Estimating ocean vertical velocities using an autonomous multipurpose profiler

International audienceA low-cost multipurpose oceanographic vertical profiler is described. Its application to the measurement of vertical currents, inspired by ocean glider flight-model methods, is presented. Preliminary results of the BioSWOT-Med campaign, carried out in April-May 2023, illustrate the instrument's ability to capture weak signals, potentially linked to sub-mesoscale oceanic structures

Fine-scale sampling unveils diazotroph patchiness in the South Pacific Ocean

Diazotrophs are important contributors to nitrogen availability in the ocean. Oceanographic cruise data accumulated over the past three decades has revealed a heterogeneous distribution of diazotroph species at regional to global scales. However, dynamic fine-scale physical structures likely affect the distribution of diazotrophs at smaller spatiotemporal scales. The interaction between fine-scale ocean dynamics and diazotrophs remains poorly understood due to typically insufficient spatiotemporal sampling resolution and the lack of parallel detailed physical studies. Here we show the distribution of five groups of diazotrophs in the South Pacific at an unprecedented resolution of 7–16 km. We find a patchy distribution of diazotrophs, with each group being differentially affected by parameters describing fine-scale physical structures. The observed variability in species abundance and distribution would be masked by a coarser sampling resolution, highlighting the need to consider fine-scale physics to resolve the distribution of diazotrophs in the ocean

Near Inertial Oscillations and Vertical Velocities Modulating Phytoplankton After a Storm in the Mediterranean Sea

Understanding the impact of storms on phytoplankton dynamics is a complex and crucial issue, both on regional and global scales. Here we address this question by conducting a numerical modeling study to represent the physical forcing and phytoplankton response of an intense storm that occurred in the northwestern Mediterranean Sea in late spring 2019. This numerical study, employing the SYMPHONIE regional circulation model, covers and complements in situ observations gathered during the FUMSECK cruise. Our realistic numerical simulation unveils that the storm event triggered robust near-inertial oscillations (NIOs) in a two-layer system, spanning a 5000 km² area and persisting for a duration of 3-4 days. We demonstrate the oscillatory pattern of the NIOs vertical velocities. Notably, our modeled vertical velocities reach a maximum of 10-3 m s-1 and coincide with a substantial 1.3-fold increase in total chlorophyll concentration. These findings underline the significance of considering the vertical dynamics linked to NIOs induced by meteorological events that are projected to grow both in frequency and intensity in the context of ongoing climate change. The outcomes of this study contribute valuable insights into the intricate relationship between storms and phytoplankton, shedding light on the potential ecological consequences of future climate shifts, and emphasizing the need for more comprehensive investigations to address this complex issue effectively

Measuring vertical velocities with ADCPs in low-energy ocean

International audienceAbstract Vertical velocities knowledge is essential to study fine-scale dynamics in the surface layers of the ocean and to understand their impact on biological production mechanisms. However, these vertical velocities have long been neglected, simply parameterized, or considered as not measurable, due mainly to their order of magnitude (< mm s −1 up to cm s −1 ), generally much lower than the one of the horizontal velocities (cm s −1 to dm s −1 ), hence the challenge of their in situ measurement. In this paper, we present an upgraded method for direct in situ measurement of vertical velocities using data from different Acoustic Doppler Current Profilers (ADCPs) associated with CTD probes, and we perform a comparative analysis of the results obtained by this method. The analyzed data were collected during the FUMSECK cruise, from three ADCPs: two Workhorse (conventional ADCPs), one lowered on a carousel and the other deployed in free-fall mode, and one Sentinel V (a new generation ADCP with four classical beams and a fifth vertical beam), also lowered on a carousel. Our analyses provide profiles of vertical velocities on the order of mm s −1 , as expected, with standard deviations of a few mm s −1 . While the fifth beam of the Sentinel V exhibits a better accuracy than conventional ADCPs, the free-fall technique provides a more accurate measurement compared to the carousel technique. Finally, this innovative study opens up the possibility to perform simple and direct in situ measurements of vertical velocities, coupling the free-fall technique with a five-beam ADCP

New insights for direct in situ measurement of oceanic vertical velocities in fine-scale studies.

International audienceVertical velocities knowledge is essential to study fine-scale dynamics in the surface layers of the ocean and to understand their impact on biological production mechanisms, in both coastal and offshore environments. Indeed, the general interest in fine-scale and, more precisely, in the determination of vertical velocities, is explained by their key role in global oceanic balance and their impact on the vertical transfer of nutrients and carbon budget despite their low intensity. With the increasing global warming issues linked to the forcing of the carbon cycle by anthropogenic activities, the estimation of vertical velocities becomes an essential information for a better representation of biogeochemical budgets. However, these vertical velocities have long been neglected, simply parameterized, or considered as not measurable, due mainly to their order of magnitude (mm s&lt;sup&gt;-1&lt;/sup&gt;), generally much lower than the one of the horizontal velocities (cm s&lt;sup&gt;-1&lt;/sup&gt;). Consequently, direct &lt;em&gt;in situ&lt;/em&gt; measurement of vertical velocities is still currently one of the biggest challenges in physical oceanography.&lt;/p&gt;&lt;p&gt;We have been working to develop a new method for direct &lt;em&gt;in situ&lt;/em&gt; measurement of vertical velocities using data from different Acoustic Doppler Current Profilers (ADCPs) associated with CTD probes, and we performed a comparative analysis of the results obtained by this method. The analyzed data were collected during the FUMSECK cruise (2019, Ligurian Sea), from three ADCPs: two Workhorse (conventional ADCPs), one lowered on a carousel and the other deployed in free-fall mode, and one Sentinel V (a new generation ADCP with four classical beams and a fifth vertical beam), also lowered on a carousel. Our analyses provided profiles of vertical velocities of the order of mm s&lt;sup&gt;-1&lt;/sup&gt;, as expected, with standard deviations of a few mm s&lt;sup&gt;-1&lt;/sup&gt;. While the fifth beam of the Sentinel V has shown a better accuracy than conventional ADCPs, the free-fall technique has provided a more accurate measurement compared to the carousel technique. Some of these measurements were gathered along the edge of the Northern Current and this new information on coastal edge currents represents a key point for the future improvement of coastal altimetry in particular.&lt;/p&gt;&lt;p&gt;Finally, this innovative study opens up the possibility to perform simple and direct &lt;em&gt;in situ&lt;/em&gt; measurements of vertical velocities, coupling the free-fall technique with a five-beam ADCP. Hence, we plan to deploy a free-falling Sentinel V in offshore areas characterized by intense fine-scale ocean dynamics, but also and above all, in coastal areas, where topographic forcings are typically the source of high amplitude vertical velocities.&lt;/p&gt

Process studies at the air-sea interface after atmospheric deposition in the Mediterranean Sea: objectives and strategy of the PEACETIME oceanographic campaign (May–June 2017)

In spring, the Mediterranean Sea, a well-stratified low nutrient low chlorophyll region, receives atmospheric deposition both desert dust from the Sahara and airborne particles from anthropogenic sources. Such deposition translates into a supply of new nutrients and trace metals for the surface waters that likely impact biogeochemical cycles. However, the quantification of the impacts and the processes involved are still far from being assessed in situ. In this paper, we provide a state of the art regarding dust deposition and its impact on the Mediterranean Sea biogeochemistry and we describe in this context the objectives and strategy of the PEACETIME project and cruise, entirely dedicated to filling this knowledge gap. Our strategy to go a step forward than in previous approaches in understanding these impacts by catching a real deposition event at sea is detailed. The PEACETIME oceanographic campaign took place in May–June 2017 and we describe how we were able to successfully adapt the planned transect in order to sample a Saharan dust deposition event, thanks to a dedicated strategy, so-called Fast Action. That was successful, providing, for the first time in our knowledge, a coupled atmospheric and oceanographic sampling before, during and after an atmospheric deposition event. Atmospheric and marine in situ observations and process studies have been conducted in contrasted area and we summarize the work performed at sea, the type of data acquired and their valorization in the papers published in the special issue

Long distance particle transport to the central Ionian Sea

In the upper layers of the Ionian Sea, young Mediterranean Atlantic Waters (MAW) flowing eastward from the Sicily channel meet old MAW. In May 2017, during the PEACETIME cruise, fluorescence and particle content sampled at high resolution revealed unexpected heterogeneity in the central Ionian. Surface salinity measurements, together with altimetry-derived and hull-mounted ADCP currents, describe a zonal pathway of AW entering the Ionian Sea, consistent with the so-called cyclonic mode in the North Ionian Gyre. The ION-Tr transect, located ~19–20° E–~36° N turned out to be at the crossroad of three water masses, mostly coming from the west, north and from an isolated anticyclonic eddy northeast of ION-Tr. Using Lagrangian numerical simulations, we suggest that the contrast in particle loads along ION-Tr originates from particles transported from these three different water masses. Waters from the west, identified as young AW carried by a strong southwestward jet, were intermediate in particle load, probably originating from the Sicily channel. Water mass originating from the north was carrying abundant particles, probably originating from northern Ionian, or further from the south Adriatic. Waters from the eddy, depleted in particles and Chl-a may originate from south of Peloponnese, where the Pelops eddy forms. The central Ionian Sea hence appears as a mosaic area, where waters of contrasted biological history meet. This contrast is particularly clear in spring, when blooming and non-blooming areas co-occur. Particle abundance in situ measurements are useful to discriminate water masses and derive circulation, together with T-S properties. Interpreting the complex dynamics of physical-biogeochemical coupling from discrete measurements made at isolated stations at sea is a big challenge. The combination of multi-parametric in situ measurements at high resolution with remote sensing and Lagrangian modeling appears as one proper way to address this challenge

Introduction: Process studies at the air–sea interface after atmospheric deposition in the Mediterranean Sea – objectives and strategy of the PEACETIME oceanographic campaign (May–June 2017)

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Particle transport in the central Ionian Sea

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A Lagrangian strategy for in situ sampling the physical-biological coupling at fine scale : the PROTEVSMED-SWOT 2018 cruise

International audienceThe term "fine scales" is generally used to refer to the ocean processes occuring on horizontal scales smaller than 10 km andcharacterized by a short lifetime (days/weeks). Fine scales have been predominantly studied with numerical simulations andsatellite observations which have highlighted their significant role on biological processes. Indeed, their short time scale is thesame as a lot of important processes in phytoplankton dynamics. Model simulations have shown that fine scales such as frontsand filaments strongly influence the distribution of phytoplankton species. Nowadays, the combination of in situ measurements,satellite observations and model simulations is a necessity to better understand these mechanisms. However these processesare particularly challenging to sample in situ because of their size and their ephemeral nature.The PROTEVSMED-SWOT cruise was performed in the Western Mediterranean Sea, in the southern region of the BalearicIslands, onboard BHO Beautemps-Beaupré, between April 30 th and May 14 th , 2018. In order to study the influence of finescales on the distribution of phytoplankton species, a satellite-based adaptive Lagrangian sampling strategy has been deployedin order to i) identify a fine scale structure of interest, ii) sample it at high spatial resolution the phytoplankton community, andiii) follow the evolution of this structure and the related distribution of phytoplankton. The SPASSO software package usessatellite altimetry, SST and surface Chl a concentration data to generate and provide near-real time daily maps of the dynamicaland biogeochemical structures present in the area. The sampling strategy was defined in order to cross a frontal zone separatingdifferent types of water. Multidisciplinary in situ sensors (hull-mounted ADCP, a Seasoar towed fish and an automated flowcytometer installed on the seawater supply of the Thermosalinograph) were used to sample at high spatial resolution physicaland biological variables. A particular attention was put in adapting the temporal sampling in different water masses to thebiological time scales in order to reconstruct the phytoplankton diurnal cycle. Such a strategy was successful in sampling two different water masses separated by a narrow front and characterized bydifferent aboundances of several phytoplankton species and functional groups. Consequently, our results highlight the role ofthe front on the physical and biological coupling confirming previous modelling and remote-sensing studies. The new generation of altimetric satellite, SWOT, will provide a 2D sea surface height at an unprecedented resolution andit will be a unique opportunity to better observe fine scale structures in the global ocean. Our methodology paves the way tofuture in situ experiments that are planned in 2022 during the SWOT fast-sampling phase, few months after its launch