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Circulation of the Thermocline Salinity Maximum Waters Off the Northern Brazil As Inferred from in Situ Measurements and Numerical Results

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Circulation of the Thermocline Salinity Maximum Waters Off the Northern Brazil As Inferred from in Situ Measurements and Numerical Results Ann. Geophys., 27, 1861–1873, 2009 www.ann-geophys.net/27/1861/2009/ Annales © Author(s) 2009. This work is distributed under Geophysicae the Creative Commons Attribution 3.0 License. Circulation of the thermocline salinity maximum waters off the Northern Brazil as inferred from in situ measurements and numerical results A. C. Silva1, B. Bourles2, and M. Araujo3 1Instituto de Cienciasˆ do Mar, Universidade Federal do Ceara,´ Av. Abolic¸ao˜ 3207, 60165-081, Fortaleza – CE, Brazil 2Institut de Recherche pour le Developpement,´ 08 B.P. 841 – Cotonou, Benin´ 3Laboratorio´ de Oceanografia F´ısica Estuarina e Costeira, Depto. Oceanografia, UFPE, CNPq Fellow. Av. Arquitetura s/n, Campus Universitario,´ 50740-550, Recife, PE, Brazil Received: 24 September 2008 – Revised: 17 March 2009 – Accepted: 6 April 2009 – Published: 4 May 2009 Abstract. High resolution hydrographic observations of retroflection into the North Equatorial Undercurrent and the temperature and salinity are used to analyse the subsurface EUC, contribute to the transport of South Atlantic high salin- circulation along the coast of North Brazil, off the Amazon ity water into the Northern Hemisphere. ◦ ◦ mouth, between 2 S and 6 N. Observations are presented Keywords. Oceanography: general (Equatorial oceanogra- from four cruises carried out in different periods of the year phy, Descriptive and regional oceanography) – Oceanogra- (March–May 1995, May–June 1999, July–August 2001 and phy: physical (Western boundary currents) October–November 1997). Numerical model outputs com- plement the results of the shipboard measurements, and are used to complete the descriptions of mesoscale circulation. The Salinity Maximum Waters are here analyzed, principally 1 Introduction in order to describe the penetration of waters originating in The Western Tropical Atlantic Ocean plays an important role the Southern Hemisphere toward the Northern Hemisphere in the inter-hemispheric transports of mass, heat, and salinity through the North Brazil Current (NBC)/North Brazil Un- (Schmitz and Mc Cartney, 1993). Along the western bound- dercurrent (NBUC). Our results show that, if the Equatorial ary, where the Deep Western Boundary Current transports Undercurrent (EUC) is fed by Northern Atlantic Waters, this cold North Atlantic Deep Water toward the Southern Hemi- contribution may only occur in the ocean interior, east of the sphere, the North Brazil Current (hereafter NBC) transports western boundary around 100 m depth. Modeling results in- warm surface waters northward from the Southern Hemi- dicate a southward penetration of the Western Boundary Un- sphere. Both currents are major components of the global dercurrent (WBUC) below the thermocline, along the North thermohaline circulation cell (Gordon, 1986). Brazilian coast into the EUC or the North Equatorial Under- The NBC and its subsurface component, the North Brazil current (NEUC) (around 48◦ W–3◦ N). The WBUC in the re- Undercurrent (NBUC), feed, through complex retroflection gion does not flow more south than 3◦ N. The northern waters patterns, eastward flowing equatorial currents at different are diverted eastward either by the NBC retroflection or by depths and latitudes (e.g., Schott et al., 1995; Bourles` et al., the northern edge of the associated clockwise rings. The ex- 1999a, b): (i) the South Equatorial Undercurrent (SEUC) be- istence of subsurface mesoscale rings associated to the NBC low the thermocline around 3◦ S, (ii) the Equatorial Under- retroflection is evidenced, without any signature in the sur- current (EUC) centered at the thermocline and the equator; face layer, so confirming earlier numerical model outputs. (iii) the North Equatorial Undercurrent (NEUC) found be- These subsurface anticyclones, linked to the NBC/NBUC low the thermocline around 3◦ N; and (iv) the North Equato- rial Counter Current (NECC) located above the thermocline north of 5◦ N. Correspondence to: A. C. Silva These eastward currents may also be fed by North Atlantic ([email protected]) waters through the recirculation of the North Equatorial Published by Copernicus Publications on behalf of the European Geosciences Union. 1862 A. C. Silva et al.: Circulation of thermocline salinity maximum waters off Northern Brazil Current (NEC) and the Western Boundary Undercurrent Amazon plume in inducting shallow pycnoclines within the (WBUC), but a direct connection between the WBUC and isothermal layers of the Northern Brazilian shelf. Observa- the EUC is still unclear (Schott and Boning,¨ 1991; Wilson et tions indicate that, along the North Brazilian shelf break, al., 1994; Bourles` et al., 1999a, b). Stramma et al. (2005), by the formation of barrier layers seems also to be associated using the 1/12◦ – Flame General Circulation Model, demon- with the advection of the subsurface maximum salinity wa- strated that part of the salinity maximum water from the ters formed in the region of the subtropical gyres and trans- Northern Hemisphere contributes to the water feeding the ported to the study area by the NBC/NBUC system (Stramma NEUC and also showed that salinity maximum water from et al., 1995; Pailler et al., 1999; Silva et al., 2005a; Stramma the South Atlantic is transported towards the north Brazil- et al., 2005). ian Shelf. The same authors also demonstrated that the NBC These data sets, along with the results obtained from a re- suffers a recirculation feeding the EUC close to the equator, alistic numerical simulation, are used in order to attempt a generating two eddy features, located near 50◦ W and 44◦ W. better description and comprehension of the complex circu- North of the equator, the NBC/NBUC retroflection may be lation within the subsurface layers, where the eastward un- linked to the formation of rings (e.g., Johns et al., 1990, 2003; dercurrents are fed through the NBC/NBUC retroflection and Didden and Schott, 1993; Richardson et al., 1994). Results where thermocline salinity maximum waters may be clearly based on satellite images and in situ observations demon- identified. strate that these rings break away from the retroflection and The paper is organized as follows. The main aspects of move northwestward along the coast of Guyana, representing the hydrographic observations and numerical simulations are an important mechanism for transporting waters from both introducted in Sect. 2. In Sects. 3 and 4, we describe the ver- equatorial and South Atlantic to the Northern Hemisphere tical and horizontal sections obtained with high resolution (Garzoli et al., 2003). During the last decade, many works hydrographic data observations of temperature and salinity, have focused on the evaluation of the genesis and the water principally in order to describe the penetration of maximum transport capacity of the NBC rings (Johns et al., 1990, 2003; salinity waters. The hydrographic data were complemented Didden and Schott, 1993; Richardson et al., 1994; Fratantoni by modelling results to obtain additional information about et al., 1995). More recently Wilson et al. (2002) described the maximum salinity waters and mesoscale transport vari- three different types of rings as follows: the first one is char- ability in this area. In these two sections, we also discuss the acterized by shallow, surface-trapped structures that are con- circulation of the seasonal and interannual variability, by us- fined to the upper ocean layers; the second type is charac- ing modelling results, to obtain additional information about terized by deep-reaching structures with significant swirl ve- the maximum salinity waters and mesoscale transport vari- − locities (∼0.2 m s 1) extending down to 2000 m; whereas the ability in this area. We conclude with a discussion relat- third type presents thermocline intensified structures with al- ing the principals results obtained from our combined hy- most no detectable surface signature. drographic data observations and numerical results. All these above-mentioned features make the Western Tropical Atlantic a very important ocean crossroads, with strong confluence of water masses coming from the north, 2 Data and methods the south and the equatorial Atlantic, and thus with intense blending processes. Therefore, a particular attention should 2.1 Shipboard surveys be given to the ocean area situated between the shelf break and the 2000 m isobath, which seems to be a key for a better The hydrographic data sets were collected during four cruises understanding of the retroflection and the formation of the performed at different times of the year, in March–May 1995, NBC/NBUC rings. Previous studies in this region have been October–November 1997, May–June 1999 and July–August either limited to the shelf region (e.g. AMASSEDS Program, 2001, carried out on board the R/V Antares, as part of the ac- Lentz and Limeburner, 1995) or focused on large scale ocean tivities of the Brazilian REVIZEE Program. The REVIZEE circulation (e.g. Schott et al., 1998; Bourles` et al., 1999a, b). Program, coordinated by the Comissao˜ Interministerial de In this paper we use high resolution in situ data from Recursos do Mar/Inter-ministerial Commission of Sea Re- four different cruises of the Brazilian REVIZEE Program sources (CIRM), is the result of the commitment made by (Programa de Avaliac¸ao˜ do Potencial Sustentavel´ de Recur- Brazil before the United Nations (UN), to ratify in 1988 the sos Vivos da Zona Economicaˆ Exclusiva) carried out along International Convention of the Law of the Sea, in effect the northern coast of Brazil limited by 2.5◦ S–8◦ N in lati- since 1994. The main objective of the REVIZEE Program tude, and 40◦ W–52◦ W in longitude, i.e. in a key area where is to perform a survey of the living resources (and of the the NBC/NBUC system penetrates into the Northern Hemi- correlated abiotic factors) present in the Brazilian Exclusive sphere. The main objective is to describe a seasonal and spa- Economic Zone (CIRM, 1994). tial variability of the thermocline salinity maximum waters Field data correspond to a total amount of 307 vertical and circulation off the Northern Brazil. From field data, Silva continuous profiles of temperature and salinity obtained us- et al.
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