Asymmetry Between Deep and Shallow Atlantic Equatorial Undercurrent Core-Position Events: Contrasting Oceanic and Atmospheric Processes

The Atlantic Equatorial Undercurrent (EUC) exhibits substantial variability in its vertical position, with implications for equatorial ocean dynamics and air–sea interactions. We investigate the oceanic and atmospheric conditions associated with anomalously deep and shallow EUC core-position events using monthly SODA oceanic and Twentieth Century Reanalysis (20CR) atmospheric data. The EUC core position is defined as the velocity-weighted vertical center of the eastward subsurface current, and events are identified from standardized position anomalies. We identify 95 independent deep and 84 shallow events, with mean core positions of 87.7 ± 4.1 and 58.1 ± 3.4 m, respectively. Despite this difference, mean maximum EUC velocities are similar (1.15 ± 0.14 and 1.13 ± 0.16 m s⁻¹). Composite analyses show contrasting reorganizations of the equatorial current system, vertical shear, temperature, SST, and SSH. Deep events exhibit stronger westward flow above the EUC, enhanced positive vertical shear, negative temperature, SST and SSH anomalies, and a steeper east–west thermocline slope than shallow events. SSH anomalies propagate eastward at approximately 1.2 m s⁻¹, consistent with Kelvin-wave-like signals. Contrasting wind-stress and sea-level-pressure anomalies further indicate distinct coupled ocean–atmosphere states rather than simple opposite phases of the same variability.