Fundamentals

The surface part of the Angola Current seems to disappear at aboutS and is separated to the south from the Benguela up-welling area by a well pronounced frontal system (MEEUWIS and LUTJEHARMS 1990). The undercurrent is believed to extend into the southern part of the eastern boundary current system and advects tropical planktonic organisms into the Benguela ecosystem. In light of the data presently available, the seaward component of the Ekman transport in the area of the Angola current plays the main role in the mass balance of the Angola current. It remains to investigate, however, whether along the front a westward recirculation into the South-Equatorial-Current (SEC) occurs, and whether filaments penetrate through the front pole-ward; symmetrically to the situation as along the Cape-Verde-Frontal Zone (FIEKAS et al. 1992, JOHN and ZELCK 1997).

The northward directed surface current of the Benguela Current system is driven by the trade-winds. Due to the off shore Ekman transport it is connected with a pronounced up-welling of cold, nutrient-rich and oxygen-deficient water masses. Reviews on the dynamics of the Benguela Current and the Benguela up-welling area can be found in NELSON and HUTCHINGS (1983), SHANNON (1985), CHAPMAN and SHANNON (1985), LUTJEHARMS et al. (1987) and FENNEL (1998).

During the southern summer (January to April) the Angola Dome is most pronounced. The feature is a doming of the shallow isotherms at about S,E, MAZEIKA, (1967), FILIPE (1998). It is undetectable in the sea surface temperature but can be clearly seen in field data at depth from 20m to 150m. According to MAZEIKA, (1967), it corresponds also to lower salinity (0.3 - 0.5 PSU) and lower oxygen values (2-3. VOITURIEZ (1981) and VOITURIEZ and HERBLAND (1982) have presented evidence that the dome is a seasonal feature in the thermocline but a permanent subthermoclinal phenomenon.

Geostrophic analysis indicates a cyclonic gyre in the Angola Basin. However, the Angola Dome itself is confined to the north by the SEUC whereas the northern boundary of the gyre is the SECC, MOROSHKIN, BUBNOV and BULATOV (1970), SHANNON, AGENBAG and BUYS, (1987), GORDON and BOSLEY (1991). The subthermoclinal doming of the isotherms seems to be induced by the pole-ward deflection of the SEUC at the Angolan continental slope, VOITURIEZ (1981).

The dynamics of the formation of the seasonal dome above the thermocline is under discussion. One hypothesis explains the occurrence of the dome with an interplay of regional forces, i.e., Ekman pumping, McCLAIN and FIRESTONE (1993), with equatorial Kelvin waves, WACONGE and PITON (1992). YAMAGATA and IIZUKA (1995) used a numerical circulation model to show that the seasonal variation of the doming isotherms can be explained by seasonal variations of the climatological wind fields.

The position of the Angola Dome is at the boundary to a so called shadow zone, where the water masses do not participate in the general circulation of the sub-tropical gyre. The oldest water masses of the south Atlantic central water are found below the thermocline in this area, TOMCZAK, (1998). They propagate with the pole-ward retroflection of the equatorial currents into the current system of the Angola dome without any ventilation in the southern sub-tropical gyre. Consequently they exhibit a significant minimum in the oxygen concentration (<1 ml/l) compared to the underlying Antarctic Intermediate Water (AAIW) and the surface layer. This oxygen deficient waters propagate pole-ward with the undercurrent into Benguela up-welling system and contributes essentially to the nutrient and oxygen budget of the Benguela up-welling area.

The Angola-Benguela Front separates the Angolan from the northern Benguela ecosystem. Although both are geographically located within tropical latitudes, by climate criteria they differ widely by being tropical and temperate ecosystems, respectively. They are characterized by a largely different spectrum of species, for fish compare e.g. LLORIS (1986), SMITH and HEEMSTRA (1986) for south of the front with PENRITH (1978) for the coastal front area and SANCHES (1966) north of it. Position and strength of the front are subject to a seasonal cycle. The position varies between S andS and has its southernmost extent in the southern summer and its northernmost extent in the southern winter. The mean off shore extension is about 250 km, see SHANNON et al. (1987), MEEUWIS and LUTJEHARMS, (1990) KOSTIANOY, (1996).

The seasonal cycle of the front shows a strong inter-annual variability. During a so called Benguela Niño warm-event, nutrient deficient Angola current water may propagate southward into the Benguela up-welling area, SHANNON et al. (1986), BOYD et al. (1987), BOYD et al. (1998), GAMMELSRØD et al. (1998), entraining tropical fish larvae of both coastal and oceanic origin, see OLIVAR and FORTUÑO (1991), JOHN et al. (1999). On the other hand reduced up-welling corresponds to recruitment failure and collapse of fish stocks off the Namibian coast. This is commercially important for the economic conditions of the riparian states of the southern Africa.

There are indications that the Benguela Niño is coupled with the Pacific El Niño events with a phase shift of about one year, SHANNON, (1998). The planned cruise can be used to prove this theory and to investigate the changes in the Angola-Benguela front during abnormal conditions. Ship-borne measurements are of special importance since satellite images are rare, due to the generally high cloudiness.