Journal of Geophysical Research-Oceans最新文献

In-situ observations indicate variations in stratified conditions over the coastal valley off the Sansha Bay in the northwestern Taiwan Strait across different years. However, dynamic processes and mechanisms governing the upwelling process over the valley, as influenced by stratification, are still unknown. By employing idealized numerical simulations, we demonstrate that compared to the unstratified case, the surface offshore flow intensifies, upwelling flux and net cross-shore transport are enhanced, upwelling area expands, and the cross-shore velocity structure is modified over the valley under stratified conditions. The primary factor controlling the vertical velocity within the valley is the relative vorticity change along a streamline (RVC). Further decomposition of the RVC reveals that the depth-averaged alongshore velocity ($��\bar{v}�$) and the depth-averaged alongshore gradient of vorticity ($��\partial �\bar{\xi }�/�\partial �y�$) primarily modulate the magnitude and spatial distribution of the vertical velocity. The negative zone of the $��\partial �\bar{\xi }�/�\partial �y�$ expands in the valley, resulting in a larger upwelling area. The magnitude of the $��\bar{v}�$ in the upper layer is slightly enhanced. The combined influence of these two factors leads to increased upwelling flux in the valley under stratified conditions. The net upslope motion over the valley is intensified under stratified conditions. The augmented advection of relative potential vorticity, originating from the increased amplitude of coastal trapped lee waves, primarily contributes to the enhanced net cross-shore transport in the valley.

Much of the Antarctic coast is covered by seasonal landfast sea ice (fast ice), which serves as an important habitat for ice algae. Fast-ice algae provide a key early season food source for pelagic and benthic food webs, and contribute to biogeochemical cycling in Antarctic coastal ecosystems. Summertime fast ice is undergoing a decline, leading to more seasonal fast ice with unknown impacts on interconnected Earth system processes. Our understanding of the spatiotemporal variability of Antarctic fast ice, and its impact on polar ecosystems is currently limited. Evaluating the overall productivity of fast-ice algae has historically been hampered by limitations in observations and models. By linking new fast-ice extent maps with a one-dimensional sea-ice biogeochemical model, we provide the first estimate of the spatio-seasonal variability of Antarctic fast-ice algal gross primary production (GPP) and its annual primary production on a circum-Antarctic scale. Experiments conducted for the 2005–2006 season provide a mean fast ice-algal production estimate of 2.8 Tg C/y. This estimate represents about 12% of overall Southern Ocean sea-ice algae production (estimated in a previous study), with the mean fast-ice algal production per area being 3.3 times higher than that of pack ice. Our Antarctic fast-ice GPP estimates are probably underestimated in the Ross Sea and Weddell Sea sectors because the sub-ice platelet layer habitats and their high biomass are not considered.

As the Arctic warms at an increased rate compared to the rest of the globe, freshwater runoff has been shown to be increasing into the Arctic Ocean. The effects of this contemporary increase in riverine freshwater into the Arctic Ocean are estimated from ocean model simulations, using two runoff data sets. One runoff data set is based on older climatological data, which has no inter-annual variability after 2007 and as such does not represent the observed increases in river runoff into the Arctic. The other data set comes from a hydrological model developed for the Arctic drainage basin, which includes contemporary changes in the climate. At the Pan-Arctic scale this new data set represents an approximately 11% increase in runoff, compared with the older climatological data. Comparing two ocean model runs forced with the different runoff data sets, overall changes in different freshwater markers across the basin were found to be between 5% and 10%, depending on the regions. The strongest increases were seen from the Siberian rivers, which in turn caused the strongest freshening in the Eastern Arctic. As the surface waters of the Arctic Ocean are sensitive to runoff, incorporating hydrological model data can help to better understand current changes and potential future impacts from increased runoff with climate change.

The controls on the development of submarine channel sinuosity are contested: slope gradient and Coriolis forcing have both been recognized as key governing factors: gradient via an inverse relationship (low sinuosity at high slope and vice versa), and Coriolis forcing through its effect on sedimentation patterns (reducing lateral bend migration, and hence sinuosity development, at high latitudes and/or in large channels). Using theoretical models to calculate the bulk properties of channelized turbidity currents, this study investigates the joint role of the Coriolis force and parameters including channel size, downchannel slope and turbidity current properties in the development of submarine channel sinuosity. Model validation is undertaken through the comparison of the calculated turbidity current tilting against the measured tilting of channel levees in the Northwest Atlantic Mid-Ocean Channel; this approach is then used to evaluate the controls on channel sinuosity in nine other modern seafloor channels. The results indicate that the Coriolis force only becomes significant when the size of the channel, the slope gradient and flow conditions are within appropriate ranges instead of solely being dependent on latitude. Thus, thick and dense (≥1% bulk sediment concentration) flows traveling within steep-gradient, small-scale channels were shown to be relatively less susceptible to flow modification by Coriolis forcing even at high latitudes. On the other hand, thin and dilute (≪1% bulk sediment concentration) flows in shallow-gradient, large-scale channels showed susceptibility to Coriolis forcing at all latitudes. These results offer new insights into submarine channel evolution and intra-channel sedimentation patterns.

The Hawaiian Island chain lies on the southern limb of the oligotrophic North Pacific Subtropical Gyre, an area characterized by low productivity. In this region, productivity is controlled by several factors, including the island mass effect and mesoscale eddies generated by wind stress curl in the lee of the islands. This study identifies high-chlorophyll events that occur periodically off the northern coasts of the Main Hawaiian Islands. These events are present in the satellite record and can be reproduced using a regional dynamical model. Model results indicate events are characterized by increases in total phytoplankton and total zooplankton, and a shift in phytoplankton size structure. We show these events are uniquely driven by the presence of cyclonic mesoscale eddies located downstream, on the opposite side of the island chain. While these eddies are known to impact productivity locally, we reveal that nutrients upwelled by these eddies can also be transported around the islands, counter to the mean background flow. These results demonstrate how leeward cyclonic eddies can have far-reaching, remote impacts on productivity around the Hawaiian Islands.

Satellite observations revealed two extremely low surface chlorophyll concentration (SCC) events with a warm sea surface temperature anomaly in the southeastern Arabian Sea (SEAS, 6°–15°N, 72°–77°E) during the summer (July–August–September) in 2015 and 2019. We find that the physical processes leading to these two similar low SCC events are remarkably different. The low SCC in the SEAS during summer 2019 is mainly related to the weakened upwelling and deepening of the thermocline depth due to the combined effects of the local wind anomalies and the arrival of westward-propagating downwelling coastal Kelvin wave driven by easterly anomalies near the eastern Sri Lanka during an extreme positive Indian Ocean Dipole (IOD) event. In summer 2015, a weaker positive IOD-induced easterly anomalies in the southern Bay of Bengal also drives downwelling coastal Kelvin waves westward, deepening the thermocline in the SEAS. But unlike that in summer 2019, the local wind stress curl anomalies in the SEAS during summer 2015 favors upwelling, which counteracts the downward motion of the coastal Kelvin waves, leading to weaker downward transport (one-third of that in 2019). Meanwhile, the upper ocean layer in the SEAS experiences extreme warming during summer owing to the development of 2015/2016 super El Niño. This substantial warming enhances upper oceanic stratification, which results in weaker vertical mixing and reduces the SCC to an extremely low level despite the much weaker IOD strength in 2015.

The North Adriatic Dense Water (NAddW)—the densest Mediterranean water generated by extreme cooling during wintertime hurricane-strength winds—drives the thermohaline circulation, ventilates the deep layers, and changes the biogeochemical properties of the Adriatic Sea. However, modeling the dynamical properties of such dense water at the climate scale has been a challenge for decades due to the complex coastal geomorphology of the Adriatic basin not properly reproduced by existing climate models. To overcome these deficiencies, a 31-year-long simulation (1987–2017) of the Adriatic Sea and Coast (AdriSC) kilometer-scale atmosphere-ocean model is used to analyze the main NAddW dynamical phases (i.e., generation, spreading and accumulation). The study highlights four key results. First, during winter, the NAddW densities are higher in the shallow northern Adriatic shelf than in the deeper Kvarner Bay—where 25%–35% of the overall NAddW are found to be generated—due to a median bottom temperature difference of 2°C between the two generation sites. Second, the NAddW mass transported across most of the Adriatic peaks between February and May, except along the western side of the Otranto Strait. Third, for the accumulation sites, the bottom layer of the Kvarner Bay is found to be renewed annually while the renewal occurs every 1–3 years in the Jabuka Pit and every 5–10 years in the deep Southern Adriatic Pit. Fourth, the NAddW cascading and accumulation is more pronounced during basin-wide high-salinity conditions driven by circulation changes in the northern Ionian Sea.

Observations have revealed the existence of persistent slope countercurrents (SCCs) that flow southwestward beneath the Kuroshio Current at several locations over the East China Sea (ECS) continental slope. It was not clear whether these flows are localized circulation features or segments of a trough-scale circulation system in the Okinawa Trough (OT). We demonstrate that there indeed exists a potentially continuous trough-scale SCC along the ECS slope that is associated with an OT-wide cyclonic circulation using high-resolution model simulations and physical interpretations. The detailed features of the deep OT circulation are illustrated by the trajectories of the Lagrangian drifters and the time-varying distributions of passive tracers. The SCC in the ECS is characterized by its weak yet persistent nature, typically located in narrow sloping regions at the isopycnal layer of 26.6–27.3 kg m⁻³. It exhibits a characteristic speed of approximately O-(1) cm s⁻¹. Analyses and experiments suggest that the divergence of upwelling in the SCC layer (26.6–27.3 σ_θ surface) gives rise to lateral potential vorticity transport, ultimately driving the deep cyclonic circulation. Furthermore, the SCC also displays a substantial connection with the onshore intrusion of the Kuroshio Current, particularly to the northeast of Taiwan Island. The SCC may potentially play a crucial role in the transport of heat and nutrients, as well as in regulating sediment distributions within the deep OT. This mechanism offers fresh insights into explaining the presence of undercurrents in semi-enclosed marginal seas.

Downstream of Cape Hatteras, the vigorously meandering Gulf Stream forms anticyclonic warm core rings (WCRs) that carry warm Gulf Stream and Sargasso Sea waters into the cooler, fresher Slope Sea, and forms cyclonic cold core rings (CCRs) that carry Slope Sea waters into the Sargasso Sea. The Northwest Atlantic shelf and open ocean off the U.S. East Coast have experienced dramatic changes in ocean circulation and water properties in recent years, with significant consequences for marine ecosystems and coastal communities. Some of these changes may be related to a reported regime shift in the number of WCRs formed annually, with a doubling of WCRs shed after 2000. Since the regime shift was detected using a regional eddy-tracking product, primarily based on sea surface temperatures and relies on analyst skill, we examine three global eddy-tracking products as an automated and potentially more objective way to detect changes in Gulf Stream rings. Currently, global products rely on altimeter-measured sea surface height (SSH), with WCRs registering as sea surface highs and CCRs as lows. To identify eddies, these products use either SSH contours or a Lagrangian approach, with particles seeded in satellite-based surface geostrophic velocity fields. This study confirms the three global products are not well suited for statistical analysis of Gulf Stream rings and suggests that automated WCR identification and tracking comes at the price of accurate identification and tracking. Furthermore, a shift to a higher energy state is detected in the Northwest Atlantic, which coincides with the regime shift in WCRs.

Abyssal T-waves are seismo-acoustic waves originating from abyssal oceans. Unlike subduction-zone-generated slope T-waves which are generated through multiple reflections between the sea surface and the gently dipping seafloor, the genesis of abyssal T-waves cannot be explained by the same theory. Several hypotheses, including seafloor scattering, sea surface scattering, and internal-wave-induced volumetric scattering, have been proposed to elucidate their genesis and propagation. The elusive mechanism of abyssal T-waves, particularly at low-frequencies, hinders their use to quantify ocean temperatures through seismic ocean thermometry (SOT) and estimate oceanic earthquake parameters. Here, using realistic geophysical and oceanographic data, we first conduct numerical simulations to compare synthetic low-frequency abyssal T-waves under different hypotheses. Our simulations for the Romanche and Blanco transform faults suggest seafloor scattering as the dominant mechanism, with sea surface and internal waves contributing marginally. Short-scale bathymetry can significantly enhance abyssal T-waves across a broad frequency range. Also, observed T-waves from repeating earthquakes in the Romanche, Chain, and Blanco transform faults exhibit remarkably high repeatability. Given the dynamic nature of sea surface roughness and internal waves, the highly repeatable T-wave arrivals further support the seafloor scattering as the primary mechanism. The dominance of seafloor scattering makes abyssal T-waves useable for constraining ocean temperature changes, thereby greatly expanding the data spectrum of SOT. Our observations of repeating abyssal T-waves in the Romanche and Chain transform faults could provide a valuable data set for understanding Equatorial Atlantic warming. Still, further investigations incorporating high-resolution bathymetry are warranted to better model abyssal T-waves for earthquake parameter estimation.