The hydrography of the mid-latitude northeast Atlantic Ocean

Deep-Sea Research I 47 (2000) 757}788

The hydrography of the mid-latitude northeast Atlantic Ocean I: The deep water masses Hendrik M. van Aken* Netherlands Institute for Sea Research, P.O. Box 59, Den Burg/Texel, The Netherlands Received 22 December 1997; received in revised form 2 April 1999; accepted 7 September 1999

Abstract The circulation of the deep water masses in the mid-latitude northeast Atlantic Ocean was studied by analysis of the distributions of potential temperature, salinity, dissolved oxygen, phosphate, nitrate, and silicate. Pre-formed nutrients were used to allow a quantitative description of the deep water masses, especially the Northeast Atlantic Deep Water, in terms of four local source water types: Iceland}Scotland Over#ow Water, Lower Deep Water, Labrador Sea Water, and Mediterranean Sea Water. Over the Porcupine Abyssal Plain between 2500 and 2900 dbar Northeast Atlantic Deep Water appears to be a mixture of mainly Iceland}Scotland Over#ow Water and Labrador Sea Water (&80%), with minor contributions of Lower Deep Water and Mediterranean Sea Water. When the Northeast Atlantic Deep Water re-circulates in the north-eastern Atlantic and #ows southwards towards the Madeira Abyssal Plain, contributions of the former two water types of northern origin diminish to about 50% due to diapycnal mixing with the overlying and underlying water masses. The observed meridional and zonal trends of dissolved oxygen and nutrients in the Northeast Atlantic Deep Water appear to be caused both by diapycnal mixing with the underlying Lower Deep Water and by mineralization of organic matter. The eastward decrease of oxygen and increase of nutrients especially require considerable mineralization of organic matter near the European continental margin. At deeper levels (&4100 dbar), where the nutrient rich Lower Deep Water is found near the bottom, the meridional gradients of oxygen and nutrients are opposite to those found between 2500 and 2900 dbar. Diapycnal mixing cannot explain this change in gradients, which is therefore

* Fax: 0031-0-222-319674. E-mail address: [email protected] (H.M. van Aken) 0967-0637/00/$ - see front matter ( 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 3 7 ( 9 9 ) 0 0 0 9 2 - 8

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considered to be a qualitative indication of ageing of the Lower Deep Water when it #ows northwards. A considerable part of the Iceland}Scotland Over#ow Water and the Lower Deep Water that enter the northeast Atlantic may be removed by deep upwelling in the Bay of Biscay and eastern Porcupine Plain. ( 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction In this paper we study the deep circulation in the mid-latitude northeast Atlantic Ocean by analysis of a data set of temperature, salinity, oxygen and nutrient observations from the 1980s and early 1990s, collected between 313 and 533N, and east of 213W (Fig. 1). We focus on the composition, circulation and modi"cation of the North-East Atlantic Deep Water, characterized by a deep salinity maximum as it is observed over the Porcupine Abyssal Plain, and also of the underlying near bottom salinity minimum. We analyse the bio-geochemical properties for indications of ageing over the meridional succession of the Porcupine Abyssal Plain, the Iberian Abyssal Plain, and the Madeira Abyssal Plain. These deep basins are separated by the Azores-Biscay Rise and by the AzoresPortugal Rise. We also study the composition of the deep water mass in terms of source water types from potential temperature H and salinity S, together with pre-formed nutrients. The zonal variation of the hydrographic properties is analysed by comparison with data from the Biscay Abyssal Plain, the West Iberian Margin, and the Seine Abyssal Plain. The hydrographic results are then discussed, and conclusions on the deep water mass of the northeastern Atlantic and its deep circulation and mixing are formulated. Part of the deep branch of the global thermohaline circulation passes through the northeastern basin of the North Atlantic Ocean (Dickson and Brown, 1994). Cold water from the Norwegian Sea #ows through the Faroe-Bank Channel and across the Iceland-Faroe Ridge into the Iceland Basin. During its descent from the sills between Iceland and Scotland into the deep Iceland Basin this water initially entrains warm and saline Sub-Polar Mode Water. At later stages in the Iceland Basin cold and less saline Labrador Sea Water (LSW) entering the eastern North Atlantic basins near the Charlie-Gibbs Fracture Zone at &523N (Talley and McCartney, 1982) is entrained as well as the overlying upper parts of the cold low salinity Lower Deep Water (LDW; van Aken and de Boer, 1995; van Aken and Becker, 1996). By this warm and cold entrainment the Iceland Scotland Over#ow Water (ISOW) is formed in the northern Iceland Basin. About 3.5 Sv (1 Sv "106 m3/s) ISOW passes south of Iceland (Saunders, 1996; van Aken and Becker, 1996). The characteristics of this water mass change considerably due to diapycnal mixing, which decreases its density, when this water proceeds further south to 523N, the latitude of the Charlie Gibbs Fracture Zone (van Aken and Becker, 1996). There a deep water mass is observed, characterized by a salinity maximum at about 2600 dbar between the low salinity cores of LSW and LDW (van Aken and Becker, 1996) and designated Northeast Atlantic Deep Water (NEADW). With a multi-stage H}S analysis Harvey and Theodorou (1986) derived

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Fig. 1. The spatial distribution of the hydrographic stations with bio-geochemical data used in this study. At each station water samples with a potential temperature below 53C were collected. The symbols indicate the regional attribution of the stations to the di!erent deep basins and are explained in the legend. The water depth in m is indicated by isobaths.

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the presence of ISOW around the deep salinity maximum between 2000 and 2600 m in the southern Iceland Basin. They however did not pursue their analysis into the Porcupine Basin and further south. Lee and Ellett (1965) assumed that NEADW contains considerable amounts of ISOW. By analysis of salinity anomalies in the deep northeast Atlantic they were able to follow the NEADW core southwards to at least a latitude of 473W over the Porcupine Abyssal Plain. At that latitude the salinity signal due to the high salinity core of Mediterranean Sea Water prevented further identi"cation of the NEADW core. The abyssal water mass, LDW, is characterized by a low salinity and a low dissolved oxygen content, and especially by its high dissolved silica content (Mantyla and Reid, 1983; McCartney, 1992). This is assumed to be due to the contribution of Antarctic bottom Water (AABW) to the LDW when it enters the eastern Atlantic basins at the Vema Fracture zone near 103N (Mantyla and Reid, 1983; McCartney, 1992; Schmitz and McCartney, 1993). Throughout the northeastern Atlantic Basin the LDW core can be recognized by a near bottom salinity minimum and a silica maximum. When the LDW meets the pure ISOW core in the Iceland Basin the ISOW has a higher potential density than the coldest LDW over the deep Porcupine Abyssal Plain. This forces LDW to overlie the ISOW core in the relatively shallow northern Iceland Basin (van Aken, 1995; van Aken and Becker, 1996). The low salinity LSW core is found at about 1850 dbar in the northern parts of the northeastern Atlantic basins, overlying the deep levels where NEADW and LDW are observed. A shallower high salinity Mediterranean Sea Water (MSW) core, which originates from over#ow near Gibraltar of sub-surface water from the Mediterranean Sea, is found at about 1000 dbar in the more southern parts of the northeastern Atlantic (Tsuchiya et al., 1992; Arhan et al., 1994). Due to diapycnal mixing of the NEADW core with the overlying water its salinity therefore may either increase or decrease, depending on the geographical location. Tsuchiya et al. (1992) have shown that near 203W the salinity minimum, characteristic for the LSW core, disappears south of 413N due to mixing with the saline MSW. This results in an increase of the salinity at NEADW levels south of that latitude. Arhan et al. (1994) have observed the same feature south of 453N near 153W. This e!ect limits the determination of the southward extent of the retraceable NEADW core in the northeastern Atlantic when one uses only a H}S analysis (Harvey and Theodorou, 1986; Harvey and Arhan, 1988). Broecker et al. (1985) tried to determine the #ushing in the deep northeastern Atlantic from northern sources by means of an analysis of dissolved oxygen and nutrients in the cold bottom layers with a potential temperature H(2.63C. Given the contrast between ISOW with low nutrient and relatively high oxygen concentrations and LDW with lower oxygen and high nutrient concentrations a northward decrease of nutrients and an increase of oxygen is expected if ISOW also contributes to the ventilation of the deep northeastern Atlantic. Broecker et al. (1985) did not "nd any evidence for a meridional trend above the scatter of the observations in the deep nutrient and oxygen concentrations south of the Charlie Gibbs Fracture Zone at 523. This led them to conclude that all ISOW leaves the northeastern Atlantic towards the

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west through this gap in the Mid Atlantic Ridge and does not contribute to the deep water mass further to the south. The results of Broecker et al. (1985) also imply that in the northward #owing LDW no bio-geochemical ageing occurs. Also Harvey and Arhan (1988) conclude, from a H}S}O analysis of the TOPOGULF data, 2 that near the Mid-Atlantic Ridge the ISOW in#uence hardly extends south of the Charlie Gibbs Fracture Zone. They ascribe any deep salinity maximum south of 483N to the in#uence of MSW. Tsuchiya et al. (1992) have analysed a meridional section at 203W, and state that no ISOW in#uence is observed south of the Rockall-Hatton Plateau at about 523N. Instead they assume that the properties of the NEADW deep salinity maximum south of 523N are caused by isopycnal mixing of MSW with North West Atlantic Deep Water (NWADW), extending the in#uence of the over#ow of Arctic waters through Denmark Strait to the eastern Atlantic basins. The lack of observational evidence for a considerable southward extension of the ISOW in#uence south of the latitude of the Charlie Gibbs Fracture Zone contrasts the observation by Dickson and Brown (1994) and van Aken and Becker (1996) that more deep water enters the Porcupine Abyssal Plain than the amount of deep water that leaves this basin westwards through the Charlie Gibbs Fracture Zone. While the total in#ow of ISOW and LDW into the deep layers of the northeastern Atlantic amounts to about 4.9 Sv (van Aken and Becker, 1996), the mean westward out#ow through the Charlie Gibbs Fracture zone is estimated from long term current measurements to be only 2.4 Sv (Saunders, 1994). The explanation of these results requires a southward transport of ISOW across the latitude of the Charlie-Gibbs Fracture (523N) Zone, and a southward recirculation of LDW in the northeastern Atlantic. This will extend the in#uence of ISOW to latitudes south of 523N. A deep southward #ow along the Mid-Atlantic Ridge has been derived with an inverse model by Gana and Provost (1993). ISOW and LDW, transported southward in this #ow, may ultimately leave the eastern basins via deep gaps in the mid-Atlantic Ridge south of the Azores (the Oceanographer Fracture Zone at &343N and the Atlantic Fracture Zone at &303N). However no evidence for such a #ow scheme exists. A more probable alternative for the ultimate removal of ISOW and LDW from the north-eastern Atlantic is deep upwelling in the northward part of a recirculation cell (Stommel and Arons, 1960) and mixing with overlying waters (Munk, 1966). Such deep recirculation was observed by Paillet and Mercier (1997) from an inverse model of the eastern North Atlantic. At depths of 2500 to 3500 m they obtain a southward #ow along the eastern side of the Mid Atlantic Ridge, which feeds cyclonic recirculation cells over the Porcupine and Biscay Abyssal Plains, the Iberian Abyssal Plain, and the Madeira Abyssal Plain. Lacking in their results is a persistent northward #ow that brings LDW to the north, as proposed by McCartney (1992). Deep long-term current meter observations con"rm the deep cyclonic recirculation over the Porcupine Abyssal Plain and the Biscay Abyssal Plain, with a characteristic poleward velocity near the continental margin of 1.2($1.0) cm/s (Dickson et al., 1985). Additionally Harvey and Theodorou (1986) derived weak cyclonic recirculation of ISOW further to the north in the Iceland Basin.

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2. The data In order to study the water mass structure in the mid-latitude northeast Atlantic Ocean by means of the analysis of the distributions of potential temperature (H), salinity (S), dissolved oxygen (O ) and the dissolved nutrients phosphate (PO ), 2 4 nitrate (NO ) and silica (Si) we compiled a data set from recent hydrographic cruises 3 in the northeastern Atlantic during which these parameters were determined (Table 1). Additional to the cruises listed in Table 1, extra hydrographic observations were obtained from the ICES oceanographic data centre. All data are restricted to the period 1981 to 1995. This temporal restriction is used in order to reduce as far as possible aliasing of temporal changes to the spatial domain. For example Sy et al. (1997) reported that from 1991 onwards large volumes of newly formed LSW, characterized by values of hydrographic parameters di!erent from the previous period, have moved from the Labrador Sea towards the Iceland Basin. This new vintage of LSW is not yet observable in our data. The hydrographic data in our data set from the whole water column were quality controlled for internal regional consistency, and individual outliers were removed. In this analysis it appeared that the phosphate concentrations from RV Oceanus cruise 202 were systematically 6% lower than the concentrations from the other cruises. In order to get a homogeneous data set, the Oceanus phosphates were multiplied by a factor 1.06. All phosphate data from the 1989 Tyro cruise showed a very large scatter and were removed from the data set. Broecker et al. (1985) noted that the TTO-NAS oxygen data were systematically 2 to 3 lmol/kg lower than the oxygen data obtained during the GEOSECS programme. Comparison of the TTO-NAS oxygen data with the other data from our data set con"rmed that the oxygen values from TTO-NAS were relatively low. Therefore 2.5 lmol/kg was added to the TTO-NAS oxygen values. The "nal accuracy of the sub-sets within the overall data set is estimated to be 0.5 lmol/kg or larger for oxygen, nitrate and silica, and 0.05 lmol/kg for phosphate.

Table 1 The cruises from which hydrographic data were used for this study Research vessel

Cruise

Period

AtlantisII Knorr Hudson Oceanus Charcot Le Noroit Tyro Tyro Pelagia Pelagia Pelagia

36 N TTO-NAS leg 4 47 N 202 Bordest 2 Bordest 3 891 903 95N1 96N1 110

June}July 1981 June 1981 April 1982 July 1988 May}June 1988 May 1989 August 1989 June 1990 1995 1996 1997

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As a measure of the oxygen content independent of the temperature dependent solubility, the apparent oxygen utilization (AOU) is used, de"ned by AOU"O !O (1) 24!5 2 where O is the dissolved oxygen concentration, and O is the saturation concen2 24!5 tration in equilibrium with the atmosphere (Broecker and Peng, 1982). In order to study the hydrography of the deep water in the northeastern Atlantic, a subset of our hydrographic data was selected, characterized by potential temperatures below 53C. This subset contains 420 stations and is subdivided further into regional groups for the di!erent deep basins (Fig. 1). 3. Non-conservative and quasi-conservative bio-geochemical hydrographic parameters For the study of the large-scale slow circulation and mixing of water masses use of conservative or quasi-conservative parameters may be advantageous in a multiparameter water mass analysis, while the study of a tracer with a known decay rate, e.g. radioactive tracers, may be a method to determine the transport rate and #ow direction of water masses. Whereas salinity and potential temperature are conservative, the concentration, of e.g. dissolved oxygen is not conservative and will decrease due to ageing of sub-surface water masses while nutrient concentrations will increase. In the following sections we will consider such changes of non-conservative parameters as a qualitative measure of ageing, re#ecting only the characteristic direction of the #ow of a water mass. Because the oxygen consumption rate is not well known and may vary strongly in the vertical and horizontal direction, equivalent changes in AOU in a water mass do not necessarily imply equivalent ages in years or equivalent transport rates of the water mass. As Johnston (1977) has argued, the rate of oxygen consumption by "sh and zooplankton is not constant, but may approximately depend linearly on the supply of organic matter, generated by primary production in the surface layers, and more or less logarithmically with depth. Additional to the respiration by these organisms, oxygen consumption by bacterial activity and by benthic processes adds to the total oxygen decay rate. This makes an accurate estimate of the decay rate of oxygen valid for a water mass in a whole deep basin quite impossible at present. However, the consumption of oxygen in sub-surface water masses due to the mineralization of organic matter is connected, with nearly constant ratios, to the production of dissolved inorganic phosphate and nitrate (Red"eld et al., 1963). This fact enables us to construct quasi-conservative bio-geochemical parameters by a linear combination of oxygen concentration (or AOU) and either dissolved inorganic nitrate or phosphate. One category of such parameters includes the pre-formed nutrients, constructed from the linear combination of AOU and nutrient concentrations (Red"eld et al., 1963). The other category includes parameters like `NOa and `POa from a linear combination of dissolved oxygen and nutrients, introduced by Broecker (1974) and used by PeH rez et al. (1993) for the study of water masses o! the Iberian Peninsula. PeH rez et al. (1993) have clari"ed that there exists a simple physical relation between, e.g. preformed nitrate and NO.

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Both categories of quasi-conservative parameters have advantages and disadvantages in hydrographic research. Generally the NO and PO are highly correlated with temperature because of the temperature dependence of the oxygen solubility. The pre-formed nutrients are hard to interpret in terms of properties of source waters, since the deep waters of both the Norwegian Sea and the Weddell Sea emerge with a marked oxygen under-saturation (Johnston, 1977). The pre-formed nutrients are however generally less well correlated with (potential) temperature than NO and PO and are therefore preferred in this analysis, since they supply a more independent water mass tracer next to temperature and salinity. The ratio of the bio-geochemical tracers involved in the mineralization of organic matter can be quanti"ed by stochiometric ratios !DO : DNO : DPO (Red"eld et 2 3 4 al., 1963). In this paper we will use the stochiometric ratios given by PeH rez et al. (1993) for the eastern North Atlantic (!DO : DNO : DPO "163 : 16.3 : 1) to determine 2 3 4 the pre-formed phosphate PO0 and pre-formed nitrate NO0 , de"ned as: 3 4 PO0 "PO !AOU/163, 4 4

(2)

NO0 "NO !AOU/10. 3 3

(3)

For dissolved silica no precise stochiometric ratio with oxygen and the other nutrients can be expected, because the ratio in living phytoplankton varies regionally, and dissolution of biogenic Si in diatom frustules occurs by a process di!erent from the process of mineralization of organic matter. However, with the ageing of water typi"ed by a decrease of dissolved oxygen and increases of dissolved nitrate and phosphate, it can be expected that the dissolved silica concentration will also increase because of dissolution of silica frustules and sediment-water #uxes. PeH rez et al. (1993) use a !DO : DSi ratio of 15 for the description of thermocline waters o! the Iberian 2 Peninsula, but also mention lower ratios in deep waters because of the strong downward decrease of the re-mineralization of organic matter. Analogous to PeH rez et al. (1993) we have determined the linear regression coe$cients of Si with both NO 3 and PO , which, combined with the !DO : DNO : DPO ratios, gave an empirical 4 2 3 4 ratio !DO : DSi"1.72 : 1 for water samples with a potential density p '36.88 2 2 kg/m3 (the density level of the core of LSW). With this ratio a parameter Si0 is de"ned, analogous to Eqs. (2) and (3), as: Si0"Si!AOU/1.72

(4)

Because of the fact that the AOU in the newly formed deep waters is generally above zero (Johnston, 1977), and since these waters, before descending to deep levels, were a!ected by higher !DO : DSi ratios, Si0 according to Eq. (4) cannot directly be 2 interpreted as the concentration of dissolved silica during the formation of source waters. In the deep waters of the northeast Atlantic Si0 de"ned according to Eq. (4) even attains negative values. To avoid misunderstanding with the interpretation of Si0, and to stress its arti"cial character, we will name Si0 `pre-formeda silica, analogous to the other pre-formed nutrients, but with `pre-formeda always between quotation marks. In the local context of this paper Si0 will be considered as simply

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a quasi-conservative tracer. Deviations of this assumption for the deep water over the Biscay Abyssal Plain are discussed below.

4. Property}property plots We assume that the deep water mass in the northeast Atlantic is formed by mixing of ISOW of northern origin and LDW of southern origin, modi"ed by admixture of the overlying LSW and MSW. These water masses involved in the #ushing of the deep northeast Atlantic each cover a small range in hydrographic parameter space. In this paper they will be approximated by idealized single point source water types or mixing end members. Their properties (Table 2) are chosen to represent the source water types as they enter the eastern North Atlantic. So the water mass description is carried out in the local context of the mid-latitude northeast Atlantic Ocean without considering the far-"eld formation history of the source water types. Because of its northern origin the properties of ISOW were estimated from hydrographic stations of RV Oceanus and RV Tyro at 203W in the Iceland Basin north of the Rockall-Hatton Plateau, not shown in Fig. 1. At lower latitudes in the Iceland Basin the characteristics of the ISOW core change due to further admixture with LSW and LDW (van Aken and de Boer, 1995; van Aken and Becker, 1996). The properties of LDW were derived from the coldest samples (H(2.03C) over the Madeira Abyssal Plain. For the LSW, observations from the western Porcupine Abyssal Plain at the latitude of the Charlie-Gibbs Fracture Zone (&523N) were used. The MSW properties were estimated from the observations over the Iberian Abyssal Plain, extrapolated to a salinity of 36.50. This salinity is found in the MSW core near Cape St. Vincent in southwestern Portugal, where the MSW core enters the North Atlantic from the Gulf of Cadiz (PeH rez et al., 1993). It is known that the properties of the source water types, e.g. LSW and ISOW, may change on inter-annual and decadal time scales (Read and Gould, 1992; Sy et al., 1997; van Bennekom, 1985; van Aken, 1995). In the context of this paper the deep water mass structure is assumed to be stationary, since not enough data, especially nutrients, are available for the analysis of the e!ects of temporal change of the source water types on the deep water mass distribution. The potential temperature}salinity diagram (Fig. 2) shows the two layer structure of the deep water mass below the salinity minimum of LSW observed at about 2000 dbar (H+3.43C) with a salinity maximum at H+2.93C (the NEADW core) and a salinity minimum near H+2.03C (the LDW core). The coldest water (H(2.03C) with clear LDW properties is found near the bottom in the deepest parts of the Madeira Abyssal Plain. In the other basins water with a potential temperature below 2.03C has not been found, probably because this water is restricted in its northward #ow by the presence of the Azores-Portugal Rise (Fig. 1). Between the LSW core and the coldest LDW the NEADW salinity maximum is observed at the stations over the Porcupine Abyssal Plain and some of the stations over the Biscay Abyssal Plain and the Iberian Abyssal Plain. This salinity maximum is generally found near the potential density level

H (3C)

1.984 2.227 3.428 12.01

Water type

LDW ISOW LSW MSW

34.889 34.975 34.890 36.50

S (lmol/kg)

245.3 283.3 279.0 177.0

O 2 (lmol/kg) 86.9 46.6 41.4 83.5

AOU (lmol/kg) 1.50 1.02 1.12 0.79

PO 4 (lmol/kg) 22.2 14.6 16.9 13.7

NO 3 (lmol/kg)

Table 2 Estimated water mass properties of the di!erent source water types discussed in this paper

46.3 9.1 10.3 7.6

Si (lmol/kg) 0.97 0.73 0.87 0.28

PO0 4 (lmol/kg)

13.5 9.9 12.8 4.7

NO0 3 (lmol/kg)

!4.3 !18.0 !13.8 !41.0

Si0 (lmol/kg)

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Fig. 2. The potential temperature (3C)-salinity diagram, based on all available water samples. The di!erent symbols show the regional attribution given in Fig. 1. The p "41.42 kg/m3, p "41.47 kg/m3, and the 3 3 p "45.845 kg/m3 isopycnals are also shown, as well as the mixing lines between the source water types 4 (large grey crosses). The full grey line is the canonical H-S line de"ned by Sanders (1986).

p +41.42 kg/m3. For the other stations over the Biscay Abyssal Plain, Iberian 3 Abyssal Plain, the Madeira Abyssal Plain, the Seine Abyssal Plain, and the West Iberian Margin the salinity minimum connected to the LSW core is absent because of competition with the high salinity MSW in#uence. Therefore also an underlying salinity maximum is absent there. Close to the mixing line between the LSW and the ISOW source water types an in#exion point near H+2.603C (p "41.47 kg/m3) is 3 present in all deep basins. Below this in#exion point, in the potential temperature range 2.0(H(2.53C, the mean di!erences between our salinity values and the `canonicala H}S line from Saunders (1986) for the northeastern Atlantic and the latitude dependent deep H}S line from Mantyla (1994) amount to !0.001 and #0.001, respectively. The di!erent non-conservative bio-geochemical properties are plotted versus potential temperature (Fig. 3). From south to north at the lowest temperatures in our data set (H(2.33C), AOU increases from the Madeira Abyssal Plain (black circles), via the Iberian Abyssal Plain (open circles) towards the Porcupine Abyssal Plain

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Fig. 3. Plots of non-conservative bio-geochemical properties (lmol/kg) versus potential temperature (3C): (a) AOU, (b) dissolved phosphate, (c) dissolved nitrate, and (d) dissolved silica. The insets in (a) and (d) show in detail the coldest water, close to the temperature of the LDW source water type. The di!erent symbols show the regional attribution given in Fig. 1.

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Fig. 3. (continued).

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(black triangles). The highest deep AOU values at LDW temperatures are found in the Bay of Biscay, and the lowest AOU values are found over the Madeira Abyssal Plain. The distribution of nitrate and phosphate at the lowest temperatures is consistent with the AOU distribution, but the di!erences between the basins hardly surpass the estimated accuracy for these nutrients. The inset in Fig. 3d shows that the enrichment of the coldest waters with dissolved silica behaves di!erently since below about 2.253C the highest Si values occur over the Porcupine Abyssal Plain, the West Iberian Margin, and the Seine Abyssal Plain. The fact that for PO and 4 NO data points from the Porcupine Abyssal Plain are observed below the mixing 3 line between the LDW and LSW source water types, despite the tendency of these non-conservative parameters to increase due to ageing, indicates that at least some in#uence of ISOW originating from the Iceland Basin reaches the Porcupine Abyssal Plain. Since AOU and nutrient concentrations are non-conservative, ageing will cause apparent non-linear mixing between the source water types, forcing data points across the above-mentioned mixing line. This may explain the lack of evidence for the presence of ISOW in the other basins when only non-conservative tracers are considered. The quasi-conservative pre-formed nutrients in the deep water mass show a clearly smaller range than the real concentrations, indicative of the importance of ageing for the observed deep nutrient distributions. For the temperature range between the source water types LDW and LSW, most data points in Fig. 4 are within the LDW}ISOW}LSW mixing triangle for all basins. This suggests that the northern ISOW in#uence in the deep water properties in the NEADW core can be observed in the northeast Atlantic to a latitude of 313N. The distribution of the data points for the di!erent basins shows a southward decreasing in#uence of ISOW. The data points from the West Iberian Margin are closest to the LDW} LSW mixing line, together with data points from the eastern Biscay Abyssal Plain. It has been indicated in the previous section that the de"nition of Si0 lacks a biochemical basis, but is based on the assumption that ageing of water masses will show a simultaneous increase in AOU, nitrate, and phosphate due to mineralization of organic matter, as well as an increase in silica due to dissolution of biogenic silica. Fig. 4c shows that at potential temperatures below 2.33C the Si0 data points from the Porcupine Abyssal Plain (black triangles), which has a clear Si excess compared to the other basins (Fig. 3d), closely follows the curve of these other basins. But it is also evident from Fig. 4c that most data points in the coldest water over the Biscay Abyssal Plain (open triangles) have low Si0 values, close to the mixing line between LDW and ISOW, contrary to what is observed for the other pre-formed nutrients. This suggests that the used empirical stochiometric !DO : DSi ratio does not hold for the Bay of Biscay as well as it does for the 2 other basins. It appears that there more oxygen is consumed and more dissolved phosphate and nitrate are released relative to the release of dissolved silica, possibly because of the transport of organic matter from the nearby continental margin, and a relative lack of diatom blooms in this basin compared with the Porcupine Abyssal Plain.

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Fig. 4. Plots of quasi-conservative pre-formed nutrient concentrations (lmol/kg) as de"ned in Eqs. (2)}(4) plotted versus potential temperature (3C): (a) pre-formed phosphate, (b) pre-formed nitrate, and (c) preformed silica. The inset in (c) shows in detail the coldest water, close to the temperature of the LDW source water type. The di!erent symbols show the regional attribution given in Fig. 1.

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Fig. 4. (continued).

5. Isopycnal analysis For each station we have determined the hydrographic properties on isopycnal surfaces by vertical linear interpolation between samples, and we have calculated the mean properties and the standard deviation in each basin. In the Bay of Biscay we discriminate the West Biscay Abyssal Plain west of 93W and the East Biscay Abyssal Plain east of this longitude. The "rst is in direct contact with the Iberian and Porcupine Abyssal Plains, while the latter is topographically more isolated. This analysis was performed for the p "41.42 kg/m3 and the p "41.47 kg/m3 isopycnal 3 3 surfaces (Tables 3 and 4). These surfaces, which are respectively close to the salinity maximum below the LSW core and close to the deep in#exion point in the H}S diagram mentioned above (see Fig. 2), are assumed to be connected with the lateral spreading of the NEADW core. These isopycnal surfaces (hereafter named the upper and lower NEADW surface) hardly "nd any obstacle between the Iberian and the Porcupine Abyssal Plains at their characteristic pressures of 2500 and 2900 dbar. But the #ow on these surfaces may be steered by the underlying topography, which includes ridges and gaps between the deep basins. A similar analysis was performed for the p "45.855 kg/m3 isopycnal surface (Table 5). This surface (hereafter named 4 the LDW surface) is well embedded in the low salinity LDW (Fig. 2) at a typical

34.957 (0.006)

34.969 (0.005)

34.978 (0.004)

34.974 (0.012)

34.995 (0.014)

34.987 (0.012)

35.020 (0.007)

Porcupine Abyssal Plain

West Biscay Abyssal Plain

East Biscay Abyssal Plain

Iberian Abyssal Plain

West Iberian Margin

Madeira Abyssal Plain

Seine Abyssal Plain

3.237 (0.029)

3.090 (0.052)

3.129 (0.062)

3.034 (0.055)

3.052 (0.017)

3.010 (0.024)

2.956 (0.026)

H (3C)

76.8 (3.7)

66.7 (2.7)

71.8 (2.5)

65.2 (3.2)

76.5 (2.4)

73.4 (2.2)

62.4 (3.2)

AOU (lmol/kg)

1.32 (0.03)

1.26 (0.03)

1.30 (0.02)

1.24 (0.04)

1.34 (0.02)

1.34 (0.02)

1.20 (0.03)

PO 4 (lmol/kg)

19.7 (0.3)

19.1 (0.4)

29.3 (0.5)

18.8 (0.6)

19.9 (0.3)

19.9 (0.3)

18.3 (0.6)

NO 3 (lmol/kg)

27.5 (1.2)

23.6 (1.3)

26.3 (1.1)

22.0 (1.8)

27.5 (1.0)

27.5 (1.0)

21.1 (2.0)

Si (lmol/kg)

!Note: Standard deviation between brackets. These values are based on all available stations in each basin.

S

Basin

0.85 (0.02)

0.85 (0.01)

0.86 (0.01)

0.84 (0.01)

0.87 (0.02)

0.87 (0.02)

0.81 (0.02)

PO0 4 (lmol/kg)

Table 3 Mean hydrographic properties in each basin on the p "41.42 kg/m3 isopycnal surface (the upper NEADW surface)! 3

12.0 (0.3)

12.4 (0.2)

12.1 (0.1)

12.3 (0.2)

12.3 (0.2)

12.3 (0.3)

12.0 (0.3)

NO0 3 (lmol/kg)

!17.2 (0.9)

!15.2 (0.2)

!15.4 (0.6)

!15.9 (0.4)

!17.0 (0.9)

!17.0 (1.1)

!15.6 (0.7)

Si0 (lmol/kg) H.M. van Aken / Deep-Sea Research I 47 (2000) 757}788 773

34.948 (0.004)

34.947 (0.002)

34.946 (0.002)

34.950 (0.005)

34.955 (0.004)

34.949 (0.004)

34.960 (0.003)

Porcupine Abyssal Plain

West Biscay Abyssal Plain

East Biscay Abyssal Plain

Iberian Abyssal Plain

West Iberian Margin

Madeira Abyssal Plain

Seine Abyssal Plain

2.674 (0.013)

2.622 (0.020)

2.653 (0.018)

2.630 (0.021)

2.611 (0.009)

2.613 (0.012)

2.619 (0.019)

H (lmol/kg)

81.5 (3.1)

74.4 (3.0)

78.1 (1.5)

74.4 (2.6)

82.7 (2.5)

81.0 (1.8)

72.8 (3.7)

AOU (lmol/kg)

1.38 (0.02)

1.33 (0.02)

1.36 (0.01)

1.32 (0.03)

1.41 (0.02)

1.39 (0.02)

1.30 (0.04)

PO 4 (lmol/kg)

20.3 (0.5)

20.1 (0.3)

20.0 (0.5)

19.9 (0.7)

20.8 (0.4)

20.6 (0.3)

19.6 (0.6)

NO 3 (lmol/kg)

34.4 (1.4)

31.3 (1.0)

33.5 (1.3)

30.1 (1.9)

33.6 (0.8)

32.1 (2.0)

29.7 (2.4)

Si (lmol/kg)

!Note: Standard deviation between brackets. These values are based on all available stations in each basin.

S (3C)

Basin

0.88 (0.02)

0.88 (0.01)

0.88 (0.01)

0.87 (0.02)

0.90 (0.02)

0.90 (0.02)

0.85 (0.02)

PO0 4 (lmol/kg)

Table 4 Mean hydrographic properties in each basin on the p "41.47 kg/m3 isopycnal surface (the lower NEADW surface)! 3

12.2 (0.6)

12.7 (0.1)

12.2 (0.1)

12.5 (0.3)

12.6 (0.4)

12.5 (0.3)

12.3 (0.3)

NO0 3 (lmol/kg)

!13.0 (1.1)

!12.0 (0.3)

!12.0 (0.5)

!13.1 (0.2)

!14.5 (0.9)

!15.0 (1.0)

!12.7 (0.6)

Si0 (lmol/kg)

774 H.M. van Aken / Deep-Sea Research I 47 (2000) 757}788

34.912 (0.004)

34.909 (0.002)

34.909 (0.002)

34.908 (0.003)

34.911 (0.003)

34.907 (0.003)

34.910 (0.002)

Porcupine Abyssal Plain

West Biscay Abyssal Plain

East Biscay Abyssal Plain

Iberian Abyssal Plain

West Iberian Margin

Madeira Abyssal Plain

Seine Abyssal Plain

2.165 (0.006)

2.150 (0.012)

2.166 (0.010)

2.154 (0.014)

2.160 (0.007)

2.161 (0.010)

2.171 (0.016)

H (lmol/kg)

88.6 (0.7)

84.9 (1.1)

86.6 (1.1)

88.6 (1.1)

90.9 (1.7)

90.4 (1.7)

88.8 (1.3)

AOU (lmol/kg)

1.47 (0.02)

1.45 (0.02)

1.46 (0.01)

1.47 (0.02)

1.50 (0.02)

1.50 (0.01)

1.49 (0.05)

PO 4 (lmol/kg)

21.6 (0.4)

21.6 (0.3)

21.2 (0.5)

21.8 (0.2)

22.0 (0.3)

22.0 (0.2)

22.0 (0.4)

NO 3 (lmol/kg)

44.4 (1.6)

42.8 (0.8)

44.0 (1.8)

43.4 (0.8)

42.1 (0.5)

42.1 (0.7)

43.5 (1.4)

Si (lmol/kg)

!Note: Standard deviation between brackets. These values are based on all available stations in each basin.

S (3C)

Basin

Table 5 Mean hydrographic properties in each basin on the p "45.855 kg/m3 isopycnal surface (the LDW surface)! 4

0.93 (0.02)

0.93 (0.04)

0.93 (0.01)

0.93 (0.03)

0.95 (0.02)

0.95 (0.01)

0.95 (0.06)

PO0 4 (lmol/kg)

12.7 (0.4)

13.2 (0.3)

12.5 (0.5)

13.1 (0.2)

12.9 (0.2)

13.1 (0.3)

13.2 (0.2)

NO0 3 (lmol/kg)

!7.2 (1.5)

!6.6 (0.4)

!6.4 (1.8)

!7.0 (0.3)

!10.7 (0.6)

!10.4 (1.0)

!8.1 (0.8)

Si0 (lmol/kg) H.M. van Aken / Deep-Sea Research I 47 (2000) 757}788 775

776

H.M. van Aken / Deep-Sea Research I 47 (2000) 757}788

pressure of 4100 dbar. This is close to the bottom over the Porcupine Abyssal Plain. On this surface the di!erent basins are connected by gaps in the ridges between the basins. The lateral distributions of salinity and AOU in the upper NEADW surface are depicted in Fig. 5. On the upper NEADW surface the salinity increases southeastwards from the Porcupine Abyssal Plain towards the Seine Abyssal Plain (Fig. 5a; Table 3). This is probably due to mixing with the overlying saline water at LSW densities containing measurable amounts of MSW (Harvey and Arhan, 1988). On the lower NEADW surface (Table 4) the salinity hardly di!ers between the basins except for the Seine Abyssal Plain and the West Iberian Margin, indicating that the downward mixing of MSW does not reach these deeper levels except near the southern continental margin, below the high salinity core of MSW. On both NEADW surfaces the meridional trend of `ageinga is to the east and south, with water with low AOU (Fig. 5b), PO , NO , and Si over the Porcupine Abyssal Plain, and high non-conservative 4 3 property values near the Northwest-African and European continental margins. The di!erence in pre-formed nutrients PO0 and NO0 between the Basins is small, 3 4 especially in the lower NEADW surface, and de"nitely smaller than the in situ concentrations. The AOU values on the characteristic isopycnal surfaces for stations within 13 of the 203W meridian are plotted versus latitude (Fig. 6). On the LDW surface (see Table 5) no signi"cant systematic trends of H and S are observed. The non-conservative properties, especially AOU (Fig. 6) and Si, show a northward increase from the Madeira Abyssal Plain towards the Porcupine Abyssal Plain. This increase cannot be caused by diapycnal mixing with the overlying water masses (Fig. 3), since these have lower AOU and nutrient values. Therefore the increase indicates that the LDW is ageing when it moves northwards. The Biscay Abyssal Plain shows the highest AOU, PO , and NO values, suggesting that the Bay of Biscay does not form the main 4 3 through-#ow for LDW from the Iberian to the Porcupine Abyssal Plain. Northwards #owing LDW probably follows a course mainly through the gap between the AzoresBiscay Rise and the Charcot Seamounts. Long-term current meter data from this gap (44.83N, 14.23W), recently obtained from R.R. Dickson (CEFAS, Lowestoft), seem to con"rm this view. They show a 353-day average north-eastward velocity vector of over 7 cm/s at a depth of 4574 m. In the Bay of Biscay Si does not di!er strongly from the neighbouring basins. Apparently the mineralization of organic matter in the deep Bay of Biscay is not associated with extra dissolution of silica. This is the cause of the deviating behaviour of Si0 in the Bay of Biscay mentioned above. The conservative parameters H and S, and the other pre-formed nutrients, hardly di!er between the basins, an indication that during its #ow to the north the water on the LDW surface is not signi"cantly in#uenced by downward mixing of overlying water masses. The meridional trend of AOU (Fig. 6) is highly signi"cant on all three isopycnal surfaces. The change in direction of the meridional AOU gradient between the LDW and NEADW water masses is clearly visible. Whereas the slopes for the upper and lower NEADW surfaces are !0.46($0.06) and !0.40($0.04) lmol/kg/deg respectively, the slope for the LDW surface is #0.28($0.04) lmol/kg/deg.

H.M. van Aken / Deep-Sea Research I 47 (2000) 757}788

777

Fig. 5. Lateral distribution of (a) the salinity and (b) AOU (lmol/kg) on the upper NEADW surface.

778

H.M. van Aken / Deep-Sea Research I 47 (2000) 757}788

Fig. 5. (continued).

H.M. van Aken / Deep-Sea Research I 47 (2000) 757}788

779

Table 6 Contribution (%) of the di!erent source water types to the water mass on (a) the upper and (b) the lower NEADW isopycnal surfaces averaged per basin! LDW

ISOW

LSW

MSW

(a) The upper NEADW surface (p "41.42 kg/m3) 3 Porcupine Abyssal Plain 22.7 (8.4) W. Biscay Abyssal Plain 37.0 (9.4) E. Biscay Abyssal Plain 41.8 (4.7) Iberian Abyssal Plain 29.7 (9.8) West Iberian Margin 35.0 (8.7) Madeira Abyssal Plain 42.4 (8.4) Seine Abyssal Plain 43.0 (6.2)

30.2 18.3 14.7 24.7 21.5 14.5 15.9

(7.9) (8.2) (4.0) (8.2) (7.7) (6.8) (5.4)

44.7 40.8 38.7 42.1 38.1 37.9 33.9

(1.8) (1.7) (0.9) (2.6) (1.8) (2.8) (1.5)

2.4 3.9 4.8 3.5 5.4 5.2 7.2

(0.6) (0.5) (0.2) (1.0) (0.7) (1.1) (0.6)

(b) The lower NEADW surface (p "41.47 kg/m3) 3 Porcupine Abyssal Plain 46.5 (10.9) W. Biscay Abyssal Plain 56.1 (7.8) E. Biscay Abyssal Plain 62.6 (4.0) Iberian Abyssal Plain 52.4 (8.6) West Iberian Margin 47.4 (8.6) Madeira Abyssal Plain 55.2 (7.6) Seine Abyssal Plain 57.0 (6.1)

27.1 18.6 12.9 21.9 26.8 19.5 18.5

(9.5) (6.9) (3.5) (7.5) (7.6) (6.6) (5.3)

24.3 22.7 21.7 23.3 23.1 22.7 21.1

(2.0) (1.1) (0.7) (1.6) (1.4) (1.5) (1.1)

2.1 2.6 2.8 2.4 2.7 2.6 3.4

(0.6) (0.3) (0.2) (0.5) (0.4) (0.5) (0.3)

!Note: The standard deviation within each basin is given between brackets.

Fig. 6. Meridional distribution of AOU (lmol/kg) near 203W on the upper NEADW surface (circles), the lower NEADW surface (triangles) and the LDW surface (crosses). The values at individual stations within 13 of the 203W are shown. The straight lines show the linear regression of AOU with latitude. The correlation coe$cients of the regression are respectively !0.79, !0.85 and 0.93.

780

H.M. van Aken / Deep-Sea Research I 47 (2000) 757}788

6. Quantitative water mass analysis The observed water mass distribution is generated by the interplay of advection and diabatic and isopycnic mixing. A quantitative analysis of conservative tracers may reveal how a source water type is diluted by mixing when it is advected from its point of origin. A simple example is the use of a mixing triangle in H}S space to determine the quantitative contributions of di!erent source water types in the formation of a water mass (Mamayev, 1975; de Boer and van Aken, 1995). Such an analysis should be interpreted carefully, since the results will depend on the a priori de"nition of the idealized source water types and their local or global context. As already expressed above, in our case the context is the source water types as they enter the eastern North Atlantic basins between the Madeira Abyssal Plain and Iceland. Moreover the analysis assumes linear mixing of conservative tracers, which may be violated when double di!usion is the dominant mixing agent (Schmitt, 1981). Although below the MSW core occasionally thermohaline staircases are observed, connected with double diffusive salt "ngers, below 2000 dbar no indications have been found for salt "nger convection. In order to analyse the systematic trends of the water mass composition of NEADW, we determined the change in the water type compositions in the upper and lower NEADW surfaces by a linear optimal multiple parameter analysis, an extension of the mixing triangle in multi-parameter space (Tomczak, 1981). For this analysis we used four end point linear mixing of the (quasi-)conservative parameters, H, S, PO0 , 4 and NO0 as well as mass conservation (de Boer and van Aken, 1995). The four source 3 water types used were ISOW, LSW, LDW and MSW, as de"ned in Table 2. Since the conservation of four conservative parameters, together with mass conservation, give "ve linear equations with four unknowns, the resulting set of equations is overdetermined, which allows the equations to be solved in a least-squares sense. The emerging meridional trend is consistent and accounts for the southward increasing salinity, temperature and pre-formed nutrients in the NEADW core (Table 6, Fig. 7). On the LDW surface the di!erences in water mass composition between the di!erent basins are hardly or not at all signi"cant because of the small di!erences in properties (see Table 5) and the relatively high LDW content (&81$3% LDW). Over the Porcupine Abyssal Plain the water in the upper NEADW layer appears to consist of about 30% ISOW, 45% LSW, 23% LDW and 2% MSW. Going south to the Madeira Abyssal Plain the contributions of ISOW and LSW decrease to 15% and 38% respectively, while the fractions of both LDW and MSW increase to 42% and 5%. The water type contributions over the Iberian Abyssal Plain have intermediate values. A similar but eastward trend in the water mass composition is also observed. ISOW and LSW contents in the upper NEADW surface decrease from the Porcupine Abyssal Plain towards the continental margin in the eastern Bay of Biscay, while in that direction both the LDW content and the MSW contents increase. The lateral distribution of the ISOW contribution (Fig. 7) indicates that the lowest percentages of ISOW ((10%) are found along the continental slope in the Bay of Biscay and over the Seine Abyssal Plain. These low ISOW contributions coincide with LDW contributions of over 50%.

H.M. van Aken / Deep-Sea Research I 47 (2000) 757}788

781

Fig. 7. Lateral distribution of the contribution (%) of ISOW to the water mass composition on the upper NEADW surface.

782

H.M. van Aken / Deep-Sea Research I 47 (2000) 757}788

Also on the lower NEADW surface a meridional trend in ISOW contribution can be recognized, decreasing from 27% over the Porcupine Abyssal Plain to 20% over the Madeira Abyssal Plain (Table 6b). Towards the east the ISOW contribution decreases to 13% over the eastern Biscay Abyssal Plain. Here we want to note that the average ISOW content on the lower NEADW surface from all basins, 21%, is slightly larger than the mean ISOW content on the upper NEADW surface, 20%, but with smaller lateral gradients Since the empirical stochiometric ratio for Si is not strictly valid for MSW, conservation of Si0 in principle cannot be applied in the multi-parameter water mass analysis. However additional inclusion of Si3 from Table 2 in the analysis hardly alters the results because of the small amounts of MSW present in the NEADW core. Only over the Biscay Abyssal Plain are signi"cant di!erences found, but as stated above, there the empirical stochiometric ratio for Si in deep water deviates from the other basins. The quantitative water mass analysis assumes stationary source water properties. We have evaluated the e!ects of varying H and S for the ISOW and LSW source water types by changing these source water types either H by 0.13C or S by 0.1. These changes are of the order of the decadal change of LSW in the eastern Atlantic basins as reported by Sy et al. (1997), and about twice the observed changes in ISOW (van Bennekom, 1985). The distribution of the ISOW and MSW contributions appear to be quite insensitive to such changes of source water properties, with an RMS di!erence relative to the contributions shown in Table 6 of less than 1%. The contributions of LDW and LSW are more sensitive to the applied changes, but the magnitudes of meridional and zonal trends of the contribution of the water types to the deep water mass are maintained.

7. Discussion The analysis of property}property plots and the quantitative water mass analysis presented above have shown that the meridional and zonal modi"cation of the deep water mass in the mid-latitude eastern North Atlantic can be described quite well with a linear mixing model. This model involves the four known deep and intermediate source water types, de"ned in the local context of the deep eastern North Atlantic. The local context implies that the deep water mass of the eastern North Atlantic Ocean is #ushed by in#ow of ISOW from a northern source, and LDW from a southern source. At shallower levels also LSW and MSW are involved. This context also implies that the amount of ISOW present in a basin does not refer to the Norwegian Sea Deep Water passing through the Faroe Bank Channel, but to the ISOW as it is formed during and after the actual over#ow by warm and cold entrainment south of Iceland. Likewise the LSW properties are the properties of the LSW core as it enters the eastern Atlantic Basin, modi"ed on its way from The Labrador Sea, while MSW is de"ned as the water mass after leaving its formation area in the Gulf of Cadiz. And the LDW properties re#ect the bottom water as it enters the Madeira Abyssal Plain from the south. The net volume #ux into the deep basins is balanced by an out#ow, either as

H.M. van Aken / Deep-Sea Research I 47 (2000) 757}788

783

westward through-#ow through the Charlie-Gibbs Fracture Zone, as large-scale deep upwelling, or as a southward recirculation out of the Madeira Abyssal Plain. During the residence time in the deep eastern North Atlantic the source water types interact by diapycnal or isopycnal mixing, thereby generating the observed deep water mass continuum. The apparent ageing, derived from isopycnal gradients in the non-conservative parameters, re#ects a net southward advective transport of ISOW at NEADW levels and a net northward transport at LDW levels. The meridionally changing water mass constitution, derived from a quantitative water mass analysis, agrees with such a circulation scheme. A quantitative water mass analysis is in principle only a linear transformation of the hydrographic parameters resulting in estimated contributing fractions of pragmatically de"ned source water types. The local or global de"nitions of the source water types determine the outcome of the analysis. And given the idealization of water masses with a range of (temporally varying) hydrographic properties by single point source water types the contributing percentages as given in Table 6 should be considered as a semi-quantitative indication of the importance of certain source regions for the local deep water mass structure, not as a de"nite and absolute percentage. The NEADW core over the Porcupine Abyssal Plain, characterized by a deep salinity maximum, appears to be a mixture of mainly LSW and ISOW, with only limited contributions from the other source water types. No water types from the western North Atlantic Basins are required, like NWADW from the near bottom layers of the western North Atlantic as proposed by Tsuchiya et al. (1992). The plots of potential temperature versus salinity and pre-formed nutrients (Figs. 2 and 4) and the analysis of water mass composition (Table 6) show that, according to the mixing model, ISOW also contributes to the deep water mass in the other deep basins but is diluted to the south and east. This result appears to be not very sensitive to changes of the properties of LSW and ISOW of the order of observed decadal changes. The signi"cant southward increase of AOU on the lower NEADW surface (Fig. 6) indicates that despite the relatively large LDW contribution on this isopycnal (O 50%) the ageing derived from this trend points to a net southward #ow, even at the level of the in#exion point in the H}S diagram (Fig. 2). Broecker et al. (1985) have searched for ISOW in#uences at potential temperatures below 2.63C, the approximate temperature of the lower NEADW surface. Our results have shown that at these low temperatures the ISOW contribution to the water mass is below 27%, decreasing to about 12% at H+2.153C on the LDW surface. Probably because of diapycnal mixing with LSW in the Iceland Basin, the potential temperature of the ISOW core has increased to levels above 2.63C before it continues into the NEADW core over the Porcupine Abyssal Plain (van Aken and Becker, 1996). This agrees with the large amount of LSW in the NEADW core (Table 6a). According to the results of the water mass analysis presented in Table 6a, the water mass in the upper NEADW surface over the Porcupine Abyssal Plain appears to consist mainly of LSW (45%) and ISOW (30%). Southward to the Madeira Abyssal Plain the ISOW content in this surface decreases more (to 15%) than does the LSW content (to 34%). This is consistent with the fact that at levels overlying the NEADW core the LSW content is even higher than in the upper NEADW surface, with LSW salinity

784

H.M. van Aken / Deep-Sea Research I 47 (2000) 757}788

minima over the Porcupine Abyssal Plain and the northern Iberian Abyssal Plain. Diapycnal mixing with overlying water therefore will increase the LSW content and decrease the ISOW content, while diapycnal mixing with LDW in the underlying water will tend to decrease both the LSW and ISOW content. A similar trend is observed eastward from the Porcupine Abyssal Plain to the East Biscay Abyssal Plain. The observation that in the lower NEADW surface the southward and eastward ISOW trends are smaller is probably explained by the fact that over the Porcupine Abyssal Plain and the northern Iberian Abyssal Plain diapycnal mixing with overlying water will increase the ISOW content on that surface, while mixing with the underlying water will tend to decrease ISOW content. The eastward and southward increase on salinity on the upper NEADW surface (Fig. 5, Table 3) seems to contradict the strongly increasing LDW contribution to the south and east, which should lead to a decrease of salinity. Responsible for the salinity increase is the minor increase in MSW content, combined with the relatively large salinity of the MSW source water type. The lateral distribution of AOU on the upper NEADW surface (Fig. 5b) as well as the distribution of ISOW (Fig. 7) are consistent with a cyclonic circulation of the NEADW core from the western Porcupine Abyssal Plain into the Bay of Biscay with a poleward transport near the continental margin as proposed by Paillet and Mercier (1997). The meridional gradient of AOU in the western basins (Fig. 6) is highly signi"cant, as is the change of the direction of this gradient between the lower NEADW surface and the LDW surface. This change can be ascribed to ageing of the water masses when NEADW #ows southwards and LDW #ows northwards. It is however also possible that the southward increase of AOU in the NEADW water mass is caused by the southward increasing admixture of LDW and MSW with high AOU. In order to estimate the latter e!ect we have reconstructed the non-conservative parameters on the upper NEADW surface for each basin from the source water properties (Table 2) and the calculated water type contributions (Table 6a). If mineralization of organic matter is not important for the apparent southward ageing of NEADW, the reconstructed AOU will have similar values and will show a meridional gradient similar to the observations. The reconstructed AOU also shows a southward and eastward increase. However, the reconstructed AOU values are on average over 5.3 lmol/kg lower than the observed values in the western basins, and over 11.4 lmol/kg lower than observed in the eastern basins. Assuming a realistic ageing of ISOW of 5 lmol/kg AOU when the ISOW moves through the Iceland Basin before it reaches the Porcupine Abyssal Plain hardly reduces the di!erence between reconstruction and observation. Similar results are obtained for the reconstructed phosphate and nitrate. This indicates, as also can be discerned from Fig. 3, that although diapycnal mixing with LDW and MSW will cause an apparent ageing of the NEADW core when it spreads from its northern origin southwards and eastwards, AOU, phosphate and nitrate also increase due to mineralization of organic matter, especially near the European continental margin. The highest salinities on the upper NEADW surface are found below the MSW core along the continental margin over the Seine Abyssal Plain (Table 3). The overlying MSW core is known to spread northwards from the Gulf of Cadiz in an eastern

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boundary current (Ambar et al., 1976; Swallow et al., 1977), while being diluted and decreasing its salinity. The latter processes are attributed mainly to lateral mixing with fresher water masses (Arhan and King, 1995). Apparently part of the high salinity MSW signal reaches the NEADW core already close to the Gulf of Cadiz near southwestern Portugal (Fig. 7). And even on the lower NEADW surface and the LDW surface the highest salinities are found over the Seine Abyssal Plain and the West Iberian Margin (Tables 4 and 5). Talley and McCartney (1982) reported a high salinity core in overlying LSW levels at &2000 m in that area. The northward decrease of salinity near the continental margin (Fig. 5a) is consistent with diapycnal mixing in a pole-ward recirculating NEADW core. This diapycnal mixing near the continental margin is probably enhanced by boundary mixing over the continental slope (Garrett, 1991). During the northward #ow of LDW the Azores-Portugal Rise forms a barrier for the coldest LDW (H(2.03C), which is not found over the Iberian Abyssal Plain. On the isopycnal LDW surface, which is found in all deep basins at an approximate pressure of 4100 dbar, AOU and nutrients increase to the north and east (Table 5). Especially over the Seine Abyssal Plain, the West Iberian Margin, and the Porcupine Abyssal Plain the silica concentration is enhanced in the LDW water mass, already characterized by its high silica concentrations (Fig. 3d). The small changes in water type constitution on this surface (LDW is everywhere &80%) also do not explain the observed zonal di!erences in non-conservative properties between the di!erent basins. Especially in the Bay of Biscay local ageing is required to explain the high observed AOU. This supports the use of AOU as a qualitative ageing parameter. The lateral distribution of the ISOW contribution (Fig. 7) appears to re#ect a cyclonic #ow from the Porcupine Abyssal Plain into the Bay of Biscay. In the resulting deep poleward #ow along the European continental margin, also established by Dickson et al. (1985), McCartney (1992), and Paillet and Mercier (1997), deep upwelling may occur (Stommel and Arons, 1960). During this process LDW and ISOW are advected upwards, while this vertical advection is balanced by diapycnal mixing (Munk, 1966). The high LDW contributions at NEADW levels in the Bay of Biscay agree with a resulting local cross-isopycnal advection, as can be expected in the case of deep upwelling. The high AOU values in the Bay of Biscay (Fig. 5b) in part re#ect the increased in#uence of LDW at NEADW levels, but are probably also enhanced by ageing. Let us, for the sake of argument, estimate the deep pole-ward #ow, required to support the deep upwelling along the continental margin. We assume that half of the surplus of deep water, which enters the deep northeast Atlantic but does not leave through the Charlie-Gibbs Fracture Zone to the west, is removed by deep upwelling in the Bay of Biscay and eastern Porcupine Abyssal Plain. That amounts to 1.25 Sv. This order of magnitude compares reasonably with the deep upwelling along the European and northwest African ocean marging of 1.6 Sv, derived from an inverse model by Arhan et al. (1995). When all deep upwelling takes place in a narrow zone of about 200 km along the continental slope, a surface of about 3]1011 m2 is involved. This requires the upwelling velocity w to be about w+4]10~6 m/s. Following Stommel and Arons (1960) we will assume that the deep upwelling is determined

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solely by the Sverdrup e!ect in the water mass below the upper NEADW surface, that is: w"H(b/f )v

(5)

H is the thickness of the water column below the NEADW surface, b is the meridional derivative of the Coriolis parameter f, and v is the mean poleward velocity component in the layer H. With a layer thickness H+2500 m we "nd for the latitude of the Bay of Biscay an expression: w"4]10~4 v.

(6)

So the upwelling of 1.25 Sv along the eastern Biscay and Porcupine continental margin can be supported by a deep poleward velocity component along the continental margin of &1 cm/s. Dickson et al. (1985) have reported poleward velocity components along the continental margin below a depth of 2000 m up to 3.4 cm/s, with a characteristic value of 1.2 cm/s. This result con"rms that indeed at least 50% of the surplus deep water entering the northeast Atlantic may be removed from the deep layer by deep upwelling along the continental margin in the eastern Bay of Biscay and Porcupine Abyssal Plain. McCartney presented evidence for a deep poleward #ow along the whole European ocean margin. Therefore this e!ect also can be expected to occur in the deep poleward #ow along the West Iberian Margin and in the Seine Abyssal Plain, and it is very likely that the whole excess of deep water in the eastern Atlantic Basins can be removed by deep upwelling along the continental margin.

8. Conclusions We can conclude that the deep water masses in the northeast Atlantic can be described adequately with a local four endpoint linear mixing model, including ISOW, LSW, MSW, and LDW as source water types. The multi-parameter hydrographic analysis used to describe the mixing of these water types forms a powerful extension of the H}S and S anomaly analysis used by Lee and Ellett (1965). It has been shown that ISOW and LSW form the main constituents of the NEADW core over the Porcupine Abyssal Plain, which is characterized there by a deep salinity maximum. Analyses of conservative and non-conservative bio-geochemical parameters have con"rmed a net southward extension and recirculation of ISOW further south in the mid-latitude North Atlantic, as proposed by van Aken and Becker (1996). ISOW can be traced southwards as far as the Madeira Abyssal Plain. This is de"nitely further south than the southward spreading to at least 473N established by Lee and Ellett (1965). At the levels of the NEADW core the AOU distribution shows a southward and eastward increase related to ageing. In the near bottom layer, where LDW is the dominant water type, a northward increase of AOU is observed, related to ageing in a net northward near bottom #ow. The lateral isopycnal distributions of conservative and non-conservative tracers are consistent with a cyclonic character of the deep recirculations, conforming with Paillet and Mercier (1997), with a poleward #ow and deep

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upwelling along the European and Northwest African continental margin in accordance with McCartney (1992) and Arhan et al. (1994). An order of magnitude estimate suggests that by this process the whole of the deep water surplus in the northeast Atlantic may be removed.

Acknowledgements This research was supported the Foundation for Geological, Oceanographic and Atmospheric Sciences (GOA), a subsidiary of the Netherlands Foundation for Scienti"c Research (NWO), and by the EU OMEX II Programme under contract number MAS3-CT97-0076. This paper is NIOZ publication 3245.

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