Distinctive Routes for Multi-Year La Niña Formation

A prevailing hypothesis for generating a multi-year La Niña (LN) compared to a single-year LN is the strong discharge of ocean heat content (OHC) associated with preceding extreme El Niño, but this mechanism explains only a small portion (~30%) of the multi-year LN events during the past century. For the majority of multi-year LN events, it is found that the meridional extent of the cold SST anomaly in the tropical Pacific, particularly the South Pacific Meridional Mode (SPMM), during the LN decaying spring plays a critical role in prolonging the LN. Idealized atmospheric general circulation model experiments confirm the role of the SPMM in generating equatorial easterlies, which may help enhance oceanic upwelling and meridional divergence at the equator, slowing down the LN damping. As a result, the cold anomaly associated with LN persists throughout the decaying summer and re-develops in the following autumn via a season-dependent coupled instability. Thus, this study emphasizes the role of the SPMM in causing multi-year LN events.