Mesoscale dynamics and transport in the North Brazil Current as revealed by the EUREC4A-OA experiment

Abstract. The North Brazil Current (NBC) rings are key features for interhemispheric water transport, facilitating exchange between the South Atlantic and North Atlantic Ocean. However, significant uncertainties persist regarding the total volume transported by these structures and the water masses they advect. In this study, we integrate high-resolution in situ observations from the EUREC4A-OA field experiment with satellite altimetry to address these knowledge gaps. Using a novel methodology, we estimate that surface NBC rings transport approximately 1.5 Sv, while subsurface eddies contribute between 0.4 and 9.7 Sv, underscoring their critical role in the regional total transport. Combined, these transports may significantly contribute to closing the Atlantic Meridional Overturning Circulation transport at low latitudes. Surface NBC rings predominantly advect Salinity Maximum Waters and fresh waters from the Amazon River, whereas subsurface NBC rings play a critical role in transporting Eastern South Atlantic Central Waters, Western South Atlantic Central Waters, and Antarctic Intermediate Waters northward. We estimate that the heat transport by surface and subsurface NBC rings is 5.8 and 0.3 TW, respectively, which is significantly lower than previous findings. Overall, these findings underscore the pivotal role of subsurface NBC rings as conduits for South Atlantic Waters across the Equator to the tropical North Atlantic. This study confirms the intricate dynamics of NBC rings and their essential role in interhemispheric water transport.