ITT Baltic+ Salinity Dynamics: Challenges of Retrieving Sea Surface Salinity over the Baltic Sea

2019 Living Planet Symposium, 13-17 May 2019, Milan, Italy

The Baltic+ Salinity Dynamics aims at advancing research and consolidating existing developments towards the generation of a dedicated Sea Surface Salinity (SSS) product suited for Baltic Earth research (https://www.baltic-earth.eu/organisation/bewg_salinity/index.html). The Baltic Sea is one of the most challenging regions for the satellite SSS retrieval. Nowadays, the availability of SMOS SSS products is quite limited over this region (temporal and spatially) due to several technical limitations related to the low sensitivity of L-band TB at SSS changes in cold waters, land-sea and ice-sea contamination, and high contamination by Radio-Frequency Interferences (RFI) sources. The assessment of the quality of the SMOS SSS products in the Baltic Sea is also an issue. On one hand, the basin is strongly stratified and therefore, differences between the first centimetres (i.e. SMOS measurements) and the few meters (i.e. measurements provided by in situ) can be noticeable. On the other hand, the basin is very shallow and then, the near surface vertical salinity gradient must be estimated from different sources. There are also seasonal salinity variations in the surface, which are caused by melting of ice and spring floods from rivers. Thus, the representativeness of satellite salinity must be assessed. Besides, some parts of the Baltic Sea such as the Gulf of Finland have estuarine regimes and therefore exhibit relatively strong horizontal salinity gradients. In the recent years, the BEC team has developed innovative algorithms for improving the quality of brightness temperatures and sea surface salinities dealing with the main processing issues: (i) Nodal Sampling [1] to mitigate the RFI contamination, (ii) Debiased non-Bayesian SSS retrieval [2] to empirically mitigate systematic biases and improve the spatial coverage of SSS maps, and (iii) Multifractal Fusion technique [3] to improve the temporal and spatial resolution to better describe the salinity dynamics. The application of these techniques has been proved to enhance SMOS SSS products over other critical regions such as the Mediterranean Sea (http://bec.icm.csic.es/ocean-experimental-dataset-mediterranean/). The work proposed in Baltic+Salinity is aimed at extending the above-mentioned methodologies for developing a dedicated SMOS SSS product, suitable for the needs of the Baltic research community. In a first stage of this project, we will develop a novel prototype SSS product (a series of 3 years of data) and we will analyze the limits and benefits of the newly generated product. We will use different subsets of in situ data for the algorithm calibration (i.e. for fixing different implementation choices and trade-offs) and for the validation of the resulting SSS products. For the algorithm calibration, we will use data from regular monitoring stations round the Baltic Sea (HELCOM dataset). These observation stations have long time series of data, which give perspectives for the temporal and spatial variabilities of the surface salinity field. Data from several Finnish coastal fixed oceanographic stations will be included to describe the coast-open sea gradients. The resulting SMOS SSS products will be validated against in situ data using the open data from ICES, which acts as data centre for HELCOM, SeaDataNet and EMODnet. We will present at the conference the up-to-date Baltic+ SSS products generated in this project. We will also analyze the impact and added-value of these products with respect to other existing EO-based datasets. The knowledge gained during the Baltic+ Salinity project will be an important added value to the on-going ESA Climate Change Initiative SSS, which aim to create the longest up to date SSS global time series. [1] V. González-Gambau, E. Olmedo, A. Turiel, J. Martínez, J. Ballabrera-Poy, M. Portabella, and M. Piles, “Enhancing SMOS brightness temperatures over the ocean using the nodal sampling image reconstruction technique,” Remote Sensing of Environment, vol. 180, pp. 205 – 220, 2016, special Issue: ESA’s Soil Moisture and Ocean Salinity Mission - Achievements and Applications. [2] E. Olmedo, J. Martínez, A. Turiel, J. Ballabrera-Poy, M. Portabella, “Debiased non-Bayesian retrieval: A novel approach to SMOS Sea Surface Salinity”, Remote Sensing of Environment, vol. 193, no. Supplement C, Pages 103-126, 2017. [3] Olmedo, E., Martínez, J., Umbert, M., Hoareau, N., Portabella, M., Ballabrera-Poy, J., Turiel, A., 2016. >Improving time and space resolution of SMOS salinity maps using multifractal fusion>. Remote Sens. Environ. 180, 246–263