Abstract The South Pacific Convergence Zone (SPCZ), the primary rainfall source for South Pacific islands, exhibits decadal variability in its core region that remains insufficiently studied. Isolating sea surface temperature internal variability (SSTIV) from external forcing (1940–2013) reveals SSTIV is the dominant driver of SPCZ core strength variability (CSV), quantified via 10‐m wind convergence. Local SSTIV modulates CSV through thermodynamic feedbacks, whereas equatorial Pacific sea surface temperature (SST) anomalies exert limited remote influence. Regression analysis shows a 9.1 × 10−6 s−1 increase in CSV and a 1.7 mm day−1 increase in precipitation per 1°C SSTIV rise on decadal scales. Although external forcing significantly shapes SPCZ SST, its direct impact on CSV is counteracted by the tropical Pacific SST gradient. The strong sensitivity of CSV to internal SST variability, compared to its muted response to external forcing, underscores the pivotal role of internal climate variability in shaping SPCZ dynamics.

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