Quantification of marine cooling effect and driving mechanism along Beibu Gulf coastal areas

Global warming and urbanization have greatly intensified the heat threats to urban residents. Marine cooling effect (MCE) is essential for mitigating urban heat threats in coastal cities, yet there is still big gap of understanding its seasonal dynamics and underlying drivers in subtropical nearshore zones. Taking the Guangxi Beibu Gulf as a case, this study combined multi-temporal Landsat 8 land surface temperature datasets with Spearman correlation, local spatial autocorrelation (LISA), and optimal-parameter geographical detector (OPGD) analyses to quantify MCE, which is indicated by marine cooling intensity (MCI) and marine cooling distance (MCD), and to dissect their spatiotemporal heterogeneity and driving mechanisms. Results reveal distinct seasonal changes regimes. MCI peaked in summer (up to 23.75 °C) and was lowest in autumn, while MCD reached its maximum inland extent in spring (750m). High intensity cooling consistently clustered around coastal port areas and dense gray-space urban core zones, whereas blue-space and green-space-rich zones exhibited weaker cooling. Gray space exerted a persistent positive influence on MCI across seasons (summer q > 0.55), while blue space extended MCD only in spring and green space generally shortened it. Landscape fragmentation (ED, LSI) weakened cooling in most seasons, underscoring the role of spatial continuity in facilitating MCE. Driver interactions varied by season. Blue-gray synergy produced the strongest nonlinear enhancement of MCI year-round (q > 0.60), indicating that mixed land-water configurations sharpen cooling contrasts at the land-sea interface. MCD propagation was governed in spring by blue-building coupling interactions (AI_BS × ABH, q = 0.34), in summer by gray-green coupling interactions (AI_IS × LPI_GS, q = 0.26), and in autumn by more and weaker balanced multi-factor effects. These findings highlight the seasonal shifts in how gray, blue, and green spaces individually and interactively regulate coastal thermal environments. The framework provides transferable metrics for diagnosing and optimizing blue-green-gray spatial patterns in climate adaptive coastal urban planning, with potential to maximize cooling benefits for densely built port cities in subtropical regions.