Vertical Diffusive Flux Research Articles

Field observations of seabed scour dynamics in front of a seawall during winter gales

Seawalls are vital for protecting coastal areas. However, the seabed in front of seawalls may undergo severe scouring. This can result in destabilization of the seawall structure, the underlying mechanisms of which remains unclear. Therefore, an integrated observation system consisting of acoustic and optical instruments was deployed in areas with severe seabed scouring. This observation system was used to observe sediment dynamics elements such as waves, currents, tides, turbulence and suspended sediment concentration (SSC) for 31 days in winter. Using advanced time–frequency analysis techniques including wavelet transform and spectrum analysis, we examined the dynamic factors associated with sediment resuspension and transport. In calm weather conditions, a notable increase in SSC was not observed indicating that the tidal dynamics were not sufficient for sediment suspension. During high winds, the SSC increased sharply to 12,222 mg/L, and the sediment vertical diffusion flux induced by turbulence was coupled to SSC, indicating that the increased SSC was predominantly attributed to local resuspension. Consistent temporal distribution of turbulence-induced sediment vertical diffusion flux and momentum flux in high wavelet power spectra highlights the important role of turbulence in sediment dynamics. Enhanced longshore currents during high wave conditions intensified sediment transportation. Horizontal net sediment fluxes notably increased to 769 t/m2 per day during winter gales, which had a significant effect on seabed erosion. This study reveals the key processes associated with seabed scouring in front of seawalls during gale events.

A global ocean state estimation using tidally induced vertical-mixing schemes

We updated the Estimated STate of the global Ocean for Climate research (ESTOC), a long-term ocean state estimation dataset that includes physical and biogeochemical fields. The physical field is optimized through a data synthesis experiment using a quasi-global four-dimensional variational data assimilation system, and under the optimized physical field, the biogeochemical field is optimized by using a Green’s function. To improve the reliability of the dynamics reproduced in the model, we implemented tidally induced turbulent mixing schemes and geothermal heat flux. This paper provides an overview of the assimilation system with an emphasis on updated elements and including details of the mixing schemes and presents preliminary results focusing on the warming in the abyssal ocean (below 3000 m) using this updated ESTOC published as Version 04a. The reproduction of both the climatological state and temporal change in abyssal-water properties is, on the whole, improved in this updated ESTOC in comparison with a previous version. This improved reproduction of the abyssal ocean state with the refined physical processes that play major roles in determining abyssal-water properties makes our new synthesized dataset valuable, at least for understanding the abyssal ocean. In this updated ESTOC, the abyssal-water warming results from a near-steady imbalance between the heating effect due to vertical diffusion and geothermal flux, and the cooling effect by advection and horizontal diffusion in the North Pacific, suggesting that the warming occurs over a long timescale. We also show that the downward heat flux related to bottom-intensified vertical mixing near the generation site of the internal tide plays an important role in determining the improved distribution of abyssal-water warming in the Pacific Ocean. These results suggest the importance of using knowledge of vertical mixing as a priori information in ocean state estimation.

Elevated Nutrient Supply Caused by the Approaching Kuroshio to the Southern Coast of Japan

Although the Kuroshio Current has been well-known for being nutrient poor in its upper layers, it sustains a great biodiversity within its region, including the Kuroshio Extension. Previous studies reported that the Kuroshio carries a large amount of nutrients in its dark subsurface layers, yet, the mechanisms of how these nutrients are brought to sunlit layers remain unclear. In a previous observational study, it is shown that when the Kuroshio flows near the shelf break, very strong turbulence with dissipation rates of O (10 -7 W kg -1), caused by trapped near-inertial internal waves, leads to the generation of large diffusive nitrate fluxes of 1-10 mmol N m -2 day -1. Nevertheless, since these results were obtained from just one transect survey, questions remain, such as how long the enhanced turbulent diffusive nitrate flux extends downstream, and how the Kuroshio path modulations affect this nutrient injection. In this study, observed features were reproduced by using a high-resolution nested simulation coupled with a N2PZD2 ecosystem model. The results show that when the model Kuroshio flows closely to the coast, it hits a small bump in the upstream region of the Hyuganada Sea producing negative potential vorticity. At the same time, trapped near-inertial internal waves in a streak of lowered lowest internal wave frequencies are reproduced near the area with low PV values, generating strong vertical mixing similar to the observations. Since more nitrate is brought upward on the continental shelf when the Kuroshio approaches to the coast, the enhanced vertical mixing effectively diffuses up the nitrate from subsurface to surface layers at a rate of ~O(1 mmol N m -2 day -1) toward 50-100 km downstream along the Kuroshio. Further analyses on how the distance between the model Kuroshio and the southeastern coast of Kyushu affects the nutrient supply, suggest that when the Kuroshio flows closer to the coast, the internal wave kinetic energy, the nitrate concentration, its vertical gradients and vertical diffusive flux increase.

Observations of turbulent mixing and vertical diffusive salt flux in the Changjiang Diluted Water

Observations of turbulent mixing and vertical diffusive salt flux in the Changjiang Diluted Water

SuppressedpCO2in the Southern Ocean Due to the Interaction Between Current and Wind

AbstractThe Southern Ocean, an important region for the uptake of anthropogenic carbon dioxide (CO2), features strong surface currents due to substantial mesoscale meanders and eddies. These features interact with the wind and modify the momentum transfer from the atmosphere to the ocean. Although such interactions are known to reduce momentum transfer, their impact on air‐sea carbon exchange remains unclear. Using a 1/20° physical‐biogeochemical coupled ocean model, we examined the impact of the current‐wind interaction on the surface carbon concentration and the air‐sea carbon exchange in the Southern Ocean. The current‐wind interaction decreased winter partial pressure of CO2(pCO2) at the ocean surface mainly south of the northern subantarctic front. It also reducedpCO2in summer, indicating enhanced uptake, but not to the same extent as the winter loss. Consequently, the net outgassing of CO2was found to be reduced by approximately 17%when including current‐wind interaction. These changes stem from the combined effect of vertical mixing and Ekman divergence. A budget analysis of dissolved inorganic carbon (DIC) revealed that a weakening of vertical mixing by current‐wind interaction reduces the carbon supply from below, and particularly so in winter. The weaker wind stress additionally lowers the subsurface DIC concentration in summer, which can affect the vertical diffusive flux of carbon in winter. Our study suggests that ignoring current‐wind interactions in the Southern Ocean can overestimate winter CO2outgassing.

Vertical Mixing and Heat Fluxes Conditioned by a Seismically Imaged Oceanic Front

The southwest Atlantic gyre connects several distinct water masses, which means that this oceanic region is characterized by a complex frontal system and enhanced water mass modification. Despite its significance, the distribution and variability of vertical mixing rates have yet to be determined for this system. Specifically, potential conditioning of mixing rates by frontal structures, in this location and elsewhere, is poorly understood. Here, we analyze vertical seismic (i.e., acoustic) sections from a three-dimensional survey that straddles a major front along the northern portion of the Brazil-Falkland Confluence. Hydrographic analyses constrain the structure and properties of water masses. By spectrally analyzing seismic reflectivity, we calculate spatial and temporal distributions of the dissipation rate of turbulent kinetic energy, ε, of diapycnal mixing rate, K, and of vertical diffusive heat flux, FH. We show that estimates of ε, K, and FH are elevated compared to regional and global mean values. Notably, cross-sectional mean estimates vary little over a 6 week period whilst smaller scale thermohaline structures appear to have a spatially localized effect upon ε, K, and FH. In contrast, a mesoscale front modifies ε and K to a depth of 1 km, across a region of O(100) km. This front clearly enhances mixing rates, both adjacent to its surface outcrop and beneath the mixed layer, whilst also locally suppressing ε and K to a depth of 1 km. As a result, estimates of FH increase by a factor of two in the vicinity of the surface outcrop of the front. Our results yield estimates of ε, K and FH that can be attributed to identifiable thermohaline structures and they show that fronts can play a significant role in water mass modification to depths of 1 km.

Decreased Stratification in the Abyssal Southwest Pacific Basin and Implications for the Energy Budget

AbstractAs the abyssal oceans warm, stratification is also expected to change in response. This change may impact mixing and vertical transport by altering the buoyancy flux, internal wave generation, and turbulent dissipation. In this study, repeated surveys of three hydrographic sections in the Southwest Pacific Basin between the 1990s and 2010s are used to estimate the change in buoyancy frequency . We find that below the °C isotherm, is on average reduced by a scaling factor of , a 12% reduction, per decade that intensifies with depth. At °C, we observe the biggest change: , or a 29% reduction per decade. Within the same period, the magnitude of vertical diffusive heat flux is also reduced by about , although this estimate is sensitive to the choice of estimated diffusivity. Finally, implications of these results for the heat budget and global ocean circulation are qualitatively discussed.

Abyssal Heat Budget in the Southwest Pacific Basin

AbstractThe abyssal Southwest Pacific Basin has warmed significantly between 1992-2017, consistent with warming along the bottom limb of the meridional overturning circulation seen throughout the global oceans. Here we present a framework for assessing the abyssal heat budget that includes the time-dependent unsteady effects of decadal warming and direct and indirect estimates of diapycnal mixing from microscale temperature measurements and finescale parameterizations. The unsteady terms estimated from the decadalwarming rate are shown to be within a factor of 3 of the steady state terms in the abyssal heat budget for the coldest portion of the water column and therefore, cannot be ignored. We show that a reduction in the lateral heat flux for the coldest temperature classes compensated by an increase in warmer waters advected into the basin has important implications for the heat balance and diffusive heat fluxes in the basin. Finally, vertical diffusive heat fluxes are estimated in different ways: using the newly available CTD-mounted microscale temperature measurements, a finescale strain parameterization, and a vertical kinetic energy parameterization from data along the P06 transect along 32.5°S. The unsteady-state abyssal heat budget for the basin shows closure within error estimates, demonstrating that (i) unsteady terms have become consequential for the heat balance in the isotherms closest to the ocean bottom and (ii) direct and indirect estimates from full depth GO-SHIP hydrographic transects averaged over similarly large spatial and temporal scales can capture the basin-averaged abyssal mixing needed to close the deep overturning circulation.

On the Effects of Mesospheric and Lower Thermospheric Oxygen Chemistry on the Thermosphere and Ionosphere Semiannual Oscillation

AbstractThis study quantifies mesosphere/lower thermosphere (MLT) oxygen chemical contributions to the global thermosphere‐ionosphere (T‐I) semiannual oscillation (SAO) using a series of numerical experiments from the National Center for Atmospheric Research (NCAR) thermosphere‐ionosphere‐mesosphere‐electrodynamics general circulation model (TIME‐GCM) that isolate essential chemical processes affecting O and O2 in the MLT region. We track the vertical dynamical, diffusive, and chemical fluxes of O and O2 in and out of two control volumes between ∼80 and 130 km using a finite volume approach to the individual species continuity equation to investigate their relative importance on the global T‐I SAO. TIME‐GCM results indicate that the global T‐I SAO amplitude and phase is fairly insensitive to significant changes in odd oxygen chemical reaction rates in the MLT. While chemistry has an appreciable effect on O in the MLT region, sensitivity to changes in odd oxygen and odd hydrogen chemical rates appear to be offset by a consequent adjustment in the vertical bulk wind and eddy diffusive transport of O locally, rendering their effects inconsequential to the global T‐I SAO aloft. The implications of our findings for reproducing a self‐consistent global T‐I SAO in the NCAR thermosphere‐ionosphere‐electrodynamics general circulation model (TIE‐GCM) with a lower boundary near ∼100 km are discussed. Specifically, including latitude‐season variations in O, O2, and N2 from NRLMSIS® 2.0 at the lower boundary of the TIE‐GCM near ∼100 km improves its representation of the climatological T‐I SAO. However, reformulating the TIE‐GCM temperature lower boundary condition could further improve its ability to simulate the T‐I SAO from first‐principles.

Oxygen Seasonality, Utilization Rate, and Impacts of Vertical Mixing in the Eighteen Degree Water Region of the Sargasso Sea as Observed by Profiling Biogeochemical Floats

AbstractSeasonal oxygen structure and utilization in the Sargasso Sea are characterized using nine profiling floats with oxygen 2021 sensors (years 2005–2008), deployed in an Eighteen Degree Water (EDW) experiment (CLIMODE). During autumn‐winter when the mixed layer is deepening, oxygen increases from the surface to the base of the EDW at 400 m. During spring‐summer, oxygen decreases except between the seasonal pycnocline and compensation depth, creating the seasonal shallow oxygen maximum layer (SOMax) with oxygen production of 0.04 μmol kg−1·day−1. In the underlying seasonal oxygen minimum (SOMin), the oxygen utilization rate (OUR) is 0.10 μmol kg−1·day−1, decreasing with depth to 0.04 μmol kg−1·day−1 in the EDW. Remineralization in May to August is double that of August to November. The Sargasso Sea is a net carbon producer; estimated annual export production from the top 100–250 m is 2.9 mol C m−2 and from the top 400 m is 4.2 mol C m−2. Below the EDW, oxygen decreases seasonally at the same time as in the EDW, indicating remineralization down to 700 m. However, on isopycnals in this deeper layer, oxygen increases during May to September, likely due to lateral advection from nonlocal surface outcrops. Summer shoaling of these isopycnals creates this paradox. The complex vertical oxygen structure in the upper 200 m enables important vertical diffusive flux that modifies the OUR calculated from oxygen change. Ignoring mixing underestimates maximum remineralization by 19% and underestimates maximum net production by 88%. However, vertical mixing is negligible in the deeper layers, so the associated total integrated remineralization error is 5%–9%.

Enhanced subsurface mixing due to near-inertial waves: observation from Seychelles-Chagos Thermocline Ridge

The Seychelles-Chagos Thermocline Ridge (SCTR) is a key region which plays a major role in various regional climatic processes at all time scales. The vertical mixing at the base of mixed layer may be climatically important at this region, where a shallow thermocline and highly variable wind activity exists. In this study, we provide evidence of strong vertical mixing at the base of mixed layer and thermocline associated with near-inertial waves (NIWs) using the microstructure profiler time series data during boreal summer 2014. During the study, it was observed that the surface turbulent layer was mostly confined in the mixed layer with turbulent kinetic energy dissipation rate of the order of 10−6 Wkg−1. Elevated dissipation rate in the order of 10−7–10−6 Wkg−1 was also observed at the base of the mixed layer and thermocline (60–100 m) during the entire period of observation. The estimated vertical diffusive heat flux across the mixed layer base (100 Wm−2) was strong and a moderate downward flux (~ 30 Wm−2) was noticed in the thermocline region. The analysis showed that the strong NIW triggered by a strong wind event were responsible for the high dissipation rate at the base of mixed layer and at deeper depths. The higher-mode NIW create elevated vertical shear at the base the mixed layer, which leads to the enhanced mixing. The study also showed the possible role of internal tides on the subsurface mixing in the region. The NIW-induced mixing may have important role on the mixed layer heat balance in a climatically active SCTR.

Surface cooling caused by rare but intense near-inertial wave induced mixing in the tropical Atlantic

The direct response of the tropical mixed layer to near-inertial waves (NIWs) has only rarely been observed. Here, we present upper-ocean turbulence data that provide evidence for a strongly elevated vertical diffusive heat flux across the base of the mixed layer in the presence of a NIW, thereby cooling the mixed layer at a rate of 244 W m−2 over the 20 h of continuous measurements. We investigate the seasonal cycle of strong NIW events and find that despite their local intermittent nature, they occur preferentially during boreal summer, presumably associated with the passage of atmospheric African Easterly Waves. We illustrate the impact of these rare but intense NIW induced mixing events on the mixed layer heat balance, highlight their contribution to the seasonal evolution of sea surface temperature, and discuss their potential impact on biological productivity in the tropical North Atlantic.

Internal phosphorus loading rate (iPLR) in a low-P stormwater treatment wetland

Low-phosphorus (P) wetland discharges (e.g., < 20 μg/L), such as those now commonly observed in Stormwater Treatment Areas (STAs) for Everglades restoration, are likely influenced by internal P loads. This study estimated for the first time the “aggregated” internal P loading rate (iPLR) (including soil and biotic vectors) in a low-P treatment wetland along with calculated vertical soil diffusive fluxes. In situ mesocosms (1.5-m diameter polyethylene cylinders) equipped with closeable windows were deployed at three locations in an STA, enclosing parcels of local vegetation and soil. The mesocosms isolated the water column P response from longitudinal P transport (flow) so the local soil-water-biota P dynamics and equilibria could be observed during no-flow conditions. The water column inside and outside the mesocosms was sampled at each location during three two-week no-flow batch incubations. A first-order model was calibrated to mesocosm batch reaction time series to estimate the iPLR (with uncertainty) at each study site. Soil diffusive flux rates were estimated from vertical porewater P concentration profiles measured at each site. Results showed that: 1) strong gradients in water column equilibrium P concentrations existed along the direction of flow (92–20 μg/L from inflow to outflow), 2) a corresponding gradient also existed for model-predicted iPLR (~6.6 to 1.9 mg/m2-d), 3) these aggregated internal loads were more than 20× greater than calculated soil diffusive fluxes, and 4) an annual extrapolation suggested that average iPLR could be approximately equivalent to the average annual inflow P loading rate to the wetland (1.3 g/m2-yr). This study demonstrates that aggregated internal P loading can greatly exceed more commonly estimated vertical soil diffusive fluxes in low-P wetlands and can be a substantial contributor to water column P concentrations. Furthermore, measuring and modeling water P equilibria during no-flow intervals was a useful method to estimate internal loading in these wetlands.

The Impact of Diffusive Water Vapor Transport on Snow Profiles in Deep and Shallow Snow Covers and on Sea Ice

Water vapor transport has been highlighted as a critical process in Arctic snowpacks, shaping the snow cover structure in terms of density, thermal conductivity, and temperature profile among others. Here, we present an attempt to describe the thermally-induced vertical diffusion of water vapor in the snow cover and its effects of the snowpack structure using the SNOWPACK model. Convection, that may also constitute a significant part of vapor transport, is not addressed. Assuming saturated conditions at the upper boundary of the snowpack and as initial condition, the vapor flux between snow layers is expressed by a 1-dimensional transient diffusion equation, which is solved with a finite difference routine. The implications on the snowpack of this vertical diffusive flux, are analyzed using metrics such as the cumulative density change due to diffusive vapor transport, the degree of over- or undersaturation, the instantaneous snow density change rate, and the percentage of snow density change. We present results for four different regions sampling the space of natural snow cover variability: Alpine, Subarctic, Arctic, and Antarctic sea ice. The largest impact of diffusive water vapor transport is observed in snow on sea ice in the Weddell Sea and the shallow Arctic snowpack. The simulations show significant density reductions upon inclusion of diffusive water vapor transport: cumulative density changes from diffusive vapor transport can reach �62 and �66 kg m�3 for the bottom layer in the shallow Arctic snowpack and snow on sea ice, respectively. For comparison, in deeper snow covers, they rarely exceed �40 kg m�3. This leads to changes in density for shallow snowpacks at the soil-snow interface in the range of �5 to �21%. Mirroring the density decease at depth is a thicker deposition layer above it with increase in density around 7.5%. Similarly, for the sea ice, the density decreased at the sea ice-snow interface by �20%. We acknowledge that vapor transport by diffusion may in some snow covers�such as in thin tundra snow�be small compared to convective transport, which will have to be addressed in future work.

Quantification of the Vertical Transport and Escape of Atomic Hydrogen in the Terrestrial Upper Atmosphere

AbstractMeasurements of the limiting escape rate of atomic hydrogen (H) atoms at Earth and the relative significance of thermal evaporation and nonthermal escape mechanisms, such as charge exchange and polar wind, have long been lacking. Our recent development of sophisticated radiative transport analysis techniques now enables the reliable interpretation of remotely sensed measurements of optically thick H emission, such as those acquired along the Earth's limb by the Global Ultraviolet Imager (GUVI) onboard the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) spacecraft, in terms of physical parameters such as exobase density and, crucially, vertical diffusive flux. In this work, we present results from a systematic investigation of H Lya emission measured by TIMED/GUVI along the Earth's dayside limb from 2002–2007, which we use to derive the vertical H flux and associated density distribution from 250 km out to 1 Earth radius. Our analysis reveals that the vertical flux of thermospheric H is nearly constant over a large range of solar activity and typically exceeds the calculated thermal evaporative flux, suggesting that terrestrial H escape is indeed limited by its vertical diffusion. The excess supply of H atoms to the exobase associated with large observed vertical fluxes requires that nonthermal escape mechanisms be operative for steady‐state continuity balance. We find that such nonthermal processes are a particularly significant component of total H escape during low solar activity, when thermal evaporation is weakest.

Air-soil diffusive exchange of PAHs in an urban park of Shanghai based on polyethylene passive sampling: Vertical distribution, vegetation influence and diffusive flux

Compared with dry and wet deposition rates, air-soil exchange fluxes cannot be directly measured experimentally. Polyethylene passive sampling was applied to assess transport directions and to measure concentration gradients in order to calculate diffusive fluxes of polycyclic aromatic hydrocarbons (PAHs) across the air-soil interface in an urban park of Shanghai, China. Seven campaigns with high spatial resolution sampling at 18 heights between 0 and 200 cm above the ground were conducted in 2017–2018. Air-to-soil deposition was observed, e.g. for phenanthrene, and soil-to-air volatilization for high molecular weight compounds, such as benzo[b]fluoranthene. Significant linear correlations between gaseous PAH concentration and log-transformed height were observed. Influence of vegetation on vertical concentration gradients of gaseous PAHs was insignificant in most cases except during the growing season. Local micrometeorological conditions resulted in a directional eddy diffusion in air and then influenced vertical diffusion of gaseous PAHs. Furthermore, the vertical eddy diffusivity was estimated as a function of distance to the air-soil surface. Air-soil exchange fluxes based on the Mackay's fugacity approach were calculated and confirmed by diffusive fluxes within air layer based on vertical concentration gradient of PAHs and eddy/molecular diffusion. Polyethylene passive sampling technology provides a useful tool to investigate air-soil exchange process.

The effects of KT ≠ KS in a Stommel-like model of the upper Atlantic Meridional Overturning Circulation under steady surface flux forcing

This study examines a simple 6-box model of a single pole-to-pole ocean basin. Each of a northern "polar gyre," a southern "polar gyre," and an "equatorial gyre," consisting of north and south subtropical gyres plus the equatorial region, is represented by two boxes: a surface box receiving constant fluxes of both temperature (heat) and salt (freshwater) and a deep box. The model includes four dominant processes: surface flux forcing, horizontal meridional advection driven by Southern Ocean winds, horizontal eddy diffusion at gyre boundaries, and convection, as well as the process of vertical diffusion by small-scale processes. Provided that heat loss from the northern polar gyre is sufficiently larger than that from the southern polar gyre, a steady-state Atlantic Meridional Overturning Circulation (AMOC)-like system, i. e., one with sinking in the north polar gyre and upwelling in a weakly stratified southern polar gyre, is obtained at present values of RF ≡ βFS / αFT, the ratio of surface forcing by fluxes of temperature (T ) and salinity (S ) in the equatorial gyre. Despite the fact that vertical diffusive fluxes are much smaller than those associated with all the other processes, it is shown that implementation in this model of a simple water mass–based representation of different vertical diffusivities for T and S, the two water properties that, with pressure, determine the density of seawater, can lead to profound change in the steady-state modes of the system. With equal diffusivities, the AMOC-like mode with north polar convection shifts abruptly to a mode with equatorial convection at sufficiently large values of RF. With unequal diffusivities, this mode boundary is replaced by an intermediate region of RF values in which all three gyres are stratified. The existence and extent of this stratified regime is shown to result predominantly from the differences between vertical turbulent diffusivities of T and S in the "salt fingering" equatorial gyre. Existence of a stratified regime at values of RF somewhat larger that present implies a tendency towards stable stratification throughout the oceans if, under climate change, the equatorial diffusivity difference were to increase as a result of water mass changes in the subtropical gyres and/or an increase in RF as a result of increased atmospheric freshwater fluxes and/or decreased heat fluxes. This tendency towards an everywhere-stratified ocean is independent of that expected from increased freshwater addition to surface polar oceans due to ice melt.

Slow acidification of the winter mixed layer in the subarctic western North Pacific

AbstractWe used carbon dioxide (CO2) system data collected during 1999–2015 to investigate ocean acidification at time series sites in the western subarctic region of the North Pacific Ocean. The annual mean pH at station K2 decreased at a rate of 0.0025 ± 0.0010 year−1 mostly in response to oceanic uptake of anthropogenic CO2. The Revelle factor increased rapidly (0.046 ± 0.022 year−1), an indication that the buffering capacity of this region of the ocean has declined faster than at other time series sites. In the western subarctic region, the pH during the winter decline at a slower rate of 0.0008 ± 0.0004 year−1. This was attributed to a reduced rate of increase of dissolved inorganic carbon (DIC) and an increase of total alkalinity (TA). The reduction of DIC increase was caused by the decline of surface water density associated with the pycnocline depression and the reduction of vertical diffusion flux from the upper pycnocline. These physical changes were probably caused by northward shrinkage of the western subarctic gyre and global warming. Meanwhile, the contribution of the density decline to the TA increase is canceled out by that of the reduced vertical diffusive flux. We speculated that the winter TA increase is caused mainly by the accumulation of TA due to the weakened calcification by organisms during the winter.

Dissolved iron and iron isotopes in the southeastern Pacific Ocean

AbstractThe Southeast Pacific Ocean is a severely understudied yet dynamic region for trace metals such as iron, since it experiences steep redox and productivity gradients in upper waters and strong hydrothermal iron inputs to deep waters. In this study, we report the dissolved iron (dFe) distribution from seven stations and Fe isotope ratios (δ56Fe) from three of these stations across a near‐zonal transect from 20 to 27°S. We found elevated dFe concentrations associated with the oxygen‐deficient zone (ODZ), with light δ56Fe implicating porewater fluxes of reduced Fe. However, temporal dFe variability and rapid δ56Fe shifts with depth suggest gradients in ODZ Fe source and/or redox processes vary over short‐depth/spatial scales. The dFe concentrations decreased rapidly offshore, and in the upper ocean dFe was controlled by biological processes, resulting in an Fe:C ratio of 4.2 µmol/mol. Calculated vertical diffusive Fe fluxes were greater than published dust inputs to surface waters, but both were orders of magnitude lower than horizontal diffusive fluxes, which dominate dFe delivery to the gyre. The δ56Fe data in the deep sea showed evidence for a −0.2‰ Antarctic Intermediate Water end‐member and a heavy δ56Fe of +0.55‰ for distally transported hydrothermal dissolved Fe from the East Pacific Rise. These heavy δ56Fe values were contrasted with the near‐crustal δ56Fe recorded in the hydrothermal plume reaching Station ALOHA in the North Pacific. The heavy hydrothermal δ56Fe precludes a nanopyrite composition of hydrothermal dFe and instead suggests the presence of oxides or, more likely, binding of hydrothermal dFe by organic ligands in the distal plume.

Equilibrium and non-equilibrium concentration fluctuations in a critical binary mixture.

When a macroscopic concentration gradient is present across a binary mixture, long-ranged non-equilibrium concentration fluctuations (NCF) appear as a consequence of the coupling between the gradient and spontaneous equilibrium velocity fluctuations. Long-ranged equilibrium concentration fluctuations (ECF) may be also observed when the mixture is close to a critical point. Here we study the interplay between NCF and critical ECF in a near-critical mixture aniline/cyclohexane in the presence of a vertical concentration gradient. To this aim, we exploit a commercial optical microscope and a simple, custom-made, temperature-controlled cell to obtain simultaneous static and dynamic scattering information on the fluctuations. We first characterise the critical ECF at fixed temperature T above the upper critical solution temperature Tc, in the wide temperature range [Formula: see text] °C. In this range, we observe the expected critical scaling behaviour for both the scattering intensity and the mass diffusion coefficient and we determine the critical exponents [Formula: see text], [Formula: see text] and [Formula: see text], which are found in agreement with the 3D Ising values. We then study the system in the two-phase region (T < T c). In particular, we characterise the interplay between ECF and NCF when the mixture, initially at a temperature Ti, is rapidly brought to a temperature T f > T i. During the transient, a vertical diffusive mass flux is present that causes the onset of NCF, whose amplitude vanishes with time, as the flux goes to zero. We also study the time dependence of the equilibrium scattering intensity I eq, of the crossover wave vector q co and of the diffusion coefficient D during diffusion and find that all these quantities exhibit an exponential relaxation enslaved to the diffusive kinetics.