Freshwater Plume Research Articles

High and medium resolution ocean models for the Great Barrier Reef

This article describes 3D medium (4km) and high (500m) horizontal resolution ocean models developed for the Coral Sea with a particular focus on the Great Barrier Reef lagoon. The medium resolution model is underpinning a 10 year reanalysis and a forecast demonstrator system for maritime monitoring. The ocean models have been developed to assist the management of the Great Barrier Reef given the existence of a number of stressors acting on the reef. The presented models aim to represent physical stressors that include thermal, salinity and flooding rivers. The river discharge is tracked by passive and age tracers. They can provide concentrations to identify the area/volume influenced by flood water. This paper gives an overview of the forward non-linear models with the aim to assess their performance.The models have been evaluated over a two year period spanning January 2009–March 2011 including the 2010/2011 La Nina and the advent of Tropical Cyclone Yasi in early 2011 that contributed to one of the largest river discharge events recorded over northern Queensland. The models’ performance is investigated on a full domain scale via means of satellite sea surface temperature (SST), expendable bathythermograph (XBT) and drifter data. Over the shelf and the Great Barrier Reef lagoon, available tide gauges, Ocean Colour, Chlorophyll-a and Queensland’s Integrated Marine Observing System (QIMOS) mooring data of temperature, salinity and velocity have been used to assess the models. Results suggest that the high resolution model gives better statistics in terms of mean absolute deviation and bias compared to the medium resolution model for subsurface temperature and salinity over the deep ocean. Moreover, sea surface temperature performance is similar for both models. Over the lagoon and shelf both models’ performance is mixed. The high resolution model displays a consistent warm bias on the order of 0.3 °C over the shallow well mixed stations throughout the year. However, the 4km model displays larger variability in this region. It tends to be too warm in summer and too cool in winter when compared to moorings. Modelled velocities are assessed via means of the QIMOS moorings. Overall the high resolution model fares better in terms of current direction with an average veering of 13° vs. 22°. The models’ performance is comparable in terms of current speed over the shallow moorings. However, the high resolution model is better equipped to represent steeper shelves and this results at times in better performance for velocity and temperature, particularly over Palm Passage.A large focus has been placed on including and assessing passive tracer dynamics emanating from river discharge. 25 river sources have been added across the models domain together with a set of one-dimensional channels representing the volume flux of brine. Two case studies are presented to demonstrate the models’ ability to reproduce mean features of observed plumes. A major river discharge event has been examined in further detail. Both models are able to reproduce the low salinity signal present in the observations in the upper level to a satisfactory degree. The 4km model matches the observed strength and duration of the fresh water plume substantially better than the high resolution model in this case.

Spatio-temporal variability and sources of volatile halocarbons in the South Yellow Sea and the East China Sea

Abstract Concentrations of volatile halocarbons (VHCs) in surface seawater of the South Yellow Sea and the East China Sea were measured during the summer and winter of 2011. The concentration maxima of CHCl3, C2HCl3, and C2Cl4 were observed in winter, and elevated concentrations of CHBr2Cl and CHBr3 were found in summer. The distributions of VHCs markedly decreased along the freshwater plume from river mouths (such as the Yangtze River estuary) to the open sea. Additionally, the effects of salinity, chlorophyll a, phytoplankton, bacterial abundance, and nutrients on the sources of VHCs were examined. The data showed evidence of spatiotemporal variability and association between the sources of these VHCs and the Yangtze River runoff and biological activity, as well as terrestrial input. The estimated fluxes indicated that the study area acted as sources of VHCs in the atmosphere during the study period.

Coupling of Estuarine Circulations in a Network of Fjords

AbstractThe Kitimat Fjord System in the northern British Columbia coast features a network of deep channels, islands, and shallow sills. In particular, the fresh water leaving Gardner Canal bifurcates at a triple junction: (1) north through a shallow sill (100‐m deep) to Devastation Channel and then out through Douglas Channel and (2) west along Verney Passage then passing over a shallow sill (35 m) to Wright Sound. The partitioning of the fresh water leaving Gardner Canal has substantial implications for interpreting estuarine circulation and water exchange of the system. In this study, the partitioning of the fresh water at the triple junction is quantified by using observations and a circulation model. The model is based on FVCOM with ~200‐m horizontal resolution in the narrow channels and forced by tides, winds, and river discharges. The remotely sensed freshwater plume, surface drifter trajectories, and temperature profiles suggest that the fresh water from Gardner Canal contributes to the estuarine circulation in the Douglas Channel. A passive tracer evolving with the advection and diffusion processes in the circulation model is used as a proxy to quantify the freshwater transport. The results show that the majority of low salinity water leaving Gardner Canal at the triple junction flows northward into the Douglas Channel through the Devastation Channel and the synoptic variability of the freshwater partition is modulated by strong wind events. A synthesis of various circulations in the abovementioned connecting channels around the Hawkesbury Island indicates a net anticyclonic circulation.

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.

Skill assessment of global, regional, and coastal circulation forecast models: evaluating the benefits of dynamical downscaling in IBI (Iberia–Biscay–Ireland) surface waters

Abstract. In this work, a multi-parameter inter-comparison of diverse ocean forecast models was conducted at the sea surface ranging from global to local scales in a two-phase stepwise strategy. Firstly, a comparison of CMEMS GLOBAL and the nested CMEMS IBI regional system was performed against satellite-derived and in situ observations. Results highlighted the overall benefits of both the GLOBAL direct data assimilation in open water and the increased horizontal resolution of IBI in coastal areas. Besides, IBI (Iberia–Biscay–Ireland) proved to capture shelf dynamics by better representing the horizontal extent and strength of a river freshwater plume, according to the results derived from the validation against in situ observations from a buoy moored in NW Spain. Secondly, a multi-model inter-comparison exercise for 2017 was performed in the Strait of Gibraltar among GLOBAL, IBI, and SAMPA (Sánchez-Garrido et al., 2013) high-resolution coastal forecast systems (partially nested to IBI) in order to elucidate the accuracy of each system to characterize the Atlantic Jet (AJ) inflow dynamics. A quantitative validation against hourly currents from high-frequency radar (HFR) highlighted both the steady improvement in AJ representation in terms of speed and direction when zooming from global to coastal scales through a multi-nesting model approach and also the relevance of a variety of factors at local scale such as a refined horizontal resolution, a tailored bathymetry, and a higher spatio-temporal resolution of the atmospheric forcing. The ability of each model to reproduce a 2 d quasi-permanent full reversal of the AJ surface inflow was examined in terms of wind-induced circulation patterns. SAMPA appeared to better reproduce the reversal events detected with HFR estimations, demonstrating the added value of imposing accurate meteorologically driven barotropic velocities in the open boundaries (imported from the NIVMAR (Álvarez-Fanjul et al., 2001) storm surge model) to take into account the remote effect of the atmospheric forcing over the entire Mediterranean basin, which was only partially included in IBI and GLOBAL systems. Finally, SAMPA coastal model outputs were also qualitatively analysed in the western Alboran Sea to put in a broader perspective the context of the onset, development, and end of such flow reversal episodes.

Effect of freshwater advection and winds on the vertical structure of chlorophyll in the northern Bay of Bengal

The northern Bay of Bengal receives a large amount of fresh water through river runoff and rainfall during the Indian summer monsoon (June–September). This fresh water spreads offshore and can modulate the upper-ocean chlorophyll via two processes: advection of river nutrients and inhibition of vertical supply of subsurface nutrients by increasing the stratification. We address these two processes during the summer monsoon of 2012 using hydrographic observations and a coupled physical-biogeochemical model. The observations show an increase in surface chlorophyll associated with the advection of a freshwater plume on the shelf. Near the slope, but over a cross-shore distance of ~70 km, a thin layer separates this elevated surface chlorophyll from the subsurface chlorophyll maximum layer (SCML). This separation disappears as the freshwater plume reaches the open ocean. The SCML shoals toward the coast and is absent on the shelf. Time-series observations from the open ocean also show an increase in chlorophyll in concurrence with the arrival of a freshwater plume. Simulations with a coupled physical-biogeochemical model show, however, that it is the wind-induced vertical processes that cause the increase in chlorophyll in the open ocean and not the nutrients brought by the horizontal advection of freshwater plumes. The increase in stratification only limits and does not completely inhibit the vertical supply of nutrients from the subsurface layers.

Factors underlying changes in salinity around the southeastern Antilles

Abstract A plume of fresh water, originating from the Amazon and Orinoco Rivers, infiltrates the southeastern Caribbean. Here, some of factors underlying changes of near-surface salinity are investigated using ocean and atmosphere reanalyses that include winds, currents, sea temperature, salinity, rainfall, and river basin run-off during pre-season (April–June) and in-season (July–September) periods 1980–2016. Composite results demonstrate that increased northeasterly trade winds and an accelerated North Brazil Current feed Amazon and Orinoco River water northwestward into the Caribbean, underpinning a 0.5C warming of sea temperature and an increase in the frequency and intensity of passing hurricanes. This chain of events is partially related to the Atlantic meridional mode.

The development and evolution of the Burdekin River estuary freshwater plume during Cyclone Debbie (2017)

This paper investigates the plume morphology and dynamics prior to and after the landfall of Cyclone Debbie (2017). The heavy rainfall and flooding produced a large buoyant coastal current, which moved southward after the cyclone landfall then advected northward with the prevailing southerly wind. The plume is simulated using the eReef GBR1 1-km model and a passive tracer is used to identify the plume behaviour. Based on the concentration of river tracers from the Burdekin River, the plume had propagated over 100 km to the north 23 days after the cyclone landfall.Based on an Empirical Orthogonal Function (EOF) analysis, the cyclone-induced river discharge is found to be the major cause for surface salinity variation with a lag time equal to 17 h. The response of surface plume area to the river flow is about 40 h lag with correlation coefficient = 0.68 and according to the wind strength index, the plume was controlled by buoyancy on c1 but could be controlled by wind in the far field. Wind also plays a very important role. Under weak downwelling wind forcing lower than 0.1 Pa, the plume thickness is sensitive to river discharge and tides. With stronger downwelling wind forcing larger than 0.1 Pa vertical mixing is generated, the plume is restricted to the coast, and high river discharge can only affect the thickness of the plume, but not its width. After Cyclone Debbie made landfall, upwelling winds developed, and the freshwater plume reversed direction from northward to southward most likely due to the influence of the northerly wind as well as the ambient current and topography.

Light availability for reef-building organisms in a plume-influenced shelf

The existence of a reef system composed by crustose coralline algae and scleractinian corals, under the influence of the world's largest freshwater plume, led us to investigate how much light is available for reef-building organisms at the bottom of the Amazon shelf. We analyzed ocean color data from MODIS Aqua to assess the plume dynamics and estimate the benthic light availability over the Amazon shelf. These methods may be applied to other river-dominated shelves. Four zones were detected regarding the bottom light regime. The seasonal changes of the dynamic forcing over the shelf reflect little in the light parameters over the Amazon shelf. Three zones presented constant light regimes along the year: dark coastal zone under the permanent influence of the plume; dim-light zone in the deeper northern shelf; and high-light zone in the shallower southern shelf. Only one zone presented a seasonally changing benthic light regime, and it was located under the seasonally variable edge of the plume over the northern mid-to-outer shelf. The minimum light level estimated in the deeper dim-light zone was ∼2 μE m−2 s−1 which is sufficient for reef-building organisms to thrive. The cause of the incipient living coverage and low vitality of crustose coralline algae, previously reported in the northern shelf, cannot be attributed to a lack of light, but might be related to sedimentation processes associated to the plume. Thus, the potential, in terms of light, for reef builders to develop along this and other river-influenced margins is much greater than previously anticipated.

Distribution of Fe isotopes in particles and colloids in the salinity gradient along the Lena River plume, Laptev Sea

Abstract. Riverine Fe input is the primary Fe source for the ocean. This study is focused on the distribution of Fe along the Lena River freshwater plume in the Laptev Sea using samples from a 600 km long transect in front of the Lena River mouth. Separation of the particulate (>0.22 µm), colloidal (0.22 µm–1 kDa), and truly dissolved (<1 kDa) fractions of Fe was carried out. The total Fe concentrations ranged from 0.2 to 57 µM with Fe dominantly as particulate Fe. The loss of >99 % of particulate Fe and about 90 % of the colloidal Fe was observed across the shelf, while the truly dissolved phase was almost constant across the Laptev Sea. Thus, the truly dissolved Fe could be an important source of bioavailable Fe for plankton in the central Arctic Ocean, together with the colloidal Fe. Fe-isotope analysis showed that the particulate phase and the sediment below the Lena River freshwater plume had negative δ56Fe values (relative to IRMM-14). The colloidal Fe phase showed negative δ56Fe values close to the river mouth (about −0.20 ‰) and positive δ56Fe values in the outermost stations (about +0.10 ‰). We suggest that the shelf zone acts as a sink for Fe particles and colloids with negative δ56Fe values, representing chemically reactive ferrihydrites. The positive δ56Fe values of the colloidal phase within the outer Lena River freshwater plume might represent Fe oxyhydroxides, which remain in the water column, and will be the predominant δ56Fe composition in the Arctic Ocean.

Observations and scaling of tidal mass transport across the lower Ganges–Brahmaputra delta plain: implications for delta management and sustainability

Abstract. The landscape of southwest Bangladesh, a region constructed primarily by fluvial processes associated with the Ganges River and Brahmaputra River, is now maintained almost exclusively by tidal processes as the fluvial system has migrated east and eliminated the most direct fluvial input. In natural areas such as the Sundarbans National Forest, year-round inundation during spring high tides delivers sufficient sediment that enables vertical accretion to keep pace with relative sea-level rise. However, recent human modification of the landscape in the form of embankment construction has terminated this pathway of sediment delivery for much of the region, resulting in a startling elevation imbalance, with inhabited areas often sitting >1 m below mean high water. Restoring this landscape, or preventing land loss in the natural system, requires an understanding of how rates of water and sediment flux vary across timescales ranging from hours to months. In this study, we combine time series observations of water level, salinity, and suspended sediment concentration with ship-based measurements of large tidal-channel hydrodynamics and sediment transport. To capture the greatest possible range of variability, cross-channel transects designed to encompass a 12.4 h tidal cycle were performed in both dry and wet seasons during spring and neap tides. Regional suspended sediment concentration begins to increase in August, coincident with a decrease in local salinity, indicating the arrival of the sediment-laden, freshwater plume of the combined Ganges–Brahmaputra–Meghna rivers. We observe profound seasonality in sediment transport, despite comparatively modest seasonal variability in the magnitude of water discharge. These observations emphasize the importance of seasonal sediment delivery from the main-stem rivers to this remote tidal region. On tidal timescales, spring tides transport an order of magnitude more sediment than neap tides in both the wet and dry seasons. In aggregate, sediment transport is flood oriented, likely as a result of tidal pumping. Finally, we note that rates of sediment and water discharge in the tidal channels are of the same scale as the annually averaged values for the Ganges and Brahmaputra rivers. These observations provide context for examining the relative importance of fluvial and tidal processes in what has been defined as a quintessentially tidally influenced delta in the classification scheme of Galloway (1975). These data also inform critical questions regarding the timing and magnitude of sediment delivery to the region, which are especially important in predicting and preparing for responses of the natural system to ongoing environmental change.

A State-of-the-Art Compact Surface Drifter Reveals Pathways of Floating Marine Litter in the German Bight

Lagrangian observations are important for the understanding of complex transport patterns of floating macroscopic litter items at the ocean surface. Satellite-tracked drifters and numerical models are an important source of information relevant to transport processes as well as distribution patterns of floating marine litter (FML) on a regional to global scale. Sub-mesoscale processes in coastal and estuarine systems have an enormous impact on pathways and accumulation zones of FML and are yet to be fully understood. Here we present a state-of-the-art, low-cost and robust design of a satellite-tracked low-cost drifter applicable in studying complex pathways and sub-mesoscale dynamics of floating litter in tidally influenced coastal and estuarine systems. It is compact, lightweight less than 5 kg, capable of refloating, easily recovered and modified. The drifter motion resolves currents of the ocean surface layer (top 0.5 m layer) taking into account wind induced motions. We further showcase findings from seven of our custom-made drifters deployed from RV Heincke and RV Senckenberg in the German Bight during spring and autumn 2017. Drifter velocities were computed from high resolved drifter position data and compared to local wind field observations. It was noted that the net transport of the drifters in areas far away from the coast was dominated by wind-driven surface currents, 1 % of the wind speed, whereas the transport pattern in coastal areas was mainly overshadowed by local small-scale processes like tidal jet currents, interactions with a complex shoreline and fronts generated by riverine freshwater plumes.

Impact of tidewater glacier retreat on the fjord system: Modeling present and future circulation in Kongsfjorden, Svalbard

Tidewater glaciers in Kongsfjorden, Svalbard, are retreating, and it is expected that they will terminate on land at some time in the future. Plumes, the submerged runoff emerging at the bases of tidewater glacier fronts, contribute to large-scale circulation within the inner fjord. A transition to a fjord with only land-terminating glaciers will affect the fjord circulation, with further implications for the biogeochemistry and ecosystem in Kongsfjorden. In this paper we compare the present and predicted future circulation using the ocean model system ROMS, and identify and quantify changes resulting from the altered forcing. Removal of subglacial discharge due to retreating tidewater glaciers causes substantial reduction of subsurface volume fluxes, and hence exchange rates, in the inner part of the fjord, and results in enhanced stratification during summer months. The increase in fjord extent due to tidewater glacier retreat results in a slight increase in tidal velocities, in particular overtide components, but this effect is not strong enough to compensate for the reduction in current velocities due to removal of freshwater plumes at glacier fronts. The freshwater content (practical salinity <34.8) in the fjord during the melting season is predicted to increase in the future, primarily due to enhanced retention of freshwater in the inner parts of the fjord. However, freshwater in the future will mostly be confined to a relatively thin surface layer whereas at present freshwater is found in a thicker layer, in particular in the proximity of the tidewater glacier fronts.

The role of water column stability and wind mixing in the production/export dynamics of two bays in the Western Antarctic Peninsula

During January and February 2017 massive phytoplankton blooms (chlorophyll > 15 mg m−3) were registered in surface waters within two bays in the Western Antarctic Peninsula (WAP). Reflecting these intense blooms, surface waters exhibited high pH (up to 8.4), low pCO2 (< 175 µatm) and low nitrate concentrations (down to 1.5 µM). These summer phytoplankton blooms consisted mainly of diatoms and were associated with the presence of shallow, surface freshwater plumes originating from glacier-melt outflow which contributed both to stratification and to iron supply, thus facilitating pronounced nitrate and CO2 drawdown. These findings suggest that with future increases in freshwater discharge around the WAP, phytoplankton blooms in the northern WAP may become more dominated by large cells, resembling the blooms occurring further south along the Peninsula. Fresher surface waters enhanced water column stability in both bays, enabling phytoplankton populations to attain high growth rates. Phytoplankton was observed to double their biomass in 2.3 days, consistent with the high net primary production rates recorded in both bays (1.29–8.83 g C m−2 d−1). Phytoplankton growth rates showed a direct mechanistic relationship with changes in water column stability, suggesting that this is a main driver of primary productivity in near-shore Antarctic coastal ecosystems, which agrees with previous findings. After wind induced mixing, the organic matter produced within both bays did not settle inside them, suggesting that it was laterally advected out of the bays. Thus, we hypothesize that highly productive near-shore bay areas in Antarctica may supply organic matter to oceanic waters.

Seasonal and spatial variations of chloroform, trichloroethylene, tetrachloroethylene, chlorodibromomethane and bromoform in the Northern Yellow Sea and Bohai Sea

Environmental contextVolatile halocarbons play significant roles as halogen carriers in atmospheric chemistry where they contribute to global warming. We studied temporal and spatial variations of volatile halocarbons in the Northern Yellow Sea and Bohai Sea, and showed that their concentrations were governed by terrestrial input, hydrographic features and biological activity. The emission of volatile halocarbons from coastal regions could have important impacts on the budgets of atmospheric reactive halogen. AbstractConcentrations of five volatile halocarbons (VHCs), namely, chloroform (CHCl3), trichloroethylene (C2HCl3), tetrachloroethylene (C2Cl4), chlorodibromomethane (CHBr2Cl) and bromoform (CHBr3), in the Northern Yellow Sea and Bohai Sea were determined during the spring of 2013 and autumn of 2012. Strong seasonality in the concentrations of VHCs (except for CHCl3) were observed. Concentrations of CHCl3 were markedly higher (1.5 fold) to coincide with the higher concentration of chlorophyll a during the spring. The elevated concentrations of C2HCl3, C2Cl4, CHBr2Cl and CHBr3 were found to match with the higher inputs of land runoff during autumn. The VHCs distributions evidently decreased along the freshwater plume from the mouth of rivers, such as Yellow and Yalu Rivers, to the open sea. The elevated concentrations of the selected halocarbons arise from terrestrial input, complicated hydrographic features, such as the Yellow River effluent, Yalu River, Yellow Sea Coastal Current and Yellow Sea Cold Water, and biological release by phytoplankton. Correlation analyses were conducted among Chl-a, salinity and the concentrations of these gases to investigate possible controls for the concentrations of these gases. Significant correlation was only observed between the concentrations of CHBr2Cl and Chl-a in the surface layer during spring. We made the tentative conclusion that phytoplankton biomass might not be the main limiting factor of sources of VHCs in the surface water. The sea-to-air fluxes indicated that the Northern Yellow Sea and Bohai Sea act as sources of gas in the atmosphere during spring and autumn.

Limited impact of eolian and riverine sources on the biogeochemical cycling of Cd in the tropical Atlantic

We present coupled cadmium (Cd) concentrations and Cd isotopic compositions (expressed as δ114Cd) in seawater profiles along the northeast-southwest GEOTRACES GA11 section in the tropical Atlantic Ocean. The GA11 section encompasses three contrasting regions including the Amazon River plume, the North African dust plume, and the Guinea Dome oxygen deficient zone (ODZ). Given the long oceanic residence time of Cd (104 to 105 yr), local inputs such as atmospheric and riverine sources are generally considered to be of little importance for the open ocean Cd budget, and the limited Cd isotope dataset available thus far has prevented any unambiguous conclusions on the importance of these processes. The GA11 section is ideally located for assessing the influence of external, natural and anthropogenic riverine and eolian inputs, as well as internal processes, on the Cd mass balance in the tropical Atlantic Ocean. As in previous Cd isotope studies, this dataset documents that both surface consumption–regeneration of micronutrients within the water column and deep water mass mixing exert the prime control on the cycling of Cd. However, we do observe some near-surface samples with lower-than-expected δ114Cd signatures along the western section of the transect. Surface waters sampled at the margin of the Amazon freshwater plume show no resolvable difference in [Cd] and δ114Cd compared to waters outside of the plume, suggesting that the Amazon River is not an important source of Cd to the open ocean. Although the eastern GA11 transect is directly downwind of the Saharan dust plume, atmospheric Cd deposition accounts for <1% of the inferred upwelling flux, indicating that atmospheric inputs to the surface North Atlantic Ocean, even within the main Saharan dust plume, only have a relatively minor influence on the Cd budget. In the subsurface tropical Atlantic (100–200 m water depth), there is a clear deviation from a tight linear Cd-PO4 correlation toward lower Cd concentrations for a given PO4 concentration. Our new Cd data show that this Cd-PO4 decoupling is likely a feature of the entire tropical Atlantic and may possibly reflect PO4 enrichment via preferential re-mineralization of organically-bound phosphorus. Alternatively, the decoupling may arise from subsurface Cd depletion caused by precipitation of Cd sulphide within sinking organic particulate microenvironments, as has been suggested in recent studies.This article is part of a special issue entitled: “Cycles of trace elements and isotopes in the ocean – GEOTRACES and beyond” - edited by Tim M. Conway, Tristan Horner, Yves Plancherel, and Aridane G. González.

Impact of freshwater plumes on intraseasonal upper ocean variability in the Bay of Bengal

The Monsoon Intraseasonal Oscillations (MISO) is known to be strongly coupled to the upper ocean variability in the Bay of Bengal (BoB). Here, we analyze high-resolution moored observations from the northern BoB (18 °N, in comparison with an array of moorings to the south along 90 °E (8, 12, and 15 °N) to examine the observed temperature and salinity variability during the 2015 summer monsoon season. The heat budget analysis indicates that the mixed-layer (ML) temperature tendency at 18 °N in late summer (August and September) deviated greatly from the tendency that is expected based on surface heat flux alone. Examination of moored ML salinity and satellite sea surface salinity (SSS) fields suggests that southward extension of riverine freshwater plumes to the mooring site in late summer plays an important role in modulating the heat balance. The associated shoaling of the ML results in enhanced penetrative heat loss and warming beneath the surface layer. Price-Weller-Pinkel (PWP) 1-D model simulations, which account for vertical mixing, suggest that the entrainment of this subsurface warm water under monsoon wind improves closure of the temperature balance. Long-term analysis of satellite-based sea surface temperature (SST) and rainfall data indicates that late summer ML shoaling is accompanied by amplified intraseasonal (10–60 day) SST variability and reduced MISO precipitation variability. Overall, these results demonstrate the critical importance of freshwater plumes to improved understanding of the upper-ocean heat budget and air-sea interaction in the BoB.

Excess attenuation caused by subducted bubbles at the ebb plume front in the Connecticut River

A field experiment was carried out in the Connecticut River in June 2017, one goal of which was to investigate the low-to-mid-frequency acoustic propagation characteristics of the fresh water plume front outside the river mouth during ebb. Linear frequency-modulated (LFM) acoustic signals in the 500-2000 Hz band were collected during several tidal cycles. Transmission loss in excess of that which is expected by sound speed gradient-driven refraction is explored via acoustic propagation modeling. The excess attenuation at the ebb plume front is attributed to large clouds of subducted bubbles by downwelling at the ebb plume front.

Sensitivity of the Bay of Bengal upper ocean to different winds and river input conditions

Abstract The sensitivity of the Bay of Bengal (BoB) upper ocean circulation and thermohaline structure to varying wind strengths and river salinity conditions is investigated using a set of long-term mesoscale simulations. The Regional Ocean Modeling System (ROMS) simulations differ in their forcing fields for winds (strong vs. weak) and in their representations of river input salinity conditions (seasonally varying estuarine salinity vs. zero salinity). The sensitivities are analyzed in terms of the responses of the surface circulation, thermohaline structure, freshwater plume dispersion, and the coastal upwelling along the western boundary. All the simulations reproduce the main broad-scale features of the Bay, while their magnitudes and variabilities depend on the forcing conditions. The impact of stronger wind is felt at greater depths for temperature than for salinity throughout the domain; however, the impact is realized with vertical distributions that are different in the northern than in the southern Bay. As expected, the stronger wind-induced enhanced mixing lowers (enhances) the upper ocean temperature (salinity) by 0.2 °C (0.3 psu), and weakens the near-surface stratification. Moreover, stronger wind enhances eddy activity, strengthens the springtime Western Boundary Current (WBC) and enhances coastal upwelling during spring and summer along the east coast of India. The fresher river input reduces the surface salinity and hence enhances the spreading and intensity of the freshwater plume, stratification, and barrier layer thickness. The lower salinity simulation leads to an eddy-dominant springtime WBC, and enhances the freshness, strength, and southward extent of the autumn East India Coastal Current (EICC). The stronger wind simulations appear to prevent the spreading of the freshwater plume during the summer monsoon due to enhanced mixing. Fresher river input reduces the overall surface salinity by ~0.4 psu; however, it significantly underestimates the salinity near the river mouths, whereas the estuarine salinity river input simulations are closer to reality. These results highlight the importance of river input salinity and realistic strong winds in reducing model biases of high-resolution simulations for the Bay of Bengal.

Phylogenetic and phylogeographic inferences based on two DNA markers reveal geographic structure of the orangeclaw hermit crab Calcinus tibicen (Anomura: Diogenidae) in the western Atlantic

ABSTRACTThe hermit crab Calcinus tibicen has an extensive distribution along the coasts of the western Atlantic Ocean, from the USA to southern Brazil. The present study aimed to test the hypothesis of the presence of phylogeographic structure of C. tibicen throughout its distribution, evaluating the genetic and morphological variabilities of the species. The molecular data, genetic diversity and demographic history were inferred from 16 different localities for the mitochondrial genes cytochrome oxidase subunit I (COI) (75 specimens) and 16S rRNA (20). The morphological information was based on 99 individuals from the same 16 localities. Results were obtained by the construction of haplotype networks and a phylogenetic tree, an Analysis of Molecular Variance (AMOVA), Tajima’s D and Fu’s Fs tests, pairwise mismatch distributions under the sudden expansion model, and by estimating divergence times. The haplotype networks show the existence of two genetically well-defined groups that do not share haplotypes. The phylogenetic tree and the matrix of genetic distances suggest lack of gene between North and South Atlantic groups, also corroborated by AMOVA for the COI gene. The genetic distance can be attributed to at least two ecological factors: the existence of a physiological barrier caused by the Amazon River freshwater plume, and the effect of the bifurcation of the westerly flowing South Atlantic oceanic currents creating a gap along the Brazilian coastline. Demographic history analyses suggest that C. tibicen experienced population expansion, probably after the Amazon River changed its direction. Despite the molecular evidence for the existence of a phylogeographic structure, no morphological pattern for each genetic group was observed. Therefore, based on the described molecular differences and the ecological and historical factors suggested, the question arises whether a cryptic species should be recognized for some of the populations of C. tibicen.