Isabelle J. Ansorge

The Agulhas Return Current

Abstract The Agulhas Return Current constitutes the intense flow along the Subtropical Convergence south of Africa. It forms the connecting link between the generically similar South Atlantic Current and the South Indian Ocean Current, thus contributing to the water exchange between these two basins. This general along-front flow is, however, substantially modified south of Africa by contributions from the Agulhas Current. We have carried out a first study of the hydrography and dynamics of the Agulhas Return Current along its full length using a collection of available modern hydrographic data. It is shown that on average the current lies at a latitude of 39°30′S south of Africa, increasing slowly downstream to a latitude of 44°30′S at 60°E, except where it crosses a number of meridional ridges where northward shifts of up to 2°30′ are occasionally observed. Geostrophic speeds relative to 1500 m demonstrate a gradual eastward decrease in the velocity of the current from an average of 75 cm/s at the Agulhas retroflection to 13 cm/s at 76°E. Volume transports are similarly reduced from 54×10 6 m 3 /s in the retroflection region to 15×10 6 m 3 /s at 76°E. Temperature/salinity properties show water mass characteristics of the Agulhas Current to extend to at least 61°E. Based on these results, we suggest that the Agulhas Return Current is zonally continuous and terminates between 66°E and 70°E. We therefore propose that the name South Indian Ocean Current be retained for the flow east of here only.

Conserving pattern and process in the Southern Ocean: designing a Marine Protected Area for the Prince Edward Islands

South Africa is currently proclaiming a Marine Protected Area (MPA) in the Exclusive Economic Zone (EEZ) of its sub-Antarctic Prince Edward Islands. The objectives of the MPA are to: 1) contribute to a national and global representative system of MPAs, 2) serve as a scientific reference point to inform future management, 3) contribute to the recovery of the Patagonian toothfish (Dissostichus eleginoides), and 4) reduce the bird bycatch of the toothfish fishery, particularly of albatrosses and petrels. This study employs systematic conservation planning methods to delineate a MPA within the EEZ that will conserve biodiversity patterns and processes within sensible management boundaries, while minimizing conflict with the legal toothfish fishery. After collating all available distributional data on species, benthic habitats and ecosystem processes, we used C-Plan software to delineate a MPA with three management zones: four IUCN Category Ia reserves (13% of EEZ); two Conservation Zones (21% of EEZ); and three Category IV reserves (remainder of EEZ). Compromises between conservation target achievement and the area required by the MPA are apparent in the final reserve design. The proposed MPA boundaries are expected to change over time as new data become available and as impacts of climate change become more evident.

An altimetry-based gravest empirical mode south of Africa: 1. Development and validation

[1] Hydrographic transects of the Antarctic Circumpolar Current (ACC) south of Africa are projected into baroclinic stream function space parameterized by pressure and dynamic height. This produces a two-dimensional gravest empirical mode (GEM) that captures more than 97% of the total density and temperature variance in the ACC domain. Weekly maps of absolute dynamic topography data, derived from satellite altimetry, are combined with the GEM to obtain a 16 year time series of temperature and salinity fields. The time series of thermohaline fields are compared with independent in situ observations. The residuals decrease sharply below the thermocline and through the entire water column the mean root-mean-square (RMS) error is 0.15C, 0.02, and 0.02 kg m �3 for temperature, salinity, and density, respectively. The positions of ACC fronts are followed in time using satellite altimetry data. These locations correspond to both the observed and GEM-based positions. The available temperature and salinity information allow one to calculate the baroclinic zonal velocity field between the surface and 2500 dbar. This is compared with velocity measurements from repeat hydrographic transects at the GoodHope line. The net accumulated transports of the ACC, derived from these different methods are within 1–3 Sv of each other. Similarly, GEM-produced cross-sectional velocities at 300 dbar compare closely to the observed data, with the RMS difference not exceeding 0.03 m s �1 . The continuous time series of thermohaline fields, described here, are further exploited to understand the dynamic nature of the ACC fronts in the region, and which is given by Swart and Speich (2010).

Physical-biological coupling in the waters surrounding the Prince Edward Islands (Southern Ocean)

Abstract The results of a macro-scale oceanographic survey conducted in the upstream and downstream regions of the Prince Edward Islands in austral autumn (April/May) 1989 are presented. During the investigation, the Subantarctic Front, upstream of the islands, was shown to lie initially south at 46°38′S, while downstream, the front remained in a northern position of approximately 46°S. Surface expressions of the front show that the Subantarctic Front forms a zonal band, while the subsurface expressions (200 m) show a distinct meander in both regions. In the upstream region of the islands, the northern branch of the Antarctic Circumpolar Current, the Subantarctic Front, influenced by the shallow bathymetry, was deflected around the northern edge of the islands. Water masses in this region were shown to modify gradually from Subantarctic Surface Water (7°C, 33.75) to Antarctic Surface Water (5°C, 33.70) as the Polar Frontal Zone was crossed. Downstream of the islands a wake was formed resulting in the generation of broad, cross-frontal meanders. As a consequence, warm Subantarctic Surface Water from north of the Subantarctic Front was advected southwards across the Polar Frontal Zone, while cooler waters, which had been modified in the transitional band of the Polar Frontal Zone, were advected northwards. In the downstream region a warm eddy consisting of Subantarctic Surface Water was observed. Its generation is possibly due to baroclinic instabilities in the meandering wake. Zooplankton species composition and distribution patterns during the investigation were consistent with the prevailing oceanographic regime. Four distinct groupings of stations were identified by numerical analysis. These corresponded to stations found north of the Subantarctic Front, within the warm eddy, located in the Polar Frontal Zone, and those stations associated with the meander. The groupings were separated by the Subantarctic Front, which appears to represent an important biogeographic boundary to the distribution of warm-water zooplankton species. Warm eddies in the downstream region of the islands may represent an effective mechanism for transporting warm water species across the Subantarctic Front.

Transport and variability of the Antarctic Circumpolar Current south of Africa

Data from five CTD and 18 XBT sections are used to estimate the baroclinic transport ( referenced to 2500 dbar) of the ACC south of Africa. Surface dynamic height is derived from XBT data by establishing an empirical relationship between vertically integrated temperature and surface dynamic height calculated from CTD data. This temperature-derived dynamic height data compare closely with dynamic heights calculated from CTD data ( average RMS difference = 0.05 dyn m). A second empirical relationship between surface dynamic height and cumulative baroclinic transport is defined, allowing us to study a more extensive time series of baroclinic transport derived from upper ocean temperature sections. From 18 XBT transects of the ACC, the average baroclinic transport, relative to 2500 dbar, is estimated at 90 +/- 2.4 Sv. This estimate is comparable to baroclinic transport values calculated from CTD data. We then extend the baroclinic transport time-series by applying an empirical relationship between dynamic height and cumulative baroclinic transport to weekly maps of absolute dynamic topography derived from satellite altimetry, between 14 October 1992 and 23 May 2007. The estimated mean baroclinic transport of the ACC, obtained this way, is 84.7 +/- 3.0 Sv. These transports agree well with simultaneous in-situ estimates ( RMS difference in net transport = 5.2 Sv). This suggests that sea level anomalies largely reflect baroclinic transport changes above 2500 dbar.

Eddies originating at the South-West Indian Ridge

Abstract The mesoscale variability of flow in the Southern Ocean is markedly intensified along the Subtropical Convergence as well as at a small number of distinct spots, the latter seemingly related to shallower parts of the bottom topography. A very prominent example of such a spot corresponds with the location where the core of the Antarctic Circumpolar Current crosses the South-West Indian Ridge. The variability in this particular spot consists of well-developed positive as well as negative anomalies in sea surface height that may represent anticyclonic and cyclonic eddies. Based on satellite altimetry, it is shown that these flow anomalies exhibit specific trajectories beyond their generation region at the South-West Indian Ridge, one of which takes them past the Prince Edward Islands. Using extensive hydrographic observations, we describe the dimensions and T/S characteristics of some of the features identified in the altimetry that have passed these islands. In this way, we successfully verify that they are indeed eddies, thus providing evidence that the fracture zone in the South-West Indian Ridge is a source of eddies for the downstream region.

Ocean warming, a rapid distributional shift, and the hybridization of a coastal fish species

Despite increasing awareness of large-scale climate-driven distribution shifts in the marine environment, no study has linked rapid ocean warming to a shift in distribution and consequent hybridization of a marine fish species. This study describes rapid warming (0.8 °C per decade) in the coastal waters of the Angola-Benguela Frontal Zone over the last three decades and a concomitant shift by a temperature sensitive coastal fish species (Argyrosomus coronus) southward from Angola into Namibia. In this context, rapid shifts in distribution across Economic Exclusive Zones will complicate the management of fishes, particularly when there is a lack of congruence in the fisheries policy between nations. Evidence for recent hybridization between A. coronus and a congener, A. inodorus, indicate that the rapid shift in distribution of A. coronus has placed adults of the two species in contact during their spawning events. Ocean warming may therefore revert established species isolation mechanisms and alter the evolutionary history of fishes. While the consequences of the hybridization on the production of the resource remain unclear, this will most likely introduce additional layers of complexity to their management.

The hydrography and dynamics of the ocean environment of the Prince Edward Islands (Southern Ocean)

Abstract The Prince Edward Islands lie in the Indian sector of the Southern Ocean at 47°S and 38°E. They lie in the path of the Antarctic Circumpolar Current (ACC), between the Subantarctic Front (SAF) to the north and the Antarctic Polar Front (APF) to the south. Two extensive hydrographic surveys (MOES 2 and MIOS 2) have been carried out to establish for the first time the mesoscale hydrography and dynamics of the oceanic surroundings of these islands. During the MOES 2, the SAF was deflected northward around the islands, while the APF lay south of the survey grid and south of the islands. Water masses in the region changed gradually from Subantarctic Surface Water (SASW) to Antarctic Surface Water (AASW) on crossing the Polar Frontal Zone (PFZ). Downstream of the islands, a wake, resulting in the generation of broad meanders, was formed. As a consequence, water masses, in particular warm SASW, were displaced from north of the SAF across the PFZ, while cooler waters, which have been modified in the transitional band of the PFZ, were displaced northwards. In contrast, during MIOS 2, the surface expression of the SAF formed an intensive frontal band. On approaching the islands, the SAF split into two branches, with a branch deflected northwards around the islands, while a second branch meandered southward. In the downstream region, an intense cold eddy consisting of AASW was observed within the PFZ, displacing the SAF northwards. South of this eddy, a warm patch of SASW water was encountered, its position possibly controlled by the meandering SAF. Evidence from both these surveys demonstrates that the ACC exhibits high degrees of mesoscale variability in the vicinity of the Prince Edward Islands. The displacement of the SAF in both instances was apparent, resulting in the advection or the entrapment of neighbouring water masses into and across the PFZ. The speed of the incident current on approaching the islands may play a role in the degree of mesoscale mixing downstream of the islands.

Basin‐Wide Oceanographic Array Bridges the South Atlantic

The meridional overturning circulation (MOC) is a global system of surface, intermediate, and deep ocean currents. The MOC connects the surface layer of the ocean and the atmosphere with the huge reservoir of the deep sea and is the primary mechanism for transporting heat, freshwater, and carbon between ocean basins. Climate models show that past changes in the strength of the MOC were linked to historical climate variations. Further research suggests that the MOC will continue to modulate climate change scenarios on time scales ranging from decades to centuries [Latif et al., 2006].

The baroclinic transport of the Antarctic Circumpolar Current south of Africa

Five hydrographic transects at nominal longitudes 0°E and 30°E, and fourteen expendable bathythermograph (XBT) sections near the former longitude are used to study the baroclinic transport of the Antarctic Circumpolar Current (ACC) between Africa and Antarctica. The bottom-referenced geostrophic transport between the Subtropical Front and the ACC Southern Boundary is 147±10 Sv. Estimating the transport from the XBTs using a technique previously employed south of Australia proves delicate because of an irregular bathymetry and water mass variations. It nevertheless confirms ACC transports around 150 Sv. Gathering these and other estimates from the Atlantic sector suggests that, while North Atlantic Deep Water is injected in the current west of 35°W, a partially compensating loss of Deep Circumpolar Water occurs east of this longitude. Another transport increase from 0°E to 30°E might reflect southward transfer across the Subtropical Front south of the Agulhas retroflection.