Plume Variability Research Articles

Variations in plume activity reveal the dynamics of water-filled faults on Enceladus

After discovering a jet activity near the south pole of Saturn’s moon Enceladus, the Cassini mission demonstrated the existence of a subsurface water ocean with a unique sampling opportunity through flybys. Diurnal variations in the observed brightness of the plume suggest a tidal control, although the existence and timing of two activity maxima seem to contradict stress analysis predictions. Here, we re-interpret the observed plume variability by combining a 3D global model of tidal deformation of the fractured ice shell with a 1D local model of transport processes within south-polar faults. Our model successfully predicts the observed plume’s temporal variability by combining two independent vapour transport mechanisms: slip-controlled jet flow and normal-stress-controlled ambient flow. Moreover, it provides a possible explanation for the differences between the vapour and solid emission rates during the diurnal cycle and the observed fractionation of the various icy particle families. Our model prediction could be tested by future JWST observations targeted when Enceladus is at different positions on its orbit and could be used to determine the optimal strategy for plume material sampling by future space missions.

Drivers of neritic water intrusions at the subtropical front along a narrow shelf

The near-ubiquitous presence of freshwater over the inner to mid-continental shelf off of the Otago Peninsula in southeast Aotearoa/New Zealand has been previously identified in long-term cross-shelf transects. Occasional influxes of this silicate-rich neritic water past the shelf break and beyond the Subtropical Front have been observed, potentially supporting elevated chlorophyll-a concentrations in SubAntarctic waters. Nearshore salinity variability has been connected to flow rates of the Clutha River, Aotearoa’s largest river by discharge, however, other physical mechanisms influencing the structure of freshwater along this coast have not been explicitly explored. A numerical model of Aotearoa’s southeast continental shelf forced with realistic discharge from the Clutha River is used to assess the drivers of river plume variability and identify conditions associated with the transport of neritic water beyond the shelf break. Event scale, lagged correlation, EOF, and composite analysis of a model passive dye tracer revealed three generalised plume structures that are dependent on river discharge, along-front wind stress, and shelf current velocities. Downfront wind stress suppresses bulge formation at the river mouth and constrains the river plume against the coast, whereas upfront wind stress causes the river plume to move offshore over the shelf and spread across the upper water column. Rarely, upfront wind stress generates a nearshore current reversal over the shelf, causing the plume to deflect straight off-shelf and across the shelf edge before meeting the frontal current and advecting to the northeast. Future observational work is required to validate the mechanisms described and address the biological implications of these findings.

CLASP: CLustering of Atmospheric Satellite Products and Its Applications in Feature Detection of Atmospheric Trace Gases

AbstractSatellite instruments have the most potential of capturing trace gas variability as they continually observe the atmosphere and its composition over wide regions. Yet the increasingly large data size of satellite products poses a challenge for their use as traditional data processing methods (e.g., averaging) may not be effective to extract the spatiotemporal variability without prior knowledge of an emission source's spatial and temporal behavior, such as location, time, and plume shape. Here, an agile clustering algorithm entitled CLustering of Atmospheric Satellite Products (CLASP) is presented to identify the spatiotemporal variability of trace gases captured in satellite observations. We find the knowledge discovery method for large data sets, clustering, is suited for identifying the variability of trace gases in satellite observations, as such CLASP is rooted in density‐based clustering methods. CLASP detects features from satellite observations and identifies their spatial, magnitude, and temporal axis leading to a better understanding of the spatiotemporal variability of atmospheric trace gases. To test the applicability of CLASP, the algorithm is applied to TROPOspheric Monitoring Instrument NO2 observations illustrating some of its different capabilities. Implementing CLASP for event identification, capturing plume variability, and source detection, CLASP identified wildfires, observed disruptions from COVID‐19 lockdown restrictions, and detected irregular emissions from oil and gas operations.

Satellite assessment of coastal plume variability and its relation to environmental variables in the Sofala Bank

Monthly composites of remote sensing reflectance at 555 nm wavelength (Rrs555) from ocean color imagery of the MODIS sensor onboard the Aqua platform were used to characterize the spatial and temporal variability of coastal plume in the Sofala Bank and its relation to river discharge, local rainfall, and wind speed. To achieve the objective, maps of monthly composites of Rrs555 over the Sofala Bank were inspected and statistical analysis was performed, including correlation, analysis of variance, and wavelet coherence between environmental variables and both plume area and Rrs555. Climatology of Rrs555 revealed that both plume dispersion and Rrs555 values are higher during June to December and lower during January to May. A positive correlation (r = 0.77) between wind speed and monthly time series of Rrs555, and a negative correlation between the Zambezi river discharge (r = −0.21) and rainfall (r = −0.67) with Rrs555 were found. These results suggest that variation of suspended matter in the Sofala Bank is mainly controlled by erosion and re-suspension by winds rather than the input of terrigenous matter by the Zambezi River discharge and rainfall, assuming that Rrs555 can be a valid proxy for the inorganic suspended matter. The southern portion of the Sofala Bank (i.e., near the mouths of the Pungue and Buzi Rivers) presented higher values of Rrs555 if compared to the center region near Zambezi river mouth and the northern region near Licungo river mouth. The higher Rrs555 values in the southern region might be associated with higher re-suspension rates due to increased tide mixing, dredging activities, and the shallower nature of bathymetry in the southern region. The dominance of wind in controlling the variability of suspended sediments and the eventual relatively greater contribution of Pungue and Buzi River than the Zambezi in supplying sediments could represent an evidence of weakening of Zambezi River supply of sediments, a process that might have started after damming the Zambezi Catchment.

Spatio-temporal variability of turbid freshwater plumes in the Inner Sea of Chiloé, northern Patagonia

Northern Patagonia is characterized by multiple rivers that discharge considerable amounts of freshwater into the coastal ocean, forming large river plumes that influence hydrographic and ecological processes. In this study, we use satellite ocean color data from MODIS (Moderate Resolution Imaging Spectroradiometer) to characterize the seasonal and interannual variability of turbid freshwater plumes in the inner waters of northern Patagonia for the first time, with a focus on the connections to river discharge and large-scale climatic variability. The turbidity signal from the surface reflectance product centered at 645 nm, Rrs(645), correlates well with peaks in river discharge data and surface salinity minima from boat-based profiles and a surface buoy, validating its use for the identification of turbid river plumes in the region. The seasonal climatology of Rrs(645) showed the presence of large river plumes throughout the year, with variability associated mainly to that of river discharge. Analysis of Rrs(645) fields under low and high discharge conditions allowed for the identification of a threshold value to delineate plume fronts and determine the probability of plume occurrence. EOF analysis reveals the dominant modes of plume variability, associated to turbidity differences between the coastal margin and deeper waters and to in-phase variability of large river plumes throughout the study area. The largest plume event occurred in year 2008, during negative (cold) phases of ENSO (La Niña) and PDO but a positive phase of SAM. The severe drought event of 2016 that occurred during positives phase of ENSO (El Niño), PDO and SAM is well captured by the EOF analysis and is characterized by large negative anomalies in the Rrs(645) signal. In general, complex co-variations are observed between monthly anomalies of Rrs(645) and the ENSO, PDO and SAM indices, highlighting the heterogeneity of climatic regulation in the region.

Remote Sensing of Landslide-Generated Sediment Plumes, Peace River, British Columbia

Quantifying the contribution of sediment delivered to rivers by landslides is needed to assess a river’s sediment load in regions prone to mass wasting. Monitoring such events, however, remains difficult. This study utilised six years of remotely sensed imagery (PlanetScope and RapidEye, Imagery courtesy of Planet Labs, Inc., San Francisco, CA, USA), topographic surveys, and field observation to examine a hydro-geologically controlled, retrogressive landslide near a tributary to the Peace River, British Columbia. The slide has been active since 2014, delivering large amounts of sediment to the Peace River, visible in a persistent plume. Here, we quantify the landslide’s sediment contribution to the Peace River, assess the hydro-meteorological drivers of plume variability, and test whether plume activity can be directly linked to landslide activity for monitoring purposes. Our results show that the landslide on average delivered 165,000 tonnes of sediment per year, a seven-fold increase of the tributary’s regular load and near half of the Peace River’s load at this location. Due to continuous erosion of landslide material, sediment supply is steady and fuelled by repeated failures. Using thresholding, the identification of ‘high’ plume activity was possible, which positively correlated with the water level in a nearby reservoir, a proxy for the state of groundwater in this region. We reason that ‘high’ plume activity is linked to increased groundwater pressure because landslide activity is groundwater-controlled and failures fuel sediment delivery to the Peace River. Using readily available imagery, it is thus possible to monitor the activity of this recurrent landslide when field data are difficult to obtain.

Dynamics of river plumes in the South Brazilian Bight and South Brazil

The plumes from the rivers of the South Brazilian Bight (SBB) and South Brazil (SB) were studied using a realistic model configuration. River plume variability on continental shelves is driven by the input of river runoff into the shelf, by wind variability, and also by ambient currents and its seasonal variability, especially the Brazil Current, which are realistically modelled in this study. It is presented a simulation of 4 years using a nested configuration, which allows resolving the region around Florianópolis with very high resolution (∼150 m). The dispersion of river plumes was assessed not only with the hydrodynamical model results but also by using passive tracers whose dynamics was analyzed seasonally. Several dyes were released together with the river discharges. This approach allowed calculating the depths of the riverine freshwater, and the resulting regions affected by the plumes. Northward intrusions of waters from the southern region, under the potential influence of the distant La Plata river plume, were evaluated with a Lagrangian approach. The local river plumes are confined to the inner shelf, except south of 30°S where discharges from Lagoa dos Patos disperse over the shelf in the spring and summer. The Brazil Current flowing southward over the slope prevents the river plumes from interaction with oceanic mesoscale dynamics. The river plumes are, thus, mainly controlled by the wind forcing. The plumes from SBB are able to disperse until SB following the southward wind regime typical of the summer. And both the SB and La Plata river plumes are also able to reach SBB, forced by the northward wind typical of the winter season, until the latitude of 25.5°S. A low salinity belt (below 35) is present along the coastal region of SB and SBB year-round, supported by contributions from both the large and small rivers. The interaction between the different plumes influences the dispersion patterns, shielding the Florianṕolis coastal region from plumes of distant rivers, and dispersing the plume of SBB rivers away from Santa Catarina Island as it disperses southward during the summer months.

Fluvial and oceanographic influences on suspended sediment dispersal in the Magdalena River Estuary

The Magdalena River is the main contributor of fresh water and sediment to the Caribbean Sea. Hydrological inputs from the Magdalena River are 10,287 and 4068 m3 s−1 at high and low flows, respectively. Sediment delivery from the Magdalena River is among the largest in the world, with 142 × 106 t yr−1 of transported suspended matter. We used MODIS images to develop a regional model and assess the spatial and temporal variability of suspended sediment concentration at the mouth of the Magdalena River, map the extent of the turbidity plume using its morphometric properties, and identify the main fluvial and oceanographic factors associated with plume variability. We analyzed monthly average suspended sediment concentration data for the period 2003–2017. MODIS images were calibrated with in situ measurements of suspended sediment concentrations in surface waters of the study area. A finite mixing model was used to identify turbidity thresholds and define areas within the plume. Calculated suspended sediment concentrations indicate high turbidity in the Magdalena River mouth (178.6 ± 78.7 mg L−1), values that are of the same order of magnitude as those measured in the Yangtze, Huang He and Amazon River mouths. Calculated plume areas (diffuse, solid and mixing) and their shapes suggest that the Magdalena River plume has a limited area, related to conditions such as wind speed and direction, streamflow, and sediment transport. Calculated areas and shapes demonstrate the ability to generate a near-field area, which indicates the presence of a clear convergence front. The data also emphasize the importance of sediment transport from the Magdalena River to the Caribbean Sea. Such fluvial transport is important in the biogeochemistry of estuarine systems, especially in turbid estuaries, where there is marked variability in the distribution of sediment in surface waters.

Riverine freshwater plume variability in the Bay of Bengal using wind sensitivity experiments

The role of winds in determining the dispersal pattern of the freshwater plume in the Bay of Bengal (BoB) is investigated using a high-resolution regional ocean model with realistic coastline, bathymetry, and river discharge data. Multiple experiments are carried out with idealized winds of different directions and speeds representing the seasonal wind patterns in the BoB. Monthly climatology of freshwater discharge from the major rivers in the domain is included at the geographic locations. The freshwater plume is channelled along the coast in cases of southeasterly and northeasterly winds but is forced towards the central BoB under southwesterly winds. The variations in equivalent depth of freshwater suggest that the high winds tend to confine the low-saline water in the northern and northeastern parts of the BoB. The momentum analysis shows that geostrophic balance dominates throughout the vertical section in the plume. The vertical viscosity also plays an important role in the upper ocean in response to the varying wind directions. The high wind condition in the bay confines the freshwater in its northernmost parts with equivalent depth >8 m. An increase in wind speed from 2 to 8 m s−1 results in the enhancement of southward meridional surface salt advective flux (SSAF) from 2 to 10 kg m−2 s−1 along the western boundary of the BoB. Thus, the distinct roles of wind speed and direction on the characteristics of the freshwater plume in the BoB are demonstrated using idealized numerical experiments.

Variability and Mixing of the Filchner Overflow Plume on the Continental Slope, Weddell Sea

AbstractA large fraction of Antarctic Bottom Water is produced in the Weddell Sea, through mixing between the cold and dense shelf water masses and the warm and saline off-shelf water. We present observations of the dense Filchner overflow plume from one mooring at the Filchner sill and two moorings located downstream, on the continental slope. The plume variability over the continental slope at a monthly time scale is related to upstream conditions at the Filchner sill, with a high correlation in density. Revised column-integrated volume transport calculations across the Filchner sill indicate 50% higher values in 2010 compared with the earlier estimates available from 1985. Over the continental slope, the plume thickness fluctuates strongly between less than 25 m and more than 250 m. Observations of elevated temperature variance and high Froude numbers at the plume interface imply high mixing rates and entrainment of ambient water masses. The mixing events typically coincide with shear spikes across the plume. The shear spikes appear quasi-periodically, when counterrotating oscillations with periods of 24 and 72 h align. The clockwise 24-h oscillation is related to diurnal, barotropic tidal currents and topographic vorticity waves, whereas the counterclockwise 72-h oscillation is related to vortex stretching or topographic vorticity waves.

Mississippi River Plume Variability in the Gulf of Mexico From SMAP and MODIS‐Aqua Observations

AbstractThe Mississippi‐Atchafalaya River System (MARS) is an important source of freshwater and terrigenous material to the Gulf of Mexico. We used sea surface salinity (SSS) measurements from the Soil Moisture Active Passive (SMAP) satellite and terrigenous dissolved organic carbon (tDOC) from Moderate Resolution Imaging Spectroradiometer (MODIS) to describe the MARS plume seasonality and interaction with the Gulf circulation. Our analyses revealed good agreement between SMAP and in situ salinity for S > 31, and that satellite‐derived tDOC can serve as a useful tracer for the river plume. A boundary detection algorithm was used to quantify the frequency of plume occurrence in the Gulf for the last 15 years. The MARS plume has its maximum offshore extension during summer, retracting to closer to shore during fall/winter. Variability at monthly time scales is controlled by river discharge and alongshore winds, which explain up to 60% of the local variance in the frequency of plume occurrence near the shelfbreak. Maximum variability in plume extension is found to the east of the Mississippi River Delta due to wind reversals that generally occur during summer. The combined use of SMAP salinity and MODIS‐tDOC allowed for a detailed view of anomalous events in 2015 and 2016, when the MARS plume was transported for 350 km from the coast due to interactions with the Loop Current and with dipole eddies. While SMAP observations allow for tracking the river plume even in cloudy conditions, high‐resolution MODIS data allow for a more accurate characterization of sharp fronts and of the width of river‐influenced filaments.

Study of Atrato river plume in a tropical estuary: Effects of the wind and tidal regime on the Gulf of Uraba, Colombia

Este estudio se enfoca en la evaluación de la importancia de los diferentes forzamientos sobre la pluma del río Atrato y su propagación en un estuario localizado en el golfo de Uraba en el Caribe Colombiano. Se realizaron seis campañas de medición entre los años 2004 y 2007 para identificar las principales características de esta pluma y para la calibración y validación del modelo numérico. Datos de campo y resultados de un modelo numérico muestran que existe una alta variabilidad espacial y temporal en la pluma de acuerdo a la magnitud de la descarga del río, el ciclo de marea y el esfuerzo cortante inducido por el viento. La descarga del río Atrato se dispersa sobre una porción considerable del golfo de Uraba pero ocupa solamente una capa superficial de alrededor de 2 m. La marea modifica la posición de la pluma a lo largo del día, induciendo una migración horizontal en el frente salino. Se identificaron dos modos de circulación en el golfo de Uraba bajo diferentes condiciones de viento.

A comprehensive analysis of ion cyclotron waves in the equatorial magnetosphere of Saturn

We present a comprehensive analysis of ion cyclotron waves in the equatorial magnetosphere of Saturn, considering all magnetic field data collected during the Cassini era (totaling to over 4 years of data from the equatorial plane). This dataset includes eight targeted flybys of Enceladus, three targeted flybys of Dione, and three targeted flybys of Rhea. Because all remaining orbits of Cassini are high-inclination, our study provides the complete map of ion cyclotron waves in Saturn's equatorial magnetosphere during the Cassini era. We provide catalogs of the radial and longitudinal dependencies of the occurrence rate and amplitude of the ion cyclotron fundamental and first harmonic wave modes. The fundamental wave mode is omnipresent between the orbits of Enceladus and Dione and evenly distributed across all Local Times. The occurrence rate of the fundamental mode displays a Fermi–Dirac-like profile with respect to radial distance from Saturn. Detection of the first harmonic mode is a rare event occurring in only 0.49% of measurements taken and always in conjunction with the fundamental mode. We also search for a dependency of the ion cyclotron wave field on the orbital positions of the icy moons Enceladus, Dione, and Rhea. On magnetospheric length scales, the wave field is independent of the moons' orbital positions. For Enceladus, we analyze wave amplitude profiles of seven close flybys (E9, E12, E13, E14, E17, E18, and E19), which occurred during the studied trajectory segments, to look for any local effects of Enceladan plume variability on the wave field. We find that even in the close vicinity of Enceladus, the wave amplitudes display no discernible dependency on Enceladus' angular distance to its orbital apocenter. Thus, the correlation between plume activity and angular distance to apocenter proposed by Hedman et al. (2013) does not leave a clearly distinguishable imprint in the ion cyclotron wave field.

Pre-melt-season sediment plume variability at Jökulsárlón, Iceland, a preliminary evaluation using in-situ spectroradiometry and satellite imagery

ABSTRACTHigh-latitude atmospheric warming is impacting freshwater cycling, requiring techniques for monitoring the hydrology of sparsely-gauged regions. The submarine runoff of tidewater glaciers presents a particular challenge. We evaluate the utility of Moderate Resolution Imaging Spectroradiometer (MODIS) imagery for monitoring turbid meltwater plume variability in the glacier lagoon Jökulsárlón, Iceland, for a short interval before the onset of the main melt season. Total Suspended Solids concentrations (TSS) of surface waters are related to remotely-sensed reflectance via empirical calibration between in-situ-sampled TSS and reflectance in a MODIS band 1-equivalent wavelength window. This study differs from previous ones in its application to an overturning tidewater glacier plume, rather than one derived from river runoff. The linear calibration improves on previous studies by facilitating a wider range of plume metrics than areal extent, notably pixel-by-pixel TSS values. Increasing values of minimum plume TSS over the study interval credibly represent rising overall turbidity in the lagoon as melting accumulates. Plume extent responds principally to consistently-strong offshore winds. Further work is required to determine the temporal persistence of the calibration, but remote plume observation holds promise for monitoring hydrological outputs from ungauged or ungaugeable systems.

Time‐series measurements of bubble plume variability and water column methane distribution above Southern Hydrate Ridge, Oregon

AbstractAn estimated 500–2500 gigatons of methane carbon is sequestered in gas hydrate at continental margins and some of these deposits are associated with overlying methane seeps. To constrain the impact that seeps have on methane concentrations in overlying ocean waters and to characterize the bubble plumes that transport methane vertically into the ocean, water samples and time‐series acoustic images were collected above Southern Hydrate Ridge (SHR), a well‐studied hydrate‐bearing seep site ∼90 km west of Newport, Oregon. These data were coregistered with robotic vehicle observations to determine the origin of the seeps, the plume rise heights above the seafloor, and the temporal variability in bubble emissions. Results show that the locations of seep activity and bubble release remained unchanged over the 3 year time‐series investigation, however, the magnitude of gas release was highly variable on hourly time scales. Bubble plumes were detected to depths of 320–620 m below sea level (mbsl), in several cases exceeding the upper limit of hydrate stability by ∼190 m. For the first time, sustained gas release was imaged at the Pinnacle site and in‐between the Pinnacle and the Summit area of venting, indicating that the subseafloor transport of fluid and gas is not restricted to the Summit at SHR, requiring a revision of fluid‐flow models. Dissolved methane concentrations above background levels from 100 to 300 mbsl are consistent with long‐term seep gas transport into the upper water column, which may lead to the build‐up of seep‐derived carbon in regional subsurface waters and to increases in associated biological activity.

Controlled boiling on Enceladus. 1. Model of the vapor-driven jets

Plumes of water vapor and ice particles have been observed from the so-called tiger stripes at the south polar terrain (SPT) of Saturn’s satellite, Enceladus. The observed high salinity (∼0.5–2%) of the ice particles in the plumes may indicate that the plumes originate from a subsurface liquid ocean. Additionally, the SPT is the source of strong infrared radiation (∼4.2 GW), which is especially intense near (within tens of meters) the tiger stripes. This could indicate that the radiation is associated with plume activity, but the connection remains unclear. Here we investigate the constraints that plume observations place on the widths of the cracks, the depth to the liquid-vapor interface, and the mechanisms controlling plume variability. We solve the fluid dynamics of the flow in the crack and the interaction between the flow and ice walls assuming that the flows of water vapor and ice particles originate from a few kilometers deep liquid ocean. For a crack with a uniform width, we find that our model could explain the observed vapor mass flow rate of the plumes when the crack width is 0.05–0.075 m. A wider crack is not favorable because it would produce a higher vapor mass flow rate than the observed value, but it may be allowed if there are some flows that do not reach the surface of Enceladus either due to condensation on the icy walls or the tortuosity of the crack. The observed heat flow can be explained if the total crack length is approximately 1.7 × 500 km. A tapering crack (a crack which is ∼1 m wide at the bottom of the flow and sharply becomes 0.05–0.075 m at shallower depths) can also explain the observed vapor mass flow rate and heat flow. Widths of 1 m or more are necessary to avoid freezing at the liquid-vapor interface, as shown in our paired paper (Ingersoll and Nakajima [2016] Icarus). The observed intense heat flow along the tiger stripes can be explained by the latent heat release due to vapor condensation onto the ice walls near the surface. The resulting buildup of ice causes the vents to seal themselves on time scales less than a year. We also find that the ice to vapor ratio of the plumes is sensitive to the ice mass fraction at the bottom of the flow (liquid–vapor interface). We find that the total mass flow rate of the plumes becomes larger when the crack width is larger, which is consistent with the observation that the flow rate increases near the orbital apocenter, where the crack is expected to be widest.

Ammonia and methane dairy emission plumes in the San Joaquin Valley of California from individual feedlot to regional scales

AbstractAgricultural ammonia (NH3) emissions are highly uncertain, with high spatiotemporal variability and a lack of widespread in situ measurements. Regional NH3 emission estimates using mass balance or emission ratio approaches are uncertain due to variable NH3 sources and sinks as well as unknown plume correlations with other dairy source tracers. We characterize the spatial distributions of NH3 and methane (CH4) dairy plumes using in situ surface and airborne measurements in the Tulare dairy feedlot region of the San Joaquin Valley, California, during the NASA Deriving Information on Surface conditions from Column and Vertically Resolved Observations Relevant to Air Quality 2013 field campaign. Surface NH3 and CH4 mixing ratios exhibit large variability with maxima localized downwind of individual dairy feedlots. The geometric mean NH3:CH4 enhancement ratio derived from surface measurements is 0.15 ± 0.03 ppmv ppmv−1. Individual dairy feedlots with spatially distinct NH3 and CH4 source pathways led to statistically significant correlations between NH3 and CH4 in 68% of the 69 downwind plumes sampled. At longer sampling distances, the NH3:CH4 enhancement ratio decreases 20–30%, suggesting the potential for NH3 deposition as a loss term for plumes within a few kilometers downwind of feedlots. Aircraft boundary layer transect measurements directly above surface mobile measurements in the dairy region show comparable gradients and geometric mean enhancement ratios within measurement uncertainties, even when including NH3 partitioning to submicron particles. Individual NH3 and CH4 plumes sampled at close proximity where losses are minimal are not necessarily correlated due to lack of mixing and distinct source pathways. Our analyses have important implications for constraining NH3 sink and plume variability influences on regional NH3 emission estimates and for improving NH3 emission inventory spatial allocations.

Interannual Caribbean salinity in satellite data and model simulations

AbstractAquarius sea surface salinity (SSS) reveals the presence of interannual variations in the Caribbean with about 0.5 psu change between salty and fresh events, which propagate westward across that basin at an average speed of 11 cm/s and are preceded by corresponding SSS anomalies east of the Lesser Antilles. These upstream SSS anomalies are produced by interannual changes in the Amazon plume. Their presence is verified using in situ measurements from the northwest tropical Atlantic station. In contrast to SSS, which displays westward propagation, SST changes almost immediately across the Caribbean, suggesting large‐scale atmospheric processes have a primary role in regulating interannual SST in contrast to SSS. A global 1/10° mesoscale ocean model is used to quantify possible origination mechanisms of the Caribbean salinity anomalies and their fate. Simulations confirm that they are produced by anomalous horizontal salt advection, which conveys these salinity anomalies from an area east of the Lesser Antilles across the Caribbean. Anomalous horizontal advection is dominated by mean currents acting on anomalous salinity. The model suggests that interannual Caribbean salinity anomalies eventually enter the Florida Current and reach the Gulf Stream 6–12 months after crossing the central Caribbean. Previous studies link the origin of salinity anomalies in the Amazon plume to variations in the annual freshwater discharge from the continent. In this model interannual discharge variations are absent while simulated SSS variability is in line with observations. This suggests that interannually forced ocean dynamics plays a key role in river plume variability and its spatial dispersion.

Water-air CO2 fluxes in the Tagus estuary plume (Portugal) during two distinct winter episodes.

BackgroundEstuarine plumes are frequently under strong influence of land-derived inputs of organic matter. These plumes have characteristic physical and chemical conditions, and their morphology and extent in the coastal area depends strongly on physical conditions such as river discharge, tides and wind action. In this work we investigate the physical dynamics of the Tagus estuary plume and the CO2 system response during two contrasting hydrological winter periods. A hydrodynamic model was used to simulate the circulation regime of the study area, thus providing relevant information on hydrodynamic processes controlling the plume.ResultsModel simulations show that for the studied periods, the major cause of the plume variability (size and shape) was the interaction between Tagus River discharge and wind. The freshwater intrusion on Tagus shelf exerted considerable influence on biochemical dynamics, allowing identification of two regions: a high nutrient region enriched in CO2 inside the estuarine plume and another warmer region rich in phytoplankton in the outer plume.ConclusionsThe Tagus estuarine plume behaved as a weak source of CO2 to the atmosphere, with estimated fluxes of 3.5 ± 3.7 and 27.0 ± 3.8 mmol C m−2 d−1 for February 2004 and March 2001, respectively.

On the role of wind and tide in generating variability of Pearl River plume during summer in a coupled wide estuary and shelf system

A numerical simulation of the buoyant river plume over the Pearl River Estuary (PRE) and adjacent shelf during a typical upwelling favorable wind period of the summer monsoon is utilized to explore the responses of the plume to wind and tide forcing. The model is forced with time-dependent river discharge, wind and tide, and it shows reasonable ability to capture the basic structure and responses of the plume. Additional numerical experiments that are forced without either wind or tide are used to evaluate the relative importance of wind and tide in generating plume variability. Results show that the vertical structure of the plume and the strength of the stratification in the estuary are determined by the combination of the buoyancy forcing associated with river discharge and tidal forcing, and vary with the advection process, while the horizontal shape and spreading of the plume over the shelf are highly influenced by the wind-driven coastal current, and are more susceptible to the change of vertical mixing. Mechanical energy analysis in each dynamical region (upper, middle, lower estuary, and shelf) reveals that this is because the system mainly gains energy from tide (wind) in the estuary (shelf), and loses energy to the bottom friction (internal-shear mixing) in the estuary (shelf). The largest forcing and dissipation terms in the middle PRE, and at the entrances of smaller estuaries such as Huang Mao Hai, are due to tidal forcing, which enables the middle PRE to serve dynamically as the entrance of an estuary, where the transition of the river plume into coastal buoyancy current usually takes place. In addition, the mixing efficiency increases from upper PRE to the shelf and from strong to weak mixing period, thus the plume in the well-mixed upper estuary is not as sensitive to the changes of wind and tide as that over the highly stratified shelf.