Review papersSurface water circulation patterns in the southeastern Bay of Biscay: New evidences from HF radar data
Introduction
Scientific outputs obtained over past decades from high frequency (HF) radar data, at different locations around the world coastal ocean (e.g. Paduan and Rosenfeld, 1996, Kohut and Glenn, 2003, Abascal et al., 2009, Gough et al., 2010, Schaeffer et al., 2011), attest to the potential of these systems such data are used not only for operational oceanography purposes, but also for undertaking research studies into ocean surface processes. The installation (in January 2009) of 2 long-range HF radar systems over-looking the southeastern Bay of Biscay, together with and its subsequent quasi-continuous functioning, has permitted the establishment of a large data base of high-frequency (hourly) and high spatial resolution (~5 km) surface current maps. This new data set incorporates a large amount of information invaluable for the study of regional surface water circulation together with the main underlying physical processes. These observations permit a description of the spatial patterns associated with the main known water circulation features, at different time scales; they are of particular interest in a study area where surface currents show marked temporal and spatial variability.
The area covered by the HF radar is located off the coast of the Basque Country, northern Spain, between 1°W–3°30'W and 43°30'N–44°40'N (Fig. 1(a)). The main morphological characteristic of the area is the large discontinuity in the orientation of the coast (east–west along the Spanish coast, north–south along the southern French coast). Further, the continental shelf offshore of the Spanish coast is narrow and is of constant width (30–40 km on average); that off the French coast increases progressively, with latitude. The continental shelf is incised by canyons; the most important are those of Cap Breton and Cap Ferret'.
The complex bathymetry of the region together with the wind variability and the thermohaline forcings, are the main physical factors responsible for the variability observed; this is in terms of the ocean circulation and the spatial distributions of biologic and environmental parameters (e.g. Bardey et al., 1999, Koutsikopoulos and Le Cann, 1996, Lavín et al., 2006).
One of the established features of the local circulation is the presence of a seasonal slope current (known as Iberian Poleward Current; hereinafter IPC) that flows eastwards at the Spanish coast and northwards at the French coast in winter (Frouin et al., 1990, Haynes and Barton, 1990, Pingree and Le Cann, 1990, Pingree and Le Cann, 1992a, Pingree and Le Cann, 1992b). Vertically, the IPC involves near-surface water masses from approx. 0 to 300 m (Le Cann and Serpette, 2009).
Pingree and Le Cann (1990) showed that the mean slope current throughout the year is relatively weak, although stronger currents are observed in winter; these are associated with warm surface water, flowing along the northern Spanish slope. From in-situ data, within the period 2008–2009, the most intense currents over the continental slope off the coasts of the Basque Country collected coasts (of 0.4–0.5 m s−1) have been observed at the surface, from November to January (Rubio et al., 2013). Other evidence, from quasi-synoptic in-situ ship-based observations in autumn–winter 2006–2007, are presented by Le Cann and Serpette (2009). These investigators revealed much stronger currents (over 0.7 m s−1) over the Spanish Cantabrian slope, to the west of the study area. Other authors have provided evidences of this winter current, together with its role in controlling surface drifters, in several investigations undertaken following the Prestige Oil Spill along the northwestern Spanish coast (González et al., 2006, González et al., 2008, Castanedo et al., 2006, Abascal et al., 2010).
During the stratified conditions in the water column (from June to October), weak currents are observed over the slope (0.1–0.2 m s−1), oriented mainly towards the south-west (SW). The flow remains highly barotropic, although the vertical gradients of the horizontal currents show higher vertical shear values over the first tens of meters (Rubio et al., 2013). Mesoscale cyclonic and anticyclonic structures, generated by the interaction of the IPC with the abrupt bathymetry irregularities, have been described also by several authors (Pingree and Le Cann, 1990, Pingree and Le Cann, 1992a, Pingree and Le Cann, 1992b, van Aken, 2002). Namely, the south east (SE) Bay of Biscay (slope and open ocean areas, between the Cap Breton and Cap Ferret canyons) is known for its intense mesoscale activity (Le Cann and Serpette, 2009).
Over the continental shelf, the water circulation is controlled principally by the wind, since the shelf is narrow with respect to tidal effects. Likewise there is no distinct influence of a large river adjacent to the study area (OSPAR, 2000). The orientation of the coastline, west–east on the western part and with a south–north axis over the eastern part of the study area, together with the seasonal distribution of the prevailing winds, explains the main directional control of coastal and shelf currents. The prevailing wind affects ocean circulation over the area, on a wide range of time-scales from seasonal variations to high-frequency processes associated mainly with breezes (Fontán et al. 2009, 2013, Discussion).
During autumn and winter, the winds are mostly southwesterly and generate (averaged) northern and eastern drift over the shelf. During spring, the wind regime changes to the north-east, causing open sea currents to be towards the west-southwest, along the Spanish coast. The summer situation is similar to that of spring, but the weakness of the winds and the greater variability of the direction of the general drift make currents more uncertain (González et al., 2002, González et al., 2004, Lazure, 1997).
Previous studies of tidal currents within the study area result from punctual records of the currents at 100 m depth (González et al., 2004); similarly from the results of regional, platform and offshore models (Le Cann, 1990, Pairaud et al., 2008). Since the intensity of the tidal currents is related strongly to the width of the continental shelf in this area, tidal currents are observed to decrease from west to east and from north to south, with values under 0.10 m s−1, for the most energetic (M2) component (Le Cann, 1990, Álvarez et al., 1997, Álvarez et al., 1998, Pairaud et al., 2008). Finally, a recently published study, based upon radar-derived currents from the same data set, but restricted to 2009, has shown that the inertial oscillations present a 4D complex distribution within the area (Rubio et al., 2011). Stronger oscillations near the surface occur in summer, with a peak in kinetic energy (KE) at the centre of the study area, over the lower part of the slope: the winter surface maximum is weaker and is located farther to the northwest.
Radar data and other in-situ measurements will be compared here with similar comparisons undertaken by other authors (Paduan and Rosenfeld, 1996, Kaplan et al., 2005, Paduan et al., 2006); this is in order to check the consistency of the results of the present study, compared with their results.
As summarised briefly above, several studies (based up on in-situ data) have been undertaken in the past, to describe the main characteristics of the surface water circulation over the shelf and slope of the study area. However, mainly because these studies were based upon punctual in-situ measurements, both in space and in time, there are still significant gaps in knowledge. Indeed, a detailed description of the dominant surface current patterns, related to different time-scales and incorporating both slope and open sea regions, has not been undertaken.
Within the above context, the main objectives of this scientific contribution are to: (a) study the consistency and applicability of HF radar data, to study the currents in the study area; (b) study and describe the main water surface circulation patterns in the area, in relation to seasonal, mesoscale and high-frequency variability such as inertial and tidal; and (c) quantify the contribution of the different scale-processes, to the “total” observed currents in the southeastern part of the Bay of Biscay.
In relation to the above objectives, the data and the analysis methods adopted are next described. Section 3 describes the main results relating to the radar-derived current data comparison, with other existing data for the region obtained in 2009. 4 Large and mesoscale circulation, 5 Tidal and inertial currents describe the results relating to seasonal, tidal and inertial variability. Finally, the main results are discussed in Section 6, whilst the conclusions are presented in the same section.
Section snippets
Data and methods
In addition to the HF radar data set, other in-situ data from independent sources are used for assessing the performances of the Basque HF radar system and to complete the information given by the radar, when suitable. The different data sets used, together with the details of the analyses performed, are described below and are summarised in Table 1.
Comparison with in-situ observations
As discussed previously by several authors (Paduan and Rosenfeld, 1996, Kohut and Glenn, 2003, Ohlmann et al., 2007, Kohut et al., 2006) the comparison between HF radar-derived currents and current data obtained from in-situ platforms is not straightforward; this is due to the specificities and own inaccuracies of the different measuring systems. It has to be noted that at 4.5 MHz frequency, the measurements made by the radar integrate currents vertically within the upper 2–3 m of the water
Large and mesoscale circulation
As explained in Section 1, the major benefit of the HF radar data is the possibility to observe surface current patterns, with high temporal and spatial resolution, in an area where until now only scarce and punctual measurements were available. Visual examination of the hourly fields provides extensive information on the observable processes in this area, ranging from mesoscale to seasonal scales. However, since the hourly surface current fields for the 3 years of available HF radar data
Tidal and inertial currents
Previous studies has demonstrated that tidal (mainly, around the semidiurnal band) and inertial currents (around the local inertial period, 17.04 h) are the main contributors to the variability in the high-frequency band of water movements in the shelf and slope areas of the SE Bay of Biscay (Rubio et al., 2011). With the objective of studying quantitatively the spatial and temporal patterns associated with these frequency bands, band pass-filters and EOF analysis have been applied to the data.
Discussion and conclusions
The data obtained from the HF radar system of the southeastern Bay of Biscay, available since the beginning of 2009, provide a large amount of information invaluable for the study of the regional surface water circulation and the controlling physical processes. Previous work using HF radar data (Paduan and Rosenfeld, 1996; Kohut and Glenn, 2003; Abascal et al., 2009, Gough et al., 2010, Schaeffer et al., 2011, among others), attest to the potential of these systems for Operational Oceanography,
Acknowledgments
This study has been undertaken with the financial support of the Spanish Ministry of Science and Innovation (National R&D&I Plan, ESTIBB CTM2009-12339 Project) and Spanish Ministry of Science and Innovation under the Research Project BIA2011-29031-C02-01. (SALTYCOR project). The work of L. Solabarrieta was supported by a PHd grant of Fundación Centros Tecnológicos Iñaki Goenaga, whilst the work of A. Rubio was supported partially by a Torres Quevedo grant (Spanish Ministry of Science and
References (53)
- et al.
Application of HF radar currents to oil spill modelling
Mar. Pollut. Bull.
(2009) - et al.
Analysis of the reliability of a statistical oil spill response model
Mar. Pollut. Bull.
(2010) - et al.
Sea level and eddy kinetic energy variability in the Bay of Biscay inferred from satellite altimeter data
J. Mar. Syst.
(2008) - et al.
Low-salinity plumes in the oceanic region of the Basque country
Cont. Shelf Res.
(2009) - et al.
Tidal and wind-induced circulation within the southeastern limit of the Bay of Biscay: Pasaia Bay, Basque coast.
(2009) - et al.
The Prestige crisis: operational oceanography applied to oil recovery, by the Basque fishing fleet
Mar. Pollut. Bull.
(2006) - et al.
Operational oceanography system applied to the prestige oil-spillage event
J. Mar. Syst.
(2008) Barotropic tidal dynamics of the Bay of Biscay shelf: observations, numerical modelling and physical interpretation
Cont. Shelf Res.
(1990)- et al.
Intense warm and saline upper ocean inflow in the southern Bay of Biscay in autumn–winter 2006–2007
Cont. Shelf Res.
(2009) - et al.
Dynamics of the semi-diurnal and quarter-diurnal internal tides in the Bay of Biscay. Part 1: Barotropic tides.
Cont. Shelf Res., 28(10-11)
(2008)
Three anticyclonic slope water oceanic eDDIES (SWODDIES) in the southern Bay of Biscay in 1990
Deep Sea Res.
Seasonal to tidal variability of currents and temperature in waters of the continental slope, southeastern Bay of Biscay
J. Mar. Syst.
Surface currents in the Bay of Biscay as observed with drifters between 1995 and 1999
Deep Sea Res. Part I
I
A description of the tides in the Eastern North Atlantic
Prog. Oceanogr.
Tide and surge dynamics along the Iberian Atlantic coast
Oceanolog. Acta
Joint analysis of temperature and ocean colour satellite images for mesoscale activities in the Gulf of Biscay
Int. J. Remote Sens.
The prestige oil spill in Cantabria (Bay of Biscay). Part I: Operational forecasting system for quick response, risk assessment, and protection of natural resources
J. Coastal Res.
Surface layer circulation derived from Lagrangian drifters in the Bay of Biscay
J. Mar. Syst.
Data Analysis Methods in Physical Oceanography
Daily scale winter-time sea surface temperature variability and the Iberian Poleward Current in the southern Bay of Biscay from 1981 to 2010
Ocean Sci. Discuss.
Variability in the air–sea interaction patterns and time-scales within the Southeastern Bay of Biscay, as observed by HF radar data
Ocean Sci.
Observations of a poleward surface current off the coasts of Portugal and Spain during winter
J. Geophys. Res.
Navidad development in the southern Bay of Biscay: climate change and swoddy structure from remote sensing and in situ measurements
J. Geophys. Res.
‘Prestige' oil spill and Navidad flow. J. Mar. Biol. Assoc. U. K.
Atlantic multidecadal oscillation (AMO) and sea surface temperature in the Bay of Biscay and adjacent regions
J. Mar. Biol. Assoc. U. K.
Cited by (52)
The coastal waters of the south-east Bay of Biscay a dead-end for neustonic plastics
2022, Marine Pollution BulletinCitation Excerpt :The water circulation over the slope is reversed and has intensities three times weaker than those observed in winter, with predominant (westerly) currents over the Spanish slope (Charria et al., 2013; Solabarrieta et al., 2014). During the transition periods (spring, early autumn) no clear pattern is observed, with weak and high variable currents (Charria et al., 2013; Solabarrieta et al., 2014). On the shelf, riverine input, wind intensity, tides, waves and local winds introduce variability in the surface circulation (Charria et al., 2013).
Reducing the error in estimates of the Sunda Strait currents by blending HF radar currents with model results
2021, Continental Shelf ResearchTidally-induced submesoscale features in the atlantic jet and Western Alboran Gyre. A study based on HF radar and satellite images
2021, Estuarine, Coastal and Shelf ScienceThe Bay of Biscay as a trapping zone for exogenous plastics of different sizes
2020, Journal of Sea Research