Frontal zones within the Western Alboran Gyre (WAG) are characterized by a density gradient resulting from the convergence of Atlantic and Mediterranean waters. Subduction along isopycnals at the WAG periphery can play a crucial role in upper-ocean ventilation and influences its stratification and biogeochemical cycles. In 2019, physical parameters (comprising temperature, salinity, turbulent kinetic energy dissipation rates) and biogeochemical data (oxygen and chlorophyll a) profiles were collected in transects along the northern edge of the WAG. Several intrusions of subducted water with elevated oxygen, chlorophyll a, and spice anomaly were identified towards the center of the anticyclone. These features had elevated kinetic energy dissipation rates on both their upper and lower boundaries. Analysis of the turbulent fluxes involving heat, salt, oxygen, and chlorophyll a demonstrated a net flux of physical and biogeochemical properties from the intrusions to the surrounding ocean. Either the turbulent or diffusive convection mixing contributed to the observed dilution of the intrusion. Other factors (e.g., water column density stability, variability of the photic layer depth, and organic matter degradation) likely played a role in these dynamics. Enhanced comprehension of the persistence and extent of these features might lead to an improved quantitative parameterization of relevant physical and biogeochemical properties involved in subduction within the study zone.

Fluorometers and backscatter sensors are essential tools in oceanography, used to measure various biogeochemical parameters, and are key components of the BGC-Argo program. The RBRtridente integrates these two sensors into a single device. In this study, we detail the calibration procedure for the RBRtridente, a combined fluorescence-backscattering sensor developed by RBR. The calibration process involves three key steps: gain calibration, temperature compensation, and optical calibration. We describe each step in detail including the methodology for generating primary standards for calibration, as well as the primary and secondary reference instruments used for calibration. The calibration procedure is validated in the field by comparing the measurements of the RBRtridente to other fluorometers and backscatter sensors commonly used as part of the BGC-Argo program.

Subtropical oceans contribute significantly to global primary production, but the fate of the picophytoplankton that dominate in these low nutrient regions is poorly understood. Working in the subtropical Mediterranean, we demonstrate that subduction of water at ocean fronts generates 3D intrusions with uncharacteristically high carbon, chlorophyll, and oxygen that extend below the sunlit photic-zone into the dark ocean. These contain “fresh” picophytoplankton assemblages that resemble the photic-zone regions where the water originated. Intrusions propagate depth-dependent seasonal variations in microbial assemblages into the ocean interior. Strikingly, the intrusions included dominant biomass contributions from non-photosynthetic bacteria and enrichment of enigmatic heterotrophic bacterial lineages. Thus, the intrusions not only deliver material that differs in composition and nutritional character from sinking detrital particles, but also drive shifts in bacterial community composition, organic matter processing, and interactions between surface and deep communities. Modeling efforts paired with global observations demonstrate that subduction can flux similar magnitudes of particulate organic carbon as sinking export, but is not accounted for in current export estimates and carbon cycle models. Intrusions formed by subduction are a particularly important mechanism for enhancing connectivity between surface and upper mesopelagic ecosystems in stratified subtropical ocean environments that are expanding due to the warming climate.

The Deep Argo program's initiative to explore the deepest regions of the world's oceans up to 6000 m necessitates the deployment of robust, precise, and stable measurements of pressure, temperature, and salinity. This study evaluates the performance of the RBR argooideep6k CTD, designed by RBR for deep-ocean applications, characterized through laboratory experiments and validated in-situ. The main challenges addressed are the errors generated by the compressibility of conductivity cell at high pressure, as well as its thermal inertia during profiling, affecting conductivity measurements. The results indicate that the compressibility calibration derived in the calibration facilities by the manufacturer performs as expected when deployed in the ocean. It also demonstrates that the corrective algorithm developed to address thermal inertia errors greatly improves the quality of salinity data, particularly when experiencing large tem-perature gradients. Finally, the stability of the RBRargo| deep6k CTD in the field is shown using the data from pilot deployments of RBRargo ideep6k CTD on Deep Argo floats.

To advance predictive skill, ocean forecast systems must exploit local high resolution observations. The recent Sub-Mesoscale Ocean Dynamics Experiment (S-MODE) provides an opportunity to examine the data assimilation issues that can limit predictive skill. A unique swarm control system guided nine ocean gliders accounting for all satellite and in situ data to provide high resolution observations for three ocean forecast experiments: 1) no glider data assimilated, 2) assimilation with glider data through a previous assimilation approach, and 3) adaptive assimilation of the glider data in which smaller scales are corrected in the vicinity of the local high resolution observations. The assimilation adaptation used a spatially varying horizontal decorrelation scale enabling corrections at smaller scales where high resolution in situ data are concentrated. When measured by spatial scales along the glider paths, skill in temperature advances from scales larger than about 220 km wavelength when utilizing the glider data with prior assimilation down to scales larger than 80 km wavelength with the adaptive assimilation. Skill in salinity advances from scales of approximately 300 km to scales of 200 km, and skill in steric height advances from 140 km to 63 km. Additionally, conductivity, temperature, and depth observations from an EcoCTD instrument provide independent data for confirmation. The results imply that as ocean observing systems advance, ocean forecast systems must adapt to use local high resolution observations.

Export of Particulate Organic Carbon (POC) is mainly driven by gravitational sinking. Thus, traditionally, it is thought that larger, faster‐sinking particles make up most of the POC export flux. However, this need not be the case for particles whose sinking speeds are comparable to the vertical velocities of a dynamic flow field that can influence the descent rate of particles. Particles with different settling speeds are released in two process‐oriented model simulations of an upper ocean eddying flow in the Northeast Pacific to evaluate the impact of (1) ocean dynamics on the respective contribution of the different sinking‐velocity classes to POC export, and (2) the particle number size‐spectrum slope. The analysis reveals that the leading export mechanism changes from gravitationally driven to advectively driven as submesoscale dynamics become more active in the region. The vertical velocity associated with submesoscale dynamics enhances the contribution of slower‐sinking particles to POC export flux by a factor ranging from 3 to 10, especially where the relative abundance of small particles is large (i.e., steep particle size‐spectrum slope). Remineralization generally decreases the total amount of biomass exported, but its impact is weaker in dynamical regimes where submesoscale dynamics are present and export is advectively driven. In an advectively driven export regime, remineralization processes counter‐intuitively enhance the role of slower‐sinking particles to the point where these slower‐sinking velocity classes dominate the export, therefore challenging the traditional paradigm for POC export. This study demonstrates that slow‐sinking particles can be a significant contribution, and at times, even dominate the export flux.

Submesoscale instabilities along oceanic fronts can cause water mass intrusions from the surface mixed layer into the stratified pycnocline. These are important drivers of vertical exchange that have a potentially significant impact on the transfer of physical properties and biological tracers.The CALYPSO (Coherent Lagrangian Pathways from the Surface Ocean to Interior) ONR research initiative focuses on observing and understanding coherent vertical pathways by which vertical exchange occurs. The Alboran Sea (located in the south-western Mediterranean, east of Gibraltar) is well known for its strong density fronts and eddies. During a research cruise, onboard R/V Pourquoi Pas? in early April 2019, we found that fronts in this area support the generation of subducting filaments. Several types of observations (using CTD, uCTD, microstructure profiles, drifters and floats) were collected along numerous cross-front transects over a period of two weeks.The analysis of the temperature profiles highlighted the presence of several intruding filaments moving along isopycnal surfaces in the proximity of the frontal area. The intrusion signal was also clearly visible in biophysical properties with elevated Chlorophyll-a concentrations, well below the deep chlorophyll maximum, in conjunction with high dissolved oxygen values. From a microstructure point of view, the upper and lower limits of the subducting filaments exhibited high turbulent dissipation rates, with values of O(10-7) W/m2. These dissipation rates are higher than what is generally observed at such depths and point to enhanced mixing activity at the boundaries of the intrusions even along isopycnal surfaces.

The study of ocean dynamics and biophysical variability at submesoscales of O(1) km and O(1) h raises several observational challenges. To address these by underway sampling, we recently developed a towed profiler called the EcoCTD, capable of concurrently measuring both hydrographic and bio-optical properties such as oxygen, chlorophyll fluorescence, and optical backscatter. The EcoCTD presents an attractive alternative to currently used towed platforms due to its light footprint, versatility in the field, and ease of deployment and recovery without cranes or heavy-duty winches. We demonstrate its use for gathering high-quality data at submesoscale spatiotemporal resolution. A dataset of bio-optical and hydrographic properties, collected with the EcoCTD during field trials in 2018, highlights its scientific potential for the study of physical–biological interactions at submesoscales.

This study analyses accuracy and stability of salinity measurements collected by four Argo autonomous drifters with RBR Ltd. inductive conductivity sensors operating in the Pacific Ocean during the recent 2-4 years. Inductive sensors have advantages over traditionally used electrode-type cells due to their better resistance to surface contamination and low power requirements, resulting in more robust and accurate measurements and extended float lifetimes. Proper assessment of the quality of the data collected by autonomous drifters is challenging due to lack of reference information. An important part of Argo program is the Delayed-Mode Quality Control process including salinity drift analysis and correction using the ‘Owens-Wong Calibration’ (OWC) method based on objective mapping of available reference data. This method, however, can misinterpret imperfect reference data as sensor drift. In this study, analyzing OWC output we introduce a combination of visualization methods focused on the locations where reference data can be treated as problematic. These methods include the analysis of spatial locations of the ‘profile correction factor’ along the float trajectory, comparing reference salinity fields calculated by the OWC method to additional reference sources (climatologies) and comparative analysis of different floats operating in the same area using the same reference datasets. The results demonstrate high level of stability of inductive conductivity cells on Argo floats, making them promising alternative for traditionally used Argo float CTDs equipped with electrode-type conductivity sensors.

The cruise was organized into 2 legs, the first without the equipment in the container and the second, with the equipment after the container was picked up in Almería.

From May 27, 2018 to June 02, 2018, a scientific campaign was conducted in the Alboran Sea as part of an ONR Departmental Research Initiative, CALYPSO. The pilot cruise involved two ships: the R/V Socib, tasked with sampling fixed lines repeatedly, and the NRV Alliance that surveyed along the trajectory of Lagrangian platforms. A large variety of assets were deployed from the NRV Alliance, with the objective to identify coherent Lagrangian pathways from the surface ocean to interior. As part of the field campaign, an Underway-CTD (UCTD) system was used to measure vertical profiles of salinity, temperature and other properties while steaming, to achieve closely spaced measurements in the horizontal along the ship’s track. Both a UCTD probe and an bio-optically augmented probe, named EcoCTD, were deployed. The EcoCTD collects concurrent physical and bio-optical observations. This report focuses exclusively on the data collected by these two underway systems. It describes the datasets collected during the pilot cruise, as well as the important processing steps developed for the EcoCTD.

Traditionally, export of particulate organic carbon (POC) is thought to be mainly driven by gravitational sinking, thus privileging faster-sinking particles. Lagrangian particles with different settling speeds are released in two process-oriented simulations to evaluate the impact of (1) the size-spectrum slope, and (2) the ocean dynamics on the respective contribution of the different sinking-velocity classes to POC export. The analysis reveals that the leading export mechanism changes from gravitationally-driven to advectively-driven as submesoscale dynamics become more important. As a result, the presence of submesoscale features strengthens the role of slower-sinking particles in POC export. The role of slower-sinking particles also strengthens as the slope of the size spectrum steepens. Implementing a remineralization scheme generally decreases the total amount of biomass exported, but its impact is weaker in dynamical regimes where submesoscale dynamics are present and export is advectively-driven. Under specific conditions, remineralization processes counter-intuitively enhance the role of slower-sinking particles to the point where these slower-sinking velocity classes dominate the export, therefore challenging the traditional paradigm for POC export. This study demonstrates that slow-sinking particles are at least significant, at most dominant in export processes.

The focus of this study is on the relative roles of winds and buoyancy in driving the Nova Scotia Current (NSC) utilizing detailed hydrographic glider transects along the Halifax Line. We define a Hydrographic Wind Index (HWI) using a simplistic two-layer model to represent the NSC and its frontal system. The HWI is based on local characteristics of the density front extracted from the glider data (e.g., frontal slope). The impact of wind-driven isopycnal tilting on the frontal slope is estimated and corrected for to accurately scale the buoyancy-driven component of the NSC. Observations from independent current profilers deployed across the NSC confirm that the HWI captures the low-frequency variability of the NSC. The monthly wind-driven flow is estimated to represent between 1.0% (+/- 0.1%) and 48% (+/- 1%) of the total alongshore currents, with a yearly mean of about 36% (+/- 1%). We demonstrate that using local conditions is more appropriate to the study of buoyancy-driven currents ranging over distances on the order of O(100 km), compared to the traditional approach based on upstream conditions. Contrary to the traditional approach, the HWI is not affected by the advective time lag associated with the downshelf propagation of the buoyant water coming from the upstream source. However, the HWI approach requires high-resolution data sets, as errors on the estimates of the buoyancy- and wind-driven flows become large as the sampling resolution decreases. Despite being data intensive, we argue that the HWI is also applicable to multisource currents, where upstream conditions are difficult to define.

Understanding how phytoplankton respond to their physical environment is key to predicting how bloom dynamics might change under future climate change scenarios. Phytoplankton are at the base of most marine food webs and play an important role in drawing CO2 out of the atmosphere. Using nearly 5years of simultaneous CTD, irradiance, chlorophyll a fluorescence and optical backscattering observations obtained from Slocum glider missions, we observed the subsurface phytoplankton populations across the Scotian Shelf, near Halifax (Nova Scotia, Canada) along with their physical environment. Bloom conditions were observed in each of the 5 springs, with the average chlorophyll in the upper 60m of water generally exceeding 3mgm−3. These blooms occurred when the upper water column stratification was at its lowest, in apparent contradiction of the critical depth hypothesis. A subsurface chlorophyll layer was observed each summer at about 30m depth, which was below the base of the mixed layer. This subsurface layer lasted 3–4months and contained, on average, 1/4 of the integrated water column chlorophyll found during the spring bloom. This suggests that a significant portion of the primary productivity over the Scotian Shelf occurs at depths that cannot be observed by satellites—highlighting the importance of including subsurface observations in the monitoring of future changes to primary productivity in the ocean.