Preprints
https://doi.org/10.5194/os-2021-101
https://doi.org/10.5194/os-2021-101

  19 Nov 2021

19 Nov 2021

Review status: this preprint is currently under review for the journal OS.

On the use of acoustic data to characterise the thermohaline stratification in a tropical ocean

Ramilla Vieira Assunção1,2, Anne Lebourges-Dhaussy2, Alex Costa da Silva1, Bernard Bourlès3, Gary Vargas4, Gildas Roudaut2, and Arnaud Bertrand1,4,5 Ramilla Vieira Assunção et al.
  • 1Laboratório de Oceanografia Física Estuarina e Costeira, Depto. Oceanografia, UFPE, Recife, Pernambuco, Brazil
  • 2Institut de recherche pour le développement (IRD), UMR 6539 LEMAR Ifremer, Université de Bretagne Occidentale, Technopole Brest Iroise, Plouzané, France
  • 3Institut de recherche pour le développement (IRD), IMAGO, Plouzané, France
  • 4Universidade Federal Rural de Pernambuco, Recife-PE, Brazil
  • 5Institut de recherche pour le développement (IRD), MARBEC, Université Montpellier, Ifremer, IRD, Sète, France

Abstract. The use of active acoustic to monitor abiotic structures and processes in the ocean have been gaining ground in oceanography. In some systems, acoustics allow the robust estimation of the depth of the pycnocline or thermocline either directly or indirectly when the physical structures drive the one of organisms. Here, we examined the feasibility of extracting the thermohaline structure (mixed-layer depth, upper and lower thermocline) from echosounder data collected in the oligotrophic Southwestern tropical Atlantic region at two seasons (spring and fall), more precisely in two areas with different thermohaline conditions, at both day and night. For that, we tested three approaches: (i) the vertical extension of the epipelagic community; (ii) the use of acoustic gradients; and (iii) a cross-wavelet approach. Results show that, even if the thermohaline structure impacts the vertical distribution of acoustic scatters, the resultant structuring did not allow for a robust estimation of the thermohaline limits indicating that other oceanographic or biological processes are acting. This result prevents for a fine-scale representation of the upper-layer turbulence from acoustic data. However, studying the proportion of acoustic biomass within each layer provides interesting insights on ecosystem structure in different thermohaline, seasonal and diel scenarios.

Ramilla Vieira Assunção et al.

Status: open (until 14 Jan 2022)

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Ramilla Vieira Assunção et al.

Ramilla Vieira Assunção et al.

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Short summary
Active acoustics has been used to characterize physical structures and processes in the ocean, typically attributed to biological dispersion or turbulent structures. We take advantage of acoustic data from the Southwest Atlantic to test the feasibility of this approach in an oligotrophic region. The results show that the thermohaline structure impacts the vertical distribution of acoustic scatterers, however the methods tested did not allow a robust estimate of the thermohaline limits.