Global warming poses a growing threat to permafrost stability worldwide. Coastal permafrost is especially vulnerable because it is exposed to both rising ocean temperatures and seawater intrusion. However, the dynamic thermo-hydro-saline coupling mechanisms governing seawater intrusion-driven permafrost thaw remain poorly understood. This study employs a fully coupled groundwater (ice)-salt-heat numerical model to investigate these dynamics in a high-latitude coastal aquifer, with sensitivity analyses on seawater temperature, aquifer permeability, and porosity. Results identify seawater temperature (<em>T</em><em><sub>s</sub></em>) as the primary driver, with permafrost thaw volume increasing by ~105% as <em>T</em><em><sub>s</sub></em> rises from 0.5 to 3.5 °C. The system evolves from an initial density-driven, clockwise saline circulation to a mature double-circulation pattern, including a thermally-driven counterclockwise cell. The onset of this double circulation occurs earlier at higher <em>T</em><em><sub>s</sub></em>, fundamentally reshaping long-term heat transfer. The salt contribution degree (<em>CD</em><em><sub>s</sub></em> < 0.5) confirms thermal forcing dominates thaw, while salinity modulates early-phase thawing. Aquifer properties strongly modulate thaw sensitivity: higher permeability accelerates heat and solute transport but promotes earlier counterclockwise circulation development, which can limit inland advance. These findings quantify the nonlinear response of coastal permafrost to ocean warming and provide a predictive framework for assessing carbon mobilization and geotechnical risks in vulnerable Arctic regions.