Observations of the vertical and temporal evolution of a Natal Pulse along the Eastern Agulhas Bank

Abstract

This study reinvestigates the work of Lutjeharms et al. (2001, 2003) who documented the properties of a Natal Pulse using isopycnal Lagrangian floats. We combined Lagrangian analyses and Eulerian maps derived from objective analysis to better describe the evolution of a Natal Pulse along three density surfaces referred to as the surface (satellite-observed), shallow (isopycnal 1026.8 kg m-3), and deep (isopycnal 1027.2 kg m-3) layer. Our observations show that this Natal Pulse extended to a depth of 1000 m and was associated with cyclonic relative vorticity values of about 6.5–8.5 × 10-5 s-1 in the surface and shallow layer and 4 × 10-5 s-1 in the deep layer. This Natal Pulse contributed to cross-shelf exchange through the offshore advection of Eastern Agulhas Bank water near the surface, onshore advection of South Indian Central Water and/or Indian Equatorial Water in the shallow layer, and Antarctic Intermediate Water in the deep layer. Sea surface temperature maps showed that the downstream progression of the Natal Pulse along the 3000 m isobath was related to a readjustment of its rotation axis. This readjustment advected Eastern Agulhas Bank water into the Natal Pulse eddy and triggered a SST cooling of about 3°C in the cyclonic area. The importance of a warm recirculating Agulhas plume originating from the Natal Pulse was highlighted. This warm water plume extended to a depth of 700 m and was associated with onshore velocities exceeding those experienced within the Natal Pulse eddy by a factor of 2. Our observations indicate that the June/July 1998 Natal Pulse and its associated plumes enhanced cross-shelf exchanges.