www.elsevier.com/locate/jmarsys

Journal of Marine System

A general view of the hydrographic and dynamical patterns of the

Rıas Baixas adjacent sea area

Ramiro A. Varela*, Gabriel Roson, Juan L. Herrera,

Silvia Torres-Lopez, Angel Fernandez-Romero

GOFUVI, Facultad de Ciencias del Mar, Universidad de Vigo, 36200 Vigo, Pontevedra, Spain

Received 28 October 2003; accepted 1 July 2004

Available online 13 October 2004

Abstract

The hydrography and dynamics of the Rıas Baixas adjacent shelf region is reviewed in this paper with the aim to serve as a

general dstate-of-the-artT reference and to help introduce several topic-related articles in this special volume. This introductory

article is structured as follows: first, a brief description of the general topographic and bathymetric characteristics and the tidal

regime is given; afterwards, the water masses existing in the region, as well as their circulation patterns are reviewed with

much more detail. In this context, we focus on the characteristics of the water masses involved in the main oceanographic

process that occurs in this area (the upwelling of Eastern North Atlantic Central Water and the Mediterranean Water). To better

reflect this purpose, bibliographic data are used, but also 46 CTD new data samplings all around the year from a station off Rıa

of Vigo are gathered. In this section, we extensively discuss the present dilemmas about the Iberian Poleward Current system,

confronting some classical and more recent views about its seasonality. After that, concise but novel and relevant information

on very recent observations of short period internal waves in the near shelf area is presented, pointing out about their

importance on the thermohaline structure of the water column. This introductory paper is closed with some thoughts on what we

consider either interesting or necessary work that should be performed in this area during the next coming years.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Rıas Baixas; Iberian Poleward Current system; Water masses

1. Introduction

As stated in the preface, this special volume has

been divided for practical purposes into three main

0924-7963/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jmarsys.2004.07.006

* Corresponding author. Fax: +34 986813792.

E-mail address: rvarela@uvigo.es (R.A. Varela).

sections: (i) Canary Islands oceanography, (ii) north-

west Spain oceanography and (iii) geological-related

manuscripts concerning northwest Spain. It was

agreed that a general, state-of-the-art review article

must introduce each section, to give the necessary

reference framework and to allow a better under-

standing of the collection of articles that will

immediately follow in the section. The present

s 54 (2005) 97–113

R.A. Varela et al. / Journal of Marine Systems 54 (2005) 97–11398

manuscript is written in this context, to serve as a

general physical oceanographical introduction of the

second section of the special volume. As an important

side—and relevant—effect, condensing all the general

introductory information in one paper will allow the

other articles in the section to reduce their introduc-

tions and concentrate in the critical scientific aspects

they deal with.

In the region, perhaps the most relevant and

frequently observed phenomenon is the recurrence

of the upwelling, or even better, the succession of

upwelling and downwelling events in accordance with

the dominant shelf winds. An overview is given to

describe the reasons of these phenomena from the

meteorological point of view, where we also included

the main characteristics of the region’s bathymetry.

Attention is then directed towards the tidal cycle in the

region, explaining not only the orientation of the tidal

ellipses in the near-shelf according to model results,

but also including real tidal measurements done in

harbours such La Coruna or Vigo, or in the Rıa of

Vigo’s mouth. In this context, we recompile a table to

show the main diurnal and semidiurnal harmonics for

the area. The next two sections of the manuscript are

devoted to the water masses present in the region, and

to its most prominent circulation features. In the

section of water masses, we focus on the Eastern

North Atlantic Central Water (with its two members—

ENACWst and ENACWsp), since these water masses

are the ones that regularly upwell and enter the Rıas,

but we also deal with the Mediterranean Water (MW)

with its different cores and time and space variability.

Both water masses cover the whole water column in

the adjacent Spanish northwest shelf, and are partic-

ularly important in understanding the whole near shelf

circulation. The Labrador and North Atlantic Deep

Waters (LW and NADW, respectively) are also

referred to, but more briefly, due to its distance from

the shelf.

Among the prominent circulation features of the

region, we were particularly concerned with the

Iberian Poleward Current (IPC) and the coastal front

that conspicuously appear during the autumn–winter

seasons. The present dilemma about the fate of the

IPC during the summer, or its relation with the

dominant winds and the density meridional gradient

are two important matters that we also dealt with.

Even when mentioned, less attention was given to the

Portugal current (PC) or the Portugal coastal current

(PCC), due again to their longer distance to the shelf.

The manuscript also takes into account what may

be one of the main directions of future work in the

region, the internal waves. Some very recent experi-

ments (still unpublished) show that the interaction of

the tides with the topography is creating short-

frequency (15 to 50 min) internal waves with vertical

displacements of 30 m or even more. Since these very

recent results can change the view on how the

physical or biogeochemical measurements are taken

in the region, we ought to include some examples in

spite they are still rather preliminary. The paper ends

with some considerations about what we think must

be done so far to increase the oceanographical

knowledge of the region, both in terms of real data

measurements and modelling attempts.

1.1. General atmospheric and topographic features.

The upwelling–downwelling cycle

The north western coast of Spain, commonly

known in the scientific literature as the bRıas BaixasQregion, extends from about 42 to 43 8N and is

characterized by a fairly regular topography and a

jagged SSE–NNW coastline with four main embay-

ments or bRıasQ (with volumes comprised between 2.7

and 4.3 km3, Fig. 1). Another distinctive topographic

feature is Cape Finisterre, at the northern limit of the

region, where the coastline abruptly changes its near

south–north orientation and continues in an east–west

direction. The bathymetry of the area up to a depth of

150 m decreases almost monotonically outwards with

an average slope of 5�10�3, the 100-m isobath

approximately being 10 km away from the Rıas’

mouth. Further seaward, the slope increases about an

order of magnitude, the 1000-m isobath being located

about 50–60 km far from the shelf. These prominent

bathymetric features of the area will produce strong

effects on the particulars of the boundary layer

circulation (Maze et al., 1997).

Freshwater inputs from the continent are rather

small, mainly limited by those coming from the Rıas

and, out the southern limit of the area, by the Mino

River. These inputs, though relatively low during the

spring–summer (71 m3 s�1), greatly increase during

the fall–winter months (313 m3 s�1), being the annual

average 201 m3 s�1. In this figure, the flow of River

Fig. 1. Map showing the main bathymetric and topographic features of the zone adjacent to the Rıas Baixas. The crosses indicate the position of

the Spanish bPuertos del EstadoQ Vilano-Sisargas (North) and Silleiro (South) buoys, while the circle close to the Silleiro buoy is where

DYBAGA St. 5 was located.

R.A. Varela et al. / Journal of Marine Systems 54 (2005) 97–113 99

Mino is explicitly excluded, as it cannot be formally

considered as a Rıa.

The meteorological dynamics of the region is

highly conditioned by the seasonal evolution of two

atmospheric systems, the Azores high and the Iceland

low (Wooster et al., 1976; Fiuza et al., 1982; Vitorino

et al., 2002). During typical summer conditions, the

Azores high is well established over the central North

Atlantic, while the Iceland low is feeble. This

situation confines winds crossing the North Atlantic

to northern latitudes, and creates a relatively low-

energy waves scenario in the near shelf region (Fig.

2a). The temperature increase of the Iberian Peninsula

during the summer also helps to create a significant

zonal gradient of atmospheric pressure between the

continent (a relative thermal low) and the Azores high,

inducing winds having a prevailing northerly compo-

nent. So, although the start of the upwelling season is

very variable (for example, in 1997 upwelling

conditions were a minority in June), the Rıas Baixas

region is generally characterized by upwelling con-

ditions from April–March to September–October,

being particularly conspicuous inside the Rıas (Woos-

ter et al., 1976; Fraga, 1981; McClain et al., 1986).

This prevailing upwelling situation transports signifi-

cant volumes of water off the shelf (the average

Ekman transport off the Rıas during the upwelling

period is about 270 m3 s�1 km�1, Alvarez-Salgado et

al., 2003) and promotes the generation of energetic

upwelling filaments, which are readily visible with the

aid of satellite images (Blanton et al., 1984; Haynes

et al., 1993; Barton et al., 2001; Smyth et al., 2001).

During the winter, the Iceland low is reinforced,

while the Azores high moves to its most southern

position (near 308N, Vitorino et al., 2002). In these

conditions, low-pressure systems moving across the

Atlantic extend their influence to the Rıas Baixas

region, causing a considerable increase on significant

wave height (Fig. 2b). In addition, southwesterly

winds are rather frequent (Fiuza et al., 1982; Fig. 2c),

and downwelling conditions may often dominate

inside the Rıas and at the coastal shelf (average

Ekman transport to the Rıas during the downwelling

period is about �330 m3 s�1 km�1, Alvarez-Salgado

et al., 2003).

The above-reported mean Ekman transports inte-

grated over distance between Capes Silleiro and

Finisterre (90 km, Fig. 1) yield 25�103 and

�30�103 m3 s�1 during the summer and winter

seasons, respectively. Those figures point out the

Fig. 2. Wind diagrams obtained from the SeaWatch buoy moored by the Spanish Institution bPuertos del EstadoQ at the southern limit of the Rıas

Baixas region (Cabo Silleiro, NW Spain), from 1998 to 2000 inclusive (see Fig. 1 to locate the buoy). (a) Histogram showing the significant

wave height during the April–September period. (b) Histogram showing the significant wave height during the October–March period. (c) Wind

rose indicating the most common wind direction during the October–March period (oceanographic convention, indicating where the wind come

from). (d) Month averaged significant wave height comparison (with error bars) between the above mentioned Silleiro buoy (solid line) and

another similar buoy moored at Vilano-Sisargas, at the northern limit of the Rıas Baixas region (shadow line).

R.A. Varela et al. / Journal of Marine Systems 54 (2005) 97–113100

greater influence of the shelf processes against the

river input, as reported continental flows are, in both

semi-annual periods, less than 1% of the Ekman

transports.

According to this general meteorological regime,

the Rıas Baixas region has been classically separated

into two main periods: an upwelling period, ranging

from April to September, and a downwelling period,

beginning in October and ending in March. This

scenario is, unfortunately, only a general and fuzzy

picture. According to Alvarez-Salgado et al. (2003),

70% of the wind regime variability observed is

concentrated in periods of less than 30 days. As a

consequence, shifting wind patterns cause a succes-

sion of upwelling and downwelling episodes (Wooster

et al., 1976; Fraga, 1981; Blanton et al., 1984), giving

place to a complex small time-scale variability of the

hydrographic and circulation structures, both on the

shelf and inside the Rıas. Intermediate periods,

defined as spin-up (preparation) or spin-down (relax-

ation) events take place in each upwelling–downwel-

ling complete cycle, significantly changing the

hydrographic, dynamic and biogeochemical character-

istics. These transitional periods may vary from 1 day

to about a week, according to the intensity of the

forcing (McClain et al., 1986; Alvarez-Salgado et al.,

2003), but are typically around 3–4 days. Results from

the near northern Portuguese shelf (Vitorino et al.,

2002) indicate that the summer wind variability at

time scales N2 days is about 25% less than in winter,

and that the main factor affecting wind variability in

summer is the sea–land breeze, at scales of 1 day. The

upwelling–downwelling and intermediate phases

described above are further complicated at the vicinity

R.A. Varela et al. / Journal of Marine Systems 54 (2005) 97–113 101

of Cape Finisterre (Fig. 1), where upwelling-favour-

able winds could extend over an angle of 2708 and

energy levels are higher than those found in the

southern limit (Fig. 2d). In this region, Torres et al.

(2003) conducted a series of QuickScat wind data

analyses, and found that the major features of the

upwelling in this area are related to the wind field

spatial structure. A particularly relevant result of that

paper is that the wind measured at the Finisterre

weather station is not always representative of the

overall wind conditions of the Galician coast, as

usually assumed.

Although this small-scale temporal variability have

been studied and described inside the Rıas in several

papers (Blanton et al., 1984; Prego and Fraga, 1992;

Figueiras et al., 1994; Tilstone et al., 1994; Alvarez-

Salgado et al., 1996, just to mention a few), hardly

any mesoscale studies have been conducted on the

adjacent shelf and on the coastal transition zone.

Alvarez-Salgado et al. (1993) described the short-time

evolution of the thermohaline properties and chemical

composition of the water column as a response of the

intermittent equatorward winds during the upwelling

season in a single station located on the shelf. Castro

et al. (1994), on the basis of thermohaline properties

(measured once along eight transects perpendicular to

the coastline), described how the relaxation of

upwelling formed a convergence front 28 km off the

coast, but no short-time evolution of this structure was

studied. More recently (1998), two revealing Lagran-

gian drift experiments in the area have been carried

out in the framework of OMEX-II (Ocean Margin

exchange) programme, concerning on, among other

goals, the study of fluxes along the Galician shelf

Table 1

Harmonic constants amplitudes (cm), phases (8) and time lags h (min), der

et al. (1997) for two relevant locations in the Spanish western Atlantic O

Harmonic constant La Coruna

Amplitude

(cm)

Phase

(8)Time

(min)

Z0 216.2 0

O1 6.6 325.4 38.

K1 7.5 72.3 �24.

P1 2.5 58.5 0.

M2 119.8 87.3 �0.

S2 42.1 118.7 1.

N2 25.4 68.6 �4.

K2 11.7 115.9 �2.

break (Joint et al., 2001). In 2002, a series of surveys

on the adjacent coast were performed on a weekly

basis all over the year, under the umbrella of the

Spanish-funded project DYBAGA (Annual DYnamics

and Biogeochemistry on GAlician shelf: Short-term

variability).

1.2. Tides in the Rıas Baixas adjacent sea area

Few direct tidal measurements are available from

the Rıas Baixas adjacent shelf region. Most of the data

historically referred to come from tidal meters located

either within the Rıas or in the harbours. Of course,

data can also be derived from the TOPEX/POSEI-

DON data sets. In general, the region could be defined

with an F factor significantly less than 0.25 (i.e.,

Vigo, F=0.026, data from the Spanish Marine Hydro-

graphic Service), indicating a pure semidiurnal

regime. A few years ago, the combined use of

REDMAR tidal meters (RED de MAReografos from

the Spanish Institution Puertos del Estado), TOPEX-

POSEIDON data and numerical models provided a

much clearer picture of the tidal dynamics of the

region (see Alvarez-Fanjul et al., 1997). From this

paper, we summarize the harmonics amplitude, phases

and tidal lags derived from tidal gauge measurements

in two locations (La Coruna and Vigo) of the Spanish

coast (Table 1), showing that the highest diurnal and

semidiurnal harmonics are, respectively, M2 and K1.

The use of models is required to give a much better

spatial resolution and to provide tidal currents. Results

of the numerical experiments, in good agreement with

observations, indicate that the tidal pattern consists

essentially in amplified ellipses in front of the capes

ived from tidal gauge measurements as described in Alvarez-Fanjul

cean coast (La Coruna and Vigo)

Vigo

lag h Amplitude

(cm)

Phase

(8)Time lag h(min)

196.0 0

3 6.5 318.7 29.7

3 7.6 59.4 2.0

8 2.5 48.4 16.0

6 112.3 76.7 7.2

4 38.8 106.2 9.4

6 23.8 58.1 3.2

6 11.0 103.8 4.0

R.A. Varela et al. / Journal of Marine Systems 54 (2005) 97–113102

(their main axis parallel to the coast), followed by

large ellipses in the lee of the capes, the main axis in a

cross-shore direction (Alvarez-Fanjul et al., 1997).

Tidal velocities in the northwest corner of the Iberian

Peninsula obtained with this numerical experiment are

relatively small (5 and 2 cm s�1 for M2 and S2,

respectively).

Alvarez-Fanjul et al. (1997) model results were,

however, in poor agreement with the barotropic theory

for tidal prediction in a continental shelf developed by

Battisti and Clarke (1982). Alvarez-Fanjul et al.

(1997), suggested that this discrepancy could be due

to the large capes present in the region, which can

significantly distort the circulation and make Battisti

and Clarke (1982) assumptions unacceptable. The

same authors found that the ellipses pattern can be

qualitatively better explained in terms of a Kelvin

wave moving along an irregular boundary (Rosenfeld

and Beardsly, 1987). In other words, the tidal

harmonics propagate from South to North as Kelvin

waves, their amplitude increasing towards the coast.

The phases are also in accordance to Kelvin wave

behaviour, increasing slowly to the North. The cape

effects—essentially to constrain the flow—are to

produce an increase of the u velocity offshore the

cape, followed by an increment of v velocity in the

lee. Finally, and according to the same authors, the

nonlinear shallow terms are almost negligible around

our area of interest (Alvarez-Fanjul et al., 1997).

According to our knowledge, no more recent tidal

data based on modelling attempts are available out of

the Spanish shelf. Instead, well on the limit of the Rıa

of Vigo and the shelf (the Rıa of Vigo’s south and

north mouths), Mıguez et al. (2001) and Souto et al.

(2001, 2003) found a tidal range of around 3 m using

data from an Aanderaa DCM-12 Doppler current

meter. 90% of the water total height changes in this

area were due to the tidal cycle, while the residual

frequencies were responsible of the remaining 10%.

1.3. Water masses in the Rıas Baixas adjacent sea

region

The surface layer comprises the mixed layer and

the seasonal thermocline. Its thermohaline character-

istics and thickness are variable, as a result of the

air–sea fluxes and runoff seasonality. The depth of

the mixed layer gradually increases from 20 m in late

summer–early autumn, to 60 m in late autumn,

reaching 125 or 150 m in early or late winter. During

the winter, the surface layer is less saline than in

summer due to mixing with waters of continental

origin. In summer, water stability is supplied mainly

by solar heating, and a seasonal thermocline devel-

ops at 10–30 m depth. Below that layer, remnants of

the earlier winter mixed layer can be found (~100 m,

Fig. 3).

A recurrent feature in the hydrography of the

Galician upper waters is that surface runoffs of the

Rıas Baixas embayments (due to the Verdugo-

Oitaven, Lerez, Ulla, Umia and Tambre rivers), as

well as the contribution of the Mino River at the

southern limit, originate a low salinity water plume

that extends along the shore. This buoyant lens,

though present all around the year, has a marked

seasonal variability, being significantly more conspic-

uous during the winter. Since this pattern is also

present in the Portuguese shelf, Peliz et al. (2002,

2003a) recently proposed to name it as Western Iberia

Buoyant Plume (WIBP), and defined it by Sb35.7.

This buoyancy lens expands all over the shelf, could

reach the 1000-m isobath and is about 20 m deep, its

thickness being reduced as it approaches to the coast

due to the raise of the isopycnals. According to Peliz

et al. (2002), the WIBP is advected both southward

with the upwelling jet and offshore in the Ekman

layer. The Ekman transport offshore during upwelling

conditions causes the WIBP to encounter with lighter

waters (due to its higher temperature) and it tends to

sink to conserve its static stability. During downwel-

ling or convergence conditions, the lighter offshore

waters tend to cover the WIBP close to the coast

(Peliz et al., 2002, 2003a).

Below the surface layer, we consider Eastern North

Atlantic Central Water (ENACW) as all mode waters

formed eastward of the Mid-Atlantic Ridge and

placed above the main thermocline. Its general

circulation roughly fits with a weak anticyclonic gyre,

but the fact that this water mass forms in a very

extensive area with markedly different climatic

features, makes the thermohaline characteristics of

this water mass not uniform. Thus, off western

Galicia, the ENACW water mass consists of the

mixing of two end members of distant origins (Fraga

et al., 1982). The lighter one, warmer and saltier, has a

subtropical origin (hereinafter ENACWst). It follows

Fig. 3. Time evolution of potential temperature (a) and salinity (b) with depth in a station in front of the Ria de Vigo, at the position 42.138N,9.58W between May 2000 and April 2001.

R.A. Varela et al. / Journal of Marine Systems 54 (2005) 97–113 103

the thermohaline relation: h=13.13+5.653(S�35.80)

for SN35.70 (Fiuza, 1984, Fig. 4). It has a distinctive

subsurface salinity maximum at its top and it is

formed along the Azores Front (~358N) by subductionof surface modal waters (Rıos et al., 1992). Close to

the Galician slope, ENACWst flow is directed to the

North mainly during autumn and winter (Huthnance

et al., 2002).

ENACWst depth range in Galician shelf (Table 2)

mainly depends on both the intensity and direction of

winds (Fig. 2). The layer bounded by the 12.5–13 8Cisotherm (representative of ENACWst, Fig. 4) appears

Fig. 4. h–S diagram of the 46 CTD samplings (once a week during 1 year) made in a station in front of the Ria of Vigo, at the position 42.138N,9.58W between May 2000 and April, 2001. The water masses seasonality can be observed by means of the following colour code: winter: blue,

spring: yellow, summer: red, autumn: green. Lines show rh isopycnals. Both ENACWsp and ENACWst lines are shown (see equations in text).

R.A. Varela et al. / Journal of Marine Systems 54 (2005) 97–113104

as a succession of peaks and valleys in Fig. 3, an

intrinsic sign of the recurrent upwelling–relaxation–

downwelling events, which are a major feature of the

region. ENACWst can occasionally outcrop the sur-

face near the coast during strong northerly wind-

induced upwelling events, frequently accompanied by

low stability in the water column. At other times,

ENACWst can be found deeper than 60 m during

Table 2

Main characteristics of the water masses in Galician Shelf/Slope

Water mass Depth (m) Potential temperature

(h, 8C)

ENACWst b300 N12.5

ENACWsp b450 11.4–12.5

MWU ~800 11.5–11.9

MWL ~1100 11.0–11.5

LSW 1500–3000 3.4–4.0

NADW N3000 b3.3

Ranges show spatial plus seasonal variability.

coastal convergence conditions. During spring–sum-

mer period, its salinity and temperature increase

respectively 0.11 and 0.98 8C in 150 days (Alvarez-

Salgado et al., 1992), showing gradually more south-

ern origin along those seasons.

Although it is common to describe the ENACW

domain using a reference h/S straight line, inter-

annual variations in the thermohaline relationships

Salinity Potential density anomaly

(rh, kg/m3)

N35.70 b27.04

35.55–35.70 27.04–27.13

36.08–36.13 27.45–27.56

36.10–36.25 27.54–27.75

34.90–34.95 27.71–27.81

34.90–34.95 N27.81

R.A. Varela et al. / Journal of Marine Systems 54 (2005) 97–113 105

can occur. The fact that Central Waters form by winter

convection implies that, depending on the climatic

conditions during the formation, air–sea fluxes may

change, and thus the h/S characteristics. In this sense,

ENACWst also presents a remarkable decadal varia-

bility (observed in the period 1972–1994), more

evident after summer when salinity reaches its

maximum value. The salinity at its typical 27.1

isopycnal is significantly and positively correlated

with the north component of wind stress of the

previous year, associated with the Azores High

(r2=0.68). In periods of reinforcement of this High

(roughly during 1970s and 1990s), anomalous dry

winds blow over the areas of formation of ENACWst,

enhancing the evaporation minus precipitation bal-

ance and thus increasing salinity (Perez et al., 1995).

However, Ellet et al. (1986) suggested that the intense

mixing with Mediterranean Water could cause the

increase of salinity. Conversely, during periods of

relaxation of anticyclonic circulation (mainly in the

1980s), wet southerly winds domain and evaporation

minus precipitation diminishes (Perez et al., 1995).

The underlying branch of ENACW, colder and less

saline, has a subpolar origin (north of 468NENACWsp, Harvey, 1982), where this water mass is

intensively modified by winter conditions and the

mixed layer reaches more than 300 m depth. Thus,

ENACWsp forms by winter cooling and deep con-

vection (McCartney and Talley, 1982) following

subduction of a weakly stratified b12 degreeQ mode

water (Paillet and Arhan, 1996). It shows a slightly

different relationship in the h/S plane: h=10.00+8.462(S�35.40) for Sb35.7 (Fiuza, 1984) and

hb12.5 8C (Castro, 1997), having a higher slope than

ENACWst. It moves slowly southward to the Iberian

Peninsula (Pollard and Pu, 1985; Huthnance et al.,

2002), with a small mean flow (b1 Sv) against the slope

(Huthnance et al., 2002).

Perhaps the most characteristic structure of the

upwelling off Galicia is the existence of an upwelling

centre between Punta Roncudo and Cape Finisterre. It

divides the area in two different sub-regimes, not only

due to the different wind regime, but also because of

the character of upwelled water. North of Punta

Roncudo, the upwelling is favoured by northeast

component winds, with lesser intensity and greater

variability than the northerly winds, which favour

upwelling off the Rıas Baixas. The water mass that

upwells in the northern zone is ENAWsp slightly

modified (also named Gulf of Biscay Central Water,

GBCW, Castro, 1997). On the contrary, in the

southern zone, at the beginning of the upwelling

season ENAWsp starts to upwell, but the more

advanced the upwelling season the more domain of

ENAWsp takes place (Alvarez-Salgado et al., 1992).

The magnitude of the upwelled southern coastal flow

was evaluated in August 1998 by means of a 4-day

experiment combining Lagrangian and ADCP meas-

urements, yielding in both cases 16 cm s�1 (Joint

et al., 2001).

In an area of ventilated thermocline like this one,

the limit between the former varieties of ENACW is

not clearly defined, both having great inter- and intra-

annual variability (Castro, 1997). During the upwell-

ing season, ENACWst and ENACWsp are adjacent

and therefore form a subsurface front off Cape

Finisterre. After that, they both follow a northwest

path (Fraga, 1987) above 400 m depth. The location

of that front is not stationary, since it can migrate

southwards, allowing the entrance of ENAWsp into

the Rıas Baixas (Rıos, 1992). However, from autumn,

the former front is displaced northward and eastward

around Finisterre, according with the northeastward

path of saline current (IPC, Castro, 1997).

Below 400 m, the lower layer of ENAWsp

(rh~27.2) reaches a salinity minimum because

between 450 and 1000 m depth, the mixing with

saline and warm Mediterranean Water (MW) begins.

The latter is characterized by anomalously high

temperatures (7–11 8C) and salinities (35.1–36.0)

centred at a depth of about 1100 m (Richardson et

al., 1989). This water mass enters the Atlantic through

the Strait of Gibraltar (with a volume transport of 0.7–

1.3 Sv and maximum velocities of ~1 m s�1). In the

Gulf of Cadiz (368N), the dense MW outflow

cascades down the shelf from ~200 m to more than

800 m and also turns northward by Coriolis force and

spreads following Portugal slope in a damped geo-

strophic balance at rh=27.6F0.1 (Price et al., 1993).

However, this current becomes unstable, generating

meddies, which propagate warm and salty anomalies

into the ocean’s interior.

MW is also divided in two cores (Fiuza et al.,

1998), with typical values of S=36.4, h=13.0 and

S=36.6, h=12.2 for the upper (MWU) and lower

(MWL) branches, respectively. Off-western Galicia,

R.A. Varela et al. / Journal of Marine Systems 54 (2005) 97–113106

these components are distinguished by a temperature

maximum (of 11.5–11.9 8C at ~800 m) and salinity

maximum (of 36.10–36.25 at ~1100 m), respectively.

An interesting, and still not in depth studied fact, is

that both constituents of MW seem to show a

thermohaline seasonal cycle (Fig. 4), being respec-

tively, warmer and saltier during spring–summer,

becoming colder and fresher during autumn–winter.

The upper core (MWU) is found close to the slope,

strongly decreasing westwards. Its dilution off Galicia

is about 45–55%, increasing towards north as a

consequence of the poleward decrease of temperature

and salinity (0.24 8C/100 km and 0.06/100 km,

respectively, between 40 and 438N). In the lower

branch (MWL), the dilution of MWover slope is about

55–60%, increasing also towards north and west. The

MWL poleward temperature decrease is smaller than

that of MWU (0.05 8C/100 km), although the

corresponding salinity decrease in both cores is

similar (0.07/100 km). This latter feature is closely

related with the appearance of salt fingering between

MWL and Labrador Sea Water (at 1300–1800 m,

Fiuza et al., 1998) and it is also associated with the

shoaling of the MWL (40 m/100 km) caused by the

poleward density decreasing. The MW plume is well

detected by observing a salinity anomaly in the range

700VpV1500 db, and may be visible as deep as 3000

db on the isopycnal r1=41.47. The influence of MW

at such depths has been justified by double-diffusive

mixing (Arhan et al., 1994). Recent estimations of the

whole volume transport of this water mass and those

above yields ~4 Sv (at 42–438N, east of 10.58E), alsodecreasing poleward (Huthnance et al., 2002). Off

Galicia, the large-scale current that carries MW shows

a large off slope flow variance. As an example, just

over the submarine mount Galician Bank (Fig. 1)

Maze et al. (1997) described quantitatively an anti-

cyclonic gyre carrying 3.8 Sv, bounded by two

northward branches, both flowing over the slope and

abyssal plain sides, respectively.

Below the MW, at depths exceeding about 1500 m,

some influence of Labrador Sea Water (LSW),

characterized by the values h=3.50 8C, S=34.89

(Talley and McCartney, 1982), is noticeable. Lower

Deep Water (zN3000 m, hb3.3 8C, rhN27.81) contains

contributions from (a) the Iceland–Scotland overflow

water, (b) the deeper levels of the LSW (that give rise

to North Atlantic Deep Water, NADW), and (c) some

Antarctic Bottom Water (AABW) that penetrates into

the North Atlantic through the Discovery Gap on the

western flank of the Madeira-Tore Rise. Neither of

these water masses found over Galician slope (LSW,

NADW) ever reaches the shelf.

1.4. Circulation in the Rıas Baixas adjacent sea area

The Rıas Baixas region is part of the western Iberia

Basin, and is well known in the oceanographic

literature because of two prominent features: first, it

is located at the northern limit of the East North

Atlantic upwelling region, which defines its character-

istics and noticeable filament activity (as a recent

quantitative example, during a 5-day Lagrangian drift

experiment in August 1998, the computed average

velocity of a drogued buoy inside an off-shore

filament was 7 cm s�1 towards the SW—Joint et al.,

2001); second, mesoscale eddies are relatively fre-

quent in the region, since it is situated on the main

path of a poleward current and a northern branch of

the Mediterranean water (MW) interacting with the

topography.

According to Barton (1998), the general circulation

of the northeast Atlantic region (Fig. 5) is determined

by the eastward flowing Azores Current (AC) in the

south (around 338N) and the relatively weak equator-

ward Portugal Current (PC, Saunders, 1982), a branch

of the North Atlantic current anticyclonic system

(NAC). There is evidence (Stramma, 1984) that the

Azores current splits into three branches when it turns

southwards and approaches the eastern ocean boun-

dary: two of these branches separate near Madeira,

while the third passes North of Madeira and will

mainly shape the Canary current (CC) with the

contribution of the Portugal current. Therefore, we

are faced with two gyres, which determines in our

region of interest two main currents, one going

southwards (the Portugal current) and one going

roughly eastwards, the northern branch of the Azores

current (34–358N). In between, lies an inter-gyre zone

of relatively low energy which includes the Rıas

Baixas region which we will describe later in more

detail.

The Portugal current is somewhat weak (Pollard

and Pu, 1985; about 3 Sv, cf. Barton, 1998), and most

of it (about 2/3) will apparently enter the Mediterra-

nean, the rest of the flow going into the Canary

Fig. 5. Schematic diagram showing the main circulation features of the studied area and the 200-, 500- and 1000-m isobaths. The possible or

alternative Iberian Poleward current (IPC) branches are shown as a dotted arrow. Other features are: the Portugal current (PC), on the far left; the

Portugal coastal current (PCC), at the vicinity of the 10–118W meridian; the western Iberian buoyant plume (WIBP), shown as a shaded region

close to the shelf; the subsurface front defined by Fraga (1987) that splits the region in two domains, the ENACWsp domain and the ENACWst

domain, is shown as a shaded region almost perpendicular to the shelf line. Finally, some anticyclonic gyres are shown in the Cantabrian sea,

symbolizing the SWODDIES that shed from the IPC in that region.

R.A. Varela et al. / Journal of Marine Systems 54 (2005) 97–113 107

current. On the other hand, Stramma (1984) calculated

a total volume transport of about 10 Sv for the three

branches of the Azores current above 800 m, between

35 W and the African coast (cf. Barton, 1998).

Another relevant quantification of the flow in the

region is that of Maze et al. (1997), who performed a

least-squares fit using a considerable set of hydro-

graphic data in three different boxes at latitudes 368N,37.28N, 408N and 438N and longitudes from the coast

to 12 or 138W. Their results yielded a depth-integrated

transport of 2 Sv to the east at the boundaries north of

37.28N, and significant northward meridional trans-

port at all levels (10 Sv at 37.28N and 12 Sv at 438N).In the north of the region, approximately parallel to

the Portugal current, but closer to the shelf break,

appears the seasonally reversing Portugal Coastal

Current (PCC, Ambar and Fiuza, 1994) as a series

of recurrent cold intrusions between the Galicia Bank

and the Iberian coast. In summer, the PCC flows

southwards, closely related to the upwelling wind

regime, carrying cold and nutrient-rich ENACWsp

waters of subpolar origin (Alvarez-Salgado et al.,

2003) while during September–April the PCC practi-

cally vanishes.

As we have seen, the inter-gyre region defined

between the southward Portugal Current and the

Portugal Coastal Current is believed to have a weak

circulation; but several oceanographers (i.e., Wooster

et al., 1976; Ambar et al., 1984; Frouin et al., 1990;

Haynes and Barton, 1990) concluded that a northward

circulation at the shelf/slope of the inter-gyre region

was also present, at least when the upwelling season

ends. Van Aken (2000a,b, 2001) characterized the

hydrography of the northeastern Atlantic Ocean

boundary, but other authors also described the

dynamics of this near-surface poleward flux using

AVHRR satellite images (Ambar and Howe, 1979;

Frouin et al., 1990; Pingree and Le Cann, 1990).

Hydrographic data and geostrophic calculations as

well as surface drifters or current meters were also

used (i.e., Haynes and Barton, 1990, 1991; Maze et

al., 1997; Fiuza et al., 1998). This poleward flow was

also confirmed by the inverse model of Maze et al.

(1997) and was recently shown to coexist with the PC

R.A. Varela et al. / Journal of Marine Systems 54 (2005) 97–113108

in the Northern part of the region according to an

analysis of chemical data performed by Perez et al.

(2001).

The poleward flow has been named in a variety of

ways in the scientific literature. While Pingree and Le

Cann (1989) named it bNavidadQ (Christmas in

Spanish) due to the season of the year when it usually

appears, Fiuza et al. (1998), Alvarez-Salgado et al.

(2003) and others extensively used the term Portugal

coastal counter current (PCCC). Recently, Peliz et al.

(2002, 2003a) suggested to name it as Iberian Pole-

ward Current system (IPCs) based on several and, for

us important, reasons. First of all, they pointed out

that the poleward flow is not limited to Portugal, but it

is a feature of the whole Iberian Atlantic coast, and it

even affects a significant portion of the bay of Biscay

region; second, they argue that it is not a true coastal

current, since it extends over the slope, further than

being limited to the continental shelf; third, it is

comprised by several branches, meanders and small

gyres, forming an actual current system. Hereafter, we

will refer to the poleward flow using this recent

terminology. It can be easily traced in Fig. 3 by either

conspicuous salinity (N35.9) and temperature (N14

8C) maxima from surface to ~130 m depth, with

maximum intensity in January 2002. This evidence

continues in the h/S diagram, where the IPC is clearly

visible and corresponds to the blue data close to the

ENACWst line (Fig. 4), suggesting that the IPC

carries ENACWst waters slightly modified by the air–

sea interactions during its northward path.

The origin, occurrence and/or reversal of the IPCs

have also been controversial; according to Frouin et al.

(1990), the IPC is mostly a consequence of seasonal

changes in wind direction combined with a significant

density meridional gradient. Pedlosky (1987) also

pointed out that a common mechanism to generate

poleward flows is the interaction between a meri-

dional density gradient and the wind forcing. In

winter, the prevailing winds come from the South

(Fig. 2), inducing a relatively strong poleward surface

transport when associated with the density gradient. In

summer, the density gradient, while being approx-

imately the same, seems not to compensate the

equatorward surface current driven by the dominating

northern winds, and the surface signature of the IPC

practically disappears. On the same line, and in the

framework of an observational program, Haynes and

Barton (1990) described the onset of the poleward

flow in autumn and correlated the annual evolution of

the poleward flow with the prevailing winds. As a

consequence, and according to these authors, one

should infer that the IPC is mostly a seasonal

phenomenon.

While in general agreement with the mechanism of

the IPC formation as stated above, Peliz (2002)

confronted the common view of a poleward reversal

during the summer. He reinterpreted Haynes and

Barton (1990) data and suggested that the IPCs still

exists after the winter and that the meandering

structure of the poleward current is the responsible

of the interpretation proposed up to now. To support

their view, Peliz et al. (2003b) developed a numerical

model aimed to test the response of the slope using

wind events at scales of a week–month and a close-to-

observed- meridional gradient as a starting initial

condition. Model results indicated that the slope flow

seems to have a considerable degree of independence

and might coexist with other equatorward currents,

both over the shelf and on the upper slope. Partic-

ularly noticeable is that the IPC is always present, and

both the traditional northward (IPC favourable) winds

and the upwelling favourable winds contribute to a

decrease in the poleward flow, by 0.2/0.3 Sv from 1.2

Sv. As interpreted by these authors, after a strong

(upwelling favourable) wind event the surface core

could be displaced offshore just a few kilometres and

being not detectable in usual coastal surveys. They

reinforce their view mentioning that Da Silva (1988)

and Santos et al. (2002), using the results of a summer

hydrological cruise, reported the existence of rem-

nants of the IPCs even after a large upwelling event.

In essence, Peliz et al. (2003a,b) suggested that the

IPC is a consequence of the interaction between the

meridional density gradient and the topography, the

seasonal winds only influencing water transport by a

factor of 20%. We should recall at this point that

Frouin et al. (1990) also stated that the wind stress

could account for only 20% of the total poleward

transport.

Typical values of temperature, salinity and depth

range of the IPC are, of course, those of central

waters of subtropical origin already described (sal-

inity core reaches up to 36.23, while temperature

fluctuates around 14.2 8C and being the depth range

between 100 and 400 m); the current flows along the

R.A. Varela et al. / Journal of Marine Systems 54 (2005) 97–113 109

west coast of Portugal and Spain and, after reaching

Cape Finisterre part of it turns eastwards at a right

angle to preserve potential vorticity (Frouin et al.,

1990) (Fig. 5).

In the Northwest Galician coast, Alvarez-Salgado

et al. (2003) found that the warm and salty surface and

central waters transported by the IPC experienced

chemical changes according to the annual stratifica-

tion/thermocline deepening cycle described before,

being enriched with nutrients originated by intense

mineralisation processes during the poleward dis-

placement. In the Cantabrian Sea, Gil (2003) exam-

ined the changes experienced during September to

December in the water masses and the dissolved

inorganic nutrients. In the same region, Garcıa-Soto et

al., 2002 analysed the IPC intrusion using advanced

very high-resolution radiometer (AVHRR) images

from 1979 to 2002. They concluded that the winter

warming during particularly strong IPC years

(dNavidad yearsT following their notation) was corre-

lated with low values of the North Atlantic Oscillation

Index (NAO) for the preceding months.

Also according to satellite AVHRR images, the

poleward flow, more or less coincident with the 27.0–

27.1 isopycnal, progressively broadens from South to

North (Fig. 6). Typical figures of the current speed are

around 0.2 m/s but a great amount of incertitude

Fig. 6. Satellite thermal image (NOAA AVHRR multichannel SST map) of

current system. (January 2001 average, courtesy of the German Aerospac

remains on its seasonal fluctuation and possible

branching on the northern Galician shelf.

1.5. Internal waves on the Rıas Baixas adjacent sea

area

Until recently, most observations done in the Rıas

Baixas adjacent shelf coast or near regions pointed to

the existence of long period internal waves, or solitons

(Jeans and Sherwin, 2001; New and Pingree, 1992,

2000). These solitons have periods of several hours

and, although severely affecting the thermohaline and

vertical turbulence structure in the water column, they

did not dramatically influence other oceanographic

properties, such as vertical nutrient and chlorophyll

distribution. Fraga (1996) and Barton et al. (2001) are

the only, to our knowledge, published work that

describe the presence of short period-internal waves in

the Rıas Baixas shelf coast, using either a series of

CTD measurements at one fixed point or a Lagrangian

drifter. Very recently, however, a series of experiments

conducted at different locations, and on different

periods of the year in the Rıas Baixas area, clearly

demonstrated the existence of internal waves of short

and medium periods (5–120 min, Fernandez-Romero,

2003). Internal waves develop through the interac-

tions between tide and stratification, and they prob-

the Rıas Baixas and Cantabric regions showing the Iberian Poleward

e Center).

R.A. Varela et al. / Journal of Marine Systems 54 (2005) 97–113110

ably enhance their amplitude (up to 30 m, Fig. 7) and

modify their characteristics due to the abrupt top-

ography close to the shelf break. To further complicate

the already complex internal waves scenario, it has

been suggested that the presence of the Galician Bank

(Fig. 1) could be a relevant factor in the internal wave

generation and propagation (Barton et al., 2001). The

occurrence of the internal waves is not only limited to

the spring–summer months but may occur all-year

Fig. 7. (a) Temperature evolution with depth during 8O h at mooring loc

spaced Seabird SBE39 T sensors). There is clear evidence of significant int

more than 30 m. (b) Spectral analysis from the same set of data but perform

and 88 min.

round, with high variability in its frequency character-

istics (Fernandez-Romero, 2003). If we can be

confident on this results, internal wave amplitudes

off Galicia can reach up to 50 m, while typical vertical

velocities associated with the waves range from 0.5 to

3 m min�1, not extraordinarily large but more than

sufficient to induce important modifications in the

thermohaline and biogeochemical structure of the

water column.

ation 4288VN, 9813.5VW (data obtained with a set of eight equally

ernal waves associated with the thermocline, sometimes extending to

ed on the 158C isotherm. Significant peaks are found at 19.5, 22, 44

R.A. Varela et al. / Journal of Marine Systems 54 (2005) 97–113 111

1.6. Some final considerations

It is clear to us that more scientific work is required,

both from the observational and modelling points of

view, to further clarify the regional circulation, the

whole process of the IPC formation, evolution and

decay, as well as the role of the tides and internal

waves in the region hydrodynamics. In particular,

operational and observational programmes are clearly

needed to cast light on the seasonal evolution of the

meridional density gradient and the Winter Iberian

Buoyancy Plume, as well as a deeper understanding of

the possible branching of the IPC current on the

northern limit of the region, the inhomogeneities of the

wind spatial structure, or the direction and speed of the

internal waves. Deeper understanding of the filaments,

meanders and gyres characteristics is also essential.

From the numerical modelling perspective, existing

shelf models need to refine their scheme and long-term

results, to better take into account the meridional

density gradient associated with IPC. Local and

regional model results need to be nested with large-

scale circulation models. Nonlinear models of the

internal wave degradation with the tide (i.e., New and

Pingree, 1992, 2000) are also needed.

Acknowledgments

Part of the data presented in this paper has been

obtained by means of DYBAGA Project: Annual

DYnamics and Biogeochemistry on GAlician shelf:

Short term variability, supported by the Spanish

Council of Science and Technology MAR99-1039-

CO2-O2 by means of the Comision Interministerial de

Ciencia y Tecnologıa (CICYT).

Partial support from to the REMODA Project:

Reactivity of the Dissolved Organic Matter in the Rıa

de Vigo upwelling system, REN2000-0880-C02-02.

Support by the Spanish Council of Science and

Technology by means of the Comision Interministe-

rial de Ciencia y Tecnologıa (CICYT) is also

acknowledged.

We also like to express our gratitude to three

anonymous reviewers who carefully read the manu-

script and pointed several flaws in an earlier version,

greatly enhancing the overall quality of the final

manuscript.

This is contribution No. 21 of the Unidad Asociada

GOFUVI-CSIC.

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