Abstract
Abstract. We document the development and public release of a new dataset (1997–2018), consisting of global maps of the Forel–Ule index, hue angle and Secchi disk depth. Source data come from the European Space Agency (ESA) Ocean Colour (OC) Climate Change Initiative (CCI), which is providing merged multi-sensor data from the mid-resolution sensors in operation at a specific time from 1997 to the present day. Multi-sensor satellite datasets are advantageous tools for ecological studies because they increase the probabilities of cloud-free data over a given region as data from multiple satellites whose overpass times differ by a few hours are combined. Moreover, data-merging from heritage and present satellites can expand the duration of the time series indefinitely, which allows the calculation of significant trends. Additionally, data are remapped consistently and analysis-ready for scientists. Also, the products described in this article have the exclusive advantage of being linkable to in situ historic observations and thus enabling the construction of very long time series. Monthly data are presented at a spatial resolution of ∼4 km at the Equator and are available at PANGAEA (https://doi.org/10.1594/PANGAEA.904266; Pitarch et al., 2019a). Two smaller and easier-to-handle test datasets have been produced from the former: a global dataset at 1∘ spatial resolution and another one for the North Atlantic at 0.25∘ resolution. The computer code for the generation of the Forel–Ule index, hue angle and Secchi disk depth from a given remote-sensing reflectance is also shared at https://doi.org/10.5281/zenodo.4439646 (Pitarch et al., 2021) and can be easily set in loop mode for batch calculations.
Highlights
Single-sensor satellite datasets are not long enough to provide significant evidence of climatic trends that become manifest over fluctuations, so the multi-sensor approach has been fostered since the last decade (IOCCG, 2007)
The source product for all derived variables presented in this dataset is the merged multi-sensor Ocean Colour (OC)-Climate Change Initiative (CCI) v4.2 Rrs, which is the primary quantity used for ocean colour studies, containing only spectral information related to the colour of the water while having all interfering factors ideally removed in the atmospheric correction and normalization calculations
Overall, adding in quadrature the uncertainties due to the satellite radiometry, we may conclude that Secchi disk depth derived from daily OC-CCI data has an uncertainty of ∼ 32 % when compared to in situ data
Summary
Single-sensor satellite datasets are not long enough to provide significant evidence of climatic trends that become manifest over fluctuations, so the multi-sensor approach has been fostered since the last decade (IOCCG, 2007). Two historic optical variables have received renewed attention: the Secchi disk depth (zSD; m) and the Forel–Ule (FU; unitless) index These two are the oldest oceanographic variables that are directly related to ocean colour, recorded for many decades before the advent of satellites (Boyce et al, 2012) and publicly available globally (NOAA, 2018). Despite the demonstrated usefulness of these optical parameters in marine physics and biogeochemistry, no public dataset of them exists far, so scientists need to calculate them from Rrs privately, which, other than creating redundant efforts, may be discouraging for non-experts This gap is filled in the present article with the provision of the monthly series from 1997 to 2018 of Secchi disk depth, hue angle and Forel–Ule index at ∼ 4 km resolution
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