A Newly Observed Quasi-stationary Subsurface Anticyclone of the North Brazil Undercurrent at 4°S: The Potiguar Eddy

This study describes a newly discovered subsurface anticyclone, called the Potiguar Eddy (PE) centered at about 4 degrees S, 36.5 degrees W, along the western flank of the North Brazil Undercurrent (NBUC). The PE is a quasi-stationary elliptical feature, with major and minor axes of approximately 330 and 130 km. It extends vertically from 100 to 400 m, with maximum velocities of 0.6m s(-1) and recirculates about 2 Sv of waters from the NBUC. Despite the presence of the PE in subpycnocline waters, the surface flow is predominantly northwestward throughout the year. Analysis of moored current meter data revealed a depth-dependent variability that reaches its highest values between about 100 and 400 m, coincident with the PE. This variability is found to be associated with a 20- to 30-day second baroclinic mode waves, which can strengthen, weaken, or even reverse the currents of the PE. We highlight the possible causes for this variability to include coastally trapped waves, instabilities, and tropical instability waves or a combination thereof-which opens up new study topics for future research. Plain Language Summary A newly discovered subsurface anticyclonic eddy centered at about 4 degrees S, 36.5 degrees W is described here. This eddy is called the Potiguar Eddy (PE) and is associated with the northward flowing North Brazil Undercurrent (NBUC) over the Potiguar basin. The PE is a quasi-stationary elliptical feature, with major and minor axes of approximately 330 and 130 km. It extends vertically from 100 to 400 m, with maximum velocities of 0.6 m s(-1) and recirculates a portion of water from the NBUC. The currents within the PE vertical extent present variability with a period of about 20-30 days, which can strengthen, weaken, or even invert the currents of the PE. We speculate the possible causes for this variability which include coastally trapped waves, instabilities, and tropical instability waves. It is possible that the presence and variability of the PE may affect the distribution of tracers and planktonic organisms in the area. Additionally, the Potiguar Bight is an important region of offshore oil and gas exploration in Brazil. The presence of the PE and the characterization of its variability are important for representing the current velocity fluctuations for the industrial activities in this region. Key Points An anticyclonic subsurface eddy along the North Brazil Undercurrent is reported The Potiguar Eddy is a mesoscale quasi-stationary feature The PE exhibits a 20- to 30-day intraseasonal variability linked to the second baroclinic mode