Lagrangian circulation of the North Atlantic Central Water over the abyssal plain and continental slopes of the Bay of Biscay: description of selected mesoscale features

Abstract

SUMMARY: Between 1994 and 2001, several experiments (ARCANE, SEFOS, INTERAFOS) were conducted to directly measure the general and mesoscale Lagrangian circulations over the Bay of Biscay abyssal plain and slopes. Two levels (~100 m and ~450 m) were selected to cover the North Atlantic Central Water range. Two types of Lagrangian instruments, drogued surface drifters tracked by satellite (Surdrift) and acoustically tracked subsurface floats (Rafos and Marvor), were used. Overall, more than 36 instrument-years were collected in the Bay of Biscay region (43-49°N, 01-12°W). The weak general circulation in the Bay of Biscay is seen to be highly influenced by the occurrence of several mesoscale coherent features, notably slope currents and eddies, and these affect the exchanges between the abyssal plain and the slopes. The objective of this paper is to depict some specific examples of the observed mesoscale field. Selected float trajectories are shown and used to discuss observations of slope currents and of both anticyclonic and cyclonic eddies. Slope currents exhibit alternation of poleward and equatorward directions, depending on both the period and the geographic area considered. Although the generation process of mesoscale eddies is difficult to observe unambiguously from Lagrangian instruments, eddies are nevertheless ubiquitous over the abyssal plain. Some characteristics of the observed cyclonic and anticyclonic eddies are presented. Smaller anticyclones, localised over the outer shelf and interpreted in terms of ajustment of slope water intrusions, are also depicted. para determinar la circulación Lagrangiana, general y de meso-escala, sobre la planicie abisal y del talud del Golfo de Vizcaya. Dos niveles fueron seleccionados para seguir el Agua Central del Atlántico-Norte (100 m y 450 m). Para esto se han usado boyas de deriva en superficie (de tipo Surdrifts de inmersión constante, seguidas por satélite) y flotadores en profundidad localizados por red acústica (Rafos y Marvor). Casi 36 años-flotadores han sido obtenidos por esas experiencias en la región del Golfo de Vizcaya. La circulación general del Golfo se muestra débil, al parecer muy influida por la presencia de procesos de meso-escala, como de corrientes del talud y remolinos que participan en los intercambios entre la planicie abisal y el talud. El objetivo de este trabajo consiste en describir específicamente algunos eventos de meso-escala que han sido observados. Algunas trayectorias de flotadores fueron usadas para describir y discutir las observaciones de las corrientes del talud y de remolinos ciclónicos y anticiclónicos. Las corrientes del talud muestran una estructura alternada, en dirección hacia el polo o el ecuador, según la época del año y la región consideradas. Aunque es difícil observar el proceso de formación de los remolinos de meso-escala con flotadores Lagrangianos, éstas estructuras son muy numerosas sobre la planicie abisal. Diferentes características pueden ser descritas con los datos recogidos de remolinos ciclónicos y anticic...

Drifter observations in the summer time Bay of Biscay slope current

“…These short-lived eddies are comparable in both size and location to those identified by Serpette et al (2006), which were noted in 1997 and 1998. The repeated observations of these features in 1997, 1998 and 2012, in isolated regions along the Armorican Slope shows that these are not one-off features, dependant on the conditions during the specific year.…”

Surface layer circulation derived from Lagrangian drifters in the Bay of Biscay

“…In particular, continental margin current instabilities yield long-lived anticyclonic eddies, called Slope Water Oceanic eDDIES (SWODDIES) by Pingree and Le Cann (1992b), which propagate slowly westward (2 cm s −1 ) with tangential speeds reaching 30 cm s −1 . Typical anticyclonic eddy core diameters are 50 km (Serpette et al, 2006). Cyclonic eddies have also been reported in the literature (Paillet, 1999;Pingree, 1979;Serpette et al, 2006;Van Aken, 2002) with an azimuthal velocity of~10 cm s −1 at a radius of~20 km.…”

Surface Salinity Measurements—COSMOS 2005 Experiment in the Bay of Biscay

“…Near the shelf breaks, there are larger gradients often associated with slope currents and shelf breaks (Pingree and Le Cann 1990;Serpette et al 2006). There was little rainfall in 2005 and anomalously low river outflow, and this is not a region with large evaporation under low-wind conditions, so that we expect no large near-surface haline stratification most of the time.…”