Fig. 1

Basic configuration and quantitative measures of the Indian Ocean Dipole (IOD). (a) Spatial distribution of sea surface temperature (shading), 10-meter wind field (vectors), and thermocline depth (stippling; red and blue dots indicate thermocline deepening or shoaling by more than 4 meters, respectively) during the peak phase of IOD (Northern Hemisphere autumn, September-November); these results are based on regression onto Dipole Mode Index (DMI) in September-November (SON); only significant results exceeding the 95% confidence level are shown; red and black boxes indicate the western pole and eastern pole of IOD; green box shows the location of equatorial zonal wind index (Ueq); (b) quantification of the dipole mode index (DMI) during the peak phase of IOD from 1941 to 2022; black line, blue bar, red bar, and yellow bar represent the contribution of original (All), EP ENSO (EP-mode), CP ENSO (CP-mode) and independent part (Non-ENSO) to the DMI; variance explanations are noted in the legend box; dashed/dotted line exhibits the one/twice standard deviation (STD, 0.42/0.84°C); (c) average and typical categorical contribution ratios of IOD events, selected based on exceeding one standard deviation in (b)"

Fig. 2

Mature phase patterns of the IOD in different components. (a) Anomalous fields of sea surface temperature (shading), 10-meter wind field (vectors), and thermocline (contours) regressed onto the DMI during the mature phase of the IOD in the EP-mode component; (b, c) similar to (a) but for the regression fields of the DMI in the CP-mode component and the internal variability component, respectively; only significant results exceeding the 95% confidence level are shaded, bold, and contoured light/dark red (blue) color indicates the thermocline deepening (shallowing) by 2/4 meters"

Fig. 3

Walker circulation patterns corresponding to IOD in different components. (a) Regression of equatorial Indo-Pacific atmospheric circulation fields onto the DMI during the peak phase of the EP-mode component, showing anomalous vertical velocity and zonal velocity; (b, c) similar to (a) but for the CP-mode component and the internal variability component, respectively"

Fig. 4

Performance of the thermocline-SST-wind positive feedback coupling efficiency in the tropical Indian Ocean across different components. (a) Scatter plot of SST at the eastern pole of the Indian Ocean Dipole (IOD) and thermocline depth (D20) from the original variations, illustrating the thermocline-SST feedback; blue/red line denotes the relationship between thermocline-shoaling and SST-cooling/thermocline-deepening and SST-warming; (b) scatter plot of the dipole mode index (DMI) and equatorial Indian Ocean zonal wind (Ueq), illustrating the temperature gradient-zonal wind feedback; (c) scatter plot of Ueq and D20, illustrating the zonal wind-thermocline feedback; considering the asymmetry of IOD, regressions are made for positive (blue) and negative (red) phases; (d-f), (g-i), (j-l) similar to (a-c), but for the EP-mode component, CP-mode component, and Non-ENSO component, respectively, showing the feedback efficiency in each component"

Fig. 5

Subsurface temperature variations in the South Indian Ocean during the peak phase of the Indian Ocean Dipole (IOD). (a) Anomalies in subsurface temperature (contours and shading) and thermocline depth (red and black lines) in the EP-mode component, based on the regression of the dipole mode index (DMI) during the peak IOD phase onto the southern tropical Indian Ocean (zonal section 8°S-12°S); (b, c) similar to (a), but for the CP-mode component and the internal variability component, respectively; (d-f) similar to (a-c), but showing the influence of each component on the meridional section (75°E-85°E)"

Fig. 6

Rossby wave propagation in the South Indian Ocean induced by IOD. (a) Lag regression of the thermocline depth anomaly (color contours and shading) and wind stress curl anomaly (black contours and stippling) in the South Tropical Indian Ocean (10°S section) on the DMI during the peak phase of the EP-mode component; (b, c) similar to (a), showing the DMI regression fields for the CP-mode component and the internal variability component, respectively; significant results exceeding the 95% confidence level are shaded and bold"

Fig. 7

Estimation of the transition from IOD to the following year's Indian Ocean basin-wide mode (IOB). (a) Monthly lagged regression of the IOB index based on the dipole mode index (DMI) during the peak phase of IOD; (b) contributions of various components in the six cases where positive IOD events (as shown in Fig. 1b) transitioned into IOB warming out of 15 events"

Fig. 8

Three typical positive IOD events and their transitions to Indian Ocean basin-wide warming in the following year. (a, b) The 1997 positive IOD event and the subsequent 1998 IOB warming event; (c, d), (e, f) similar to (a, b), showing the cases for 2015-2016 and 2019-2020, respectively. These three events represent IOD and IOB events dominated by EP-mode, CP-mode, and internal variability, respectively"