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The Atlantic Subtropical Front/Current Systems of Azores and St. Helena

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The Atlantic Subtropical Front/Current Systems of Azores and St. Helena VOLUME 37 JOURNAL OF PHYSICAL OCEANOGRAPHY NOVEMBER 2007 The Atlantic Subtropical Front/Current Systems of Azores and St. Helena MANUELA F. JULIANO AND MÁRIO L. G. R. ALVES Laboratory of Marine Environment and Technology, University of the Azores, Praia da Vitória, Azores, Portugal (Manuscript received 28 September 2004, in final form 9 January 2007) ABSTRACT A large-scale climatic ocean circulation model was used to study the Atlantic Ocean circulation. This inverse model is an extension of the ␤-spiral formulation presented in papers by Stommel and Schott with a more complete version of the vorticity equation, including relative vorticity in addition to planetary vorticity. Also, a more complete database for hydrological measurements in the Atlantic Ocean was used, including not only the National Oceanographic Data Center database but also World Ocean Circulation Experiment data and cruises near the Azores, Angola, and Guinea-Bissau. A detailed analysis of the Northern Hemisphere Azores Current and Front shows that this new database and the model results were able to capture all major features reported previously. In the Southern Hemisphere, the authors have identified fully and described the subtropical front that is the counterpart to the Azores Current, which they call the St. Helena Current and Front. Both current systems of both hemispheres have similar intensities, depth penetration, volume transports, and zonal flow. Both have associated subsurface adjacent counter- current flows, and their main cores flow at similar latitudes (ϳ34°N for the Azores Current and 34°S for the St. Helena Current). It is argued that both current systems and associated fronts are the poleward 18°C Mode Water discontinuities of the two Atlantic subtropical gyres and that both originate at the correspond- ing hemisphere western boundary current systems from which they penetrate into the open ocean interior. Thus, both currents should have a similar forcing source, and their origin should not be linked to any geographical peculiarities. 1. Introduction A large anticyclonic gyre is found in the subtropical The atmospheric and oceanic circulations each show region of both hemispheres. In the Atlantic Ocean the a significant meridional symmetry in both hemispheres subtropical gyre of the Northern Hemisphere is delim- of the globe. In the ocean this symmetry can be noted ited to the south by the North Equatorial Current, to through the existence of western boundary currents, the west by the Caribbean Sea, the “Loop” Current, the eastern boundary currents, and open-ocean systems in Florida Current, and the Gulf Stream, to the north by both hemispheres. Despite these similarities, there are the North Atlantic Current, and to the east by eastern also a few differences, due either to continental geog- boundary current system. The Southern Hemisphere raphy peculiarities like the coastline tilting differences subtropical gyre is delimited by the South Equatorial between the Gulf Stream and the Brazil Current zones Current, the Brazil Current, the South Atlantic Current (see, e.g., Da Silveira et al. 1999) or to the existence of and Subantarctic Front (SAF), and the Benguela Cur- the interhemispheric meridional overturning circula- rent. The North Atlantic subtropical gyre is zonally tion that contributes to larger transports in the Gulf crossed by the Azores Current, which appears to be the Stream than in the Brazil Current. In the Southern poleward boundary of the 18°C Subtropical Mode Wa- Hemisphere, the absence of geographic barriers gives ter (Alves and Colin de Verdière 1999; Jia 2000). In the rise to the Antarctic Circumpolar Current, which has South Atlantic Ocean, previous studies (Gordon et al. no Northern Hemisphere counterpart. 1992; Tsuchiya et al. 1994; Belkin 1994; Belkin and Gor- don 1996; Provost et al. 1999) identified a subtropical zonal feature but did not note its similarity and corre- Corresponding author address: Manuela F. Juliano, LAMTec- spondence to the Azores Current in the North Atlantic. Laboratory of Marine Environment and Technology, University of the Azores, Apartado 64, Edifícios da Marina, 9760-412 Praia Therefore there had appeared to be an asymmetry be- da Vitória, Azores, Portugal. tween the North and South Atlantic gyres that could E-mail: [email protected] not be explained by asymmetry in atmospheric circula- DOI: 10.1175/2007JPO3150.1 © 2007 American Meteorological Society 2573 Unauthenticated | Downloaded 10/10/21 03:18 PM UTC JPO3121 2574 JOURNAL OF PHYSICAL OCEANOGRAPHY VOLUME 37 tion, because both hemispheres are dominated by anti- dressed, and the Jia (2000) arguments are revisited. In cyclonic zones: the Azores anticyclone in the Northern section 5, some conclusions are presented. Hemisphere and the St. Helena anticyclone in the Southern Hemisphere. To explain the apparent asym- 2. Data metry, Thompson (1971) suggested a combined effect of the Coriolis parameter and the gradient of topogra- For our Atlantic Ocean climatological description phy as being mainly responsible for the absence or (80°S–80°N, 100°W–35°E), we have used hydrological weakening of these zonal eastward currents in the data (temperature T and salinity S) as a function of Southern Hemisphere. We note also that even the well- pressure, longitude, latitude, and time that were ob- observed Azores Front/Current system has been poorly tained through historical databases: NODC, Global reproduced by general circulation numerical models, Temperature Salinity Profile Project (GTSPP), and possibly because of the weakness of the meridional WOCE. To these databases we have also added data density gradient in the models (Jia 2000). Also, the nec- from oceanographic cruises carried out in the region of essary condition for the existence of baroclinic instabil- the Azores,1 Angola,2 and Guinea-Bissau.3 The result- ity, one of the primary mechanisms now known to be ing database covered the period from 1940 to 1999. The responsible for the major variability of this system entire dataset was submitted to procedure for quality (Alves 1996; Beckmann et al. 1994a), is not verified in and validation control (the same as described in the these general circulation models, even when horizontal World Ocean Database 1998; Conkright et al. 1999), resolution is increased (Beckmann et al. 1994b). followed by objective interpolation using an algorithm, The upgrade of historical National Oceanographic described by Papoulis (1991), with a correlation radius Data Center (NODC) databases with data of higher of 500 km (Alves et al. 1994; Juliano 2003). In this way, resolution and quality [e.g., World Ocean Circulation optimized fields of temperature and salinity were ob- Experiment (WOCE)] for the South Atlantic and the tained, evenly distributed in five three-dimensional ma- implementation of an innovative method for large-scale trices for the Atlantic Ocean—one for each season and circulation determination now permits a much more one for the annual average. For the seasonal time complete description of the counterpart of the Azores blocks, the following aggregation was used: trimester 1 Current in the Southern Hemisphere. We have given it consisted of January–March; trimester 2 covered April– the name St. Helena Current. This name was chosen by June; trimester 3 was for July–September; and trimester analogy with the Azores Current because of the prox- 4 consisted of October–December. All of these matri- imity to St. Helena Island and because the atmospheric ces have 89 vertical levels unevenly distributed between 5 and 6000 dbar, as listed in Table 1, with a horizontal St. Helena anticyclone in the Southern Hemisphere is resolution of 0.5° ϫ 0.5° (latitude ϫ longitude) covering the counterpart of the northern Azores anticyclone. the Atlantic Ocean. This current has also been called the North Subtropical As an example of this procedure, Fig. 1 shows the Front (Belkin 1994; Belkin and Gordon 1996; Provost position of each validated data point at the surface and et al. 1999), the Brazil Current Front (Tsuchiya et al. for the annual average of the horizontal distributions of 1994), and the Benguela/South Atlantic Front (Gordon temperature and salinity. It is clear that there is much et al. 1992). more data for the Northern Hemisphere than for the The St. Helena Current appears to be a zonal cur- Southern Hemisphere and that, closer to the shore, the rent, flowing eastward with horizontal and vertical structure similar to the Azores Current. Ours appears to be a complete description of it and the first to rec- 1 Circulac¸a˜ o Oceaˆ nica e Dinaˆ mica Frontal na Regia˜ o dos ognize its similarity with the Azores Current and Front. Ac¸ores (CODFRA) project (JNICT-PMCT/C/MAR/941/90); The Azores and St. Helena Current/Front systems will Frente/Corrente dos Ac¸ores (FCA) cruises and the Frontoge´nese both be described and compared herein. e Instabilidade Baroclı´nica na Regia˜o dos Ac¸ores (FIBRA) A brief description of the data used is given in section project (STRIDE-STRDB/C/MAR/230/92), Simulac¸a˜o Nume´rica da Variabilidade e Clima da Frente–Corrente dos Ac¸ores 2, and in section 3 the large-scale ocean circulation (CVFCA) project (Praxis/3/3.2/EMG/1956/95) and Canary Is- model used is briefly described. In section 4 a compari- lands Azores Gibraltar Observations (CANIGO) project (MAST son is made between the well-known Azores Current/ III-MAS3-CT96–0060). 2 Front system and that of the less well known “St. Hel- Instituto de Investigac¸a˜o das Pescas e do Mar (IPIMAR) Re- port, cruise N/E “Capricórnio,” in Angola, October–December ena Current.” We emphasize similarities between the 1995 (in Portuguese). currents and note some differences. The cause for ex- 3 IPIMAR Report, cruise N/E “Capricórnio,” in Guinea- istence of these current/front systems is briefly ad- Bissau, May–June 1995 (in Portuguese). Unauthenticated | Downloaded 10/10/21 03:18 PM UTC NOVEMBER 2007 J U L I A N O A N D ALVES 2575 TABLE 1.
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