment. Within the mixed layer, our findings reveal a consistent response to the passage of the medicanes, as they move across regions of warmer or colder sea surface temperatures (SST). Upon moving to warmer SST regions, the response is characterized by an increase in chlorophyll a (Chl a), phytoplankton biomass, nutrients, and dissolved oxygen, as well as a greater drop in the sea temperature, relative to cold SST regions. The presence of warm-core eddies and marine heat waves along the cyclone's track before maximum cyclone intensity significantly affects the dynamics of the medicane with a more pronounced deepening that drives stronger vertical mixing and upwelling than cold-core eddies. These processes favor the injection of nutrients into the ocean's upper layers, driving the observed increase in Chl a concentration and phytoplankton biomass. These findings provide new insights into how ocean-atmosphere coupling may affect extreme Mediterranean cyclones and how they can drive regional marine productivity and ecosystem dynamics.

The investigation of oceanic submesoscale phenomena is a vital aspect of comprehending the multiscale fractal structure of the ocean. This study presents a systematic analysis of submesoscale sea surface temperature (SST) variability in the Kuroshio-Oyashio Extension region, combining high-resolution along-track satellite observations (Visible Infrared Imaging Radiometer Suite L2 product) with Massachusetts Institute of Technology General Circulation Model simulations. The spatial variance method effectively captures power-law scaling and energy density across specific scale ranges (5–300 km), even when applied to fragmented satellite SST data. The analysis reveals a $��k^{�-�5�/�3�}�$ spectral slope in SST spatial variance, consistent with surface quasi-geostrophic turbulence theory. Furthermore, submesoscale SST variability (5–50 km) exhibits a marked seasonal cycle, peaking in April and reaching a minimum in August. The strongest variance occurs near 42°N, collocated with a pronounced background SST frontal zone in the Oyashio Extension region. To unravel the underlying dynamics, we develop a dynamic diagnostic framework that quantifies the competing generation and dissipation processes controlling submesoscale SST variance. The combined forcing of large-scale/mesoscale SST fronts and submesoscale turbulent heat transport drives wintertime intensification and an April maximum in submesoscale SST variance, while nonlinear dynamics and horizontal mixing dissipate variance, resulting in an August minimum. The seasonal variation of the frontogenesis function leads the submesoscale SST variance by 1 month, which proves that frontal instability plays an important role in generating submesoscale horizontal SST gradients.

The Kuroshio-Oyashio Extension (KOE) region has experienced increasingly frequent extensive summer marine heatwaves (MHWs), yet the governing physical mechanisms remain unclear, particularly regarding the role of mixed layer depth (MLD). This study investigates seven major MHW events between 2001 and 2024 using a composite heat budget analysis, providing a generalized basin-scale assessment of dominant drivers beyond single-event perspectives. Anomalies in the surface heat flux term, calculated as the net surface heat flux divided by the MLD, explain approximately 65% of the warming during the development phase, while the same term (34%) and vertical processes (39%) contributed comparably to cooling during the decay phase. MLD variability modulates upper ocean warming through two distinct processes. Shallow MLD increases the anomalous surface heat flux term, and MLD shoaling induces the detrainment effect. These MLD-related contributions (47%) are comparable in magnitude to the surface heat flux anomaly effect (36%). In the decay phase, the surface heat flux anomaly effect (40%) and entrainment associated with MLD deepening (36%) both contribute comparably to the total cooling. Reduced low cloud cover and intensified wind speed played key roles in driving these processes during the development and decay phases, respectively. This study highlights not only the multiple atmospheric and oceanic processes that shape the evolution of extensive summer MHWs in the KOE region, but also the distinct and quantitatively assessed role of MLD, which can exert an influence comparable to that of surface heat flux.

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.

The standing meanders of the Antarctic Circumpolar Current are key sites of mixing and meridional transport in the Southern Ocean, but their long-term variability and trends remain poorly understood. Here, we investigate the Pacific-Antarctic Ridge meander using satellite altimetry data from 1993 to 2023. While its mean position has remained broadly stable, the meander has widened by 1.44 km per decade and accelerated by 0.01 m $��s^{�-�1�}�$ per decade, with the largest changes concentrated downstream of the first meander trough. Analysis indicates that the dominant contributor to these trends is a multi-decadal increase in eddy kinetic energy, reflecting enhanced along-jet variability and intensified eddy-mean flow interactions. A secondary but coherent influence arises from upper-ocean warming, which strengthens meridional thermal gradients and is consistent with a barotropic-baroclinic acceleration of the jet. Additional contributions include modest poleward shifts and intensification of westerly winds, and a basin-scale strengthening of the South Pacific Gyre that reinforces the downstream acceleration. The Pacific-Antarctic Ridge meander exhibits structural changes distinct from those documented over the Campbell Plateau, highlighting regional contrasts in standing-meander sensitivity to forcing. These findings carry implications for cross-frontal exchange, jet stability, and the evolving dynamical role of the Southern Ocean in the global climate system.

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.