Fig. 4: Application of the North Pacific Meridional Mode (PMM) mechanism to the formation of triple-dip La Niña (LN).

a Composite evolutions of Niño3.4 (blue curve) and PMM (black curve) indices for the selected triple-dip LN events. The indices are smoothed with a 3-month running-mean filter and the shaded areas represent their ±1 s.d. b‒d Composite structures of anomalous sea surface temperature (SST; shading, in °C) and surface wind (vector with minimum intensity > 0.4 m s⁻¹) for the triple-dip LN during the second (b), third (c), and fourth springs (d). The black parallelograms delineate the region where a negative PMM occurs and exerts its influence. e, f Longitude-time plots of anomalous SST and surface wind for the triple-dip LN over the tropical (5°S‒5°N) (e) and subtropical (15°N‒25°N) (f) Pacific basins. The green contour in e denotes the climatological 28°C isotherm line at the equator, which depicts the eastern edge of the western Pacific warm pool. g Same as (e) except for anomalous sea surface height (SSH; shading, in cm). The gray stippled areas indicate a significance level of 0.05 as determined by a two-tailed Student’s t-test.