Abstract
SUMMARY: River runoff off northwest Iberia generates a low-density buoyant structure with a strong influence on shelf and coastal circulation. This study estimates the runoff to the shelf of the ten largest rivers in the region based on the furthest downstream gauge records available, and also takes into account the basin area downstream from the station (22% of the basin area for the entire study region). Monthly statistics were computed to obtain mean values for each river to cover the recurrent lack of runoff data in the region. In order to reconstruct gaps in the time series on a daily scale, a method based on the observed discharge of a nearby river basin was used. In addition, the influence of runoff on the shelf was analyzed using monthly CTD data sampled during a 12-year period in the Ria de Vigo and the adjacent shelf. The CTD series shows the existence of a buoyant structure with maximum growth during winter and with large variability of its thermal anomaly. The salinity anomaly correlated significantly with mean winter monthly values of the North Atlantic Oscillation (NAO) index. This atmospheric index integrates both the influence of precipitation —and therefore runoff— and the predominant winds during winter that contribute to the accumulation of fresh water over the shelf.
Highlights
Fresh water introduced into the coastal region by river discharge creates a budget for both buoyancy and inertia as a consequence of the drainage of a light fluid into a denser environment
In the western Iberian region, many rivers contribute to generating river plumes, which have the largest extension during the downwelling season when river discharge is high and S-SW winds are predominant
Table 3. – Correlation matrixes (r) of runoff estimated at the mouth of river basins (95% significance level)
Summary
Fresh water introduced into the coastal region by river discharge creates a budget for both buoyancy and inertia as a consequence of the drainage of a light fluid into a denser environment. River plumes are known to strongly influence circulation in the area (e.g. Santos et al, 2004; Varela et al, 2005; Ruiz-Villarreal et al, 2006; Álvarez-Salgado et al, 2006; Torres and Barton, 2007; Herrera et al, 2008; Otero et al, 2008). The thickness and the extension of the plume, which has typical values of 10 to 40 m and 15 to 30 km respectively, respond to wind event variability (Otero et al, 2008). Typical plume speeds are of around 0.1 to 0.3 m s-1, but can reach values over 1 m s-1 in response to extreme meteorological events combined with high river discharge to the shelf (Ruiz-Villarreal et al, 2005)
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