Intermediate circulation variability in the equatorial Indian Ocean

The Equatorial Intermediate Current (EIC) impacts the distribution and the transport of biogeochemical tracers such as oxygen. The EIC in Indian Ocean, covering the range from 200 to 1000 m between 2°S and 2°N , has higher velocity and a lower-frequency variability in the central basin than in the east. The EIC variability is forced by the wind stress forming equatorial beams and is also strengthened by basin resonance. We use zonal current velocity timeseries of 2015-2023 obtained from different equatorial moorings and a continuous timeseries of 2000-2022 years derived from a global NEMO ocean model configuration at 0.25° horizontal resolution with 46 z-levels (ORCA025.L46) and apply the method of vertical mode decomposition aiming to characterize equatorial zonal velocity variability of the Indian Ocean by equatorial beams, baroclinic modes, and equatorial basin resonance.

From west to east, the Indian Ocean is divided by the topography into three subbasins. The west basin is from the western boundary to the Maldives Islands at 73°E; the central basin is from 73°E to the 90°E ridge; the east basin is from 90°E to the eastern boundary. The frequency – baroclinic mode decomposition of the velocity field shows that semiannual and annual signals are the most significant components. For semiannual signals, the second to fourth baroclinic modes contribute at the mooring locations at 80°E and 85°E, while the fifth to eighth modes dominate at 93°E, indicating the essential role of higher baroclinic modes in the eastern basin. For annual signals, lower baroclinic modes are more significant in the east than in the central subbasin. The model output agrees with the observed distribution of contributing baroclinic modes. Observations further reveal several strong EIC events occurring in 2015-2016 and 2020-2021. Atmospheric data showed corresponding strong anomalies in zonal wind stress and outgoing long-wave radiation. Sea surface temperature anomalies happened along with them. With the distribution of the contributing baroclinic mode, the equatorial beams could explain the strong current events at intermediate depths. The energy input from atmospheric forcing propagates along beams, which are predominantly formed by the second baroclinic mode in the central basin and by the superposition of several higher baroclinic modes in the eastern basin. Future research would focus on the role of equatorial beams in the deeper current variability with the knowledge of contributed baroclinic modes in the Indian Ocean.