Influences of the Tropical Multi‐Basin SSTA on the Seasonal Forecast of Summer Western Pacific Subtropical High Based on the NUIST‐CFS1.0

Abstract Western Pacific subtropical high (WPSH) modulates East Asian summer climate variability and is influenced by tropical sea surface temperature anomalies (SSTA). However, the relative contribution and combined effects of tropical multi‐basin SSTA on summer WPSH prediction skill remain poorly quantified. Using decoupling experiments with the global seasonal‐interannual Climate Forecast system of Nanjing University of Information Science and Technology (NUIST‐CFS1.0), this study quantifies the distinct roles of tropical Pacific Ocean (TPO), Indian Ocean (TIO), and Atlantic Ocean (TAO) SSTA in the summer WPSH prediction based on the hindcasts initialized in February and May during 1983–2018. Results demonstrate that while both TAO and TPO SSTA are critical drivers of WPSH prediction, their influence dynamics differ significantly. TPO's influence decreases as the lead time shortens and originates from both local air‐sea interactions in the western Pacific and remote forcing linked to El Niño events. TAO's influence remains stable across all lead times, contributing to WPSH variability through both direct (Kelvin wave–induced Ekman divergence) and indirect pathways, including an eastward route via northern Indian Ocean warming (∼20% contribution) and a westward route through central Pacific cooling (declining from 25% at 4‐month lead to 9% at 1‐month lead). In contrast, the TIO SSTA have weaker effects, with varying mechanisms: direct influence dominates in short‐lead forecasts from May, while indirect influence via TPO cooling emerges in longer‐lead forecasts from February. These results highlight the necessity of prioritizing TAO and TPO SSTA predictions as well as the inter‐basin effects in improving the prediction skill of summer WPSH.