Temporal and spatial variability of river plumes in the NW and N Iberian shelf (1987–2007)

• We developed a method to characterize river plumes based on salinity gradient.
• We studied the spatial and temporal variability of river plumes from 1987 to 2007 in the North Iberian Shelf.
• The area influenced by river plumes was correlated with precipitation and river runoffs.

The analysis of the salinity gradient estimated from in situ data was applied to characterize the spatial and temporal variability of low density buoyant structures in the North Iberian Shelf (NIS) (i.e. Galicia and Cantabrian Sea). To this aim, we have used salinity data acquired in 20 cruises carried out in spring (around April) between 1987 and 2007 and in 5 across-shelf sections sampled monthly between 1994 and 2005 in the studied area. The time series of the salinity gradient from the across-shelf sections data highlighted the seasonal pattern of occurrence of river plumes. This pattern, mainly related with the seasonality of the precipitation rate, varied along the NIS: high probability of occurrence (>70%) took place between November and February in the Central Cantabrian Sea, while it extended to March in the western and to May in the eastern parts of the studied area. The analysis of the spatial distribution of river plumes during spring pointed out two areas, off Miño and Adour river mouths, where the probability of occurrence of river plumes was high during this period. The presence of plumes in the Central Cantabrian Sea was more sporadic and localized preferentially in the proximity of Nalón and Pas-Besaya river mouths. The time series of plume descriptors, such as the area affected by plumes at 10 m depth (AP10, km2) and the mean depth of the plume (zP), differed along the NIS. In the westernmost part, AP10 and zP were significantly lower between 1987 and 1996 than between 1997 and 2006. In the Cantabrian Sea, the values of these descriptors were lower in the central than in the eastern part, and in the latter of these zones the values kept persistently moderate/high during whole period. Plume descriptors were correlated with precipitations and river discharges of the previous month (lag k=1, March) and, in some cases, with the winter values of these forcing factors (k between 5 and 3, November and January respectively).