Latent Flux Research Articles

The Impact of Cold-Air Outbreaks and Oceanic Lateral Fluxes on Dense-Water Formation in the Greenland Sea from a 10-Year Moored Record (1999–2009)

Abstract The Greenland Sea produces a significant portion of the dense water from the Nordic seas that supplies the lower limb of the Atlantic meridional overturning circulation. Here, we use a continuous 10-yr hydrographic record from moored profilers to examine dense-water formation in the central Greenland Sea between 1999 and 2009. Of primary importance for dense-water formation is air–sea heat exchange, and 60%–80% of the heat lost to the atmosphere during winter occurs during intense, short-lived events called cold-air outbreaks (CAOs). The long duration and high temporal resolution of the moored record has for the first time facilitated a statistical quantification of the direct impact of CAOs on the wintertime mixed layer in the Greenland Sea. The mixed layer development can be divided into two phases: a cooling phase and a deepening phase. During the cooling phase (typically between November and January), CAOs cooled the mixed layer by up to 0.08 K day−1, depending on the intensity of the events, while the mixed layer depth remained nearly constant. Later in winter (February–April), heat fluxes during CAOs primarily led to mixed layer deepening of up to 38 m day−1. Considerable variability was observed in the mixed layer response, indicating that lateral fluxes of heat and salt were also important. The magnitude and vertical distributions of these fluxes were quantified, and idealized mixed layer simulations suggest that their combined effect is a reduction in the mixed layer depth at the end of winter of up to several hundred meters.

Joint Distributions of Local Pore Space Properties Quantitatively Explain Simulated Air Flow Variations in Paper

The gas flow through sheet-like porous materials such as paper can show marked lateral variations due to a heterogeneous, locally varying microstructure. Hence, reliable predictions of such lateral flux variations require an appropriate consideration of local variations in the microstructure. The flow through such sheet-like materials is commonly described with Darcy’s law in which permeances are formulated in terms of microstructure properties, such as porosities, tortuosities, or hydraulic radii. This work proposes an extension of existing permeance models that directly considers the variation and the cross-dependence between local microstructure properties. The extended model is applied to local air fluxes through a paper sheet to exemplarily reveal the joint impact of local porosities and local tortuosities on the air flux. The key extension is to consider a joint distribution of porosity and tortuosity. The latter is constructed from the univariate property distributions using a copula approach and yields local tortuosities including their variation for any encountered local porosity. These values jointly enter any permeance model that qualitatively captures the dependence of the air flux on the porosity. To assess the merit of the model, variations in the air flux and in the pore space properties are independently determined from the same measured microstructure of paper. Air flux variations are provided by computational fluid dynamics simulations on multiple, nonoverlapping segments taken from the microstructure. A statistical analysis of the entire microstructure provides the distribution of local porosity, tortuosity, and thicknesses. Our model quantitatively explains that porosity-dependent variations in the tortuosity, in particular the ones associated with high-volume pathways, decisively determine air flux variations.

Natural convection due to lateral uniform heat flux in a slender porous cavity saturated with nanofluid: Departure from LTE state

The present article deals with the influence of the local thermal nonequilibrium (LTNE) state on natural convection in an enclosure filled with nanofluid-saturated porous medium. The flow is induced due to the maintenance of constant heat flux on the vertical walls and insulation of horizontal walls. The Darcy model has been adopted to describe the flow governing equations. These equations are solved numerically by using the alternate direction implicit method and analytically by applying parallel flow assumptions valid for large aspect ratios. Boundary layer analysis is performed to report the boundary layer thickness ( δ x ∼ 2 1 / 2 ( k nf / k f ) 1 / 10 ( R a T ) − 1 / 5 ) as well as the heat transfer rate of nanofluid ( N u nf ∼ ( k nf / k f ) − 1 / 5 R a T 2 / 5 ) . The combined effect of LTNE state parameters (interface heat transfer coefficient (H), porosity scaled thermal conductivity ratio (γ)) and volume fraction of nanoparticle ( ϕ ) is addressed on the flow dynamics and heat transfer mechanism. Rigorous analysis, indicates that the average heat transfer rate of nanofluid and solid porous matrix are increasing function of H and γ, however, the role of γ is accelerated for a relatively high value of H. For increasing the value of γ from 1 to 10, the enhancement in the heat transfer rate of nanofluid is reported to be up to 27% at 10% volume fraction of nanoparticles. The average Nusselt number of nanofluid, as well as solid phase, are reduced when the value of ϕ is increased under the LTNE circumstances. Furthermore, the flow structure is controlled by unicellular structure with rotating anticlockwise direction in the entire study. For the very small value of H the temperature distribution in the solid porous matrix is mainly by conduction mode and distribution is linear, however, for a relatively high value of H the distribution in the same is controlled by both conduction as well as convection, and it will be more pronounced for the higher value of γ.

Quantifying 3D Gravity Wave Drag in a Library of Tropical Convection‐Permitting Simulations for Data‐Driven Parameterizations

AbstractAtmospheric gravity waves (GWs) span a broad range of length scales. As a result, the un‐resolved and under‐resolved GWs have to be represented using a sub‐grid scale (SGS) parameterization in general circulation models (GCMs). In recent years, machine learning (ML) techniques have emerged as novel methods for SGS modeling of climate processes. In the widely used approach of supervised (offline) learning, the true representation of the SGS terms have to be properly extracted from high‐fidelity data (e.g., GW‐resolving simulations). However, this is a non‐trivial task, and the quality of the ML‐based parameterization significantly hinges on the quality of these SGS terms. Here, we compare three methods to extract 3D GW fluxes and the resulting drag (Gravity Wave Drag [GWD]) from high‐resolution simulations: Helmholtz decomposition, and spatial filtering to compute the Reynolds stress and the full SGS stress. In addition to previous studies that focused only on vertical fluxes by GWs, we also quantify the SGS GWD due to lateral momentum fluxes. We build and utilize a library of tropical high‐resolution (Δx = 3 km) simulations using weather research and forecasting model. Results show that the SGS lateral momentum fluxes could have a significant contribution to the total GWD. Moreover, when estimating GWD due to lateral effects, interactions between the SGS and the resolved large‐scale flow need to be considered. The sensitivity of the results to different filter type and length scale (dependent on GCM resolution) is also explored to inform the scale‐awareness in the development of data‐driven parameterizations.

The quantitative response of pollutant spatial distribution and mixing dynamics to confluent conditions in an asymmetric open-channel confluence

River confluences are key nodes of pollutant transport in fluvial ecosystems and they are accompanied by complex hydrodynamic and mixing processes. The generalization and quantitative response of mixing to highly variable confluent conditions remain unclear. This studyinvestigates the spatial distribution and mixing process of pollutants through a series of meticulous experiments in an asymmetric open-channel confluence. Furthermore, we propose quantitative responses and characterization equations for the mixing rate and mixing metric for various confluent conditions, namely junction angle, momentum flux ratio, and width-depth ratio. The spatial distribution of pollutants have a strong three-dimensional structure, with a narrow-long pollution belt near the tributary and an almost vertical mixing interface adjacent to the downstream junction corner; this interface exhibits a logarithmic growth trend downstream and is affected by shear layer distortion. The standard deviation of the cross-sectional pollutant concentration at the confluence decreases gradually along with the downstream mixing of convergent flows, and the fitting curve slope increases as the junction angle, momentum ratio, and width-depth ratio increase. The mixing rate is promoted by the enhancements of helical motion, lateral advection, and turbulent diffusion caused by the strengthening of mutual deflection and lateral momentum flux under a large junction angle and momentum ratio, as well as the reductions of vertical velocity gradient and cross-sectional area under large width-depth ratio conditions. Sufficient and complete mixing occurs under a larger momentum ratio condition of >1 within a short distance of 4–5 widths of the main channel. In addition, the mixing metric shows a strong linear relationship with each of the three confluent conditions. In summary, the significant (p < 0.001) quantitative responses of the mixing rate and mixing metric can be characterized by a polynomial equation. These results have major implications for the systematic understanding of the three-dimensional distribution characteristics, mixing patterns, and mixing dynamics of pollutants at asymmetric confluences, as well as for the prediction or calculation of the downstream mixing metric.

An inventory of salty and warm subsurface intrusions in the South Brazil Bight

Subsurface intrusions of salty and warm waters onto the shelf can potentially impact the thermodynamics and biogeochemistry of these relatively shallow water environments. Such thermohaline intrusions are usually associated with double diffusive processes and can enhance the vertical and lateral fluxes. In this work, we characterized subsurface intrusions of Tropical Waters (TW) at the South Brazil Bight (SBB) - a mid-latitude continental shelf, which were detected based on salinity vertical profiles from a historical dataset. We found the intrusions tend to follow the pycnocline and are relatively common year-round, occurring at 27% of the dataset, with a maximum salinity highly correlated with the maximum stratification. Intrusions have different along-shelf characteristics, varying with the shelf width and depth. North of São Sebastião Island (SSI) – at the central portion of the SBB –, intrusions were thicker, saltier, and shallower than the ones found at the south of SSI. These differences could be related to the variations of the Coastal Water (CW) and South Atlantic Central Water (SACW) volume along the shelf and throughout the year. We found that intrusions’ thickness presents an inverse correlation with stratification, resulting in thinner intrusions in more stratified environments. This inverse correlation suggests double-diffusively driven intrusions. We apply the Turner Angle method to investigate whether the environment is susceptible to double diffusive processes. This analysis suggests that thermohaline intrusions in the SBB might not be primarily double diffusive. However, the dominance of different double diffusion processes - diffusive convection (salt fingering) at the upper (lower) layer - suggests that these processes influence intrusion growth, steady-state, and erosion. Nonetheless, the mechanism responsible for the initial density perturbation remains unknown and further studies are needed to explore the relationship between the Brazil Current dynamics and thermohaline warm and salty subsurface intrusions.

Internal tides in the central Mediterranean Sea: observational evidence and numerical studies

Internal tides are studied in the central Mediterranean Sea using observational data and numerical experiments. Both numerical results and observations indicate that the baroclinic variability in this area is dominated by the K1 diurnal tide. In agreement with previous studies, the diurnal internal tides have the characteristics of Kelvin-like bottom trapped waves. They are mainly generated by the interaction of the induced barotropic tidal flow with the steep bathymetric gradient connecting the Ionian Sea with the shallow Sicily Channel. The bathymetric gradient appears to be the major forcing shaping the propagation paths of the internal tides. The most energetic internal tides follow the steep bathymetric gradient, propagating southward and tending to dissipate rapidly. Other waves cross the continental shelf south of Malta and then split with one branch moving toward the southern coast of Sicily and the other moving toward the west. Internal tides propagate with a variable phase velocity of about 1 ms−1 and a wavelength of the order of 100 km. During their journey, the internal waves appear to be subject to local processes that can modify their characteristics. The induced vertical shear strongly dominates the vertical turbulence and generates vertical mixing that alters the properties of the water masses traversing the area. Barotropic and internal tides remove heat from the ocean surface, increasing atmospheric heating, and redistributing energy through increased lateral heat fluxes. Lateral heat fluxes are significantly greater in the presence of internal tides due to the simultaneous increase in volume fluxes and water temperatures.

Construction of a spatially gridded heat flux map based on airborne flux Measurements using remote sensing and machine learning methods

Acquiring the relative truth values of land surface sensible (H) and latent (LE) heat fluxes at the regional scale is important for monitoring and simulating evapotranspiration at a global scale using satellite data or process-based models. However, it is difficult to directly obtain these values with fine spatial representation at regional scales using conventional ground-based measurement methods. Therefore, airborne measurements of turbulent flux are highly advantageous. By leveraging airborne measurements of H and LE fluxes collected in the Netherlands in August 2008, in this study we evaluated five machine learning methods for the construction of a regional gridded heat flux map, including artificial neural networks, boosted regression trees, random forest regression, deep neural networks, and support vector regression. The models were trained and tested using a dataset compiled from observations of H and LE, the digital elevation model, MODIS land surface temperature, enhanced vegetation index, and albedo data for each flux footprint. The best performing model was used to construct a regional gridded heat flux map over a case region located in the center of the Netherlands. The results shown that the support vector regression performed better than other models, with R2=0.91,RMSE=9.6W/m2 for H, and R2=0.89,RMSE=26.32W/m2 for LE. The constructed heat flux maps achieved a relative prediction error of ≤32.8% (R2>0.71) for H and ≤43.5% (R2>0.53) for LE compared to aircraft measurements. The constructed heat flux maps were then aggregated for each land cover type, providing individual estimates of source strength (52< H< 66 W/m2 and 108< LE< 218 W/m2) and spatial variability (52< σH< 66 W/m2 and 108< σLE< 218 W/m2) with an ensemble precision of < 6% (1σ). Finally, the limitations and future prospects of the current study were summarized and discussed.

Abyssal Circulation From the Yap‐Mariana Junction to the Northern Philippine Basin

AbstractThe lower deep branch of the Pacific Meridional Overturning Circulation (L‐PMOC) is a crucial element of the ocean's climate and biogeochemical systems through carrying the Lower Circumpolar Water (LCPW). For the first time, the pathway and volume transport of L‐PMOC from the Yap‐Mariana Junction (YMJ) to the Northern Philippine Basin (NPB) are revealed by a six‐mooring array measurement over 2019–2021. The L‐PMOC seasonally intrudes into the western Pacific at the YMJ. Then, it is directed into the West Mariana Basin (WMB) through YMJ‐Northern Channel with 1.41 ± 1.43/0.26 (mean ± standard deviation/total rms error) Sv, and further into the NPB through Kyushu‐Palau Ridge (KPR) Channel with 0.75 ± 0.53/0.18 Sv. Their difference 0.65 ± 1.35/0.28 Sv is the net lateral flux of LCPW into the WMB over the 2.5 yr. Analyses of a data‐assimilative ocean model solution suggest that the L‐PMOC transports through the deep channels are consistent with deep pressure gradients forced mainly by upper ocean processes.

Evaluation of thermo-diffusion and diffusion-thermo phenomenon on the reactive micropolar fluid motion over an extending device

ABSTRACT The dynamics of thermal-migration and micro-rotation of tiny particles are very essential in thermal engineering to improve the performance of their devices. This has inspired scientists to examine different micro-particles of fluid materials. As such, this article examines thermo-diffusion and diffusion-thermo effects on the motion of hydromagnetic reacting micropolar fluid along an elongated surface with lateral mass flux in porous media. This body of work also presents a report on thermophoretic phenomenon together with viscous dissipation and heat source. The model equations are firstly transformed from partial derivatives to ordinary ones by the use of some similarity quantities and subsequently tracked numerically. The nonlinearity of the involving equations has compelled the use of stable Runge-Kutta-Fehlberg approach with shooting technique to provide the needed solution. To emphasize the influence of the governing parameters impacting the flow fields, variety of graphs have been plotted and discussed qualitatively. Verification of the numerical code with existing data in the literature shows an excellent agreement as checked under limiting conditions. It is evidently shown that the material quantity causes the fluid motion to accelerate, both thermo-diffusion and diffusion-thermo strengthen the heat distribution in the system while the concentration profile declines with chemical reaction.

High Dissolved Carbon Concentration in Arid Rocky Mountain Streams.

Warming in mountains is known to intensify aridity and threaten water availability globally. Its impacts on water quality, however, have remained poorly understood. Here we collate long-term (multi-year to decadal mean), baseline stream concentrations and fluxes of dissolved organic and inorganic carbon, two essential indicators of water quality and soil carbon response to warming, across more than 100 streams in the United States Rocky Mountains. Results show a universal pattern of higher mean concentrations in more arid mountain streams with lower mean discharge, a long-term climate measure. A watershed reactor model revealed less lateral export of dissolved carbon (via less water flow) out of the watersheds in more arid sites, leading to more accumulation and higher concentrations. Lower concentrations typically occur in cold, steep, and compact mountains with higher snow fraction and lower vegetation cover, which generally have higher discharge and carbon fluxes. Inferring from a space-for-time perspective, the results indicate that as warming intensifies, lateral fluxes of dissolved carbon will decrease but concentrations will increase in these mountain streams. This indicates deteriorating water quality and potentially elevated CO2 emission directly from the land (instead of streams) in the Rockies and other mountain areas in the future climate.

National CO2budgets (2015–2020) inferred from atmospheric CO2observations in support of the global stocktake

Abstract. Accurate accounting of emissions and removals of CO2 is critical for the planning and verification of emission reduction targets in support of the Paris Agreement. Here, we present a pilot dataset of country-specific net carbon exchange (NCE; fossil plus terrestrial ecosystem fluxes) and terrestrial carbon stock changes aimed at informing countries' carbon budgets. These estimates are based on “top-down” NCE outputs from the v10 Orbiting Carbon Observatory (OCO-2) modeling intercomparison project (MIP), wherein an ensemble of inverse modeling groups conducted standardized experiments assimilating OCO-2 column-averaged dry-air mole fraction (XCO2) retrievals (ACOS v10), in situ CO2 measurements or combinations of these data. The v10 OCO-2 MIP NCE estimates are combined with “bottom-up” estimates of fossil fuel emissions and lateral carbon fluxes to estimate changes in terrestrial carbon stocks, which are impacted by anthropogenic and natural drivers. These flux and stock change estimates are reported annually (2015–2020) as both a global 1∘ × 1∘ gridded dataset and a country-level dataset and are available for download from the Committee on Earth Observation Satellites' (CEOS) website: https://doi.org/10.48588/npf6-sw92 (Byrne et al., 2022). Across the v10 OCO-2 MIP experiments, we obtain increases in the ensemble median terrestrial carbon stocks of 3.29–4.58 Pg CO2 yr−1 (0.90–1.25 Pg C yr−1). This is a result of broad increases in terrestrial carbon stocks across the northern extratropics, while the tropics generally have stock losses but with considerable regional variability and differences between v10 OCO-2 MIP experiments. We discuss the state of the science for tracking emissions and removals using top-down methods, including current limitations and future developments towards top-down monitoring and verification systems.

Relationship between coherent Reynolds shear stress and large-scale vortical structures

An equation that relates the principal derivative of the Reynolds shear stress and large-scale vorticity fluxes is developed and validated using experimental data in a cylinder near wake. When large-scale vortices prevail, the Reynolds shear stress associated with the coherent motion is a good approximation to the conventional Reynolds shear stress. The latter is essentially determined by the imbalance between the dynamic pressure arising from the streamwise non-homogeneity caused by large-scale vortical structures and the lateral coherent vorticity flux.

Agricultural practices drive elevated rates of topsoil decline across Kenya, but terracing and reduced tillage can reverse this

As agricultural land area increases to feed an expanding global population, soil erosion will likely accelerate, generating unsustainable losses of soil and nutrients. This is critical for Kenya where cropland expansion and nutrient loading from runoff and erosion is contributing to eutrophication of freshwater ecosystems and desertification. We used the Revised Universal Soil Loss Equation (RUSLE) to predict soil erosion rates under present land cover and potential natural vegetation nationally across Kenya. Simulating natural vegetation conditions allows the degree to which erosion rates are elevated under current land use practices to be determined. This methodology exploits new digital soil maps and two vegetation cover maps to model topsoil (top 20 cm) erosion rates, lifespans (the mass of topsoil divided by erosion rate), and lateral nutrient fluxes (nutrient concentration times erosion rate) under both scenarios. We estimated the mean soil erosion rate under current land cover at ~5.5 t ha−1 yr−1, ~3 times the rate estimated for natural vegetation cover (~1.8 t ha−1 yr−1), and equivalent to ~320 Mt yr−1 of topsoil lost nationwide. Under present erosion rates, ~8.8 Mt, ~315 Kt, and ~ 110 Kt of soil organic carbon, nitrogen and phosphorous are lost from soil every year, respectively. Further, 5.3 % of topsoils (~3.1 Mha), including at >25 % of croplands, have short lifespans (<100 years). Additional scenarios were tested that assume combinations of terracing and reduced tillage practices were adopted on croplands to mitigate erosion. Establishing bench terraces with zoned tillage could reduce soil losses by ≥75 %; up to 87.1 t ha−1 yr−1. These reductions are comparable to converting croplands to natural vegetation, demonstrating most agricultural soils can be conserved successfully. Extensive long-term monitoring of croplands with terraces and reduced tillage established is required to verify the efficacy of these agricultural support practices as indicated by our modelling.

Two-dimensional simulation of island permafrost degradation in Northeastern Tibetan Plateau

Despite the response of ground temperatures to increases in air temperature, discontinuous and isolated bodies of permafrost are strongly affected by lateral heat fluxes from adjacent unfrozen zones. However, many current simulations consider heat flow in only one-dimension (1D) and thus cannot represent two or three-dimensional effects. To address this issue, we use fine-scale in situ measurements and two-dimension (2D) heat conduction simulations and compare the simulated ground thermal evolution and degradation of island permafrost to a 1D simulation. The 2D simulation reasonably represented seasonal and interannual features of the ground thermal regime, such as temperature profile, thawing/frozen depth, and permafrost body width. Lateral thawing of the island permafrost was dominant, and was about one order of magnitude greater than the modelled vertical thaw. In the vertical direction, downward permafrost thaw caused by increase in air temperature rise was restricted by phase change of ground ice near the permafrost table during the simulation period, contributing little to the overall permafrost degradation. Rates of permafrost degradation in the 1D simulation were 1.6–1.8 times lower than the simulation considering lateral heat fluxes. The field monitoring and numerical simulation highlight the importance of coupling lateral heat transfer in permafrost models.

Characteristics and Drivers of Marine Heatwaves in 2021 Summer in East Korea Bay, Japan/East Sea

Marine heatwaves (MHWs) are persistent, discrete, extreme high-temperature events in the ocean, which can destructively affect marine ecosystems. Using satellite remote sensing data and reanalysis data from 1982 to 2021, we find that six indices characterizing the MHWs are in a remarkable increasing trend in the Japan/East Sea (JES), which shows that the most severe MHW events take place in the East Korean Bay (EKB) in the summer of 2021. Based on this finding, the present study focuses on the characteristics and mechanisms of the MHWs in the EKB and its adjacent areas from June to August 2021. The analysis reveals that the total days and mean intensity of MHWs that occur in the EKB are 1.84 and 1.47 times more than those averaged in the JES, respectively. It is shown that mechanisms for the occurrences of the MHWs in the summer of 2021 are caused by the atmospheric high-pressure system moving to the EKB area. Other reasons also decrease the water cooling: the net positive lateral heat fluxes across open boundaries, and the weak sea surface wind over the EKB area. Other possible reasons which cause the summer MHW events in 2021 need the oceanic numerical models to further investigate the issue.

Turbulence structures and entrainment length scales in large offshore wind farms

Abstract. The flow inside and around large offshore wind farms can range from smaller structures associated with the mechanical turbulence generated by wind turbines to larger structures indicative of the mesoscale flow. In this study, we explore the variation in turbulence structures and dominant scales of vertical entrainment above large offshore wind farms located in the North Sea, using data obtained from a research aircraft. The aircraft was flown upstream, downstream, and above wind farm clusters. Under neutrally stratified conditions, there is high ambient turbulence in the atmosphere and an elevated energy dissipation rate compared to stable conditions. The intensity of small-scale turbulence structures is increased above and downstream of the wind farm, and it prevails over mesoscale fluctuations. But in stable stratification, mesoscale flow structures are not only dominant upstream of the wind farm but also downstream. We observed that the vertical flux of horizontal momentum is the main source of energy recovery in large offshore wind farms, and it strongly depends on the magnitude of the length scales of the vertical wind velocity component. The dominant length scales of entrainment range from 20 to ∼60 m above the wind farm in all stratification strengths, and in the wake flow these scales range from 10 to ∼100 m only under near-neutral stratification. For strongly stable conditions, negligible vertical entrainment of momentum was observed even just 2 km downstream of large wind farms. We also observed that there is a significant lateral momentum flux above the offshore wind farms, especially under strongly stable conditions, which suggests that these wind farms do not satisfy the conditions of an “infinite wind farm”.

Incorporating temperature into seepage loss estimates for a large unlined irrigation canal

Quantifying seepage losses from unlined irrigation canals is necessary to improve water use and conservation. The use of heat as a tracer is widely used in quantifying seepage rates across the sediment–water interface. In this study, field observations and two-dimensional numerical models were used to simulate seepage losses during the 2018 and 2019 irrigation season in the Truckee Canal system. Nineteen transects were instrumented with temperature probes and stage recording devices for inverse modeling to derive seepage flux and volumetric losses over the 39 km length of canal. The numerical models for each transect were calibrated and validated using the two-year dataset. Soil zones and observation data were used in each numerical model to help guide calibration of vertical and lateral heat and fluid fluxes. Model simulations were used to derive multivariable regression equations that consider stage, temperature, and hydraulic gradient. The results demonstrate the value of long-term datasets that illustrate the seasonality of groundwater levels, siltation, stage, and temperature on seepage rates. Seepage rates estimated by the numerical models range from 0.16 to 4.6 m3/d m−1. Total annual volumetric losses estimated for 2018 and 2019 were 1.6 × 10-2 to 1.2 × 10-2 km3, respectively. The seepage losses estimated by this study account for 32 % to 41 % of the inflow volumes. Regression models were able to reproduce seepage time-series simulated by the numerical models reasonably well. In arid environments, water diverted into irrigation canals may be influenced by seasonal variations in temperature sufficient to influence the water accounting of conveyed surface flows.

Biogeochemical cycling of dissolved manganese in the Arabian Sea

The study of marine biogeochemical cycling of Mn, a bio-essential trace metal, is crucial to decipher its relative role in controlling the production and diversity of marine phytoplankton. Furthermore, Mn oxides and oxyhydroxides act as sinks for other trace elements in the water column, and Mn redox cycling may exert a major control on the vertical distribution of these elements. Dissolved Mn (dMn) concentrations are measured along two GEOTRACES-India (GI) transects, GI05 and GI10, for a basin-wide investigation of biogeochemical controls on the dMn distribution in the Arabian Sea (AS). Surface dMn distribution shows a strong spatial gradient, with relatively dMn-rich levels in the eastern AS (3.4–5.7 nmol kg−1) compared to the central and western AS (1.2–2.3 nmol kg−1). This correlates with the higher model estimates of soluble Mn input from the atmospheric dust deposition in the eastern AS (∼1.5–2.0 times of input in the western and central AS), and probable additional Mn input from riverine discharge and shallow shelf region of the eastern AS. In the south-eastern AS, a correlated increase in surface dMn with a decrease in salinity indicates the Mn input by the advection of dMn-rich, low-salinity Bay of Bengal (BoB) surface waters to the AS under the influence of the Northeast Monsoon Current. Our mass balance estimates for dMn in the euphotic waters (upper 100 m) suggest important contributions from atmospheric Mn deposition (14–99% of the total estimated input flux), and dMn input/dilution (13–74% of the total estimated input or removal flux) due to lateral/vertical advective-eddy diffusive mixing. Mn-oxidation and passive scavenging are determined to be the dominant processes for the dMn removal (61–99%) from the euphotic zone (upper 100 m), while export via Mn bio-assimilation is comparatively negligible.In the AS denitrification zone, the observed dMn maxima in the upper thermocline depths (∼200–400 m) result from the in situ reductive dissolution of Mn oxides and oxyhydroxides, and lateral advective-eddy diffusive Mn flux from the reducing sediments of the western Indian margin. Outside the denitrification zone, in the northern and western AS, a shift of dMn maxima to the lower thermocline waters (∼500 m) indicates the change in the source region and dMn input from the reducing margins of the north (Pakistan Margin) and north-western AS (Oman Margin), intersecting the core of the oxygen minima at the depth range of 400–600 m.Deep water (>1500 m) dMn concentrations are low (<0.3 nmol kg−1) in most of the AS regions. Anomalous dMn enrichment in the deep waters is observed in the southern and western AS, presumably indicating the influence of hydrothermal plumes emanating from the active hydrothermal vents over the Carlsberg Ridge and spreading ridges in the Gulf of Aden. A correlated and localized increase in dMn and dAl levels close to the Murray Ridge hints at dMn input from the sediment resuspension over the ridge and subsequent Mn dissolution in low-oxygen waters (DO < 20 µM) and/or diffusive flux from the Mn-rich pore waters.

Shallow Cumulus Entrainment Dynamics in a Sheared Environment

Abstract Vertical wind shear has long been known to tilt convective towers and reduce thermal ascent rates. The purpose of this study is to better understand the physical mechanisms responsible for reduced ascent rates in shallow convection. In particular, the study focuses on cloud-edge mass flux to assess how shear impacts mass-flux profiles of both the ensemble and individual clouds of various depths. A compositing algorithm is used to distill large-eddy simulation (LES) output to focus on up- and down-shear cloud edges that are not influenced by complex cloud geometry or nearby clouds. A direct entrainment algorithm is used to estimate the mass flux through the cloud surface. We find that the dynamics on the up- and down-shear sides are fundamentally different, with the entrainment of environmental momentum and dilution of buoyancy being primarily responsible for the reduced down-shear ascent rates. Direct estimates of fluid flow through the cloud interface indicate a counter-shear organized flow pattern that entrains on the down-shear side and detrains on the up-shear side, resulting from the subcloud shear being lifted into the cloud layer by the updraft. In spite of organized regions of entrainment and detrainment, the overall net lateral mass flux remains unchanged with respect to the no shear run, with weak detrainment present throughout cloud depth.