Whatever its origin is, a floating particle at the sea surface is advected by ocean currents. Surface currents could be derived from in situ observations or combined with satellite data. For a better ...resolution in time and space, we use satellite-derived sea-surface height and wind stress fields with a 1/3° grid from 1993 to 2001 to determine the surface circulation of the South Pacific Ocean. Surface currents are then used to compute the Lagrangian trajectories of floating debris. Results show an accumulation of the debris in the eastern-centre region of the South Pacific subtropical gyre (120°W; 80°W–20°S; 40°S), resulting from a three-step process: in the first two years, mostly forced by Ekman drift, the debris drift towards the tropical convergence zone (∼30°S). Then they are advected eastward mostly forced by geostrophic currents. They finally reach the eastern-centre region of the South Pacific subtropical gyre from where they could not escape.
The Biogeochemical-Argo program (BGC-Argo) is a new profiling-float-based, ocean wide, and distributed ocean monitoring program which is tightly linked to, and has benefited significantly from, the ...Argo program. The community has recommended for BGC-Argo the addition of six measurements in addition to pressure, temperature and salinity measured by Argo, to include oxygen, pH, nitrate, downwelling light, chlorophyll fluorescence and the optical backscattering coefficient. The purpose of this addition is to enable the monitoring of ocean biogeochemistry and health, and in particular, monitor major processes such as ocean deoxygenation, acidification and warming and their effect on phytoplankton, the main source of energy of marine ecosystems. Here we describe the salient issues associated with the operation of the BGC-Argo network, with information useful for those interested in deploying and using the data it produces. These include, float testing, deployment and increasingly, recovery. Aspect of data management, processing and quality control are covered as well as specific issues associated with each of the six BGC-Argo sensors. In particular, it is recommended that water samples be collected during float deployment to be used for validation of sensor output.
The North Western Mediterranean Sea exhibits recurrent and significant autumnal and spring phytoplankton blooms. The existence of these two blooms coincides with typical temperate dynamics. To ...determine the potential control of physical and biogeochemical factors on these phytoplankton blooms, data from a multiplatform approach (combining ships, Argo and BGC‐Argo floats, and bio‐optical gliders) were analyzed in association with satellite observations in 2012–2013. The satellite framework allowed a simultaneous analysis over the whole annual cycle of in situ observations of mixed layer depth, photosynthetical available radiation, particle backscattering, nutrients (nitrate and silicate), and chlorophyll‐a concentrations. During the year 2012–2013, satellite ocean color observations, confirmed by in situ data, have revealed the existence of two areas (or bioregions) with comparable autumnal blooms but contrasting spring blooms. In both bioregions, the ratio of the euphotic zone (defined as the isolume 0.415 mol photons m−2 d−1, Z0.415) and the MLD identified the initiation of the autumnal bloom, as well as the maximal annual increase in Chl‐a in spring. In fact, the autumnal phytoplankton bloom might be initiated by mixing of the summer shallowing deep chlorophyll maximum, while the spring restratification (when Z0.415/MLD ratio became >1) might induce surface phytoplankton production that largely overcomes the losses. Finally, winter deep convection events that took place in one of the bioregions induced higher net accumulation rate of phytoplankton in spring associated with a diatom‐dominated phytoplankton community principally. We suggest that very deep winter MLD lead to an increase in surface silicates availability, which favored the development of diatoms.
Key Points
The initiation of the autumnal bloom is linked to the mixing of the shallowing summer DCM
Heat fluxes and restratification of the mixed layer control the spring bloom dynamics
Deep winter convection events enhance the spring bloom's magnitude by favoring diatoms development
Since 2010, an intense effort in the collection of in situ observations has been carried out in the northwestern Mediterranean Sea thanks to gliders, profiling floats, regular cruises, and mooring ...lines. This integrated observing system enabled a year‐to‐year monitoring of the deep waters formation that occurred in the Gulf of Lions area during four consecutive winters (2010–2013). Vortical structures remnant of wintertime deep vertical mixing events were regularly sampled by the different observing platforms. These are Submesoscale Coherent Vortices (SCVs) characterized by a small radius (∼5–8 km), strong depth‐intensified orbital velocities (∼10–20 cm s−1) with often a weak surface signature, high Rossby (∼0.5) and Burger numbers O(0.5–1). Anticyclones transport convected waters resulting from intermediate (∼300 m) to deep (∼2000 m) vertical mixing. Cyclones are characterized by a 500–1000 m thick layer of weakly stratified deep waters (or bottom waters that cascaded from the shelf of the Gulf of Lions in 2012) extending down to the bottom of the ocean at ∼2500 m. The formation of cyclonic eddies seems to be favored by bottom‐reaching convection occurring during the study period or cascading events reaching the abyssal plain. We confirm the prominent role of anticyclonic SCVs and shed light on the important role of cyclonic SCVs in the spreading of a significant amount (∼30%) of the newly formed deep waters away from the winter mixing areas. Since they can survive until the following winter, they can potentially have a great impact on the mixed layer deepening through a local preconditioning effect.
Key Points
Small‐scale eddies (5–10 km radius) formed by deep convection are described from in situ observations in the NW Mediterranean Sea
Bottom‐reaching convection might favor the formation of cyclonic vortices characterized by surface‐ intensified or depth‐intensified velocities
Long‐lived vortices can transport a significant fraction of the convected waters and influence the convection onset the following winter
During winter 2012–2013, open‐ocean deep convection which is a major driver for the thermohaline circulation and ventilation of the ocean, occurred in the Gulf of Lions (Northwestern Mediterranean ...Sea) and has been thoroughly documented thanks in particular to the deployment of several gliders, Argo profiling floats, several dedicated ship cruises, and a mooring array during a period of about a year. Thanks to these intense observational efforts, we show that deep convection reached the bottom in winter early in February 2013 in a area of maximum 28 ± 3
109m2. We present new quantitative results with estimates of heat and salt content at the subbasin scale at different time scales (on the seasonal scale to a 10 days basis) through optimal interpolation techniques, and robust estimates of the deep water formation rate of 2.0
±0.2 Sv. We provide an overview of the spatiotemporal coverage that has been reached throughout the seasons this year and we highlight some results based on data analysis and numerical modeling that are presented in this special issue. They concern key circulation features for the deep convection and the subsequent bloom such as Submesoscale Coherent Vortices (SCVs), the plumes, and symmetric instability at the edge of the deep convection area.
Key Points
We study deep convection and subsequent bloom in the northwestern Mediterranean Sea based on modern observation techniques
We provide estimates of mass and energy fluxes over a period of a year in the deep convection area and deep water formation rates
We highlight small‐scale circulation features that are important for deep convection and subsequent bloom and introduce this special issue
Diazotrophs regulate marine productivity in the oligotrophic ocean by alleviating nitrogen limitation, contributing to particulate organic carbon (POC) export to the deep ocean. Yet, the ...characterization of particles composing the sinking POC flux has never been explored in such ecosystems. Moreover, the contribution of the direct gravitational export of diazotrophs to the overall flux is seldom assessed. Here we explore the composition of the sinking POC flux in a hot spot of N
2
fixation (the western sub-tropical South Pacific) using polyacrylamide gel-filled traps deployed at two stations (S05M and S10M) and three depths (170 m, 270 m, 1000 m) during the TONGA expedition (November-December 2019). Image analyses of particles collected in the gels was used to classify them into 5 categories (fecal aggregates, phytodetrital aggregates, mixed aggregates, cylindrical fecal pellets, and zooplankton carcasses). Fecal aggregates were the most abundant at both stations and all depths and dominated the flux (average of 56 ± 28% of the POC flux), followed by zooplankton carcasses (24 ± 19%), cylindrical fecal pellets (15 ± 14%) and mixed aggregates (5 ± 4%), whereas phytodetrital aggregates contributed less (<1%). Since N isotope budgets show that export is mainly supported by diazotrophy at these stations, these results suggest that the diazotroph-derived N has been efficiently transferred to the foodweb up to zooplankton and fecal pellets before being exported, pleading for an indirect export of diazotrophy. However, random confocal microscopy examination performed on sinking particles revealed that diazotrophs were present in several categories of exported particles, suggesting that diazotrophs are also directly exported, with a potential contribution to overall POC fluxes increasing with depth. Our results provide the first characterization of particle categories composing the sinking flux and their contribution to the overall flux in a hot spot of N
2
fixation.
During the winter from 2009 to 2013, the mixed layer reached the seafloor at about 2500 m in the northwestern Mediterranean Sea. Intense fronts around the deep convection area were repeatedly sampled ...by autonomous gliders. Subduction down to 200–300 m, sometimes deeper, below the mixed layer was regularly observed testifying of important frontal vertical movements. Potential Vorticity dynamics was diagnosed using glider observations and a high resolution realistic model at 1-km resolution. During down-front wind events in winter, remarkable layers of negative PV were observed in the upper 100 m on the dense side of fronts surrounding the deep convection area and successfully reproduced by the numerical model. Under such conditions, symmetric instability can grow and overturn water along isopycnals within typically 1–5 km cross-frontal slanted cells. Two important hotpspots for the destruction of PV along the topographically-steered Northern Current undergoing frequent down-front winds have been identified in the western part of Gulf of Lion and Ligurian Sea. Fronts were there symmetrically unstable for up to 30 days per winter in the model, whereas localized instability events were found in the open sea, mostly influenced by mesoscale variability. The associated vertical circulations also had an important signature on oxygen and fluorescence, highlighting their under important role for the ventilation of intermediate layers, phytoplankton growth and carbon export.
The necessity of wide, global-scale observing systems for marine biogeochemistry emerged dramatically in the last decade. A global network based on Biogeochemical (BGC) Argo floats is considered as ...one of the most promising approaches for reaching this goal. As a first step, pilot studies were encouraged to test the feasibility of a global BGC-Argo array, to consolidate the methods and practices under development, and to set up the array’s characteristics. A pilot study in the Mediterranean Sea—deemed a suitable candidate for a test case because it combines a relatively large diversity of oceanic biogeochemical conditions in a reduced open-ocean basin—was consequently approved as a part of the NAOS (“Novel Argo ocean Observing System”) project, a French national initiative to promote, consolidate and develop the Argo network. We present here a first assessment of the NAOS Mediterranean array, in view of scientific choices on observing-system strategy, on implementation and statistics on network performances, and on data-quality control.
The seasonal variability of the carbonate system in the eastern Mediterranean Sea (EMed) was investigated based on discrete total alkalinity (A
T
), total dissolved inorganic carbon (C
T
), and pH ...measurements collected during three cruises around Crete between June 2018 and March 2019. This study presents a detailed description of this new carbonate chemistry dataset in the eastern Mediterranean Sea. We show that the North Western Levantine Basin (NWLB) is unique in terms of range of A
T
variation vs. C
T
variation in the upper water column over an annual cycle. The reasons for this singularity of the NWLB can be explained by the interplay between strong evaporation and the concomitant consumption of C
T
by autotrophic processes. The high range of A
T
variations, combined to temperature changes, has a strong impact on the variability of the seawater
p
CO
2
(
p
CO
2
S
W
). Based on Argo float data, an entire annual cycle for
p
CO
2
S
W
in the NWLB has been reconstructed in order to estimate the temporal sequence of the potential “source” and “sink” of atmospheric CO
2
. By combining this dataset with previous observations in the NWLB, this study shows a significant ocean acidification and a decrease in the oceanic surface pH
T
25
of −0.0024 ± 0.0004 pH
T
25
units.a
–1
. The changes in the carbonate system are driven by the increase of atmospheric CO
2
but also by unexplained temporal changes in the surface A
T
content. If we consider that the EMed will, in the future, encounter longer, more intense and warmer summer seasons, this study proposes some perspectives on the carbonate system functioning of the “future” EMed.
The international array of profiling floats known as Argo is a major component of the global ocean- and climate-observing system. In 2010, the NAOS (Novel Argo Observing System) project was selected ...as part of France’s Equipex “Investissement d’Avenir” program. The objectives of NAOS were to consolidate the French contribution to the Argo core mission (global temperature and salinity measurements down to 2,000 m) as well as to develop the future generation of French Argo profiling floats and prepare the next phase of the Argo program with an extension to the deep ocean (Deep-Argo), biogeochemistry (BGC-Argo) and polar seas. This paper summarizes the main technological advances and at-sea validations carried out as part of NAOS: development of a deep (4,000 m) float, a new BGC float for R&D applications, and a BGC float for deployments in Arctic areas; assessment of a new density and Absolute Salinity optical sensor; improvement of the reliability of the standard Argo float; and upgraded satellite-transmission performance.
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