Poleward along-shore current pulses on the inner shelf of
the Bay of Biscay from subinertial to inter-annual periods
Marion Kersalé(1), Louis Marié(2), Bernard Le Cann(1), Alain Serpette (3), Cyril Lathuilière (3) and Arnaud Le Boyer (3)
Laboratoire de Physique des Océans, UMR 6523 CNRS­Ifremer­UBO­IRD, UBO, Brest, France (2)
Laboratoire de Physique des Océans, UMR 6523 CNRS­Ifremer­UBO­IRD, Ifremer, Plouzané, France (3)
Division HOM, Service Hydrographique et Océanographique de la Marine, Brest, France
(1) Introduction ­ Former studies
[Batifoulier et al., 2012]
Trajectories of three drifters
drogued at 65 m in 1998
Eastern boundary current systems (EBCS) are characterized by the generation of strong poleward currents. These poleward flows appear as intermittent and locally variable currents, defined as pulses.
● Despite its complex topography, the Bay of Biscay (BoB), is a classical EBCS, under the influence of several forcings for the generation of these pulses on the shelf, such as:
­ Large scale oceanic basin circulation
­ Local forcings: Seasonal wind regimes, river discharges
● In the framework of the ASPEX and LEFE/GMMC ENIGME projects, the dynamical characteristics and generation processes of these pulses, from July 2009 to August 2011, have been investigated from subinertial to inter­annual periods.
● Focus on 4 moored current meters over the inner­shelf:
Penmarc’h section ­ ASPEX 1 (N47°48’, W4°30’)
Loire section ­ ASPEX 4 (N46°52’, W2°58’)
Shelf mooring positions. Thick black line denote the positions where the component 44°N section ­ ASPEX 7 (N44°, W1°31’)
of the wind stress is calculated and Biscay Marine Energy Platform – BIMEP (N43°28', W2°53’) projected. Thin black lines indicate the 60, ●
Climate analysis showed that the seasonality characteristics during the period are similar, but anomalous relative to long­term weather conditions
● The time series evidenced alongshore poleward currents with a repeated occurrence over 2 years.
Events Inventory
The large number of events and their repeated occurrence allowed us to separate events into 3 distinct types.
1 Southern events
3
2
1
Events occurring principally at BIMEP and ASPEX 7
Strong wind event
●
Depth averaged along­shore current velocity (cm s­1)
This average circulation had marked seasonal variability Summer/Autumn: Strong barotropic events more important in the south
Winter: The circulation in the north is characterized by a strong surface [Le Boyer et al., 2013] intensified variability.
●
­1
Depth averaged along­shore current velocity (cm s )
+ Along­shore component of the wind stress (Pa) in Spring­Summer 2011
3 Strong events
Alongshore current velocity at BIMEP larger than 40 cm s−1 Strong and persistent wind
coast
−∞
l
m
c
m
K
K
2 impulse response kernels and Intrinsic response of the currents system independent of the statistical characteristics of the wind forcing
l
c
Shoreward
K7
Poleward K 7
x10-3
ASPEX 7 current impulse response functions
Dynamics in the south (north) is controlled by the along­shore wind stress along the Spanish (French) coast at short time lags ~1 to 2 days
● Some influence of along­shore winds at longer time lags (2 to 3 days) ● Remote forcing through coastal­trapped wave propagation ?
●
Propagation events in term of CTWs dynamics
●
2 Shelf events
Currents detected in the southern and northern part of the BoB
Variable winds
2011
d=c , l
­ Repeated occurrence of the poleward currents over two years ­ Classification of these currents into three types:
Southern events, Shelf events and Strong events
­ Relation between the along­shore currents to along­shore wind stress from equatorward locations at short time lags
­ Propagation events explained through CTW dynamics
●
2010
The contribution of wind forcing to currents at a mooring “m” can be 0
d
d
expressed as : um (t)=∑
ds
d
δ
K
(s
,
δ
t
)
τ
(s ,t +δ t )
∫
∫
m
130 and 450 isobaths.
Conclusions
Inter­annual and seasonal variabilities
2009
●
K c7 , l (δ t , d )[ cm . s −1 . Pa −1 . km − 1 . h− 1 ]
Hydrodynamic surveys revealed poleward coastal jets along the Aquitaine shelf and the Armorican shelf
During Summer During Autumn
➔ Westerlies wind + Strong stratification
➔ Stratification breakdown ➔ Downwelling circulation along the ➔ Cross­shore density gradient
[Lazure et al., 2008]
Spanish coast
➔ High­speed CTWs + internal baroclinic Kelvin wave
●
Along­shore velocities [cm s­1] + bottom temperature and SSH
Wind stress­to­current impulse response function
Motivations – Area of study
Propagation evaluated with the alongshore phase speed of the maxima of the vertically integrated alongshore current:
­In spring­summer, phase velocities are less than ~5 m s−1, and can be as low as 0.3 m s−1.
­In autumn­winter, phase velocities are much faster (~10 m s−1).
● Explanation of these propagation features in term of Coastal Trapped Waves (CTWs). Properties of CTWs computed using the matlab codes ● Events propagations from Brink [2006]
explained by the different baroclinic mode phase speeds ● Computed alongshore velocity is almost barotropic :
NH 2
)
Weak value S=(
fL
Phase speed [m s­1] function of wavelength [km]
for typical BoB cross shelf topographies
References
Lazure et al. [2008], Circulation on the Armorican shelf (Bay of Biscay) in autumn. J. Mar. Sys. 72, 218–237.
Batifoulier et al. [2012], Poleward coastal jets induced by westerlies in the Bay of Biscay. J. Geophys. Res. 117.
Le Boyer et al. [2013] Circulation on the shelf and the upper slope of the Bay of Biscay. Cont. Shelf Res. 55, 97­107.
Brink [2006], Coastal­trapped waves with finite bottom friction. Dyn. Atmos. Oceans 41, 172–190.
Kersalé et al. [submitted], Subinertial poleward along­shore current pulses on the inner shelf of the Bay of Biscay. Est. Coast. Shelf Sci.
~ 0.01­0.1 (winter)
~ 0.03­0.2 (summer)
Acknowledgements
Special thanks go to IFREMER and Météo­France that provided the data for this study. We thank Julien Mader (AZTI, Spain) for BIMEP data, which gives us access to an invaluable and unique database. The authors thank Pascal Lazure for precious help, comments and useful discussions.