Vertical fluxes in the upper ocean

Oceanic fronts at the mesoscale and submesoscale are associated with enhanced vertical motion, which strengthens their role in global biogeochemical cycling as hotspots of primary production and subduction of carbon from the surface to the interior. Using process study models, theory, and field observations of biogeochemical tracers, this thesis improves understanding of submesoscale vertical tracer fluxes and their influence on carbon cycling. Unlike buoyancy, vertical transport of biogeochemical tracers can occur both due to the movement of isopycnals and due to motion along sloping isopycnals. We decompose the vertical velocity below the mixed layer into two components in a Lagrangian frame: vertical velocity along sloping isopycnal surfaces and the adiabatic vertical velocity of isopycnal surfaces and demonstrate that vertical motion along isopycnal surfaces is particularly important at submesoscales (1-10 km). The vertical flux of nutrient, and consequently the new production of phytoplankton depends not just on the vertical velocity but on the relative time scales of vertical transport and nutrient uptake. Vertical nutrient flux is maximum when the biological timescale of phytoplankton growth matches the vertical velocity frequency. Export of organic matter from the surface and the interior requires water parcels to cross the mixed layer base. Using Lagrangian analysis, we study the dynamics of this process and demonstrate that geostrophic and ageostrophic frontogenesis drive subduction along density surfaces across the mixed layer base. Along-front variability is an important factor in subduction. Both the physical and biological modeling studies described above are used to interpret observations from three research cruises in the Western Mediterranean. We sample intrusions of high chlorophyll and particulate organic carbon below the euphotic zone that are advected downward by 100 meters on timescales of days to weeks. We characterize the community composition in these subsurface intrusions at a lateral resolution of 1–10 km. We observe systematic changes in community composition due to the changing light environment and differential decay of the phytoplankton communities in low-light environments, along with mixing. We conclude that advective fluxes could make a contribution to carbon export in subtropical gyres that is equal to the sinking flux.

Evaluating the signature of oceanic striations on the distribution of biogeochemical properties in the Eastern Pacific Ocean off Chile

<p>In recent years, quasi-zonal mesoscale jet-like features, also called “striations”, have been ubiquitously detected in the time-mean circulation of the world ocean using satellite altimetry and in situ data. These alternating bands of eastward/westward flow are able to advect and mix physical properties. Yet, their impact on biogeochemistry, and potentially marine ecosystems, has not been assessed yet.</p><p>In eastern boundary upwelling systems, a mesoscale structuration of biogeochemical properties may be associated to striations through the interaction of zonal flows with sharp coastal-offshore background gradients (stirring). Transport patterns by mesoscale eddies (trapping) may also be involved as striations were noticeably suggested to result from the organization of the mesoscale eddy field as preferred eddy tracks in the eastern South Pacific upwelling system (off central Chile).</p><p>In this region, we evaluate the expression of striations in satellite records of ocean color and in a set of numerically simulated biogeochemical tracers (chlorophyll, carbon, primary production, oxygen, nutrients). A multi-decadal hindcast simulation of the physical-biogeochemical dynamics was run over the period 1984-2013 using the ROMS-PISCES (for Regional Oceanic Modeling System - Pelagic Interactions Scheme for Carbon and Ecosystem Studies) platform at an eddy-resolving resolution. High-pass spatial filtering is used to remove the large-scale signal in time-averaged satellite data and model outputs, and subsequently evaluate the match between striations and biogeochemical tracer anomalies in the model and observations. The relation between striations and the shape of the coastal-offshore gradient of the phytoplankton biomass and the oxygen-minimum zone is then deduced in the CTZ, and further in the open ocean region. The fraction of tracer anomalies associated to striations is quantified, and the respective potential role of stirring and eddy trapping is explored by matching quasi-zonal bands of sea level anomaly, geostrophic currents and biogeochemical tracers on moving frames of variable widths from 3 months to several years. The role played by eddy trapping is then confirmed by a composite analysis based upon automated eddy tracking.</p>