Winter dynamics on the northern Portuguese shelf. Part 2: bottom boundary layers and sediment dispersal

Climate of the last millennium at the southern pole of the North Atlantic Oscillation: an inner-shelf sediment record of flooding and upwelling

“…As a whole, the above-described results imply similar seasonality and/or persistent mixed water column conditions. Based on the present-day observations of Vitorino et al (2002) and Oliveira et al (2007), this suggests the dominance of strong winter mixing and/or summer upwelling turbulence for AD 900− 1730, followed by a transition period between AD 1730 and AD 1740 marked by dry winter conditions. Under the present interglacial conditions and with no indication of considerable changes in river flux within the last ± 300 yr, this radical change at the end of the LIA may reveal: (1) changes in the seasonality of the main oceanographic processes; (2) the presence of water masses with different thermohaline properties; and (3) changes in the oceanic circulation pattern.…”

“…In the Douro area, the low salinity river plume and the prevailing westerly−southerly winds cause a downwelling regime over the shelf and a rather homogeneous water column, with a deepening of the thermocline to 80−100 m. Furthermore, observations by Oliveira et al (2007) showed a strong cross-shelf salinity gradient at the surface and the inshore penetration of warm oceanic waters. Winter episodic northerly winds generate upwelling events, offshore expansion of the lowsalinity plume and large onshore transport in the bottom Ekman layer that results in a rapid thermal restratification at mid-shelf (Vitorino et al 2002). In spring, low salinity waters still cover a significant part of the continental shelf; however, due to the prevalence of northerly winds from spring to early fall, equatorward currents prevail (Portuguese Current [PC]) and near the shore, upwelling of subsurface cooler and nutrient-rich waters becomes the most significant hydrodynamic process ( Fig.…”

“…Under the present interglacial conditions and with no indication of considerable changes in river flux within the last ± 300 yr, this radical change at the end of the LIA may reveal: (1) changes in the seasonality of the main oceanographic processes; (2) the presence of water masses with different thermohaline properties; and (3) changes in the oceanic circulation pattern. Vitorino et al (2002) and Oliveira et al (2007) show that episodic fall−winter upwelling events result in thermal stratification in the upper water column. Additionally, the analysis of 40 yr of data (AD 1967(AD −2006 has shown that the number of such events has increased in the last decade, whereas the total annual and summer upwelling intensity is de creasing , Gómez-Gesteira et al 2008.…”

Spatial and temporal variability in coccolithophore abundance and distribution in the NW Iberian coastal upwelling system

“…In this context, southerly wind regime is accompanied by downwelling favourable conditions and a consequent decrease in primary productivity. Strong southwesterly winds at this time might also induce highly energetic storms that can lead to wavedriven mobilization of fine sediments from mid-shelf depths (Dias et al, 2002a;Vitorino et al, 2002;Oberle et al, 2014). During these periods, fluvial discharges from Minho and Douro rivers, located south, can contribute terrestrial sediments to the study area (Dias et al, 2002a, b;Zúñiga et al, 2016).…”

25. Climate variability of the last five isotopic interglacials: Direct land-sea-ice correlation from the multiproxy analysis of North-Western Iberian margin deep-sea cores

“…On the shelf, the fine-particle sediments in suspension are transported northwards by poleward currents, and some are deposited in the Douro and Galicia Mud Patches . Extreme storm events can produce resuspension of some sediment from the mud patches, and transport of sediments off the shelf can occur Vitorino et al, 2002). Pollen grains belonging to the fine-particle fraction have a similar behaviour as fine sediments during the sedimentary processes (Muller, 1959;Chmura and Eisma, 1995).…”