Thermohaline measurements in the continental shelf zone of the NW Iberian Peninsula, 1994–2006

Abstract

From 1994 to 2006, we conducted a monthly series of CTD sampling along a coastal- shelf transect, in the Ria de Vigo and the adjacent shelf (NW Iberian Upwelling System; 42 to 44° N and 8 to 10° W). Our aim was to determine the spatial variability of the hydrographic processes over different timescales (seasonal and inter-annual) at 3 stations (ria, near-shore and shelf edge), and their relationship with other related oceanographic and climatic indices: upwelling index (UI) and North Atlantic Oscillation index (NAO). At a seasonal scale, a change in water temperature from inversion (December to February) to stratification (April to September) was found at all stations. The maximum vertical gradient in temperature is reached in July-August, coinciding with the middle of the upwelling season. The seasonal salinity cycle is influenced by continental runoff and, in winter time, by salty subtropical water carried alongshore by the Iberian Poleward Current. At an inter- annual scale, during spring-summer (upwelling season), in the upper layer of the ria station, a warm- ing (0.06°C yr -1 ) is observed. This is in accordance with the multidecadal decreasing trend in upwelling intensity (~5 m 3 s -1 km of coast -1 yr -1 ) that would have favoured a warming of surface coastal waters. In contrast, a surface temperature decrease (0.11°C yr -1 ) was observed at the shelf- edge station in the autumn-winter period (downscaling season). We hypothesise that the observed reduction in temperature is due to the combined effect of the increase in upwelling events during the downwelling season and the decline of NAO since 1995. No significant temporal trends on an inter- annual scale were found in salinity values in surface or bottom layers of the stations.