Linking North Pacific eastern subtropical mode water to ENSO: precursor signatures and subtropical cell pathways

Mode waters play a crucial role in ocean heat and carbon storage, as well as in climate variability. Here we reveal a strong relationship between the North Pacific Eastern Subtropical Mode Water (NPESTMW) and El Niño-Southern Oscillation (ENSO) events. NPESTMW volume anomalies exhibit significant correlations with the Ocean Niño Index up to nine months in advance. Our analysis identifies two distinct pathways connecting NPESTMW and ENSO development. First, NPESTMW serves as a footprint of the Pacific Meridional Mode, a well-established ENSO precursor. Second, NPESTMW influences tropical Pacific sea surface temperature through the Subtropical Cell. Notably, our findings indicate that stronger NPESTMW volume anomalies are more closely tied to multi-year ENSO events than to single-year development, especially for the La Niña phase. These discoveries offer new insights into the roles of subtropical mode water in shaping ENSO development.