A Time Series of Agulhas Undercurrent Transport

Abstract

A 550-day record of Agulhas Undercurrent transport between March 2003 and August 2004 is constructed from five deep moorings placed on the continental shelf off South Africa at nominally 32°S. The vertical and lateral scales of the undercurrent are estimated to be 2000 m and 40 km, respectively, using the average of seven direct velocity sections, predominantly taken in austral autumn over a 10-yr period from 1995 to 2005. Peak speeds in the undercurrent are some of the greatest ever seen at depth: over 90 cm s?1 at 1400 m. The undercurrent has a transport of 4.2 ± 5.2 Sv (1 Sv = 1 ? 106 m3 s?1), in close agreement with a previous estimate from a single current meter record during 1995 of 4.2 ± 2.9 Sv. Records below 1800 m, within the North Atlantic Deep Water (NADW) layer of the undercurrent, show a higher level of coherence and less variance than those at intermediate depths. On average, 2.3 ± 3.0 Sv of NADW is carried northeastward within the undercurrent, an amount similar to that estimated previously by analyses of deep water mass characteristics around South Africa. Short-term variability in the undercurrent peaks at the semidiurnal period, at a local shear-adjusted inertial period (21.6 h), and at 4.5, 6.5, and 9.5 days. The latter may be associated with topographic Rossby waves, although no evidence for enhanced onshore velocities was found at these periods. The variability of the undercurrent is highly topographically controlled, strongly aligned in the along-stream direction, with significant variance in cross-stream velocity only at 2-day periods or less and isotropic variance only at the (effective) inertial period. For the mesoscale, the deeper current meters within the NADW layer all exhibit broad peaks at 50?60 days, which matches the periodicity of solitary meanders of the Agulhas Current (so-called Natal pulses) presented previously in the literature. The results of this study demonstrate that these meanders are highly barotropic in nature.