Surface waters of the NW Iberian margin:: Upwelling on the shelf versus outwelling of upwelled waters from the Rias Baixas

A set of hydrographic surveys were carried out in the Ria of Vigo (NW Spain) at 2-4 d 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 ria can be explained by the non-steady-state term that, in turns, is correlated (R-2>75%) with the offshore Ekman transport. Conversely, 96% of the variability of exchange fluxes in the inner ria rely on the steady-state term. The outer and middle ria are under the direct influence of coastal upwelling, which enhances the positive residual circulation pattern by an order of magnitude: from 10(2) to 10(3) m(3) s(-1). On the contrary, downwelling provokes a reversal of the circulation in the outer ria. The position of the downwelling front along the embayment depends on the relative importance of Ekman transport (Q(x), m(3) s(-1) km(-1)) and continental runoff (R, m(3) s(-1)). When Q(x)/R>7+/-2 the reversal of the circulation affects the middle ria. Our results are representative for the 'Rias Baixas', four large coastal indentations in NW Spain. During the upwelling season (spring and summer), 60% of shelf surface waters off the 'Rias Baixas' consist of fresh Eastern North Atlantic Central Water (ENACW) upwelled in situ. The remaining 40% consists of upwelled ENACW that previously enters the rias 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 rias. (C) 2000 Academic Press.