JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. Cll, PAGES 25,985-26,009, NOVEMBER 15, 1999

Signatures of the Mediterranean outflow from a North Atlantic climatology 1. Salinity and density fields

Michaela Ciobotaru Iorga Department of Mechanical Engineering and Material Science, Duke University, Durham, North Carolina

M. Susan Lozier

Earth and Ocean Sciences, Duke University, Durham, North Carolina

Abstract. Using historical data from the National Oceanic Data Center, the climatology of the eastern North Atlantic basin has been investigated for the purpose of detailing the Mediterranean outflow water in terms of its salinity, density, and flow patterns. Part 1 of this work is a descriptive analysis of the fate of the Mediterranean Water once it flows out of the Strait of Gibraltar. Tracing the salinity and density signatures, high-resolution maps of the climatological outflow are presented, with an emphasis on the continuity of the water from its source. From the climatological fields a continuous signal of Mediterranean Water is tracked northward to ---50ø20'N, yet its westward advection is limited to the Tagus Basin. Recirculations of Mediterranean Water in the Gulf of Cadiz and in the Bay of Biscay, deduced from property signals, are also detailed. Iorga and Lozier [this issue] present absolute velocity fields from a diagnostic model constrained by geostrophic dynamics, conservation of mass, and no-flux boundary conditions.

1. Introduction

Warm and salty waters flowing from the Mediterranean Sea into the North Atlantic Ocean form one of the most pro- nounced tongue-like property distributions in the global ocean. At ---1000-1200 m the temperature and salinity signatures of Mediterranean Water nearly fill the entirety of the subtropical North Atlantic basin (Figure 1). The distinct temperature and salinity characteristics of Mediterranean Water can be traced westward to the Bermuda Rise [Armi and Bray, 1982] and northward to the Rockall Channel [Reid, 1979; Harvey, 1982]. During the past 2 decades an interest in Mediterranean Water has focused on its possible influence on deep water formation processes in the northern North Atlantic. Although it has been suggested [Reid, 1979, 1994; Harvey, 1982] that the source waters to the formation sites are influenced by the warm and saline Mediterranean Water at middepths, the particular venue of this influence remains unknown. The influence is

assumed to be either directly through advection or indirectly through mixing. The direct route hypothesis, first promulgated by Reid [1979], is that a branch of the Mediterranean outflow penetrates northward along the eastern boundaries to the Greenland-Scotland Sill where its salt content influences the

production of deep waters in the Nordic Seas. Alternatively, it is proposed that the warm and salty source waters in the Nor- wegian/Greenland Sea derive mainly from the upper waters of the North Atlantic Current, which carry salty waters into the Nordic Seas (M. S. McCartney and C. Mauritzen, On the origin of the warm water inflow to the Nordic Seas, submitted to Deep-Sea Research, Part I, 1999, hereinafter referred to as

Copyright 1999 by the American Geophysical Union.

Paper number 1999JC900115. 0148-0227/99/1999JC900115509.00

McCartney and Mauritzen, submitted manuscript, 1999). With this latter scenario, Mediterranean Water is relegated to an indirect role on the source waters in the Norwegian/Greenland Sea since it is assumed that the waters carried by the Gulf Stream/North Atlantic Current System gain salt by mixing with the adjoining Mediterranean Water at subtropical latitudes [Lozier et al., 1995]. Ambiguity also surrounds the issue of whether the Mediterranean Water extends into the subtropical basin (where it is likely to mix with the Gulf Stream or North Atlantic Central Waters) by advective or diffusive means. Reid [1994] has suggested a westward advective pathway for Medi- terranean Water in the range 35ø-45øN, and this has been supported by the model results of Hogg [1987] and Bogden et al. [1993]. However, from a study of zonal transports in the eastern North Atlantic, Maze et al. [1997] argued that there was no direct advection of Mediterranean Water into the ocean interior. Model

results from Paillet and Mercier [1996] support this argument. Before the question of whether and how Mediterranean

Water influences deep water processes in the North Atlantic can be answered a detailed analysis of the fate of the Medi- terranean outflow is needed. Since this influence is presumably on a climatological scale, we have chosen to investigate histor- ical hydrographic data in order to detail the climatological signature of the Mediterranean Water in the eastern North Atlantic. Specifically, we use a recent high-resolution climato- logical database [Lozier et al., 1995] of the North Atlantic to examine the salinity and density fields of the eastern basin. Our primary goal is to establish the continuity of the Mediterranean signal into the open Atlantic on a climatological scale. High- resolution isopycnal maps and meridional and zonal cross sec- tions of salinity, density, potential temperature, and oxygen are presented in this descriptive analysis of the climatological flow pattern of the Mediterranean Water. Of particular interest are the possible northward and westward penetrations of this

25,985

25,986 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

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I ,-,, ....

... :.•

m m mmm m--•m m m m ß O* 80'W 70'W 60'W 50'W 40'W $O'W 20'W 10'W O'

Figure 1. Property distributions at middepth in the North Atlantic basin. These property fields were created using the hydrographic database described by Lozier et al. [1995]. The spatial resolution of these fields is 0.5 ø.

water mass. An attempt has been made to compare and con- trast the climatological signals to those derived from synoptic surveys or, as is the case with Reid [1979, 1994], from quasi- synoptic studies. Iorga and Lozier [this issue] (hereinafter re- ferred to as part 2) present results from a diagnostic model that uses geostrophy and mass conservation, in conjunction with the Lozier et al. [1995] database, to estimate the flow field in the eastern North Atlantic. Background for our study is given in section 2, followed by a discussion of our database and meth- ods in section 3. Section 4 contains the results of our analysis, and a summary is found in section 5.

2. Background

Past studies concerning the fate of the Mediterranean Water have generally focused on synoptic descriptions of the flow in the local vicinity of the Strait of Gibraltar and/or the Gulf of Cadiz. A brief survey of the collective knowledge from these studies is given here as background for the larger context of our study. The reader is referred to Daniault et al. [1994] for a more complete summary of these studies and to Figure 2a for the location of topographic features discussed in the text. A detailed map of the bathymetry in the Gulf of Cadiz is given in

IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 25,987

I I I I I I I ..! I .. I !•. ß ' 55øN

I

• .. Promontorjr

I: ! ß St. Vincent Canyon Cape St. Vincent ' .•/ Sto...Vincent Spur ...... Cape St. Maria

....• ....... • •. .•••.•

-.. :. '/:•.•;•';?C"•nyo n

Bank:' •:•;•2).. Seine Strait of Gibraltar I 35øN A.P.

'•' :•: ....... ::?/•' ..... ;.• •...•4•?:: ......

':'•:•' I 30'N I

Bank

Horseshoe A.P.

I I

Figure 2a. Topographic features in the eastern North Atlantic basin.

Figure 2b. Mediterranean Water exits the Strait of Gibraltar as a single, dense plume with typical properties of (0, S) = (13øC, 38.4 practical salinity units (psu) [Baringer, 1993; Bar- inger and Price, 1997]. The plume moves down the northern continental slope of the Gulf of Cadiz, flowing around complex topography as a bottom-driven boundary current. As the out- flow spreads northwestward along the southern Spanish coast, it slowly loses its high salinity as it mixes with fresh North Atlantic Central Water [Madelain, 1967, 1970; Zenk, 1975; Ambar, 1984, 1985; Zenk and Armi, 1990; Baringer and Price, 1997; Bower et al., 1997]. By the time the flow reaches the vicinity of Cape St. Vincent it is neutrally buoyant [Baringer, 1993; Ochoa and Bray, 1991; Zenk and Arrni, 1990].

Downstream from the Strait of Gibraltar, two main cores of the outflow have been observed. The upper core of the outflow follows the northern slope of the Gulf of Cadiz at a depth ranging from 600 [Zenk and Arrni, 1990] to 750 m [Bower et al.,

1997], while a second core, generally transporting a larger volume, is guided by various channels down to ---1200 m [Zenk and Armi, 1990; Baringer, 1993; Bower et al., 1997]. The two branches, or cores, converge near Cape St. Vincent. A more detailed synoptic description of the outflow in the Gulf of Cadiz is presented by Zenk [1975]. Briefly, Zenk identifies four branches of Mediterranean Water that are determined by the local bathymetry: a shelf branch in the depth range of 400-600 m; a main offshore branch, at a depth varying between 800 and 1000 m; an intermediate branch described as the southern core of the shelf branch; and a jet-like branch, guided by a narrow channel near the Strait of Gibraltar (36øN, 7ø08'W). Ochoa and Bray [1991] also report several branches that flow down small local canyons and eventually rejoin the main branch halfway to Cape St. Vincent.

After the cores converge in the vicinity of Cape St. Vincent the Mediterranean water flows through the gap between Get-

25,988 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

38øN

37øN

36øN

35øN

34øN

33øN

10øW 9'W 8'W 7'W 6'W 5'W

Figure 2b. Bathymetry in the Gulf of Cadiz. The contour interval is 100 dbar.

tysburg Bank and Cape St. Vincent [Madelain, 1967, 1970; Zenk, 1975; Ambar, 1984, 1985; Zenk and Armi, 1990; Bower et al., 1997], termed St. Vincent's "gateway" by Zenk and Armi [1990]. From this gateway, part of the outflow is trapped along the continental slope, flowing northward [Reid, 1979, 1994; Daniault et al., 1994], while another part has been reported to flow westward into the ocean interior [Reid, 1994; Daniault et al., 1994; Arhan et al., 1994]. Daniault et al. [1994] describe a reservoir of Mediterranean Water situated east of 13øW, be- tween 37 ø and 40øN, with a westward exit and several north- ward exits. The northward penetration of dilute Mediterra- nean Water as an eastern poleward boundary current as far north as Porcupine Bank (---50øN) is generally accepted [Reid, 1979, 1994; Bogden et al., 1993; Hill and Mitchelson-Jacob, 1993; McCartney and Mauritzen, submitted manuscript, 1999]. However, its further poleward penetration into the Norwegian/ Greenland Sea is doubted by those who contest the possibility of a rise of the isopycnals from ---1200 m northwest of the Porcupine Bank to <500 m over the sill of the Wyville- Thomson Ridge [McCartney and Mauritzen, 1999; Hill and Mitchelson-Jacob, 1993].

3. Database and Methods

3.1. Bathymetry

To facilitate the description of the property fields in section 4, the complex bathymetry in our spatial domain is briefly discussed in this section. The reader is again referred to Figure 2b for the detail of the bathymetry in the Gulf of Cadiz. The Gulf of Cadiz has an irregular bottom topography along the continental slope of Portugal and immediately offshore, where the majority of the Mediterranean Water has been observed. The slope varies in steepness and is deeply indented by several submarine canyons. Downstream from the Strait of Gibraltar, the isobaths diverge as the coast rounds to the north, thus

creating a broad, irregular slope in the eastern Gulf of Cadiz. Near Cape St. Maria (---8øW) the isobaths converge sharply, creating a steep slope that extends to Cape St. Vincent. In the vicinity of Cape St. Vincent the continental slope protrudes in a feature called the Cape St. Vincent Spur that creates the southern wall of the wide St. Vincent's Canyon. The slope is irregular and relatively broad north of this canyon. At ---38øN the Lisbon and Setubal Canyons extend far into the slope. Farther north, the slope turns northwestward and steepens along the southern flank of the Estremadura Promontory, known also as Tejo or Tagus Plateau, and then sharply reverses direction, following its northern flank. Nazar• Canyon is lo- cated to the north of this promontory. Galicia Bank, west of the Iberian Peninsula at --•43øN, is another major topographic feature in this region that rises to the Mediterranean Water's depth.

The Seine and Horseshoe Abyssal Plains, which are north- eastern extensions of the larger Cape Verde-Madeira Abyssal Plain, are located south of 36øN. They are separated from the Tagus Abyssal Plain in the north by a quasi-zonal ridge at 36ø-37øN, the summit of which is the Gorringe Bank, which rises to within 25 m of the surface. Located between the Gor-

ringe Bank and Cape St. Vincent is the wide St. Vincent's Canyon, with a depth of --•3800 m. Finally, the Tagus Abyssal Plain communicates to the north with the Iberian Abyssal Plain through a sill of depth 4600 m, just west of the Estremadura Promontory.

3.2. Hydrographic Database

Our data source is a recently assembled database of the North Atlantic [Lozier et al., 1995], which consists of the cli- matological mean property fields averaged on isopycnal sur- faces. Data from ---144,000 hydrographic stations in the North Atlantic, spanning the period 1904-1990, were retrieved from the National Oceanic Data Center and quality controlled to eliminate erroneous data. The reader is referred to Lozier et al.

[1995] for a detailed discussion of the quality control and processing of the station data. Whereas Lozier et al. [1995] focused on the North Atlantic basin in its entirety, our focus on the Mediterranean outflow dictates the use of different isopy- cnal surfaces, different projections, and generally higher reso- lution grids. Specifically, we use the quality-controlled, un- evenly distributed set of station data to investigate the details of the salinity and density fields in the eastern North Atlantic, which for our purpose is defined as 25ø-60øN and 40ø-0øW. The spatial distribution of the 26,533 stations in our domain of interest are shown in Figure 3a. The irregularly distributed data are binned and ensemble-averaged to produce property maps on isopycnal surfaces, as well as meridional and zonal cross sections of the eastern North Atlantic basin. Details

concerning mapping techniques and resolution are given in section 3.3.

The ensemble averaging used to produce the maps in this paper does not take into account temporal changes in data density. To guard against the possibility that features observed in the climatological database are the result of sampling over a short time period only, the database's temporal representative- ness was examined. Figure 3b shows the spatial coverage of the data for each of the 8 decades represented in the database. Even though the database is temporally weighted toward the last 4 decades, there are no spatial domains that were exclu- sively measured during one time period only. In an effort to be more explicit about the temporal coverage of our database we

IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 25,989

60øN have attached an appendix that shows the data coverage for each cross section presented in section 4. Our concern about temporal aliasing is also allayed by a statistical analysis of the data that shows little change in the climatological mean when properties were averaged such that all years received equal weighting [Lavine and Lozier, 1999].

50•N

40øN

30øN

40•W 30øW 20øW 10øW 0 ø

Figure 3a. Spatial distribution of the stations in the eastern North Atlantic for the period 1904-1990.

3.3. Mapping

Isopycnal property maps and zonal and meridional cross sections of the water properties are produced using a variable grid size to account for differences in local data density. Gen- erally, a smoothing radius of 1/10 ø is used near the Strait of Gibraltar. This radius increases toward the interior of the

ocean to a maximum of 30'. The vertical increment of the grid varies between 25 m in the Gulf of Cadiz and increases to a

maximum of 100 m in the interior of the ocean. Because our

analysis of the Mediterranean Water dictates much smaller grid increments (to resolve boundary currents, for example) than a basin-wide climatology [Lozier et al., 1995], data density is compromised in some locales. When analyzing the fields, data gaps were masked, as seen in Figures 4c and 4d for two representative cross sections. These masked sections were then compared to sections produced from a smoothing of the local data (Figures 4a and 4b). A subjective determination was made as to whether the smoothed representation of the data was sufficiently supported by the masked plot. For the two exam- ples presented, one (8ø15'W) was selected as representative, while the other (37øN) was selected as unrepresentative. We

60'N

50'N

30'N

40 'W

1910-1920 1920-1930 1930-1940 1940-1950

30'W 20'W 10'W 0' 40'W 30'W 20'W 10'W 0' 40'W 30'W 20'W 10øW 0' 40'W 30'W 20'W 10'W 0'

60'N

50'N

40'N

30'N

60'N

50'N

40'N

30'N

40'W

1950-1960 1960-1970 1980-1990

30'W 20'W 10'W O' 40'W 30'W 20'W 10'W O' 40'W 30'W 20'W 10'W O' 40'W 30'W 20'W 10'W O'

Figure 3b. Spatial distribution of stations in the eastern North Atlantic for selected decades, as marked.

60'N

50'N

40øN

30'N

25,990 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

b

Figure 4. (a) and (b) Salinity fields plotted without masking for two representative cross sections. (c) and (d) Same fields as in Figures 4a and 4b but with data gaps shown.

have chosen to show the smoothed fields in this paper to facilitate the representation of the flow and its continuity. However, the fields we present have been selected only in the case that they are representative of the masked property fields. Because of this choice, the sections we show in this paper are not always evenly spaced, rather they are selected to maximize data density and representativeness. The locations of each of the 46 cross sections discussed in this paper are shown in Figure 5.

4. Results

In the following subsections the Mediterranean outflow sig- nal is analyzed on the basis of the climatological salinity sig- nature of the Mediterranean Water in the eastern North At-

lantic. An overall view of the salinity signal in the Gulf of Cadiz is given in section 4.1, followed by a collection of meridional and zonal cross sections of density and salinity fields and isopy- cnal maps of pressure, salinity, and oxygen. These salinity and density fields are investigated in sections 4.2-4.5 to establish the continuity of the Mediterranean signal into the open At- lantic. Our discussion begins at the Strait of Gibraltar and proceeds downstream. A schematic (Plate 2), presented as part of section 5, provides a large-scale distillation of our results. To place the following discussions (sections 4.1-4.5) of local areas

in a larger context, the reader may find the schematic of some aid.

4.1. Plan View of Salinity Signal

Salinity fields for eight isopycnals, which span the depth of the Mediterranean outflow (from •o.5 = 29.40 at -500 dbar to •o.5 -- 30.10 at -1400 dbar), have been used to compose a high-resolution plan view of Mediterranean Water spreading into the Gulf of Cadiz (Plate 1). Salinities >36.25 psu, which approximately define the minimum limits of Mediterranean Water in the vicinity of its outflow [Baringer and Price, 1997], are marked on the map. Specifically, for each isopycnal surface all 0.1 ø grid elements that have a salinity >36.25 psu have been assigned a color that corresponds to the depth of that mea- surement. The isopycnal surfaces are then stacked one on top of the other, with the deepest isopycnal at the bottom and the shallowest on the top. This arrangement creates a plan view of the spreading of the salt signal. As seen in Plate 1, the high- salinity Mediterranean outflow leaves the Strait of Gibraltar and flows over the Camarinal and Spartel Sills as a trapped vein. The outflow, sinking rapidly, turns northward at -35ø40'N and 6ø40'W and then follows the 400 m isobath to -6ø50'W and 36ø20'N. The northward turn of the outflow has

been attributed to topographic steering [Ochoa and Bray, 1991] and to a Coriolis deflection JAmbar and Howe, 1979; Baringer

IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 25,991

!

35 øW

Figure 5. Plan view of the location and span of all cross sections presented and discussed in section 4.

and Price, 1997; Ochoa and Bray, 1991]. Further downstream, the northern edge of the outflow descends slightly and follows the isobaths between 500 and 700 m, while the southern edge descends below 1000 m. The onshore, shallow portion of the outflow is commonly designated as the "upper core," while the offshore, deep portion is designated as the "lower core" [Zenk andArmi, 1990; Daniault et al., 1994; Bower et al., 1997]. As the flow follows the continental slope, the plume widens, reaching its maximum width in the Gulf of Cadiz of over 50 km at

7ø30'W. This width is in rough agreement with the Baringer and Price [1997] estimate of 65 km at this site, which they also describe as a local maximum. The agreement between the synoptic and climatological jet's width presumably attests to the strong boundary trapping of this outflow. From this loca- tion of maximum width both cores narrow in the horizontal

plane as the flow approaches Cape St. Vincent. Even though the data are sparse near Cape St. Vincent, the vein of Medi- terranean Water turning northward along the Iberian Penin- sula, at a depth below 900 m, is evident.

4.2. Gulf of Cadiz Observations

4.2,1, The Mediterranean outflow subdivision into two

cores, Tracking the signal of Mediterranean Water through the eastern Gulf of Cadiz is facilitated by a sequence of three meridional cross sections of salinity starting near the Spartel Sill at 6ø30'W and progressing westward at an interval of 20' (Figure 6). Five isopycnals, chosen to represent the upper (O'o.s = 29.50), middle (O'o. s = 29.70 and O'o. s = 29.80), and lower (O'o. s = 29.90 and O'o. s = 30.00) portion of the Mediter- ranean outflow, are superposed on the salinity fields for this and all subsequent cross sections.

The cross section at 6ø30'W (Figure 6a) shows the outflow just downstream of the Spartel Sill. The outflow is bottom- trapped, lying between ---400 m and the bottom (at ---600 m) and has a maximum salinity of 38.00 psu. The outflow main- tains a fairly constant thickness of ---250 m, from section 6030 ' to ---6ø50'W (Figure 6b), but by 7ø10'W it has thickened to over 500 m. The maximum salinity decreases to 37.60 psu by 6ø50'W

25,992 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

and to 37.00 psu by 7ø10'W. In addition to a freshening and thickening of the plume as it moves downstream, the sequence of cross sections shows the plume gaining buoyancy and, by 7ø10'W, existing as two connected but distinct cores. In sum, over a horizontal distance of--•100 km the climatological plume is diluted, thickens, and is vertically differentiated.

There are three main hypotheses for the subdivision of the Mediterranean outflow into two vertical cores. First, Madelain [1970] suggested that variable bottom topography and subma- rine canyons are responsible for subdividing the outflow along different paths. Second, Siedler [1968] and later Zenk [1975] proposed that tidal mixing in the Strait of Gibraltar produces waters with two distinct peaks in temperature and salinity re- lationships that then move out onto the continental slope. Finally, the Gulf of Cadiz Experiment data indicated that two distinct cores of outflow evolve through differential mixing within the Gulf of Cadiz [Baringer, 1993; Baringer and Price, 1997]. Because the climatological flow exits the strait as a single, dense plume, the second hypothesis is apparently not supported by our work. However, it must be kept in mind that spatial and/or temporal averaging of the flow field could create a smoothing of two cores such that they could not be distin- guishable. This possibility, however, is weakened by the fact that the use of the same smoothing scales downstream does not prevent the appearance there of separate, discernable cores. However, the difference rests on the relative vertical distance between the cores at each site and on the strength of the temperature and salinity differences at each site. Further sup- port for the argument that two cores are not exported from the Strait of Gibraltar is provided by an analysis of the historical O-S relationship, which does not show any bimodality.

While our analysis cannot discern the effect of bottom to- pography on the evolution of the cores, our analysis does suggest, as will be argued, that the Mediterranean outflow could be subdivided into two cores as a result of differential

mixing with North Atlantic waters. The use of the climatolog- ical salinity field allows for a study of the Mediterranean out- flow in the context of the surrounding waters. As seen in Figure 6, fresh North Atlantic waters overlie the Mediterranean out- flow along the southern Iberian coast. Additionally, relatively fresh waters (--•35.6 psu), centered at --•600 m, are found at the outflow's southern boundary, as seen at the southern edge of each cross section in Figure 6. The source of this low-saline water at the southern edge of the Mediterranean outflow is evident from the maps of salinity and oxygen on the isopycnal surface, 0-0. 5 = 29.50, which lies at a nominal depth of 500 m in the Gulf of Cadiz (Figure 7). Low-oxygen waters extend west- ward into the open Atlantic from the northwestern African coast, centered along 14øN [Reid, 1994; Lozier et al., 1995]. Along the African coast, north of 25øN, the oxygen isopleths are directed northward along the coastline, suggesting the ex- istence of a poleward current advecting waters low in salinity (--•35.60 psu) and oxygen (--•3.5 mL L -h) along the continental slope (Figure 7 inset). The continuation of this boundary cur- rent into the Gulf of Cadiz is supported by the southward rise of the isopycnal surface 0-0. 5 = 29.50 near the African conti- nental slope, seen on the meridional section at 7ø10'W. A poleward flowing undercurrent off northwest Africa was first noted by Wooster and Reid [1963], and nearly two decades later, Tomczak and Hughes [1980] reported the existence of a core of fresh water of southern origin advected poleward along the continental slope at several latitudes. They found traces of southern water north of 20øN at depths down to at least 600 m.

Meridional cross sections in the eastern Gulf of Figure 6. Cadiz showing the Mediterranean outflow subdivision into two cores. The background waters are contoured with dashed lines with an interval of 0.1 psu. Salty waters are contoured with shading, every 0.05 psu, starting with a minimum salinity of 36.0 psu. Dashed shaded lines designate areas where there are data gaps. The isopycnals, 0-0. 5 = 29.50, 0-0. 5 = 29.70, 0-0. 5 = 29.80, 0-0. 5 = 29.90, and 0-0. 5 = 30.00, are designated by the solid lines in this and all subsequent cross sections.

IORGA AND LOZIER: MEDITERRANEAN OUTFLOWmSALINITY AND DENSITY FIELDS 25 993

11 øW 10øW 9øW 8øW 7øW 6øW

Estremadura

39ON Promontory 39øN

IBERIAN

PENINSULA

38ON I •, 38ON .• Cape St. Vincent

! / CapeSt. Maria I 37øN 37øN 400 rn •

• •00 ß •'0o, •

[' Gorringe 36 øN Bank ' 36 øN

Gulf of Cadiz

35øN 35øN

11 øW 10øW 9øW 8øW 7øW 6øW

1500 m 900 rn 100 rn

Plate 1. Plan view of the Gulf of Cadiz, showing the spreading of Mediterranean Water with a salinity greater than 36.25 practical salinity units (psu). The depth of salinity is indicated by the color code.

The existence of this undercurrent as far north as 32øN at -700

m has been reported by Barton [1989]. The depth of the cli- matological boundary current, at 600 m, is within the range reported from synoptic studies of this current [Barton, 1989]. Thus we hypothesize that the source of the fresh waters at this depth in the Gulf of Cadiz is the boundary current along the African coast.

Downstream changes in salinity indicate mixing between the outflow waters, the overlying surrounding waters, and those to the south. The 36.00 psu salinity isopleth (the outer shaded contour defining the plume of Mediterranean waters) lies on the isopycnal surface fro. s = 29.50 at 6ø30'W (Figure 6a), but changes its position to fro. s = 29.70 by 6ø50'W as the outflow penetrates into the Gulf of Cadiz (Figure 6b). At -6ø50'W and 36øN (Figure 6b) the Mediterranean outflow reaches the same depth as the poleward flowing fleshwater. The meridional sec- tion at 6ø50'W indicates the intersection of the fresh Atlantic

water with the southwestern edge of the Mediterranean out-

flow, an intersection that allows for horizontal, cross-isopycnal mixing. The upper waters are simultaneously mixing with the overlying waters derived from the open Atlantic. This differ- ential mixing is one potential mechanism, as explained by Bar- inger and Price [1997], for the creation of two cores from the Mediterranean plume, observed at 7ø10'W. Additionally, at this site the upper core now has a maximum salinity of-36.3 psu, and the lower core has a maximum salinity of-37.0 psu.

4.2.2. The Mediterranean outflow in the western Gulf of

Cadiz. Westward penetration of the Mediterranean outflow along the continental slope in the Gulf of Cadiz is represented by a sequence of six meridional cross sections in Figure 8. For a better understanding of the vertical distribution of the salin- ity signal the same five isopycnal surfaces as in Plate 1 are superposed on the salinity fields. The outflow reaches its max- imum width of over 50 km at -7ø30'W, before it encounters the steep slope of Cape St. Maria (at 8øW). Three salinity maxima, at -400 and 900 m and a new one at -1200 m, are

25,994 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

80øW 60'W ..•O'W 40'W 30'W 20'W 10'W O' 70 'W ttO'W 50'W 40'W 30'W

10*W 9'W 8*W 38'N

10'W

38'N

lOW

70'W 60'W $O'W 40'W 30'W 20'W 10'W O*

10'N

10'W 9'W 8*W 7*W

3TN

36W

Slit

Figure 7. Pressure, salinity, and oxygen in the North Atlantic on the isopycnal surface cro. s = 29.50. Insets show details of the oxygen and salinity fields.

IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 25,995

d

-> No• -> North

Figure 8. Meridional cross sections in the western Gulf of Cadiz. Contouring is as used in Figure 6. The solid lines designate the isopycnals, as in Figure 6.

found from 7025 ' to 7ø40'W. It is possible that the coexistence of the three local salinity maxima is created by the temporal variability of the flow; however, an examination of the data does not support this possibility. Instead, these cores appear to be climatological manifestations of cores observed from syn- optic surveys [Zenk, 1975; Ambar, 1983; Hinrichsen and Rhein,

1993]. Zenk [1975] noted the existence of a slope branch at this site with two vertical salinity maxima at -400 and 700 m [Zenk, 1975, Figure 3] and a main branch found between -7ø30 '- 7ø40'W and 36ø10'-36ø20'N at -1000 m (moorings 20 and 22) [from Zenk, 1975, Figures 4 and 7]. These depths approxi- mately match those found from the climatological representa-

25,996 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

tion. Of note in Figures 8a and 8b is the southward penetration of the lower core's salinity signal across the Gulf of Cadiz. This extended deep salinity maxima could result from the presence of a jet-like branch of dilute Mediterranean Water that is created by a sharp canyon upstream of 7ø25'W at 36øN and 7ø08'W [Zenk, 1975; Baringer, 1993; Baringer and Price, 1997]. Alternatively, the southward penetration could result from lat- eral mixing induced by strong velocities associated with the southern edge of the main branch [Zenk, 1975]. It is noted that as the outflow turns sharply westward following the continental slope around 7øW, meridional sections and the stream's axes are not orthogonal; therefore events from upstream/down- stream of one meridional section could be averaged onto the same section. Such averaging could explain why a jet-like branch of Mediterranean Water observed at -7ø08'W and

36øN [Zenk, 1975] is not detected on the 7ø10'W climatological representation of the outflow (Figure 6c), yet its dilute saline signal appears downstream from the formation site (Figure 8) as a deep salinity maximum centered on the isopycnal surface •ro. s = 30.00.

As reported by Daniault et al. [1994], two salinity maxima are sometimes encountered on a single vertical profile downstream of Portimio Canyon. The climatological fields (Figures 8d and 8e) also display two salinity maxima in this locale. At approx- imately this longitude the lower core of the outflow detaches from the continental slope, becoming neutrally buoyant at -1100-1200 m. From Figure 8f it is apparent that the upper core is flowing along the continental shelf with its maximum salinity located slightly below •ro. s = 29.80 (-850 dbar), in agreement with synoptic measurements [Daniault et al., 1994]. The lower core, carrying a substantially larger volume of Med- iterranean Water, is centered on the isopycnal surface •ro. s = 30.00 (---1250 dbar), the deepest isopycnal shown in Figure 8. This deep isopycnal is approximately equivalent at this site to the isopycnal •r• = 32.25, which was used as the targeted isopycnal for the deployment of floats used in A Mediterra- nean Undercurrent Seeding Experiment (AMUSE) [Bower et al., 1997]. In sum, three salinity maxima, at 400, 900, and 1200 m, are detected from 7025 ' to 7ø40'W; yet by Cape St. Maria, only two salinity maxima, centered at ---850 and 1250 m, are found.

4.2.3. The eyeIonic circulation of Mediterranean Water south of 36øN. A sequence of four meridional sections be- tween 9030 ' and 12ø30'W (Figure 9) and three zonal sections at 33050 ', 34030 ', and 35øN (Figure 10) are used to describe the Mediterranean signal south of 36øN. These cross sections also have five isopycnal surfaces superposed on the salinity fields (•ro. s = 29.50, •ro. s = 29.70, •ro. s = 29.80, •ro. s = 29.90, and •ro. s = 30.00). The deepest isopycnal surface, •ro. s = 30.00 (equivalent to % = 32.25), has been defined as the location of the lower (main) core of the Mediterranean outflow in the western Gulf of Cadiz [Bower et al., 1997; Arhan et al., 1994].

In the meridional section at 9ø30'W (Figure 9a), where Med- iterranean Water has two local salinity maxima, 36.15 psu at -35ø30'N and 36.05 psu at -34ø30'N, the isopycnal slope indicates a local cyclonic flow, assuming a shallow level of no motion at •ro. s = 29.50. Similar elongated salinity signals, as- sociated with the bowling of the isopycnal, can be identified in the zonal sections at 35øN and, substantially diluted, at 34030 ' and 33ø50'N between -10030 ' and 8øW (Figures 10a-10c). This apparent cyclonic gyre is centered at -35øN and 9øW and has a diameter of ---250 km (Figure 10c). Daniault et al. [1994] also found southward geostrophic flow along 36øN, south of the

saddle that connects the Tagus and Horseshoe Abyssal Plains. They described a southward recirculation of Mediterranean Wa- ter, with a diameter of -150 km, in this area. The slight difference in size and location between the climatological feature and Da- niault et al.'s [1994] observation might be the result of discrepan- cies between synoptic and climatological data.

4.3. Mediterranean Water in the interior

of the North Atlantic

The sequence of cross sections used in section 4.2 is further analyzed to detail the path of the Mediterranean Water into the interior of the North Atlantic. Additional information is

provided by a sequence of six meridional sections extending toward the interior of the North Atlantic (Figure 11) and six zonal sections spanning the Tagus Basin (Figure 12). The same five isopycnal surfaces used in section 4.1.1 and Figure 6 are superposed over the salinity fields for all these sections.

4.3.1. The Mediterranean Water in the Tagus Basin. The meridional sections in Figure 9 show the northward penetra- tion of Mediterranean Water along the western Iberian shelf, concentrated at 1100-1200 m. Also apparent from these sec- tions is the dilution of the salinity signal (-0.15 psu) from Cape St. Vincent to Cape Finisterre, a distance of -400 km. The salinity maximum at -37øN and 1200 m, shown in Figure 9a, can be traced westward, appearing in the meridional sec- tions 10ø30 ', 11ø30 ', and 12ø30'W. The plume is diluted west- ward by -0.15 psu (from 36.3 to 36.15 psu) over a horizontal distance of -200 km (Figures 9b, 9c, and 9d). This westward penetration of Mediterranean Water along the northern flank of the Gorringe Bank is consistent with a westward current (6 cm s -•) north of Gorringe Bank, reported by Daniault et al. [1994] and by Maz• et al. [1997].

Another feature of interest in the sequence of cross sections in Figures 9 and 11 is the appearance of another Mediterra- nean core between 39 ø and 39ø30'N from 10ø30 ' to 18øW. The

distinction between this core and the branch of Mediterranean

Water that flows westward along the Gorringe Bank is partic- ularly evident at 12ø30'W. Here the core at -39øN can be clearly identified and distinguished from the generally diffuse salinity signal from 35 ø to 41øN. That the salinity maximum defining this second core is located on the same isopycnal surface (rr0. s = 29.90) as the Mediterranean core penetrating westward along the northern flank of the Gorringe Bank and that this core is less saline than the westward core suggest an anticyclonic turning of the westward flowing Mediterranean Water. This anticyclonic pathway creates a "reservoir" of Med- iterranean Water in the Tagus Basin, similar to Daniault et al.'s [1994] description. A more detailed analysis of the Mediterra- nean Water pathway west of Iberian Peninsula will be provided in section 4.3.3, where the possibility of a mean westward Mediterranean flow across the Atlantic is investigated.

4.3.2. The Mediterranean Water exit from the Tagus Basin into the Horseshoe Basin. On the basis of observations from

the Bord-Est 3 program, Daniault et al. [1994] suggested that Mediterranean Water flows northwestward into the Tagus Ba- sin, circulates cyclonically, and then exits the basin southward, west of the Gorringe Bank. Float trajectories from AMUSE [Bower et al., 1997] illustrate that this southward deflection out of the Tagus Basin is a preferred Meddy pathway. Our analysis also finds evidence for a southward exit of Mediterranean

Water from the Tagus Basin, although this exit is believed to be associated with an anticyclonic turning of the waters in the

IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 25,997

½

4i000

b

d

Figure 9. Meridional cross sections in the eastern North Atlantic basin from 9030 ' to 12ø30'W. Contouring is as used in Figure 6, except that the minimum salinity contour plotted with shading is 35.90 psu. The solid lines designate the isopycnals, as in Figure 6.

Tagus Basin, rather than a cyclonic turning. Such a circulation is supported by the diagnostic model presented in part 2. Ev- idence for the southward Mediterranean Water/Meddy path- way comes from the climatological signal of high salinity cen- tered at ---15øW (Figure 10c) and at ---14øW (Figure 10b) along 34ø30'N. Such an exit can explain these salinity maxima as well as the local salinity maximum centered at ---11ø30'W on the zonal section at 36ø15'N (Figure 12b). The southward pene- tration of this water is apparently influenced by the Ampere Bank (located along 35øN latitude), which has a seamount that reaches to the Mediterranean outflow at 13øW (Figure 10c). A series of zonal cross sections south of this locale does not

exhibit a continuous signal of southward penetrating Mediter- ranean water. Thus we are led to believe that this southward

exit is principally reserved for Meddies and does not constitute a mean advective pathway.

4.3.3. Westward penetration of Mediterranean Water into the North Atlantic. A branch of Mediterranean Water also

exists along the southern flank of the Gorringe Bank, as de- scribed in this section. The elongated 36.0 psu contour on the 35ø50'N (Figure 12a) and 36ø15'N (Figure 12b) sections illus- trate this branch. The meridional section at 14øW (Figure 11a)

exhibits two salinity maxima, south and north of the seamount that defines the northern wall of the Horseshoe Abyssal Plain. We can associate the southernmost maximum with a westward

flow along the southern flank of the Gorringe Bank, while the northernmost maximum is assigned as its counterpart along the northern flank of the Gorringe Bank. The convergence of these branches into a single branch centered on --•36øN is evident on the meridional sections at 15 ø and 18øW (Figures lib and 11c). The fate of this outflow branch is of particular interest as it represents a potential advective pathway for Med- iterranean Water into the interior of the Atlantic, as suggested by Reid [1994]. Our climatological analysis, however, suggests that this branch does not penetrate much past 22øW, as evi- denced by the extremely weak signal at 26øW (Figure lie) and 30øW (Figure 11f). As mentioned earlier, it is believed that these waters turn anticyclonically within the Tagus Basin to rejoin the branch of northward flowing Mediterranean Water near the Estremadura Promontory. Evidence for this supposi- tion is found in the cross sections of Figures 11 and 12. The elongated tongue on 36ø15'N (Figure 12b) exhibits a local maximum at 15ø30'W that can be associated with the local

salinity maxima also at 15ø30'W on the 36055 ' and 37ø15'N

25,998 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

Figure 10. Zonal cross sections showing the cyclonic recir- culation of the Mediterranean Water, south of 36øN. The con- tour intervals for the background waters and for the Mediter- ranean outflow are the same as Figure 6 with 35.90 psu as the minimum salinity plotted with shading. The solid lines desig- nate the isopycnals, as in Figure 6.

zonal sections (Figures 12c and 12d). This signal suggests that the branch of Mediterranean Water present along the south- ern flank of the Gorringe Bank turns cyclonically after -15ø30'W. The distribution of salinity maxima on the merid- ional sections at 14 ø, 15 ø, and 18øW supports this view. The local salinity maximum centered at -39øN on all these sections is believed to indicate the return pathway of the westward branch defined by the local salinity maximum centered at •37øN. The western limit of this anticyclonic deflection of the Mediterranean Water is seen at •22øW (Figure lid) as a single local salinity maximum of 35.7 psu between 37 ø and 39øN. The cross sections west of this site (Figures lie and 11f) show a generally diffuse salinity signal (35.5 psu) rather than a distinct core of Mediterranean Water. Our interpretation of this spatial change in the salt core is that the salinity signal carried westward beyond 20øW is mainly spread by diffusive processes rather than by a direct advective pathway. Addition- ally, salt could be spread westward by Meddies as Arhan and King's [1995] conceptual model suggests. Arhan and King [1995] found that turbulent mixing by mesoscale eddies is the main cause of the observed large-scale westward penetration of Mediterranean salt. Our conclusion of no advective pathway past •20øW for waters emanating from the Strait of Gibraltar is in agreement with the circulation estimated by Maillard [1986] and by Paillet and Mercier [1996], and it is supported by results of the diagnostic model in part 2.

4.4. The Fate of Mediterranean Water North

of the Estremadura Promontory

A sequence of 10 zonal sections and six meridional sections that span the eastern North Atlantic basin north of 40øN are collected in Figures 13, 14, and 15. The climatological salinity fields north of 39ø25'N (Figures 13a-13f) show a divergence of the flow that could be associated with a divergence of the isobaths south of Galicia Bank. A comparison of the salinity field at 39ø25'N (Figure 12f) to that at 40ø50'N (Figure 13a) shows a westward spreading of the salinity signal as this branch moves northward. Apparently, such spreading contributes to a bifurcation of the flow south of the Galicia Bank, as is evident in the sequence of sections in Figure 13. The bifurcation, with branches on either side of Galicia Bank, is exhibited most clearly in the 42ø50'N section. The offshore, western branch is visible between 15 ø and 11ø30'W, with salinities of 35.8-35.85 psu, while the eastern branch remains trapped along the con- tinental shelf, preserving its higher salinity of more than 36.00 psu. The western branch of Mediterranean Water, more dilute than the along-shelf flow, is the climatological manifestation of a branch observed to the west of Galicia Bank by Daniault et al. [1994]. The weaker salinity signal carried by this western branch, relative to the shelf branch, is possibly due to its longer advective pathway from the Strait of Gibraltar and/or due to stronger lateral mixing processes. North of the Galicia Bank (43006 ' and 43ø30'N sections, Figures 13e and 13f, respective- ly), the salinity field shows a convergence of the western and eastern branches into a wide vein.

The zonal section at 44ø30'N (Figure 13f), which falls along the northern shelf of the Iberian Peninsula, and the six merid- ional sections within the Bay of Biscay (Figures 14a-14f) show the eastward turn of the Mediterranean Water into the Bay of Biscay. West of Cape Ortegal (section 9øW, Figure 14a) the maximum salinity signal is offshore. As the Cape Ortegal shelf protrudes into the ocean (section 8ø30'W, Figure 14b), the Mediterranean Water vein is trapped again near the upper

IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 25,999

C

'"' 11 'x

d

30 3• 32 33 34 35 3• •C/ 3t• • 40 41} 4• 4S 44 45

e

Figure 11. Meridional cross sections in the eastern North Atlantic basin from 14 ø to 30øW. Contouring is as used in Figure 9. The solid lines designate the isopycnals, as in Figure 6.

slope, flowing eastward along the northern Iberian coast (sec- tions 7055 ' to 5ø10'W, Figures 14c-14f). The dilution of the local maximum salinity from ---35.9 psu at 7ø55'W (Figure 14c), near the Iberian shelf, to ---35.8 psu at 5ø10'W (Figure 14f) indicates the eastward penetration of the flow along the north- ern Iberian shelf.

In the Bay of Biscay, north of 46øN (section 46ø06'N, Figure 15a, and section 47øN, Figure 15b), salinity maxima are found near the shelf at ---6øW and offshore at ---8øW and at 11-

12ø30'W. The isopycnal slopes in these sections, from ---6 ø to 8øW, are consistent with a local cyclonic recirculation of Med- iterranean Water in the Bay of Biscay, if a shallow level of no

26,000 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

d

o

East e

o

East --> East

40 -9 .,•

f

o ;35.8

-> East *-> East

Figure 12. Zonal cross sections showing Mediterranean Water pathways west of the Iberian Peninsula. Contouring is as used in Figure 9. The solid lines designate the isopycnals, as in Figure 6.

(slow) motion is assumed at cro. 5 = 29.50. It is believed that the two salinity maxima at 12030 ' and 11øW in the 46ø06'N zonal section (Figure 15a) result from the presence of Meddies at these locales. This supposition is deduced from the fact that

these salinity maxima are isolated and found only during the early 1950s. Meddies in this vicinity are not as common as those found to the south and west of Gibraltar, but some have been noted. In a study of historical data, Richardson et al.

IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 26,001

• a 60'N

35'W

Plate 2.

upper core of MW

lower core of MW

reservoir of MW

preferred Meddy pathways

conjectured pathways

possible flow of non-Mediterranean water

Rockall Plateau

Porcupine Bank

.. . 55ON

50'N

45'N

IBERIAN PENINSULA I 40'N

I 35'N

AFRICA • •

" ! ß -. I -- • ,,,, ,,,, ,,,, • • • a25'N

30'W 25'W 20'W 15øW 10øW 5øW 0 ø

Composite summary of the Mediterranean Water pathways in the eastern North Atlantic basin.

[1991] found 3 Meddles in the region bounded by 44ø-46øN and 15ø-18øW. In regions of low data density a mean may be skewed by the presence of a Meddy.

As the continental shelf protrudes into the ocean basin north of 47øN, Mediterranean Water penetrates poleward, confined along the continental slope. The northern edge of the cyclonic recirculation of Mediterranean Water in the Bay of Biscay is also identified offshore in this zonal cross section. Farther

north, the salinity signal can be traced along the continental slope up to 48ø30'N (Figures 15c and 15d). However, the salinity maximum is substantially diluted from more than 35.7

psu at 47øN to 35.675 psu at 47ø30'N and to 35.65 psu at 48ø30'N (Figures 15b, 15c, and 15d). Past 48ø30'N there is no longer a strong distinct core of Mediterranean Water in the climatological mean; yet a weak salinity signal can be traced to ---50ø20'N, as shown in the 14øW cross section (Figure 16b). This alongshore climatological salinity signal, found near Por- cupine Bank, is confirmation of the northward penetration of Mediterranean Water reported by Arhan et al. [1994].

The cross sections in Figure 16 also reveal a salinity maxi- mum to the south of the alongshore maximum. The steep slope of the isopycnals in the 11ø40'W meridional section (Figure

26,002 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

C

0

4•

d

f

Figure 13. Zonal cross sections in the eastern North Atlantic basin, north of Estremadura Promontory. Contouring is as used in Figure 6. The minimum salinity contour plotted with shading is 35.85 psu. The solid lines designate the isopycnals, as in Figure 6.

16a) associated with the two distinct salinity maxima indicates a possible bifurcation of the Mediterranean Water as it flows out of the Bay of Biscay. One branch is apparently deflected westward, while the other branch penetrates northward as a boundary current [Harvey, 1982]. As will be demonstrated in part 2, a diagnostic model of the eastern basin indicates that

the westward branch converges with the deep North Atlantic Current, thus eventually entering the Rockall Channel.

4.5. The Mediterranean Outflow: A Cross-Isopycnal Flow

Near the Strait of Gibraltar the isopycnal surface fro. s = 29.50 is approximately at the depth of the interface between

IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 26,003

a

c

d

40OO

44 45 4• 47 48

Figure 14. Meridional cross sections in the Bay of Biscay. The contour interval is 0.1 psu for the background waters and 0.05 psu for the Mediterranean Water. The minimum salinity contour plotted with shading is 35.70 psu. Two additional shaded contours (35.65 and 35.675 psu) are used to show the Mediterranean Water along the northern coast of the Bay of Biscay. The solid lines designate the isopycnals, as in Figure 6.

the inflow of North Atlantic waters and the outflow of Medi-

terranean Water. In the interior of the Gulf of Cadiz the main

core of the Mediterranean outflow (corresponding to the low- er-salinity maximum) is centered on the isopycnal fro. 5 = 30.00. Downstream, north of the Estremadura Promontory, the local

salinity maximum is shifted upward and centered on fro. s = 29.90, indicating a gain of buoyancy for the Mediterranean Water. Past this promontory, as the flow turns eastward, north of 45øN, the salinity tongue is found centered on the isopycnal surface fro. s - 29.80. A change in isopycnals from rr I = 32.23

26,004 IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS

46 ø 06'

->East

b

Figure 15. Zonal cross sections showing the fate of Mediterranean Water in the Bay of Biscay. The contour interval for the background waters is 0.1 psu. For the Mediterranean outflow the contour interval is 0.025 psu, and the minimum salinity plotted with shading is 35.7 (46ø06'N), 35.65 (47 ø and 47ø30'N) and 36.6 psu (48ø30'N). The solid lines designate the isopycnals, as in Figure 6.

(1200 dbar, equivalent to -0-0. 5 = 30.03) to o h = 32.12 (1000 dbar, equivalent to -0-0. 5 = 29.92) for the maximum salinity anomaly associated with the lower core of the Mediterranean Water is also described by Arhan et al. [1994]. They suggest a relationship between this change and the presence of the un- derlying Labrador Sea Water north of 44øN.

5. Summary A summary of this climatological analysis, given below, is

aided by a schematic of the pathway of the salinity signal from its source (Plate 2). The schematic represents the overall path- way of the climatological Mediterranean salinity signal based on an analysis of historical hydrographic data. It is essentially a composite of the 46 sections shown and discussed in this paper. As sketched, the Mediterranean Water enters the Gulf of Cadiz through the Strait of Gibraltar, below 150 m. It rap- idly sinks as it turns northward, flowing along the continental slope as a bottom trapped vein, lying between -400 m and the

bottom at -600 m. Downstream of the Spartal Sill, the clima- tological maximum salinity is 38.00 psu. The outflow of Med- iterranean Water maintains a fairly constant thickness of -250 m, from 6030 ' to -6ø50'W, but by 7ø10'W it has thickened to over 500 m. The maximum salinity decreases from 38.00 to 37.60 psu by 6ø50'W and to 37.00 psu by 7ø10'W. In addition to a freshening and thickening of the plume as it moves down- stream, the plume gains buoyancy and, by 7ø10'W, splits into two connected but distinct cores. Our analysis of the climato- logical data supports the hypothesis that two distinct cores of outflow evolve through differential mixing within the Gulf of Cadiz, one of the mechanisms suggested by Baringer and Price [1997]. The source of fresh waters involved in the horizontal, cross-isopycnal mixing process appears to be tropical waters, advected northward along the African continental slope into the Gulf of Cadiz at 550-650 m depth. Additionally, it is suggested that the upper layer of Mediterranean Water verti- cally mixes with the inflowing North Atlantic Central Water,

IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 26,005

b

Figure 16. Meridional cross sections showing the fate of Mediterranean Water south of Porcupine Bank. The contour interval for the background waters is 0.1 psu. For the Medi- terranean outflow the contour interval is 0.025 psu and the minimum salinity plotted is 35.575 psu for 11ø40'W and 35.525 psu for 14øW. The solid lines designate the isopycnals, as in Figure 6.

while the deeper layer mixes with the waters to the south. It is supposed that this differential mixing of the upper and lower layers of Mediterranean Water could produce the two clima- tological cores.

As the Mediterranean Water flows along the continental shelf from 7ø25 ' to 7ø40'W, three salinity maxima, at ---400, 900, and 1200 m, are found. It has been suggested that these cores result either from strong mixing processes associated with a jet-like branch of Mediterranean Water [Baringer and Price, 1997; Zenk, 1975], from mixing induced by a swift main branch [Zenk, 1975], or from topographic steering [Baringer and Price, 1997]. By Cape St. Maria the Mediterranean waters have converged again to two vertical salinity maxima centered at ---800 and 1200 m. These cores correspond to the two cores of Mediterranean outflow extensively mentioned in the litera- ture [Baringer and Price, 1997; Bower et al., 1997; Ochoa and Bray, 1991; Daniault et al., 1994]. The climatological salinity field in the western Gulf of Cadiz contains a signature of a

cyclonic recirculation that is centered at ---34ø30'N, 9ø30'W and is ---250 km in diameter. This recirculation acts to spread the salinity signal south of 34øN.

From Cape St. Vincent a part of the Mediterranean outflow turns northward and enters the Tagus Basin through Zenk and Armi's [1990] gateway. Within this basin the climatological flow diverges, with part of the water flowing northward along the Iberian continental slope and the other part deflected west- ward. The westward flow is divided by the Gorringe Bank into a southern and northern component. The collective westward branch turns anticyclonically within the Tagus Basin, creat- ing a reservoir of Mediterranean Water. After the anticy- clonic turn this branch rejoins the northward, shelf branch of Mediterranean Water west-northwest of the Estremadura

Promontory. From the Estremadura Promontory the Mediterranean Wa-

ter penetrates northward along the continental slope and di- verges downstream into two branches at Galicia Bank. North of Galicia Bank, the two branches converge and mainly turn eastward into the Bay of Biscay, following the northern Iberian slope. The Mediterranean Water branch reaches Porcupine Bank after it has penetrated poleward along the continental shelf and after it has been partially recirculated in the Bay of Biscay. Its saline signal is tracked as far north as 50ø20'N. Finally, we speculate that south of the Porcupine Bank, a branch of Mediterranean Water is deflected westward until it

converges with the North Atlantic Current, while the rest of it continues its northward penetration as a boundary current into the Rockall Channel. Additional support for this hypothesis will be provided in part 2, which provides results from a diag- nostic model of the ocean circulation in the eastern North

Atlantic basin.

The main contribution of this work is the placement of the Mediterranean outflow signal in a broad temporal and spatial context. The strength of the climatological signal lies in its spatial continuity. Though this continuity is compromised to a degree by nonuniform data density, the overall signal is not seriously affected. The fields shown here are resolved with a grid spacing of no more than 0.5 ø , far exceeding the expected length scale of climatological flows. Specific main contributions from this work are the verification of the northward penetra- tion of the Mediterranean Water into the Rockall Channel and

the suggestion of their possible penetration poleward over the sill of the Wyville-Thomson Ridge. Additionally, this analysis, in conjunction with the diagnostic modeling work presented by Iorga and Lozier (submitted manuscript, 1999) suggests that the westward penetration of Mediterranean Water past ---20øW is principally diffusive on climatological scales.

Appendix

The climatological data were grouped into four time peri- ods, each containing 2 decades. Data coverage at each grid in a given cross section is indicated by a gray shade that corre- sponds to its measurement date. Because the time periods are superposed on each other (beginning with the most recent period), some data are partially covered. A data coverage section is provided for each cross section shown in section 4 either in Figures A1, A2, or A3. Color images for all figures can be accessed at www.eos.duke.edu/Faculty/Lozier/lozier. html.

7o25 ' W

B •

Figure A1. (a) The temporal coverage of the cross sections presented in Figure 6. The two-decade time periods are color coded as follows: 1970-1990, solid; 1950-1970, dark shading; 1930-1950, light shading; and 1910-1930, medium shading. The dots have been sized to give additional information on the temporal span of the data. The size of the dot indicates when the measurement was made relative to the first measurement (smallest dot) and the latest measurement (largest dot) in each section. So that the data from one time period do not cover data from another time period the largest dots have been plotted first and the smallest last. (b) Same as for Figure Ala but for Figure 8. (c) Same as Figure Ala but for Figure 9. (d) Same as Figure Ala but for Figure 11.

IORGA AND LOZIER: MEDITERRANEAN OUTFLOW--SALINITY AND DENSITY FIELDS 26,007

-9600

4OOO

-,S -,7 -16 -,S -,4 4* .,2 -, -m -9 a -* -•

-> East

4 I; -14 43 -12 11 -10 -9 ß -7 4i

A -1, 47 -I, -->Ea6t

o

i - i - i - i - i ß i ß

=> Nod1 ->

Figure A2. Same as Figure A1 but for Figures (a) 10, (b) 12, (c) 13, and (d) 14.

26,008 IORGA AND LOZIER: MEDITERRANEAN OUTFLOWmSALINITY AND DENSITY FIELDS

... ':•: ?.:;;:.: .:}

11040 ' W

.

.: ,

ß :::

14 ø W

B _>•

Figure A3. Same as Figure A1 but for Figures (a) 15 and (b) 16.

Acknowledgments. Support from the National Oceanic and Atmo- spheric Administration (NA 46GPO184) and the National Science Foundation (OCE-96489) is gratefully acknowledged.

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M. C. Iorga, Department of Mechanical Engineering and Material Science, Duke University, Durham, NC 27708.

M. S. Lozier, Earth and Ocean Sciences, Duke University, Durham, NC 27708. (s.lozier@duke.edu)

(Received January 8, 1998; revised February 15, 1999; accepted February 24, 1999.)