Journal of Geophysical Research-Oceans最新文献

Phytoplankton play a crucial role in regulating marine biogeochemical pathways but studying their spatial and temporal dynamics often requires extensive and laborious oceanographic expeditions. Here, we report a novel use of Biogeochemical-Argo data to delineate planktonic habitats in the Tropical North Atlantic Ocean (TNA). We then investigated the phytoplankton biomass via in situ measurement of the BGC-Argo floats and phytoplankton community structure of each habitat using Phytoplankton Functional Types derived from satellite observations. Our habitat delineation approach provided a finer scale and dynamic overview of phytoplankton assemblages and their surrounding environment, complementary to the widely used and static biogeochemical provinces of Longhurst. While picophytoplankton remained dominant for most communities, we found elevated abundance of microphytoplankton, including diatoms and dinophytes, and nanophytoplankton, including haptophyte and green algae, following the seasonal displacement of the Intertropical Convergence Zone, and the Amazon and Orinoco River discharges into the Tropical Atlantic and Eastern Caribbean Sea. At midlatitude, our method was able to capture both the seasonal and spatial variations of the oligotrophic region of the North Atlantic subtropical gyre. These findings shed light on the seasonality of phytoplankton distribution driven by meteorological and oceanic forcings and demonstrated the potential of the BGC-Argo in studying and monitoring marine ecosystem.

The slope current in the northwest South China Sea (SCS) plays a vital role in regulating mass and nutrient exchange between the coastal region and the open ocean. Over the past three decades, this current has exhibited a significant increasing trend, indicating a much stronger marginal sea-open ocean interaction. Using observational data, we show that the intensified anticyclonic warm eddy shedding from the Luzon Strait propagates along the continental slope, leading to a stronger slope current. During 1993–2022, a reduction in Subtropical Mode Water was accompanied by decreases in sea surface height (SSH) and Kuroshio transport in the Luzon Strait, as well as an enhanced looping pathway of the Kuroshio in the northern SCS. These conditions favor anticyclonic warm eddies (positive SSH anomaly) shedding from the northwestern Luzon Strait and propagating along the continental slope southwestward. At the same time, the accompanying cold eddies (negative SSH anomaly) propagate westward towards western SCS. The combined propagation of these dipole-like eddies further strengthens the SSH gradient between the shelf and inner ocean in the SCS, thereby intensifying the slope current. Importantly, it is the increased mean strength of the anticyclonic eddies, rather than their number, that drives this long-term current intensification. In contrast, neither local surface wind nor buoyancy forcing can account for the observed enhancement of the slope current, although wind forcing likely contributes to strengthening its southern segment. These findings highlight the crucial role of Kuroshio intrusion and eddy-current interaction in regulating the long-term variability of circulation in the SCS.

The northward propagation of the Monsoon Intraseasonal Oscillations (MISOs) in the Bay of Bengal (BoB) is an intrinsic characteristic of the Indian summer monsoon (ISM). Previous studies have demonstrated the critical role of air-sea interactions in modulating MISO propagation. This study elucidates the intraseasonal variability of ocean heat content (OHC) in the upper 200 m of the BoB and its dynamic relationship with MISO. Similar to sea surface temperature (SST), the positive OHC anomalies lead MISO's northward propagation, showing two prominent maxima located east of Sri Lanka and in the northwestern BoB. The OHC anomalies are stronger east of Sri Lanka, penetrating the thick barrier layer during MISO events, whereas temperature anomalies in the northwestern BoB remain confined to the mixed layer. Diagnostic analyses reveal that the intraseasonal OHC variability, unlike that of SST, stems from intensified downward vertical advection driven by intraseasonal vertical velocity. In contrast to the wind-dominated intraseasonal vertical velocity in NBOX, the pronounced intraseasonal OHC variability east of Sri Lanka stems from sea level anomaly generated by both westward-propagating Rossby waves and MISO-related winds. Subsequently, with the arrival of MISO, upward vertical advection and thick barrier layer prolong warm SST anomalies east of Sri Lanka, providing additional heat and moisture to enhance MISO convection and rainfall intensity. These results highlight the essential role of the memory effect of upper ocean heat exchange and redistribution processes in sustaining MISO propagation.

The world-renowned tidal bore forms in the Qiantang River Estuary (QRE) and leads to distinctive hydro-sediment dynamics and the reshaping of coastal geomorphology. This study presents a time series of in situ data of wave, current, and suspended sediment concentration (SSC) in the tidal bore, covering the spring-neap tidal cycle in 2020. Field data analysis reveals the spring-neap and flood-ebb asymmetries in hydro-sediment dynamics. Current-induced bed shear stress is mostly larger than that induced by waves. The interactions of semi-diurnal tide and shallow water tide play a leading role in the tidal asymmetry. Turbulence, particularly ejection and sweep, contributes to the sediment inception and increased turbidity. Peak turbulent kinetic energy (TKE) and strong high-frequency water level oscillations occur during both flood and ebb tides, driven by different mechanisms. During the flood tides, they are initiated by the breaking of the tidal bore and its secondary waves. During ebb tides, the wave-current interactions enhance TKE and generate intense high-frequency oscillations, which is a process previously under-documented. The findings reveal the dynamic mechanism of turbulence asymmetry and high-frequency oscillation in water level due to current-wave interactions and shed light on the evolution of dynamic geomorphology in macro-tidal turbid estuaries.

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.