Fig. 2: Spatiotemporal evolution of oceanic wave propagations and their relationship with WWB and MJO during 1997/98, 2015/16, and 2023/24 El Niños.

Longitude-time plots of equatorial (5°S‒5°N) Pacific monthly \(\triangle D20\,({\rm{color\; shading}},\,\triangle D20\left(t\right)={D20}_{t}-{D20}_{t-1},{\rm{t\; is\; time}}\), unit in meter overlaid monthly with zonal surface wind anomalies (purple contours, unit in m/s) and daily OLR (dashed black contours, unit in W/m²) average over 10°S‒10°N during 1997 El Niño (a), 2015 El Niño (b), and 2023 El Niño (c). The \(\triangle D20\) and zonal surface wind anomalies plots show only the Pacific region (120°E-80°W), with the OLR anomalies filtered using the temporal-spatial filter with a 20–100-day bandpass and eastward wavenumber 1–5. OLR values below -5 W/m² are displayed, with a contour interval of 5 W/m². Zonal surface wind anomalies are shown only for values exceeding the WWB threshold (2 m/s) with a contour interval of 1 m/s. The blue dashed arrows in (a–c) indicate the direction of oceanic downwelling wave propagation, defined when \(\triangle D20\) > 10 m. The straight dashed lines denote the westerly wind burst region (135°E–180°E). The horizontal dashed line indicates the dividing line between the developing year and decay year. Panels (d–f) display the time series of the normalized daily westerly wind burst (WWB) index (blue solid line) and MJO index (red dashed line) for the 1997/98 El Niño, 2015/16 El Niño, and 2023/24 El Niño. The black solid line and gray shading in (d–f) represent the mean and 95% confidence interval of other El Niño events from 1980–2024. Red shading in (d), yellow in (e), and blue in (f) highlight the episodes of ocean downwelling wave propagation.