• Membre du conseil du laboratoire

• Planète et Univers/Sciences de la Terre/Océanographie
  

The temporal evolution of a surface chlorophyll a bloom sampled in the western tropical South Pacific during the 2015 Oligotrophy to UlTra-oligotrophy PACific Experiment (OUTPACE) cruise is examined. This region is usually characterized by largely oligotrophic conditions, i.e. low concentrations of inorganic nutrients at the surface and deep chlorophyll a maxima. Therefore, the presence of a surface bloom represents a significant perturbation from the mean ecological state. Combining in situ and remote sensing datasets, we characterize both the bloom's biogeochemical properties and the physical circulation responsible for structuring it. Biogeochemical observations of the bloom document the bloom itself, a subsequent decrease of surface chlorophyll a, significantly reduced surface phosphate concentrations relative to subtropical gyre water farther east, and a physical decoupling of chlorophyll a from a deep nitracline. All these characteristics are consistent with nitrogen fixation occurring within the bloom. The physical data suggest surface mesoscale circulation is the primary mechanism driving the bloom's advection, whereas balanced motions expected at submesoscales provide little contribution to observed flow. Together, the data provide a narrative where subtropical gyre water can produce significant chlorophyll a concentrations at the surface that is stirred, deformed, and transported great distances by the mesoscale circulation. In this case, for the time period considered, the transport is in an easterly direction, contrary to both the large-scale and mean mesoscale flow. As a result, future studies concerning surface production in the region need to take into account the role complex mesoscale structures play in redistributing subtropical gyre water.

The circulation within the Southwest Pacific Ocean is today well established from a climatological point of view. The northern branch of the anticyclonic South Pacific gyre creates the South Equatorial current, a major westward current controlling the circulation in the Southwest Pacific. The complex topography as well as the barotropic instabilites cause intense mesoscale activity that is well observed with satellites but strongly undersampled with in situ observations. The ocean dynamics at mesoscale can have an important impact on the ecosystem of this oligotrophic region, and in particular on the development of species involved in the biological carbon pump. We use the in situ dataset of the OUTPACE cruise (ADCP, TSG, SVP data) to validate satellite data (altimetry, Sea Surface Temperature, Chlorophyll-a concentration) that allow to characterize the overall conditions during the cruise. In particular we used specifically designed high resolution (1/8) regional altimetric product (summing Absolute Geostrophic currents and Ekman currents) produced by CLS (with support from CNES) to study the circulation at different scale : large, mesoscale and submesoscale. Lagrangian numerical experiments performed with this altimetric product allow us to identify the general surface circulation. Mesoscale activity is examined through structure identification to determine its influence on the water masses encountered during the cruise. Finally the computation of Finite Size Lyapunov Exponents (FSLE), a Lagrangian diagnostic that identify frontal areas, allow to quantify the impact of these structure on the surface distribution of biogeochemistry quantities (temperature, salinity, chlorophyll..). Our results show that despite the strong mesoscale activity in this area, the meso- and submesoscale structures had a small, but non-negligable, influence on the water masses sampled during the cruise. Meso- and submesoscale participate in the surface distribution of tracers such as SST, SSS or chlorophyll-a but also of some micro-organismes such as bacteria or Prochlocorococcus.