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Fifani, Gina, Alberto Baudena, Milad Fakhri, Georges Baaklini, Yannice Faugère, Rosemary Morrow, Laurent Mortier et Francesco d’Ovidio. « Drifting Speed of Lagrangian Fronts and Oil Spill Dispersal at the Ocean Surface ». Remote Sensing 13, no 22 (9 novembre 2021) : 4499. http://dx.doi.org/10.3390/rs13224499.
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Due to its dire impacts on marine life, public health, and socio-economic services, oil spills require an immediate response. Effective action starts with good knowledge of the ocean dynamics and circulation, from which Lagrangian methods derive key information on the dispersal pathways present in the contaminated region. However, precise assessments of the capacity of Lagrangian methods in real contamination cases remain rare and limited to large slicks spanning several hundreds of km. Here we address this knowledge gap and consider two medium-scale (tens of km wide) events of oil in contrasting conditions: an offshore case (East China Sea, 2018) and a recent near-coastal one (East Mediterranean, 2021). Our comparison between oil slicks and Lagrangian diagnostics derived from near-real-time velocity fields shows that the calculation of Lagrangian fronts is, in general, more robust to errors in the velocity fields and more informative on the dispersion pathways than the direct advection of a numerical tracer. The inclusion of the effect of wind is also found to be essential, being capable of suddenly breaking Lagrangian transport barriers. Finally, we show that a usually neglected Lagrangian quantity, the Lyapunov vector, can be exploited to predict the front drifting speed, and in turn, its future location over a few days, on the basis of near-real-time information alone. These results may be of special relevance in the context of next-generation altimetry missions that are expected to provide highly resolved and precise near-real-time velocity fields for both open ocean and coastal regions.
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Heron, Mal, Roberto Gomez, Bernd Weber, Anna Dzvonkovskaya, Thomas Helzel, Nicolas Thomas et Lucy Wyatt. « Application of HF Radar in Hazard Management ». International Journal of Antennas and Propagation 2016 (2016) : 1–14. http://dx.doi.org/10.1155/2016/4725407.
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A review is given of the impact that HF radars are having on the management of coastal hazards. Maps of surface currents can be produced every 10–20 minutes which, in real time, improve navigation safety in restricted areas commonly found near ports and harbours. The time sequence of surface current maps enables Lagrangian tracking of small parcels of surface water, which enables hazard mitigation in managing suspended sediments in dredging, in emergency situations where flotsam and other drifting items need to be found, and in pollution control. The surface current measurement capability is used to assist tsunami warnings as shown by the phased-array data from Chile following the Great Tohoku Earthquake in 2011. The newly launched Tsunami Warning Center in Oman includes a network of phased-array HF radars to provide real-time tsunami monitoring. Wind direction maps can be used to locate the position of cold fronts in the open ocean and to monitor the timing and strength of sea-breeze fronts in key locations.
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Nemoto, Masaki, Kouichi Nishimura, Syunichi Kobayashi et Kaoru Izumi. « Numerical study of the time development of drifting snow and its relation to the spatial development ». Annals of Glaciology 38 (2004) : 343–50. http://dx.doi.org/10.3189/172756404781815202.
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AbstractThe time evolution of drifting snow under a steady wind is estimated using a new numerical model of drifting snow. In the model, Lagrangian stochastic theory is used to incorporate the effect of turbulence on the motion of drifting-snow particles. This method enables us to discuss both the saltation and the suspension process. Aerodynamic entrainment, grain/bed collision (splash process), wind modification and particle size distribution are also taken into account. The calculations show that the time needed by the total mass flux to reach a steady state appears to be 3–5 s. Vertical profiles of horizontal mass flux near the surface show a similar tendency. In contrast, it takes >50 s for the wind speed and the whole mass-flux profile to reach a steady state. This longer time depends on the time-scale of the turbulent diffusion, which is responsible for the mass flux extending to an order of a few meters in height. Applying Taylor’s hypothesis, the estimated length scale at which drifting snow reaches equilibrium is around 400 m. This result is comparable with previously reported field observations.
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Lee, Daniel, Amandine Schaeffer et Sjoerd Groeskamp. « Drifting dynamics of the bluebottle (<i>Physalia physalis</i>) ». Ocean Science 17, no 5 (1 octobre 2021) : 1341–51. http://dx.doi.org/10.5194/os-17-1341-2021.
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Abstract. Physalia physalis, also called the bluebottle in Australia, is a colonial animal resembling a jellyfish that is well known to beachgoers for the painful stings delivered by its tentacles. Despite being a common occurrence, the origin of the bluebottle before reaching the coastline is not well understood, and neither is the way it drifts at the surface of the ocean. Previous studies used numerical models in combination with simple assumptions to calculate the drift of this species, excluding complex drifting dynamics. In this study, we provide a new parameterization for Lagrangian modelling of the bluebottle by considering the similarities between the bluebottle and a sailboat. This allows us to compute the hydrodynamic and aerodynamic forces acting on the bluebottle and use an equilibrium condition to create a generalized model for calculating the drifting speed and course of the bluebottle under any wind and ocean current conditions. The generalized model shows that the velocity of the bluebottle is a linear combination of the ocean current velocity and the wind velocity scaled by a coefficient (“shape parameter”) and multiplied by a rotation matrix. Adding assumptions to this generalized model allows us to retrieve models used in previous literature. We discuss the sensitivity of the model to different parameters (shape, angle of attack and sail camber) and explore different cases of wind and current conditions to provide new insights into the drifting dynamics of the bluebottle.
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Poulain, Pierre-Marie, Luca Centurioni et Tamay Özgökmen. « Comparing the Currents Measured by CARTHE, CODE and SVP Drifters as a Function of Wind and Wave Conditions in the Southwestern Mediterranean Sea ». Sensors 22, no 1 (4 janvier 2022) : 353. http://dx.doi.org/10.3390/s22010353.
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Instruments drifting at the ocean surface are quasi-Lagrangian, that is, they do not follow exactly the near-surface ocean currents. The currents measured by three commonly-used drifters (CARTHE, CODE and SVP) are compared in a wide range of sea state conditions (winds up to 17 m/s and significant wave height up to 3 m). Nearly collocated and simultaneous drifter measurements in the southwestern Mediterranean reveal that the CARTHE and CODE drifters measure the currents in the first meter below the surface in approximately the same way. When compared to SVP drogued at 15 m nominal depth, the CODE and CARTHE currents are essentially downwind (and down-wave), with a typical speed of 0.5–1% of the wind speed. However, there is a large scatter in velocity differences between CODE/CARTHE and SVP for all wind and sea state conditions encountered, principally due to vertical and horizontal shears not related to the wind. For the CODE drifter with wind speed larger than 10 m/s and significant wave height larger than 1 m, about 30–40% of this difference can be explained by Stokes drift.
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Ochoa, José, et Peter P. Niiler. « Vertical Vorticity Balance in Meanders Downstream the Agulhas Retroflection ». Journal of Physical Oceanography 37, no 6 (1 juin 2007) : 1740–44. http://dx.doi.org/10.1175/jpo3064.1.
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Abstract The Agulhas Current flows poleward along the western boundary of the southeastern Indian Ocean where, at the southernmost latitude of the African continent, it executes a dramatic anticyclonic turn, or retroflection, to the east. Since 1978, a large number of drifting buoys have passed through this eastward-flowing Agulhas Return Current (ARC), or the zonal frontal boundary between subtropical and subpolar waters of the south Indian Ocean. The spatial distribution of the ensemble-averaged near-surface velocity along the ARC axis reveals a series of steady-state meanders of 700-km wavelength and amplitudes that decrease from 170 km in the first meander to 50 km in the following four meanders. Here an analysis of vorticity balance of the meandering ARC speed axis is presented that demonstrates a balance between the β term and advection of curvature vorticity. This balance implies that the ARC axis, or frontal region, is horizontally nondivergent in agreement with the other observations of flow in the surface layers of near-zonal oceanic fronts.
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Wang, Tianyu, Yan Du et Minyang Wang. « Overlooked Current Estimation Biases Arising from the Lagrangian Argo Trajectory Derivation Method ». Journal of Physical Oceanography 52, no 1 (janvier 2022) : 3–19. http://dx.doi.org/10.1175/jpo-d-20-0287.1.
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Abstract An Argo simulation system is used to provide synthetic Lagrangian trajectories based on the Estimating the Circulation and Climate of the Ocean Model, phase II (ECCO2). In combination with ambient Eulerian velocity at the reference layer (1000 m) from the model, quantitative metrics of the Lagrangian trajectory–derived velocities are computed. The result indicates that the biases induced by the derivation algorithm are strongly linked with ocean dynamics. In low latitudes, Ekman currents and vertically sheared geostrophic currents influence both the magnitude and the direction of the derivation velocity vectors. The maximal shear-induced biases exist near the equator with the amplitudes reaching up to about 1.2 cm s−1. The angles of the shear biases are pronounced in the low-latitude oceans, ranging from −8° to 8°. Specifically, the study shows an overlooked bias from the float drifting motions that mainly occurs in the western boundary current and Antarctic Circumpolar Current (ACC) regions. In these regions, a recently reported horizontal acceleration measured via Lagrangian floats is significantly associated with the strong eddy–jet interactions. The acceleration could induce an overestimation of Eulerian current velocity magnitudes. For the common Argo floats with a 9-day float parking period, the derivation speed biases induced by velocity acceleration would be as large as 3 cm s−1, approximately 12% of the ambient velocity. It might have implications to map the mean middepth ocean currents from Argo trajectories, as well as to understand the dynamics of eddy–jet interactions in the ocean.
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Romero, Leonel, J. Carter Ohlmann, Enric Pallàs-Sanz, Nicholas M. Statom, Paula Pérez-Brunius et Stéphane Maritorena. « Coincident Observations of Dye and Drifter Relative Dispersion over the Inner Shelf ». Journal of Physical Oceanography 49, no 9 (septembre 2019) : 2447–68. http://dx.doi.org/10.1175/jpo-d-19-0056.1.
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AbstractCoincident Lagrangian observations of coastal circulation with surface drifters and dye tracer were collected to better understand small-scale physical processes controlling transport and dispersion over the inner shelf in the Gulf of Mexico. Patches of rhodamine dye and clusters of surface drifters at scales of O(100) m were deployed in a cross-shelf array within 12 km from the coast and tracked for up to 5 h with airborne and in situ observations. The airborne remote sensing system includes a hyperspectral sensor to track the evolution of dye patches and a lidar to measure directional wavenumber spectra of surface waves. Supporting in situ measurements include a CTD with a fluorometer to inform on the stratification and vertical extent of the dye and a real-time towed fluorometer for calibration of the dye concentration from hyperspectral imagery. Experiments were conducted over a wide range of conditions with surface wind speed between 3 and 10 m s−1 and varying sea states. Cross-shelf density gradients due to freshwater runoff resulted in active submesoscale flows. The airborne data allow characterization of the dominant physical processes controlling the dispersion of passive tracers such as freshwater fronts and Langmuir circulation. Langmuir circulation was identified in dye concentration maps on most sampling days except when the near surface stratification was strong. The observed relative dispersion is anisotropic with eddy diffusivities O(1) m2 s−1. Near-surface horizontal dispersion is largest along fronts and in conditions dominated by Langmuir circulation is larger in the crosswind direction. Surface convergence at fronts resulted in strong vertical velocities of up to −66 m day−1.
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Pingree, R. D., et Linda Maddock. « Stokes, Euler and Lagrange Aspects of Residual Tidal Transports in the English Channel and the Southern Bight of the North Sea ». Journal of the Marine Biological Association of the United Kingdom 65, no 4 (novembre 1985) : 969–82. http://dx.doi.org/10.1017/s0025315400019445.
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INTRODUCTIONThe dust cart on the Isles of Scilly did not end its service at the local tip, over the cliffs at Deep Point on the eastern side of St Mary's on 14 October 1983. Instead it floated off and continued in service as a Lagrangian drifter, indicating the residual transport of water around the islands. As a hazard to shipping its position was given out as a navigation warning and it was allegedly sighted on the western side of Scilly. The interesting oceanographic question is whether the dust cart was (or would have had it not sunk) circumnavigating the islands in a clockwise or anticlockwise sense. Accordingly this experiment has been repeated using satellite tracked Argos drifting buoys, drogued with parachutes (~ 10 m diameter), and set at depths of about 15 m. The releases showed that the residual transport was clockwise around the islands. In one instance a drogue circumnavigated the islands in about 12 h (i.e. ~ one tidal period) at an average speed of about 100 cm s−1 (Fig. 1).
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Dhareshwar, L. J., P. A. Naik, T. C. Kaushik et H. C. Pant. « Study of laser-driven shock wave propagation in Plexiglas targets ». Laser and Particle Beams 10, no 1 (mars 1992) : 201–11. http://dx.doi.org/10.1017/s0263034600004328.
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An experimental study of laser-driven shock wave propagation in a transparent material such as Plexiglas using a high-speed optical shadowgraphy technique is presented in this paper. A Nd:glass laser was used to produce laser intensity in the range 1012-1014 W/cm2 on the target. Optical shadowgrams of the propagating shock front were recorded with a second-harmonic (0.53-μm) optical probe beam. Shock pressures were measured at various laser intensities, and the scaling was found to agree with the theoretically predicted value. Shock pressure values have also been obtained from a one-dimensional Lagrangian hydrodynamic simulation, and they match well with experimental results. Shadowgrams of shock fronts produced by nonuniform spatial laser beam irradiation profiles have shown complete smoothing when targets with a thin coating of a material of high atomic number such as gold were used. Shock pressures in such coated targets are also found to be considerably higher compared with those in uncoated targets.
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Duan, Lian, Meelan M. Choudhari et Chao Zhang. « Pressure fluctuations induced by a hypersonic turbulent boundary layer ». Journal of Fluid Mechanics 804 (13 septembre 2016) : 578–607. http://dx.doi.org/10.1017/jfm.2016.548.
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Direct numerical simulations (DNS) are used to examine the pressure fluctuations generated by a spatially developed Mach 5.86 turbulent boundary layer. The unsteady pressure field is analysed at multiple wall-normal locations, including those at the wall, within the boundary layer (including inner layer, the log layer, and the outer layer), and in the free stream. The statistical and structural variations of pressure fluctuations as a function of wall-normal distance are highlighted. Computational predictions for mean-velocity profiles and surface pressure spectrum are in good agreement with experimental measurements, providing a first ever comparison of this type at hypersonic Mach numbers. The simulation shows that the dominant frequency of boundary-layer-induced pressure fluctuations shifts to lower frequencies as the location of interest moves away from the wall. The pressure wave propagates with a speed nearly equal to the local mean velocity within the boundary layer (except in the immediate vicinity of the wall) while the propagation speed deviates from Taylor’s hypothesis in the free stream. Compared with the surface pressure fluctuations, which are primarily vortical, the acoustic pressure fluctuations in the free stream exhibit a significantly lower dominant frequency, a greater spatial extent, and a smaller bulk propagation speed. The free-stream pressure structures are found to have similar Lagrangian time and spatial scales as the acoustic sources near the wall. As the Mach number increases, the free-stream acoustic fluctuations exhibit increased radiation intensity, enhanced energy content at high frequencies, shallower orientation of wave fronts with respect to the flow direction, and larger propagation velocity.
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Robinson, T. O., I. Eames et R. Simons. « Dense gravity currents moving beneath progressive free-surface water waves ». Journal of Fluid Mechanics 725 (23 mai 2013) : 588–610. http://dx.doi.org/10.1017/jfm.2013.112.
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AbstractThe characteristics of dense gravity currents in coastal regions, where free-surface gravity waves are dominant, have yet to be studied in the laboratory. This paper provides a first insight into the dynamics of dense saline gravity currents moving beneath regular progressive free-surface water waves. The gravity currents were generated by releasing a finite volume of saline into a large wave tank with an established periodic wave field. After the initial collapse, the gravity currents propagated horizontally with two fronts, one propagating in the wave direction and the other against the wave direction. The fronts of the gravity currents oscillated with an amplitude and phase that correlated with the orbital velocities within a region close to the bed. To leading order, the overall length of the gravity current was found to be weakly affected by the wave action and the dynamics of the current could be approximated by simply considering the buoyancy of the released fluid. Other characteristics such as the position of the gravity current centre and the shape of the two leading profiles were found to be significantly affected by the wave action. The centre was displaced at constant speed dependent on the second-order wave-induced mean Lagrangian velocity. For long waves, the centre was advected downstream in the direction of wave propagation owing to the dominance of Stokes drift. For short waves, the gravity current centre moved upstream against the wave direction, as under these wave conditions Stokes drift is negligible at the bed. An asymmetry in the shape of the upstream and downstream current heads was observed, with the gravity current front moving against the waves being much thicker and the front steeper, similar to the case of a current moving in a stream.
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Lund, Björn, Brian K. Haus, Jochen Horstmann, Hans C. Graber, Ruben Carrasco, Nathan J. M. Laxague, Guillaume Novelli, Cédric M. Guigand et Tamay M. Özgökmen. « Near-Surface Current Mapping by Shipboard Marine X-Band Radar : A Validation ». Journal of Atmospheric and Oceanic Technology 35, no 5 (mai 2018) : 1077–90. http://dx.doi.org/10.1175/jtech-d-17-0154.1.
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AbstractThe Lagrangian Submesoscale Experiment (LASER) involved the deployment of ~1000 biodegradable GPS-tracked Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) drifters to measure submesoscale upper-ocean currents and their potential impact on oil spills. The experiment was conducted from January to February 2016 in the Gulf of Mexico (GoM) near the mouth of the Mississippi River, an area characterized by strong submesoscale currents. A Helmholtz-Zentrum Geesthacht (HZG) marine X-band radar (MR) on board the R/V F. G. Walton Smith was used to locate fronts and eddies by their sea surface roughness signatures. The MR data were further processed to yield near-surface current maps at ~500-m resolution up to a maximum range of ~3 km. This study employs the drifter measurements to perform the first comprehensive validation of MR near-surface current maps. For a total of 4130 MR–drifter pairs, the root-mean-square error for the current speed is 4 cm and that for the current direction is 12°. The MR samples currents at a greater effective depth than the CARTHE drifters (1–5 m vs ~0.4 m). The mean MR–drifter differences are consistent with a wave- and wind-driven vertical current profile that weakens with increasing depth and rotates clockwise from the wind direction (by 0.7% of the wind speed and 15°). The technique presented here has great potential in observational oceanography, as it allows research vessels to map the horizontal flow structure, complementing the vertical profiles measured by ADCP.
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Jakobson, L., T. Vihma, E. Jakobson, T. Palo, A. Männik et J. Jaagus. « Low-level jet characteristics over the Arctic Ocean in spring and summer ». Atmospheric Chemistry and Physics Discussions 13, no 1 (22 janvier 2013) : 2125–53. http://dx.doi.org/10.5194/acpd-13-2125-2013.
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Abstract. Low-level jets (LLJ) are important for turbulence in the stably stratified atmospheric boundary layer, but their occurrence, properties, and generation mechanisms in the Arctic are not well known. We analysed LLJs over the central Arctic Ocean in spring and summer 2007 on the bases of data collected in the drifting ice station Tara. Instead of traditional radiosonde soundings, data from tethersonde soundings with a high vertical resolution were used. The Tara results showed a lower occurrence of LLJs (46%) than many previous studies over polar sea ice. Strong jet core winds contributed to growth of the turbulent layer. Complex relationship between the jet core height and the temperature inversion top height were detected: substantial correlation (r = 0.72; p < 0.01) occurred when the jet core was above the turbulent layer, but inside the turbulent layer there was no correlation. The most important forcing mechanism for LLJs was baroclinicity, which was responsible for generation of strong and warm LLJs, which on average occurred at lower altitudes than other jets. Baroclinic jets were mostly associated to transient cyclones instead of the climatological air temperature gradients. Besides baroclinicity, cases related to inertial oscillations, gusts, and fronts were detected. In approximately 50% of the observed LLJs the generation mechanism remained unclear, but in most of these cases the wind speed was strong in the whole vertical profile, the jet core representing only a weak maximum. Further research needs on LLJs in the Arctic include investigation of low-level jet streams and their effects on the sea ice drift and atmospheric moisture transport.
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Novelli, Guillaume, Cédric M. Guigand, Charles Cousin, Edward H. Ryan, Nathan J. M. Laxague, Hanjing Dai, Brian K. Haus et Tamay M. Özgökmen. « A Biodegradable Surface Drifter for Ocean Sampling on a Massive Scale ». Journal of Atmospheric and Oceanic Technology 34, no 11 (novembre 2017) : 2509–32. http://dx.doi.org/10.1175/jtech-d-17-0055.1.
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AbstractTargeted observations of submesoscale currents are necessary to improve science’s understanding of oceanic mixing, but these dynamics occur at spatiotemporal scales that are currently challenging to detect. Prior studies have recently shown that the submesoscale surface velocity field can be measured by tracking hundreds of surface drifters released in tight arrays. This strategy requires drifter positioning to be accurate, frequent, and to last for several weeks. However, because of the large numbers involved, drifters must be low-cost, compact, easy to handle, and also made of materials harmless to the environment. Therefore, the novel Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) drifter was designed following these criteria to facilitate massive sampling of near-surface currents during the Lagrangian Submesoscale Experiment (LASER). The drifting characteristics were determined under a wide range of currents, waves, and wind conditions in laboratory settings. Results showed that the drifter accurately follows the currents in the upper 0.60 m, that it presents minimal wave rectification issues, and that its wind-induced slip velocity is less than 0.5% of the neutral wind speed at 10 m. In experiments conducted in both coastal and deep ocean conditions under wind speeds up to 10 m s−1, the trajectories of the traditional Coastal Ocean Dynamics Experiment (CODE) and the CARTHE drifters were nearly identical. Following these tests, 1100 units were produced and deployed during the LASER campaign, successfully tracking submesoscale and mesoscale features in the Gulf of Mexico. It is hoped that this drifter will enable high-density sampling near metropolitan areas subject to stress by the overpopulation, such as lakes, rivers, estuaries, and environmentally sensitive areas, such as the Arctic.