**WATER MASSES IN UPPER AND MIDDLE NORTH ATLANTIC OCEAN EAST OF AZORES**

Aida F. Ríos, Fiz F. Pérez and F. Fraga

Instituto de Investigaciones Marinas. (C.S.I.C.)

c/Eduardo Cabello, 6

36208-Vigo. Spain

KEY WORDS: water masses, North Atlantic Central Water, Eastern North
Atlantic Water

**SUMMARY**

During the \"ANA\" cruise in November 1988, Western North Atlantic Water
(WNAW) was found linked to the Azores Current (AC) at 23° W, where
according to various authors Eastern North Atlantic Water (ENAW) forms a
boundary with WNAW east of the Mid-Atlantic Ridge (MAR). This boundary
changes position during the year.

The vein of Mediterranean Water (MW) moving towards the MAR seems to be
connected with the AC, and restricts the penetration of ENAW of subpolar
origin (ENAW~P~).

A subsurface front has been found along 42° N, separating ENAW of
subtropical origin (ENAW~T~) that moves northeastwards generating a
cyclonic eddy that is confined between the lower limit of surface layer
and approximately isopycnal 27.06, and modified ENAW~P~ that moves
southwards forming various anticyclonic eddies.

**INTRODUCTION**

The central water masses in the Northeast Atlantic are characterized by
weak circulation and little marked structure in contrast with the
northwestern Atlantic where Western North Atlantic Central Water (WNAW,
Iselin 1936) dominates and characterizes the main thermocline. The North
Atlantic Current (NAC) crosses the Mid-Atlantic Ridge (MAR) at 48° N,
and the Azores Current (AC) crosses the ridge between 32° and 36° N
(Harvey and Arhan, 1988, Stramma and Müller, 1989). Klein and Siedler
(1989) found and described a strong seasonal variability in the flow of
AC. As these currents spread eastwards, characteristic changes are
produced in the thermocline by winter ventilation and by mixing with
surface waters (Pollard and Pu, 1985).

The first studies in this area were mainly concerned with Mediterranean
Water (MW) (Lacombe and Tchernia, 1960; Parrilla and Moron,1971;
Madelain,1972). This water is characterized by a strong salinity maximum
that extends northwards close to the continental slope of the Iberian
Peninsula, and also westwards. Recently, the MW distribution has been
found to be connected with meddies formation whose presence north of 36°
N off Portugal have been demonstrated by Käse et al. (1989). With regard
to the NACW, the most commonly described characteristic of this region
(Maillard, 1986; Saunders, 1982) is a weak easterly current without
clear continuity. The thermocline of NACW is defined in different ways
and with different nomenclatures. The introduction of subpolar mode
water formed in the northeastern cyclonic gyre (McCartney and Talley,
1982) produces Eastern North Atlantic Central Water (ENAW) defined by
Harvey (1982) for the large volumes of Central Water formed north of
46°N, with temperatures and salinities between (4°C, 34.96 and 12°C,
35.66). We call this water ENAW~P~ (for subpolar). The introduction of
these mode waters is an improvement in the T/S relationships given for
NACW by Sverdrup et al (1942), since it differenciates between two
different water bodies of NACW in this area. As these mode waters
advance or diffuse to the south-southeast they form a front with the MW
that restricts its presence in the southeast North Atlantic (Pollard and
Pu, 1985, Fig. 3; Coste et al. 1986)

In contrast, the coast of the Iberian Peninsula is characterized by
upwelling between April and September (Wooster et al, 1976). Fiuza
(1984) found that this upwelled water originates in a front in the
Azores area (about 36°N and 22°W) the preceding winter. Fiuza (1984)
defined ENAW from a T-S segment delimited by 10.00°C, 35.40 and 12.20°C,
35.66 (Le Group Tourbillon, 1983) and another T-S segment between
13.15°C, 35.80 and 18.50°C, 36.75 (Fiuza and Halpern, 1982). These two
segments are linked to a curve that coincides, between 12.50°C, 35.70
and 12.825°C, 35.75, with NACW defined by Helland-Hansen and Nansen
(1926). Emery and Meincke (1986) call this water ENACW, but they define
it with a greater range (8-23°C and 35.2-36.7). Nevertheless it does not
correspond with the observational data (Fig.1). The segment of
Tourbillon Group included in the ENAW definition of Fiuza (1984)
coincides in part with those defined by Harvey (1982). We call H (12.2°
C, 35.66) the limit between ENAW~P~ and ENAW~T~ (for subtropical
origin). So, this point corresponds to the upper limit of ENAW~P~.

In the area of Finisterre, Fraga et al. (1982) found a quasi-permanent
upwelling in summer due to the presence of a subsurface front between
central water of distinct origins. The water to the south of the front
is similar to that defined by Fiuza (1984) as ENAW, while that to the
north is ENAW modified by surface mixing, and whose origin is at 47°N
(Fraga et al. 1982). Collins et al (1983) also found fronts in the same
area (44°N, 14°W). Fiuza (1984) regarded this front as the northern
limit of ENAW~T~.

The aim of the present work is to determine the origin and thermohaline
characteristics of the waters with temperatures greater than 5°C that
flow along or to the western coast of the Iberian Peninsula. Special
reference is made to the distribution and displacement of different
bodies of central water present in close areas, which generate distinct
frontal and upwelling processes near Cape Finisterre.

**MATERIAL**

During the \"ANA\" cruise of the \"Biomass-IV\" expedition on R/V
\"Profesor Siedlecki\" in November 1988, 20 stations were occupied
between 42° 53\'N - 9° 28.5\'W and 23° 29\'N - 23° 40.1\'W. Nine
stations lay on a transect perpendicular to the NW coast of Galicia
(Spain); the other eleven stations lay on a meridional transect
perpendicular to the first. The positions of stations are shown in
Figure 12.

At each station, salinity, temperature and pressure were measured with a
\"Neil Brown\" CTD model S-N01/1132. Samples for salinity determination
were collected at 0, 50, 100, 150, 200, 250, 300, 400, 500, 600, 700,
800, 1000 and 1100 m depth with Nansen bottles, of which four (at 150,
300, 700 and 1100 m) had three thermometers, two protected and one
unprotected, to check the CTD calibration. The salinities were measured
with an induction salinometer (Plessey Environmental Systems Model
6230N).

**WATER MASS DEFINITIONS**

Figure 1 shows a T-S diagram with the data from all the stations. The
water column between 150 and 600 db that corresponds approximately with
26.5 and 27.2 isopycnals is dominated by central water, but the
thermohaline characteristics are those of either ENAW or WNAW according
to the depth and/or geographical position. The central water with
S\<35.66 lies closer to ENAW as defined by Harvey (1982), and shows that
waters of subpolar origin best characterize the deeper levels of central
water in the whole area. Furthermore, this water mass mixes with MW in
different ratios.

In the transect along the 42°N zonal section, the surface water has
higher temperatures due to heating. The salinity on the surface is
determined by the salinity of the winter mixing layer. On the other hand
in the meridional transect there is a clear north-south thermal
gradient, and high salinity surface water of tropical origin is present
in the form of a wedge (Worthington, 1976).

All T-S diagrams on the zonal section (42°N) indicate a segment of
central water corresponding to ENAW as defined by Fiuza (1984), except
at stations 8 and 9 (Figs. 2 and 3) where the salinity maximum at 100 db
has a temperature higher than that corresponding to ENAW. The maximum
salinity values of ENAW are found in coastal stations 1 and 2, and in
stations 5 and 8. In stations 3, 4 and 7 there are marked changes where
the maximum salinity values decrease to similar values to H, which
corresponds with the upper saline limit of ENAW~P~.

The T-S diagrams of the meridional stations (Figs. 3, 4 and 5) show a
clear increase of maximum salinity in the segment corresponding to the
central water according to how the stations are situated towards the
south. This increase is due to a greater contribution of the more saline
component of WNAW, although ENAW~P~ is present at all stations at
Gamma-q\>26.9 (Gamma-q =density-10^-3^ at q° of potential temperature.
UNESCO, 1985). Exactly at station 14 the T/S relationship
(Gamma-q\<26.9) is coincident with WNAW, whereas south of 32°N (station
15) we note: i) the increase in the less saline component of central
water due to the presence of intermediate water of subpolar origin
(Emery and Meincke (1986) have shown that the northern limit of the
distribution of AAIW is about 20°N), ii) the saline edge of subtropical
origin in the surface layer and, iii) the presence of saline subsurface
water (S\>36.4). Fraga (1973) also found a saline wedge at 23°N, 23°W,
that he calls A with salinity 36.84 and 18.5°C and the origin of which
he locates further to the north. A water with similar characteristics
has been described by Siedler et al (1987) as Madeira Mode Water
spreading to the southwest. The MMW probably makes up the component of
maximum salinity of ENAW~T~ which differentiates it from WNAW and which
is similar to the ENAW~T~ found by Käse and Rathlev (1982).

The tongue of MW is characterized by a salinity maximum at depths
between 1000 and 1300 db. The maximum value observed corresponds to a
salinity of 36.220 at 10.91°C at a level of 1276 db at station 2 (Fig.
2). The highest percentages of MW are close to the slope of the Iberian
Peninsula (Fig. 2), due to the Coriolis efect as the MW moves northwards
(Madelain, 1967). The influence of MW diminishes to the west and the
presence of ENAW increases. Nevertheless, higher salinity values have
been observed at station 12 (Fig. 3), at 36°N, 22°W.

**SUBSURFACE FRONT IN THE AZORES ZONE**

The distribution of isopycnals (Fig. 6) shows a surface layer of
seasonal warming, that reaches a depth of 150 to 170 db. This layer
varies latitudinally with greater vertical gradients to the south.
Although the isopycnals gradually reach the surface, a divergent zone
associated to the AC is observed between stations 12 and 15 (see below).
The thermohaline characteristics of surface water in stations 13 and 14
are similar. However, those of subsurface water are clearly different.
The surface water of station 13 is substituted for surface water of
station 14, and therefore a displacement of surface water to the north
is situated in the divergent zone.

The outstanding dynamic characteristic is the marked elevation of the
subsurface isopycnals from stations 13 to 14 which affects the entire
water column. This elevation at 33°N is associated with the Azores
Current (AC), which produces (by the Coriolis effect) an accumulation of
water on its southern edge (station 14) and elevated isopycnals at its
northern edge (station 13) as it moves eastwards through the MAR and
subsequently towards the southeast. The core of AC is centered at about
200 m, in the same position that the maximum divergence between 26.6 and
26.7 isopycnals has been found (Fig. 6). This fact is shown in Fig. 7
where the calculated geostrophic velocity is represented, taking as
reference layer 1400 db given by Sy (1988) for the Azores region. The
accumulation of water on the southern border of the AC (33°N) produces a
slight sinking of the isopycnals with values greater than 26.7, and a
slight elevation of the isopycnals with values of less than 26.5, which
in the most superficial layer causes the lateral displacement described
above.

The salinity distribution of central water mainly coincides with the
isopycnals represented by horizontal lines in Fig. 8. However between
stations 13 and 14 there is an important elevation of the isohalines
towards the surface layer, which clearly shows an important horizontal
frontal process of mixing between WNAW and subsurface water. The TS
diagrams in Figure 4 also show a marked change in the 26.6 and 26.7
isopycnals between stations 13 and 14 in contrast to the close proximity
of the salinity maximum at stations 10-13 in the 26.7 isopycnal (Figs. 3
and 4), and which also can be by the horizontal homogeneity of WNAW
included between 36 and 36.1 isohalines (Fig 8b).

To the north of station 10 (Fig. 3), between 26.7 and 26.8 isopycnal, an
important subsurface gradient in the salinity maximum exists, which
corresponds with WNAW. This feature also can be observed in Figure 8.
This frontal process is similar to that described in the same area by
Käse and Siedler (1982) and Fiuza (1984, in his Figure 1.9).

To the north of station 13, between 200 and 600 db (Fig. 6) the greater
separation of the 26.9 and 27.2 isopycnals shows an increase of vertical
homogeneity of central water which is predominantly ENAW~P~ (S\<35.66).
Located between these same levels, just to the north of AC, is a
westward current core (Fig. 7). Also in November, Stramma and Müller
(1989) found a current system very similar to that shown in Fig. 7 in a
vertical section projected onto 26°30\'W. To the north and south of the
MW tongue, opposite currents, separated by about 200 Km, suggest the
possibility of an anticyclonic gyre, like a meddy. Käse et al (1989)
found meddies with similar diameters at 36°N, 17°W.

We do not have meridional transects located farther west, but taking
into account the Topogulf 1 and Topogulf 2 profiles (Harvey and
Arhan,1988, their figs. 3 and 4) located on both sides of MAR, the MW
tongue and the AC are related in such a way that the MW tongue lies
slightly farther north than the AC. Furthermore, in the case where the
AC has two branches, the two tongues of MW are situated farther north.
Also, Sy (1988) shows the spreading of MW limited by the current system
of AC and NAC.

**SUBSURFACE FRONTS OF ENAW**

In contrast to the meridional section, the zonal section shows a much
weaker salinity gradient. At the surface the thermal gradient determines
the isopycnal distribution, creating a pycnocline at about 50-75 db
(Fig. 9), and several frontal features exist due to lateral
displacements (station 2, Fig. 2).

In the zonal section, two groups of T-S diagrams that repeat the same
dynamic processes, have been separated (Fig. 2). Thus, the thermohaline
characteristics of stations 5, 1, 2 and 3 are similar to those of the
stations 8, 5, 6 and 7. The stations 3 and 5 are repeated in order to
facilitate the comparison. On the other hand, the isopycnal distribution
(Fig. 9) also shows repeated dynamic processes. The stations 3, 4 and 7
have high values of Gamma-q, with salinity maxima close to 35.66 that
correspond to the absence or weak influence of ENAW~T~. However, the
presence of ENAW~T~ is notable at stations 1, 2, 5 and 8 where there are
various convergences. Taking into account the subtropical origin of this
water and the form of the isopycnal gradients between 26.8 and 27.0,
Figure 9 shows the currents that must correspond with two cyclonic gyres
centered on 12°W and 19°W (see Fig. 10), coinciding with the current
vectors given by Swallow et al (1977) in their Figure 1, for
January-February. The T-S diagrams of station 6, 1 and 2, situated near
the frontal zone, show the total or partial loss of the most saline
parts of ENAW~T~ by mixing with subsurface water in its evolution
towards the north.

The maintenance of ENAW~T~ in latitudes higher than that of its
formation and the higher isopycnal elevation of the eastern part of the
zonal section are probably due to the geostrophic adjustment of the
eastward oceanic flow driven by the large-scale meridional baroclinic
pressure gradient in the eastern North Atlantic as the flow reaches the
continental slope of the western Iberian Peninsula (Frouin et al, 1990).
The maximum salinity values of ENAW~T~ during the year at points close
to the continental slope (42°N, 9°W) (Table 1). The continuous increase
since the preceding winter indicates the progressive course of central
water of southern origin during the year. The maximum values of ENAW~T~
at the end of the year are higher than those at 40°N, 10°W in February.

A strong current of ENAW~T~, at the most easterly edge, occurs close to
the slope of the Iberian Peninsula (Fig 10). Using infrared satellite
images, Frouin et al (1990) found a similar salty surface current
flowing along the Iberian Peninsula to Biscay Bay during late November
and early December 1983. Pérez et al. (1985) found ENAW~T~ further to
the north at 47°N, 10°W. Between 15° and 18°W (Fig. 10) a weak and wide
ENAW~T~ displacement towards the north also exists. On the other hand,
in the PHYGAS cruises in the Bay of Biscay there is a general tendency
for an anticyclonic gyre although cyclonic and anticyclonic gyres occur
on a smaller scale (Fruchaud-Laparra et al 1976a; Fruchaud-Laparra et al
1976b). These authors showed that the presence of ENAW~T~ is restricted
to the southern part of the Bay of Biscay.

The isopycnal distribution of salinity (Fig. 11) indicates that the
northerly penetrations of a ENAW~T~ layer are confined between the lower
limit of the surface layer and approximately the 27.06 isopycnal, whit
its maximum thickness at station 8 between 100 and 270 db (Fig. 9). The
existence of zero current between 250 and 400 m depth (Madelain, 1967)
corresponds with the 27 and 27.1 isopycnals that define the vertical
boundary between ENAW~P~ and ENAW~T~ proceeding from the north and
south, respectively. Below this level there is clear evidence of a front
between ENAW~P~ and MW, with maximum values of ENAW~P~ at station 9
(salinity minimum less than 35.4) and a maximum of MW at station 2 with
36.1 PSU.

Under the 27.2 isopycnal (Fig. 9) the shape of the isopycnals changes in
response to the presence of the MW core. In this way, there is a slight
sinking of the isopycnals close to 27.4 at stations 4, 6 and 9, giving
rise to 3 minima coincident with the increased presence of ENAW~P~
detected by lower salinities. The most important constriction of MW is
situated at 15°W and associated with a strong horizontal salty gradient
(Fig. 11); an important geostrophic current southwards is located
exactly at this position (Fig. 10) in contrast to the core of maximum
salinity which shows a northward current. It is interesting to note that
in this frontal zone, the calculated geostrophic current keep the salty
edge to its left, separating two tongues of MW. This important front at
15°W also was observed by Harvey (1982) at 16°W along 46°N. Thus, the
frontal edge of the MW tongue together with ENAW~P~ indicates a strong
current in the opposite direction to the normal spread of MW.

Another constriction of the tongue of MW at 13°W can observed. Harvey
(1982) in a section in 46°N also found this constriction at 12° W with a
detached core of MW at 18°W. This pattern of MW distribution, on the
same latitude, coincides with Madelain (1967) who showed the separation
of two tongues as a consequence of the topographical influence of the
Galicia Bank (42°45\'N, 11°40\'W). The MW distribution further to the
west is limited by ENAW~P~.

**SUMMARY**

Like Fiuza, we have found ENAW with salinity values greater than 35.66
to the east of 18°W near the Iberian coast. But during the \"ANA\"
cruise (November 1988) WNAW was found linked to AC along the 23^rd^
meridian, where, at the end of winter the presence of ENAW is indicated
by various authors (Käse and Rathlev, 1982; Pollard and Pu, 1985). Thus
the boundary between WNAW and ENAW east of the MAR varies during the
year with its maximum penetration to the northeast in December.

The position of the tongue of MW that moves towards the MAR seems to be
linked to the AC and limits the penetration of ENAW~P~ Near Finisterre,
the spread of MW produces a strong salty gradient. The current
established keeps the salty edge to its left.

According to Maillard (1986), after crossing the MAR, the AC and NAC
produce an area of divergence west of Cape Finisterre where the
circulation is very poorly defined. On the southern margin of NAC,
ENAW~P~ moves southwards generating the various anticyclonic eddies that
have been described by various authors (47°N and 18°W in Harvey and
Glynn,1985; and in the Biscay Bay by Fruchaud-Laparra et al., 1976). On
the other hand, ENAW~T~ is formed at the northern margin of the AC and
moves partly northeastwards towards the Iberian coast, generating
meanders or probably, as have been described above, various cyclonic
eddies (see Fig. 12). There is an area of subsurface convergence between
43° and 44°N where ENAW~P~, modified by summer warming, can mix with
ENAW~T~ between 26.9 and 27.1 isopycnals. In this way, the presence of
ENAW~T~ is strongly reduced north of Finisterre. Furthermore this
convergence is well marked at Finisterre (Fraga et al, 1982), where
there is a permanent subsurface front responsible for the intense
upwelling which takes place in spring and summer (Fraga, 1980; Mouriño
et al, 1985; Fraga et al, 1987).

**AKNOWLEDGEMENTS**

The ANA cruise of the \"BIOMASS IV\" expedition was made possible thanks
to the agreement on Scientific Cooperation between PAN (Polish Academy
of Sciences) and CSIC (Consejo Superior de Investigaciones Científicas)
of Spain. Part of the material used during the cruise was supported by
grant number MAR88-245 of the \"Comisión Interministerial de Ciencia y
Tecnología\" (CICYT). We thank Dr. A. Grelowski in charge of the CTD
during the cruise, the other participants in the expedition, and the
crew for their help. We also thank Ramón Penín and Trinidad Rellán for
drawing the Figures. We would like to thank Dr. D. Quadfasel and the
another anonymous reviewer for their valuable suggestions and comments
on an earlier version.

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