Intrusion of warm surface water along the Angolan-Namibian coast in February–March 1995: the 1995 Benguela Nino

Abstract

The upper ocean temperatures in the Angolan-Namibian coastal waters were anomalously high during March 1995, with positive temperature anomalies of up to 8°C. Maximum temperature differences were 30–50 m deep, reflecting a deepening of the thermocline from normal depths of 10–30 m. The unusually warm water mass covered the Angolan coast from Cabinda (5°S), the northern limit of the survey area, to at least 24°S off central Namibia. Higher than normal temperatures were observed as far south as Luderitz (27°S). Satellite-derived SST and direct observations indicated that the seaward distribution of warm water extended more than 300 km from the coast. Surface drogues released inshore along the central Namibian shelf suggested a maximum southward extension by 3 March 1995. The warm event was associated with observed mortalities in sardine Sardinops sagax, horse mackerel Trachurus trachurus capensis and kob Argyrosomus inodorus off the coast. It also caused a southward displacement of sardine stocks from Angola, resulting in an increased availability of pelagic fish in Namibian waters. Conditions have occasionally been anomalously warm in Angolan and Namibian waters in the past, with the last major event in 1984. These events are known as Benguela Ninos, because of their resemblance to the well known Pacific El Nino. The 1995 Benguela Nino appeared to be associated with a positive subsurface salinity anomaly of 0.5 × 10–3 in Namibian waters and a negative (-4.0 × 10–3) surface salinity anomaly in Angolan waters, thought to be derived from the freshwater input of the Congo River.