The North Brazil Undercurrent

Abstract

The flow field in the area of what was thought to be the source region of the North Brazil Current (NBC) off the northeast coast of Brazil between 5°30'S and 10°S was investigated in austral spring during November 1992 and compared with observations in October 1990. The data were taken with several different instruments, including vessel-mounted ADCP, lowered-ADCP, Pegasus, CTD and XBTs. The flow was found off the coast at 5°30'S as well as at 10°S as an undercurrent, the North Brazil Undercurrent (NBUC). The NBUC shows a subsurface core at about 200 m depth with velocities of up to 90.0 cm s -', resulting in large northward transports of more than 22 Sv in the upper 1000 m. The transport is about the same at 5°30'S and 10°S, hence no net inflow from the east is required to feed the NBUC. The climatological Ekman transport is to the south between 5°30'S and 10°S, and in consequence the northward flow near the surface was reduced and might be one reason for the existence of the undercurrent. The flow near the coast was to the north at 10°S, therefore the Brazil Current had to start as a coastal current south of 10°S. For the zonal sections at 5°30'S and 10°S the geostrophic computations relative to the density surface a, = 32.15 kg m-=` (about 1150 m depth) resulted in transports comparable to those obtained from direct measurements. The results further show that the choice of a correct level of no motion can be supported by the direct observations. A shallower reference based on water mass boundaries alone would reduce the NBUC transport to almost zero. Computations with data from the historical data base for austral fall resulted in a weaker NBUC of less than 20 Sv near 10°S, indicating a possible seasonal signal in the NBUC with a stronger NBUC in austral spring.