Vertical Motions and Their Effects on a Biogeochemical Tracer in a Cyclonic Structure Finely Observed in the Ligurian Sea

Abstract

Vertical velocities can be estimated indirectly from in situ observations by theoretical frameworks like the ω‐equation. Direct measures of vertical exchanges are challenging due to their typically ephemeral spatiotemporal scales. In this study we address this problem with an adaptive sampling strategy coupling various biophysical instruments. We analyze the 3‐D organization of a cyclonic mesoscale structure finely sampled during the Observing Submesoscale Coupling At High Resolution cruise in the Ligurian Sea during fall 2015. The observations, acquired with a moving vessel profiler, highlight a subsurface low‐salinity layer (≃50 m), as well as rising isopycnals, generated by geostrophic cyclonic circulation, in the structure's center. Reconstructed 3‐D fields of density and horizontal velocities are used to estimate the vertical velocity field down to 250 m by applying the adiabatic QG ω‐equation, for the first time in this region. The vertical motions are characterized by multipolar patterns of downward and upward velocities on the edges of the structure and significantly smaller vertical velocities in its center. Both the 3‐D distribution of particles (size ≥100 μm), measured with a laser optical plankton counter, and the Synechococcus and Prochlorococcus abundances (cell per cubic meter) measured by flow cytometry are consistent with the 3‐D velocity field. In particular, a secondary vertical recirculation is identified that upwells particles (from 250 to 100 m) along isohalines to the structure's center. Besides demonstrating the effect of vertical patterns on biogeochemical distributions, this case study suggests to use particle matter as a tracer to assess physical dynamics.

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)

“…At FAST, where the impact was modest under present conditions (+41 %-49 %), the future conditions significantly enhanced the increase in N 2 fixation after dust seeding (+97 %-120 %). These experiments also permitted the study of the fate of some lithogenic elements associated with dust, allowing a first direct assessment of the low percentage of dissolution for thorium (∼ 1 %) and protactinium (< 6 %) after Saharan dust deposition in seawater (Roy-Barman et al, 2020). Unforeseen effects of temperature and/or pH on the release of thorium and rare earth elements in seawater led to a lower Th release and a higher light rare earth element (REE) release under increased greenhouse conditions.…”

“…The idea behind this approach was to aim at a situation where the air-sea exchanges dominate and lateral advection and diffusion can be neglected. Such an approach was already successfully adopted during several previous cruises such as LATEX (Nencioli et al, 2011;Doglioli et al, 2013;Petrenko et al, 2017), KEOPS2 (d'Ovidio et al, 2015), OUTPACE (Moutin et al, 2017;de Verneil et al, 2018) and OSCAHR (Rousselet et al, 2019). During PEACETIME, we used the following datasets: (1) near-real-time altimetry data from the AVISO Mediterranean regional product (now distributed by CMEMS, https://resources.marine.copernicus.eu/?option= com_csw&view=details&product_id=SEALEVEL_MED_ PHY_L4_NRT_OBSERVATIONS_008_050, last access: 11 November 2020), where the altimetry-derived currents were then processed by SPASSO to derive Eulerian and Lagrangian diagnostics of ocean circulation: Okubo-Weiss parameter, particle retention time and advection, finite-size Lyapunov exponent (e.g Fig.…”

Observation of a mesoscale eddy dipole on the northern Madagascar Ridge: Consequences for the circulation and hydrography in the vicinity of a seamount

“…The coarse resolution of the CTD casts during the two transects does not allow any diagnosis of frontogenesis (Capet et al, 2008) nor vertical velocities though inversion of the ω-equation (Pollard and Regier, 1992;Legal et al, 2007;Rousselet et al, 2019). However, the frontal region between the two eddies showed high positive values of the Okubo-Weiss quantity, of the order of 1.7 × 10 -10 s -2 , a marker for areas characterized by growth of horizontal tracer gradient (Okubo, 1970;Weiss, 1991).…”

Impact of moderate energetic fine-scale dynamics on the phytoplankton community structure in the western Mediterranean Sea

“…Following Allen et al (2001)'s suggestions to preserve as much as possible synopticity, four transects of the NS hippodrome (see Fig. 1) have been selected between 11 May and 12 May 2018, to obtain a "butterfly" design as in Cotroneo et al (2016) and Rousselet et al (2019). The σ, u and v fields have been interpolated onto a 3D grid, using objective analysis (Le Traon, 1990;Rudnick, 1996).…”