Eddy-induced cross-shelf export of high Chl-a coastal waters in the SE Bay of Biscay

Abstract

Different remote sensing data were combined to characterise a winter anticyclonic eddy in the southeastern Bay of Biscay and to infer its effects on cross-shelf exchanges, in a period when typical along shelf-slope currents depict a cyclonic pattern. While the joint analysis of available satellite data (infrared, visible and altimetry) permitted the characterisation and tracking of the anticyclone properties and path, data from a coastal HF radar system enabled a quantitative analysis of the surface cross-shelf transports associated with this anticyclone. The warm core anticyclone had a diameter of around 2 50 km, maximum azimuthal velocities near 50 cm s-1 and a relative vorticity of up to-0.45 f. The eddy generation occurred after the relaxation of a cyclonic wind-driven current regime over the shelf-slope; then, the eddy remained stationary for several weeks until it started to drift northwards along the shelf break. The surface signature of this eddy was observed by means of high-frequency radar data for 20 consecutive days, providing a unique opportunity to characterise and quantify, from a Lagrangian perspective, the associated transport and its effect on the Chl-a surface distribution. We observed the presence of mesoscale structures with similar characteristics in the area during different winters within the period 2011-2014. Our results suggest that the eddy-induced recurrent cross-shelf export is an effective mechanism for the expansion of coastal productive waters into the adjacent oligotrophic ocean basin.

Long‐Term Trends in Phytoplankton Chlorophyll a and Size Structure in the Benguela Upwelling System

“…Various responses in phytoplankton biomass and productivity have also been documented at eddy peripheries in relation to stronger vertical velocities, enhanced mixing, as well as advection and entrainment (Lévy et al, ; Mahadevan, ; Quartly & Srokosz, ). However, in Eastern Boundary Upwelling Systems, mesoscale eddies are often observed to have a negative impact, with mixing and stirring (Rossi et al, , ; Y. Wang et al, ), as well as offshore transport and subduction of biomass (Rubio et al, ; Y. Wang et al, ) and nutrients (Gruber et al, ), resulting in a reduction of phytoplankton biomass and production in shelf regions. Since we investigated seasonal means of Chla and phytoplankton size classes, we could not examine the direct impacts of mesoscale features in the current study.…”

“…However, in Eastern Boundary Upwelling Systems, mesoscale eddies are often observed to have a negative impact, with mixing and stirring (Rossi et al, 2008(Rossi et al, , 2009Y. Wang et al, 2018), as well as offshore transport and subduction of biomass (Rubio et al, 2018;Y. Wang et al, 2018) and nutrients (Gruber et al, 2011), resulting in a reduction of phytoplankton biomass and production in shelf regions.…”

Tide‐Induced Periodic Sea Surface Temperature Drops in the Coral Reef Area of Nanwan Bay, Southern Taiwan

“…Recent studies have demonstrated the use of this land‐based remote sensing technology for ocean dynamics in coastal oceanography. Rubio et al (2018) used high‐frequency radar to analyze eddy‐induced high chlorophyll‐a concentrations in the Bay of Biscay. Combining radar and satellite altimetry data further helps in understanding the current dynamics within the bay.…”

Requirements for a Coastal Hazards Observing System

“…The value of improved altimeter data in detecting coastal SSHa sea level anomalies due to river discharge has been demonstrated by Gómez-Enri et al (2017) and appears promising to study linkages between the land-sea domains (Piecuch et al, 2018). Another example of application concerns the characterization of mesoscale coastal dynamics in the southeastern Bay of Biscay (Spain) in combination with other remote sensing platforms and High-Frequency Radar observations (Rubio et al, 2018).…”