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El Nino-like warm events in the Eastern Atlantic (6oN, 20oS) and fish availability from Congo to Angola (1964-1999)

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... This variability is also associated with substantial modulations of nutrients supply (Bachèlery et al., 2016) and of the oxygen content (Monteiro and van der Plas, 2006;Monteiro et al., 2008Monteiro et al., , 2011Bachèlery et al., 2016) during several months, as illustrated on Fig. 1b. The extended duration of non-suitable habitat conditions (hypoxia, lack of food supply) is thought to affect the marine biodiversity and the abundance of the fish stocks (Woodhead et al., 1997a(Woodhead et al., , 1997bHamukuaya et al., 1998;Gammelsrød et al., 1998;Binet et al., 2001;Boyer and Hampton, 2001;Monteiro et al., 2008). ...
... This remarkable correspondence (71% in surface and 100% in subsurface) confirmed the high dynamical connection between CTW and extreme temperature conditions along the continental shelf. Benguela Niños and Niñas are associated with fluctuations in the oxygen content of the shelf water ( Fig. 1b; Monteiro and van der Plas, 2006;Monteiro et al., 2008Monteiro et al., , 2011 and significantly affect the marine ecosystem and fish resources (Binet et al., 2001;Gammelsrød et al., 1998). Therefore, it is likely that extreme low-oxygen episodes along the continental shelf and their ecological consequences are connected to CTW propagations. ...
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We investigate the dynamics of the interannual Coastal Trapped Waves (CTW) propagations along the south-western African coast and their role in triggering Benguela Niño and Niña events from 1958 to 2008. Using regional ocean model sensitivity experiments, we track equatorially-forced CTW down to the Southern Benguela Upwelling System (SBUS), where they account for 70% of the coastal sea level anomalies (SLA), temperature, and salinity variability. We then decompose the model coastal variability into individual CTW modal contributions and identify periods of energetic downwelling and upwelling propagations. A composite analysis allows for documenting and quantifying the oceanic response (circulation, temperature, and salinity) on the shelf during the passage of remotely-forced CTW. Results reveal that North of ~19°S, the coastal interannual variability is dominated by the second and third CTW modes. In the BUS, their amplitudes decrease and the interannual fluctuations are largely explained (> 70%) by the faster and weakly-dissipative first CTW mode. This dynamic explains the peculiar propagative pattern associated with SLA propagations, in which equatorially-forced fluctuations in the SBUS peak before the waves imprint the variability at ~19°S. The impact of CTW on the temperature in the SBUS is drastically lower than in the NBUS and Angolan regions. At last, we show that 71% of the extreme Benguela Niño and Niña events, in the surface layer, are associated with remotely-forced CTW propagations. The coherence between our CTW index and these extreme events increases when detecting temperature anomalies in the sub-surface rather than at the sea surface.
... Binet [12] suggested that the increased landings during the 1980s may have resulted from stronger and more frequent ENSO events which widened coastal eddies in the Guinea Current thereby increasing survival of larval Sardinella. ENSO-linked warm events during this same period have also been observed to influence Sardinella distributions and catches from Congo to Angola [19]. Landings of demersal finfish off Ghana also increased from 1960 to 1980 but have been variable in recent years (Fig. 3). ...
... Both the inter-annual variability in SST and its decadal scale pattern off Ghana appear to be influenced by ENSO events in the tropical Pacific Ocean and by variability in the tropical Atlantic. These processes are also likely to affect the entire tropical west coast of Africa [19] and terrestrial ecosystems in West Africa through changes in rainfall. In contrast, the livelihood strategies employed by coastal communities to respond to environmental variability have local, regional, and national spatial dimensions. ...
... La variabilité de l'environnement marin méditerranéen, liée aux variations des caractéristiques des gyres de la mer Alboran ainsi qu'aux conditions climatiques (sécheresses et précipitations), peut entraîner des changements de la distribution des poissons sur de courtes échelles de temps, avec des implications considérables sur l'exploitation et la pêche (Fréon et al., 2005 ;Binet et al., 2001 ;Boyer et al., 2001). ...
... For example, Roy and Reason (2001) found significant correlations between ENSO, NAO, and the effect on upwelling intensity in the Canary Current Ecosystem, and thus annual catch of sardine (Sardina pilchardus). El Niño-like warm events in the Eastern Atlantic (68N, 208S) were observed and suggested to affect fish availability from Congo to Angola (Binet et al., 2001). ...
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Chang, Y.-J., Sun, C.-L., Chen, Y., Yeh, S.-Z., DiNardo, G., and Su, N.-J. 2013. Modelling the impacts of environmental variation on the habitat suitability of swordfish, Xiphias gladius, in the equatorial Atlantic Ocean. - ICES Journal of Marine Science, 70: 1000-1012.We developed a habitat suitability index (HSI) model to identify the optimal habitats of swordfish (Xiphias gladius) in the equatorial Atlantic Ocean. Environmental variables, including sea surface temperature (SST), mixed layer depth (MLD), chlorophyll-a concentrations, and sea surface height anomaly, as well as catch and effort data from Taiwanese longline fisheries, were used. The geometric mean model including all the above environmental variables was identified as the most parsimonious model for yielding HSI predictions coinciding with productive fishing grounds with high fishing effort. Swordfish mainly aggregated in the northwest region during March-May and spread southeast thereafter in response to seasonal shifts in oceanographic conditions. There was annual variation in the distribution of habitat patches, and the habitat quality was reduced in the northwest region of the equatorial Atlantic Ocean during 2005. The apparent spatial shifts in optimal habitats might be linked to reduced MLD and elevation in sea surface height, which might be related to climate variability (e.g. Niño-Southern Oscillation and/or Northern Atlantic Oscillation). Because environmental data regarding climate change scenarios are becoming readily available, we can utilize the proposed HSI models to evaluate possible changes in habitat suitability resulting from climate change and provide scientific advice for the development of management regulations. © 2013 © 2013 International Council for the Exploration of the Sea. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected] /* */
... Whereas migrations of the two dominant clupeid species in Angola, Sardinella aurita and Sardinella maderensis, have been documented (Boley and Fr eon, 1980), and the effects of seasonality and anomalous events on their catches have been described (Binet et al., 2001;Ostrowski, 2007;Ostrowski et al., 2009), no long-term changes in their distributions and range sizes have been documented so far. ...
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The NansClim project (2010-2013) represented a regional collaboration to assess the effects of climate on Benguela dynamics. Based on in situ (since the 1960s in Namibia and South Africa and 1985 in Angola) and satellite (since the 1980s) observations, the project focussed on four subsystems, namely the Angola subtropical, northern Benguela upwelling, southern Benguela upwelling and Agulhas Bank. This contribution summarizes the findings for selected key questions, ranging from changes in the physico-chemical habitats, plankton, pelagic and demersal fish communities, to cross-cutting evaluation at subsystem and regional scales. The results underline the overriding importance to of considering the combined effects of climate and fishing as drivers of the dynamics of the ecosystem components. Each subsystem currently continues to function largely as a separate entity as described in earlier reviews. However, some changes have been observed across several subsystems, e.g., a coherent shift from one relatively stable period to another occurred in the northern and southern Benguela in the mid-1990s. Future climate change could weaken the boundaries between the four subystems. The findings underline the need for continued regional research collaboration and regional surveys focussed at ecosystem, rather than resource, assessment. Our conclusions include implications for ecosystem-based fisheries management, and recommendations for future regional research.
... There is, however, also a climate mode on interannual time scales in the tropical Atlantic, the so-called Atlantic zonal mode or Atlantic Niño mode [see, e.g., Chang et al. (2006) for a review of the tropical Atlantic climate modes], which is focused in the cold tongue region of the eastern tropical Atlantic and along the southwestern African coast (Fig. 1b). The Atlantic Niño mode plays an important role in the onset of the West African monsoon (Brandt et al. 2011) while SST variability in the southeastern tropical Atlantic is related to rainfall anomalies (Reason and Rouault 2006) and to effects on the marine ecosystem along the West African coast (Binet et al. 2001). Warmest temperatures in the tropical Atlantic are on average found in the western basin and to the north of the equator coincident with the position of the intertropical convergence zone (ITCZ, Fig. 1a). ...
Article
El Nino-Southern Oscillation (ENSO) in the Pacific and the analogous Atlantic Nino mode are generated by processes involving coupled ocean-atmosphere interactions known as the Bjerknes feedback. It has been argued that the Atlantic Nino mode is more strongly damped than ENSO, which is presumed to be closer to neutrally stable. In this study the stability of ENSO and the Atlantic Nino mode is compared via an analysis of the Bjerknes stability index. This index is based on recharge oscillator theory and can be interpreted as the growth rate for coupled modes of ocean-atmosphere variability. Using observational data, an ocean reanalysis product, and output from an ocean general circulation model, the individual terms of the Bjerknes index are calculated for the first time for the Atlantic and then compared to results for the Pacific. Positive thermocline feedbacks in response to wind stress forcing favor anomaly growth in both basins, but they are twice as large in the Pacific compared to the Atlantic. Thermocline feedback is related to the fetch of the zonal winds, which is much greater in the equatorial Pacific than in the equatorial Atlantic due to larger basin size. Negative feedbacks are dominated by thermal damping of sea surface temperature anomalies in both basins. Overall, it is found that both ENSO and the Atlantic Nino mode are damped oscillators, but the Atlantic is more strongly damped than the Pacific primarily because of the weaker thermocline feedback.
... Rapid horizontal and vertical migrations can be induced, altering the distribution of fish and therefore their availability to fishing. While many of these shifts in distribution may be relatively local and temporary, they have been observed to persist for several months and over large areas, greatly influencing the exploitation of populations (Schwartzlose et al., 1999;Binet et al., 2001;Boyer et al., 2001;Bertrand et al., 2004). ...
Conference Paper
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Populations of small pelagic fish form important fisheries Spanish Mediterranean waters, particularly sardine (Sardina pilchardus) and anchovy (Engraulis encrasicolus), and are distributed along the entire length of the Spanish continental shelf. Using annual acoustic survey data for the years 2003 - 2005, a GIS-based environmental modelling approach was used to investigate the distribution and abundance of small pelagic fish in relation to environmental variables. Generalised additive models showed bathymetry, sea surface chlorophyll-a concentration and sea surface temperature to be related to both the presence/absence and relative abundance of fish. The strength and significance of these relationships varied spatially and temporally. An extension of the same modelling procedure was applied to sardine and anchovy specifically. Models were used to produce high resolution essential fish habitat (EFH) maps, based on the predicted probability of presence of each species. Substantial inter-annual variability in the distribution and quality of EFH was observed, particularly for anchovy. Shifts in fish distribution associated with variability in EFH could affect the catchability of fish with considerable fishery implications.
... [2] The Angola Benguela Frontal Zone (ABFZ) is an important frontal feature situated between 15-17°S off the west coast of Africa [e.g., Shannon et al., 1987;Meeuwis and Lutjeharms, 1990;Kostianoy and Lutjeharms, 1999;Lass et al., 2000;Veitch et al., 2006]. It separates the warm saline equatorial current system from the cold and highly productive Benguela upwelling system and hence is a vital system boundary, whose fluctuation in position or intensity seems to affect local fisheries and rainfall variability [e.g., Gammelsrod et al., 1998;Binet et al., 2001;Rouault et al., 2003]. ...
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From January to May 2001, several countries of Southern Africa experienced above normal rainfall and floods. 23 000 people were displaced in Southern Angola after a flood in April. In March, an inundation killed several people and displaced 5,000 others in eastern Zambia's. The situation in Zambia was aggravated when authorities had to open the spillway gates at the Kariba Dam, the main source of electricity for Zambia and Zimbabwe. Water discharged from the Kariba dam ran into neighbouring Mozambique, aggravating floods in that country. At the same time warm sea surface anomalies were measured off the Angolan and Namibian coast. Warm events in the Southeast Tropical Atlantic off Angola and Namibia called "Benguela Nino" are known to affect the fisheries of the region but they also affect the rainfall. In 1995, the warmest recorded Benguela Nino happened with anomalies off up to 8°C extending 300 km offshore with a southward extension to 27°S. During the 1984, 1986, 1995 and 2001 warm events, above average rainfall occurred near the sea surface temperature anomalies and extended inland from the coast to an extent that appeared to depend on the intensity of the regional moisture convergence and atmospheric circulation anomalies. Rainfall over western Angola / Namibia is greatest for those events for which the local circulation anomalies act to strengthen the climatological westwards flux of Indian Ocean sourced moisture across low latitude southern Africa and which flow anticyclonically over the warmest SST off the northern coast. The significance of the warm events occurring during the February to April period is that this is the time when SST reaches its maximum in the annual cycle (up to 28oC off northern Angola) and this favours more intense local evaporation and convection and a greater impact on late austral summer rainfall.
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