The asymmetry of air-sea coupled strength between the first-year and second-year La Niña events

Compared to the La Niña event in the first-year, the air-sea system is not well coupled during the second-year. This is evidenced by the generally weaker anomalous sea surface temperature (SST) and the significantly stronger easterly wind anomalies. According to the Bjerknes positive feedback, the strong easterly wind anomalies are challenging to explain using only the zonal gradient of the sea level pressure (SLP) anomaly. This is due to the incursion of the off-equatorial southeasterly wind originating from the southeast Pacific before the onset of the second-year event. It results in a distinctive development feature in the event, distinguishing it from the first-year event, which is a typical result of equatorial ocean–atmosphere interaction. The different roles of the off-equatorial signal and equatorial easterly wind indicate a relay process in which the former mainly plays a triggering role, while the latter primarily acts as a reinforcing role in the second-year event. Furthermore, the zonal advective feedback plays a crucial role in determining the amplitude of the second-year cold event, which is a main difference from the first-year event. The reduced intensity produces a relatively weaker SST amplitude during the second-year La Niña event due to the thicker mixed layer depth.