Transformation in the Stability of Tide‐Induced Upper Saline Plume Driven by Transient External Forcing.

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. Plain Language Summary: The fluctuation in sea level induced by tides generates an upper saline plume (USP) in the seaward upper end of coastal aquifers. Inland fresh groundwater flows seaward and mixes with the saltwater within the USP, forming a saltwater‐freshwater mixing zone at the edge of the USP. This zone influences the fate of land‐sourced pollutants prior to their ocean discharge and eventually affects the coastal water quality and ecosystems. Therefore, it is important to study the USP configuration and its influencing factors for the sustainable management of marine and groundwater resources in coastal areas. The USP observed in the previous research maintains a stable or unstable structure, thus ignoring the effect of the timescale changes on external forcing conditions, such as tidal stage, spring‐neap variability in tidal amplitude, and seasonality in subsurface inflow. In this study, we investigated the stability of the USP over the timescale of external forcing conditions in coastal aquifers. In contrast to earlier studies, the USP was not found to be always stable or unstable, but experienced a dynamic transformation of stability over time. These results provide new insights into the complex intertidal environment in coastal aquifers. Key Points: Dynamic transformation of stable‐unstable tide‐induced upper saline plume is identified in coastal aquifersIgnoring the timescale changes in external forcing conditions may underestimate the generation of an unstable flowThere is an inverse relationship between the frequency contrast of inland and seaward fluctuations and the duration of the unstable flow [ABSTRACT FROM AUTHOR]