Monitoring Black Sea environmental changes from space: New products for altimetry, ocean colour and salinity. Potentialities and requirements for a dedicated in-situ observing system

Abstract

In this paper, satellite products developed during the Earth Observation for Science and Innovation in the Black Sea (EO4SIBS) ESA project are presented. Ocean colour, sea level anomaly and sea surface salinity datasets are produced for the last decade and validated with regional in-situ observations. New data processing is tested to appropriately tackle the Black Sea’s particular configuration and geophysical characteristics. For altimetry, the full rate (20Hz) altimeter measurements from Cryosat-2 and Sentinel-3A are processed to deliver a 5Hz along-track product. This product is combined with existing 1Hz product to produce gridded datasets for the sea level anomaly, mean dynamic topography, geostrophic currents. This new set of altimetry gridded products offers a better definition of the main Black Sea current, a more accurate reconstruction and characterization of eddies structure, in particular, in coastal areas, and improves the observable wavelength by a factor of 1.6. The EO4SIBS sea surface salinity from SMOS is the first satellite product for salinity in the Black Sea. Specific data treatments are applied to remedy the issue of land-sea and radio frequency interference contamination and to adapt the dielectric constant model to the low salinity and cold waters of the Black Sea. The quality of the SMOS products is assessed and shows a significant improvement from Level-2 to Level -3 and Level-4 products. Level-4 products accuracy is 0.4-0.6 psu, a comparable value to that in the Mediterranean Sea. On average SMOS sea surface salinity is lower than salinity measured by Argo floats, with a larger error in the eastern basin. The adequacy of SMOS SSS to reproduce the spatial characteristics of the Black Sea surface salinity and, in particular, plume patterns is analyzed. For ocean colour, chlorophyll-a, turbidity and suspended particulate materials are proposed using regional calibrated algorithms and satellite data provided by OLCI sensor onboard Sentinel-3 mission. The seasonal cycle of ocean colour products is described and a water classification scheme is proposed. The development of these three types of products has suffered from important in-situ data gaps that hinder a sound calibration of the algorithms and a proper assessment of the datasets quality. We propose recommendations for improving the in-situ observing system that will support the development of satellite products.

Earth Observation-Based Cyanobacterial Bloom Index Testing for Ecological Status Assessment in the Open, Coastal and Transitional Waters of the Baltic and Black Seas

“…The total average freshwater runoff in the region is 272 km 3 year −1 , i.e., 80% of the total river flow of the entire Black Sea basin, with the Danube River-the largest European river, representing 61% of the total river flow to the Black Sea [5,56,57]. The river runoff results in considerable desalination zones in the coastal areas with surface salinities ranging from less than 15 PSU in the river influence zone while in the open sea, it reaches about 18-19 PSU [56,58]. These features of desalination and geomorphological heterogeneity of the seabed topologically represented by paleo-valleys and watersheds of flooded paleo-rivers led to the formation of a thermo-haline structure with pycnocline [59].…”

“…Winds of north-western, western, and south-western directions, connected with circulation in temperate latitudes of the northern hemisphere predominate in the region [60]. The annual SST cycle range in the Black Sea is around 17 • C with the coldest temperature in winter, i.e., water temperatures in the north-western shelf < 5 • C with the presence of sea ice in some regions, and the warmest temperature of around 25 • C in summer, mainly in July and August [58,61,62]. The dominant phytoplankton include diatoms, dinoflagellates, and coccolithophores [8,63].…”

“…The exception might be relatively dense cyanobacterial bloom areas due to uncorrected C2RCC-derived R rs at red/near-infrared wavelengths [90,91]. The same processor was evaluated in the Black Sea by Grégoire et al [58] against R rs measured by the two Black Sea Aeronet-OC stations "Gloria" and "Galata" located in the western part. The agreement between in situ measured and retrieved from OLCI after application of C2RCC R rs was significantly high for all investigated bands (i.e., 413, 443, 490, 560 and 665 nm), where R 2 ranged from 0.64 to 0.90 for the blue bands, and R 2 ranged from 0.83 to 0.88 for green and red bands.…”