Surface Convergence Zones Research Articles

Surface Convergence Zones due to Lagrangian Residual Flow in Tidally Driven Estuaries

Abstract Buoyant material, such as floating debris, marine organisms, and spilled oil, is aggregated and trapped within estuaries. Traditionally, the aggregation of buoyant material is assumed to be a consequence of converging Eulerian surface currents, often associated with lateral (cross-estuary) density gradients that drive baroclinic lateral circulations. This study explores an alternative aggregation mechanism due to tidally driven Lagrangian residual circulations without Eulerian convergence zones and without lateral density variation. In a tidally driven estuary, the depth-dependent tidal phase of the lateral velocity varies across the estuary. This study demonstrates that the lateral movement of surface trapped material follows the tidal phase, resulting in a lateral Lagrangian residual circulation known as Stokes drift for small-amplitude motions. For steeper bathymetry, the lateral change in tidal phase is greater and the corresponding lateral Lagrangian residual flow faster. At local depth extrema, e.g., in the thalweg, depth does not vary laterally, so that the associated tidal phase is laterally constant. Therefore, the Stokes drift is weak near depth extrema resulting in Lagrangian convergence zones where buoyant material concentrates. These ideas are evaluated employing an idealized analytic model in which the along-estuary tidal flow is driven by an imposed barotropic pressure gradient, whereas cross-estuary flow is induced by the Coriolis force. Model results highlight that convergence zones due to Lagrangian residual velocities are efficient in forming persistent aggregation regions of buoyant material along the estuary. Significance Statement Our study focuses on the aggregation of buoyant material (e.g., debris, oil, organisms) in estuaries. Traditionally, the aggregation of buoyant material is assumed to be a consequence of converging Eulerian surface currents, often associated with lateral (cross-estuary) density gradients that drive baroclinic lateral circulations. Our study explores an alternative aggregation mechanism due to tidally driven Lagrangian residual circulations without Eulerian convergence zones and without lateral density variation. Our results highlight that convergence zones due to Lagrangian residual velocities are efficient in forming persistent aggregation regions of buoyant material along the estuary.

Large quantities of small microplastics permeate the surface ocean to abyssal depths in the South Atlantic Gyre

Hundreds of studies have surveyed plastic debris in surface ocean gyre and convergence zones, however, comprehensive microplastics (MPs, ≤5mm) assessments beneath these surface accumulation areas are lacking. Using in situ high-volume filtration, Manta net and MultiNet sampling, combined with micro-Fourier-transform-infrared imaging, we discovered a high abundance (up to 244.3 pieces per cubic meter [n m-3 ]) of small microplastics (SMPs, characteristically <100μm) from the surface to near-sea floor waters of the remote South Atlantic Subtropical Gyre. Large horizontal and vertical variations in the abundances of SMP were observed, displaying inverse vertical trends in some cases. SMP abundances in pump samples were more than two orders of magnitude higher than large microplastics (LMPs, >300μm) concurrently collected in MultiNet samples. Higher-density polymers (e.g., alkyd resins and polyamide) comprised >65% of the total pump sample count, highlighting a discrepancy between polymer compositions from previous ocean surface-based surveys, typically dominated by buoyant polymers such as polyethylene and polypropylene. Contrary to previous reports stating LMP preferentially accumulated at density gradients, SMP with presumably slower sinking rates are much less influenced by density gradients, thus resulting in a more even vertical distribution in the water column, and potentially longer residence times. Overall, our findings suggest that SMP is a critical and largely underexplored constituent of the oceanic plastic inventory. Additionally, our data support that weak current systems contribute to the formation of SMP hotspots at depth, implying a higher encounter rate for subsurface particle feeders. Our study unveils the prevalence of plastics in the entire water column, highlighting the urgency for more quantification of the deep-ocean MP, particularly the smaller size fraction, to better understand ecosystem exposure and to predict MP fate and impacts.

High-resolution simulations of interactions between surface ocean dynamics and frazil ice

Abstract. Frazil and grease ice forms in the ocean mixed layer (OML) during highly turbulent conditions (strong wind, large waves) accompanied by intense heat loss to the atmosphere. Three main velocity scales that shape the complex, three-dimensional (3D) OML dynamics under those conditions are the friction velocity u* at the ocean–atmosphere interface, the vertical velocity w* associated with convective motion, and the vertical velocity w*,L associated with Langmuir turbulence. The fate of buoyant particles, e.g., frazil crystals, in that dynamic environment depends primarily on their floatability, i.e., the ratio of their rising velocity wt to the characteristic vertical velocity, which is dependent on w* and w*,L. In this work, the dynamics of frazil ice is investigated numerically with the high-resolution, non-hydrostatic hydrodynamic model CROCO (Coastal and Regional Ocean COmmunity Model), extended to account for frazil transport and its interactions with surrounding water. An idealized model setup is used (a square computational domain with periodic lateral boundaries, spatially uniform atmospheric and wave forcing). The model reproduces the main features of buoyancy- and wave-forced OML circulation, including the preferential concentration of frazil particles in elongated patches at the sea surface. Two spatial patterns are identified in the distribution of frazil volume fraction at the surface: one related to individual surface convergence zones, very narrow, and oriented approximately parallel to the wind/wave direction and one in the form of wide streaks with a separation distance of a few hundred meters, oriented obliquely to the direction of the forcing. Several series of simulations are performed, differing in terms of the level of coupling between the frazil and hydrodynamic processes, from a situation when frazil has no influence on hydrodynamics (as in most models of material transport in the OML) to a situation in which frazil modifies the net density, effective viscosity, and transfer coefficients at the ocean–atmosphere interface and exerts a net drag force on the surrounding water. The role of each of those effects in shaping the bulk OML characteristics and frazil transport is assessed, and the density of the ice–water mixture is found to have the strongest influence on those characteristics.

Horizontal Transport of Buoyant Material by Turbulent Jets in the Upper Ocean

AbstractCurrents in the ocean surface boundary layer (OSBL) determine the horizontal transport of submerged buoyant material, such as pollutants, plankton, and bubbles. Commonly, the mean horizontal transport, that is, the transport that changes the horizontal position of the material’s center of mass, is assumed to be accomplished by horizontal mean currents. However, surface convergence zones due to OSBL turbulence organize both wind-driven horizontal currents and near-surface concentrated buoyant material. In such surface convergence zones, concentrations of buoyant material are enhanced (e.g., apparent as windrows) and collocate with increased horizontal turbulent currents, here referred to as turbulent jets. In turn, the correlation of turbulent jet flow and material concentrations leads to a net mean horizontal transport due to turbulent motion. To examine this turbulent jet transport, an idealized model is devised for a wind-driven flow that is perturbed by prescribed cellular flow structures with crosswind surface convergence zones. Model solutions of the jet flow and material concentrations reveal that turbulent jet transport is comparable to the transport by horizontal mean currents for sufficiently strong cellular flow and material buoyancy. To test this model, we also perform more realistic turbulence-resolving large-eddy simulations (LESs) of wind and wave-driven OSBL turbulence. LES results are consistent with many features of the idealized model and suggest that the commonly overlooked turbulent jet transport is about 20%–50% of the traditional transport by horizontal mean currents. Thus, turbulent jet transport should be taken into account for accurate transport models of buoyant material in the OSBL.

Hydraulically controlled front trapping on a tidal flat

[1] We report observations of baroclinic surface fronts in Skagit Bay. The fronts formed repeatedly in very shallow (<0.7 m deep) water along the edge of a tidal channel shortly after the incoming tide flooded the adjacent tidal flat and eventually departed from channel edge as the flood tide progressed. The fronts were surface convergence zones, trapping GPS drifters and other flotsam. Across-front salinity (temperature) jumps were a few practical salinity units (a few degrees) with freshwater on the channel side. Across-front transects of velocity (from mobile and fixed acoustic Doppler current profilers) and density (from mobile and fixed conductivity-temperature-depth profilers) are presented. Composite Froude numbers indicate that supercritical flow over the flats initially held fronts near the channel edge. As flood tide progressed, depth-averaged flow speeds decreased, while internal wave speeds increased, owing to increasing depths and density gradients. Flow over the flats eventually became subcritical, at which point fronts departed from the channel edge. The front then formed the leading edge of a shallow (around 0.3 m thick) freshwater plume which propagated across the flats against the incoming tidal flow. Water was then exchanged between the channel and flats with a channel edge composite Froude number near 0.5, consistent with mixing-induced reduction in channel-flats exchange. The across-channel velocity shear was correlated with the across-channel wind, consistent with a simple eddy viscosity model. Strong winds prevented plume propagation.

Mass tracking for chemical analysis: the causes of ozone formation in southern Ontario during BAQS-Met 2007

Abstract. A three-level nested regional air pollution model has been used to study the processes leading to high ozone concentrations in the southern Great Lakes region of North America. The highest resolution simulations show that complex interactions between the lake-breeze circulation and the synoptic flow lead to significant enhancements in the photochemical production and transport of ozone at the local scale. Mass tracking of individual model processes show that Lakes Erie and St. Clair frequently act as photochemical ozone production regions, with average mid-day production rates of up to 3 ppbv per hour. Enhanced ozone levels are evident over these two lakes in 23-day-average surface ozone fields. Analysis of other model fields and aircraft measurements suggests that vertical circulation enhances ozone levels at altitudes up to 1500 m over Lake St. Clair, whereas subsidence enhances ozone over Lake Erie in a shallow layer only 250 m deep. Mass tracking of model transport shows that lake-breeze surface convergence zones combined with the synoptic flow can then carry ozone and its precursors hundreds of kilometers from these source areas, in narrow, elongated features. Comparison with surface mesonet ozone observations confirm the presence, magnitude, and timing of these features, which can create local ozone enhancements on the order of 30 ppbv above the regional ozone levels. Sensitivity analyses of model-predicted ozone and HOx concentrations show that most of the region is VOC-limited, and that the secondary oxidation pathways of aromatic hydrocarbons have a key role in setting the region's ozone and HOx levels.

Precipitation-generated oscillations in open cellular cloud fields

Cloud fields adopt many different patterns that can have a profound effect on the amount of sunlight reflected back to space, with important implications for the Earth's climate. These cloud patterns can be observed in satellite images of the Earth and often exhibit distinct cell-like structures associated with organized convection at scales of tens of kilometres. Recent evidence has shown that atmospheric aerosol particles-through their influence on precipitation formation-help to determine whether cloud fields take on closed (more reflective) or open (less reflective) cellular patterns. The physical mechanisms controlling the formation and evolution of these cells, however, are still poorly understood, limiting our ability to simulate realistically the effects of clouds on global reflectance. Here we use satellite imagery and numerical models to show how precipitating clouds produce an open cellular cloud pattern that oscillates between different, weakly stable states. The oscillations are a result of precipitation causing downward motion and outflow from clouds that were previously positively buoyant. The evaporating precipitation drives air down to the Earth's surface, where it diverges and collides with the outflows of neighbouring precipitating cells. These colliding outflows form surface convergence zones and new cloud formation. In turn, the newly formed clouds produce precipitation and new colliding outflow patterns that are displaced from the previous ones. As successive cycles of this kind unfold, convergence zones alternate with divergence zones and new cloud patterns emerge to replace old ones. The result is an oscillating, self-organized system with a characteristic cell size and precipitation frequency.

Large Eddy Simulations of Upper-Ocean Response to a Midlatitude Storm and Comparison with Observations*

Abstract A large eddy simulation (LES) model is used to investigate an upper-ocean response to a fall storm in the open ocean of the North Pacific Ocean. The storm is characterized by rapid increases in wind speed and surface heat loss but a relatively steady wave field. The LES model shows that surface convergence zones or windrows organize into line patterns aligned with the wind direction, evolving from nearly parallel lines to irregular structures featuring Y junctions as the wind speed increases. The downwelling-to-upwelling velocity ratio ranges between 1.2 and 1.6, indicating a moderate level of asymmetry between the downwelling and upwelling plumes in Langmuir circulation. During the storm, the turbulent Langmuir number Lat increases from 0.2 to 0.5 while the vertical turbulence intensity σw2 decreases from 1.4 to 0.7 u*2, where u* is the friction velocity. The order of turbulence intensities in three directions switches from crosswind ≈ vertical &amp;gt; downwind directions to downwind &amp;gt; crosswind &amp;gt; vertical directions. This suggests a transition from Langmuir to shear turbulence as the storm progresses. The Hoennikker number (Ho) remains below 0.1 and the strong evaporative heat loss does not contribute much to the turbulence generation in the ocean mixed layer. The LES results are compared with in situ and acoustic measurements collected during the storm. Patterns of model-predicted near-surface downwelling zones are in good agreement with horizontal distributions of bubble clouds revealed in sidescan sonar images. Striking similarity is also found in the temperature anomalies between the LES model and high-resolution thermistor chain measurements.

Langmuir circulation driving sediment entrainment into newly formed ice: Tank experiment results with application to nature (Lake Hattie, United States; Kara Sea, Siberia)

Langmuir circulation (Lc) was generated under freezing conditions in saltwater tank experiments through surface wind stress and cross‐waves interacting with subsurface return flow. Fine‐grained sediments distributed in the tank prior to frazil crystal formation were aligned in parallel streaks in Lc bottom convergence zones. Downwelling at Lc surface convergence zones aligned floating frazil in wind‐parallel rows, and individual crystals rotated on helical paths down to the tank bottom and up again to the surface. The crystals interacted with suspended particles in the water column, and with sediment on the tank bottom, preferentially collecting fine‐grained particles and enhancing their entrainment into new ice. Evidence includes higher sediment concentrations in ice and ice‐interstitial water (ice pore water) as compared to the tank water. Both tank ice and ice interstitial water contain more silt‐sized particles than tank water suspension load and tank bottom sediment. Sand is reduced in the ice, and clay is about the same concentration in all samples. This points to preferential entrainment of fine particles in newly formed ice supported by Lc‐driven circulation. Comparable results of Lc‐supported ice particle entrainment were found in Lake Hattie. Comparison of ice sediment from tank experiments run with Kara Sea material to ice particles from the natural Kara setting showed both types of ice sediment have very similar grain size distributions and mineralogical compositions. Results from experiments and nature help to better understand the potentially Lc‐driven entrainment of sediment into ice formed in shallow freezing waters.

Phalaropes feeding at a coastal front in Santa Monica Bay, California

The spinning behavior often exhibited by phalaropes when feeding at freshwater sites is rarely observed at sea. Instead, phalaropes are typically observed slowly swimming forward while foraging on marine neuston concentrated in surface convergence zones. Small-scale coastal ocean fronts, eddies and internal waves capable of generating such convergences are extremely common, albeit ephemeral, features in the Southern California Bight. This region is marked by a complex flow regime, resultant in part from its variable coastal morphology. We used satellite data (AVHRR) and in situ measurements (CTD, surface drifters) to describe and track a coastal front in Santa Monica Bay, California, centrally located in the Southern California Bight. A high number of Red-necked Phalaropes ( Phalaropus lobatus) were associated with this feature over the course of several days. Neuston tows and gut content analyses revealed these phalaropes were primarily feeding on fish eggs and assorted debris that were abundant at the sea surface in this front. No phalaropes were observed spinning anywhere in the vicinity. Previously unpublished metabolic activity rates for phalaropes indicate that spinning is much more energetically expensive than is swimming at a comparable speed. Convergences associated with fronts (or eddies, internal waves, etc.) in the Southern California Bight apparently provide phalaropes with a rich, easily accessible and steady supply of food without having to resort to the energetically costly behavior of spinning.

Comparison between Global Latent Heat Flux Computed from Multisensor (SSM/I and AVHRR) and from In Situ Data

The accurate estimate of the latent heat flux (LHF) is important to understand better the coupling between the atmosphere and the ocean and their respective circulation. In the near future, the availability of satellite-derived datasets over long periods will allow us to perform studies that, so far, have only been possible with historic in situ datasets. Therefore, a natural issue to explore is how the computation derived from both data types agree on LHF estimates, Comprehensive Ocean-Atmosphere Data Set (COADS) on one hand and satellite-derived parameters on the other hand are input to a similarity theory-based model and treated in completely equivalent ways to compute global latent heat flux. In order to compute latent heat flux exclusively from satellite measurements, an empirical relationship (Q−W relationship) is used to compute the air mixing ratio from Special Sensor Microwave/Imager precipitable water W and a new one is derived to compute the air temperature also from retrieved W (T−W relationship). First analyses indicate that in situ and satellite LHF computations compare within 40%, but systematic errors increase the differences up to 100% in some regions. By investigating more closely the origin of the discrepancies, the spatial sampling of ship reports has been found to be an important source of error in the observed differences. When the number of in situ data records increases (more than 20 per month), the agreement is about 50 W m−2 rms (40 W m−2 rms for multiyear averages). Limitations of both empirical relationships and W retrieval errors strongly affect the LHF computation. Systematic LHF overestimation occurs in strong subsidence regions and LHF underestimation occurs within surface convergence zones and over oceanic upwelling areas. The analysis of time series of the different parameters in these regions confirms that systematic LHF discrepancies are negatively correlated with the differences between COADS and satellite-derived values of the air mixing ratio and air temperature. To reduce the systematic differences in satellite-derived LHF, a preliminary ship-satellite blending procedure has been developed for the air mixing ratio and air temperature. The T−W relationship is not used any more and the air temperature is computed by adding the 3-yr-averaged COADS air-sea temperature difference to the satellite SST maps. The method to get the air mixing ratio is based on a weighted combination of COADS and satellite values according to the number of COADS observations available. After the blending proem is applied, large improvements are observed in the Northern Hemisphere where both datasets are complementary. At midlatitudes, the blending procedure does not modify LHF values since satellite and COADS air mixing ratio do not differ by more than the expected satellite uncertainty (1 g kg−1). In the eastern and northern part of the basin, where the air mixing ratio difference is large and ship observations are numerous, blended LHF values are efficiently corrected toward in situ estimates compensating for the limitations of Q−W relationship. In the Southern Hemisphere, the number of in situ observations rarely exceeds four values per month, and without “ground truth,” more confidence is given to the satellite-derived values. Statistically, the rms difference drops to 28 W M−2 when 20 ship observations are available, which is likely a good approximation of the lowest bound of the blended LHF uncertainty compared with optimal in situ estimates. However, along the eastern boundaries in the southern oceans, local differences are expected to be much larger.

Static and dynamic response of natural multicomponent oceanic surface films to compression and dilation: Laboratory and field observations

The quasi‐static elasticity (Gibbs' elasticity) of sea surface films collected in North American coastal waters has been examined using a large number of surface pressure‐area isotherms. The films examined show considerable variability in elastic properties. Differences among films appear to be the result of chemical variability due to differences in source; they are also affected by dynamic physical processes (e.g., film compression). Elasticities of adsorbed films from subsurface and microlayer water are observed to exhibit a bimodal distribution at 0.5 mN m−1 reference spreading pressure; films from organized banded slicks are shown to have generally higher elastic moduli. Mechanisms of surface selectivity and competitive adsorption are suggested to explain this effect; results of film aging and work hardening studies are presented in support. Significant hysteresis effects are observed for cyclical compression and dilation of the films suggesting the prevalence of relaxation processes that lead to compositional changes and higher elastic moduli. Field measurements of capillary and ripple spectra are presented to infer information on wave damping and dynamic viscoelasticity. Results obtained in regions where surface convergence is absent imply damping enhancement over a broad frequency range consistent with theory for films of low elastic moduli. Films found in surface convergence zones, the result of surface upwelling, exhibit damping enhancements consistent with higher elastic moduli. Damping maxima are observed in multiple frequency bands; this banding is not readily explained with current theories. Results similar to these have been observed in laboratory studies with other chemical systems. In both the laboratory and field experiments, the tendency of the films to group into a few subsets with similar surface characteristics implies that natural films might be modeled by a small number of representative end members. This would facilitate remote sensing applications by providing a basis for determining the effects of films on surface waves under various conditions (e.g., diurnal variation, history of variability of wind stress).

Moisture and latent heat flux variabilities in the tropical Pacific derived from satellite data

A method of determining ocean‐atmosphere moisture and latent heat fluxes using spaceborne sensors is presented and evaluated. The satellite data used are the geophysical parameters: sea surface temperature, precipitable water, and surface wind speed, derived from observations of the microwave radiometer on the Nimbus 7 spacecraft from 1980 to 1983 in the tropical Pacific. The period included one of the most intense El Niño‐Southern Oscillation (ENSO) episodes of this century. The surface humidity is first derived from the precipitable water using a global empirical relation and is found to be in close agreement with in situ measurements in the ascending and the descending branches of the Hadley and Walker circulations during both ENSO and non‐ENSO periods. The latent heat flux is then computed with a bulk parameterization model. At monthly and 2° latitude by 2° longitude resolution, the random error of the computed latent heat flux is estimated to approximately 26 W/m2 with the flux variability in the order of 100 W/m2. In areas covered by surface convergence, the moisture flux is found to be low, with the temporal variability due mostly to the variability of wind speed. Elsewhere, the variability of sea‐air humidity difference plays a more important role. During the early phase of the 1982–1983 ENSO, a surface convergence center moves east leading the anomalous equatorial westerlies. At this center, the low wind and high humidity caused negative (low) latent heat flux anomalies despite anomalously high sea surface temperatures. Latent heat flux is found to play an important role in the seasonal cooling of the upper ocean except in areas covered by major surface convergence zones where the variability of insolation is expected to be large and in areas of ocean upwelling where ocean dynamics perturb the upper ocean heat balance. The latent heat flux from the general circulation model of the European Center of Medium Range Weather Forecast is found to be inadequate in capturing the temporal and spatial variations computed from the satellite data.