Seasonal and interannual variability of the Douro turbid river plume, northwestern Iberian Peninsula

Abstract

The Douro River represents the major freshwater input into the coastal ocean of the northwestern Iberian Peninsula. The seasonal and interannual variability of its turbid plume is investigated using ocean color composites from MODIS (Moderate Resolution Imaging Spectroradiometer) sensor aboard the Aqua and Terra satellites (2000–2014) and long-term records of river discharge, wind and precipitation rate. Regional climate indices, namely the Eastern Atlantic (EA) and North Atlantic Oscillation (NAO), were analyzed to identify the influence of atmospheric variability on the generation of anomalous turbid river plume patterns. The connection between the monthly time series of normalized water leaving radiance at 555nm (nLw(555)) and river discharge is high (r=0.81), which indicates a strong link between river outflow and turbidity levels in the river plume. The equivalent result is found between precipitation and nLw(555) time series, but the peak correlation was found with a 1-month lag, revealing the importance of the river dams on the outflow regulation (r=0.65). Lag correlations between nLw(555) and EA index show a peak at 1-month lag (r=0.51). The relation between NAO index and Douro river discharge is considerable (−0.50), for a time lag of 1-month as well. However, the correlation coefficient between nLw(555) and NAO index presents a maximum peak for a longer period (r=−0.42 at 3-month). Anomalous turbid plume patterns, not related with estuarine outflow, are found during autumn 2004. A coccolithophore bloom is proposed as a plausible explanation for these unexpected turbid patches.