Large- to submesoscale surface circulation and its implications on biogeochemical/biological horizontal distributions during the OUTPACE cruise (southwest Pacific)

Abstract

Abstract. The patterns of the large-scale, meso- and submesoscale surface circulation on biogeochemical and biological distributions are examined in the western tropical South Pacific (WTSP) in the context of the OUTPACE cruise (February–April 2015). Multi-disciplinary original in situ observations were achieved along a zonal transect through the WTSP and their analysis was coupled with satellite data. The use of Lagrangian diagnostics allows for the identification of water mass pathways, mesoscale structures, and submesoscale features such as fronts. In particular, we confirmed the existence of a global wind-driven southward circulation of surface waters in the entire WTSP, using a new high-resolution altimetry-derived product, validated by in situ drifters, that includes cyclogeostrophy and Ekman components with geostrophy. The mesoscale activity is shown to be responsible for counter-intuitive water mass trajectories in two subregions: (i) the Coral Sea, with surface exchanges between the North Vanuatu Jet and the North Caledonian Jet, and (ii) around 170∘ W, with an eastward pathway, whereas a westward general direction dominates. Fronts and small-scale features, detected with finite-size Lyapunov exponents (FSLEs), are correlated with 25 % of surface tracer gradients, which reveals the significance of such structures in the generation of submesoscale surface gradients. Additionally, two high-frequency sampling transects of biogeochemical parameters and microorganism abundances demonstrate the influence of fronts in controlling the spatial distribution of bacteria and phytoplankton, and as a consequence the microbial community structure. All circulation scales play an important role that has to be taken into account not only when analysing the data from OUTPACE but also, more generally, for understanding the global distribution of biogeochemical components.

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

“…On B side of the AB transect the vertical velocities seem to be associated with peculiar distributions of salinity and particles. Horizontal and ageostrophic vertical velocities have already been shown to shape the distribution of tracers or planktonic organisms horizontally (Dandonneau et al, 2003;d'Ovidio et al, 2010;Rousselet et al, 2018) or to advect nutrients to upper layers. In the present study, the vertical distribution of particles (around 43.85 • N) follows the isopycnals and, in particular, the salinity isolines, similarly to submesoscale vertical flux of nutrient (Mahadevan, 2016).…”

Dynamics of phytoplankton and heterotrophic bacterioplankton in the western tropical South Pacific Ocean along a gradient of diversity and activity of diazotrophs

“…reflected by T DIP variability ranging two orders of magnitude in the top-40 m layers within MA (2 to 700 h). The role of the submesoscale activity largely explained such variability (Rousselet et al, 2017).…”

Intermediate water flows in the western South Pacific: as revealed by individual Argo floats trajectories and a model re-analysis

“…More detailed works are underway on such striations in order to observe them more precisely and to understand their physics (Cravatte et al, 2017;Belmadani et al, 2017). During the OUTPACE cruise, Rousselet et al (2018) highlighted the velocity anomaly of such striations at the surface in one specific area close to the OUTPACE domain (18-22 • S, 160 • E-150 • W). Thanks to the floats, we can extend this observation at intermediate levels and several years after the cruise.…”

“…Current speeds at the parking depth of the floats are then calculated, leading to horizontal gridded maps of velocities either global (Davis, 2005;Ollitrault and Rannou, 2013) or regional (Cravatte et al, 2012). Other approaches are developed in order to study the intermediate circulation, for example, by taking into account the spreading of different float trajectories from a common position (Sevellec et al, 2017). Moreover, when Argo floats are biogeochemical floats (BGC Argo), on top of allowing the study of intermediate circulation, they provide measurements of biogeochemistry and biology parameters over the first thousand meters of the water column.…”