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Papers by annalisa bracco

AGU Fall Meeting Abstracts, Dec 1, 2010

ABSTRACT Positively buoyant organisms such as the macroalgae Sargassum and the cyanobacteria Tric... more ABSTRACT Positively buoyant organisms such as the macroalgae Sargassum and the cyanobacteria Trichodesmiumm often form surface accumulations visible in satellite imagery. Here we discuss the accumulation of floating material in the ocean in presence of meso- and submesoscales activity. Using high resolution simulations of the ocean mesoscale in both idealized and realistic domains (specifically in a 3D box where coherent eddies are forced by small-scale winds, and in the western Gulf of Mexico, where extensive concentrations of floating Sargassum have been recorded in satellite images) we show that the distribution of tracers at ocean surface departs rapidly from the one observed few tens of meters below it. Such distribution does not resemble what observed for passive tracers in quasigeostrophic turbulence. The strong divergence and convergence zones generated at the surface by the eddy field are responsible for the creation of `lines' where the floating material accumulates. Floating particles are expelled from the core of the eddies, and concentrate in convergence regions of size and strength comparable to the ones observed through the satellite images. In light of those results, Sargassum and/or Trichodesmium may provide a useful proxy to track convergence/divergence processes at the ocean surface.

Progress in Oceanography, Mar 1, 2023

Journal Of Geophysical Research: Oceans, May 6, 2022

Bulletin of the American Physical Society, Mar 6, 2018

Journal of the Acoustical Society of America, Oct 1, 2022

Accurate numerical simulations of underwater acoustic propagation in a dynamic ocean—and its asso... more Accurate numerical simulations of underwater acoustic propagation in a dynamic ocean—and its associated uncertainty—require using realistic environmental parameters as inputs and especially a high-fidelity representation of the expected spatio-temporal variability of the ocean sound speed in the volume of interest. In areas characterized by strong temperature and salinity variations (e.g., associated with long-living mesoscale eddies in the Gulf of Mexico), the approximate simulation of the 3D sound-speed field and its variability requires predictive oceanographic models capable of resolving such variations. This study investigates the impact of vertical resolution focusing on how it shapes the representation of the 3D sound speed variability through a suite of simulations of the northern Gulf of Mexico performed with a regional ocean model run at submesoscale permitting horizontal resolution (0.5 km) using increasing vertical resolution from 30 to 200 terrain-following layers over a one-month simulation interval (as described in the companion paper presented by Touret et al.). Geo-acoustic parameters were matched to the existing sediment database. In selected areas influenced by mesoscale eddies, ray tracing is used to determine the significance of increased resolution on acoustic propagation as a function of the sensor configurations and the expected sound speed variability.

Journal of the Acoustical Society of America, Oct 1, 2022

Vertical resolution affects the representation of ocean sound speed according to a suite of regio... more Vertical resolution affects the representation of ocean sound speed according to a suite of regional simulations of the De Soto Canyon circulation in the Gulf of Mexico. Simulations have identical horizontal resolution of 0.5 km, partially resolving submesoscale dynamics, and increasing vertical resolution from 30 (i.e., comparable to what commonly used in mesoscale permitting or resolving hindcast and forecast products such as HYCOM) to 200 terrain-following layers. Simulations with 30- and 70-layers underestimate the ageostrophic contributions in and around the eddies below the mixed-layer and do not reproduce the sharp vorticity and density variations associated with the mesoscale circulations compared to the 140- and 200-layers runs. The ocean sound speed (based on the classical MacKenzie formula) was found to be far more variable when the submesoscale, ageostrophic circulations are captured also in their vertical structure and vertical contributions to the density field. Hence, the results of this study indicate that to better predict the influence of the submesocale oceanic circulation on ocean sound speed variability, model simulations should consider enhancing both horizontal and vertical resolution to resolve at least the first 3 baroclinic modes. To do so, more than 100 vertical layers were found to be needed in this study.

Authorea (Authorea), Mar 26, 2023

Diatoms are among the most efficient marine organisms for primary production and carbon sequestra... more Diatoms are among the most efficient marine organisms for primary production and carbon sequestration, absorbing at least 10 billion tonnes of carbon dioxide every year. Yet, the spatial distributions of these planktonic organisms remain puzzling and the underlying physical processes poorly known. Here we investigate what dynamical conditions are conductive to episodic diatom blooms in oligotrophic waters based on Lagrangian diagnosis and satellite-derived phytoplankton functional types and ocean currents. The Lagrangian coherence of the flow is diagnosed in space and time simultaneously to identify which structures favor diatom growth. Observations evidence that flow structures with a high degree of coherence (40 days or longer) in high turbulent kinetic energy and vorticity sustain high concentrations of diatoms in the sunlite layers. Our findings show that the integration of Eulerian kinematic variables into a Lagrangian frame allows revealing new dynamical aspects of geophysical turbulence and unveil transport properties having large biological impacts.