Journal of Geophysical Research-Oceans最新文献

The deep waters of the Amerasian Basin in the Arctic Ocean are among the most isolated in the modern Arctic Mediterranean Sea (e.g., Schlosser et al., 1997, https://doi.org/10.1016/s0168-583x(96)00677-5). In this study, we use a suite of tracers spanning a range of timescales—including chlorofluorocarbons (CFCs), SF₆, radiocarbon (¹⁴C), and the radioactive noble gas argon-39 (³⁹Ar)—to assess the mean age and renewal rates of deep and bottom waters in the Canada and Makarov basins. Measurements from the 2015 US Arctic GEOTRACES expedition (GN01), combined with data from samples collected since 1979, reveal a homogeneous deep layer below ca. 2,500 m depth characterized by limited ventilation and gradual warming, consistent with estimates of geothermal heat fluxes. Apparent tracer ages in this layer average ca. 450 years, with a standard deviation of about ±40 years. Sparse but detectable CFC maxima in bottom waters observed at several stations suggest episodic inputs of dense, shelf-derived waters, likely resulting from the downslope cascading of brine-enriched plumes. Salinity and oxygen isotope analyses indicate that these inputs originate primarily from the Chukchi Borderland and Beaufort Sea shelf. Time-dependent mass balance calculations show that present-day ¹⁴C and ³⁹Ar concentrations can be explained by radioactive decay since a single deep water renewal event. These results indicate that the most recent basin-wide deep water renewal occurred approximately 450 years ago and constrain present deep ventilation rates to no more than approximately 0.01 Sv.

Global warming intensifies coastal phytoplankton blooms (CPBs) and marine heatwaves (MHWs), elevating risks to marine ecosystem health. However, the impacts of regional warming on CPBs in the coastal seas around China (CSAC) remain inadequately quantified, which impedes the development of targeted strategies to mitigate the increasing bloom frequency. To address this gap, we analyzed 1 km-resolution daily CPB records (2003–2020) from the CSAC, combined with concurrent abiotic data sets, to quantify their responses to warming. Our results indicate that bloom frequency increased in 72.1% of the affected CSAC areas, with 57.6% of these increases exhibiting a positive correlation with rising sea surface temperature. Blooms typically expanded in coverage during temperate springs and tropical autumns under moderate-intensity, long-duration MHWs. Key hotspots, such as the Pearl River Delta and Leizhou Bay, experienced earlier bloom timing and higher bloom frequency, as well as greater spatial extent during these seasonal MHW events. Our results highlight that both gradual warming and discrete MHW events are key drivers of the increased frequency and expanded spatial coverage of blooms in productive coastal zones. Therefore, although climate warming is projected to strengthen water column stratification and reduce nutrient availability, efforts to reduce coastal eutrophication remain crucial for mitigating future CPB intensification.

Estuarine-shelf systems contribute substantially to global dimethyl sulfide (DMS) emissions, yet the seasonal dynamics and cycling processes of DMS in these regions remain poorly understood. This study conducted seasonal field investigations (summer and winter) in the Yangtze River Estuary and adjacent sea to elucidate the sources, sinks, and environmental controls of DMS. Our findings reveal that elevated temperature and irradiance promote the biological production of DMS, resulting in a summer net accumulation rate approximately five times higher than that in winter. Seasonal variations in environmental drivers (e.g., temperature, light) did not significantly alter the relative importance of DMS sinks, with microbial consumption dominating removal processes in both seasons, followed by sea-air exchange and photodegradation. Approximately 80% of DMS was recycled within the mixed layer, while ∼20% was emitted to the atmosphere. The sea-air fluxes of DMS substantially exceed climatological mean fluxes, suggesting the survey region is a potential hotspot for global oceanic DMS emissions. By delineating seasonal source-sink dynamics, this study provides comprehensive insights into DMS cycling in estuarine-shelf systems, contributing to refined bottom-up models for reducing uncertainties in global DMS atmospheric budgets.

The ocean has absorbed over 90% of the excess heat trapped in the Earth system due to rising greenhouse gas emissions, with upper layers playing a crucial role. This study finds that 35% of the total ocean heat content (OHC) in the western Indian Ocean is stored within the upper 300 m. From 2000 to 2023, this layer shows a significant warming trend of $��0.87��G�J�/�m^2�$ over 24 years, making it the only tropical ocean basin with such a persistent rise. In contrast, the net surface heat flux into the ocean shows a declining trend of $��-�15.90��W�/�m^2�$ over 24 years, suggesting that direct atmospheric forcing is not the primary driver. Instead, seasonal ocean dynamics explains the observed increase in OHC and surface heat loss. During the winter monsoon, enhanced westward heat transport from the eastern equatorial Indian Ocean, driven by strengthened northeast monsoon currents, leads to heat accumulation in the western Indian Ocean. In the summer monsoon, the Great Whirl, a large anticyclonic eddy, plays a central role. Although northward heat transport associated with the Great Whirl has weakened, the southward transport has declined more sharply, resulting in net heat gain. Additionally, a northward shift in monsoon winds displaces the Great Whirl closer to the Socotra Islands, altering upwelling patterns and further redistributing heat. These findings underscore the dominant role of ocean circulation in driving long-term upper-ocean warming in the western Indian Ocean, contrasting with the expected influence of surface heat fluxes.

The vertical distribution of subsurface chlorophyll (Chla) is crucial for understanding the marine primary productivity estimation and the ocean carbon cycle. Here we developed a dual-branch neural network (TCB-MHA) that integrates satellites, reanalysis, and BGC-Argo data to accurately estimate Chla profiles in tropical-subtropical oceans. The model shows exceptional performance in estimating subsurface Chla concentration (R² = 0.824–0.859, RMSE = 0.0525–0.0980 mg/m³), capturing the depth (R² = 0.919, MAPE = 7.23%) and intensity (R² = 0.795, MAPE = 12.97%) of the subsurface chlorophyll maximum (SCM). However, SCM thickness prediction remains challenging (R² = 0.513, MAPE = 16.82%), likely due to the lack of phytoplankton photoacclimation parameterization. Independent validation in the South China Sea confirmed the model's strong generalizability. Climatological analysis reveals clear latitudinal SCM patterns across tropical-subtropical oceans: deeper and weaker in subtropical regions (SCM depth >100 m, SCM intensity = 0.32 ± 0.10 mg/m³) versus shallower and stronger in tropical zones (SCM depth <80 m, SCM intensity = 0.48 ± 0.10 mg/m³). Seasonal and vertical SCM variations are also consistently captured. The model further quantifies the impact of mesoscale processes and spring blooms of the Western Mediterranean on Chla distribution. This study highlights the advantages of deep learning in integrating multi-source heterogeneous ocean data, and the resulting monthly 3D Chla products can be applied to improve carbon cycle and productivity studies.

Oceanic fronts significantly affect primary production. While surface fronts are well-studied, subsurface fronts have received relatively little attention. The impacts and underlying mechanisms of subsurface fronts on phytoplankton distribution and nitrogen cycle remain unclear, limiting our understanding of primary production. Based on data from in situ sampling, satellite, and reanalysis, pronounced thermal fronts occurred in the subsurface layers but weakened and disappeared toward the surface and deeper layers of the northern South China Sea. Despite differing formation mechanisms (i.e., dipole eddies and warm offshore water intrusion), both frontal zones exhibited substantially higher chlorophyll a (Chl a) levels than non-frontal zones (on average, Chl a concentrations increased by 77.78% and inventories rose by 88.56%). Positive correlations between frontal intensities and Chl a concentrations, along with enhanced convergence-divergence and vertical processes, suggested that Chl a aggregate relates to physical accumulation. Additionally, evident nitrate (NO₃⁻) loss and isotope enrichment factors (¹⁵$��\epsilon �$ = 3.4‰ and ¹⁸$��\epsilon �$ = 4.5‰) supported that Chl a increase were also associated with NO₃⁻ assimilation. However, 22.7% of the total NO₃⁻ pool in frontal zones was from nitrification, representing an increase of 15.2% compared to non-frontal zones. Elevated regeneration was attributed to enhanced oxygen exchange related to frontal dynamics, as suggested by comparable dissolved oxygen and ammonium levels in both zones, yet with elevated apparent oxygen utilization and NO₃⁻ in frontal zones. This study highlights that subsurface fronts not only facilitate phytoplankton aggregation but also drive active NO₃⁻ regeneration, thereby leading to an overestimation of new production in oligotrophic oceans.

With the rapid loss of Arctic sea ice in recent decades, the pack ice zone (PIZ) is gradually transitioning to the marginal ice zone (MIZ), but its impacts on heat exchanges within the atmosphere-sea ice-ocean system remain unquantified. This study identifies the transition region from PIZ to MIZ using a positive difference in MIZ occurrence frequency between 1979–2010 and 1992–2023, and compares changes in heat exchanges in this region to those over the pan Arctic Ocean. The transition from PIZ to MIZ in summer increases shortwave radiation absorption by the ice-ocean surface (0.7 W m⁻² yr⁻¹, P < 0.05), further increasing upper ocean heat content (0.04 × 10⁸ J m⁻² yr⁻¹, P < 0.05); while that in winter enhances the exchanges of longwave radiation (about 0.8 W m⁻² yr⁻¹, P < 0.05) and surface upward latent and sensible heat fluxes (0.4 and 0.5 W m⁻² yr⁻¹, P < 0.05), partly leading to an increase in 2-m air temperature (about 0.2 K yr⁻¹, P < 0.05), more than twice the average of the pan Arctic Ocean, although the transition region occupies less than 25% of the pan Arctic Ocean. The amplification of heat exchanges is more pronounced in the transition region from PIZ to MIZ than in the MIZ defined by ice concentration between 15% and 80%. Results highlight that the transition process from PIZ to MIZ is more critical for the thermodynamic coupling of the Arctic atmosphere-sea ice-ocean system, compared to changes in the location and extent of MIZ.

The Labrador Current is a major pathway for export of North Atlantic Deep Water across 53°N. While direct measurements of the integrated transport of the current were made from 1997 to 2014 at the 53°N Moored Observatory and since 2014 as part of the Overturning in the Subpolar North Atlantic Program (OSNAP), the calculations of integrated transport are typically delayed by years due to the time to recover and process moored instrumentation. Here, we examine a method to compute variations in the transport from ocean bottom pressure gradients determined from the GRACE and GRACE-FO satellite missions since 2002, which will allow near-real-time monitoring. After verifying the method using an ocean state estimate that captures the mean and variable transport similar to the observations, we calculate the variability using the satellite observations. While there is a notable degradation at the end of the GRACE mission after 2012, results from 2002 to 2011 are consistent with interannual variations seen in the 53°N Moored Observatory and results from the GRACE Follow-on mission (starting in 2018) are consistent with observations made by the OSNAP array. Monthly differences are of the order of 3–4 Sv compared to overall variability of ±6 Sv (one standard deviation). No statistically significant trend since 2002 was found.

Ocean currents flowing over the seabed produce a turbulent bottom boundary layer (BBL) which extracts energy from the overlying flow and mediates sedimentary, chemical, and biological processes at the seafloor. The role of the BBL is especially important in relatively shallow shelf seas where stratification, mean flow, and tidally-driven oscillating currents are typically strong. Despite this importance, the dynamics and even the height of the BBL formed under stratified, tidal conditions are not well understood. We address this knowledge gap by performing large-eddy simulations of the evolution of a tidally forced and stratified BBL, initialized from rest, over a range of geophysically relevant parameter values. The BBL has two distinct regions: an actively stirred, near-well-mixed bottom mixed layer (BML) and a weakly stirred and strongly stratified capping pycnocline. A simple steady-state empirical model, dependent on stratification, tidal period, and bottom turbulence intensity, provides a useful estimate of BML height. Motivated by appreciable growth in the BML that can occur on timescales of days to weeks, we also develop a time-dependent model for the BML height. This model accounts for the vertical structure of the BBL turbulence and reproduces the BML growth with high fidelity. We also present a simple model for the cycle-averaged eddy diffusivity, which depends on the BML height. Both the steady-state and time-dependent models of BML height—and the accompanying expression for eddy diffusivity—can be used to develop tidal BBL parameterizations and inform grid resolution requirements within ocean models, as well as guiding sampling for future observational studies.