Water and nutrient fluxes off Northwest Africa
Introduction
The 1970s was the International Decade for Ocean Exploration. This interest in international cooperation was combined with the relevance of fisheries off Northwest Africa to impulse a major program, the Cooperative Investigation of the Northern Part of the Eastern Central Atlantic (CINECA), aimed at understanding the ocean environment of this highly productive region. The program involved near 100 scientific cruises from many nations (Hempel, 1982), mainly focusing in the continental shelf and slope. As part of this program the Institut de Ciències del Mar together with other CSIC research teams (at that time Instituto de Investigaciones Pesqueras, IIP) participated with eight cruises. Despite some of these cruises were consecutive in time and had good coverage of the upper ocean in adjacent areas, so far they have always been analyzed separately. A first objective of our work has been to produce synoptic data sets, of good quality and large-scale coverage, from these old data sets.
Our region of interest is the upper ocean between the Canary Islands (28°N) and Cape Verde (15°N). This is a very complex oceanographic region, with the top 700 or 800 m occupied by surface and central waters of either northern (North Atlantic Central Waters, NACW) or southern (South Atlantic Central Waters, SACW) origin (Fraga, 1974; Tomczak, 1981, Tomczak, 1984; Zenk et al., 1991). NACW have their source in the North Atlantic subducting zone, defined as where the Ekman pumping velocity is negative, and reach subsurface waters at lower latitudes through the thermocline circulation (Sarmiento et al., 1982; Kawase and Sarmiento, 1985). Only water masses formed in the subducting zone during late winter and early spring may escape the surface layers and get injected in the permanent thermocline; during the rest of the year, waters subducted remain in the mixed-layer as the escape velocities caused by Ekman pumping are less than the seasonal advance of the mixed-layer thermocline. The vertical extension of the NACW in lower latitudes will be determined by the densest winter-outcrop isopycnal within the subducting zone, near σθ=27.3 (Kawase and Sarmiento, 1985; Reid, 1994). SACW, on the other hand, are originated in the subantarctic zone of the southern ocean, and travel through the South Atlantic thermocline into the equatorial and tropical regions (Frantantoni et al., 2000; Zhang et al., 2003; Sarmiento et al., 2004; Williams et al., 2006). Intermediate waters (Antarctic Intermediate Waters in the 600–1000 m range and Mediterranean Waters down to nearly 1500 m) and deep waters complete the water column, but their study falls beyond the scope of this paper.
The time-dependent response in the region is characterized by the passage of different waves (Hagen, 2001, Hagen, 2005). Along the coast these should be northward propagating waves, fast as the Kelvin wave and slow as topographically trapped Rossby waves (Hagen, 2001). At a certain latitude we may find westward propagating Rossby waves (Price and Maagard, 1986; Müller and Siedler, 1992; Siedler and Finke, 1993; Hagen, 2001, Hagen, 2005). In this paper we will be concerned with the steady-state response, for a thorough review of these free traveling waves the reader is referred to Hagen (2001).
The North Atlantic subtropical gyre is a large-scale anticyclone, with its margins raising towards the surface. The southeastern margin is the Cape Verde front while the eastern one is the eastern boundary current system, its easternmost branch taking place in the coastal transition zone. The steady-state connection between the offshore and coastal upwelling fronts is controlled by three main factors: the location of the Cape Verde frontal zone, the intensity and latitudinal extension of coastal upwelling, and the size and location of the Guinea Dome open-ocean upwelling area.
The Cape Verde frontal zone corresponds to the southern limit of the North Atlantic thermocline recirculation (Kawase and Sarmiento, 1985; Stramma and Siedler, 1988; Zenk et al., 1991; Arhan et al., 1994). It stretches southwest from Cape Blanc to the Cape Verde Islands, and effectively separates relatively new (salty, warm, nutrient-poor, and oxygen-rich) NACW from the older (fresh, cold, nutrient-rich, and oxygen-poor) SACW. The position of the front is linked to the westward veering of the Canary Current as it becomes the North Equatorial Current, and displays relatively small meridional excursions, north in summer and south in winter (Stramma and Siedler, 1988). Hagen (2001) has proposed that the frontal position is fixed by the vanishing of the sea-surface wind-stress curl.
As a result of the surface trade winds, coastal upwelling is present between the Strait of Gibraltar and Cape Blanc all year long, being most intense south of the Canary Islands (Wooster et al., 1976; Speth and Detlefsen, 1982; Nykjaer and Van Camp, 1994). During summer upwelling intensifies north of the Canary Islands and reaches the Iberian Peninsula, and during winter it reaches south past Cape Verde, e.g. Fig. 14 in Pelegrí et al. (2006). Pelegrí et al. (2005) have discussed the double role played by the coastal upwelling zone, both linking the surface and subsurface waters through the vertical upwelling cell and providing a meridional connection through the coastal upwelling jet. One important consideration is that this upwelling boundary current must accommodate an interior flow of some 2–3 Sv north of the Canary Islands, so it must have an inertia that goes well beyond the response to synoptic atmospheric variability. This is why we may speak of the Canary Upwelling Current as the permanent eastern boundary current of the North Atlantic subtropical gyre (Pelegrí et al., 2005, Pelegrí et al., 2006).
South of the Cape Verde frontal zone we find a cyclonic circulation region, composed by the eastward North Equatorial Counter Current (NECC) and the westward North Equatorial Current (NEC). This current system flows around the Guinea Dome, an open-ocean upwelling region forced by the surface cyclonic winds (Siedler et al., 1992; Yamagata and Iizuka, 1995; Elmoussaoui et al., 2005). The dome and associated circulation move offshore, towards the central Atlantic, during summer. At this time it appears natural to expect that the cyclonic pattern will close through a northward current off Africa, between Capes Verde and Blanc. During winter Ekman pumping intensifies and moves east (Nykjaer and Van Camp, 1994), effectively merging with the coastal upwelling zone and possibly breaking the cyclonic coastal connection between NECC and NEC. The SACW in this region are transported north along the slope, typically centered at depths of 300 m, via the poleward undercurrent (Hughes and Barton, 1974). This current may reach the Canary Islands and even the Iberian Peninsula (for a review see Hagen, 2001), displaying substantial seasonal variability (Machín et al., 2006).
When considering the main controls on the steady-state dynamics, it is important to keep in mind that the mean field also experiences remotely forced interannual and interdecadal variations. Arfi (1985) used local data at 20°N to suggest that upwelling intensified from the 1960s to the 1970s. Roy (1991) also presented results supporting that the 1970s was a period of relative intense upwelling, which decreased in the 1980s. More recently satellite-derived sea surface temperature (SST) measurements have shown that the 1980s was a period of little interannual variability of the upwelling index (difference in SST between the coast and the open ocean) off Northwest Africa, with oscillations of a few tenths of a degree (Nykjaer and Van Camp, 1994; Hernández-Guerra and Nykjaer, 1997), but that there existed a major shift by the end of the 1980s into the mid 1990s, with a change in the upwelling index greater than one degree (Santos et al., 2005). These changes are quite important as compared with the mean upwelling indexes for the 18–26°N band, which range between 1 and 2 °C (Nykjaer and Van Camp, 1994; Hernández-Guerra and Nykjaer, 1997; Santos et al., 2005). The interannual oscillations in the upwelling index appear to be associated to individual North Atlantic Oscillation (NAO) events while the interdecadal variations may be related to sustained NAO events during several consecutive years (Santos et al., 2005). Our period of interest (March–April 1973 and October–November 1975) had moderate 3-month averaged NAO indexes (Climate Prediction Center, http://www.cpc.ncep.noaa.gov), which are characteristic for the mild intensification observed during the 1970s.
The above brief description of the main circulation patterns immediately points at several key issues that control the fluxes and large-scale patterns of central waters along Northwest Africa. The Cape Verde frontal system is characterized by sharp mesoscalar intrusions of both NACW and SACW (named interleaving after Barton and Hughes, 1982), how effective is it as a barrier between northern and southern waters? The coastal upwelling front stretches meridionally until the Cape Verde front, beyond in winter, is there a connection between these two frontal systems? The coastal transition zone is characterized by vertical upwelling cells and ubiquitous filaments that effectively transfer nutrient-rich waters into the nutrient-depleted surface waters of the subtropical gyre, where and at what rates these mass and nutrient exchanges take place?
In this work we have recovered the data of four historical cruises to produce two large-scale synoptic data sets. These data sets are carefully analyzed to investigate the above questions, so that we can improve our understanding and quantification of the water-mass and nutrient connections in the central waters all along Northwest Africa. In Section 2 we briefly describe the original data, and how it has been assembled to produce spring and fall data sets, and in Section 3 we plot the data to show the hydrographic conditions in the area during these two seasons. In Section 4 we explore what processes are responsible for enhancing mixing at the frontal system, in Section 5 we use the data to infer the surface and subsurface dynamic fields in the region, and in Section 6 we compute the along and cross-shore mass and nutrient transports.
Section snippets
Data set
The data used in this work were collected by IIP as part of four different hydrographic cruises: Atlor II (March 1973), Atlor III (April 1973), Atlor VI (October 1975), and Atlor VII (November 1975). These cruises stretched along the African coastline, between 16.5° and 26.1°N (Fig. 1), and the data were compiled and published in Cruzado and Manríquez (1974), Fraga and Manríquez (1974), Manríquez and Rucabado (1976), and Manríquez and Fraga (1978). Cruises Atlor III and Atlor VI covered the
Property–property diagrams
Fig. 2 illustrates the potential temperature–salinity (θ–S) relationships during both seasons, as obtained using all available stations (hereafter, when talking about temperature we will actually refer to potential temperature, calculated using the surface as the reference level). In this figure, as well as in Fig. 3, we have included straight lines that define NACW and SACW (Tomczak, 1981).
The hydrographic stations may be divided in three groups, following a North-to-South θ–S transition from
Mixing in the frontal system
The results in last section suggest that the Cape Verde frontal system effectively behaves as a barrier between NACW and SACW. This front, however, undergoes two major instabilities acting at very different spatial and temporal scales, which are responsible for enhancing the exchange of properties. To quantify the fluxes of central waters off Northwest Africa we need to better understand the character and magnitude of these processes, this is the purpose of this section.
Water and nutrient fluxes
The vertical upwelling cell drives nutrient-rich subsurface waters to the baroclinic zone, where they become accommodated by along-shore water and nutrient fluxes all the way to the Cape Verde front. In this manner the upwelling jet, if deep enough, may play a decisive role in the along-shore advection of the northern water masses. The Cape Verde frontal system is usually thought to be the southern limit reached by the northern waters. This system moves seasonally several degrees (Stramma and
Water and nutrient balances
The vertical upwelling cell off Northwest Africa would cause the southward water and nutrient transports to progressively increase towards the equator. This recirculation ends at the Cape Verde frontal zone, in what constitutes the southern limit of the horizontal cell (Pelegrí et al., 2005, Pelegrí et al., 2006). In this section we use the available data, together with some simplifying hypothesis, to obtain a gross perspective of the mass and nutrient balances in our study domain. Convergence
Conclusions
We have combined data from four hydrographic cruises during the 1970s in order to produce two seasonal data sets (spring 1973 and fall 1975) with meridional coverage off Northwest Africa from 17° to 26°N. The region is divided in three areas: southern (18–21°N), central (21–23.5°N), and northern (23.5–26°N). The data sets have been used with three main purposes: to describe the hydrography of the zone, to examine mixing processes at the frontal system, and to estimate water-mass and nutrient
Acknowledgments
We would like to thank all scientists, technicians, and crew personnel that participated in Atlor II, III, VI, and VII cruises. We gratefully acknowledge Schlitzer (2007) for making available Ocean Data View, used to prepare many figures in this paper. This research has been supported by the Spanish Ministerio de Educación y Ciencia, through project CANOA (Ref. no. CTM2005-00444/MAR). MVP would like to acknowledge Consejo Superior de Investigaciones Científicas for funding through an I3P grant.
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