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International Symposium on Benguela - Opportunity, Challenge and Change, Univ Cape Town, Cape Town, SOUTH AFRICA, NOV, 2016; International audience; Otolith elemental signatures can be used to identify when individual or groups of fish are spending a significant amount of time in different environments. Elemental signatures of juvenile sardine Sardinops sagax caught in winter 2008 and 2009 around the coast of South Africa were measured using inductively-coupled plasma mass-spectroscopy. The otolith elemental signatures of 34 fish caught in 2008 and of 52 fish caught in 2009 were measured at the edge (to represent conditions 20-30 days prior to capture). Principal component analysis was used to visualise the relationships of individuals to each other, in terms of their otolith chemistry, in two-dimensional space, and multiple ANOVAs were used to investigate spatial and temporal variations among samples collected in 2008 and 2009. Significant differences among sites were found in MANOVAs, but the between-site differences varied among the elements. Magnesium concentration tended to decrease whereas barium concentration tended to increase from the west to the east coast. Barium indicate upwelling impact but for 2008 samples on the northern part of the west coast. Otolith microchemistry provides evidence of large and small-scale differentiation in sardine, but differences between years indicates that this is not necessarily temporally stable. The ocean off South Africa is a dynamic and variable environment and this is reflected in the inter-site, and also inter-annual, differences in elemental signatures of juvenile sardine.

The dynamics and seasonal variability of jet currents on the southern Benguela shelf-edge are investigated using a climatologically forced Regional Ocean Modelling System (ROMS) model. The jet is primarily forced by the intense horizontal gradients that exist across the southern Benguela shelf. These gradients are set up by nearshore cooling via the strongly seasonal upwelling regime and variable offshore warming by the advection of Agulhas waters. While the nearshore cooling is prevalent only during the spring and summer upwelling season, the offshore warming exists throughout the year. As a result intensified geostrophically adjusted currents exist throughout the year, particularly off the Cape Peninsula and Cape Columbine. However, the distinct shelf-edge jet features are most intense during upwelling seasons and extend, more or less continuously, from Cape Agulhas, the southern-tip of the continent, to Cape Columbine. The spring and summer jet off the Cape Peninsula reaches speeds of at least 0.7 m.s −1 , bifurcates as it moves northward. The branch that continues northward over the shelf goes on to feed the offshore branch of the Cape Columbine jet (over the 500 m isobath) and to a less extent the nearshore branch (over the 200 m isobath) that is locally enhanced by upwelling processes. During winter, the Cape Peninsula jet is more confined to the shelf region and goes on to feed the whole outer-shelf (200–500 m) region off and beyond Cape Columbine. An ageostrophic component associated with offshore non-linearities related to Agulhas influx causes the mean manifestation of the Cape Peninsula jet to broaden slightly (60 km) compared to its 40 km-wide geostrophic core which is situated over the 350 m isobath. The ageostrophic component is related to the generation of eddies that cause the isopycnals to flatten out.

A Benguela Nino developed in November 2010 and lasted for 5 months along the Angolan and Namibian coastlines. Maximum amplitude was reached in January 2011 with an interannual monthly Sea Surface Temperature anomaly larger than 4°C at the Angola Benguela Front. It was the warmest event since 1995. Consistent with previous Benguela Niños, this event was generated by a relaxation of the trade winds in the western equatorial Atlantic, which triggered a strong equatorial Kelvin wave propagating eastward along the equator and then southward along the southwest African coast. In the equatorial band, the associated ocean sub-surface temperature anomaly clearly shows up in data from the PIRATA mooring array. The dynamical signature is also detected by altimetry derived Sea Surface Height and is well reproduced by an Ocean Linear Model. In contrast to previous Benguela Niños, the initial propagation of sub-surface temperature anomalies along the equator started in October and the associated warming in the Angolan Benguela Front Zone followed on as early as November 2010. The warming was then advected further south in the Northern Benguela upwelling system as far as 25°S by an anomalously strong poleward sub-surface current. Demise of the event was triggered by stronger than normal easterly winds along the Equator in April and May 2011 leading to above normal shoaling of the thermocline along the Equator and the south-west African coastline off Angola and an associated abnormal equatorward current at the Angola Benguela Front in April and May 2011.