Buoyant Water Research Articles

Extreme wind events responsible for an outsized role in shelf-basin exchange around the southern tip of Greenland.

The coastal circulation around Southern Greenland transports fresh, buoyant water masses from the Arctic and Greenland Ice Sheet near regions of convection, sinking, and deep-water formation in the Irminger and Labrador Seas. Here, we track the pathways and fate of these fresh water masses by initializing synthetic particles in the East Greenland Coastal Current on the Southeast Greenland shelf and running them through altimetry-derived surface currents from 1993 to 2021. We report that the majority of waters (83%) remain on the shelf around the southern tip of Greenland. Variability in the shelf-basin exchange of the remaining particles closely follows the number of tip jet wind events on seasonal and interannual timescales. The probability of a particle exiting the shelf increases almost fivefold during a tip jet event. These results indicate that the number of tip jets is a close proxy of the shelf-basin exchange around Southern Greenland.

Thermal-induced buoyant water jet discharge under shallow coastal water conditions

A novel discharge dispersion model is developed to simulate the complex three-dimensional flow behaviour of thermal-induced buoyant water jets under current-wave coexisting conditions. The model solved the governing fluid flow and energy equations for two immiscible and incompressible phases (water and air) which were weakly coupled by applying the Boberbeck-Boussinesq approximation. Different turbulence models, such as k−ε multiphase, k-ω SST, k-ω SST-multiphase, k-ω SST-stable, and realizable k−ε were applied. Extensive verification of the model's performance is conducted by comparing the developed model results against a diverse range of analytical and experimental data. First, a series of simulations are carried out to evaluate the performance of the model in reproducing the results of the wave hydrodynamic and interactions with the submerged trapezoid bar. This is followed by numerically replicating the experimental results of a vertical non-buoyant submerged jet under current-only and current-wave environments. Finally, the potency of the coupled hydro-thermal algorithm is assessed by validating against different thermal-induced buoyant submerged jet experimental tests. For this purpose, numerical prediction of the developed model is tested against physical experiments for a series of tests for thermal-induced buoyant submerged horizontal jets in stationary water and inclined thermal-induced buoyant water jet under the influence of current-wave environments. Results showed that the k-ω SST-multiphase provides the best agreement with the laboratory measured data in terms of flow, temperature distribution field, plume trajectory and dilution. The findings confirmed that the developed model can be used as a reliable tool in precisely modelling characteristic of thermal-induced buoyant water jet in shallow coastal waters.

Forcing conditions of cross-shelf plumes on a wide continental shelf, Winyah Bay, South Atlantic Bight

Buoyant cross-shelf river plumes can extend far offshore through the combined effect of buoyancy and wind forcing, creating a critical land-ocean link in global biogeochemical cycles. On the Carolinas continental shelf, cross-shelf plume structure has been analyzed using satellite imagery, with forcing conditions represented by an estuarine Richardson number, wind stress, and alongshore pressure gradient. Three distinct cross-shelf plume patterns emerged, each occurring under an upwelling-favorable wind: (1) The separated plume, when a single filament of buoyant water spreads offshore (a prototypical cross-shelf plume structure); (2) The upwind-curving plume, which turns against the wind at some offshore distance and is created by stronger buoyancy forcing; and (3) The multi-lobe plume, which is partially trapped by the coast with multiple streaks protruding offshore and is created by stronger wind forcing, and further aided by a coincident alongshore pressure gradient force. The latter two regimes represent a low-wind, high discharge limit and a strong-wind limit of cross-shelf plumes. High-resolution satellite images reveal rich submesoscale variability associated with each plume type. Results suggest plume transport extends farthest offshore in low-energy separated plumes through a balance of weak buoyancy and weak wind forcing.

Influence of glacial influx on the hydrodynamics of Admiralty Bay, Antarctica - study based on combined hydrographic measurements and numerical modeling

This study investigates the impact of glacial water discharges on the hydrodynamics of a glacial bay in Antarctica, comparing it to well-studied northern hemisphere fjords. The research was carried out in Admiralty Bay (AB) in the South Shetland Islands, a wide bay adjacent to twenty marine-terminating glaciers. From December 2018 until February 2023, AB water properties were measured on 136 days. This dataset showed that a maximally two-layered stratification occurs in AB and that glacial water is always the most buoyant water mass. Using the Delft3D Flow, a three-dimensional hydrodynamical model of AB was developed. During tests, the vertical position and initial velocity of glacial discharges have been shown to be insignificant for the overall bay circulation. Fourteen model scenarios have been calculated with an increasing glacial influx added. The AB general circulation pattern consists of two cyclonic cells. Even in scenarios with significant glacial input, water level shifts and circulation are predominantly controlled by the ocean. Glacial freshwater is carried out of AB along its eastern boundary in a surface layer. Freshwater thickness in this outflow current is maximally 0.27-0.35 m. Within the inner AB inlets, significant glacial influx produces buoyancy-driven vertical circulation. Using an approach combining hydrographic and modeling data, a four-year timeseries of glacial influx volumes into AB has been produced. On average, glacial influx in summer is 10 times greater than in spring and winter and 3 times higher than in autumn. The annual glacial influx into AB was estimated at 0.434-0.632 Gt. Overall, the study demonstrated the unique characteristics of the topography and forcings that influence the hydrodynamics of an Antarctic glacial bay.

The effect of alongshore wind stress on a buoyancy current’s stability

Buoyancy fronts reach from the surface to the bottom over continental shelves, separating light inshore water from denser offshore water, and are known to be responsive to Ekman transport (and associated return flow at depth) driven by alongshore winds. The consequent changes in frontal structure are clearly related to changes in the gravitational Available Potential Energy (APE), so it is reasonable to expect that these winds will affect the eddy field that results from baroclinic instabilities. Idealized numerical experiments and scaling analyses are brought to bear on this problem. It is found that several days of wind-driven downwelling (which creates more nearly vertical isopycnals) generally leads to an enhancement in the time maximum of volume-averaged Eddy Kinetic Energy (EKE). Upwelling-favorable winds (which tend to flatten isopycnals) usually lead to a decrease in APE, hence in eddy energy. The exception to this rule occurs when the winds are strong enough that an upwelling front forms inshore of the buoyant water, in which case APE and EKE may increase.

Cross‐Shelf Penetrating Fronts of Buoyant Coastal Currents Around the Headland

AbstractBuoyant coastal current induced by riverine outflow often propagates along the coast in the direction of coastal trapped wave. However, it is uncertain how the buoyant coastal current interacts with irregular coastal bathymetry. Measured surface salinity distributions indicate that the buoyant coastal current, which is induced by the Changjiang River plume, frequently penetrates offshore around the curved headlands in the Zhoushan Islands. This signal can also be seen in satellite chlorophyll data, reflecting its importance in cross‐shelf exchange both physically and ecologically. To understand the variation and underlying dynamic mechanisms of these penetrating fronts, realistic and idealized numerical experiments were conducted. The results indicated that the penetrated water salinity was about 30–32 psu, with a horizontal penetration scale of tens of kilometers. Curvature and self‐organizing map analyses suggested that it was a frequent event with an approximately 2‐week spring‐neap cycle from generation to decay. It more likely occurred in transition seasons such as spring and autumn, owing to the runoff accumulation and gentle wind conditions. Smoothing the topography would substantially eliminate the penetrating fronts. Idealized model experiments suggested that headland‐induced river plume penetration was a type of curved topography‐induced current separation. The criterion for this to occur is u/fRc > 1, in which u is the current velocity, f is the Coriolis parameter, and Rc is the curvature radius. In this case, the headland is too sharp that the Coriolis force could not balance the centrifugal force. Once penetrated, the buoyant water was further transported by the ambient Taiwan Warm Current.

Dynamics of deep-submarine volcanic eruptions

Deposits from explosive submarine eruptions have been found in the deep sea, 1–4 km below the surface, with both flow and fall deposits extending several km’s over the seafloor. A model of a turbulent fountain suggests that after rising 10–20 m above the vent, the erupting particle-laden mixture entrains and mixes with sufficient seawater that it becomes denser than seawater. The momentum of the resulting negatively buoyant fountain is only sufficient to carry the material 50–200 m above the seafloor and much of the solid material then collapses to the seafloor; this will not produce the far-reaching fall deposits observed on the seabed. However, new laboratory experiments show that particle sedimentation at the top of the fountain enables some of the hot, buoyant water in the fountain to separate from the collapsing flow and continue rising as a buoyant plume until it forms a radially spreading intrusion higher in the water column. With eruption rates of 10^6–10^7hbox {kg s}^{-1}, we estimate that this warm water may rise a few 100’s m above the fountain. Some of the finer grained pyroclasts can be carried upwards by this flow and as they spread out in the radial intrusion, they gradually sediment to form a fall deposit which may extend 1000’s m from the source. Meanwhile, material collapsing from the dense fountain forms aqueous pyroclastic flows which may also spread 1000’s m from the vent forming a flow deposit on the seabed. Quantification of the controls on the concurrent fall and flow deposits, and comparison with field observations, including from the 2012 eruption of Havre Volcano in the South Pacific, open the way to new understanding of submarine eruptions.

Numerical Simulations of the Cabbeling Phenomenon in Surface Gravity Currents in Cold Fresh Water

The behaviour of warm water discharge at a temperature higher then Tm horizontally into a homogeneous body of cold fresh water at a temperature lower then Tm was investigated by means of a numerical model. Water density here was taken to be a quadratic function of temperature. Thus cabbeling process was inevitable as positively buoyant water form surface current while penetrating the ambient water. The current halted as mixture became dense and sink. These results are very similar to the experimental study of warm discharge into cold water by Marmoush et al. [14] and Bukreev [22]. The results showed an initially sinking water at the point where the two water bodies meets within the first few time interval. Development of Rayleigh-Taylor instabilities was observed at the lower part of the surface current as lighter fluid penetrate further. The frontal head was found to being replenished by a surface flow of warm unadulterated water, but after much entrainment of ambient fluid and cabbeling then, this head halted and sink. On the floor, denser fluid advance in the same direction as the original surface current, with some degree of Kelvin-Helmholtz instability as it penetrates further. Relations were also drawn that describes the speed, the spread length of both surface current were obtained.
 Relation were also drawn that describes the final spread length of the surface current Lsm and the time taken to reach that final spread length Tsm as a function ϕin. This work as presented here is practical and relevant to many fields of study and also enhances policy making towards the protection of the aquatic ecosystems.

Offshore Spreading of a Supercritical Plume Under Upwelling Wind Forcing: A Case Study of the Winyah Bay Outflow

In this study, we present observations of the Winyah Bay (WB) plume (SC, United States) formed by high freshwater discharge and a moderate upwelling-favorable wind acting continuously for ∼1.5 days prior to the shipboard survey. If a similar wind forcing persists over a longer period, the plume turns upstream (against its natural propagation) and curves offshore forming a “filament” with minimal transverse spreading, as seen in numerous satellite images. The observed plume comprises a train of tidal sub-plumes undergoing rotational adjustment and being transported offshore by Ekman dynamics. The WB outflow is supercritical in terms of the interior Froude number. Moderate wind extends this supercritical regime farther offshore. The plume is characterized by interior fronts associated with consecutive tidal pulses. Age of the buoyant water can be distinguished by the buoyant layer mixing (evident in the layer’s thickness and salinity anomaly) along with the transformation of its TS properties. However, relatively little transverse (lateral) spreading of buoyant water occurs: the equivalent freshwater layer thickness remains surprisingly consistent, approximately 0.8 m, over more than 20 km in the direction of the bulge extension. It is hypothesized that the supercritical regime constrains the transverse spreading of a plume. Microstructure measurements reveal higher dissipation rates below the base of the older (offshore) part of the plume. This is attributed to internal wave radiation from a newly discharged tidal pulse into an older plume, with the buoyant layer acting as a waveguide. Theoretical estimations of the internal wave properties show that the interior front is highly supercritical, while the observed dissipation maximum agrees with the theoretical wave structure.

Tracking Irminger Rings’ properties using a sub-mesoscale ocean model

A 1/60° numerical simulation is carried out within the Labrador Sea to investigate eddies produced along the western coast of Greenland. These eddies, known as Irminger Rings, carry relatively buoyant water from the West Greenland Current system into the interior Labrador Sea. These eddies can survive for up to 2 years; we detect and track 232 eddies produced within our 14 year simulation to investigate how they evolve during their lifetime. Irminger Rings start with a significant layer of freshwater (median 4.4 m) that quickly erodes during the convective winter. The freshwater layer, as opposed to the warm Irminger Water layer, constitutes the majority of the stratification within each eddy. Eddies generally travel southwestwards after formation, and eddies whose trajectory is close to the continental slope tend to have a reduced lifespan and quicker speed than those which drift into the interior deep basin. We find that eddies which spawn further north are more likely to end up influenced by the boundary currents, while those which form to the south are more likely to live longer and enter the deep interior basin. While the formation rate of eddies is generally uniform across our 2005–2018 simulation, Irminger Rings are far more likely to decay during the convective wintertime.We find that most eddies quickly decay within a few months, although some survive long enough to endure two convective winters. All Irminger Rings increase the local stratification in the Labrador Sea, limiting convection. However, the eddies which endure some part of two winters experience a significant buoyancy loss over a long time span such that they may produce Labrador Sea Water within their core during their second winter. This constitutes a small but non-negligible volume of Labrador Sea Water (0.02 to 0.09 Sv) and updates our understanding of Irminger Ring’s role on stratifying the Labrador Sea.Plain language summary:The Labrador Sea, between Canada and Greenland, experiences deep convection, a process where the surface water cools and becomes denser than the water at depth. This forces the surface water to sink and mix with the water below, homogenizing the water column and producing deepwater, an important component in the distribution of heat between the equatorial and polar regions. Deep convection is also important in ventilating the deep ocean with oxygen and carbon dioxide, sequestering these gasses for thousands of years. Deepwater is strongly influenced by the local weather as well as the current systems which surround the Labrador Sea. Oceanic eddies are produced from these current systems and bring their water mass properties into the region where deepwater is formed. These eddies are relatively buoyant, hindering deepwater formation, but also survive up to 2 years before decaying. We use a high-resolution ocean simulation to resolve these eddies so we can further understand how they evolve after being formed. Our simulation suggests that these eddies decay faster during winter, although eddies which survived to experience two winters often experienced deep convection and the production of Labrador Sea Water. This paints the picture that these eddies not only significantly reduce deepwater formation, but can also act as local sources of deepwater formation.

Hypersalinity drives convergent bone mass increases in Miocene marine mammals from the Paratethys

Pachyosteosclerosis-a condition that creates dense, bulky bones-often characterizes the early evolution of secondarily aquatic tetrapods like whales and dolphins1-3 but then usually fades away as swimming efficiency increases.4 Here, we document a remarkable reversal of this pattern, namely the convergent re-emergence of bone densification in Miocene seals, dolphins, and whales from the epicontinental Paratethys Sea of eastern Europe and central Asia. This phenomenon was driven by imbalanced remodeling and inhibited resorption of primary trabeculae and coincided with hypersaline conditions-the Badenian salinity crisis-that affected the Central Paratethys between 13.8 and 13.4 Ma.5 Dense bones acting as ballast would have facilitated efficient swimming in the denser and more buoyant water and hence were likely adaptive in this setting. From the Central Paratethys, pachyosteosclerosis subsequently spread eastward, where it became a defining feature of the endemic late Miocene whale assemblage.6,7.

The Antarctic Coastal Current in the Bellingshausen Sea

Abstract. The ice shelves of the West Antarctic Ice Sheet experience basal melting induced by underlying warm, salty Circumpolar Deep Water. Basal meltwater, along with runoff from ice sheets, supplies fresh buoyant water to a circulation feature near the coast, the Antarctic Coastal Current (AACC). The formation, structure, and coherence of the AACC has been well documented along the West Antarctic Peninsula (WAP). Observations from instrumented seals collected in the Bellingshausen Sea offer extensive hydrographic coverage throughout the year, providing evidence of the continuation of the westward flowing AACC from the WAP towards the Amundsen Sea. The observations reported here demonstrate that the coastal boundary current enters the eastern Bellingshausen Sea from the WAP and flows westward along the face of multiple ice shelves, including the westernmost Abbot Ice Shelf. The presence of the AACC in the western Bellingshausen Sea has implications for the export of water properties into the eastern Amundsen Sea, which we suggest may occur through multiple pathways, either along the coast or along the continental shelf break. The temperature, salinity, and density structure of the current indicates an increase in baroclinic transport as the AACC flows from the east to the west, and as it entrains meltwater from the ice shelves in the Bellingshausen Sea. The AACC acts as a mechanism to transport meltwater out of the Bellingshausen Sea and into the Amundsen and Ross seas, with the potential to impact, respectively, basal melt rates and bottom water formation in these regions.

Advances on Coastal and Estuarine Circulations Around the Changjiang Estuary in the Recent Decades (2000–2020)

Advances on the circulation in the Changjiang Estuary and adjacent East China Sea (ECS) and Yellow Sea (YS) coastal waters in the recent decades (2000–2020) are synthesized in this review. The circulation over the complicated bathymetry in the region is locally driven by winds, tides, as well as riverine discharge, and is remotely influenced by shelf currents between the 50 and 100-m isobaths through the cross-shelf exchanges. The interchange of the momentum and the freshwater pathway inside the Changjiang Estuary are jointly determined by tides and seasonally varying discharge and winds over the shelf. The buoyant waters are trapped inside the bulge that forms and expands over the shelf to the west of the 30-m isobath in the vicinity of Hangzhou Bay and the Changjiang Estuary. These buoyant waters are exported offshore by the shelf current, tidal mixing, and variations of wind patterns, forming the Changjiang River plume, which shows notable seasonality due to the reversal of both winds and shelf currents in the ECS and YS. Extensive spatial irregularities in the form of freshwater patches are present along its pathway to the Tsushima Strait in summer and to the Taiwan Strait in winter, respectively. Tides and the bathymetry irregularity have recently been found to play critical roles in determining the cross-shelf exchanges of water mass and momentum along the pathway of the ECS coastal current, and along this pathway, a year-round upslope intrusion of shelf waters appears in both summer and winter. Tides also play an important role in altering the expansion of the Changjiang River plume, cross-shelf extrusion of waters, and variation in the Yellow Sea Coastal Current over the shallow Subei Shoal.

Numerical simulation of the flow rate effect on the evolution of a negatively buoyant water jet

A numerical simulation study was done on a penetrating pure water jet injected into another surrounding salt water miscible with negative buoyancy conditions. For the incompressible filtered Navier-Stokes equations and the sum of the fluid model volume, we used a transient Computational Fluid Dynamics (CFD) solver (VOF model). A finite volume discretization method using Open Source code given in Open Foam 2.3.0. was used to solve these equations. The flow has a significant impact in the laminar system on the evolution of the jet in terms of subsequent permanent phase as well as the transient regime.

Estuarine salt-plug induced by freshwater pulses from the inner shelf

Estuarine salt plugs can cause water quality problems and are usually attributed to hypersaline conditions in low-inflow estuaries, where evaporation rates exceed or match freshwater input rates. However, salt plugs may also develop in typical estuaries, in brackish waters, by an additional freshwater influence from the estuary's mouth. In this study, month-long and years-long data showed the formation of a salt plug by a freshwater pulse from the ocean into Barataria Bay, in the Mississippi River Delta. Month-long currents showed an inverse estuarine circulation at Barataria Pass with inflow in the surface and outflow underneath. A dimensionless analysis showed that the dynamics governing the inverse circulation was related to a density gradient reversal, which was related to a salt-plug. Empirical Orthogonal Functions applied to nine years of salinity values along the estuary showed that Mode 1 (EOF1) represented the salt-plug structure and explained 86% of the variability. The salt-plug structure was most pronounced during winter, under minimum river discharge, and least evident in summer. Wavelet coherence analysis showed an annual relationship between EOF1 weights and both, river discharge and along-channel wind-stress. This study shows evidence of a salt-plug development by buoyant water pulses from outside an estuary.

Upwelling in the Ocean Basins North of the ACC: 2. How Cool Subantarctic Water Reaches the Surface in the Tropics

AbstractLarge volumes of cool water are drawn up to the surface in the tropical oceans. A companion paper shows that the cool water reaches the surface in or near the upwelling zones off northern and southern Africa and Peru. The cool water has a subantarctic origin and spreads extensively across the Atlantic and Pacific basins after it reaches the surface. Here, we look at the spreading in two low‐resolution ocean general circulation models and find that the spreading in the models is much less extensive than observed. The problem seems to be the way the upwelling and the spreading are connected (or not connected) to the ocean's large‐scale overturning. As proposed here, the cool upwelling develops when warm buoyant water in the western tropics is drawn away to become deep water in the North Atlantic. The “drawing away” shoals the tropical thermocline in a way that allows cool subantarctic water to be drawn up to the surface along the eastern margins. The amounts of upwelling produced this way exceed the amounts generated by the winds in the upwelling zones by as much as 4 times. Flow restrictions make it difficult for the warm buoyant water in our models to be drawn away.

Tidal Mixing Sustains a Bottom‐Trapped River Plume and Buoyant Coastal Current on an Energetic Continental Shelf

AbstractConventional wisdoms on river plume dynamics suggest that a down‐shelf buoyant coastal current will ultimately be trapped at a specific depth, that is, the trapping depth, as constrained by riverine outflow and offshore bottom Ekman transport. Theoretically, a prerequisite down‐shelf current is necessary to form a stable bottom‐trapped river plume. In this study an alternative is described by carrying out a modeling study on the Zhe‐Min Coastal Current (ZMCC). Buoyant water from the Changjiang River is a major factor driving the ZMCC, as is common in bottom‐trapped river plumes; however, the trapping depth is more determined by tidal mixing. When the plume water comes to the sloping topography, strong tidal mixing induces a mixing front, shoreward of which the bottom Ekman layer occupies the entire water column. Such a tidal‐induced front maintains a down‐shelf frontal current, which is intensified both at the surface due to the thermal wind balance and on the top of bottom boundary layer due to the tidal rectification. Direct wind‐induced transport only covers a small fraction of the ZMCC; however, it redistributes the plume water and, thus, affects the coastal current. The tide‐induced frontal trapping depth varies much less between seasons than that predicted by previous plume theories. Instead, it fluctuates strongly in the spring‐neap cycle. Even in summer when upwelling‐favorable winds prevail, the mixing front still sustains a down‐shelf coastal current. Intense tidal mixing exists in many coastal waters, which might be an alternative mechanism in forming bottom‐trapped river plumes and their associated buoyant coastal current.

Two-phase SPH simulation of vertical water entry of a two-dimensional structure

In view of the fact that the SPH model is easy to handle the flows with the free surface of large deformation, a 2-D flow induced by vertical water entry of a 2-D structure is simulated using the two-phase SPH model. The local pressure of the boundary particles is obtained by pressure of the fluid particles nearby through a modified kernel approximation. To evaluate the accuracy of the method, water entry of a 2-D symmetric wedge with fixed separation point of the free surface on the wedge surface is simulated. The pressure distribution of the wedge at the initial stage agrees well with the analytical results available. Evolution of the free surface and the air flow in the cavity induced by the water entry are obtained. A higher speed air jet is found at the neck of the cavity when the neck of the cavity becomes smaller. For the case of a horizontal cylinder entering the water with an unknown separation point of flow on the model surface, the early stage of the water entry is simulated for the rigid body with different density. Evolution of the free surface deformation of the half-buoyant cylinder and neutrally buoyant cylinder water entry is compared with the experimental data. The effects of the density ratio and Froude number on the pinch-off of the cavity are discussed. It is found that the pinch-off time remains almost constant for different density ratio and Froude number. Meanwhile, for a given Froude number, the dimensionless pinch-off depth and the location of the cylinder at the time of pinch-off increase with the density ratio. Further, for a given density ratio, these two parameters increase with the Froude number and, however, the relative cavity shape appears to be a self-similar shape when Fr ≥ 8.35.

Frontal Dynamics of a Buoyancy‐Driven Coastal Current: Quantifying Buoyancy, Wind, and Isopycnal Tilting Influence on the Nova Scotia Current

AbstractThe focus of this study is on the relative roles of winds and buoyancy in driving the Nova Scotia Current (NSC) utilizing detailed hydrographic glider transects along the Halifax Line. We define a Hydrographic Wind Index (HWI) using a simplistic two‐layer model to represent the NSC and its frontal system. The HWI is based on local characteristics of the density front extracted from the glider data (e.g., frontal slope). The impact of wind‐driven isopycnal tilting on the frontal slope is estimated and corrected for to accurately scale the buoyancy‐driven component of the NSC. Observations from independent current profilers deployed across the NSC confirm that the HWI captures the low‐frequency variability of the NSC. The monthly wind‐driven flow is estimated to represent between 1.0% (±0.1%) and 48% (±1%) of the total alongshore currents, with a yearly mean of about 36% (±1%). We demonstrate that using local conditions is more appropriate to the study of buoyancy‐driven currents ranging over distances on the order of (100 km), compared to the traditional approach based on upstream conditions. Contrary to the traditional approach, the HWI is not affected by the advective time lag associated with the downshelf propagation of the buoyant water coming from the upstream source. However, the HWI approach requires high‐resolution data sets, as errors on the estimates of the buoyancy‐ and wind‐driven flows become large as the sampling resolution decreases. Despite being data intensive, we argue that the HWI is also applicable to multisource currents, where upstream conditions are difficult to define.

Circulation Induced by Isolated Dense Water Formation over Closed Topographic Contours

AbstractThe problem of localized dense water formation over a sloping bottom is considered for the general case in which the topography forms a closed contour. This class of problems is motivated by topography around islands or shallow shoals in which convection resulting from brine rejection or surface heat loss reaches the bottom. The focus of this study is on the large-scale circulation that is forced far from the region of surface forcing. The authors find that a cyclonic current is generated around the topography, in the opposite sense to the propagation of the dense water plume. In physical terms, this current results from the propagation of low sea surface height from the region of dense water formation anticyclonically along the topographic contours back to the formation region. This pressure gradient is then balanced by a cyclonic geostrophic flow. This basic structure is well predicted by a linear quasigeostrophic theory, a primitive equation model, and in rotating tank experiments. For sufficiently strong forcing, the anticyclonic circulation of the dense plume meets this cyclonic circulation to produce a sharp front and offshore advection of dense water at the bottom and buoyant water at the surface. This nonlinear limit is demonstrated in both the primitive equation model and in the tank experiments.