Explore the vast range of titles available.

The Indonesian seas are a renowned global biodiversity hotspot, yet nutrient sources and fluxes (especially the vertical flux) sustaining this richness remain unclear. Here, we used non‐atmospheric helium‐3 (3He) to constrain the vertical diffusion coefficient (Kd) in the Indonesian seas, which ranges from 5.2 × 10−5 to 2.3 × 10−3 m2 s−1 and averages 6.6 × 10−4 m2 s−1, a value notably higher than those found in the open ocean and in most marginal seas. We estimated that 6.9 ± 7.9 mmol m−2 d−1 of nitrate (NO3−) is vertically transported into the surface mixed layer, that is, >90% of the total NO3− required to support a net community production (NCP) of 470 ± 467 mg‐C m−2 d−1. Regions with narrow straits, steep topography and dynamic circulation with strong vertical mixing display high NCP and chlorophyll‐a, suggesting that vertical nutrient transport dominates biological productivity. Findings highlight the importance of vertical mixing in supplying nutrients and maintaining the extraordinary biological productivity and diversity in the Indonesian seas. Plain Language Summary The Indonesian seas, at the center of the Indo‐Pacific Intersection, are recognized as a global hotspot of marine biodiversity. However, nutrient (e.g., nitrate) cycling in the Indonesian seas is poorly understood, such that nutrient sources and fluxes sustaining biological production remain unknown. The Indonesian seas lie on a plate tectonic belt, where intense submarine hydrothermal venting releases abundant primordial isotopic helium (3He) into the ocean interior that outgasses at surface providing an ideal tracer of vertical transport. We find that vertical diffusion in the Indonesian seas is notably stronger than those found in the open ocean and in most marginal seas, with a mean vertical diffusion coefficient (Kd) of 6.6 × 10−4 m2 s−1. Nitrate is vertically transported into the surface mixed layer at a rate of 6.9 ± 7.9 mmol m−2 d−1, which supports >90% of net community production (NCP) in the Indonesian seas. Here, narrow straits, steep and irregular topography and dynamic circulation with strong vertical mixing result in high chlorophyll‐a concentrations (a measure of primary producers' biomass) and NCP. This vertical nutrient transport supplies essential conditions for algal growth and “fuels” food web biological productivity. Thus, we suggest that strong vertical mixing plays a key role in making the Indonesian seas a global biodiversity hotspot. Key Points The Indonesian seas present strong and spatially variable vertical mixing Vertical mixing supplies >90% of nutrients in the surface mixed layer of Indonesian seas Here, vertical nutrient supply provides essential biogenic elements supporting high net community production and a biodiversity hotspot

Flow in the stable boundary layer is examined at four contrasting sites with greater upwind surface roughness. The surface heterogeneity is disorganized and in some cases weak as commonly occurs. With low wind speeds, the vertical divergence (or convergence) of the momentum and heat fluxes can be large near the surface in what is normally assumed to be the surface layer where such divergence is neglected. For the two most heterogeneous sites, a shallow “new” boundary layer is captured by the tower observations, analogous to an internal boundary layer but more complex. Above the new boundary layer, the magnitudes of the downward fluxes of heat and momentum increase with height in a transition layer, reach a maximum, and then decrease with height in an overlying regional boundary layer. Similar structure is observed at the site with rolling terrain where the shallow new boundary layer at the surface is identified as cold-air drainage generated by the local slope above which the flow undergoes transition to an overlying regional flow. Significant flux divergence near the surface is generated even over an ice floe for low wind speeds and in a shallow Ekman layer that forms during the polar night. For higher wind speeds, the magnitude of the downward fluxes decreases gradually with height at all levels as in a traditional boundary layer.

Experimental closure of the surface energy balance during convective periods is a long-standing problem. With experimental data from the Idealized horizontal Planar Array experiment for Quantifying Surface heterogeneity, the terms of the temperature-tendency equation are computed, with an emphasis on the total derivative. The experiment occurred at the Surface Layer Turbulence and Environmental Science Test facility at the U.S. Army Dugway Proving Ground during the summer of 2019. The experimental layout contained an array of 21 flux stations over a 1 km2 grid. Sensible heat fluxes show high spatial variability, with maximum variability occurring during convective periods. Maximum variability in the vertical heat flux is 50–80 W m-2 (median variability of 40%), while in the horizontal flux, it is 200–500 W m-2 (median variability of 48% for the streamwise and 40% for the spanwise fluxes). Ensemble averages computed during convective afternoon periods show large magnitudes of horizontal advection (48 W m-3 or 172 K h-1) and vertical flux divergence (13 W m-3 or 47 K h-1). Probability density functions of the total derivative from convective cases show mean volumetric heating rates of 43 W m-3 (154 K h-1) compared to 13 W m-3 (47 K h-1) on non-convective days. A conceptual model based on persistent mean flow structures from local-surface-temperature heterogeneities may explain the observed advection. The model describes the difference between locally-driven advection and advection driven by larger-scale forcings. Of the cases examined, 83% with streamwise and 81% with spanwise advection during unstable periods are classified as locally driven by nearby surface thermal heterogeneities.

Eddy-covariance measurements made in the marine atmospheric boundary layer above a high Arctic fjord (Adventfjorden, Svalbard) are analyzed. When conditions are unstable, but close to neutral −0.1 < z/L < 0, where z is the height, and L is the Obukhov length, the exchange coefficient for sensible heat CH is significantly enhanced compared with that expected from classical surface-layer theory. Cospectra of the vertical velocity component (w) and temperature (T) reveal that a high-frequency peak develops at f ≈ 1 Hz for z/L > − 0.15. A quadrant analysis reveals that the contribution from downdrafts to the vertical heat flux increases as conditions become close to neutral. These findings are the signature of the evolving unstable very-close-to-neutral (UVCN) regime previously shown to enhance the magnitude of sensible and latent heat fluxes in the marine surface layer over the Baltic Sea. Our data reveal the significance of the UVCN regime for the vertical flux of the carbon dioxide (CO2) concentration (C). The cospectrum of w and C clearly shows how the high-frequency peak grows in magnitude for z/L > − 0.15, while the high-frequency peak dominates for z/L > − 0.02. As found for the heat flux, the quadrant analysis of the CO2 flux shows a connection between the additional small-scale turbulence and downdrafts from above. In contrast to the vertical fluxes of sensible and latent heat, which are primarily enhanced by the very different properties of the air from aloft (colder and drier) during UVCN conditions, the increase in the air–sea transfer of CO2 is possibly a result of the additional small-scale turbulence causing an increase in the water-side turbulence. The data indicate an increase in the gas-transfer velocity for CO2 for z/L > − 0.15 but with a large scatter. During the nearly 2 months of continuous measurements (March–April 2013), as much as 36% of all data are associated with the stability range −0.15 < z/L < 0, suggesting that the UVCN regime is of significance in the wintertime Arctic for the air–sea transfer of heat and possibly also CO2.

This study presents analysis of the nearshore marine atmospheric surface layer during Super Typhoon Yagi using detailed observational data. As the typhoon's peripheral circulation intensified, wind speeds increased across the observed heights (16.4–35.1 m), while the vertical wind profile remained relatively uniform, likely due to the limited vertical observation range and strong turbulent mixing. Turbulence in the near‐surface layer transitioned from being dominated by large‐scale vortices to being governed by more active, smaller‐scale vortices. Scale analysis, including ensemble empirical mode decomposition and spectral analysis, revealed that the characteristic turbulent length scale decreased during the typhoon process, suggesting under extreme nonequilibrium conditions, motions that contributed significantly to vertical flux transport were concentrated toward smaller scales. These multiscale turbulent processes played a crucial role in reshaping the wind profile and regulating vertical energy transport, thus offering new insights into mechanisms governing surface layer dynamics under the peripheral circulation of a typhoon.

The vertical flux of particulate organic carbon (POC) from the surface to the deep ocean regulates the ocean carbon uptake, with implications for the Earth's carbon cycle. It is debated in the literature what functional form best describes the attenuation of this flux with depth. The wide scatter found in measurements of the flux has impeded progress on this question. A theoretical model is proposed, which treats this scatter as key information rather than noise. Based on the evidence that the POC flux data follow a lognormal distribution, the model predicts the vertical POC flux profile as a function of three parameters: log‐mean and log‐standard deviation of the POC export flux, and a depth scaling term consistent with previous functional forms. The model captures the large variability observed in individual POC flux profiles and illustrates that large POC flux events contribute substantially to the vertical transfer of POC.

Thwaites Glacier is one of the fastest‐changing ice‐ocean systems in Antarctica. Basal melting beneath Thwaites' floating ice shelf, especially around pinning points and at the grounding line, sets the rate of ice loss and Thwaites' contribution to global sea‐level rise. The rate of basal melting is controlled by the transport of heat into and through the ice–ocean boundary layer toward the ice base. Here we present the first turbulence observations from the grounding line of Thwaites Eastern Ice Shelf. We demonstrate that contrary to expectations, the turbulence‐driven vertical flux of heat into the ice–ocean boundary layer is insufficient to sustain the basal melt rate. Instead, most of the heat required must be delivered by lateral fluxes driven by the large‐scale advective circulation. Lateral processes likely dominate beneath the most unstable warm‐cavity ice shelves, and thus must be fully incorporated into parameterizations of ice shelf basal melting. Plain Language Summary Our knowledge of the response of Thwaites Glacier to the changing climate of the 21st century remains highly uncertain despite the significant risk it poses to global sea‐level if it were to collapse entirely. The rate of ice loss from Thwaites over at least the next 100 years will be controlled by the rate at which its ice shelf—the portion of the glacier that floats on the ocean—is melted from below by ‘warm’ ocean water. Understanding the processes that drive this basal melting is therefore essential. Here we present the first observations of vertical mixing and small‐scale ocean turbulence right at the critical point that Thwaites Glacier first begins to float (referred to as the grounding line). Unexpectedly, we find that the heat required to maintain basal melting is not brought to the ice base via vertical mixing, but rather it is delivered horizontally via the large‐scale ocean circulation. The dominance of lateral processes is likely to be important for many of Antarctica's most unstable glaciers which overlie warm ocean water, and therefore must be fully incorporated into models of basal melting to reliably predict the rate of future sea‐level rise. Key Points First observations of oceanic turbulence from the grounding line of Thwaites Eastern Ice Shelf Rather than the vertical turbulent heat flux, the lateral advective heat flux within 4 m of the ice base sustains the basal melt rate Lateral heat fluxes driven by the large‐scale advective circulation likely dominate melting beneath many West Antarctica's ice shelves

Top‐down entrainment shapes the vertical gradients of sensible heat, latent heat, and CO2 fluxes, influencing the interpretation of eddy covariance (EC) measurements in the unstable atmospheric surface layer (ASL). Using large eddy simulations for convective boundary layer flows, we demonstrate that decreased temperature gradients across the entrainment zone increase entrainment fluxes by enhancing the entrainment velocity, amplifying the asymmetry between top‐down and bottom‐up flux contributions. These changes alter scalar flux profiles, causing flux divergence or convergence and leading to the breakdown of the constant flux layer assumption (CFLA) in the ASL. As a result, EC‐measured fluxes either underestimate or overestimate “true” surface fluxes during divergence or convergence phases, contributing to energy balance non‐closure. The varying degrees of the CFLA breakdown are a fundamental cause for the non‐closure issue. These findings highlight the underappreciated role of entrainment in interpreting EC fluxes, addressing non‐closure, and understanding site‐to‐site variability in flux measurements. Plain Language Summary In the atmosphere over a heated surface, water vapor, carbon dioxide, and heat are transported from both the ground (bottom‐up) and the top of the air column (top‐down). The swirling motion of air within the column helps to even out the distribution of these quantities, known as “scalars.” Scalar fluxes measure how many molecules of these substances cross a unit area over time. At the surface, energy balance and plant processes control heat, water vapor, and carbon dioxide fluxes. However, fluxes at the top of the air column do not follow the same rules and abide by the same constraints as their ground counterpart. This study uses numerical simulations to show that when the temperature difference across a layer at the top of the boundary layer decreases, the boundary layer becomes deeper, increasing the transport of heat from the top. This causes changes in the slopes of flux profiles, disrupting the assumption that fluxes remain constant with height even close to the ground surface. As a result, measurements near the surface often underestimate or overestimate true surface fluxes, contributing to the much‐debated surface energy balance non‐closure problem. Key Points Entrainment‐modulated top‐down transport influences the slopes of the scalar flux profiles in the unstable atmospheric surface layer Variations in scalar flux profiles lead to differing degrees of failure in the constant flux layer assumption (CFLA) for different scalars The failure of the CFLA explains the non‐closure issue in the surface energy balance

Diel vertical migration (DVM) can enhance the vertical flux of carbon (C), and so contributes to the functioning of the biological pump in the ocean. The magnitude and efficiency of this active transport of C may depend on the size and taxonomic structure of the migrant zooplankton. However, the impact that a variable community structure can have on zooplankton-mediated downward C flux has not been properly addressed. This taxonomic effect may become critically important in highly productive eastern boundary upwelling systems (EBUSs), where high levels of zooplankton biomass are found in the coastal zone and are composed by a diverse community with variable DVM behavior. In these systems, presence of a subsurface oxygen minimum zone (OMZ) can impose an additional constraint to vertical migration and so influence the downward C export. Here, we address these issues based on a vertically stratified zooplankton sampling at three stations off northern Chile (20–30∘ S) during November–December 2015. Automated analysis of zooplankton composition and taxa-structured biomass allowed us to estimate daily migrant biomass by taxa and their amplitude of migration. We found that a higher biomass aggregates above the oxycline, associated with more oxygenated surface waters and this was more evident upon a more intense OMZ. Some taxonomic groups, however, were found closely associated with the OMZ. Most taxa were able to perform DVM in the upwelling zone withstanding severe hypoxia. Also, strong migrants, such as eucalanid copepods and euphausiids, can exhibit a large migration amplitude (∼500 m), remaining either temporarily or permanently within the core of the OMZ and thus contributing to the release of C below the thermocline. Our estimates of DVM-mediated C flux suggested that a mean migrant biomass of ca. 958 mg C m−2 d−1 may contribute with about 71.3 mg C m−2 d−1 to the OMZ system through respiration, mortality and C excretion at depth, accounting for ca. 4 % of the net primary production, and so implies the existence of an efficient mechanism to incorporate freshly produced C into the OMZ. This downward C flux mediated by zooplankton is however spatially variable and mostly dependent on the taxonomic structure due to variable migration amplitude and DVM behavior.

A new three-dimensional (3D) turbulent kinetic energy (TKE) subgrid mixing scheme is developed using the Advanced Research version of the Weather Research and Forecasting (WRF) Model (WRF-ARW) to address the gray-zone problem in the parameterization of subgrid turbulent mixing. The new scheme combines the horizontal and vertical subgrid turbulent mixing into a single energetically consistent framework, in contrast to the conventionally separate treatment of the vertical and horizontal mixing. The new scheme is self-adaptive to the grid-size change between the large-eddy simulation (LES) and mesoscale limits. A series of dry convective boundary layer (CBL) idealized simulations are carried out to compare the performance of the new scheme and the conventional treatment of subgrid mixing to the WRF-ARW LES dataset. The importance of including the nonlocal component in the vertical buoyancy specification in the newly developed general TKE-based scheme is illustrated in the comparison. The improvements of the new scheme with the conventional treatment of subgrid mixing across the gray-zone model resolutions are demonstrated through the partitioning of the total vertical flux profiles. Results from real-case simulations show the feasibility of using the new scheme in the WRF Model in lieu of the conventional treatment of subgrid mixing.