Fig. 1: Observed decadal La Niña occurrences and decadal WWV skewness.

A Niño3.4 SST index (170°W–120°W, 5°S–5°N) averaged over December, January, and February from HadISST⁶³. El Niño (red dots) and La Niña (blue dots) events are defined as when Niño3.4 is greater than 0.75 standard deviation (s.d.). The consecutive La Nina sequence is indicated by the light blue vertical shades. Among those events, CP ENSO event is indicated by triangle when Niño4 (160°E–150°W, 5°S–5°N) is greater than Niño3 (150°W–90°W, 5°S–5°N) in amplitude and EP ENSO is indicated by circle when Niño4 is smaller than Niño3 (ref. ⁴⁴). Beyond ENSO events captured by Niño3.4 index, additional CP and EP ENSO events are defined as when Niño4 and Niño3 index is greater than 0.75 s.d., respectively. B Time evolution of decadal La Niña occurrences and WWV skewness in 11-year sliding windows. The WWV is defined as the water volume with temperature above 20 °C over the domain of 120°E–80°W, 5°S–5°N⁴⁵. The blue and orange shade indicates when WWV skewness is negative and positive, respectively. The blue dot indicates consecutive La Niña sequence over that 11-years window (center year) with the big size indicating two sequences and the small size indicating one sequence. The correlation coefficient between the two curves and the p value are also indicated. In observation, there is a tendency for multi-year La Niña event to occur more often when the tropical Pacific Ocean is easier to discharge than recharge.