The three-dimensional transport pathways, the time scales of vertical transport, and the dispersion characteristics of submesoscale currents at an upper-ocean front are investigated using material points (tracer particles) that advect with the local fluid velocity. Coherent submesoscale vortex filaments and eddies which dominate submesoscale (0.1 - 10 km) dynamics are found to play a crucial role which is quantified here. These coherent structures are generated and sustained through nonlinear evolution of baroclinic instability. The collective motion of particles helps identify common features of transport at the front. It is found that the particles in the central region organize into inclined lobes, each associated with an eddy, and the filaments associated with the heavy- and light-edges of the front transfer edge particles to the lobes. This flux of new particles into the lobe causes local particles to adjust, which leads to slumping of the front. The particle motion in the vertical shows multiple time scales -- a fast time scale with O(10) m vertical displacement within an hour and a slower near-inertial time scale, comparable to the intrinsic time scale of the growing instability. The fast time scale motions typically occur in the vortex filaments. The overall slumping process is slower than what one might anticipate from the large magnitude of vertical velocity in the filaments and requires a sustained correlation over time between the lateral and the vertical motion. By tracking clouds of particles, we show that their centers of mass downwell/upwell over 1-2 inertial time periods, after which an adjustment follows with a sub-inertial time scale. The dispersion characteristics of the submesoscale turbulent currents using single- and pair-particle statistics have been investigated. The shape change in clusters of four particles reveals deformation into thin, needle-like structures.
Reliable forecasts for the dispersion of oceanic contamination are important for coastal ecosystems, society and the economy as evidenced by the Deepwater Horizon oil spill in the Gulf of Mexico in 2010 and the Fukushima nuclear plant incident in the Pacific Ocean in 2011. Accurate prediction of pollutant pathways and concentrations at the ocean surface requires understanding ocean dynamics over a broad range of spatial scales. Fundamental questions concerning the structure of the velocity field at the submesoscales (100 meters to tens of kilometers, hours to days) remain unresolved due to a l…
The spindown of a geostrophically balanced density front in an upper-ocean mixed layer is simulated with a large eddy simulation (LES) model that resolves O(1000) m down to O(1) m scale. Our goal is to examine the interaction between the submesoscale and the turbulent finescale, and better characterize vertical transport, frontogenesis and dissipative processes. The flow passes through symmetric and baroclinic instabilities, spawns vortex filaments of O(100) m thickness as well as larger eddies, and develops turbulence that is spatially localized and organized. A O(100) m physical-space filter…
We extracted ~2.8M nearly cloud-free 144x144 km^2 cutout images from the 2012-2020 Level-2 VIIRS Sea Surface Temperature (SST) dataset to quantitatively compare with MIT ocean general circulation model outputs, specifically the one year LCC4320 1/48 deg global-ocean simulation starting on November 17, 2011, matched in geography and day-of-year to VIIRS observations. We analyzed these cutouts using an unsupervised probabilistic autoencoder (PAE) to learn the SST structure on ~10-to-80 km scales (submesoscale-to-mesoscale). A key finding is that, in general, the LLC4320 simulation accurately rep…
Linking Horizontal and Vertical Submesoscale Lagrangian Dynamics South of the Ibiza Channel
Institute for Cross Disciplinary and Complex Systems (IFISC), CSIC-UIB（交叉学科与复杂系统研究所 (IFISC)，CSIC-UIB）；Mediterranean Institute for Advanced Studies (IMEDEA), CSIC-UIB（地中海高级研究所 (IMEDEA)，CSIC-UIB）；Instituto de Ciencias Marinas de Andalucia (ICMAN-CSIC)（安达卢西亚海洋科学研究所 (ICMAN-CSIC)）
Along-track wavenumber spectral densities of sea surface height (SSH) are estimated from Jason-2 altimetry data as a function of spatial location and calendar month, to understand the seasonality of meso- and submesoscale balanced dynamics across the global ocean. Regions with significant mode-1 and mode-2 baroclinic tides are rejected, restricting the analysis to the extratropics. Where balanced motion dominates, the SSH spectral density is averaged over all pass segments in a region for each calendar month, and is fit to a 4-parameter model consisting of a flat plateau at low wavenumbers, a …
Generative data assimilation highlights fronts as key regulators of ocean energy cascade
University of Washington（华盛顿大学）；California Institute of Technology（加州理工学院）；ENS, PSL Université, Ecole Polytechnique, Sorbonne Université, CNRS（法国高等师范学院、巴黎文理研究大学、巴黎综合理工学院、索邦大学、法国国家科学研究中心）
Near the ocean surface, river plumes influence stratification, buoyancy and transport of tracers, nutrients and pollutants. The extent to which river plumes influence the overall circulation, however, is generally poorly constrained. This work focuses on the South China Sea (SCS) and quantifies the dynamical impacts of the Mekong River plume, which is bound to significantly change in strength and seasonality in the next 20 years if the construction of over hundred dams moves ahead as planned. The dynamic impact of the freshwater fluxes on the SCS circulation are quantified by comparing submeso…
Ocean flows are routinely inferred from low-resolution satellite altimetry measurements of sea surface height assuming a geostrophic balance. Recent nonlinear dynamical systems techniques have revealed that surface currents derived from altimetry can support mesoscale eddies with material boundaries that do not filament for many months, thereby representing effective transport mechanisms. However, the long-range Lagrangian coherence assessed for mesoscale eddy boundaries detected from altimetry is constrained by the impossibility of current altimeters to resolve ageostrophic submesoscale motio…
Persistent Lagrangian transport patterns at the ocean surface are revealed from Lagrangian Coherent Structures (LCSs) computed from daily climatological surface current velocities in the northwestern Gulf of Mexico (NWGoM). The velocities are produced by a submesoscale permitting regional ocean model of the Gulf of Mexico. The significance of the climatological LCSs (cLCSs) is supported with ensemble-mean drifter density evolutions from simulated and historical satellite-tracked drifter trajectories. A persistent attracting barrier between the NWGoM shelf and the deep ocean is effectively iden…
In the framework of Monitoring by Ocean Drifters (MONDO) Project, a set of Lagrangian drifters were released in proximity of the Brazil Current, the western branch of the Subtropical Gyre in the South Atlantic Ocean. The experimental strategy of deploying part of the buoys in clusters offers the opportunity to examine relative dispersion on a wide range of scales. Adopting a dynamical systems approach, we focus our attention on scale-dependent indicators, like the finite-scale Lyapunov exponent (FSLE) and the finite-scale (mean square) relative velocity (FSRV) between two drifters as function …
Vertical transport in the ocean plays a critical role in the exchange of freshwater, heat, nutrients, and other biogeochemical tracers. While there are situations where vertical fluxes are important, studying the vertical transport and displacement of material requires analysis over a finite interval of time. One such example is the subduction of fluid from the mixed layer into the pycnocline, which is known to occur near density fronts. Divergence has been used to estimate vertical velocities indicating that surface measurements, where observational data is most widely available, can be used …
Sea surface height anomalies observed by satellites in 1993--2012 are combined with simulation and observations by surface drifters and Argo floats to study water flow pattern in the Near Strait (NS) connected the Pacific Ocean with the Bering Sea. Daily Lagrangian latitudinal maps, computed with the AVISO surface velocity field, and calculation of the transport across the strait show that the flow through the NS is highly variable and controlled by mesoscale and submesoscale eddies in the area. On the seasonal scale, the flux through the western part of the NR is negatively correlated with th…
The Southern Ocean plays an integral role in the global climate system, exchanging heat, salt, and carbon throughout the major ocean basins via the deep, fast-flowing Antarctic Circumpolar Current. The Antarctic Circumpolar Current is bounded by spatially and temporally varying fronts that partition distinct water masses. Locating and quantifying cross-front transport is crucial for understanding global patterns of inter-basin exchange; however, this is challenging because fronts are typically defined by hydrographic properties, such as temperature or salinity, which are subject to external so…
The frontal structure of the Southern Ocean is investigated using a sophisticated frontal detection methodology, the Wavelet/Higher Order Statistics Enhancement (WHOSE) method, introduced in \cite{Chapman2014}. This methodology is applied to 21 years of daily gridded sea-surface height (SSH) data to obtain daily maps of the locations of the fronts. By forming `heat-maps' of the frontal occurrence frequency and then approximating these heat-maps by a superposition of simple functions, the time-mean locations of the fronts, as well as a measure of their capacity to meander, are obtained and rela…
Lagrangian fronts (LF) in the ocean delineate boundaries between surface waters with different Lagrangian properties. They can be accurately detected in a given velocity field by computing synoptic maps of the drift of synthetic tracers and other Lagrangian indicators. Using Russian ship's catch and location data for a number of commercial fishery seasons in the region of the northwest Pacific with one of the richest fishery in the world, it is shown statistically that the saury fishing grounds with maximal catches are not randomly distributed over the region but located mainly along those LFs…
We study three dimensional oceanic Lagrangian Coherent Structures (LCSs) in the Benguela region, as obtained from an output of the ROMS model. To do that we first compute Finite-Size Lyapunov exponent (FSLE) fields in the region volume, characterizing mesoscale stirring and mixing. Average FSLE values show a general decreasing trend with depth, but there is a local maximum at about 100 m depth. LCSs are extracted as ridges of the calculated FSLE fields. They present a "curtain-like" geometry in which the strongest attracting and repelling structures appear as quasivertical surfaces. LCSs aroun…
The Earth system is accumulating energy due to human-induced activities. More than 90 percent of this energy has been stored in the ocean as heat since 1970, with about 64 percent of that in the upper 700 m. Differences in upper ocean heat content anomaly (OHCA) estimates, however, exist. Here, we evaluate spread in upper OHCA estimates arising from choices in instrumental bias corrections and mapping methods, in addition to the effect of using a common ocean mask. The same dataset was mapped by six research groups for 1970 to 2008, with six instrumental bias corrections applied to expendable …
We present the results of in-situ measurements of $^{134}$Cs and $^{137}$Cs released from the Fukushima Nuclear Power Plant (FNPP) collected at surface and different depths in the western North Pacific in June and July 2012. It was found that 15 month after the incident concentrations of radiocesium in the Japan and Okhotsk seas were at background or slightly increased level, while they had increased values in the subarctic front area east of Japan. The highest concentrations of $^{134}$Cs and $^{137}$Cs up to 13.5 ${\pm}$ 0.9 and 22.7 ${\pm}$ 1.5 Bq m$^{-3}$ have been found to exceed ten time…
Ocean wave energy is a new renewable energy resource which is going to become one of the reliable and alternative resources for fossil fuels during recent decades. The majority of studies have focused on extract wave energy at an effective rate; whilst, there are a few studies to explore the hydrodynamic of wave and surge wave energy converters. In this study a 2D numerical model based on RANS equations is closured with SST turbulence model employed to simulate the hydrodynamic of the flap type wave energy devices. The results indicate that a partially standing wave induced by the interaction …
The estimate of individual wave run-up is especially important for tsunami warning and risk assessment as it allows to evaluate the inundation area. Here as a model of tsunami we use the long single wave of positive polarity. The period of such wave is rather long which makes it different from the famous Korteweg-de Vries soliton. This wave nonlinearly deforms during its propagation in the ocean, what results in a steep wave front formation. Situations, when waves approach the coast with a steep front are often observed during large tsunamis, e.g. 2004 Indian Ocean and 2011 15 Tohoku tsunamis.…
We present an idealized study of the baroclinic structure of a rotating, stratified throughflow across a finite amplitude ridge in the f-plane that is forced by a steady, uniform inflow and outflow of equal magnitude. The resulting equilibrated circulation is characterized by unstable, western-intensified boundary currents and flow along f/H contours atop the ridge near the crest that closes the forced circulation. We find that bottom Ekman dynamics localized to lateral boundary currents amplify the stratification expected by simple stretching/squashing: a strongly stratified bottom front (hig…
On October 14th, 2016, the station of Tofino (British Columbia, Canada) issued the first ever real-time tsunami alert triggered by a coastal High-Frequency Radar system, based on the identification of abnormal surface current patterns. The detection occurred in the absence of any reported seismic event but coincided with a strong atmospheric perturbation, which qualified the event as meteo-tsunami. We re-analyze this case in the light of a new radar signal processing method which was designed recently for inverting fast-varying sea surface currents from the complex voltage time series received…
The three dimensional structure, dynamics and dispersion characteristics of a simulated upwelling filament in the Iberian upwelling system are analyzed using Lagrangian tools. We used a realistic regional simulation of the western Iberian shelf which is concomitant with an in-situ oceanographic campaign that surveyed the area. We compute 3d fields of finite--size Lyapunov exponents (FSLE) from 3d velocity fields and extract the field's ridges to study the spatial distribution and temporal evolution of the Lagrangian Coherent Structures (LCSs) evolving around the filament. We find that the most…
