Journal of Geophysical Research-Oceans最新文献

Large amplitude and unexpected waves are a regular source of natural disasters. Among them, impulse waves generated by landslides can represent a significant threat. Therefore, predicting and measuring the generation of such waves is essential. In this study, the phenomenon is modeled by a 2D-experimental setup using a steady non-uniform granular flow along a slope as a forcing wave generator. The present device provides a continuous supply of grains to avoid finite volume effects, as the part of the landslide actually involved in the wave generation strongly depends on the configuration and is not necessarily available in geophysical events. This system consists of an energy transfer between the granular flow and the wave generation which is characterized by a Froude number. It is found that the latter cannot be defined only based on the dry flow properties to characterize the wave. In particular, the dynamics underwater influence wave generation during a finite time. Accordingly, the present study shows that the wave maximum amplitude is governed by a newly defined Froude number, based on both dry and underwater granular flow properties. Moreover, it is shown that the granular deposit, specifically its runout, can be thought as a proxy of the immersed granular dynamics as long as the impact properties are still considered.

The South Scotia Ridge, in the Atlantic sector of the Southern Ocean, is a key region for water mass modification. It is the location of the Weddell-Scotia Confluence, an area of reduced stratification which separates the Weddell Gyre to the south and the Antarctic Circumpolar Current to the north, and which receives input of shelf waters from the tip of the Antarctic Peninsula. To elucidate the transformations over the ridge, we focus on one of its largest seamounts, Discovery Bank, which has previously been observed as hosting a stratified Taylor column that retains water for months to years, during which time water masses are entrained from north and south of the Weddell Front and steadily mixed. Data from ship-deployed sensors and autonomous platforms are analyzed to quantify and understand the diapycnal mixing, heat fluxes and water mass transformations over the bank. Ocean glider and free-profiling drifting float data show that the mid-depth temperature maximum of the Circumpolar Deep Water (CDW) is eroded between the northern and southern sides of the bank, while diapycnal diffusivity is enhanced by up to an order-of-magnitude over its steeply sloping portions. This is accompanied by heat fluxes from the CDW layer being increased by up to a factor of six, which may contribute to a reduction in mid-depth stratification. Tidal model analysis shows that the southern side of the bank hosts strong barotropic to baroclinic energy conversion (>150 N m⁻²), emphasizing the role of internal tides in modulating water mass transformations in the Confluence.

The diurnal cycling of the surface mixing/mixed layer (ML) depth, air-sea heat flux, and vertical profiles of the temperature and turbulent kinetic energy dissipation rate in the tropical central South China Sea was observed in summer (June 2017) and winter (January 2018). In the daytime, solar heating warmed and stabilized the ML, and the thickness of the ML can be well characterized by the Zilitinkevich scale as noted in previous studies. By contrast, in the nighttime the ML was deepened by convective turbulence generated by surface cooling. Guided by these observations, we have derived a simple scaling for the nighttime deepening of the ML by simplifying the classic Kraus-Turner type model. We show that the variation of the ML depth can be scaled by a function of the wind speed, air-sea heat flux and the temporal variation of the sea surface temperature, all of which are observable variables at the sea surface. It is found that the scaling works well in reproducing observed variations of the ML depth from hydrographic data. As such, this study advances our understanding of the response of the upper ocean to atmospheric forcing and provides a simple way for predicting the ML depth with solely surface observations.

The Loire estuary (France) was extensively deepened during the 20th century. Coincidentally, suspended sediment concentrations increased drastically from ∼0.1 g/l to ∼1–5 g/l at the surface and the estuarine turbidity maximum (ETM) moved upstream. In this study we, for the first time, brought together a century of observations of estuary bed level, tidal amplitude, and sediment concentration to demonstrate these large changes. Next, we analyzed a minimal set of physical mechanisms that explain the dramatic increase in sediment concentration. To this end, we used the iFlow model representing dynamic equilibrium conditions in the Loire. Novel in the model is that it dynamically resolves salt stratification and corresponding damping of turbulence. For conditions representing the year 2000, high sediment concentrations were found with satisfactory correspondence to observations. Low sediment concentrations were found when using the year 1900 bed level but keeping all other model parameters the same. Varying the bed level gradually between these two extremes, the equilibrium solution suddenly increases for intermediate bed level, constituting an abrupt regime shift. Robustness of this result was established in an extensive sensitivity study featuring 13,200 model experiments. The regime shift is enabled by a feedback between increasing sediment concentration, reducing turbulence due to sediment and salt stratification, and increasing sediment importing capacity of the estuary. The essential sediment importing mechanisms in this feedback are related to the tidal asymmetry and gravitational circulation. This is the first time gravitational circulation and salt stratification are shown to be important factors in a transition to hyperturbidity.

Sea level rise is challenging for coastal communities and land management decision makers. Understanding the patterns of regional variations at different temporal and spatial scales is key to implement adaptation plans that mitigate the local impacts of sea level rise. In this study, in situ observations from 14 tide gauges were complemented with satellite altimetry data to assess seasonality, multidecadal variability and long-term trends in mean sea level around the Western Iberian Coast (WIC) and the Portuguese archipelagos (Azores and Madeira). Results show varying spatial seasonal patterns between regions, with minimum (maximum) sea level observed in April (September) at the islands and minimum (maximum) observed in July (November) at the WIC. The influence of coastal upwelling on the seasonal mean sea level variations was detected over mainland. Although the influence of atmospheric patterns was observed on sea level inter-annual variability, the Atlantic Multidecadal Oscillation (AMO) showed a greater correlation with the sea level inter-decadal patterns. Finally, the trend analysis confirmed widespread sea level rise along the mainland and around the islands, which has intensified in recent decades. The regions of La Coruña and Cascais showed trends that were similar to the global average sea level rise since 1993, but the mainland regional average pointed to lower rates of rise (2.00 ± 0.06 mm/year). This work reinforces the need for long-term monitoring networks of sea level, ensuring the vertical stability of instruments and platforms. The implementation of regional adaptation plans to sea level rise is deeply dependent on high quality information.

The Atlantic Meridional Overturning Circulation entails vigorous thermohaline transformations in the subpolar North Atlantic Ocean (SPNA). There, warm and saline waters originating in the (sub)tropics are converted into cooler and fresher waters by a combination of surface fluxes and sub-surface mixing. Using microstructure measurements and a small-scale variance conservation framework, we quantify the diapycnal and isopycnal contributions –by microscale turbulence and mesoscale eddies, respectively– to thermohaline mixing within the eastern SPNA. Isopycnal stirring is found to account for the majority of thermal (65%) and haline (84%) variance dissipation in the upper 400 m of the eastern SPNA. A simple dimensional analysis suggests that isopycnal stirring could account for $��O�$(5–10) Sv of diahaline volume flux, suggesting an important role of such stirring in regional water-mass transformations. Our mixing measurements are thus consistent with recent indirect estimates in highlighting the importance of isopycnal stirring for North Atlantic overturning.

A 3D large eddy simulation coupled with a free surface tracking scheme was used to simulate cross-shore hydrodynamics as observed in a large wave flume experiment. The primary objective was to enhance the understanding of wave-backwash interactions and the implications for observed morphodynamics. Two simulation cases were carried out to elucidate key processes of wave-backwash interactions across two distinct stages: berm erosion and sandbar formation, during the early portion of a modeled storm. The major difference between the two cases was the bathymetry: one featuring a berm without a sandbar (Case I), and the other, featuring a sandbar without a berm (Case II) at similar water depth. Good agreement (overall Willmott's index of agreement greater than 0.8) between simulations and measured data in free surface elevation, wave spectrum, and flow velocities validated the model skill. The findings indicated that the bottom shear stress, represented by the Shields parameter, was significant in both cases, potentially contributing substantial sediment transport. Notably, the occurrence of intense wave-backwash interactions were more frequent in the absence of a sandbar. These intense wave-backwash interactions resulted in a pronounced horizontal pressure gradient, quantified by high Sleath parameters, exceeding the criteria for momentary bed failure. Additionally, a more vigorous turbulence-bed interaction, characterized by near-bed turbulent kinetic energy, was observed in the case lacking a sandbar, potentially augmenting sediment suspension. These insights are pivotal in understanding the mechanisms underlying berm erosion and how sandbar formation serves to protect further beach erosion.

Bottom Temperature anomalies (BTA) along the North American West Coast strongly influence benthic and demersal marine species. However, to date seasonal BTA forecast efforts have been limited and sources of BTA predictability largely undiagnosed. Here, an empirical model called a Linear Inverse Model (LIM), constructed from a high-resolution ocean reanalysis, is developed to predict North American West Coast BTAs and diagnose sources of predictive skill. The LIM is considerably more skillful than damped persistence, particularly in winter, with anomaly correlation (AC) skill values of 0.6 at 6-month lead. Analysis of the LIM's dynamics shows that elevated BTA forecast skill is linked to developing El Niño-Southern Oscillation (ENSO) events, driving predicted BTA responses whose peaks occur at longer leads with increasing latitude. Weaker ENSO-related signals in the northern coastal region still yield high BTA skill because noise there is also weaker. Likewise, the LIM's forecast signal-to-noise ratio is highest for bathymetry depths of ∼50–150 m, maximizing forecast skill there. Together, these predictive components lead to “forecasts of opportunity” when LIM anticipates especially high prediction skill. For the top 20% of events identified by the LIM as the forecasts of opportunity, 6-month lead BTA hindcasts have AC skill averaging 0.7, while the remaining 80% hindcasts have mean skill of only 0.4, suggesting that the LIM can leverage ENSO-related predictability of BTA to produce skillful forecasts.

Oceanic mesoscale eddies (with scale 10¹–10² km) and their submesoscale fine structures (with scale 10⁰–10¹ km) can effectively induce vertical motions and bring nutrients into the oceanic euphotic layer, which leaves abundant footprints on the ocean surface chlorophyll distributions and have the potential to promote primary productivity of oceanic ecosystem. In return, these surface chlorophyll footprints observed by ocean color satellites can serve as a useful tool to reveal the spatial structures of mesoscale eddies and their submesoscale fine structures. By combining artificial intelligence (AI) algorithms to develop a series of identification strategies for typical surface chlorophyll patterns around mesoscale eddies, we find that over 20% of mesoscale eddy observations exhibit identifiable typical chlorophyll patterns, which tends to regulate an increase of the surface chlorophyll concentration within the corresponding eddies, especially enhancing by about 30% in nutrient-restricted subtropical regions compared with the background values. Based on their geometric features, typical chlorophyll patterns are primarily classified as Core, Spiral, Tail, Ring, Loop, and Eye respectively by clustering algorithm. Further spatial-spectral analysis found that the typical patterns on eddies exhibit a much steeper wave-number spectral slope about −3, compared to the non-typical distributions on eddies and the non-eddy background distribution (about −2.7–−2.2). This implies that the occurrence of different typical chlorophyll patterns may correspond to specific mesoscale and submesoscale dynamic processes.

The Amundsen Sea in West Antarctica features rapidly thinning ice shelves, large polynyas, and sizable spring phytoplankton blooms. Although considerable effort has gone into characterizing heat fluxes between the Amundsen Sea, its associated ice shelves, and the overlying atmosphere, the effect of the phytoplankton blooms on the distribution of heat remains poorly understood. In this modeling study, we implement a feedback from biogeochemistry onto physics into MITgcm-BLING and use it to show that high levels of chlorophyll—concentrated in the Amundsen Sea Polynya and the Pine Island Polynya—have the potential to increase springtime surface warming in polynyas by steepening the attenuation profile of solar radiation with depth. The chlorophyll-associated warm anomaly (on average between +0.2$��°�$C and +0.3$��°�$C) at the surface is quickly dissipated to the atmosphere, by increases in longwave, latent and sensible heat loss from open water areas. Outside of the coastal polynyas, the summertime warm anomaly leads to an average sea ice thinning of 1.7 cm across the region, and stimulates up to 20% additional seasonal melting near the fronts of ice shelves. The accompanying cold anomaly, caused by shading of deeper waters, persists year-round and affects a decrease in the volume of Circumpolar Deep Water on the continental shelf. This cooling ultimately leads to an average sea ice thickening of 3.5 cm and, together with associated changes to circulation, reduces basal melting of Amundsen Sea ice shelves by approximately 7% relative to the model scenario with no phytoplankton bloom.