Vertical structure, turbulent mixing and fluxes during Lagrangian observations of an upwelling filament system off Northwest Iberia

Abstract

In August 1998, a recurrent filament located near 42°N off Galicia was sampled as part of the OMEX-II project. Lagrangian and other observations were made on the shelf where the filament arose and offshore in the filament itself under upwelling favourable but fluctuating winds. The shelf drift experiment monitored a change from southward to weak northward net flow as the winds decreased to zero. Shipborne ADCP measurements showed that the shelf was supplying decreasing volumes of water to the filament as the wind speeds decreased. At the shelf edge the internal tide was larger than can be explained by local forcing and there were many unusually large high frequency internal waves with a quasi-sinusoidal form. Turbulence observations revealed enhanced dissipation rates and vertical eddy diffusion coefficients within the shelf thermocline (of order 1 cm 2 s Ϫ1), which appeared to be caused by the breaking of internal wave. A second Lagrangian experiment was executed in the filament some 120 km offshore, which again coincided with a period of wind relaxation. Cross-sections revealed a double cold core and that the offshore flow was limited to a thin surface layer. Substantial onshore flow occurred below 50 m in the centre of the filament, while the strongest and deepest offshore flow coincided with its northern boundary. Turbulent kinetic energy dissipation rate measurements showed very weak mixing below 15 m in the filament core, but enhanced mixing at its boundaries. Four mixed layer drifters released in the filament initially indicated convergence at its southern boundary, marked by strong temperature and salinity contrasts. After the wind became more favourable for upwelling, the drifters accelerated. One drifter traced the full extent of the filament, while the other three escaped from it and began to circulate cyclonically over 28 days in a 100 km diameter loop back towards their release point. Although strong mesoscale activity linked the shelf and ocean regimes, offshore transport in the filament was weak at the time of the experiment and vertical and horizontal re-circulations on a variety of time scales were important. There was sufficient vertical mixing in the thermocline to cause it to thicken and draw some heat into the lower layers during the summer months on the shelf. The amount of heat involved was too little to have a significant impact on the development of a filament over a typical lifetime of a week. 

Short-term variations of the partial pressure of CO2 in surface waters of the Galician upwelling system

“…During the second Lagrangian experiment the behaviour of this drifting buoy was compared with mixed layer Argos drifters drogued at 15m depth. This showed that the drifting buoy from PML was an efficient water mass marker (Joint et al, 2001a;Barton, Inall, Sherwin, & Torres, 2001). The first Lagrangian experiment started over the continental shelf on 3 August (42.62°N, 9.40°W) and the drifter moved southwards until 7 August (42.07°N, 9.44°W) when the experiment ended.…”

Deep ocean exchange with west-European shelf seas

“…Well-developed upwelling can form filaments taking surface water off-shelf, with complementary on-shelf flow (beside and below). Filament fluxes are typically O(0.6 Sv) each (Barton et al, 2001) exceeding the Ekman transport over the (semi-regular) spacing between filaments.…”

Galicia upwelling revisited: out-of-season events in the rias (1967–2009)

“…Numerosos estudios han caracterizado diferentes eventos de afloramiento en la plataforma adyacente a estas rías (Blanton et al 1984;Tenore et al 1984;McClain et al 1986;Alvarez-Salgado et al 1993, 2000Perez et al 1995;Prego y Bao 1997;Prego et al 1999;Barton et al 2001;Alvarez et al 2008a) y también en su interior (Blanton et al 1987;Prego y Fraga 1992;Prego 1993aPrego , 1993bPrego , 1994Rosón et al 1995;Nogueira et al 1997aNogueira et al , 1997bDoval et al 1998;Pardo et al 2001;Prego et al 1995Prego et al , 2001deCastro et al 2004;Alvarez et al 2005). Consecuentemente, se puede resumir que el afloramiento en esta costa occidental gallega es un fenómeno frecuente durante los meses de primavera y verano que es resultado del régimen de vientos de componente norte que soplan a lo largo de la plataforma.…”