Longitudinal contrast in Turbulence along a ∼ 19S section in the Pacific and its consequences on biogeochemical fluxes
|Author(s)||Bouruet-Aubertot Pascale1, Cuypers Yannis1, Doglioli Andrea2, Caffin Mathieu2, Yohia Christophe, de Verneil Alain2, Petrenko Anne2, Lefèvre Dominique2, Le Goff Hervé1, Rougier Gilles2, Picheral Marc3, Moutin Thierry2|
|Affiliation(s)||1 : Sorbonne Université – UPMC Univ. Paris 06 – LOCEAN, France
2 : Aix Marseille Univ, Université de Toulon, CNRS, IRD, MIO UM 110, 13288, Marseille, France
3 : LOV, Villefranche sur mer, France
|Source||Biogeosciences (1726-4170) (Copernicus GmbH), 2018 , Vol. 15 , N. 24 , P. 7485-7504|
|Note||Special issue Interactions between planktonic organisms and biogeochemical cycles across trophic and N2 fixation gradients in the western tropical South Pacific Ocean: a multidisciplinary approach (OUTPACE experiment) Editor(s): T. Moutin, S. Bonnet, K. Richards, D. G. Capone, E. Marañón, and L. Mémery|
Microstructure measurements were performed along the OUTPACE longitudinal transect in the tropical Pacific (Moutin and Bonnet, 2015). Small-scale dynamics and three-dimensional turbulence in the first 800m surface layer were characterized based on hydrographic and current measurements at fine scale and turbulence measurements at cm scale using a vertical microstructure profiler. The possible impact of turbulence on biogeochemical budgets in the surface layer was also addressed in this region of increasing oligotrophy to the East. The dissipation rate of turbulent kinetic energy, ε, showed an interesting contrast along the longitudinal transect with higher turbulence level in the West, i.e. the Melanesian Archipelago, compared to the East, within the South Pacific Subtropical Gyre. The surface layer with enhanced turbulence decreased in vertical extent traveling eastward. This spatial pattern was correlated to the energy level of the internal wave field, higher in the West compared to the East. The difference in wave energy mostly resulted from enhanced wind power input into inertial motions in the West. Moreover, three long duration stations were sampled along the cruise transect, each over three inertial periods. The analysis from the western long duration station gave evidence of an energetic baroclinic near-inertial wave that was responsible for the enhanced ε. This was in strong contrast with the observations at the two eastern long duration stations where semi-diurnal internal tides dominate. Averaged nitrate turbulent diffusive fluxes were at least twice as large at the western station than at the two eastern stations due to the higher vertical diffusion coefficient. In the surface layer, above the nitracline, phosphate turbulent diffusive fluxes were significant except at the very eastern part of the transect. This input may have an important role in sustaining the development of N2-fixing organisms that were evidenced as the main primary contributors to the biological pump in the area. The time-space intermittency of mixing events, intrinsic to turbulence, was underlined but its consequences on micro-organisms would deserve a dedicated study.