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3.5. Canary Islands Eddies and Coastal Upwelling Filaments Off North‐West Africa

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3.5. Canary Islands Eddies and Coastal Upwelling Filaments Off North‐West Africa Canary Islands eddies and coastal upwelling filaments off North-west Africa Item Type Report Section Authors Sangrà, Pablo Publisher IOC-UNESCO Download date 24/09/2021 16:06:32 Link to Item http://hdl.handle.net/1834/9181 3.5. Canary Islands eddies and coastal upwelling filaments off North‐west Africa For bibliographic purposes, this article should be cited as: Sangrà, P. 2015. Canary Islands eddies and coastal upwelling filaments off North‐west Africa. In: Oceanographic and biological features in the Canary Current Large Marine Ecosystem. Valdés, L. and Déniz‐ González, I. (eds). IOC‐UNESCO, Paris. IOC Technical Series, No. 115, pp. 105‐114. URI: http://hdl.handle.net/1834/9181. The publication should be cited as follows: Valdés, L. and Déniz‐González, I. (eds). 2015. Oceanographic and biological features in the Canary Current Large Marine Ecosystem. IOC‐UNESCO, Paris. IOC Technical Series, No. 115: 383 pp. URI: http://hdl.handle.net/1834/9135. The report Oceanographic and biological features in the Canary Current Large Marine Ecosystem and its separate parts are available on‐line at: http://www.unesco.org/new/en/ioc/ts115. The bibliography of the entire publication is listed in alphabetical order on pages 351‐379. The bibliography cited in this particular article was extracted from the full bibliography and is listed in alphabetical order at the end of this offprint, in unnumbered pages. ABSTRACT Mesoscale eddies are almost continuously spun off from the Canary Islands constituting a unique eddies source that is not present in other Eastern Boundary Upwelling Systems. The main forcing mechanism is the Canary Current perturbation by the islands topography. Wind forcing also contributes to their generation lowering the threshold of the Canary Current intensity for triggering of the eddy shedding process. They rotate initially in solid body rotation losing it when they reach their mature stage. They are also the main source of long lived eddies for the NE subtropical Atlantic building up the Canary Eddy Corridor. This eddy corridor plays an important role as a zonal conduit carrying both physical and biogeochemical properties from the cold nutrient‐rich upwelling eastern boundary towards the interior ocean. Coastal upwelling filaments are recurrently observed near Cape Ghir, Cape Jubi, Cape Bojador and Cape Blanc. Although they have common characteristics, such as low temperature and high chlorophyll‐a signals, their structure and origin are different. They play a key role in transferring biogeochemical properties from the coastal upwelling eutrophic region towards the interior oligotrophic subtropical gyre, contributing thus to its enrichment and to the setting up of the Coastal Transition Zone. Keywords: Canary Islands · Mesoscale eddies · Upwelling filaments · Canary Eddy Corridor · Canary Current Large Marine Ecosystem · Northwest Africa IOC TECHNICAL SERIES, No. 115, pp. 105‐114. URI: http://hdl.handle.net/1834/9181. 2015 CANARY ISLANDS EDDIES AND COASTAL UPWELLING FILAMENTS OFF NORTH‐WEST AFRICA Pablo SANGRÀ Instituto de Oceanografía y Cambio Global (IOCAG), Universidad de Las Palmas de Gran Canaria. Spain 3.5.1. INTRODUCTION A distinctive feature of the Canary Current Large Marine Ecosystem (CCLME) is the presence of an archipelago, the Canary Islands, extending ca. 500 km zonally that act as a topographic barrier perturbing the southward flowing of the prevailing winds (Trade Winds) and currents (Canary Current). As a result, oceanic mesoscale eddies are almost continuously spun off by the islands of the archipelago. The CCLME therefore includes a source of mesoscale eddies that is not present in other Eastern Boundary Upwelling Systems (EBUS) and, as we will review in this work, this may have a great impact on the modulation and distribution of physical and biogeochemical properties, both locally and regionally. The first observations of Canary Island induced eddies were from remote sensing (La Viollete, 1974; Hernández‐Guerra, 1990; Hernández‐Guerra et al., 1993). The sea surface temperature signature of warm core anticyclonic eddies has been observed southeast of Gran Canaria, Tenerife and El Hierro islands. The sea surface temperature signature of cold core cyclonic eddies has been observed southwest of Gran Canaria, La Palma and El Hierro islands. Arístegui et al. (1994) first described the Canary Island eddies from in situ data, revealing their principal characteristics such as diameter, depth and property anomalies in the hydrographic fields. These and further studies were motivated by the search for an explanation of the observed local enhancement of biological productivity by islands, known as the Island Mass Effect (Doty and Ogury, 1956). Later, after a first phase of study concentrated on the eddies hydrography, the focus shifted towards an understanding of their biological effects, taking as a case study the eddies shed from the island of Gran Canaria (Arístegui et al., 1997; Barton et al., 1998; Arístegui and Montero, 2005; Alonso‐ González et al., 2013; Lasternas et al., 2013). Simultaneously, progress was also made in determining the hydrographic and kinematic characteristics of these eddies, their generation mechanisms, and establishing their Lagrangian evolution (Sangrà et al., 2005, 2007; Piedeleu et al., 2009). Sangrà et al. (2009) observed that Canary Island induced eddies contribute to a zonally oriented eddy corridor that can extend as far as the Mid Atlantic Ridge; this was named the Canary Eddy Corridor (CEC, Figure 3.5.1). This study represented a turning point in understanding of the role that Canary Island induced eddies may play, as their impact may be felt far from the islands, affecting the whole northeastern subtropical Atlantic region. Upwelling filaments are also typical mesoscale features of the CCLME. They are related with the variability of the coastal upwelling and consist of narrow, O(10 km), and elongated, O(100 km), structures of upwelled water that stretch zonally offshore from the coast, often rooted to major capes (Figure 3.5.2). They are shallow structures, O(100 m), identifiable by low surface temperature and high chlorophyll‐a concentration signals. Since the very first remote sensing observations of the northwest African Upwelling System (NAUS), upwelling filaments have been recurrently observed near Cape Ghir (3038’N), Cape Jubi (2740’N), Cape Bojador (2607’N) and Cape Blanc (Figure 3.5.1) (Van Camp et al., 1991; Gabric et al., 1993; Hagen et al., 1996; Pacheco and Hernández Guerra, 1999). In this work we review a number of studies on the Canary Island induced eddies and on the NAUS upwelling filaments system. We also discuss the impact that such mesoscale structures may have on the CCLME. 105 Sangrà, P. Eddies and filaments off North‐west Africa Figure 3.5.1. Westward‐propagating eddy trajectories lasting over 6 months, as obtained from 14 years (1992–2006) of merged altimeter data, showing the Canary Eddy Corridor (CEC) extending from 22°N to 29°N. Black arrows indicate the sites of recurrently observed upwelling filaments. They are located in the region near Cape Ghir (C.G.), near and between Cape Jubi (CJ) and Cape Bojador (C.bo.), and near Cape Blanc (C.B.). Adapted from Sangrà et al. (2009). 3.5.2. ISLAND INDUCED EDDIES: FORCING MECHANISMS Early observational studies of island induced eddies associated the perturbation of the oceanic flow with the island topography, a process referred to as topographic forcing (La Violette, 1974; Hernández‐Guerra et al., 1993; Arístegui et al., 1994; Heywood et al., 1996). Laboratory and numerical experiments that consider a flow past an obstacle show that, if the impinging flow is sufficiently energetic, the boundary layer around the obstacle detaches, alternating between flanks and giving rise to the sequential spin off of cyclonic and anticyclonic eddies (Berger and Whille, 1972; Bearman, 1984; Sangrà, 1995; Dong et al., 2007). As a result a Von Kármán like vortex street is formed downstream of the obstacle. This flow regime, and thus the eddy shedding process, is dependent on the Reynolds number, (1) where U is the characteristic velocity of the incident flow, L is the obstacle diameter, AH the horizontal eddy diffusivity coefficient, ε the Rossby number, and EH the horizontal Ekman number. Field and laboratory observations indicate that the eddy shedding process starts at Re>50‐60 (e.g. Pattiaratchi et al., 1986). Assuming AH and L constant, the flow regime will depend ultimately on the intensity of the impinging oceanic flow, U. Therefore the topographic forcing of oceanic island induced eddies requires a relatively energetic incident flow. However, island induced eddies have been observed in regions where an energetic incident oceanic flow is absent or very weak, such as is the case for the Hawaii Islands (Calil et al., 2008; Jia et al., 2011). Therefore, 106 IOC TECHNICAL SERIES, No. 115, pp. 105‐114. URI: http://hdl.handle.net/1834/9181. 2015 there must be an alternative or additional mechanism to topographic forcing to explain eddy generation during periods of low intensity incident flow. This second mechanism is known as atmospheric forcing, and is associated with wind field perturbation by the orography of tall islands. In the case of tall islands, prevailing wind perturbations lead to the generation of a downwind wind wake where two counter‐rotating stationary wind stress curl cells develop (Chavanne et al., 2002; Jiménez et al., 2008; Mason, 2009). These are the result of the strong wind shear between the wind sheltered region just downstream of the island, and the wind exposed region further away. Observations for the Canary Islands have reported a gradient of the wind intensity between these regions as high as 15 m s‐1 (Piedeleu, 2014). As proposed by Jiménez et al. (2008), the anticyclonic/cyclonic wind curl cell will induce a divergence/convergence of the Ekman transport that will lead to a downward/upward linear Ekman pumping velocity, which shrinks/stretches the planetary vorticity tubes, spinning up anticyclonic/cyclonic relative vorticity.
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