Strong Horizontal Density Gradients Research Articles

Vertical Motions Driven by Geostrophic Stress in Surface Boundary Layer under a Finite Ekman Number Regime

Abstract Geostrophic stress caused by a strong horizontal density gradient embedded in the surface boundary layer plays an important role in generating vertical motion and associated tracer transport. However, dependence of this frictionally driven vertical velocity on the Ekman number (Ek), a key dimensionless parameter for frictional flows in a rotating reference frame, has not been systematically analyzed, especially for a finite Ek. In this study, we theoretically demonstrate that the geostrophic stress always induces an ageostrophic stress acting to offset itself, and such an offsetting effect becomes more evident with increasing Ek. When Ek approaches unity or larger, vertical motion driven by geostrophic stress is much weaker than that derived by Garrett and Loder (GL81), who neglect effects of ageostrophic stress and predict a vertical velocity magnitude scaled with curl of geostrophic stress. Although the cancellation tendency between geostrophic and ageostrophic stress is universal, its underlying dynamics depends on vertical structures of turbulent viscosity and geostrophic flows. A realistic simulation in the winter Kuroshio Extension is conducted to validate the theoretical results and examine which regime, a small versus finite Ek, is more relevant in this region. It is found that the characteristic vertical scale involved in the definition of Ek is primarily determined by the vertical structure of turbulent viscosity and evidently smaller than that of geostrophic flow. The value of Ek in the winter Kuroshio Extension is generally larger than unity. Correspondingly, the GL81 model results in severe overestimation of the geostrophic stress-driven vertical velocity and tracer transport.

Frontal Dynamics in the Alboran Sea: 1. Coherent 3D Pathways at the Almeria‐Oran Front Using Underwater Glider Observations

AbstractOcean fronts are areas that can support phytoplankton production through fertilization in the sunlit layer and the subduction of biogeochemical properties from the surface to the interior of the ocean. The Almeria‐Oran (AO) front is formed from the juxtaposition of fresh inflowing Atlantic waters and more saline re‐circulating Mediterranean waters. A fleet of three gliders flying in parallel lines was deployed across the AO to obtain observations in the CALYPSO project. These observations were combined with remote sensing and modeling simulations, thus providing a novel approach to identifying the three‐dimensional transport and the submesoscale across‐front circulation. The resulting 33 cross‐front sections reveal spatial and temporal changes in the frontal boundary, with isopycnals steepening and/or relaxing. The observations revealed strong horizontal density gradients (up to ∼1.4 kg m−3) and the spatial variability was observed over different length scales (∼10–45 km). The potential vorticity decreased across the front due to the vorticity component in the horizontal density gradient direction. The predominant cyclonic relative vorticity on the dense side of the AO is associated with downwelling processes. The biogeochemical observations also suggest vertical transport along coherent pathways through baroclinic instability. Phytoplankton biomass enhancement occurs as a result, and is subducted below the euphotic layer. The observed oxygen filaments show upwelling and downwelling, providing a mechanism for oxygenating deeper layers and reducing the ventilation of deep low‐oxygenated waters. Understanding the mechanisms of vertical transport can help us evaluate the dynamics of ocean fronts and their impacts on biological carbon storage.

Reproducibility and variability of submesoscale frontal eddies on a broad, low-energy shelf of freshwater influence

A multi-decadal simulation of ocean circulation in the northern Gulf of Mexico produces strong submesoscale instabilities in the Mississippi/Atchafalaya plume fronts. The model skill in reproducing these submesoscale frontal eddies over the Texas-Louisiana shelf is assessed using simulated and observed salinity and velocity fields as a way to investigate simulation accuracy and quantify the variability of frontal eddies. The model successfully reproduces mean salinity structure observed in multi-year densely sampled CTD profiles. Variability associated with submesoscale eddies is the largest source of error in predicted salinity. On the other hand, the model is statistically able to reproduce the magnitude and characteristics of frontal eddies; metrics for eddy kinetic energy are similar between the observations and simulation, and observed horizontal salinity gradients have similar occurrence rates in the model when sampled in a manner similar to the observations. Seasonal and inter-annual variability of frontal eddies is associated with the volume of freshwater onto the shelf and wind stress. Wind stress, the highest in winter and lowest in summer, contributes to the suppression of baroclinic instability during non-summer seasons. River streamflow, highest in spring, creates strong horizontal and vertical density gradients. These strong horizontal density gradients, along with weak seasonal upwelling-favorable winds that tend to broaden the plume, are the primary factors in exciting submesoscale instabilities during summer on the Texas-Louisiana shelf. At decadal scales, streamflow, EKE, and salinity gradients have a positive correlation suggesting that long-term variability of frontal eddies may be influenced remotely by inter-annual variability in the Mississippi River outflow.

Langmuir turbulence in horizontal salinity gradient

Langmuir circulation (LC) is believed to be one of the leading causes of turbulent mixing in the upper ocean. Large eddy simulation (LES) models that solve the Craik–Leibovich equations are used to study LC in the upper ocean, yielding new insights that could not be obtained from field observations or turbulent closure models alone. The present study expands our previous LES modeling investigations of LC to real ocean conditions with large-scale environmental motion due to strong horizontal density gradient, which is introduced to the LES model through scale separation analysis. The model is applied to field observations in the Gulf of Mexico when a measurement site was impacted by fresh water inflow. Model results suggest that LC can enhance turbulence in the water column and deepen the mixed layer (ML) with or without the large scale motions, being consistent with previous studies. The strong salinity gradient is shown to be able to reduce the mean flow in the ML, align Langmuir cells with the pressure gradient direction and inhibit turbulence in the ocean surface boundary layer.

Downfront Winds over Buoyant Coastal Plumes

AbstractDownfront, or downwelling favorable, winds are commonly found over buoyant coastal plumes. It is known that these winds can result in mixing of the plume with the ambient water and that the winds influence the transport, spatial extent, and stability of the plumes. In the present study, the interaction of the Ekman velocity in the surface layer and baroclinic instability supported by the strong horizontal density gradient of the plume is explored with the objective of understanding the potential vorticity and buoyancy budgets. The approach makes use of an idealized numerical model and scaling theory. It is shown that when winds are present the weak stratification resulting from vertical mixing and the strong baroclinicity of the front results in near-zero average potential vorticity q. For weak to moderate winds, the reduction of q by diapycnal mixing is balanced by the generation of q through the geostrophic stress term in the regions of strong horizontal density gradients and stable stratification. However, for very strong winds the wind stress overwhelms the geostrophic stress and leads to a reduction in q, which is balanced by the vertical mixing term. In the absence of winds, the geostrophic stress dominates mixing and the flow rapidly restratifies. Nonlinearity, extremes of relative vorticity and vertical velocity, and mixing are all enhanced by the presence of a coast. Scaling estimates developed for the eddy buoyancy flux, the surface potential vorticity flux, and the diapycnal mixing rate compare well with results diagnosed from a series of numerical model calculations.

Global Estimates of Lateral Springtime Restratification

AbstractSubmesoscale frontal dynamics are thought to be of leading-order importance for stratifying the upper ocean by slumping horizontal density gradients to produce vertical stratification. Presented here is an investigation of submesoscale instabilities in the mixed layer—mixed layer eddies (MLEs)—as a potential mechanism of frontal slumping that stratifies the upper ocean during the transition from winter to spring, when wintertime forcings weaken but prior to the onset of net solar warming. Observations from the global Argo float program are compared to predictions from a one-dimensional mixed layer model to assess where in the world’s oceans lateral processes influence mixed layer evolution. The model underestimates spring stratification for ~75% ± 25% of the world’s oceans. Relationships between vertical and horizontal temperature and salinity gradients are used to suggest that in 30% ± 20% of the oceans this excess stratification can be attributed to the slumping of horizontal density fronts. Finally, 60% ± 10% of the frontal enhanced stratification is consistent with MLE theory, suggesting that MLEs may be responsible for enhanced stratification in 25% ± 15% of the world’s oceans. Enhanced stratification from frontal tilting occurs in regions of strong horizontal density gradients (e.g., midlatitude subtropical gyres), with a small fraction occurring in regions of deep mixed layers (e.g., high latitudes). Stratification driven by MLEs appears to constrain the coexistence of sharp lateral gradients and deep wintertime mixed layers, limiting mixed layer depths in regions of large lateral density gradients, with an estimated wintertime restratification flux of order 100 W m−2.

A generalized mathematical model of geostrophic adjustment and frontogenesis: uniform potential vorticity

AbstractFronts, or regions with strong horizontal density gradients, are ubiquitous and dynamically important features in the ocean and atmosphere. In the atmosphere, fronts are associated with some of the most severe weather events, while in the ocean, fronts are associated with enhanced turbulence, water mass transformation and biological activity. Here, we examine the dynamics involved in the formation of fronts, or frontogenesis, in detail using a generalized mathematical framework. This extends previous work which has generally revolved around two limiting cases: fronts generated through forcing due to a convergent large-scale flow, and fronts generated spontaneously during the geostrophic adjustment of an initially unbalanced flow. Here, we introduce a new generalized momentum coordinate to simultaneously describe forced and spontaneous frontogenesis. The nonlinear, inviscid, Boussinesq, hydrostatic governing equations for uniform PV flow are solved for arbitrary Rossby and Froude number. The solution is then examined in three distinct cases. Firstly, for a zero potential vorticity (PV) flow bounded by rigid lids, a general solution is derived for the transient response of the fluid to an arbitrary initial mass imbalance and deformation field. The deformation frontogenesis solution of Hoskins & Bretherton (J. Atmos. Sci., vol. 29, 1972, pp. 11–37) and the mass imbalance solution of Blumen (J. Phys. Oceanogr., vol. 30, 2000, pp. 31–39) emerge as two limits of this general solution. Secondly, the problem of geostrophic adjustment of an initial mass imbalance (no deformation field) is considered for uniform PV flow bounded by rigid lids. The general solution is derived, composed of an adjusted state and a transient component describing the propagation of inertia–gravity waves. The criteria for the occurrence of a frontal discontinuity is determined in terms of the Rossby and Froude numbers. The uniform PV solution reduces identically to the zero PV solution of Blumen in the limit of vanishing background stratification. Thirdly, we examine the more general case of uniform PV flow with a deformation field and either balanced or unbalanced initial conditions. In this case the solution is composed of a time-varying mean state – matching the Hoskins & Bretherton solution in the limit of small strain – and an inertia gravity wave field, the dynamics of which are examined in detail. Our analysis provides a unifying framework capable of describing frontal formation and geostrophic adjustment in a wide variety of settings.

Dynamics of wind‐driven circulation in a shallow lagoon with strong horizontal density gradient

Albemarle‐Pamlico Sound (APS) is a shallow lagoonal estuary with weak stratification but strong horizontal density gradient. Idealized and realistic model simulations are conducted to examine the dynamics of wind‐driven circulation in APS. Two dimensionless parameters, the Ekman (Ek) and Wedderburn (W) numbers, are found to be two key parameters in controlling the flow pattern and dynamics. Circulation in Albemarle Sound and northern Pamlico Sound is primarily driven by wind‐forcing, and shows laterally sheared flows withEk approaching 1 and Wexceeding 1. Analysis of the along‐channel momentum equation shows a balance among the wind stress, barotropic pressure gradient due to sea level setup, and bottom friction, while the vertical vorticity is primarily determined by the balance between the wind stress curl and bottom stress curl. In contrast, circulation in southern Pamlico Sound is driven by both wind and gravitational forces, and shows vertically sheared two‐layer flows with lowEk and Waround 1. Both baroclinic and barotropic pressure gradients act against the wind stress to drive the two‐layer flows. A regime diagram in the parameter space ofEk and W is used to summarize variability of circulation pattern on the seasonal time scale. Northern Pamlico Sound is characterized by high Ek and high W values and laterally sheared currents. Southern Pamlico Sound is usually characterized by low Ek and low W values and vertically sheared flows but featuring laterally sheared flows during the fall when Ek averages to over 0.5.

Mixing in the Barents Sea Polar Front near Hopen in spring

Observations were made of hydrography, nutrients and shear microstructure in the water column across the Barents Sea Polar Front near Hopen. Profiles were collected in early May 2008 along a 125-km section extending from the Hopen Trench, across the front between the Arctic and Atlantic origin waters, and on toward the Spitsbergenbanken. Additionally a 10-h time series station was undertaken near the front. The Polar Front was identified near the 150m isobath, coinciding with the 1°C isotherm, with strong horizontal gradients in the temperature and salinity fields, which compensated in density. A second tidally-generated front with a strong horizontal density gradient was detected on the bank, dominated by salinity gradients due to ice melt. Biological activity was elevated between the two fronts. Nutrients were depleted in the euphotic zone where the chlorophyll concentrations were significantly enhanced relative to the stations on the warm side of the Polar Front and in the tidally mixed area. Below a turbulent surface layer, the water column on the warm side of the front was quiescent. Farther on the bank the turbulent boundary layers near the surface and seabed merged and the entire water column was turbulent. Energy arguments show that tidal straining together with stirring due to wind and tides can overcome the stratification induced by melting and heating, maintaining the tidal front. A mechanism is proposed whereby high nitrate concentrations on the warm side of the front are transported along the isopycnals onto the bank where tidal mixing then effectively mixes the nutrient-rich deep water upward, sustaining the phytoplankton bloom.

Modeling the seasonal variability of hydrography and circulation in the Kara Sea

In the frame of a project on transport of contaminants in the Arctic, the Hamburg shelf ocean model (HAMSOM) is applied to the Kara Sea. The HAMSOM system consists of a three‐dimensional, baroclinic circulation model coupled to a thermodynamic and dynamic sea ice model. The Kara Sea model is forced with climatological winds, atmospheric heat fluxes, river runoff, and tides. The obtained results reveal no typical Kara Sea circulation that prevails throughout the year. Instead, the model showed a strong seasonal variability in circulation and hydrography due to winds, freshwater runoff, and ice formation. The circulation is weakest in spring when the wind speeds are low and horizontal density gradients are small. Fresh water from the rivers spreads toward the north and northwest rather than forming a coastally trapped current that flows to the east. In autumn, the circulation is significantly enhanced because of increasing wind speeds and strong horizontal density gradients. Good agreement was found between model results and recent observations. The “classical” cyclonic current pattern in the southern Kara Sea, however, was not reproduced by the model.

Variability of the shelf water off the northeast Spanish coast

During the seasonal stratification of 1983 and 1984, intensive sampling of the shelf region of the northeast Spanish coast was carried out. This unique dataset is used to define the most salient features of the shelf water dynamics. We show that region has a strong spatial and temporal variability associated with the southward spreading of continental waters from the Rhone river, in the northern gulf of Lyons. Southward spreading begins in early spring and was particularly obvious during June 1983 when strong horizontal density gradients were observed. Spreading almost completely disappears in August when observed upper layer salinities are uniform. We also show that significant shelf/slope water exchange takes place associated with the abrupt topographic canyons found in the region. In particular we observed in June 1983 a small positive geostrophic vorticity region at the Palamós canyon while a larger anticyclonic eddy was observed over the shelf south of this canyon. The biological implications of these two features have been demonstrated. We conclude indicating that in this region where no dominant winds exist, the shelf water dynamics is dominated by the southward spreading of low salinity continental waters and by the intrusion of high salinity open ocean water through submarine canyons.

Dolomitization by Thermal Convection in Carbonate Platforms: ABSTRACT

Thermal may be a significant dolomitizing ground-water flow process in active carbonate platforms. In platform margins there is an upward transfer of seawater from the surrounding ocean depths. This motion is induced by the strong horizontal density gradient that exists between the warmer platform interior (warmed by geothermal heat flux) and the cold surrounding ocean waters. The cold dense seawater flows inward displacing the platform pore waters upward. This flow process was discovered and studied in south Florida by Francis Kohout. Here these upwelling waters discharge from well-known submarine springs on the shelf and shelf edge. The waters are of seawater composition but with increased Ca/Mg suggesting that they have acted as dolomitizing fluids. In Flori a this flow pattern is distorted by and mixed with the large regional flow of the Floridan aquifer. In isolated platforms this distortion should not occur. To test the idea that this kind of might be an important diagenetic agent in platform settings in general, I have used ground-water flow theory to model this process in isolated platforms. Approximate theory suggests that Darcy velocities of 1 m/yr (3 ft/yr) occur. I am using boundary layer theory to determine the fully developed flow pattern and the distance from the margin to which the flow penetrates. The rate of dolomitization that could be developed by this process is comparable to and may exceed that of other flow processes in active platforms. I conclude that this Kohout convection is a large-scale flow process tha appears capable of pervasive dolomitization of platform margins. End_of_Article - Last_Page 528------------

The winter circulation of the Adriatic Sea

The winter circulation of the northern Adriatic Sea is examined in the light of 1965–1966 Bannock data. A vertically integrated numerical model of the flow is used to study the relative importance of evaporation, wind stress, coastal river runoff, and exchange with the Southern Adriatic Sea for the wintertime fields of vertically integrated mass transport and density. A strong and sudden outbreak of cold, dry central Asian air over the Adriatic Sea during January, 1966, resulted in such rapid evaporation rates that water columns evidently overturned continuously. Changes in the density and temperature in the interior of the basin were dominated by surface heat fluxes. Initially sea temperatures were high and evaporation was rapid but as it continued, sea temperatures fell and the rate of evaporation decreased. Model calculations suggest that the strong horizontal density gradient set up by evaporation and coastal inflow of fresh water is an important source of the large scale wintertime circulation. This circulation, counterclockwise in the northern Adriatic Sea with a net transport of about 0·4 × 10 6m 3 s −1, is closed upon itself in most of the model solutions because of the relief of the ocean bottom. Mass exchange with the Southern Adriatic Sea appears to be of secondary importance for this circulation.