The fluctuation in sea level induced by tides generates an upper saline plume (USP) in the intertidal region, which is closely related to groundwater flow and solute transport processes in coastal aquifers. Thus, a clearer understanding of the USP configuration is needed to accurately predict the extent of seawater intrusion and water chemical fluxes to the ocean. This study experimentally and numerically examined the effect of transient external forcing conditions on the stability of tide-induced USP, represented as seasonal subsurface inflow. In contrast to earlier studies, the USP was not always stable or unstable, but experienced a dynamic transformation of stability-instability as the seasonal subsurface inflow fluctuated. The same intensity of subsurface inflow caused a stable USP in the fixed cases but a dynamic USP state in the seasonal cases. The USP responded rapidly to the fluctuation of seasonal subsurface inflow and was drawn to expand into the intertidal zone. These features of the USP contributed to the increased potential for the onset of an unstable flow when the subsurface inflow varied seasonally. Therefore, ignoring the timescale changes on external forcing conditions may underestimate the generation of unstable USPs. The frequency contrast between seasonal subsurface inflow and tide also influenced the dynamic transformation process occurring in the intertidal region. There was an inverse relationship between the frequency contrast and the duration of the unstable flow. These findings provide a new understanding of the complex intertidal environment in coastal aquifers.