The continental slope current system between Cape Verde and the Canary Islands

We use hydrographic, velocity and drifter data from a cruise carried out in November 2008 to describe the continental slope current system in the upper thermocline (down to 600 m) between Cape Verde and the Canary Islands. The major feature in the region is the Cape Verde Frontal Zone (CVFZ), separating waters from tropical (southern) and subtropical (northern) origin. The CVFZ is found to intersect the slope north of Cape Blanc, between 22°N and 23°N, but we find that southern waters are predominant over the slope as far north as 24°N. South of Cape Blanc (21.25°N) the Poleward Undercurrent (PUC) is a prominent northward jet (50 km wide), reaching down to 300 m and indistinguishable from the surface Mauritanian Current. North of Cape Blanc the upwelling front is found far offshore, opening a near-slope northward path to the PUC. Nevertheless, the northward PUC transport decreases from 2.8 Sv at 18°N to 1.7 Sv at 24°N, with about 1 Sv recirculating ofshore just south of Cape Blanc, in agreement with the trajectory of subsurface drifters. South of the CVFZ there is an abrupt thermohaline transition at σ ϴ =26.85 kg m –3 , which indicates the lower limit of the relatively pure (low salt and high oxygen content) South Atlantic Central Water (SACW) variety that coexists with the dominant locally-diluted (salinity increases through mixing with North Atlantic Central Water but oxygen diminishes because of enhanced remineralization) Cape Verde (SACWcv) variety. At 16°N about 70% of the PUC transport corresponds to the SACW variety but but this is transformed into 40% SACWcv at 24°N. However, between Cape Verde and Cape Blanc and in the 26.85 ϴ < 27.1 layer, we measure up to 0.8 Sv of SACWcv being transported south. The results strongly endorse the idea that the slope current system plays a major role in tropical-subtropical water-mass exchange.

[1]  E. Mittelstaedt.  The ocean boundary along the northwest African coast: Circulation and oceanographic properties at the sea surface , 1991 .

[2]  M. Arhan,et al.  Model-inferred upper ocean circulation in the eastern tropics of the North Atlantic. , 2005 .

[3]  J. Font,et al.  Water and nutrient fluxes off Northwest Africa , 2008 .

[4]  E. Barton.  Meanders, eddies and intrusions in the thermohaline front off northwest africa , 1987 .

[5]  J. Pelegrí,et al.  Seasonal Flow Reversals of Intermediate Waters in the Canary Current System East of the Canary Islands , 2010 .

[6]  M. Tomczak.  A multi-parameter extension of temperature/salinity diagram techniques for the analysis of non-isopycnal mixing , 1981 .

[7]  D. Fratantoni,et al.  The mid-depth circulation of the northwestern tropical Atlantic observed by floats , 2009 .

[8]  E. Mittelstaedt.  The upwelling area off Northwest Africa—A description of phenomena related to coastal upwelling , 1983 .

[9]  J. Sarmiento,et al.  Nutrients in the Atlantic thermocline , 1985 .

[10]  L. Kohlmeier,et al.  Some dietary predictors of plasma organochlorine concentrations in an elderly German population. , 1998, Archives of environmental health.

[11]  Fernando Fraga.  Distribution des masses d'eau dans l'upwelling de Mauritanie , 1974 .

[12]  Arne Körtzinger,et al.  Oxygen minimum zone in the North Atlantic south and east of the Cape Verde Islands , 2008 .

[13]  C. Donlon,et al.  OSTIA : An operational, high resolution, real time, global sea surface temperature analysis system , 2007, OCEANS 2007 - Europe.

[14]  L. Stramma,et al.  Upper-level circulation in the South Atlantic Ocean , 1991 .

[15]  François G. Meyer,et al.  High levels of PCBs in breast milk of inuit women from arctic quebec , 1989, Bulletin of environmental contamination and toxicology.

[16]  K. Denman,et al.  Least squares multiple tracer analysis of water mass composition , 1987 .

[17]  Charles Troupin,et al.  Seasonal variability of the Canary Current: A numerical study , 2011 .

[18]  J. Karstensen,et al.  The oxygen minimum zones in the eastern tropical Atlantic and Pacific oceans , 2008 .

[19]  P. Oliveira,et al.  Seasonal and interannual variability of surface circulation in the Cape Verde region from 8 years of merged T/P and ERS-2 altimeter data , 2005 .

[20]  A. Gabric,et al.  Offshore export of shelf production in the cape blanc (Mauritania) giant filament as derived from coastal zone color scanner imagery , 1993 .

[21]  '. M.Arhana,et al.  Hydrographic sections across the Atlantic at 7°30N and 4°30S , 2002 .

[22]  Matthias Tomczak,et al.  Identification of diapycnal mixing through optimum multiparameter analysis: 2. Evidence for unidirectional diapycnal mixing in the front between North and South Atlantic Central Water , 1994 .

[23]  L. Stramma,et al.  Water masses and currents in the upper tropical northeast Atlantic off northwest Africa , 2005 .

[24]  J. Pelegrí,et al.  Meridional changes in water mass distributions off NW Africa during November 2007/2008 , 2012 .

[25]  Simón Ruiz,et al.  Vortex Merger in Oceanic Tripoles , 2011 .

[26]  J. Pelegrí,et al.  Nutrient irrigation of the North Atlantic , 2006 .

[27]  Pablo Sangrà,et al.  Coupling between the open ocean and the coastal upwelling region off northwest Africa: water recirculation and offshore pumping of organic matter , 2005 .

[28]  J. Pedlosky,et al.  The Ventilated Thermocline , 1983 .

[29]  R. Onken,et al.  Seasonal changes in the tropical Atlantic circulation: Observation and simulation of the Guinea Dome , 1992 .

[30]  G. Csanady.  Intermittent ‘full’ upwelling in Lake Ontario , 1977 .

[31]  L. V. Camp,et al.  Seasonal and interannual variability of coastal upwelling along northwest Africa and Portugal from 1981 to 1991 , 1994 .

[32]  W. Zenk,et al.  Cape Verde Frontal Zone , 1991 .

[33]  E. Hagen.  Northwest African upwelling scenario , 2001 .

[34]  J. Pelegrí,et al.  Mass fluxes in the Canary Basin , 2006 .

[35]  M. Tomczak.  Problems of physical oceanography in coastal upwelling investigations , 1972 .

[36]  E. Mittelstaedt.  On the currents along the Northwest African coast South of 22° North , 1976 .

[37]  B. Voituriez,et al.  Influence of the Southern Atlantic Central Water on the distribution of salinity and oxygen in the northeast tropical Atlantic Ocean , 1978 .

[38]  M. Tomczak,et al.  Optimum multiparameter analysis of mixing in the thermocline of the eastern Indian Ocean , 1989 .

[39]  F. Schott,et al.  The mean flow field of the tropical Atlantic Ocean , 1999 .

[40]  P. Schlittenhardt,et al.  Upwelling and boundary circulation off Northwest Africa as depicted by infrared and visible satellite observations , 1991 .

[41]  E. D. Barton.  The Poleward Undercurrent On The Eastern Boundary Of The Subtropical North Atlantic , 1989 .

[42]  J. Pelegrí,et al.  Northward Penetration of Antarctic Intermediate Water off Northwest Africa , 2009 .

[43]  Philip L. Richardson,et al.  Mapping climatological seasonal variations of surface currents in the tropical Atlantic using ship drifts , 1986 .