Downwelling Currents Research Articles

Effects of currents and waves on the morphologies of coastal sandy clinoforms: sediment mobility calculations based on current meter and wave data from Southern California, U.S.A.

ABSTRACT In some wave-exposed coasts, sandy clinoforms occur with rollovers (locations where their surfaces steepen) at ∼ 20–60 m depth. They have been suggested to have formed from sand mobilized by strong wave agitation in shallow water that has deposited in more tranquil deeper water beyond the rollover, although other suggested origins of clinoforms might also apply. In situ information on active sediment transport is needed to address their origins. Here, we assess sediment transport across a sandy clinoform rollover at 25–30 m depth using legacy data from current meters installed across the Southern California shelf near Del Mar in depths of 15, 30, and 60 m. Although lasting only 25 hours, the data captured conditions during the passage of a cyclone, which occurs frequently along this coast. Information from a global meteorological model reveals that waves were 1–2 m in height during the cyclone passage. Using the mean particle size from vibracores (0.129 mm), sand at the 15-m-depth site would have been continuously agitated above its threshold of motion during the 25-hour period, whereas sand at the 30-m site was mobile over 17 hours. Total-load fluxes of sand estimated from the waves and currents were strong at the 15-m site and comprised mostly suspended particles. The cyclone winds drove water, including bottom water, mostly parallel to the coast, but that movement was accompanied by a downwelling component over the first ∼ 10 hours. That downwelling likely moved suspended particles from the topset of the clinoform onto the foreset. A later upwelling component may have partially reversed that particle movement but was unlikely to have been so effective. The results suggest how wave effects, coupled with downwelling currents, dominated the sand transport and deposition over this short 25-hour period, contributing to the clinoform morphology.

Initial understanding and assessment of role of oceanographic features for ferromanganese crusts and nodules in the East Vietnam Sea

The iron and manganese content in marine water is very small but the volume of ferromanganese nodules contributes 30% of the total mass of polymetallic nodules and crusts in marine and ocean floor. This suggests that the process of enrichment of ferromanganese crusts and nodules is not only contributed by chemical processes but also by oceanographical and biological processes. The article indicates the initial results of analyzing oceanographic, biological, and environmental features to understand their roles in the growing ferromanganese crusts and nodules and to predict the distribution of ferromanganese crusts and nodules in the East Vietnam Sea. As a result, ferromanganese crusts and nodules in the East Vietnam Sea can be distributed in the continental slopes, where upwelling and downwelling currents occur, to ensure enough dissolved oxygen concentration for the enrichment of ferromanganese crusts and nodules as well as to meet required conditions for microbial activity, which is involved in these processes. However, due to the limitations of the results of studying the enrichment of ferromanganese crusts and nodules in the East Vietnam Sea, the paper just shows the prediction of the distribution of ferromanganese crusts and nodules. Thus, it is necessary to carry out the expedition for enrichment processes of ferromanganese crusts and nodules and to determine the factors that impacted the growing ferromanganese crusts and nodules in the East Vietnam Sea.

Storm‐flood‐dominated delta: A new type of delta in stormy oceans

AbstractStorms are increasingly identified as a key depositional process. Their deposits show unique sedimentary characteristics, distinct from deposits of fair‐weather wave processes, but are not currently separated in classic ternary deltaic classification schemes. The term ‘storm‐flood‐dominated delta’ is proposed for a new type of delta that more fully represents deltas dominated by storm‐flood processes. This study describes several outcrop cliffs of the Gallup Formation, deposited during the Late Turonian to Early Coniacian in north‐west New Mexico, that exemplify storm‐generated facies and provide examples that can be used to generate a facies model for storm‐flood‐dominated deltas. Key identification criteria include extensive sharp‐based planar to hummocky cross‐stratified sandstone beds, commonly presenting as large‐sized gutter casts. They are interbedded with mudstones that show low bioturbation intensity and normally and inversely graded beds, suggesting direct deposition from river plumes. Major types of gutter casts are classified based on geometry and dimension. The gutter casts are interpreted as storm channels of storm‐flood‐dominated deltas and formed by erosion and infill of submerged channels resulting from offshore‐oriented downwelling currents that may include localized rip currents. These amalgamated channels are likely linked to multiple feeding rivers and distributary channels that ensured high sediment supply rates and highly efficient offshore‐directed sediment transport through hyperpycnal flows. A four‐component pyramidal classification scheme of deltaic deposition is employed to emphasize storms as a distinct process in contrast to the traditional classification that lumps storm and fair‐weather processes in a single ‘wave’ end‐member. The four‐component depositional model can be readily applied to interpret storm‐dominated environments and provide new insights into depositional processes of the marine realm. Recognition of storm processes in deciphering depositional evolution will also help to better understand control mechanisms of sequence stratigraphy, such as relative sea‐level changes, and to analyze sedimentary processes governing sediment transport in ‘source to sink’ systems.

Seismic evidence for a chemically distinct thermochemical reservoir in Earth's deep mantle beneath Hawaii

Nearly antipodal continent-sized zones of reduced seismic shear wave velocities exist at the base of Earth's mantle, one beneath the Pacific Ocean, the other beneath the South Atlantic Ocean and Africa. Geophysicists have attributed the low velocity zones to elevated temperatures associated with large-scale mantle convection processes, specifically, hot mantle upwelling in response to cooler subduction-related downwelling currents. Hypotheses have included superplumes, isochemical heterogeneity, and stable as well as metastable basal thermochemical piles. Here we analyze waveform broadening and travel times of S waves from 11 deep focus earthquakes in the southwest Pacific recorded in North America, resulting in 8500 seismograms studied that sample the deep mantle beneath the Pacific. Waveform broadening is referenced to a mean S-wave shape constructed for each event, to define a relative “misfit”. Large misfits are consistent with multipathing that can broaden wave pulses. Misfits of deep mantle sampling S-waves infer that the structure in the northeast part of the low velocity province beneath the Pacific has a sharp side as well as a sloping sharp top to the feature. This sharp boundary morphology is consistent with geodynamic predictions for a stable thermochemical reservoir. The peak of the imaged pile is below Hawaii, supporting the hypothesis of a whole mantle plume beneath the hotspot.

The red coral populations of the gulfs of Naples and Salerno: human impact and deep mass mortalities

The existence of deep red coral (Corallium rubrum) banks in the gulfs of Naples and Salerno (South Tyrrhenian Sea, Mediterranean Sea) is well known in historical records due to the heavy coral harvesting that occurred during the 19th and 20th centuries, by both trawling gears and scuba diving. In 2010 and 2012, during two Remotely Operated Vehicle (ROV) surveys on board of the Research Vessel (R/V) Astrea, red coral banks were detected in 16 of the 25 visited localities, between 45 and 150 m depth. Seven of these banks, located in the inner part of the Gulf of Naples, were already explored in 1918 by a scientific survey reporting the occurrence of red coral. Healthy populations (densities > 90 colonies m−2) were present only around the coasts of the Phlegrean Islands (Ischia and Procida Islands). Very low densities (< 5 colonies m−2) or the absence of coral were recorded in all other sites of the Gulf of Naples (including all historical re-visited banks), and a variable percentage of dead colonies was observed. This evidence suggests a huge state of stress likely favoured by the hydrodynamic conditions in the Gulf, enhancing water pollution and sedimentation rate. Finally, the documented high fishing pressure plays a major role in the hard-bottom communities’ degradation. A recent mass mortality episode was also recorded along the Amalfi coast, around Li Galli Islands (Gulf of Salerno), at a depth range between 80 and 100 m, where the mortality affected 80% of the largest colonies, estimated to be around 70 years old. Several possible reasons for this mortality have been hypothesised, such as the formation of local down-welling currents inducing an unusual drop of the thermocline, or sudden warm water emissions (sulphur springs) in an area characterised by important volcanic activities, or local landslides generating turbidity currents along the steep slopes.

3-D process modelling of ancient storm-dominated deposits by an event-based approach: Application to Pleistocene-to-modern Gulf of Lions deposits

Abstract A new numerical model was developed for 3-D characterization of deposits generated by storms along clastic coasts away from major rivers. It is based on the assumption that the preserved sedimentary heterogeneities result mainly from a succession of deposits generated during both fair-weather periods and major events (storms in our case). Hydro-sedimentary processes are first modelled during these major storms: (i) transport parallel and transverse to the coast of suspended sediment by the strongest unidirectional currents (littoral drift and downwelling currents), (ii) sea-bed erosion due to shear stress generated by unidirectional and wave-induced oscillatory currents, (iii) deposition of sediment. Fair-weather deposits are modelled by an average deposition process. The results of the numerical model, i.e. the description of the sedimentary deposits, are multi-lithologic, in three dimensions and with a flexible vertical resolution. The latter is one of the main strengths of the model because the vertical description of the deposits can be adapted to the duration and extent of the simulation in order to save computation time. Very precise results (definition of less than 1 mm) can be obtained at a small time-scale (for storms lasting several hours to several days), and reservoir-scale results at a longer time scale. The numerical model was applied to the Gulf of Lions, which is located off the southern French coast, in the north-western Mediterranean. First, present-day storm deposits are simulated in order to validate the storm-process modelling. Then sedimentary deposits formed over several thousands of years, during the Pleistocene, are simulated in order to validate the sediment-deposit modelling. The model results fit well with observations and validate the approach of simulating the shelf deposits and their associated sedimentary heterogeneities at reservoir scale with an event- and process-based method. This innovative approach seems promising for providing future geological models with realistic petrophysical properties.

Effect of Langmuir cells on bubble dissolution and air‐sea gas exchange

Gases are exchanged between the atmosphere and ocean by diffusion through the sea surface and by dissolution of air bubbles injected by breaking wind waves. Langmuir cells enhance the contribution from bubbles by keeping them under water for longer thus increasing their dissolution. We determine the importance of Langmuir cells by using a bubble model to calculate the amount of gas that dissolves from bubbles as a function of wind speed, gas saturation, and injection depth and a Langmuir cell model to estimate the effect of the associated downwelling currents on bubble dissolution. The calculations are preformed for the eight gases N2, O2, CO2, He, Ne, Ar, Kr, and Xe, and the results are then compared with the total gas exchange determined by common descriptions of air‐sea gas exchange. The contribution of gas bubbles to air‐sea gas exchange increases with wind speed and can reach 77% for O2, 98% for N2, and 16% for CO2 at a wind speed of 20 m s−1 and a gas saturation of 95%. The additional effect of Langmuir cells on the total gas exchange at a gas saturation of 95% is 19% for O2, 35% for N2, and 0.7% for CO2. The importance of Langmuir cells for air‐sea gas exchange generally increases with gas saturation and decreases with the solubility of the gas.

On the Upwelling of Downwelling Currents

Abstract The term “downwelling currents” refers to currents with a downslope mass flux in the bottom boundary layer. Examples are the Malvinas and Southland Currents in the Southern Hemisphere and the Oyashio in the Northern Hemisphere. Although many of these currents generate the same type of highly productive ecosystems that is associated with upwelling regimes, the mechanism that may drive such upwelling remains unclear. In this article, it is postulated that the interaction between a downwelling current and the continental slope generates shelfbreak upwelling. The proposed mechanism is relatively simple. As a downwelling current flows along the continental slope, bottom friction and lateral diffusion spread it onto the neighboring shelf, thus generating along-shelf pressure gradients and a cross-shelf circulation pattern that leads to shelfbreak upwelling. At difference with previous studies of shelfbreak dynamics (e.g., Gawarkiewicz and Chapman, Chapman and Lentz, and Pickart), the shelfbreak upwelling in the proposed model is not controlled by the downslope buoyancy flux associated with the presence of a shelf current but by the along-shelf pressure gradient associated with the presence of a slope current. As these experiments demonstrate, shelfbreak upwelling will occur in flat-bottomed domains or even in the absence of a bottom boundary layer. The shelfbreak upwelling, moreover, is not evidence of the separation of the bottom boundary layer but of the downstream divergence of the slope currents, and its magnitude is proportional to the volume transport of that current. To prove this hypothesis, the results of a series of process-oriented numerical experiments are presented.

Possible effects of downwelling on the recruitment of coral reef fishes to the Eilat (Red Sea) coral reefs

We compared water current measurements with data on the recruitment of reef fishes to a coral reef in Eilat, Red Sea for two consecutive recruitment seasons. There was a clear correlation between the daily number of recruits and the magnitude of the downwelling flow, suggesting that an increase in offshore (near bottom) flow induces higher recruitment. The higher recruitment may be the result of larval swimming against the flow. Although the exact mechanism explaining this correlation is unclear, it may involve odor transport from the reef by the downwelling currents, which reaches competent larvae in deeper water and directs them to potential settlement sites.

Tidal fronts and their role in air-sea gas exchange

Tidal fronts are a common feature of many coastal environments. They are characterized by a surface convergence zone that enhances wave breaking and the generation of gas bubbles due to wave-current interaction. The associated downwelling currents carry bubbles to depths of up to 160 m and increase the amount of air that dissolves from them. An energetic tidal front is formed at the entrance to the Strait of Georgia, BC, Canada, by a hydraulically controlled sill flow with vertical velocities of up to 0.75 m s −1 . Extensive ship-board measurements during two cruises are interpreted with models of wave-current interaction and gas bubble behavior. The observations suggest that tidal fronts may contribute significantly to the aeration of the subsurface waters in the Fraser Estuary. This process may be also of importance for other coastal environments with plunging sill flows of dense water that deliver aerated surface water to intermediate depths.

Possible Sources Driving the Potential Vorticity Structure and Long-Wave Instability of Coastal Upwelling and Downwelling Currents

AbstractIt is well known that upwelling and downwelling currents are unstable to perturbations. Less is, however, known about the physical mechanism responsible for the observed and modeled instabilities. It is shown that the origin of the long-wave barotropic/baroclinic instability observed on upwelling currents has to be sought among diabatic or thermobaric mechanisms. In particular, the role of mixing associated with Kelvin–Helmholtz instability and of wind forcing is investigated. Low Richardson numbers occur when the pycnocline outcrops at the sea surface. The criterion for instability (Ri ≤ 1/4) can be reached in a narrow region close to the upwelling front, permitting Kelvin–Helmholtz instability and mixing. This can precondition the current for long-wave instability by transforming the current's potential vorticity. A constant wind can likewise modify the potential vorticity. The resulting potential vorticity anomaly is always negative for both upwelling and downwelling currents, and this anomaly interacts with the outcropped front, destabilizing it. Examples are provided via numerical calculations using an idealized front. A wind stress is an effective means of inducing the negative PV necessary for instability; with wind, Kelvin–Helmholz instability, when present, merely modifies the instability characteristics. In addition, upwelling fronts are always less stable than comparable downwelling fronts.

Geoengineering Downwelling Ocean Currents: A Cost Assessment

Downwelling ocean currents carry carbon into the deep ocean (the solubility pump), and play a role in controlling the level of atmospheric carbon. The formation of North Atlantic Deep Water (NADW) also releases heat to the atmosphere, which is a contributor to a mild climate in Europe. One possible response to the increase in anthropogenic carbon in the atmosphere and to the possible weakening of the NADW is modification of downwelling ocean currents, by an increase in carbon concentration or volume. This study assesses the costs of seven possible methods of modifying downwelling currents, including using existing industrial techniques for exchange of heat between water and air. Increasing carbon concentration in downwelling currents is not practical due to the high degree of saturation of high latitude surface water. Two of the methods for increasing the volume of downwelling currents were found to be impractical, and four were too expensive to warrant further consideration. Formation of thicker sea ice by pumping ocean water onto the surface of ice sheets is the least expensive of the methods identified for enhancing downwelling ocean currents. Modifying downwelling ocean currents is highly unlikely to ever be a competitive method of sequestering carbon in the deep ocean, but may find future application for climate modification.

Swimming Against the Flow: A Mechanism of Zooplankton Aggregation

Zooplankton reside in a constantly flowing environment. However, information about their response to ambient flow has remained elusive, because of the difficulties of following the individual motions of these minute, nearly transparent animals in the ocean. Using a three-dimensional acoustic imaging system, we tracked >375,000 zooplankters at two coastal sites in the Red Sea. Resolution of their motion from that of the water showed that the animals effectively maintained their depth by swimming against upwelling and downwelling currents moving at rates of up to tens of body lengths per second, causing their accumulation at frontal zones. This mechanism explains how oceanic fronts become major feeding grounds for predators and targets for fishermen.

Exploring the persistence of sorted bedforms on the inner-shelf of Wrightsville Beach, North Carolina

Geological studies offshore of Wrightsville Beach, North Carolina reveal subtle large-scale regions of coarse sand with gravel and shell hash (widths between 100 and 200 m and negative relief of ∼1 m) that trend obliquely to the coast. It was previously suggested that these regions serve as conduits for sand exchange between the shoreface and inner shelf during storm-associated downwelling. Consequently they were classified as rippled scour depressions. More recently, the role of alongshore flows and self-organization as a result of inhibited settling of fine sand has been discussed. In this study, 45 days of near-bed current measurements were analyzed using benthic boundary layer and sediment transport models to examine the role of along- and cross-shore flows in driving sediment transport at this site. The wind climate was found to be a dominant influence on near-bed flows. Six distinct sediment transport events were recognized. During these events, sediment transport models show that bedload transport is directed mainly in the cross-shore direction, while suspended sediments are directed alongshore to the southwest. Current observations during these sediment transport events provide no evidence of cross-shore sediment transport caused by steady downwelling currents. Instead, benthic boundary layer model results are used to show that differences in bed roughness between the coarse areas of the seabed within the “rippled scour depressions” and the finer areas of the inner shelf are more pronounced during increasingly energetic wave and current conditions. The enhanced difference in roughness results in increased turbulence intensities over coarse regions inhibiting the deposition of the fine sand that is resuspended over the shelf during these events relative to finer areas over the shelf. Findings from this study contribute to explaining the observed long-term persistence of these features.

Circulation and larval distribution in internal tidal bore warm fronts

Internal tidal bore warm fronts were observed during the summer of 1996 off the coast of Southern California. Warm bore fronts had concentrating currents resulting from high‐frequency internal motions and from a larger two‐way flow; the two‐way flow featured surface currents onshore and bottom currents offshore. A sharp thermocline depression and high‐frequency, large amplitude internal motions followed the leading edge of the bore, with down‐welling currents on the trailing side of the crest of the nonlinear internal waves and upwelling currents in front of the crest. Warm bores propagated onshore with a propagation speed, c, that ranged from 10.6 to 19.6 cm s−1, while time‐averaged frontal currents, ū, varied from 11.2 to 17.6 cm s−1 in the shallowest bin. In one out of three cases ū &gt; c, which implied that there were faster currents than the rate of advance of the front and which implied that the origin of surface frontal material is behind the front, not in front of it. Three invertebrate larval taxa were found at all sites across fronts, but only two intertidal barnacles, Pollicipes polymerus and Chthamalus spp., were concentrated at the front's surface, while the subtidal bryozoan Membranipora spp. was not. Frontal Pollicipes were more concentrated than were Chthamalus. The frontal downwelling currents observed suggested that concentrated larvae would have to swim upward in order to maintain depth. Pollicipes were abundant on the offshore warm side of the fronts but were absent or rare on the onshore colder side, suggesting that the origin of frontal Pollicipes was behind the front, although an alternative cannot be ruled out conclusively. Chthamalus were more abundant at depth than at the surface at all sites except at the front, where this pattern was reversed. The origin of frontal Chthamalus is uncertain. Membranipora were more abundant on the onshore colder side of the fronts, and abundances were usually higher at depth than at surface. Lack of accumulation in this species may be due to its limited swimming capability.

On the penetration of the 660 km phase change by mantle downflows

We present a simple analytic model of the interaction of cold convective downwelling currents with an endothermic phase change. The model describes the ponding and lateral spreading of downflows along the phase transition interface. A simple comparison of the vertical forces on the ponding material provides a necessary condition for a downflow to penetrate the phase boundary. This condition is fundamentally dependent on the geometry of the downflow. For planar downwellings, the model predicts a minimum ponding time before the structure can penetrate the phase boundary. For columnar (axisymmetric) downflows, there is no minimum time of spreading required before penetration can proceed. The model thus provides an explanation for the observation that in numerical models of three‐dimensional convection with an endothermic phase change, cylindrical downflows penetrate the phase interface while planar ones do not. Since descending slabs in the Earth‧s mantle display a wide spectrum of geometries between planar and cylindrical (given various trench curvatures, as well as intersections of two or more subduction zones), this phenomenon may explain, in part, why some slabs appear to extend into the lower mantle while others are deflected at the 660 km discontinuity.

Mantle dynamics in Mars and Venus: Influence of an immobile lithosphere on three‐dimensional mantle convection

Mantle dynamics in Mars and Venus: Influence of an immobile lithosphere on three‐dimensional mantle convection

Locomotion of the coelacanth Latimeria chalumnae in its natural environment

The coelacanth Latimeria chalumnae is the only living relic of a fossil group of crossopterygian lobe-finned fish1–4. We describe observations of its locomotion in a natural environment using six individuals observed from the research submersible Geo in the Indian Ocean at a depth of between 117 and 198 m. Past speculation on the pattern of locomotion has included crawling with the paired fins on the rocky ocean bottom, stalking like a large piscivorous grouper, or even fast swimming in open water4–6. Our observations show L. chalumnae to be a nocturnal piscivorous drift-hunter, moving very slowly in up- or downwelling currents, while paired fins stabilize and correct the drift motion. Paired and unpaired lobed fins are able to generate thrust. Fast starts are performed with the large caudal fin. Paired fins were not used for locomotion along the bottom. They alternate synchronously in a pattern common in tetrapod locomotion.