The annual sea level cycle (ASLC), characterized by its amplitude and phase, modulates coastal flood risk by altering the background water level upon which episodic storm surges superimpose. However, the future evolution of this seasonal cycle under climate change and its localized interactions with storm surges remain poorly constrained at high spatial resolutions. Here, we investigate future spatiotemporal shifts in the ASLC along the global coastline and evaluate their implications for extreme coastal flooding. To project future variations, we analyze historical simulations and future climate scenarios (SSP1-2.6, SSP2-4.5, and SSP5-8.5) from the Coupled Model Intercomparison Project Phase 6 (CMIP6).By leveraging high-resolution dynamic sea level fields to overcome the spatial limitations of previous coarse-grid models, we map global changes in annual amplitude and phase relative to the historical baseline. Our findings highlight that shifts in the seasonal peak of background sea levels can either amplify or mitigate extreme coastal flooding, depending on their temporal alignment with the local storm surge season. This high-resolution global assessment provides crucial insights for regional coastal adaptation and resilient infrastructure planning.