Fig. 1

Locations of TIOON mooring arrays (Q5, Q4, Q2; red triangles) and the bathymetry of the tropical eastern Indian Ocean"

Fig. 2

Time series of zonal velocity (a, b, c) and meridional velocity (d, e, f) observed by TIOON moorings (Q5, Q4, Q2) in the eastern tropical Indian Ocean, along with their corresponding observation depths (g, h, i)"

Tab. 1

Summary of TIOON mooring observations and statistical characteristics of zonal (u; units: cm·s−1) and meridional (v; units: cm·s−1) current velocities at each mooring site, including geographical location and depth (units: m), observation time range, and current velocity, with data presented as mean values ± standard deviation of velocities"

Fig. 3

Wavelet amplitude spectra of deep zonal currents (a, b, c) and meridional currents (d, e, f) observed by TIOON moorings Q5, Q4, and Q2 in the equatorial Indian Ocean. The analysis reveals the periodic characteristics and spatial variability of deep equatorial circulation. The x-axis represents observation time (from March 2015 to May 2021), and the y-axis indicates the period (in days, shown on a logarithmic scale). Wavelet power is color-shaded on a linear scale. Statistically significant regions are outlined by contours at the 85% (pink) and 90% (magenta) confidence levels"

Fig. 4

Temporal and spatial characteristics of wind stress in the equatorial Indian Ocean (2°S - 2°N). Panels (a, b) show the time-longitude distributions of zonal (τax) and meridional (τay) wind stress anomalies, respectively. Panels (c, d) present the period-longitude distributions of τax and τay derived from fast Fourier transform (FFT), respectively, with colors indicating spectral power. Wind stress data are derived from the JRA-55 reanalysis, based on daily wind fields from January 2015 to December 2021"

Fig. 5

Lagged correlation between intraseasonal deep zonal currents in the equatorial Indian Ocean (based on mooring observations at Q2, Q4, and Q5) and intraseasonal wind stress (derived from the JRA-55 reanalysis), illustrating the spatiotemporal lag characteristics of wind stress in modulating deep equatorial zonal circulation. Panels (a, b, c) show the correlation between zonal currents with intraseasonal zonal wind stress anomalies; panels (d, e, f) show the correlation between zonal currents with meridional wind stress anomalies. Statistically significant regions are outlined by black contours at the 99% confidence level (P < 0.01)"

Fig. 6

Lagged correlation between intraseasonal deep meridional flow in the equatorial Indian Ocean (based on mooring observations at Q2, Q4, and Q5) and intraseasonal wind stress (derived from the JRA-55 reanalysis). Statistically significant regions are outlined by black contours at the 99% confidence level (P < 0.01)"

Fig. 7

Zonal wavenumber [1·(°)−1]-frequency (d−1) power spectral density (m2·s−2) of intraseasonal zonal flow in the eastern equatorial Indian Ocean (a) and theoretical propagation trajectories of equatorial waves in the Indian Ocean basin (b). In (a), the theoretical dispersion relationships of equatorial waves are plotted based on the first and fifth baroclinic modes, including Yanai waves (dashed lines), Kelvin waves (dotted lines), and Rossby waves (solid lines). In (b), the theoretical propagation trajectories of equatorial waves are calculated using the second baroclinic mode; the gray shaded areas indicate seafloor topography, and the black bars mark the locations of observation moorings, from left to right: Q2, Q4, Q5; x0: the starting longitude of equatorial wave propagation; WB: western boundary; T: period"