Journal of Geophysical Research-Oceans最新文献

A tidal model based on altimeter observations reveals that first-mode diurnal internal tides (DITs) propagate approximately 2,100 km eastward from the Luzon Strait (LS) into the Pacific Ocean. As they radiate over long distances, the DITs refract equatorward due to the beta effect. In this study, we utilize in situ round-trip acoustic echo time measurements between the seafloor and the sea surface, obtained from an array of 10 pressure-recording inverted echo sounders (PIES), to investigate the variability of DITs in the eastern Philippine Sea (EPS). The observations conducted over 1-year and 1.5-year periods during 2020–2021 reveal a clear weakening of DIT amplitudes in summer, in contrast to the barotropic diurnal tides, which show maximum spring tide amplitudes at the solstices and minimum amplitudes at the equinoxes. The observed seasonal variation in DIT energy flux shows a significant correlation with the relative vorticity averaged over regions of energetic warm eddies. Ray-tracing using HYCOM ocean model outputs indicates that the warm eddies in the upstream region of the ray path during summer (July to September) enhance the equatorward refraction of DITs. This study suggests that the superposition of the K₁ and P₁ constituents induces a pronounced semi-annual cycle in the DITs, even over considerable propagation distances. In addition, warm eddies exert a substantial influence on the DIT propagation path. Our results imply that the pronounced temporal variability of DITs should be considered to improve the parameterization of internal-wave-induced ocean mixing in oceanic and climate models.

Submesoscale processes in the ocean typically peak in winter, driven by mixed-layer instability and intensified atmospheric forcing. However, coastal upwelling regions can deviate from this paradigm due to region-specific dynamics. Based on validated high-resolution simulations, we investigate the seasonal and regional variability of submesoscale activity in the Arabian Sea. Our results reveal that the western Arabian Sea exhibits a pronounced summer peak in submesoscale activity, primarily associated with wind-driven upwelling, enhanced frontogenesis, and mixed-layer baroclinic instability. Although earlier studies have reported intensification of submesoscale processes in coastal upwelling regions, detailed dynamical interpretations remain limited. Our work advances this understanding by explicitly diagnosing the regional physical mechanisms driving submesoscale variability under monsoon-influenced upwelling system. This regional contrast becomes more evident when considering the broader basin. In the northern open ocean, submesoscale processes exhibit the canonical winter-intensified pattern, whereas in the eastern Arabian Sea near the Maldives, they display a distinct bimodal structure with both summer and winter peaks. These findings highlight the importance of adopting region-specific frameworks to interpret submesoscale seasonality, moving beyond the winter-intensified paradigm dominant in open-ocean settings. Our results provide novel insights into how coastal and open-ocean submesoscale dynamics coexist in the Arabian Sea, with implications for seasonally varying energy cascades, vertical heat and nutrient fluxes, and air-sea exchange in the upper ocean.

This study employs the coupled conditional nonlinear optimal perturbation (C-CNOP) method, which incorporates initial coupling uncertainties, to identify sensitive areas of targeted observations for positive Indian Ocean Dipole (IOD) events. Results show that the initial errors most likely to yield large prediction uncertainties of IOD events are mainly concentrated in sea temperatures near the thermocline in the eastern Indian Ocean (IO_Temp: 70–110 m depth, 5°S–5°N, 85°E−105°E) and western Pacific (PO_Temp: 120–160 m depth, 5°S–5°N, 130°E−150°E), as well as zonal winds (UWind), exhibiting an east–west dipole pattern over the tropical Indo-western Pacific. Through sensitivity experiments—designed to assess the impact of initial uncertainties in different areas on IOD predictions while bypassing the assimilation process and avoiding initial shock effects—we find that prediction uncertainties are more sensitive to initial errors in the UWind area than in the IO_Temp and PO_Temp areas, demonstrating a stronger impact on forecast skill, particularly in winter and summer. Further analysis demonstrated that the IO_Temp & PO_UWind coupled area involving the eastern Indian Ocean subsurface temperature and western Pacific zonal winds, exhibits greater sensitivity than the UWind area alone, emerging as the most sensitive area of positive IOD events. This key area highlights both the Pacific's remote influence and the crucial role of local ocean on IOD development. These results underscore the critical role of coupled initialization in IOD predictability, offering a theoretical basis for advancing coupled data assimilation.

This study evaluates the performance of dynamical downscaling in the Northern Adriatic Sea, focusing on eddy kinetic energy spectra and dense water formation. Using the perfect model framework, a high-resolution (2 km) reference simulation of the entire Adriatic Sea serves as the benchmark for a series of one-way nesting downscaling experiments reaching the same horizontal resolution in the Northern Adriatic. Results show that a downscaling ratio of 1:3 effectively reproduces the local energy budget and multiscale features. However, the absence of feedback from small to large scales limits the downscaling performance. This limitation is evident in dense water formation, which is controlled by the interplay between local and remote drivers in the Northern Adriatic Sea. When local drivers, such as buoyancy fluxes, dominate, the dense water formation process is well reproduced. In contrast, when remote influences, particularly the inflow of salty Levantine Intermediate Water through the Otranto Strait, are not properly resolved by the parent model, reproducibility of dense water formation deteriorates. Our experiments indicate that a 2 km horizontal resolution effectively captures cross-scale interactions at the strait, while a 6 km resolution is insufficient. These interactions, particularly feedback from small scales to large scales, lead to changes in thermohaline dynamics that propagate toward the Northern Adriatic Sea.

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.