The on-shelf penetration of low-frequency open-ocean signals makes a significant contribution to the variability of coastal sea level. However, owing to the complicated coupling, the high-frequency tidal effects on the shoreward penetration of the low-frequency signals are generally overlooked. This study revisits the classic β-plane arrested topographic wave model aiming to more explicitly reveal the role of tides in modulating the open-ocean sea-level transmission over the continental shelf. The bottom friction coefficient (BFC) is re-expressed as r =fδ_B=κ(τ/ρ)^1/2, relating the thickness of bottom boundary layer δ_B to Coriolis parameter f, or relating the bed shear stress (BSS) τ to constant κ and water density ρ. A data-based method is used to estimate the total BSS considering the non-linear interaction which combines the current-alone BSS and tide-alone BSS. It provides a novel perspective to bring in the superimposed effect of tidal currents on the signals transmission. In the western North Atlantic, the combined BFC exhibits an abrupt change between 30°N~33°N based on the above redefined equation. Modeling results show that this peak BFC would enhance the on-shelf penetration of open-ocean signals especially in the downstream vicinity of 31°N, and this enhancement is more evident for shorter-wavelength signals. Such a pronounced coastal response from modeling result is verified and clearly manifested in the tide-gauge measurements along the east coast of North America. Hence both predictions and observations verify a strong coupling between high-frequency tidal processes and the on-shelf penetration of low-frequency signals from deep ocean.
 

coastal sea level,tidal current,boundary layer