Characteristics of intermediate water flow in the Benguela current as measured with RAFOS floats

Seven floats (not launched in rings) crossed over the mid-Atlantic Ridge in the Benguela extension with a mean westward velocity of around 2 cm/s between 22S and 35S. Two Agulhas rings crossed over the mid-Atlantic Ridge with a mean velocity of 5.7 cm/s toward 285°. This implies they translated at around 3.8 cm/s through the background velocity field near 750 m. The boundaries of the Benguela Current extension were clearly defined from the observations. At 750 m the Benguela extension was bounded on the south by 35S and the north by an eastward current located between 18S and 21S. Other recent float measurements suggest that this eastward current originates near the Trindade Ridge close to the western boundary and extends across most of the South Atlantic, limiting the Benguela extension from flowing north of around 20S. The westward transport of the Benguela extension was estimated to be 15 Sv by integrating the mean westward velocities from 22S to 35S and multiplying by the 500 m estimated thickness of intermediate water. Roughly 1.5 Sv of this are transported by the ∼3 Agulhas rings that cross the mid-Atlantic Ridge each year (as observed with altimetry). This value of the Benguela extension transport is the first one to have been obtained from long-term (two-year) observations and across the full width of the Benguela extension.