The Mediterranean Sea is often considered as a miniature ocean for climatic studies (Béthoux et al., 1999;Durrieu de Madron et al., 2011;Tsimplis et al., 2006). Its semi-enclosed geometry fosters a zonal overturning circulation that connects remote areas of the easternmost Levantine Sea with the North Atlantic Ocean, in compensation for the evaporation excess over the basin (Hopkins, 1978). This open thermohaline cell sets the stratification of the whole Mediterranean Sea according to a two-layer flow (Wüst, 1961): the non-return flow of fresh waters of Atlantic origin (AW) in the surface layer, and the formation and westward spreading in the intermediate layer of salty waters, namely the Levantine Intermediate Water (LIW). This latter water mass plays a major role in the vertical exchanges of physical and biogeochemical properties as well as their arrangement and transport over the different sub-basins of the Mediterranean Sea. LIW is involved in the deep water formation processes that drive the two internal thermohaline cells in the western and eastern Mediterranean basins

The ocean’s meso- and submeso-scales (1-100 km, days to weeks) host features like filaments and eddies that have a key structuring effect on phytoplankton distribution, but that due to their ephemeral nature, are challenging to observe. This problem is exacerbated in regions with heavy cloud coverage and/or difficult access like the Southern Ocean, where observations of phytoplankton distribution by satellite are sparse, manned campaigns costly, and automated devices limited by power consumption. Here, we address this issue by considering high-resolution in-situ data from 18 bio-logging devices deployed on southern elephant seals (Mirounga leonina) in the Kerguelen Islands between 2018 and 2020. These devices have submesoscale-resolving capabilities of light profiles due to the high spatio-temporal frequency of the animals’ dives (on average 1.1 +-0.6 km between consecutive dives, up to 60 dives per day), but observations of fluorescence are much coarser due to power constraints. Furthermore, the chlorophyll a concentrations derived from the (uncalibrated) bio-logging devices’ fluorescence sensors lack a common benchmark to properly qualify the data and allow comparisons of observations. By proposing a method based on functional data analysis, we show that a reliable predictor of chlorophyll a concentration can be constructed from light profiles (14 686 in our study). The combined use of light profiles and matchups with satellite ocean-color data enable effective (1) homogenization then calibration of the bio-logging devices’ fluorescence data and (2) filling of the spatial gaps in coarse-grained fluorescence sampling. The developed method improves the spatial resolution of the chlorophyll a field description from ~30 km to ~12 km. These results open the way to empirical study of the coupling between physical forcing and biological response at submesoscale in the Southern Ocean, especially useful in the context of upcoming high-resolution ocean-circulation satellite missions.