Surface Waters of the NW Iberian Margin: Upwelling on the Shelf versus Outwelling of Upwelled Waters from the Rı́as Baixas

Abstract

A set of hydrographic surveys were carried out in the Rı́a of Vigo (NW Spain) at 2–4d intervals during four 2–3 week periods in 1997, covering contrasting seasons. Residual exchange fluxes with the adjacent shelf were estimated with a 2-D, non-steady-state, salinity–temperature weighted box model. Exchange fluxes consist of a steady-state term (dependent on the variability of continental runoff) and a non-steady-state term (dependent on the time changes of density gradients in the embayment). More than 95% of the short-time-scale variability of the exchange fluxes in the middle and outer rı́a can be explained by the non-steady-state term that, in turns, is correlated (R2>75%) with the offshore Ekman transport. Conversely, 96% of the variability of exchange fluxes in the inner rı́a rely on the steady-state term. The outer and middle rı́a are under the direct influence of coastal upwelling, which enhances the positive residual circulation pattern by an order of magnitude: from 102to 103m3s−1. On the contrary, downwelling provokes a reversal of the circulation in the outer rı́a. The position of the downwelling front along the embayment depends on the relative importance of Ekman transport (Qx, m3s−1km−1) and continental runoff (R, m3s−1). When Qx/ R>7±2 the reversal of the circulation affects the middle rı́a. Our results are representative for the ‘Rı́as Baixas’, four large coastal indentations in NW Spain. During the upwelling season (spring and summer), 60% of shelf surface waters off the ‘Rı́as Baixas’ consist of fresh Eastern North Atlantic Central Water (ENACW) upwelled in situ. The remaining 40% consists of upwelled ENACW that previously enters the rı́as and it is subsequently outwelled after thermohaline modification. During the downwelling season (autumn and winter), 40% of the warm and salty oceanic subtropic surface water, which piled on the shelf by the predominant southerly winds, enters the rı́as.