THE NORTH BRAZIL UNDERCURRENT

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 b etween 5 degrees 30'S and 10 degrees S was investigated in austral spr ing during November 1992 and compared with observations in October 199 0. The data were taken with several different instruments, including v essel-mounted ADCP, lowered-ADCP, Pegasus, CTD and XBTs. The flow was found off the coast at 5 degrees 30'S as well as at 10 degrees 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(-1), resulting in large northward transports of more than 22 Sv in t he upper 1000 m. The transport is about the same at 5 degrees 30'S and 10 degrees S, hence no net inflow from the east is required to feed t he NBUC. The climatological Ekman transport is to the south between 5 degrees 30'S and 10 degrees S, and in consequence the northward flow n ear 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 de grees S, therefore the Brazil Current had to start as a coastal curren t south of 10 degrees S. For the zonal sections at 5 degrees 30'S and 10 degrees S the geostrophic computations relative to the density surf ace sigma(1) = 32.15 kg m(-3) (about 1150 m depth) resulted in transpo rts comparable to those obtained from direct measurements. The results further show that the choice of a correct level of no motion can be s upported by the direct observations. A shallower reference based on wa ter mass boundaries alone would reduce the NBUC transport to almost ze ro. Computations with data from the historical data base for austral f all resulted in a weaker NBUC of less than 20 Sv near 10 degrees S, in dicating a possible seasonal signal in the NBUC with a stronger NBUC i n austral spring.