Pacific decadal variability (PDV) influences not only circum-Pacific regions but also global climate, yet its evolution and controlling factors on geological timescales remain unclear. The response of PDV amplitude to greenhouse gas (GHG) forcing has been relatively well studied, while little is known about how paleogeographic changes regulate PDV amplitude. Using CESM paleoclimate simulations spanning 18 geological time slices from 170 Ma to the present, we quantitatively decompose the relative importance of GHGs versus land-sea configuration in regulating PDV evolution. We divide the entire period into two phases: Post-Pangaea Dispersal (170-90 Ma) and Pre-Amasia Convergence (80-0 Ma). During the earlier phase, GHGs and land-sea configuration have comparable influences on PDV amplitude (-0.06 and -0.07 C/100 Ma, respectively). In contrast, the later phase is dominated by GHG forcing (+0.07 C/100 Ma), with land-sea configuration playing a dampening role (-0.02 C/100 Ma). We further show that paleogeography modulates PDV amplitude through two mechanisms: background temperature is negatively correlated with PDV amplitude, whereas ocean basin width is positively correlated. Idealized experiments confirm that narrowing the Pacific basin alone (from 200 to 140 of longitude) weakens PDV by about 15% (from 0.32 to 0.27 C) through enhanced stratification. Our findings highlight that both GHGs and paleogeographic evolution regulate Pacific decadal variability amplitude through modulation of upper-ocean stratification.
 

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