Hydrodynamic characterization of the bottom boundary layer in a coastal upwelling system (Ría de Vigo, NW Spain)

Abstract

The hydrodynamics of the bottom boundary layer (BBL) in the Ría de Vigo (NW Iberian Peninsula) are studied for the first time, identifying their possible forcing factors and filling a previous lack of knowledge about the coastal upwelling system of NW Iberian Peninsula. For tackling this subject, high resolution time series of bottom currents by means of a downwards looking Acoustic Doppler Current Profiler (3 m above the bottom) were recorded in the inner part of the Ría de Vigo covering the four seasons of the climate year 2004–2005.

Our results point to a coupling among bottom residual currents, shelf winds and runoff supporting previous works not directly focused on BBL. Most of the time, 64.5%, 75.4% and 61.6% for residual, tidal and total currents respectively, the bottom currents respond to a logarithmic profile being possible to apply the law of the wall and estimate shear stress.

Shear stress variability is controlled by the frequency of wind reversals and modulated by runoff and thermal stratification. Rapid shifts of wind direction favors the generation of shear stress. Intense summer stratification or strong winter mixing of the water column reduces shear stress levels. Runoff by increasing the variability of the velocity pulse direction can favor the development of shear stress. Bottom shear stress reaches levels that are strong enough (~5% of the time in autumn) to re-suspend fine cohesive sediments. This fact comes out to support previous biogeochemistry studies suggesting the occurrence of intense resuspension processes during the autumn season.