Surface Mixed Layer Research Articles

Influence of atmospheric deposition on biogeochemical cycles in an oligotrophic ocean system

Abstract. The surface mixed layer (ML) in the Mediterranean Sea is a well-stratified domain characterized by low macronutrients and low chlorophyll content for almost 6 months of the year. In this study we characterize the biogeochemical cycling of nitrogen (N) in the ML by analyzing simultaneous in situ measurements of atmospheric deposition, nutrients in seawater, hydrological conditions, primary production, heterotrophic prokaryotic production, N2 fixation and leucine aminopeptidase activity. Dry deposition was continuously measured across the central and western open Mediterranean Sea, and two wet deposition events were sampled, one in the Ionian Sea and one in the Algerian Basin. Along the transect, N budgets were computed to compare the sources and sinks of N in the mixed layer. In situ leucine aminopeptidase activity made up 14 % to 66 % of the heterotrophic prokaryotic N demand, and the N2 fixation rate represented 1 % to 4.5 % of the phytoplankton N demand. Dry atmospheric deposition of inorganic nitrogen, estimated from dry deposition of nitrate and ammonium in aerosols, was higher than the N2 fixation rates in the ML (on average 4.8-fold). The dry atmospheric input of inorganic N represented a highly variable proportion of biological N demand in the ML among the stations, 10 %–82 % for heterotrophic prokaryotes and 1 %–30 % for phytoplankton. As some sites were visited on several days, the evolution of biogeochemical properties in the ML and within the nutrient-depleted layers could be followed. At the Algerian Basin site, the biogeochemical consequences of a wet dust deposition event were monitored through high-frequency sampling. Notably, just after the rain, nitrate was higher in the ML than in the nutrient-depleted layer below. Estimates of nutrient transfer from the ML into the nutrient-depleted layer could explain up to a third of the nitrate loss from the ML. Phytoplankton did not benefit directly from the atmospheric inputs into the ML, probably due to high competition with heterotrophic prokaryotes, also limited by N and phosphorus (P) availability at the time of this study. Primary producers decreased their production after the rain but recovered their initial state of activity after a 2 d lag in the vicinity of the deep chlorophyll maximum layer.

Automatic Segregation of Pelagic Habitats

It remains difficult to segregate pelagic habitats since structuring processes are dynamic on a wide range of scales and clear boundaries in the open ocean are non-existent. However, to improve our knowledge about existing ecological niches and the processes shaping the enormous diversity of marine plankton, we need a better understanding of the driving forces behind plankton patchiness. Here we describe a new machine-learning method to detect and quantify pelagic habitats based on hydrographic measurements. An Autoencoder learns two-dimensional, meaningful representations of higher-dimensional micro-habitats, which are characterized by a variety of biotic and abiotic measurements from a high-speed ROTV. Subsequently, we apply a density-based clustering algorithm to group similar micro-habitats into associated pelagic macro-habitats in the German Bight of the North Sea. Three distinct macro-habitats, a “surface mixed layer,” a “bottom layer,” and an exceptionally “productive layer” are consistently identified, each with its distinct plankton community. We provide evidence that the model detects relevant features like the doming of the thermocline within an Offshore Wind Farm or the presence of a tidal mixing front.

Determination and uncertainty of spring net community production estimated from O2/Ar measurements in the northern East China Sea and southern Yellow Sea

The East China Sea (ECS) and Yellow Sea (YS) are East Asian marginal seas and mainly cover shallow continental shelves with depths of <200 m. These seas have high biological productivities with large spatial and temporal variabilities. Nevertheless, there are a limited number of studies on the biological pump in the ECS and YS. We observed net community production (NCP) for the first time in this region based on high resolution O2/Ar measurements, which act as a measure of biological pump activity. Observations were conducted underway using an equilibrator inlet mass spectrometer in April 2018. Three distinct water masses were identified in the study area: a cold and less saline water mass in the southern YS (Yellow Sea Cold Water; YSCW); a warm and saline water mass along a branch of the Kuroshio current (Eastern Kuroshio Branch Water; EKBW); and a water mass near the southern coast of the Korean Peninsula (Southern Coast Water; SCW). The average NCP of 49 ± 19 mmol O2 m−2 d−1 in the YSCW was almost twice as high as that of 26 ± 13 and 23 ± 12 mmol O2 m−2 d−1 in the EKBW and SCW, respectively. Vertical mixing between the surface mixed layer and subsurface layers was assessed using vertical measurements of microscale turbulence and discrete O2/Ar values and appeared to bias the NCP estimates by ∼2%. In contrast, horizontal advection, which was assessed using the horizontal O2/Ar gradient and current speeds, could be a major source of uncertainty up to 38% in the estimates. The spatial variability of the NCP, shown in the standard deviations (39–52% of the three averages), may largely be ascribed to the horizontal advection in the region, which is affected by strong surface currents.

Surface evolution of aluminosilicate glass fibers during dissolution: Influence of pH, solid-to-solution ratio and organic treatment

Materials made of synthetic vitreous mineral fibers, such as stone wool, are widely used in construction, in functional composites and as thermal and acoustic insulation. Chemical stability is an important parameter in assessing long term durability of the products. Stability is determined by fiber resistivity to dissolution, where the controlling parameters are solid surface area to solution volume ratio (S/V), pH and composition of the fibers and organic compounds used as binders. We investigated stone wool dissolution under flow through conditions, far from equilibrium, at pH range of 2 to 13, as well as under batch conditions, close to equilibrium, for up to 28 days, where S/V ranged from 100 to 10000 m−1. The dissolution rate of stone wool shows minimum at pH 8.5 and increases significantly at pH < 4.5 and pH > 12. In close to equilibrium conditions, S/V defines the steady state concentration for the leached components. Decreased dissolution rate could result from evolution of a surface leached layer or the formation of secondary surface phases or both. We suggested three dissolution rate controlling mechanisms, which depend on pH. That is, dissolution is controlled by: a SiO2 rich surface layer at pH < 4.5; by adsorption of an Al and Al-Si mixed surface layer at 5 < pH < 11 and by divalent cation adsorption and formation of secondary phases (silicates, hydroxides) at pH ∼ 13. The organic compounds, used to treat the stone wool fibers during manufacture, had no influence on their dissolution properties.

Horizontal and vertical spreading of dye in the coastal ocean of the northern Mid-Atlantic bight

The paper investigates the horizontal and vertical spreading of a neutrally-buoyant chemical tracer (rhodamine) that was released into the surface mixed layer in the coastal ocean. Rhodamine concentrations from the dye release experiment were combined with observations of water column properties and velocity measurements from the high-frequency radar (HFR) and ship-based ADCP in order to investigate and interpret the downward penetration of dye from the mixed layer into the stratified part of the water column and to quantify the horizontal diffusivity associated with the lateral dye spreading. Of the total amount of dye encountered during the 33-h long survey, 99% was observed within the mixed layer, suggesting that the downward penetration of dye from the mixed layer into the pycnocline is a slow process that occurs on the time scale of at least multiple days. The observed elevated dye concentrations below the mixed layer were always co-located with the elevated concentrations within the mixed layer. However, the distribution of dye below the mixed layer was patchy, and the below-mixed-layer dye concentrations were poorly correlated with those within the mixed layer. Combined with the absence of strong vertical shear of horizontal velocity across the base of the mixed layer, this suggests that the observed dye could have been ejected from the mixed layer earlier and at a different location, and then advected to the observed location by the horizontal currents below the mixed layer. The vertical diffusivity in the mixed layer is estimated to be approximately 7x10−4m2s, about two orders of magnitude larger than the diffusivity across the base of the mixed layer, which is 5x10−6m2s. Lateral spread of dye was found to be well represented by the HFR velocities (with roughly 2-km resolution) with the added small-scale turbulent diffusivity of 5 m2s.Plain Language Summary.The top part of the water column, called the surface mixed layer, is well mixed to a nearly uniform density. Below lays the pycnocline where density changes rapidly with depth. The exchange of water and properties between the mixed layer and pycnocline is an important but poorly understood oceanographic process and is the main subject of the current observational study. Specifically, this paper describes and interprets the initial 36 h of the vertical and horizontal spreading of a dye plume, released into the surface mixed layer of the coastal ocean south of Martha's Vineyard, MA. Our results indicated that the ejection and penetration of dye down from the mixed layer is a slow process that occurs over a time scales longer than multiple days. The episodic nature of the downward dye ejection from the mixed layer and the resulting patchy distribution of the subsurface dye is another important finding from this work, which has far-reaching implications for the parameterization of these small-scale vertical exchange processes in numerical models. The vertical diffusivity within the mixed layer (7x10−4m2s) is estimated to be about two orders of magnitude larger than the diffusivity across the base of the mixed layer (5x10−6m2s). Finally, the lateral spreading of the dye plume was found to be well represented by advection by the high-frequency-radar-based velocities with the added small-scale turbulent diffusivity of 5 m2s – the value that is in line with our prior estimates for this coastal region, but is larger than many prior estimates from different regions of the ocean.

Air-Sea Gas Fluxes and Remineralization From a Novel Combination of pH and O2 Sensors on a Glider

Accurate, low-power sensors are needed to characterize biogeochemical variability on underwater glider missions. However, the needs for high accuracy and low power consumption can be difficult to achieve together. To overcome this difficulty, we integrated a novel sensor combination into a Seaglider, comprising a spectrophotometric lab-on-a-chip (LoC) pH sensor and a potentiometric pH sensor, in addition to the standard oxygen (O2) optode. The stable, but less frequent (every 10 min) LoC data were used to calibrate the high-resolution (1 s) potentiometric sensor measurements. The glider was deployed for a 10-day pilot mission in August 2019. This represented the first such deployment of either type of pH sensor on a glider. The LoC pH had a mean offset of +0.005±0.008 with respect to pH calculated from total dissolved inorganic carbon content, c(DIC), and total alkalinity, AT, in co-located water samples. The potentiometric sensor required a thermal-lag correction to resolve the pH variations in the steep thermocline between surface and bottom mixed layers, in addition to scale calibration. Using the glider pH data and a regional parameterization of AT as a function of salinity, we derived the dissolved CO2 content and glider c(DIC). Glider surface CO2 and O2 contents were used to derive air-sea fluxes, Φ(CO2) and Φ(O2). Φ(CO2) was mostly directed into the ocean with a median of −0.4 mmol m–2 d–1. In contrast, Φ(O2) was always out of the ocean with a median of +40 mmol m–2 d–1. Bottom water apparent oxygen utilization (AOU) was (35±1) μmol kg–1, whereas apparent carbon production (ACP) was (11±1) μmol kg–1, with mostly insignificant differences along the deployment transect. This deployment shows the potential of using pH sensors on autonomous observing platforms such as Seagliders to quantify the interactions between biogeochemical processes and the marine carbonate system at high spatiotemporal resolution.

Oceanic Circulation Drives the Deepest and Longest Marine Heatwaves in the East Australian Current System

AbstractAlthough the impacts of marine heatwaves (MHWs) can extend well below the ocean surface, little is known about how oceanic and atmospheric forcings control their vertical structure. Here, we relate the MHW drivers to their sub‐surface characteristics in different dynamical regimes including the East Australia Current. We detect MHWs in the depth‐dependent surface mixed layer over 30‐years and use a heat budget to identify the dominant mechanisms driving them. We show that MHWs in the Western Boundary Current (WBC) jet are predominantly driven by air‐sea heatflux whilst in the WBC extension, MHWs are advection‐driven. The deepest and longest MHWs are advection‐driven and are more prevalent in autumn and winter. Surface (latent) flux‐driven MHWs are shallower and occur predominantly in summer. Demonstrating how MHW characteristics are linked to their drivers facilitates their prediction through driver diagnosis, especially below the surface where observations are sparse and ecological impact is high.

The influence of merger and convection on an anticyclonic eddy trapped in a bowl

We investigate the impact of several parameters on the lifecycle of an anticyclonic eddy lying in a topographic depression (a bowl), similar to the Lofoten Vortex and the Rockall Trough eddy cases. We observe that the vortex merger with submesoscale coherent vortices generated at depth allows the eddy to grow in size, and intensify at depth. Wintertime convection is also shown to directly intensify the eddy by deepening isopycnals. Also, convection indirectly affects the shape of the eddy. It enhances the number of merger (1) at the surface, with small vortices generated in the convectively-deepened surface mixed-layer, and (2) at depth, because the vertical distance between the main eddy’s core and small companion vortices is reduced, thus increasing the merging efficiency. These processes altogether contribute to the maintaining of the eddy. On the other hand, the bottom drag is the main process contributing to the decay of the eddy. Our study thus shows that the sustaining for several years of such eddies trapped in a bowl is mainly due to the balance between merger and bottom drag.

Atmospheric Cold Pools and Their Influence on Sea Surface Temperature in the Bay of Bengal

AbstractRecent observations show that atmospheric cold pool (ACP) events are plentiful in the Bay of Bengal (BoB) during summer (May–September) and fall (October–November) and that these events can significantly modify local air‐sea interaction processes on sub‐daily time scales. In this study, we examine whether the magnitude of sea surface temperature (SST) drop associated with ACP events shows any diurnal variability during summer and fall. For this purpose, we use moored buoy data with a 10‐min temporal resolution at 8°, 12°, and 15°N along 90°E and a one‐dimensional mixed layer (ML) model. The analysis shows a reduction in SST (ΔSST) due to ACPs in the BoB during summer and fall, with a maximum magnitude of ΔSST during the afternoon (1200–1600 LST). However, the maximum magnitude of ΔSST during the afternoon is a factor of two higher during fall (∼−0.14°C) than summer (∼−0.07°C). Analysis based on observations and ACP sensitivity experiments indicates that the shallow daytime thermocline and associated thin surface ML is the primary factor regulating the day to night difference in ΔSST associated with ACPs. The presence of this shallow daytime thermocline and thin ML amplifies the effects on SST of net surface heat loss and entrainment of cold sub‐surface water associated with enhanced ACP wind speeds.

Volume and Heat Transport in the South China Sea and Maritime Continent at Present and the End of the 21st Century

AbstractOcean transports through the Southeast Asian Seas connect the western tropical Pacific and Indian Oceans, thereby exerting an important role in regional and global climate. High‐resolution regional ocean model simulations over the South China Sea (SCS) and maritime continent are used to study the mean and seasonally varying thermohaline structure and volume transport through the straits surrounding the SCS. Diversity in the vertical structure of these straits is not only indicative of the role of widely varying bathymetry but also strong seasonality associated with monsoonal currents. The presence of a Pacific water mass in intermediate and deep layers of the Luzon Strait points to a key pathway between the Pacific and Indian Oceans. Further, examining a suite of global, high‐resolution model simulations reveals the projected changes in the regional upper ocean transports due to anthropogenic radiative forcing by the end of the 21st century. The global models predict an increase in heat and volume transport through the Luzon and Karimata Straits, and a decrease thereof through the Makassar and Lombok Straits by the end of the century. Overall, these changes impute additional net convergence of heat and volume in the SCS, a significant reduction of sea surface salinity and mixed layer depth, and an increase in the upper‐ocean heat content of the region. As the SCS serves as a regional heat capacitor and is impacted by the global thermohaline circulation locally via Indonesian Throughflow, these predicted changes have the potential to impact climate over the Indo‐Pacific region and globally.

Striped texture of submesoscale fields in the northeastern Baltic Proper: Results of very high-resolution modelling for summer season

Very high-resolution modelling of the northeastern Baltic Proper shows that preferentially elongated along the flow, submesoscale inhomogeneities of hydrodynamic fields or stripes of the order of 10–20 km in length and 1 km in width, are typical for summer season both in surface mixed layer and for interior layers which are not directly exposed to atmospheric forcing. In surface layer, the presence of stripes is supported by the remote sensing imagery and their vertical extension is comparable with the mixed layer depth (approx. 5–8 m). In the interior layers, the vertical extension of stripes is considerably larger (approx. 10–50 m) and their slopes exceed the isopycnal slope. Four competitive mechanisms of formation of the mesoscale striped texture are considered: stirring of large-scale inhomogeneities by the eddy field, the classic, inviscid adiabatic fluid symmetric instability, the McIntyre instability, and the strain-induced frontogenesis. Based on the instability criteria and the growth rates and geometry of the disturbances, the classic symmetric instability and the strain-induced frontogenesis are probably responsible for the formation of submesoscale striped texture in the surface layer, while in the interior layers, the strain-induced frontogenesis and hypothetically the McIntyre instability can be essential. Stirring of large-scale inhomogeneities by the eddy field could be responsible for formation of striped texture in a passive tracer concentration and in temperature and salinity in the presence of thermohaline gradients on isopycnic surfaces (thermoclinicity), but it does not imply formation of stripes in dynamically active tracers, such as vertical vorticity, horizontal gradients of buoyancy, etc.

Spring Accumulation Rates in North Atlantic Phytoplankton Communities Linked to Alterations in the Balance Between Division and Loss.

For nearly a century, phytoplankton spring blooms have largely been explained in the context of abiotic factors regulating cellular division rates (e.g., mixed-layer light levels). However, the accumulation of new phytoplankton biomass represents a mismatch between phytoplankton division and mortality rates. The balance between division and loss, therefore, has important implications for marine food webs and biogeochemical cycles. A large fraction of phytoplankton mortality is due to the combination of microzooplankton grazing and viral lysis, however, broad scale simultaneous measurements of these mortality processes are scarce. We applied the modified dilution assay along a West-to-East diagonal transect in the North Atlantic during spring. Our results demonstrate positive accumulation rates with losses dominated by microzooplankton grazing. Considering the dynamic light environment phytoplankton experience in the mixed surface layer, particularly in the spring, we tested the potential for incubation light conditions to affect observed rates. Incubations acted as short-term ‘light’ perturbations experiments, in which deeply mixed communities are exposed to elevated light levels. These “light perturbations” increased phytoplankton division rates and resulted in proportional changes in phytoplankton biomass while having no significant effect on mortality rates. These results provide experimental evidence for the Disturbance-Recovery Hypothesis, supporting the tenet that biomass accumulation rates co-vary with the specific rate of change in division.

The Influence of Load on Dry and Tribocorrosive Sliding of AISI 4330 and 15-5PH against Cast Iron

Environmental conditions, testing variables, and material properties significantly influence the sliding wear behavior of all tribosystems. Such parameters affect the mechanism of wear occurring, govern how the wear scar will be categorized, and control whether transitions occur throughout the test. The present study investigated this through sliding wear tests of AISI 4330, 15-5PH, and heat-treated cast iron in the dry and NaCl regime. Loads of 2, 4, and 6 kg were applied to the cast iron pin in both regimes, with the volume loss results, wear scar morphology, and microstructural evolution analyzed. It was found that the AISI 4330 and 15-5PH discs produced higher volume losses in the dry regime than in the NaCl regime, indicating that the solution had a beneficial effect rather than a detrimental effect versus the dry tribosystem. Beneficial oxidative wear allowed for AISI 4330 to lose less than 15-5PH in the dry regime, with their respective cast iron pins following the same result. Microstructurally, the 15-5PH wear scar cross sections exhibited a mechanically mixed surface layer of refined material mixed with oxides above a layer of grains distorted in the direction of sliding. This highlighted the significant difference between the wear of 15-5PH and AISI 4330, because the latter did not exhibit any areas of interest.

Niche partitioning by photosynthetic plankton as a driver of CO2-fixation across the oligotrophic South Pacific Subtropical Ocean

Oligotrophic ocean gyre ecosystems may be expanding due to rising global temperatures [1–5]. Models predicting carbon flow through these changing ecosystems require accurate descriptions of phytoplankton communities and their metabolic activities [6]. We therefore measured distributions and activities of cyanobacteria and small photosynthetic eukaryotes throughout the euphotic zone on a zonal transect through the South Pacific Ocean, focusing on the ultraoligotrophic waters of the South Pacific Gyre (SPG). Bulk rates of CO2 fixation were low (0.1 µmol C l−1 d−1) but pervasive throughout both the surface mixed-layer (upper 150 m), as well as the deep chlorophyll a maximum of the core SPG. Chloroplast 16S rRNA metabarcoding, and single-cell 13CO2 uptake experiments demonstrated niche differentiation among the small eukaryotes and picocyanobacteria. Prochlorococcus abundances, activity, and growth were more closely associated with the rims of the gyre. Small, fast-growing, photosynthetic eukaryotes, likely related to the Pelagophyceae, characterized the deep chlorophyll a maximum. In contrast, a slower growing population of photosynthetic eukaryotes, likely comprised of Dictyochophyceae and Chrysophyceae, dominated the mixed layer that contributed 65–88% of the areal CO2 fixation within the core SPG. Small photosynthetic eukaryotes may thus play an underappreciated role in CO2 fixation in the surface mixed-layer waters of ultraoligotrophic ecosystems.

Descriptive Capability of Datasets as Proxy of Sea Water Temperature in Coastal Systems: An Evaluation from the Aegean Sea

Scientific and technological progresses have introduced diverse data sources for seawater temperature over broad temporal and spatial ranges. Here, we investigated the performance of satellite and model-based seawater temperature data for different temporal composites and depths. We applied an in-situ temperature time-series obtained in a coastal bottom in the Aegean Sea over three years, as the reference. Both datasets showed largely significant relationships based on cross-correlation analyses and presented descriptive properties of the in-situ conditions at corresponding depths. Based on the results of analyses, the modeling datasets presented more reliable results and representations of in-situ conditions than the datasets obtained from satellite for the coastal region. However, the datasets obtained from the satellite also provided reliable data for all time frames investigated, particularly in the mixed surface layer. Monthly datasets were more effective in providing descriptive values in long term studies. This is the first detailed study to explore the descriptive capacities of modeling for water temperature in coastal environments. According to the results, the selection of a dataset as a proxy for seawater temperature requires careful consideration. The present study provides an extensive baseline for evaluating the suitability of the application of specific datasets as proxies in coastal ecosystems.

Minor Contribution by Biomineralizing Phytoplankton to Surface Ocean Biomineral Pools in the Late Stratified Period

Vertical distributions of biogenic silica (bSi), particulate inorganic carbon (PIC) and key biomineral-forming phytoplankton indicate vertical zoning, or partitioning, during the late summer stratified period in the northeast Atlantic. Coccolithophores were generally more numerous in the surface mixed layer, whilst PIC concentrations were more homogenous with depth throughout the euphotic zone. Diatoms were notably more abundant and more diverse in the lower euphotic zone beneath the mixed layer in association with subsurface maxima in chlorophyll-a, bSi and oxygen concentrations. The four dominant coccolithophore species (Emiliania huxleyi, Gephyrocapsa muellerae, Syracosphera spp., and Rhabdosphaera clavigera) represented 78 ± 20% (range 31–100%) of the observed community across all sampled depths yet simultaneously contributed an average of only 13% to measured PIC pools. The diatom community, which was dominated by Pseudo-nitzschia spp. and by a species tentatively identified as Nanoneis longta, represented only ~1% of the bSi pool on average, with contributions increasing within the chlorophyll maximum. Despite a slow gradual deepening of the surface mixed layer in the period prior to observation, and adequate nutrient availability beneath the mixed layer, biomineral pools at this time consisted largely of detrital rather than cellular material.

Coupled and decoupled stratocumulus-topped boundary layers: turbulence properties

Abstract. We compare turbulence properties in coupled and decoupled marine stratocumulus-topped boundary layers (STBLs) using high-resolution in situ measurements performed by the helicopter-borne Airborne Cloud Turbulence Observation System (ACTOS) platform in the region of the eastern North Atlantic. The thermodynamically well-mixed coupled STBL was characterized by a comparable latent heat flux at the surface and in the cloud-top region, and substantially smaller sensible heat flux in the entire depth. Turbulence kinetic energy (TKE) was efficiently generated by buoyancy in the cloud and at the surface, and dissipated with comparable rate across the entire depth. Structure functions and power spectra of velocity fluctuations in the inertial range were reasonably consistent with the predictions of Kolmogorov theory. The turbulence was close to isotropic. In the decoupled STBL, decoupling was most obvious in humidity profiles. Heat fluxes and buoyant TKE production at the surface were similar to the coupled case. Around the transition level, latent heat flux decreased to zero and TKE was consumed by weak stability. In the cloud-top region, heat fluxes almost vanished and buoyancy production was significantly smaller than for the coupled case. The TKE dissipation rate inside the decoupled STBL varied between its sublayers. Structure functions and power spectra in the inertial range deviated from Kolmogorov scaling. This was more pronounced in the cloud and subcloud layer in comparison to the surface mixed layer. The turbulence was more anisotropic than in the coupled STBL, with horizontal fluctuations dominating. The degree of anisotropy was largest in the cloud and subcloud layer of the decoupled STBL. Integral length scales, of the order of 100 m in both cases, indicate turbulent eddies smaller than the depth of the coupled STBL or of the sublayers of the decoupled STBL. We hypothesize that turbulence produced in the cloud or close to the surface is redistributed across the entire coupled STBL but rather only inside the sublayers where it was generated in the case of the decoupled STBL. Scattered cumulus convection, developed below the stratocumulus base, may play a role in transport between those sublayers.

Surface atmospheric forcing as the driver of long-term pathways and timescales of ocean ventilation

Abstract. The ocean takes up 93 % of the excess heat in the climate system and approximately a quarter of the anthropogenic carbon via air–sea fluxes. Ocean ventilation and subduction are key processes that regulate the transport of water (and associated properties) from the surface mixed layer, which is in contact with the atmosphere, to the ocean's interior, which is isolated from the atmosphere for a timescale set by the large-scale circulation. Utilising numerical simulations with an ocean–sea-ice model using the NEMO (Nucleus for European Modelling of the Ocean) framework, we assess where the ocean subducts water and, thus, takes up properties from the atmosphere; how ocean currents transport and redistribute these properties over time; and how, where, and when these properties are ventilated. Here, the strength and patterns of the net uptake of water and associated properties are analysed by including simulated seawater vintage dyes that are passive tracers released annually into the ocean surface layers between 1958 and 2017. The dyes' distribution is shown to capture years of strong and weak convection at deep and mode water formation sites in both hemispheres, especially when compared to observations in the North Atlantic subpolar gyre. Using this approach, relevant to any passive tracer in the ocean, we can evaluate the regional and depth distribution of the tracers, and determine their variability on interannual to multidecadal timescales. We highlight the key role of variations in the subduction rate driven by changes in surface atmospheric forcing in setting the different sizes of the long-term inventory of the dyes released in different years and the evolution of their distribution. This suggests forecasting potential for determining how the distribution of passive tracers will evolve, from having prior knowledge of mixed-layer properties, with implications for the uptake and storage of anthropogenic heat and carbon in the ocean.

Turnover in a small Canadian shield lake

AbstractIn dimictic lakes, the stable density stratification during summer and winter inhibits vertical mixing of nutrients and oxygen. This favors the development of hypolimnetic hypoxia, which degrades cool‐water fish habitat and enhances nutrient mineralization from the sediments. Fall and spring turnover events, therefore, provide a crucial biannual link between surface and bottom waters. However, the physical processes occurring to mix lakes during turnover events remain comparatively uninvestigated. In this study, long‐term field observations were supplemented with output from a three‐dimensional numerical model, to better understand turnover events within a small temperate lake during 2011–2017. Mid‐basin penetrative convection, sidearm convection, and wind‐induced mixing were identified, with mid‐basin convection likely contributing the most to turnover events; a typical side‐arm convective plume had 2% (spring) to 4% (fall) of the mass flux compared to those at mid‐basin. During fall turnover, wind shear only mixed the upper 35% of the surface mixed layer, with convection acting to deepen below. During spring turnover, ice‐cover sheltered the lake from wind, causing convection to be the only process occurring and lengthening the turnover duration (~ 51 d) compared to fall (~ 13 d).

Impacts of strong wind events on sea ice and water mass properties in Antarctic coastal polynyas

Strong offshore wind events (SOWEs) occur frequently near the Antarctic coast during austral winter. These wind events are typically associated with passage of synoptic- or meso-scale cyclones, which interact with the katabatic wind field and affect sea ice and oceanic processes in coastal polynyas. Based on numerical simulations from the coupled Finite Element Sea-ice Ocean Model (FESOM) driven by the CORE-II forcing, two coastal polynyas along the East Antarctica coast––the Prydz Bay Polynya and the Shackleton Polynya are selected to examine the response of sea ice and oceanic properties to SOWEs. In these polynyas, the southern or western flanks of cyclones play a crucial role in increasing the offshore winds depending on the local topography. Case studies for both polynyas show that during SOWEs, when the wind speed is 2–3 times higher than normal values, the offshore component of sea ice velocity can increase by 3–4 times. Sea ice concentration can decrease by 20–40%, and sea ice production can increase up to two to four folds. SOWEs increase surface salinity variability and mixed layer depth, and such effects may persist for 5–10 days. Formation of high salinity shelf water (HSSW) is detected in the coastal regions from surface to 800 m after 10–15 days of the SOWEs, while the HSSW features in deep layers exhibit weak response on the synoptic time scale. HSSW formation averaged over winter is notably greater in years with longer duration of SOWEs.