The South Pacific Convergence Zone (SPCZ) serves as the primary rainfall source for South Pacific islands, yet the decadal-scale variability within its core region remains insufficiently understood. By isolating sea surface temperature variability attributed to internal climate processes (SST_IV) from externally forced signals over the period 1940–2013, we demonstrate that SST_IV is the dominant driver of core strength variability (CSV) in the SPCZ, as quantified by 10‑m wind convergence. Local SST_IV modulates CSV through thermodynamic feedbacks, whereas equatorial Pacific 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℃ SST_IV 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 pronounced sensitivity of CSV to internal SST variability, in contrast to its comparatively weak response to external forcing, underscores the critical role of internally generated climate variability in governing SPCZ dynamics and, by extension, the hydroclimate of the surrounding island regions.