Seasonal modulator: The effect of high-speed rail connectivity on intercity mobility seasonal fluctuation in China

Intercity mobility seasonality reflects the temporal concentration of travel demand shaped by natural cycles, socio-cultural practices, institutional calendars, and transport supply conditions. While existing studies on high-speed rail (HSR) mainly focus on accessibility, spatial connectivity, and aggregate mobility growth, less attention has been paid to how HSR is associated with the seasonal organization of intercity mobility. To address this gap, this study conceptualizes HSR as a seasonal modulator and examines its relationship with intercity mobility seasonality in China using daily intercity mobility data and HSR service data. The results show that intercity mobility seasonality is spatially uneven. Central and western China generally exhibit stronger seasonal fluctuations, while major cities in eastern coastal urban agglomerations show more stable year-round mobility. At the directed OD-pair level, short-distance and high-frequency flows within major urban agglomerations tend to have lower seasonality, whereas long-distance and low-frequency corridors are more likely to show stronger seasonal concentration. HSR connectivity is significantly associated with intercity mobility seasonality, but the association varies across service types and spatial contexts. At the national level, HSR access and connectivity are positively associated with seasonal concentration, with G-series services showing the most stable association. Within urban agglomerations, HSR is more closely linked to routine and evenly distributed mobility, while in long-distance and non-intra-agglomeration corridors it is more often associated with stronger seasonal peaks related to tourism, labor mobility, and family visits. These findings extend the discussion of HSR impacts from spatial accessibility to temporal organization and provide implications for seasonal capacity allocation, holiday demand management, and differentiated intercity transport planning.