Volume of North Pacific Eastern Subtropical Mode Water as a Precursor to ENSO Events

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 Ocean Niño Index up to nine months in advance. This relationship is driven by two distinct mechanisms. First, NPESTMW acts as a footprint of another ENSO precursor — the Pacific Meridional Mode — thereby indirectly linking to ENSO occurrence. Second, NPESTMW influences tropical Pacific sea surface temperature through the Subtropical Cell, creating a direct link to ENSO. Moreover, we find that stronger NPESTMW volume anomalies are more closely tied to multi-year ENSO events than to single-year occurrences, especially for the La Niña phase. These findings offer new insights into the roles of subtropical mode water in shaping both ENSO occurrence and its multi-year behavior.