Transient oceanic and tidal contributions to water exchange and residence times in a coastal upwelling system in the NE Atlantic: the Pontevedra Ria, Galicia

An analysis of the intra-annual variability of hydrographic residence times, τ, is presented for the Pontevedra Ria––a large coastal system in NW Spain with a deep, unobstructed connection with the Atlantic Ocean. Ria-offshore water exchange and τ were quantified with an iterative approach to the fraction of freshwater method using 23 quasi-steady state water–salt budgets. Excluding prolonged flushing due to downwelling events at the end of the upwelling season, mean was 6 ± 2 day in the central ria and 2 ± 1 day in the internal ria. The tidal contribution, equal to 10–25% of total water renewal, was quantified for the first time for a Galician ria with the hydrodynamic model estuarine contaminant simulator (ECoS). The relationship between τ and residual freshwater input, QZ, was atypical, since QZ played a minor role in water exchange in comparison to periodic upwelling of East North Atlantic Central Water (ENACW) from the continental shelf mainly during spring and summer. During positive upwelling stress, the intrusion of ENACW into the ria produced enhanced flushing and τ<4 day in the central ria and <2 days in the internal ria. Under downwelling favourable conditions, water was retained and τ prolonged to >8 and >5 days, respectively. A quantitative parameterisation of τ with upwelling and QZ was weakly significant, due to the short-term variability in ria salinity. This paper elucidates problems associated with the use of water budgets to transient coastal systems such as the Pontevedra Ria.