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EASTERN BOUNDARY CIRCULATION and HYDROGRAPHY OFF ANGOLA Building Angolan Oceanographic Capacities

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EASTERN BOUNDARY CIRCULATION and HYDROGRAPHY OFF ANGOLA Building Angolan Oceanographic Capacities EASTERN BOUNDARY CIRCULATION AND HYDROGRAPHY OFF ANGOLA Building Angolan Oceanographic Capacities P. TCHIPALANGA, M. DENGLER, P. BRANdt, R. KOPTE, M. MACUÉRIA, P. COELHO, M. OSTROWSKI, AND N. S. KEENLYSIDE The seasonal circulation and interannual hydrographic variability off the coast of Angola is revealed by biannual research cruise data (1995–2017) from the Nansen Programme. ngola is located at the Atlantic coast in south- development. Currently, the fishing sector is third western Africa between 5° and 17°20ʹS, with in importance to the national economy after the oil Aborders to the Democratic Republic of Congo in and mining industries and supplies about 25% of the the north and to Namibia in the south. Its coastline total animal protein intake of the Angolan popula- stretches over a distance of 1,600 km. The Angolan tion (FAO 2011). However, fish resources are found territorial waters support a highly productive ecosys- to be affected by climate variability and changes tem. Seasonal upwelling occurs in large parts of its of the eastern boundary upwelling ecosystems coastal zone during austral winter (Fig. 1). Coupled (e.g., Gammelsrød et al. 1998; Parrish et al. 2000; with a dense coastal population, this marine ecosys- Lehodey et al. 2006; Gruber 2011). There is urgent tem plays a key socioeconomic role in the country’s need to understand these impacts to help sustainable AFFILIATIONS: TCHIPALANGA—Departamento do Ambiente Norway; KEENLYSIDE—Geophysical Institute, Bjerknes Centre for e Ecossistemas Aquáticos, Instituto Nacional de Investigação Climate Research, University of Bergen, Bergen, Norway Pesqueira, Moçâmedes, Angola; DENGLER AND KOPTE—Physical Ocean- CORRESPONDING AUTHOR: Marcus Dengler, ography, Ocean Circulation and Climate Dynamics, GEOMAR [email protected] Helmholtz Centre for Ocean Research, Kiel, Germany; BRANdt— The abstract for this article can be found in this issue, following the table Physical Oceanography, Ocean Circulation and Climate Dynamics, of contents. GEOMAR Helmholtz Centre for Ocean Research, and Mathe- DOI:10.1175/BAMS-D-17-0197.1 matisch-Naturwissenschaftliche Fakultät, Christian-Albrechts-Uni- versität zu Kiel, Kiel, Germany; MACUÉRIA AND COELHO—Instituto In final form 1 February 2018 ©2018 American Meteorological Society Nacional de Investigação Pesqueira, Luanda, Angola; OSTROWSKI— Oceanography and Climate, Institute of Marine Research, Bergen, This article is licensed under a Creative Commons Attribution 4.0 license. AMERICAN METEOROLOGICAL SOCIETY AUGUST 2018 | 1589 Unauthenticated | Downloaded 10/06/21 07:28 PM UTC climate system (Stramma et al. 2008; Monteiro et al. 2011; Schmidtko et al. 2017). The ocean currents at the continental margin repre- sent a typical tropical east- ern boundary circulation system dominated by the poleward-flowing Angola Current. Predominately during austral summer, the Angola Current is thought to transport warm and saline, low-oxygen South Atlantic Central Water (SACW) southward into the coastal upwelling region, possibly resulting in hy- poxic to anoxic conditions on the shelf (Mohrholz et al. 2008; Monteiro et al. 2008; Rouault 2012). In turn, the Angola Current is thought to be supplied by the east- ward currents in the equa- torial region (Peterson and Stramma 1991; Wacongne and Piton 1992; Rouault et al. 2007). Historically (see sidebar for a detailed description of the history of physical ocean- ography in Angola), knowl- edge of the velocity structure and transport of the Angola Current at the coast had been gained exclusively from few syn optic hydrographic data FIG. 1. Average sea surface temperature (SST) and sea surface salinity (SSS) (Moroshkin et al. 1970; Dias in (a),(c) Feb–Apr and (b),(d) Jun–Aug determined from microwave fusion 1983a,b; Mercier et al. 2003). Optimum Interpolation (OI) SST (www.remss.com) and Soil Moisture Ocean These earlier studies found Salinity level 3, version 2 (www.catds.fr), satellite data, respectively. geostrophic poleward flow between the surface and management of the marine resources and to assist 300–400-m depth having velocities between 30 and Angola in meeting its sustainable management goals. 50 cm s−1. Transport estimates of the Angola Cur- Below the productive upper ocean, thermo- rent determined by Dias (1983a) were 1.2 and 3.7 Sv cline waters are depleted in oxygen and form one (1 Sv ≡ 106 m3 s−1) from data collected in September of the most pronounced oxygen minimum zones 1970 and July 1971, respectively. However, Mercier et al. (OMZs) in the global ocean. Recent observations (2003), applying an inverse model to hydrographic sec- suggest a decline in oxygen content of the oceans, tions, reported an Angola Current transport of 11 Sv particularly in the southeastern Atlantic, and an in January–March 1995 corresponding to the peak expansion of OMZs, modulated by the interan- phase of the 1995 Benguela Niño. During a survey in nual, decadal, and multidecadal variability of our April 1999, a secondary, offshore branch of southward 1590 | AUGUST 2018 Unauthenticated | Downloaded 10/06/21 07:28 PM UTC HISTORY OF PHYSICAL OCEANOGRAPHY IN ANGOLA he German R/V Meteor expedition in measurements. Early data were used the Benguela Niños. Under the T 1925–27 might be considered as the to demonstrate the seasonal march of umbrella of BENEFIT, several research starting point of physical oceanography temperature, indicating the presence of cruises with German R/Vs Poseidon, studies off Angola. This survey included primary and secondary upwelling and Meteor, Petr Kottsov, and Alexander von two trans-Atlantic transects with an downwelling seasons off Angola (Berrit Humboldt and the South African R/V inserted port call in Moçâmedes in and Dias 1977). Similar time series Africana were carried out. A new moni- southern Angola. The observational stations were established at 13°45´S toring program was also established program included hydrographic mea- (Santa Marta) and 15°S (Moçâmedes), along a section off Moçâmedes, with 11 surements, providing a general picture both discontinued in 1975. surveys completed until now and still of the tropical and South Atlantic wa- The turmoil of the war for indepen- ongoing. Different bilateral agreements ter mass and oxygen distribution (Wat- dence in 1975 saw the most difficult allowed frequent visits by research tenberg 1938; Wüst 1935). The USSR’s period in the history of the Angolan vessels from different countries, mostly R/V Akademik Kurchatov carried out a oceanography. Most of the infrastruc- from Germany, Spain, Portugal, Great first detailed hydrographic survey off ture, instruments, and equipment were Britain, Russia, and Cuba. Angola and northern Namibia between lost and much of the collected litera- More recently, the NansClim April and June 1968. The obtained ture, reports, and data were dispersed. project (2009–14) was implemented data were used for a first description In the early 1980s, the Angolan Minis- between Norway, Angola, Namibia, of the eastern boundary circulation try of Fishery signed an agreement with and South Africa (Jarre et al. 2015). (Moroshkin et al. 1970). At about the the Swedish International Cooperation By using data collected through the same time, the Portugal Navy and the Agency to, among others, rebuild the Nansen Programme and other relevant Mission of Bio-Oceanological Studies R/V Goa. Sporadic research surveys regional data, the project aimed at of Angolan Fisheries (MEBPA) was es- were carried out with the rebuilt vessel studying ocean and climate variability tablished; it later became the INIP. INIP or with Cuban and USSR research ves- and corresponding changes in ma- responsibilities included acquisition of sels aiming to monitor the main fishing rine biodiversity. The cooperation oceanographic data at fixed stations, resources. between Germany, Angola, Namibia, along monitoring lines, and during The arrival of the Nansen Pro- and South Africa was fostered through coastal surveys. Two research vessels, gramme to Angola in the mid-1980s the Geochemistry and Ecology of the R/V Goa and R/V Sardinella, were em- marked the beginning of a new era Namibian Upwelling System (GE NUS, ployed. Starting in 1967, a transect at in the monitoring of the Angolan 2009–15) and Science Partnerships about 12°S across the continental slope coastal ocean and its living resources. for the Assessment of Complex Earth and shelf was taken 4–6 times per year, The surveys with the R/V Dr. Fridtjof System Processes (SPACES, 2012–18) yielding a first estimate of seasonal Nansen were first conducted in 1985. programs, which included additional variability of the boundary circula- Since 1995, these surveys have been cruises with R/Vs Maria S. Merian and tion (Dias 1983a). A systematic study carried out twice a year, covering the Meteor as well as first direct velocity covering the shelf of southern Angola entire extent of the Angolan continen- observations from moorings in the An- started in 1968 with three cruises of tal shelf. gola Current (Kopte et al. 2017). The R/V Goa per year. Daily data acquisi- The regional cooperation among latter was performed also in coopera- tion was performed at the Lobito Angola, Namibia, and South Africa was tion with the EU-PREFACE (2013–17) time series station (12°19´S, 13°35´E) improved through the programs Ben- project, having 28 partners across 18 at about 40-m water depth. These guela Environment Fisheries Interaction countries in Europe and Africa. EU- measurements, including several physi- and Training (BENEFIT, 1999–2009) PREFACE has enhanced the Angolan cal and biogeochemical parameters, and Benguela Current Large Marine observing system by funding additional started in 1968, stopped in 1975, and Ecosystem (BCLME, 2002–08; now cruises, autonomous measurement were restarted in 1978. They are run- Benguela Current Commission). These programs, and equipment for an ocean- ning to this day, although showing many programs particularly addressed meteorology buoy in the PIRATA gaps and a general reduction in the climate variability associated with ex- network (Bourlès et al. 2008) at 6°S, parameter range compared to the early treme environmental events, including 8°E (PIRATA Southeast extension).
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