GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L08301, doi:10.1029/2004GL019618, 2004
Extreme cross-shelf transport induced by eddy
interactions southwest of Iberia in winter 2001
Álvaro Peliz,1 A. Miguel P. Santos,2 Paulo B. Oliveira,2 and Jesus Dubert1
Received 30 January 2004; accepted 23 March 2004; published 16 April 2004.
[1] Unusually high for winter time satellite-derived
chlorophyll-a concentrations southwest of Iberia were
detected in February 2001. Analysis of satellite data
(SeaWiFS, AVHRR, sea level anomaly from TOPEX/
POSEIDON and ERS) revealed that the phytoplanktonrich waters were associated with an extremely long filament
(up to 400 km) induced by eddy-eddy interactions occurring
in the vicinity of the continental margin. The eddy
interaction occurred simultaneously with an accumulation
of coastal fresh water over the shelf. The event promoted an
export of biogenic material offshore along with an estimated
shelf flushing in a 5 to 6 day period that resulted in a
dramatic offshore growth of phytoplankton. A description
of the event is given and preliminary estimates of the crossINDEX
shelf transport are presented and discussed.
TERMS: 4520 Oceanography: Physical: Eddies and mesoscale
processes; 4219 Oceanography: General: Continental shelf
processes; 4855 Oceanography: Biological and Chemical:
Plankton. Citation: Peliz, Á., A. M. P. Santos, P. B. Oliveira,
and J. Dubert (2004), Extreme cross-shelf transport induced by
eddy interactions southwest of Iberia in winter 2001, Geophys.
Res. Lett., 31, L08301, doi:10.1029/2004GL019618.
1. Introduction
[2] Phytoplankton biomass in the surface layers southwest of the Iberian Peninsula is low in wintertime. Chlorophyll-a concentrations (Chl-a) are typically below
0.3 mg m3 in this area [e.g., Peliz and Fiúza, 1999]. The
poor phytoplankton biomass concentrations are associated
with relatively deep surface mixed layers (about 100 m to
150 m in wintertime) and with the northward advection of
southern oligotrophic water originating in the recirculation
of the Azores Current Eastern Branch. Phytoplankton biomass enrichment in the offshore zones is usually observed
during summer after persistent upwelling (filaments, separated coastal jets, etc.). Santos et al. [2004] report a winter
situation off the northwestern coast of Iberia where relatively high Chl-a offshore concentrations were induced by
the joint effect of wind-driven dynamics and buoyant
plumes. However, off the southwestern coast the influence
of buoyant plumes is usually much less. In this paper, we
analyse an extreme event of high offshore concentrations of
Chl-a detected in satellite imagery. We show that the high
concentrations are associated with an extreme offshore
transport promoted by eddy interactions.
2. Data
[ 3 ] Surface Chl-a concentrations are derived from
SeaWiFS L1A-LAC scenes obtained from NASA’s GSFC,
USA. Sea surface brightness temperature (calibrated channel 4-SST) images were processed from NOAA-14;
AVHRR-HRPT level 1b data from Dundee RSF (UK).
Sea surface level anomalies (SLA) result from a combination of TOPEX/POSEIDON and ERS data produced by the
CLS Space Oceanography division. Winds from re-analyses
by the National Center for Environment Prediction (NCEPUSA), were provided by the NOAA-CIRES Climate Diagnostics Center, Boulder, USA.
3. The Chl-a Filament
[4] Figure 1 shows a Chl-a image from 11 February
2001. A southwestward oriented pigment-rich filament
protruding into the offshore oligotrophic waters is clearly
visible. The difference in concentrations is on average one
order of magnitude (around 0.1– 0.2 mg m3 for the
background and 1.0– 1.5 mg m3 inside the filament). The
filament appears to be rooted in the coastal zone between
Cape Sines and Cape S. Vicente (S and SV in Figure 1). The
pigment structure is about 400 km long reaching as far as
13W and 35N, and shows clear signs of turbulent
interactions along its path. Unfortunately, visible and
infrared imagery from the preceding days is strongly
contaminated by clouds and it is not possible to follow
the development of the filament. The clearest image of the
zone corresponds to 16 January and no evident signs of the
filament were observed on that date. The filament is
still visible on 18 February and appears to be decaying by
28 February. This indicates that the structure took less than
a month to develop and remained active for a period of two
to three weeks.
[5] Filaments off western Iberia usually develop after
long and persistent upwelling driven by northerly winds.
Alternatively, this filament could have developed as a
consequence of the separation of a northward coastal
flow driven by long and intense east-southeasterly winds
on the southern coast. This situation is, however, unlikely and NCEP data for the period from the beginning
of January to mid-February indicate that neither of these
scenarios occurred. The winds were predominantly from
the southwest. Therefore, the hypothesis that the filament relates to wind-driven dynamics alone should be
eliminated.
1
Departamento de Fı́sica (CESAM), Universidade de Aveiro, Aveiro,
Portugal.
2
IPIMAR-DAA, Lisbon, Portugal.
4. The Eddy-Eddy Interaction Event
Copyright 2004 by the American Geophysical Union.
0094-8276/04/2004GL019618$05.00
[6] Analysis of SST imagery (Figure 2) reveals that
significant mesoscale activity was taking place southwest
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Figure 1. SeaWiFS-derived surface Chl-a concentrations
(mg m3) southwest of the Iberian Peninsula on 12 February
2001 (E, Cape Espichel; S, Cape Sines; and SV, Cape
St. Vincente).
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of the filament. However, the SLA maps (Figure 3) show
that by 17 January a large anticyclonic feature was already
present off southwest Iberia. It appears to be composed of
two anticyclones (A1 and A2) which lie on the edge of a
clearly defined cyclonic zone to the south. The large
gradient in SLA is coincident with the axis of the filament
in Figure 1. By 31 January (Figure 3b) the cyclonic zone
extends offshore and to the north and (C1) starts developing. The sequence of weekly SLA maps show that at the end
of January, A1 and A2 separate, and by 14 February
(Figure 3c) the SLA maps match the situation described
in the SST images. In this context, the preconditions for the
generation of the filament had started by 17 January, and
were promoted by the interaction of the two large anticyclones with the cyclonic flow south of Iberia. After
31 January the positive anomalies (anticyclones) apparently
lose intensity and the SLA gradient becomes zonally oriented in the vicinity of the coast. Also the
two anticyclones are then separated, giving rise to a north-
of Iberia during January and February 2001. In the
infrared images, two counter-rotating eddies (A1 and C1)
are observed to interact in the slope zone between Cape S.
Vicente and Cape Espichel. An anticyclone to the north
(eddy A1, about 80 km wide) was already evident in the
SST image of 11 February over the slope off Cape Sines.
Later, a cyclone is generated (C1) as a result of a
developing northward intrusion from the Gulf of Cadiz
contouring the southwestern tip of Iberia. This current
intensifies and rotates cyclonically between 16 and
25 February (Figures 2b and 2c). This northwestward flow
interacts with the anticyclonic eddy (A1) and both contribute to the further development of the offshore jet
(marked with a black arrow). Afterwards the dipolar
interaction slowly migrates northward, pushing the crossshelf jet in the same direction several tens of kilometers
from Cape S. Vicente to the zone off Cape Sines.
[7] During the two week period (covered by the available
IR data), while the squirt is advected northward, it acts to
drain mass from the shelf into the deep ocean. On the ocean
side, two larger eddies (A2 and A3) further induce the
evolution of the filament. The large anticyclone (A2) in
the southwest pushes the filament to the south while A3
together with C1 promote an eastward branching (Figure 1).
Finally, it should be noted that the source waters for
the filament are colder (14.5C) than the offshore waters
(16.5C) (absolute SST not shown). Taking a climatological value of salinity (36.35) the filament waters must be
less than 35.7 to ensure static stability. This is a clear
indication of their runoff origin (most likely from the
southern coasts given the prevailing wind direction). The
winter of 2001 was also characterized by a high precipitation rate (the second highest weekly values after 1990 from
NCEP data).
5. The Origin of the Filament
[8] Although it is relatively easy to track the peak of
development and decay of the filament, the lack of data for
the development period as noted above precludes a clear
description of the origin of the eddies and of the generation
Figure 2. Sequence of NOAA-14 AVHRR SST (channel 4
brightness temperatures) images of the SW Iberian
Peninsula: (a) 11, (b) 16 and (c) 25 February 2001. SST
values increase from light (colder waters) to dark grey
(warmer waters).
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experiments. In the first (Figure 4a) it is assumed that the
particles are advected within the Ekman layer and the
geostrophic velocity is zero (v = vek). It was observed that
all particles migrate uniformly in a southeasterly direction
(coastward). For the second experiment (Figure 4b) the
Ekman velocity was added to the velocities derived from
the SLA values (v = vek + vgeo). In this case, a considerable
number of particles drift offshore in the direction of the
filament, but the generated filament is rather short. Finally,
in the last experiment (Figure 4c), the SLA-derived
velocities were multiplied by a factor of two and the
influence of the wind was disregarded (v = 2 * vgeo). In
this case it is observed that a number of patterns of the real
filament could have been simulated (compare Figure 4c
with Figure 1). Notably the southwestward extension of
the filament is very similar to that which is observed in the
satellite imagery. Also the particles describe paths
Figure 3. Maps of Sea Surface Level Anomalies (cm) for
southwestern Iberia on 17 and 31 January and 14 February.
Labels indicate eddy structures discussed in the text.
ward branching of the advective path (see A1 in Figures 1
and 2a).
6. Simulations of the Filament With Lagrangian
Trajectories
[9] Velocity fields were calculated for the period between
1 January and 15 February 2001. Space and time interpolation has been used to regrid the SLA to a 5 5 km grid
for the domain shown in Figure 4. The geostrophic velocities derived from SLA were calculated according to
fvgeo ¼ g@h=@x;
fugeo ¼ g@h=@y;
ð1Þ
where h represents SLA. The Ekman velocities, vek = t/(rdf ),
with d (Ekman layer depth) being 15 m, f the Coriolis
parameter for the given latitude, and the surface stress
(tx, ty) = raCdjwj(wx, wy), with ra = 1.22 kg m3 being an
average air density, w wind velocities from NCEP
reanalysis and Cd = 0.0012, a drag coefficient. Lagrangian
trajectories were calculated with the estimated velocity
fields using a 2nd order Runge-Kutta integration scheme.
The paths were calculated for sets of particles released off
southwestern coast of Iberia at the beginning of January.
Different experiments with varying parameters were
conducted. Figure 4 shows the results for three of these
Figure 4. Estimated Lagrangian trajectories from
1 January to 15 February 2001. Open circles indicate the
start of the trajectory. (a) Lagrangian trajectories estimated
with Ekman velocity only. (b) Trajectories estimated with
Ekman and geostrophic velocities (calculated from SLA)
summed. (c) Trajectories estimated with geostrophic
velocities (from SLA) multiplied by two.
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recirculating eastward, resembling an eastward branching
(contouring cyclone C1) of the filament (see Figure 2). We
conclude that, off the coast, the wind has little influence on
the generation of the advective path. Secondly, the
geostrophic velocities derived from the SLA seem to be
rather weak in relation to the ones that were necessary to
generate the filament. This may be caused by the smoothing
used to produce the SLA maps from the altimeter tracks.
Hernandez and Le Traon [1995] also found a factor of 2 for
the difference in the steepness of the altimeter dynamic
height circulations maps in the Azores Region. Also, the
advective path created between the eddies may in reality be
rather narrow, and so the filament widens due to mixing and
Ekman transport.
7. Estimates of Offshore Transport
[10] Assuming an average cross-shelf flow of about
0.15 m/s (estimates from SLA values), 30 km wide and
150 m deep (shelfbreak depth), the estimated offshore
transport is 0.675 106 m3/s (58 km3/day). The shelf zone
where the core of the squirt was located (between Capes
Espichel and S. Vicente) is about 20 km wide and 140 km
long. Taking a mean depth of 80 m (half shelf depth) the
total volume of this zone is approximately 280 km3. The
estimated period for shelf flushing is about 5 to 6 days.
Assuming that the filament was active for about two weeks
it is expected that shelf waters would have been completely
renewed at least once. By means of image classification it
was possible to estimate the area of the filament; 89260 km2
and its average Chl-a concentration, 0.7 mg/m3 (from which
we subtract the background concentration 0.2 mg/m3).
Given a depth of 20 m for the vertical extension of the
plume [e.g., Santos et al., 2004], a total of 893 t of Chl-a
were contributed by the filament to the offshore surface
waters. The net Chl-a transport crossing the slope may be
estimated using Ctot = (Cshelf Cocean) Vp where Cshelf and
Cocean are respectively reference values for Chl-a concentrations over the shelf and offshore (1.2 and 0.2 mg/m3) and
Vp is the cross-slope transport calculated as above but now
for a depth compatible with the Chl-a plume (20 m). With
these values the obtained cross slope Chl-a transport is only
8 t/day. Taking 20 days for the development of the filament
(assuming a constant transport) the expected biomass within
the filament due to cross-shore transport would be 160 t.
Though the values used are arguable, the difference
suggests that a dramatic growth of Chl-a was registered
inside the filament along with its development into deep
ocean side.
8. Discussion and Conclusions
[11] Coastal summer filaments are known to be the most
effective mechanisms of cross-shelf exchange in the Iberian
region with lengths of up to 200– 250 km from the coast
[e.g., Haynes et al., 1993; Peliz et al., 2002]. These
filaments relate to strong coastal fronts and jets that are
generated in persistent summer upwelling conditions. The
upwelled water is denser than offshore surface water and the
filaments tend to subduct and dissipate as they cross
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the slope (no significant phytoplankton production is
observed at large distances from the coast). In the present
case, a set of eddies southwest of Iberia created an advective
path that provided a conduit to transport coastal waters as
far as 400 km from the origin. Buoyancy input from coastal
river plumes (particularly intense in that winter) provided
stratification and nutrients for an extensive phytoplankton
development. In summary, the export mechanism from shelf
to deep ocean described here is intrinsically different from
that reported in literature for the western Iberia. Firstly, the
process driving the cross-shelf transport is not related to
wind but to extra-coastal mesoscale dynamics that generated
an advective path between the offshore eddies, much longer
than the ones observed in summer filaments. Secondly,
coastal fresh water provided buoyancy along the length of
the filaments, enabling phytoplankton production to occur
very far from the coast.
[12] In this region, several sources of mesoscale eddy
activity exist (Meddies [e.g., Serra et al., 2002], the Iberian
Poleward Current [e.g., Peliz et al., 2003], and the Azores
Current Eastern Branch [e.g., Pingree et al., 1999])
rendering this narrow shelf zone particularly exposed to
eddy-eddy interactions. It is possible that similar events
occur frequently with large impacts on the local ecosystems.
[13] Acknowledgments. Support from ProRecruit (POCTI/1991/
BSR/36663) and SIGAP (22-05-01-FDR-00013) is acknowledged. Institutions that preprocessed and distributed data: DAAC/GSFC (in the frame of
PICOLOR - NASA’s Mission to Planet Earth Program 902 - SeaWiFS); the
CLS Space Oceanography division/EU ENACT project (ENVH2 - CT
2001-0117) and CNES; and the NOAA-CIRES, CDC for providing NCEP
re-analysis.
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P. B. Oliveira and A. M. P. Santos, IPIMAR-DAA, Av. Brası́lia s/n, 1449006 Lisboa, Portugal.
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