Seasonal distributions of ocean particulate optical properties from spaceborne lidar measurements in Mediterranean and Black sea

Abstract

Assessing the oceanic surface layer's optical properties through CALIOP has been one of the reasons of the extension of the CALIOP mission for 3 more years (2018–2020). This is the first work evaluating the potential use of CALIOP for ocean applications at regional scale in mid-latitude regions (i.e. Mediterranean, MED, and Black Sea, BS) and investigating the added information on ocean particles given by the column integrated depolarization ratio (δT) parameter. We implemented and refined a retrieval procedure to estimate this parameter at 1/4 degree of spatial resolution, comparing 7 years of CALIOP observations (2011–2017) to the corresponding Copernicus multi-sensor L3 ocean colour products of the surface particle backscattering coefficient (bbp) and chlorophyll-a concentration (Chl-a). This study pointed out that the current CALIOP sampling is inadequate to detect subtle day-night difference due to plankton diel variability for these basins. At a basin scale, δT covaries with bbp for bbp ≥ 0.0015 m−1. This is more evident for BS (R = 0.84) than for MED (R = 0.61). The analysis of seasonal distributions confirm this result for BS, where δT has a semi-annual cycle in very good agreement with bbp. In the MED, characterized by different trophic regimes, δT shows also some similarities with Chl-a annual cycle. The combined characterization in the MED bioregions of the annual patterns of bbp:Chl-a, δT:Chl-a and δT:bbp ratios suggested that δT parameter can provide valuable information about the non-sphericity and the size of ocean particles.