Longitudinal contrast in turbulence along a ∼ 19° S section in the Pacific and its consequences for biogeochemical fluxes

Microstructure measurements were performed along the OUTPACE longitudinal transect in the tropical Pacific (Moutin and Bonnet, 2015). Small-scale dynamics and turbulence in the first 800m surface layer were characterized based on hydrographic and current measurements at fine vertical scale and turbulence measurements at centimeter 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, epsilon, showed an interesting contrast along the longitudinal transect with stronger turbulence in the west, i.e., the Melanesian Archipelago, compared to the east, within the South Pacific Subtropical Gyre, with a variation of epsilon by a factor of 3 within [100-500 m]. The layer with enhanced turbulence decreased in vertical extent travelling eastward. This spatial pattern was correlated with 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 epsilon observed within a 50-250 m layer, with a value of 8 x 10(-9) Wkg(-1), about 8 times larger than at the eastern long-duration stations. Averaged nitrate turbulent diffusive fluxes in a 100 m layer below the top of the nitracline were about twice larger west of 170 degrees W due to the higher vertical diffusion coefficient. In the photic layer, the depthaveraged nitrate turbulent diffusive flux strongly decreased eastward, with an averaged value of 11 mu molm 2d(-1) west of 170 degrees Wcompared with the 3 mu molm(-2)d(-1) averaged value east of 170 degrees W. Contrastingly, phosphate turbulent diffusive fluxes were significantly larger in the photic layer. This input may have an important role in sustaining the development of N-2-fixing organisms that were shown to be the main primary contributors to the biological pump in the area. The timespace intermittency of mixing events, intrinsic to turbulence, was underlined, but its consequences for micro-organisms would deserve a dedicated study.