Using ocean colour remote sensing products to estimate turbidity at the Wadden Sea time series station Spiekeroog

Time series measurements at the Wadden Sea time series station Spiekeroog (WSS) in the southern North Sea were used to empirically develop approaches for determining turbidity from ocean colour remote sensing products (OCPs). Turbidity was observed by a submerged optical sensor. Radiometric quantities were collected using hyperspectral radiometers. Surface reflected glint correction was applied to the radiometric quantities to compute remote sensing reflectance (RRS) and the RRS was converted into perceived colour of seawater matching the Forel-Ule colour Index (FUI) scale. The empirical approaches for determining turbidity from OCPs showed good least squares linear correlations and statistical significance (R2 > 0.7, p < 0.001). These OCP approaches had relatively low uncertainties in predicting turbidity with encouraging mean absolute percent difference less than 31 %. The problem of bio-fouling on submerged sensors and the potential application of OCPs to monitor or correct for sensor drifts was evaluated. A protocol is proposed for the acquisition and processing of hyperspectral radiometric measurements at this optically complex station. Use of the classic FUI as a time series indicator of surface seawater changes did show promising results. The application of these OCPs in operational monitoring changes in water quality was also explored with the aim to evaluate the potential use of the WSS datasets in calibration and validation of satellite ocean colour remote sensing of these very turbid coastal waters

Copernicus Marine Service ocean state report, issue 4

This is the final version. Available from Taylor & Francis via the DOI in this record. FCT/MCTE

Temperature and Salinity Anomalies in the Sea Surface Microlayer of the South Pacific During Precipitation Events

AbstractWe present the results of salinity (ΔS) and temperature (ΔT) anomalies in the sea surface microlayer (SML) in relation to the underlying mixed bulk water (bulk). Several light to moderate rain events were recorded in the southern Pacific near Fiji using our remotely operated catamaran. Precipitation and evaporation drive freshwater fluxes across the sea surface (i.e., the SML) and are the most essential processes of the hydrologic cycle. However, measurements of the SML during precipitation are rare, but necessary to fully understand freshwater exchange at the air‐sea interface. Here we show that freshwater can mix rapidly with the bulk water through wind‐induced mixing, as ΔS and ΔT show a clear dependence on wind speed. At high wind speeds (5.1–11.6 m s−1), anomalies approach zero (ΔS = −0.02 ± 0.49 g kg−1, ΔT = −0.09 ± 0.46°C) but can reach ΔS = 1.00 ± 0.20 g kg−1 and ΔT = −0.37 ± 0.09°C at lower wind speeds (0–2 m s−1). We find shallow freshwater lenses and fronts, likely caused by past rainfall, with ΔS and ΔT of up to −1.11 g kg−1 and 1.77°C, respectively. Our observations suggest that freshwater lenses can be very shallow (<1 m depth) and missed by conventional measurements. In addition, the temperature and salinity in the SML respond to freshwater fluxes instantaneously. It highlights the role of the SML in a mechanistic understanding of the fate of freshwater over the ocean and, therefore, the global hydrologic cycle.Plain Language Summary: Rain and evaporation are the most important processes in the global water cycle, causing either the supply to or the removal of freshwater from the upper ocean, thereby changing the salinity of the sea surface. Evaporation also removes heat and lowers the temperature on the ocean surface. We used the measurements of sea surface microlayer (SML) salinity and temperature as key indicators to study hydrologic cycle processes during our cruise with the RV Falkor in the South Pacific and found that freshwater mixes rapidly with the underlying bulk water during strong winds (5.1–11.6 m s−1). We also detected shallow freshwater lenses and fronts, most likely caused by past rainfall, with ΔS and ΔT of up to −1.11 g kg−1 and 1.77°C, respectively. Our observations suggest that freshwater lenses can occur at the sea surface and that the SML respond to freshwater fluxes instantaneously. It highlights the role of the SML for future studies of the global hydrologic cycle.Key Points: Small scale air‐sea interactions (freshwater fluxes) during precipitation were investigated in the southern Pacific. Temperature and salinity anomalies occur with a high spatial variability.Measurements with remote controlled catamaran revealed shallow freshwater lenses, which were not detectable with ship based measurements.German Research Foundationhttps://doi.org/10.7284/908805https://www.rvdata.us/search/cruise/FK191120https://bec.icm.csic.es/https://smos-diss.eo.esa.int/https://doi.org/10.48670/moi-00165https://disc.gsfc.nasa.gov/datasets/GPM_3IMERGHH_06/summar

Acoustic and optical methods to infer water transparency at Time Series Station Spiekeroog, Wadden Sea

Water transparency is a primary indicator of optical water quality that is driven by suspended particulate and dissolved material. A data set from the operational Time Series Station Spiekeroog located at a tidal inlet of the Wadden Sea was used to perform (i) an inter-comparison of observations related to water transparency, (ii) correlation tests among these measured parameters, and (iii) to explore the utility of both acoustic and optical tools in monitoring water transparency. An Acoustic Doppler Current Profiler was used to derive the backscatter signal in the water column. Optical observations were collected using above-water hyperspectral radiometers and a submerged turbidity metre. Bio-fouling on the turbidity sensors optical windows resulted in measurement drift and abnormal values during quality control steps. We observed significant correlations between turbidity collected by the submerged metre and that derived from above-water radiometer observations. Turbidity from these sensors was also associated with the backscatter signal derived from the acoustic measurements. These findings suggest that both optical and acoustic measurements can be reasonable proxies of water transparency with the potential to mitigate gaps and increase data quality in long-time observation of marine environments