Journal of Geophysical Research-Oceans最新文献

Identifying and quantifying submesoscale stirring in the ocean remains a significant challenge. This study investigates the lateral dispersion and vertical transport effects within a cyclonic mesoscale eddy, using a combination of submesoscale-permitting in situ observations and model data. By analyzing water mass variations along cross-eddy isopycnals, distinct dispersion behaviors are identified at different depth layers. The analysis reveals an equivalent submesoscale along-isopycnal diffusivity larger than 100 m² s⁻¹ in the eddy's subsurface layer. To further examine transport pathways, Lagrangian particle tracking experiments are conducted using LLC4320 model output, presenting the features of lateral spreading and vertical penetration at various depths and locations within the eddy. Our results show that in the mixed layer, submesoscale stirring associated with submesoscale instabilities drives efficient lateral dispersion away from the eddy core. While in the subsurface stratified layer, internal wave motions significantly enhance vertical shear, which drives isopycnal submesoscale stirring. This process disrupts the eddy's coherence and causes vertical tracer exchange. These findings emphasize the essential role of submesoscale stirring processes in driving both lateral and vertical tracer dispersion and consequently affecting the eddies' capacity to trap water masses.

The ocean beneath ice shelves plays a critical role in their evolution and resilience. Despite this, direct observations of circulation within ice shelf cavities remain scarce. Here, we examine 4.5 years of moored hydrographic data (2018–2022) from the HWD2 borehole, which represent the first multi-year measurements of currents, temperature, and conductivity from the central Ross Ice Shelf cavity. The data set resolves distinct temporal variability across the water column. While the basal meltwater layer circulates differently from the deeper layers, the upper mid–water column is characterized by complex thermohaline structure that represents intrusions of supercooled water driven by sub-seasonal processes rather than the previously hypothesized spring–neap tidal cycle. These intrusions also exhibit a seasonal cycle. In contrast, the lower mid-depth region more closely reflects the open ocean signal. Multi-year records reveal inter-annual variability, highlighted by enhanced melting and refreezing from September to November 2019. Observations suggest that the enhanced melting and refreezing in late 2019 were influenced by strong Ross Ice Shelf polynya activity in 2018. Together, these records provide the only direct evidence of inter-annual variability in the central Ross Ice Shelf cavity, and further reveal seasonally recurring intrusions of supercooled water that highlight a critical pathway by which ocean and climate variability can influence heat and freshwater redistribution beneath the ice shelf, with important implications for its stability.

Resuspension of fine-grained bottom sediment under wind-driven currents and waves is a key process in shaping nearshore environments. Commonly, resuspension is quantified for predicting the dispersion of contaminants and nutrients affecting water quality by numerical modeling and field measurements. Although a large body of research deals with this topic, unique field observations from hypersaline environments coupled with conceptual-quantitative description of the process are lacking. Here, we present high-resolution direct measurements of winds, waves, currents, and turbidity conducted along the Dead Sea shores and derivations of an integrated 1D-numerical model based on mass and momentum conservation laws. Comparing the model predictions and the observations determine, for the first time, that depth-averaged turbulent viscosity during Dead Sea storms is of order of 10⁻³ m² s⁻¹. Resuspension of bottom clay to fine sand is governed primarily by waves inducing shear stress three orders of magnitude larger than current-induced shear stress, a ratio which is rather constant during Dead Sea storms. The observed spatiotemporal turbidity pattern is reproduced and accounts for the effect of grain-size distributions on the lake floor. Additionally, we highlight the importance of wave-induced resuspension as an additional source of sediment involved in the formation of thin, muddy layers that are traditionally interpreted as indicators of inflowing sediment plumes. The novelty of the manuscript lies in the combination of rare observations and modeling, which provides comprehensive physics of the studied processes, an approach that can be used in other nearshore environments of lakes or oceans.

Marine-derived volatile sulfur compounds (VSCs) play a critical role in global climate regulation and the sulfur biogeochemical cycle. To characterize their biogeochemical dynamics, a comprehensive investigation on VSCs, including dimethyl sulfide (DMS), carbonyl sulfide (OCS), and carbon disulfide (CS₂), was conducted in the upper seawater and the overlying atmosphere in the Kuroshio-Oyashio confluence region. The average concentrations of DMS, OCS and CS₂ were 1.03 ± 1.40, 0.075 ± 0.042, 0.029 ± 0.024 nmol L⁻¹ in seawater, and 125.9 ± 58.9, 497.9 ± 157.1, 67.4 ± 56.9 pptv in the atmosphere, respectively. Mean sea-to-air fluxes of DMS, OCS and CS₂ were 5.67 ± 6.20, 0.151 ± 0.273, and 0.125 ± 0.148 μmol m⁻² d⁻¹, respectively. The elevated OCS and CS₂ in subsurface waters of the Kuroshio Extension (KE) were attributed to phytoplankton abundance and temperature-dependent hydrolysis. The production of DMS, as the chemical signaling molecule and antioxidant, was promoted in the KE and eddy-active regions in warm but oligotrophic conditions. Co-occurrence of high DMS and CS₂ concentrations with elevated chlorophyll levels in upper waters of the Oyashio Current highlighted the pivotal role of phytoplankton in the production of DMS and CS₂. Notably, the edges of oceanic eddies were observed as hotspots for OCS and CS₂. Mixing ratios of VSCs in the atmosphere were comprehensively modulated by sea-air interactions, air mass transport, and oxidation processes. These findings provide valuable insights into the complex interplay between biological and physical processes that govern the biogeochemistry of VSCs in dynamic oceanic environments.

We present a new 27-year record of landfast sea ice extent in northern Alaska and adjacent waters, which uses ice chart data to extend a previous analysis based on synthetic aperture radar (SAR) imagery. This new climatology provides updated information on the decline of landfast ice in a region of the Arctic that has seen extensive losses of sea ice in recent summers. By comparing our results with early satellite data analysis from the 1970s, we find that trends in the timing of landfast ice have been ongoing for at least 50 years. Over the period 1996–2023, the landfast season shortened by 19 days/decade in the Chukchi Sea and 13 days/decade in the Beaufort Sea, primarily due to later formation of landfast ice. Also, the time between onset of freezing air temperatures and landfast ice formation is increasing, which is consistent with a coastal ocean that takes longer to freeze. While it was previously reported that the typical annual maximum width of landfast ice in the Chukchi Sea declined by 13 km between periods 1970–1976 and 1996–2008, we find this retreat has slowed with a decline of 3.3 km over the course of our data set as few areas of extensive landfast remain to be lost. Conversely, landfast sea ice extent in the Beaufort Sea had previously been found to have remained constant since the 1970s, but we find an average reduction of 2.5 km. We attribute this emergent phenomenon to a reduction in the number grounded ridges forming offshore.

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.