Journal of Geophysical Research-Oceans最新文献

Diurnal warming (DW) at the ocean surface occurs when there is a combination of solar heating in the absence of vertical mixing typically derived from wind stress. DW has been well described, mostly from satellite data, but also with some in situ observations. Evidence of DW has mostly been restricted to the subtropics, and there are very few reports of DW at northerly latitudes. We present here observations of a DW event of 1.5°C confined to the upper 2 m in the Labrador Sea at $��>�55�°�$N. These measurements were conducted with the Air-Sea Interaction Profiler (ASIP), an upwardly rising, ocean microstructure instrument. Cloud cover obscured the ocean surface to passive remote-sensing instruments and as a result no evidence of this particular DW event was available from the nine independent satellite products that were analyzed. Therefore, the event would have gone undetected without the deployment of ASIP at precisely this time and location. The ASIP observations were used to derive a heuristic set of criteria for potential occurrences of DW in the Labrador Sea region: (a) shortwave radiation above 600 W m⁻² and (b) 10-m wind speed below 4 m s⁻¹. These criteria were subsequently applied to $��\sim �$40 years of the ERA5 reanalysis product indicating that DW events in the Labrador Sea have the potential to occur more frequently than satellites observe. Attaching microstructure temperature sensors on Argo floats would provide a more accurate assessment of the occurrence of DW events globally as well as their effect on surface mixing rates.

The influence of a subsurface ridge on a subinertial coastal Kelvin wave is investigated using a numerical model and a simplified analytical framework. A first baroclinic mode Kelvin wave is made to impinge on progressively complex subsurface geometries. For an idealized submerged shelf that extends infinitely in both offshore and alongshore directions, the over-shelf response includes low vertical modes of the shallower domain and an upward-propagating beam from the shelf-top corner, inducing alongshore variability in the shelf flow. When the alongshore extent is truncated to form a ridge between two deep basins, the downstream response strongly depends on ridge width. Phase patterns reveal upward and downward energy propagation from the downstream ridge corner, along with horizontal propagation of multiple vertical modes. As the ridge narrows, beam amplitudes decrease, though vertical propagation remains visible in phase patterns. If the ridge width is less than approximately twice the Rossby radius of deformation of the ridge-top domain, evanescent modes along the ridge flanks begin to overlap and interact, promoting more horizontal energy transmission into the downstream basin. Thus, the Rossby radius of deformation sets a natural scale governing the balance between vertical scattering and horizontal transmission. In the most realistic configuration, a submerged ridge of finite offshore and alongshore extent, waves trapped to and propagating around its periphery affect the downstream basin through combination with the signals that propagate over the top of the ridge.

High-resolution mooring observations at 17.8°N, 89.5°E in the northern Bay of Bengal (BoB) are used to investigate the fraction of near-inertial (NI) wind power input ($��{\Pi }_W�$) at the ocean surface that propagates below the mixed layer (ML) and its role in modulating diapycnal heat flux-induced ML temperature (MLT) cooling during Tropical Cyclone (TC) Bulbul (5–9 November 2019). The $��{\Pi }_W�$ induces a sharp rise in ML NI Kinetic Energy (NIKE) (∼80 Jm⁻³), followed by its downward propagation, with subsurface NIKE (∼40 Jm⁻³) about half of the ML value and lagging by approximately 2 days. Though the ML NIKE budget indicates that approximately 10% of the $��{\Pi }_W�$ is radiated downward from the ML, the enhancement of NIKE below the ML is plausible due to stalling of NIWs at the base of the ML. Radiation of NIKE at the ML base increases vertical shear of currents, leading to a fourfold enhancement of diapycnal diffusivity to 1.25 × 10⁻³ m²s⁻¹ compared to the pre-cyclonic value (3.3 ± 0.04 × 10⁻⁴ m² s⁻¹). As a result, diapycnal heat flux at the ML base increased to −126.4 ± 11.4 Wm⁻² from the pre-cyclonic value of ∼2.7 ± 1.9 Wm⁻². During the peak cooling phase of ML, diapycnal heat flux (−200 Wm⁻²) shows a comparable contribution with respect to the net heat flux term. During the post-TC phase, the ML heat storage term has exhibited a pronounced high-frequency variability ranging between −4,000 and −2,000 Wm⁻² due to the NI current in the presence of cold wake.

Upper Circumpolar Deep Water (UCDW) is carbon-rich, oxygen-poor, nutrient-rich, and relatively warm and salty compared to waters above and below it. Where it is entrained into the surface mixed layer or impinges on the Antarctic Continental Shelf, it can outgas carbon, promote productivity, and melt sea ice and marine terminating ice sheets. Here, we analyze 20 years (2005–2024) of temperature and salinity profile data from CTD (conductivity-temperature-depth) instruments, both mounted on Argo floats and deployed during ship-based campaigns, to map mean water-property distributions and temporal trends at the UCDW temperature maximum. This circumpolar analysis of mean pressure, temperature, salinity, and density fields show, consistent with previous studies, the cyclonic subpolar gyres as domes in pressure with warm temperatures swirling in from the north and cold temperature from the coastal regions. Also consistent with previous studies, the westward-flowing Antarctic Shelf Current is characterized by a relatively cold and deep temperature maximum adjacent to the continental shelf present in all but the Amundsen and Bellingshausen seas. The analysis also reveals that over the past 20 years, UCDW has generally shallowed, warmed, and freshened in the Weddell Sea and off East Antarctica, whereas it has deepened, cooled, and gotten saltier from Drake Passage westward to the eastern edge of the Ross Sea, a striking regional dichotomy in the trends.

Icebergs play an important role in the climate system through their temporally and spatially distributed injection of freshwater into the ocean. Waterline melting from surface wave action accounts for a substantial amount of iceberg mass loss and drives iceberg fragmentation, yet it is poorly constrained and lacks physics-based model implementation, particularly in considering the ice-ocean boundary layer. Here we develop a hierarchy of models that couple wave-induced iceberg melt with the ice-ocean interfacial boundary layer, an approach traditionally used in modeling the ocean-driven melting of ice shelves. We find that the flux of meltwater from wave-induced ice loss into the proximal ocean acts to lower further wave-induced melting by 10%–20%. Depth-averaged wave-induced melting increases sublinearly with increasing wave height and wavelength, and approximately linearly with respect to far-field ocean temperatures. Furthermore, the most widely used wave erosion parameterizations overestimate depth-average melt rates by a factor greater than two compared to the models developed here. We derive an analytical solution for wave-induced melt rate that more accurately represents thermal forcing and wave-driven heat transfer at the ice boundary, and which can be readily applied in future modeling studies. Furthermore, numerical solutions of box models developed here for wave-induced melting and meltwater plume transport can model sub-grid near-ice mixing and can be used in larger-scale ocean models simulating icebergs. We conclude by proposing the modification of a traditionally used simple power-law function describing wave-driven melt rates to more accurately model bulk iceberg mass loss.

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.