Deepening of the mixed layer may also lower phytoplankton photosynthesis by reducing the average light intensity in the upper water column. Such short-term events may lead to interannual variations in the properties of seasonal blooms (i.e., onset, magnitude, and decline), and possibly cause “secondary” blooms. Simultaneously, the phytoplankton biomass interacts with the light field through attenuation of irradiance, which modifies the amount of light available for photosynthesis (i.e. the so-called “self-shading”; reported in the Southern Ocean by de Baar et al., 2005; Blain et al., 2013). The vertical distribution of the phytoplankton biomass results from complex interactions between physical and biological factors. Deep or subsurface chlorophyll maxima (DCM) are recurrent features in the Southern Ocean (Uitz et al., 2009). Nevertheless, the processes responsible for their development, formation, and decay vary with seasons and regions and are often poorly characterized. Although DCMs are typically attributed to photoacclimation of phytoplankton to low light intensity in oligotrophic conditions (Cullen, 1982), complex mechanisms associated with the sedimentation and accumulation of particles from the upper mixed layer (Quéguiner et al., 1997) or the development of active algal cells supported by a deep nutricline (Holm-Hansen et al., 2004) have been observed in the SO. Further characterizing these processes is crucial to improve our understanding of the dynamics of DCMs and quantifying their contribution to primary production and associated carbon fluxes. The important process of bloom triggering has long been attributed to the seasonal vertical displacement of the mixed layer depth relative to the critical depth, as explained by Sverdrup’s theory (1953). Owing to recent technological developments and new observational approaches, this classical theory is becoming strongly challenged (Behrenfeld, 2010; Taylor and Ferrari, 2011). Achieving a comprehensive understanding of the effects of light-mixing conditions on phytoplankton dynamics requires concurrent in situ observations of both physical forcing and biological variables in the ocean interior (i.e. not just in the surface layer “seen” by satellite-borne sensors) over a broad range of timescales. For SOCLIM, we propose to use extensively the recently developed Bio-Argo floats together with surface instrumented moorings, which would provide the high-quality time-resolved data over the large spatio-temporal scales needed to really understand bloom dynamics and the underpinning factors. Intensive investigations will be conducted in contrasted biogeochemical regions of the SO.

... To carbon sequestration in the deep-sea

The attenuation of the downward flux of particulate organic carbon is a key process to be quantified as it is an important factor of control for exchanges of CO2 between the ocean and the atmosphere (Kwon et al. 2009). Estimation of downward particulate fluxes mostly relies on deployment of sediment traps or thorium deficit measurements (Buesseler, 2009) with unavoidable large uncertainties because of the difficulties in acquiring coherent and interoperable datasets. More accurate estimations of the attenuation flux exponent (b), and a better understanding of its spatio-temporal variability represent one of the “hot topic” in marine biogeochemistry (Kwon et al., 2009). Better estimations of b may significantly reduce the uncertainties in quantifying and modeling the strength of CO2 sink/source in the SO. Such determination are highly needed because there is recent evidence suggesting that the SO has a lower carbon export potential than what is predicted by models (Maiti et al., 2013). Some real hopes now come from the use of Bio-Argo floats that drift most of the time (90%) in the mesopelagic zone and record optical proxies of POC. Recent refinements, (Briggs et al., 2011, Estapa et al., 2013) open the extremely attractive perspective to get highly resolved estimation of b. The combination of data from floats and moorings hosts a great potential for major scientific advances on this issue.

The potential energy created in the ACC is converted into vigorous nonlinear dynamical structures such as eddies, jets and filaments (Nikurashin et al. 2012). This turbulent energy can power strong air-sea transfers, water-mass formation, transformation, and water properties transport and mixing (Lambaerts et al. 2013).

However, because the observation of such small-scale oceanic structures is very difficult, the exact nature of these processes and their impact are poorly understood, and quantified. This assessment is essential to evaluate, for example, the ocean uptake of climate-relevant tracers such as heat and carbon. Indeed, these oceanic processes drive tracers transport into the ocean (Resplandy et al. 2014). They are also functionally linked to the vertical transfers (i.e., subduction) of water masses and related properties, and they are the rate-limiting step in estimating the ocean sequestration of anthropogenic CO2. The understanding of these processes is therefore capital to accurately estimate the meridional fluxes of mass, heat, fresh-water and biogeochemical tracers.

The project will make it possible (1) to focus on the meso and submeso-scale dynamics, and enhance our understanding and estimates of their role in mixing, and (2) to recover the three-dimensional transport (i.e., lateral, and vertical) of water properties generated by this scale dynamics in this Indian sector of the SO which is characterized by the highest turbulence of the world’s oceans.

Because of their very high variability in space and time, the observations of such processes are challenging, as they cannot be achieved through the classical observational in situ strategy. Within SOCLIM we propose an innovative approach that consists in using the quasi-lagrangian nature of the proposed observing platforms such as profiling floats (Argo and Bio-Argo) and the PolarPod. The deployment of profiling floats will be carried out in selected fine-scale features that will be defined judiciously beforehand by analyses of real-time satellite data. The Lagrangian observations will be associated with measurements provided by others innovative instruments (Seasoar, UCTD, gliders, wave gliders) deployed in collaboration with our foreign partners. These field data, in combination with satellite products, will give us the ability to observe for relatively long spans of time the selected ocean fine-scale structures, and the evolution of their dynamical context as well as that of their properties. These innovative observations will enable us to understand the role of the small–scale ocean dynamics in mixing and 3D transfers of properties in the upper 2000 m of the ocean.