Overemphasized role of preceding strong El Niño in generating multi-year La Niña events

Previous studies have emphasized the significance of a strong El Niño amplitude preceding La Niña (LN) in the generation of multi-year LN events on account of the slow ocean heat content recharge-discharge process in the equatorial Pacific. However, our analysis of observational data spanning from 1900 to 2022 reveals that the majority of multi-year LN events (7 out of 11, or 64%) did not require a preceding strong El Niño to generate their 2nd-year LN, suggesting that there has been an overemphasis on the traditional view. Instead, we identify that a negative phase of the Pacific Meridional Mode (PMM) in the spring, when the 1st-year LN begins to decay, activates the mechanism responsible for triggering another LN and producing a multi-year LN event. This negative PMM is characterized by cold surface temperature anomalies extending southwestward from the Baja California coast towards the equator. The westward extension of the 1st-year LN cold anomalies, which interact directly with the eastern edge of the western Pacific warm pool, is highlighted as a crucial factor in the occurrence of a negative PMM. Additionally, our findings further suggest that the PMM mechanism can cause the formation of a 3rd-year LN, resulting in triple-dip LN events.

既往研究强调，受赤道太平洋缓慢的海洋热含量充放电过程影响，拉尼娜（La Niña, LN）事件发生前的强厄尔尼诺（El Niño）振幅对于多年拉尼娜事件的生成具有重要意义。然而，我们对1900年至2022年的观测数据分析结果显示，多数多年拉尼娜事件（11例中的7例，占比64%）在形成次年拉尼娜时，并不需要前期伴随强厄尔尼诺事件，这表明学界此前对传统观点存在过度强调的问题。取而代之的是，我们发现当第一年拉尼娜开始衰减的春季，太平洋经向模（Pacific Meridional Mode, PMM）的负相位会激活触发新一轮拉尼娜事件、进而形成多年拉尼娜事件的机制。该负相位的太平洋经向模表现为从下加利福尼亚海岸向赤道西南方向延伸的冷海表温度异常。第一年拉尼娜冷异常的西伸过程会直接与西太平洋暖池东缘发生相互作用，这一过程被认定为触发负相位太平洋经向模的关键因素。此外，我们的研究结果进一步表明，太平洋经向模机制可引发第三年拉尼娜事件，最终形成三重拉尼娜事件。