Intraseasonal variability and dynamical mechanisms of equatorial deep currents in the Indian Ocean^*

This paper examines the intraseasonal variability of equatorial deep currents (below 1200 m) in the Indian Ocean, utilizing time series data spanning from 2015 to 2021. These data were obtained from TIOON (tropical Indian Ocean observation net) moorings positioned at 80°E, 85°E, and 93°E on the equator, supplemented by continuous current velocity data from BRAN2022, wind velocity data from JRA-55, and temperature-salinity data from WOA23. Observations reveal that the standard deviation (STD) of zonal current velocities at these locations ranges from 2.5 to 3.1 cm·s−1, while the STD of meridional current velocities ranges from 2.6 to 3.1 cm·s−1. Notably, intraseasonal variability accounts for 88%-91% and 74%-84% of the total current variability for the zonal and meridional currents, respectively, underscoring its significance. Wavelet analysis indicates that the primary period of intraseasonal deep zonal currents is 10-100 days, with a longer period (50-90 days) observed at 80°E and a shorter period (T4) analysis reveals strong nonlinear dynamics near the 90°E ridge, with significant energy transfer from intraseasonal variability to the mean flow. In contrast, over flat topography, the energy transfer is reversed—from the mean flow to intraseasonal variability—though notably weak. This study enhances our understanding of deep current dynamics and provides observational evidence to improve deep ocean circulation simulations.