The Equatorial Intermediate Current (EIC) is a significant part of the equatorial deep circulation and influences the distribution of heat and water mass. The Indian Ocean EIC is observed throughout the water column spanning from 200 to 1000 m between 2°S and 2°N. At 200-400m, RAMA observed the zonal current has a mean velocity of 11 cm·s⁻¹. At 1000 m, the Argo floats reveal the mean zonal velocities typically range from 1–5 cm·s⁻¹. The EIC has strong seasonal variability and intraseasonal variability (ISV). The EIC also appears interannual variation with exceptionally strong amplitude. Investigating the exceptional EIC benefits deepens our understanding of the atmosphere-intermediate currents response, and serves the study of the deep equatorial circulation. This study examines the exceptional EIC characteristics and causes and found it is a snapshot of an intermediate meander.

In 2019, an exceptional EIC event is caught by a deep-sea mooring at (0, 80°E). The exceptional EIC flows eastward with strong amplitude at depths ranging from 200 to 800 m. The velocity exceeding 2 STD of EIC lasts from January to February (STD = 0.11 m s^-1). Mode decomposition suggests the second baroclinic mode contributes most to the exceptional EIC, and it explains why the velocity in the entire intermediate layer is positive instead of flowing with several cores along the depths. While the exceptional EIC peaks, the seasonal variability, and the ISV reach a maximum at the same time. The EIC is inspired by the collaboration of seasonal and intraseasonal variability, which contribute 65% and 30%, respectively. In the eastern Indian Ocean, two important atmospheric forcings originate the wind stress for the EIC generation: the monsoon and Madden–Julian Oscillation. They are responsible for the two variations’ unique features. Generally, the EIC reverses twice annually but we found the kinetic energy in the first cycle is 1.5 times stronger than that in the second cycle. The seasonal difference is induced by the monsoon transition, associated with oceanic wave propagation. Derived from the wind stress, a strong anomaly westerly occurs in the eastern basin 40 days before the exceptional EIC. Model simulation reveals when the strong EIC ISV appears, MJO has a 70% probability to appear under phases 3-5 with strong amplitude and long-lasting. Interestingly, the model simulation also reveals the exceptional EIC situates in an intermediate meander, that flows around the equator and swings within 2°S-2°N from January to February and from September to October.