This paper not to be cited without prior referenee to the author
ICES C.M. 1996/S:34
Theme Session S
CURRENT. MEASUREMENTS OVER THE UPPER SLOPE OFF
THE WEST COAST OF PORTUGAL, 1994-1995
••
Antonio Jorge da Silva
Instituto Hidrografieo, Rua das Trinas, 49, 1296 lISBOA CODEX. PORTUGAL
Abstract
In an attempt to study the eharaeteristies of the slope eurrent off the Iberian
Peninsula, as part of its contribution to the EU funded projeet SEFOS (Shelf Edge
Fisheries and Oeeanography Study), the Instituto Hidrografico, Lisbon, Portugal. has
been earrying out eurrent measurements in the upper 600 m of the water eolumn over
the upper eontinental slope off the west eoast of Portugal.
At latitude 39° 45' N, a elear northward slope eurrent was observable in the
upper 300-400 m over 910 m bottom depth, from Oetober 1994 to January 1995, and
again in April 1995. The mean monthly signal was baroelinie, with maximum values
close to 11 cm s·1 around 200 m in Oetober, and 100 m in December 1994. The
northward (residual) current reached 35 em 5.1 in December 1994. 16 km offshore,
over 2300 m bottom depth, the eurrent showed larger cross-slope fluctuations than over
910 m, exeept in April 1995. when the current was c1early polarized along the slope at
both moorings. with maximum veloeity between 100 m and 150 m. This suggests that
the core of the current is usually dose to (ar inshore of) the 900 m isobath, with
oecasional meanders, as might have happened in April 1995.
INTRODUCTION
Siope eurrents along eastem oeean boundaries, either as undercurrents or with surface
expression, have been widely reported in the literature (e.g. Neshyba, Mooers, Smith and Barber, .
1989). Off Portugal, such current has been identified on satellite images as a warm filament a few
tens of kilometers wide, as weil as on hydrographie data collected during winter (Frouin, Fiuza.
Ämbar arid Boyd. 1990). The existing direct current measurements are, however, sparse and of
short duration.
As part of its eontribution to the European projeet SEFOS (Shelf Edge Fisheries and
Oceanography Studies), the Instituto Hidrografico, Lisbon, began aseries of direct current
measurements over the upper slope off the Portuguese west eoast. This is a short report on the
eorresponding findings. This paper reports the results that were sueeessfully gathered from May
1994 to Oetober 1995 in three moorings: one at the latitude of SEFOS Standard Seetion 1 (37 0 45'
N), over 491 m depth, and the other two at the latitude of Seetion 2 (39 0 45' N), over 910 m and
2300 m, eaeh one eonsisting of 4 Aanderaa eurrent meters at nominally 50, 100, 300 and 600 m
(or 480 m).
DATA AND METHODS
In May-June 1994 two pairs of moorings were deployed along the Iines defined by the
SEFOS Standard Seetions 1 and 2 (TurreI et al., 1995). Assistanee t6 the moorings was provided
in November-Deeember 1994 and again in Oetober 1995. Due to aeeidents or malfunetions some
of the series may have been signifieantly redueed as shown in the table below. The mooring
loeations are indieated in Figure 1. Mooring 1A was never reeovered and therefore is not displayed
in the figure.
Mooring
SEFOS Section
Sottom depth
Actual depth of the
valid period of the
(m)
observations
observations
50-100-190
LOST
59
104
21 Jun - 15 Sep 1994(a)
295
21 Jun - 15 Sep 1994
482
21 Jun - 09 Dec1994
145
10 May - 23 Nov 1994
1A
1
200
18
1
491
2A
2
I ..
910
.
21 Jun - 12 Jul 1994
195
16 Jun - 23 Nciv 1994
390
10 May - 23 Nov 1994
680
10 May - 23 Nov 1994
28
2
2300
50-100-300-600
LOST
2A
2
908
97
23 Nov 94 - 29 May 95
28
2
2300
145
23 Nov 94 - 29 May 95
342
23-26 Nov 1994
639
23 Nov 94 - 29 May 95
72-120-312-615
14 Dec 94 - 11 Oet 95
a) An interruption of ca. 24 hours occurred in the series on 16 July
Aanderaa ReM 7, or ReM 4, have been used in all moorings, set for a sampling rate of 1
per hour. Following validation, the series have been submitted to a speetral analysis in order to
find the best cutoff period to remove tidal and inertial frequencies. Residual currents were thus
obtained by passing a seventh order Butterworth filter with a eutoff period of 35 hours.
2
•
RESULTS AND DISCUSSION
. Comparative statisties of the original and ihe filtered time series in all moorings provide
indieation of the amount of vaiianee eontained in the low-frequeney domain. As ean be seeri from
the table below,. the residual eurrent eontains a majo.r part of ttie varianee in the N-S eoniponent
at the latitude of Seetion 2. The varianee of the E-W eomponent of tlle residual eurrent is rritieh
smaller than that of the N-S eomponent in mooring 2A, but of similar magnitude in mooring 28,
whieh may be assoeiated with different topographie steeiing (see Figure 1). It is, however,
interesting that in the lowest level of mooring 2A, whiie the varianee level (not shown) is mueh
smaller than above, the pereentage eontained in the low frequeney is similar in both eurrent
eomponents. On the other hand, while the varianee of the E-W eurrent eomponent in moonng 28
has ci similar magnitude throughout the watl~r eolumn, its residual part looses importanee. In
mooring 18 the varianee retained in the low frequeney is nearly the same in both .current
eomponents, being quite smaller in the N-S eomponent than in the time series from the two
northem moorings, whieh may be a result of a mueh srnaller topographie steering.
..
.
''''
"-""
\'''.'".
.-
'. ..
'
.•...
...•,,<, ..
."
:.
'.
",'
.
... varianee in the low frequencY domain (residual eurrent) as a percentage of the väriance in the original series .'
Mooring
1A (Jun - Dee 1994)
2A (May - Nov 1994)
e
'0'
2A (Nov 94 - May 95)
28 (Dee 94 - Oet 95)
Depth of observation (m)
E·W component
N-S eomponent
59
32.7
48.4
104
33.9
49.2
295
35.3
29.7
482
27.7
27.7
145
10.3
62.3
195
9.9
61.2
390
12.5
51.4
680
28.3
.27.5
97
24.6
64.4
145
21.9
72.0
(a)
(a)
(a)
639
21.0
30.8
61.2
72·
63.4
120
66.0
68.2
312
52.7
57.1
615
39.4
.
66.0,
(a) The series is only 4 days long
Figure 2 shows eurrent speetra of the original. unfiltered series obtained in the upper eurrent
meters in moorings 2A and 28 in .1995, showing a detail of their low-frequeney domain. Obvious
peaks of variability eorresporid, as expected, to the semi-diurnal tide and the inertial frequeney. A
tidal harmonie is also identifiable, although with a mueh lower energy level. The energy level in the
3
-------------~----
-
low frequency domain is quite significant and some energetic bands are observable: that around
5 days in mooring 28 is quite prominent, and present throughout the water column. Also to be
noticed the higher energy in N-S component in' mooring 2A and the reverse situation in mooring
28.
In most of the spectra obtained with blocks of 1024 hours the second spectral estimate (21.3
days) appeared quite prominent. When the analysis was made with half the fundamental frequency
the peak was centered either around the same period or around 42.6 days. Although it was not
possible to obtain a goöd statistical significance at such periods given the length of the series. one
was led to look tor basic periods around 1 month to perform another statistical analysis. The actual
time intervals for the statistics have been chosen in such a way that, whenever possible. they would
not contain any major currerit reversals in the upper three series of mooring 2A. The results of
such statistic calculations are presented in Figures 3-7.
When analyzing Figures 3-7 it is necessary to bare in. mind that the surface layer understood as the upper 50-100 m, - was never covered by the measurements. In mooring 18,
almost 40 km away from the coast, it is just possible to observe what might have beEm the onset
of the upwelling by early July and its further development until September. A southward jet, or its
outer border (5 cm S-1), appeared in the area and progressively extended downward, whilE:i
eastward compensation current (2-5 cm S·1) could be seen down to about 300 m. No evidence was
found of a northward undercurrent.
•
an
Until mid-September 1994, the situation observed in mooring 2A (Figure 4) was not mucti
different from that in mooring 18, except tor the negligible cross-slope component. In late
September a northward current became evident, a situation that persisted until the end of January
1995 (Figure 5), with the tendency for the largest values to be found at shallower levels... In
February, while the monthly mean longshore' current component was riegligible at mooring 2A, it
became evident in Mooring 28 throughout the whole observed water column (Figure 6), which
points to the possibility of a long periodic meander. In April 1995 the longshore current was similar . .
in both moorings, in a suggestion that the current core might have been centered. in between them.· In May the situation observed the previous year at the same season was apparently resumed. agairi
lasting the whole summer during which the motion was basically southward at mooring 28 (Figura
7). Throughout the observational period the mean monthly cross-slope component was negligible,
except in January 1995, whe!n it was important and towards the coast in both moorings, particularly
28.
At the deepest observational level, below 600 m. the monthly mean was usually very small
at both moorings, pointing·to a very weak circulation at the distribution levels of the Mediterranean
water. that was probably shifted offshore due to the effect of the submarine promentory south of
Section 2 (Figure 1).
In some disagreement with the generally accepted idea that the slope current is maximum
in winter. the results of the investigations now being reported point to a maximum expression of the
current in late autumn. which is quite evident in the progressive vector diagrams of the residual
the ether hand, the usual absence of the current signal in one
current presented in Figure 8.
of the ;"oorings when it is present in the other (which is only 16 km apart) points to its filament
On
4
nature. Finally, the recurrent presence of the slope current in autumn and winter at mooring 2A
together with the proximity of the 200 m isobath, which roughly coincides with the shelf edge,
provides reasons to suspect that the transport at the outer shelf may be non-negligible.
REFERENCES
•
Frouin R, A.F.G. Fiuza, I. Ambar and T.J.Boyd (1990) Observations of a poleward surface current
off the coasts of Portugal and Spain during winter. Journal of Geophysical Research, 95,
679-691.
Neshyba S.J., C.N.K. Mooers, RL. Smith and RT. Barber (Eds.) (1989) Poleward f10ws along
eastern ocean boundaries. Coastal and Estuarine Studies, 34, Springer-Verlag, New York,
374 pp.
•
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.,
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Figure 1. Bathymetric chart of the W Iberian Peninsula and positions of the current meter moorings.
Station positions tor SEFOS Standard Sections 1 and 2 are also reterenced. Isobaths correspond to
200 m, 1000 m, 2000 m, 3000 m, and 4000 m.
•
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Figure 3. Near monthly means, standard deviations and extremes of N-S (v - upper pannel)
and E-W (u - (ower pannel) current components at mooring 1B, June - September 1994.
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Figure 4. Near monthly means, standard deviations and extremes of N-S (v - upper pannel) and E-W (u - lower pannel) current components at mooring 2A,
May - December 1994. Note the different scales of u and v.
v (cmI')
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Figure 5. Near monthly means, standard deviations and extremes of N-S (v - upper pannel) and E-W (u -Iower pannel) current components at mooring 2A,
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Figure 6. Near monthly means, standard deviations and extremes of N-S (v - upper pannel) and E-W (u - lower pannel) current components at
mooring 2B, December 1994 - May 1995
v (cm'S)
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Figure 7. Near monthly means, standard deviations and extremes of N-S (v - upper pannel)
and E-W (u - lower pannel) current components at mooring 28, May - September 1995.
150
125
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24 Oct· 22 Noy 1994
- 2~~OY8~~4 Dec 1994
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Figure 8. Mooring 2A - progressive vector diagrams for selected periods in late autumn and winter 1994-95
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