North Brazil Current Ring Generation and Evolution Observed with SeaWiFS*

Abstract

The earth’s largest oceanic rings are formed by the retroflecting North Brazil Current (NBC) near 8 8N in the western tropical Atlantic. The NBC flows northward across the equator and past the mouth of the Amazon River entraining river-influenced shelf water along its nearshore edge. Enhanced phytoplankton production associated with the nutrient-rich Amazon discharge results in near-surface chlorophyll gradients that delineate the trajectory of the retroflecting NBC. These large-scale gradients, visible from space using Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color imagery, enable visualization of NBC rings during the initial phases of their evolution and northwestward translation. Observations of 18 NBC rings identified between September 1997 and September 2000 are summarized. Six rings formed each year. Although nearly circular at formation the rings quickly deformed as they translated at speeds near 15 cm s 21 toward the Caribbean Sea. Typical core radii of rings near 558W were 100 km and 150 km in the across- and alongshore dimensions, respectively. The contribution of each ring to intergyre mass transport (1.0 6 0.4 Sv) was estimated using SeaWiFS derived surface areas and an estimate of vertical penetration (600 m) based on in situ tracer observations. Several rings were observed (using satellite-tracked surface drifters in combination with SeaWiFS imagery) to violently collide with the Lesser Antilles. At least one ring maintained an organized circulation while passing directly over the island of Barbados.