Water Masses and Oceanic Circulation of the Brazilian Continental Margin and Adjacent Abyssal Plain

This chapter presents an overview of the water masses and oceanic circulation of the Brazilian Continental Margin (BCM) and the adjacent abyssal plain. Six major water masses compose the stratification over 5000 m of water column in the Southwestern Atlantic: the Tropical Water (the surface water TW, 0–150 m), the South Atlantic Central Water (the pycnoclinic water SACW, 150–500 m), the Antarctic Intermediate Water and the Upper Circumpolar Water (the intermediate waters AAIW and UCPW, 500–1300 m), the North Atlantic Deep Water (the deep water NADW, 1300–3500 m), and the Antarctic Bottom Water (the bottom water AABW, > 3500 m). The Lower Circumpolar Water (LCPW) may be also considered as an upper limb of the AABW and therefore characterized as such. These water masses are transported either equatorward or poleward by an intricate system of western boundary currents. There are three major current systems within the BCM limits. The Brazil Current (BC) system extends from 14oS to about 40oS and closes the South Atlantic Subtropical Gyre. This current starts as a weak 2 Sv (1 Sv = 106 m3 s−1) jet transporting only TW at 15oS and exits the BCM as a jet with a vertical extent of more than 1000 m, transporting 25 Sv of TW, SACW, AAIW, and UCPW. Along its southward path, the BC develops large meanders and eddies, which may neck off and form vortical rings shed to more interior portions of the gyre. The North Brazil Undercurrent/North Brazil Current (NBUC/NBC) is the western boundary system which closes the Equatorial Gyre (14oS-6oN). This current system is more intense than that of the BC, with core speeds greater than 1 m s−1 and transports of 30–35 Sv. Unlike the BC, the NBUC presents its core at depths between 150 and 250 m. In its path toward the northern hemisphere, the current undergoes a series of retroflections, at which it loses volume to the Tropical Atlantic interior and gains volume from shallower branches of South Equatorial Current (SEC). As a result, the NBUC loses its pycnoclinic core and becomes the surface-intensified NBC. This latter jet crosses the equator transferring TW, SACW, and AAIW to the northern hemisphere. Its large inertia inhibits the formation of large meanders and eddies while attached to the Brazilian continental slope. The only known exception is the recently described Potiguar Eddy at 4oS, an anticyclonically rotating permanent lens of SACW. The third current system is due to the Deep Western Boundary Current (DWBC), the main component of the Atlantic Meridional Overturning Circulation. This current flows southward along the Americas’ continental border and reaches the BCM as a NADW-transporting, 20Sv deep jet. It exits the BCM as a 40 Sv jet after being fed by distinct interior sources of volume and recirculations. The AABW flow is not organized as a jet. Its northward motion occurs along the whole region of interest as a northward sluggish flow rubbing itself against the ocean floor of the abyssal plain.