Baltic Sea Chlorophyll-a time series and trend from Observations Reprocessing

'''DEFINITION'''The time series are derived from the regional chlorophyll reprocessed (MY) product as distributed by CMEMS which, in turn, result from the application of the regional chlorophyll algorithm over remote sensing reflectances (Rrs) provided by the Plymouth Marine Laboratory using an ad-hoc configuration for CMEMS of the ESA OC-CCI processor version 6 (OC-CCIv6) to merge at 1km resolution (rather than at 4km as for OC-CCI) MERIS, MODIS-AQUA, SeaWiFS, NPP-VIIRS and OLCI-A data. The chlorophyll product is derived from a Multi-Layer Perceptron neural-net (MLP) developed on field measurements collected within the BiOMaP program of JRC/EC (Zibordi et al., 2011). The algorithm is an ensemble of different MLPs that use Rrs at different wavelengths as input. The processing chain and the techniques used to develop the algorithm are detailed in Brando et al. (2021a; 2021b). Monthly regional mean values are calculated by performing the average of 2D monthly mean (weighted by pixel area) over the region of interest. The deseasonalized time series is obtained by applying the X-11 seasonal adjustment methodology on the original time series as described in Colella et al. (2016), and then the Mann-Kendall test (Mann, 1945; Kendall, 1975) and Sens’s method (Sen, 1968) are subsequently applied to obtain the magnitude of trend. '''CONTEXT'''Phytoplankton and chlorophyll concentration, as a proxy for phytoplankton respond rapidly to changes in environmental conditions, such as light, temperature, nutrients and mixing (Gregg and Rousseaux, 2014). The character of the response in the Baltic Sea depends on the nature of the change drivers, and ranges from seasonal cycles to decadal oscillations (Kahru and Elmgren 2014). Therefore, it is of critical importance to monitor chlorophyll concentration at multiple temporal and spatial scales, in order to be able to separate potential long-term climate signals from natural variability in the short term. In particular, in the Baltic Sea phytoplankton is known to respond to the variations of SST in the basin associated with climate variability (Kabel et al. 2012).'''CMEMS KEY FINDINGS'''Baltic Sea shows a positive trend in the time interval 1997-2022 with a slope of 0.67±0.49% per year and it confirms the trend retrieved in the previous release. Maxima and minima values are quite similar year-by-year. Absolute maximum is clear in 2008 while absolute minimum is achieved during 2004. Since 2019 chlorophyll seems to increase weakly.'''Figure caption'''Baltic Sea time series and trend (1997-2021) of satellite chlorophyll, based on CMEMS product OCEANCOLOUR_BAL_BGC_L3_MY_009_133. The monthly regional average (weighted by pixel area) time series is shown in grey, with the deseasonalized time series in green and the trend in blue.'''DOI (product):'''https://doi.org/10.48670/moi-00197