Abstract: The intraseasonal variability of the equatorial deep currents (below 1200 m) in the Indian Ocean are investigated in this paper based on the 2015-2020 deep current velocity timeseries obtained from TIOON (Tropical Indian Ocean Observation Net) mooring at 80°E, 85°E, and 93°E on the equator, along with a continues current velocity from BRAN2022. Observations show the zonal current velocities at three locations range from 0.21 ± 3.05 cm s^-^1 to 0.29 ± 2.50 cm s^-^1, and the meridional current velocities range from -0.03 ± 2.61 cm s^-^1 to 0.05 ± 3.14 cm s^-^1. The intraseasonal variability of the zonal and meridional currents accounts for 88%-91% and 74%-84% of the total current variability, respectively, highlighting the significance of intraseasonal variability. Wavelet analysis indicates that the main period of the intraseasonal deep zonal currents is 10-100 days, with a longer period (50-90 days) at 80°E and a shorter period (< 50 days) at 93°E. This suggests blue-shift phenomenon that the variability of the deep currents shifts to a higher frequencies when being more eastern. The intraseasonal meridional currents exhibits a significant peak at the 60-day period. The equatorial wind stress anomaly is an important forcing to drive the intraseasonal variability in deep currents by direct and reflected wave processes. In the central basin (80°E and 85°E), the intraseasonal variability is primarily driven by the zonal wind stress anomaly and modulated through a reflected wave process. Energy is transferred to the deep layers via Rossby waves that are formed by Kelvin waves reflected at the eastern boundary, predominantly based on low-order baroclinic modes. In the eastern basin (93°E), the intraseasonal variability of deep currents is mainly driven by both zonal and meridional wind stress anomalies and modulated through a direct wave process. Energy is transferred to the deep layers via Yanai waves directly generated west of the current, involving multi-order baroclinic modes. Based on the linear wave theory, this study depicts the energy propagation rays of equatorial beams, showing that topography plays an important role in the dynamics of deep circulation by affecting equatorial beam energy propagation. The flat topography at central basin enables energy propagating downward and westward by reflected wave process, while the ridge near 90°E may obstruct energy propagating downward and eastward by the direct waves process. This study deepens the understanding of deep currents dynamics and offers observational evidence for improving deep ocean circulation simulations.

Key words: Indian Ocean circulation, equatorial deep current, intraseasonal variability, equatorial wave