Academic literature on the topic 'Eddy-freshwater interaction'

Abstract. Eddy covariance data are widely used for the investigation of surface–air interactions. Although numerous datasets exist in public depositories for land ecosystems, few research groups have released eddy covariance data collected over lakes. In this paper, we describe a dataset from the Lake Taihu eddy flux network, a network consisting of seven lake sites and one land site. Lake Taihu is the third-largest freshwater lake (area of 2400 km2) in China, under the influence of subtropical climate. The dataset spans the period from June 2010 to December 2018. Data variables are saved as half-hourly averages and include micrometeorology (air temperature, humidity, wind speed, wind direction, rainfall, and water or soil temperature profile), the four components of surface radiation balance, friction velocity, and sensible and latent heat fluxes. Except for rainfall and wind direction, all other variables are gap-filled, with each data point marked by a quality flag. Several areas of research can potentially benefit from the publication of this dataset, including evaluation of mesoscale weather forecast models, development of lake–air flux parameterizations, investigation of climatic controls on lake evaporation, validation of remote-sensing surface data products and global synthesis on lake–air interactions. The dataset is publicly available at https://yncenter.sites.yale.edu/data-access (last access: 24 October 2020) and from the Harvard Dataverse (https://doi.org/10.7910/DVN/HEWCWM; Zhang et al., 2020).

Abstract The cyclonic circulation of the Atlantic subpolar gyre is a key mechanism for North Atlantic climate variability on a wide range of time scales. It is generally accepted that it is driven by both cyclonic winds and buoyancy forcing, yet the individual importance and dynamical interactions of the two contributions remain unclear. The authors propose a simplified four-box model representing the convective basin of the Labrador Sea and its shallow and deep boundary current system, the western subpolar gyre. Convective heat loss drives a baroclinic flow of relatively light water around the dense center. Eddy salt flux from the boundary current to the center increases with a stronger circulation, favors the formation of dense waters, and thereby sustains a strong baroclinic flow, approximately 10%–25% of the total. In contrast, when the baroclinic flow is not active, surface waters may be too fresh to convect, and a buoyancy-driven circulation cannot develop. This situation corresponds to a second stable circulation mode. A hysteresis is found for variations in surface freshwater flux and the salinity of the near-surface boundary current. An analytical solution is presented and analyzed.

There is broad consensus among ecosystem experts that river dolphins, as top predators, can structure riverine ecosystems and benefit biodiversity. The effects of dolphins on rivers and vice versa do not operate in isolation, but are context dependent, being conditioned by other factors such as human interference. Based on the aforesaid presumption, the focus of the present study was on quantifying the factors responsible for the decline of the Indus River dolphin and species tolerance towards these factors in the upper sub-basin of the Indus River system (River Beas, India). Dolphins avoided (r=–0.667; P=0.001) the study section with higher disturbance, until some other factors, such as prey availability, came into play. Species occupancy was significantly different for different flow seasons and was associated with deep pools, eddy currents, and low disturbance index. The results indicated a marked decrease in species occurrence above a disturbance index level of 44. The findings of the present study contribute towards a better understanding of the complex ecological interactions of river dolphins with their environment and provide valuable insights into the wider conservation status of other threatened components of sympatric freshwater biodiversity that can further help in designing effective conservation measures for the ecosystem as a whole.

Abstract. The key processes driving the air–sea CO2 fluxes in the western tropical Atlantic (WTA) in winter are poorly known. WTA is a highly dynamic oceanic region, expected to have a dominant role in the variability in CO2 air–sea fluxes. In early 2020 (February), this region was the site of a large in situ survey and studied in wider context through satellite measurements. The North Brazil Current (NBC) flows northward along the coast of South America, retroflects close to 8∘ N and pinches off the world's largest eddies, the NBC rings. The rings are formed to the north of the Amazon River mouth when freshwater discharge is still significant in winter (a time period of relatively low run-off). We show that in February 2020, the region (5–16∘ N, 50–59∘ W) is a CO2 sink from the atmosphere to the ocean (−1.7 Tg C per month), a factor of 10 greater than previously estimated. The spatial distribution of CO2 fugacity is strongly influenced by eddies south of 12∘ N. During the campaign, a nutrient-rich freshwater plume from the Amazon River is entrained by a ring from the shelf up to 12∘ N leading to high phytoplankton concentration and significant carbon drawdown (∼20 % of the total sink). In trapping equatorial waters, NBC rings are a small source of CO2. The less variable North Atlantic subtropical water extends from 12∘ N northward and represents ∼60 % of the total sink due to the lower temperature associated with winter cooling and strong winds. Our results, in identifying the key processes influencing the air–sea CO2 flux in the WTA, highlight the role of eddy interactions with the Amazon River plume. It sheds light on how a lack of data impeded a correct assessment of the flux in the past, as well as on the necessity of taking into account features at meso- and small scales.

Abstract Models of lake physical processes provide the lower flux boundary conditions for numerical predictions of weather and climate in lake basins. So far, there have been few studies on evaluating lake model performance at the diurnal time scale and against flux observations. The goal of this paper is to evaluate the National Center for Atmospheric Research Community Land Model version 4–Lake, Ice, Snow and Sediment Simulator using the eddy covariance and water temperature data obtained at a subtropical freshwater lake, Lake Taihu, in China. Both observations and model simulations reveal that convective overturning was commonplace at night and timed when water switched from being statically stable to being unstable. By reducing the water thermal diffusivity to 2% of the value calculated with the Henderson–Sellers parameterization, the model was able to reproduce the observed diurnal variations in water surface temperature and in sensible and latent heat fluxes. The small diffusivity suggests that the drag force of the sediment layer in this large (2500 km2) and shallow (2-m depth) lake may be strong, preventing unresolved vertical motions and suppressing wind-induced turbulence. Model results show that a large fraction of the incoming solar radiation energy was stored in the water during the daytime, and the stored energy was diffused upward at night to sustain sensible and latent heat fluxes to the atmosphere. Such a lake–atmosphere energy exchange modulated the local climate at the daily scale in this shallow lake, which is not seen in deep lakes where dominant lake–atmosphere interactions often occur at the seasonal scale.

AbstractAmongst the variety of oceanic processes running the gamut of spatiotemporal scales, mesoscale eddies are the most common and often have region-specific characteristics. The large kinetic energy inherent to eddies themselves is a strong modulator of the global climate, ocean circulation, productivity, and freshwater transport. This study uses multi-source satellite remote sensing observation data to construct a multi-parameter eddy dataset for the 1993–2019 period, which differs significantly from a few of previous published eddy datasets that include only basic sea surface eddy physical features. Eddies within the dataset have life cycles of greater than four weeks, and their corresponding sea surface chlorophyll, sea surface temperature, and wind fields are provided. Atmospheric and oceanic variables are used to present a comprehensive picture of a given mesoscale eddy’s impact on the local physical, but also biological environment. The dataset would find immense value in research on mesoscale eddies, their impact on the atmosphere, and related biological processes.

Estuarine outflow can have a significant impact on physical and ecological systems in the coastal ocean. Along southeastern Australia, inshore of the East Australian Current, the shelf is narrow, the coastal circulation is advection dominated, and river estuarine outflow tends to be low, hence river plumes have largely been ignored. For these reasons, we lack an understanding of the spatial and temporal evolution of river plumes during large rainfall events (which are projected to increase in frequency and intensity), and the interaction of the mesoscale circulation with the estuarine outflow remains to be explored. Using a high-resolution (750 m) hydrodynamic model, we simulate idealized plumes from 4 estuaries during three different mesoscale circulation scenarios and investigate the spatial and temporal evolution of the estuarine outflow under two contrasting rainfall events (normal and large). We explore the plume from the largest of the 4 rivers, the Hawkesbury River, to understand the impact of the mesoscale circulation. During the first EAC mode, the plume spreads both northward and southeastward. The offshore spread of the plume is the largest in this scenario (~12.5 km east of the river mouth) in the wet event. In the second EAC mode, this plume dispersal is toward the north and east, driven by the proximity of a cyclonic eddy on the shelf, with an eastward extension of 11 km. In the third EAC mode, most of this river plume spreads southward with some to the north, again dictated by the position of the cyclonic eddy. The cross-shelf dispersal is a minimum of 9.5 km from the river mouth. It takes around 6 days for the freshwater spatial extent of the plume in the wet event to return to the base case. These results show the importance of mesoscale EAC circulation on the shelf circulation when considering river plumes dispersal. Knowledge of the ultimate fate of riverborne material, dilution and cumulative effects will enable better environmental management of this dynamic region for the local government.

AbstractHeat and momentum exchanges at the Southern Ocean surface are crucial for the Earth’s Climate, but the importance of the small-scale spatial variability of these surface fluxes is poorly understood. Here, we explore how small-scale heterogeneities of the surface conditions due in particular to ocean eddies affect the atmosphere–sea ice–ocean interactions off Adélie Land, in East Antarctica. To this end, we use a high-resolution regional atmosphere–sea ice–ocean coupled model based on the NEMO-LIM and MAR models. We explore how the atmosphere responds to small-scale heterogeneity of the ocean or sea ice surface conditions, how eddies affect the sea ice and atmosphere, and how the eddy-driven surface fluxes impact the heat, freshwater, and momentum budget of the ocean. The atmosphere is found to be more sensitive to small-scale surface temperature gradients above the ice-covered than above the ice-free ocean. Sea ice concentration is found to be weaker above anticyclonic than cyclonic eddies due to increased sea ice melting or freezing (0.8 cm/day) partly compensated by sea ice convergence or divergence. The imprint of ice-free eddies on the atmosphere is weak, but in the presence of sea ice, air warming (+ 0.3 $$^{\circ }$$ ∘ C) and wind intensification (+ 0.1 m/s) are found above anticyclonic eddies, while cyclonic eddies have the opposite effects. Removing the interactions of eddies with the sea ice or atmosphere does not affect the total sea ice volume, but increases the ocean kinetic energy by 8% and weakens northward advection of sea ice, leading to a 15% decrease in freshwater flux north of 62.5 $$^{\circ }$$ ∘ S and weaker ocean restratification.

The stirring of passive tracers driven by altimetry-derived daily surface geostrophic currents is studied on subseasonal timescales in the Bay of Bengal. Advection of latitudinal and longitudinal bands highlights the chaotic nature of stirring in the Bay via repeated straining and filamentation of the tracer field. An immediate finding is that stirring is local, i.e., of the scale of the eddies, and does not span the entire basin. Further, stirring rates are enhanced along the coast of the Bay and are relatively higher in the pre-and post-monsoonal seasons. The spatially non-uniform stirring at the surface of the Bay is reflected in long-tailed probability density functions of Finite-Time Lyapunov Exponents (FTLEs), which become more stretched for longer time intervals. Quantitatively, advection for a week shows that mean FTLEs lie between 0.13$\pm$0.07 day$^{-1}$, while extremes reach almost 0.6 day$^{-1}$. Averaged over the Bay, relative dispersion initially grows exponentially, followed by a power-law at scales between approximately 100 and 250 km, which finally transitions to an eddy-diffusive regime. Quantitatively, below 250 km, a scale-dependent diffusion coefficient is extracted that behaves as a power-law with cluster size, while above 250 km, eddy-diffusivities range from $6 \!\times \!10^3$ $\!-\!$ $1.6\times 10^4$ m$^2$s$^{-1}$ in different regions of the Bay. These estimates provide a useful guide for resolution-dependent diffusivities in numerical models that hope to properly represent surface stirring in the Bay.\\ A particularly important tracer field in the Bay is the sea surface salinity; indeed, freshwater from rivers influences Indian summer monsoon rainfall and tropical cyclones by stratifying the upper layer and warming the subsurface ocean in the Bay of Bengal. We use {\it in situ} and satellite data with reanalysis to showcase how river water experiences a significant increase in salinity on sub-seasonal timescales. This involves the trapping and homogenization of freshwater by a cyclonic eddy in the Bay. Using a specific example from 2015, river water is shown to enter an eddy along its attracting manifolds within a period of two weeks. This leads to the formation of a highly stratified subsurface layer within the eddy. When freshest, the eddy has the largest sea-level anomaly, spins fastest, and supports strong lateral gradients in salinity. Subsequently, observations reveal a progressive increase in salinity inside the eddy within a month. In particular, salty water spirals in, and freshwater is pulled out across the eddy boundary. Lagrangian experiments elucidate this process, whereby horizontal chaotic mixing provides a mechanism for the rapid increase in surface salinity.\\ The eddy-freshwater interaction, or adjustment, is then studied using a high-resolution Regional Ocean Modeling System. Apart from lateral advection, a mixed layer salinity budget shows the importance of ageostrophic vertical advection during the evolution of salinity within the eddy. An analysis of the depth-integrated eddy kinetic energy indicates the development of both barotropic and baroclinic instabilities. The vertical profile associated with these conversion terms reveals that the surface freshwater was likely involved in developing baroclinic terms in the mixed layer. In addition, an eddy available potential energy (EPE) budget suggests that the entrainment of the river water raises the EPE, which is reflected in the development of gradients in salinity within the eddy. The EPE is lowered with homogenization, signifying irreversible mixing. Further, EPE rates are modulated by the correlation of buoyancy fluxes with density anomalies, which involves lateral advection of freshwater associated with surface cooling and local, regional rainfall. Finally, the adjustment of this freshwater eddy triggers submesoscale dynamics that appear to be an integral part of salinity homogenization. The observation and reanalysis data also showcase the presence of these events across different years, thus bringing out the broader impact of mixing freshwater into high salinity ambient water by eddies in the Bay. This pathway is distinct from vertical diffusive mixing and is likely to be important for the evolution of salinity in the Bay of Bengal.
Ministry of Education (MoE), Indian Institute of Science, Bengaluru; University Grants Commission; National Monsoon Mission, Indian Institute of Tropical Meteorology, Pune; Divecha Centre for Climate Change, Indian Institute of Science, Bengaluru