The North Atlantic Subtropical Front and Countercurrent: Role of Mode Water in Their Seasonal Variability and Decadal Changes Forced by NAO

Abstract While the variability of the North Pacific Subtropical Countercurrent has been widely studied, its counterpart, the North Atlantic Subtropical Countercurrent, remains poorly understood. Using RG Argo and OISST data from 2004 to 2024, we investigate the mean state, seasonal and decadal‐scale variability of the North Atlantic Subtropical Countercurrent and its associated Subtropical Front, and their dynamical linkage to the North Atlantic Subtropical Mode Water. The North Atlantic Subtropical Countercurrent/Front is located within 70°–46°W and 24°–34°N, from surface to 150 m depth. The meridional section of potential density across the North Atlantic Subtropical Front exhibits a wedge‐shaped vertical pattern, characterized by northward deepening of the lower pycnocline and shoaling of the upper pycnocline, resulting from the North Atlantic Subtropical Mode Water intrusion. Through the thermal wind relation, the northward shoaling of the upper pycnocline induces eastward shear, giving rise to the North Atlantic Subtropical Countercurrent and Front. The North Atlantic Subtropical Mode Water peaks in April, while the North Atlantic Subtropical Front peaks in May, indicating a 1‐month lag in the front's response to the mode water change. The North Atlantic Subtropical Mode Water and North Atlantic Subtropical Countercurrent/Front exhibit synchronous decadal‐scale change, with the mode water further modulated by the North Atlantic Oscillation. This study systematically illustrates the dynamic linkage between the North Atlantic Subtropical Countercurrent/Front and North Atlantic Subtropical Mode Water, offering a new mechanistic framework for understanding how subsurface mode‐water variability regulates upper‐ocean circulation and air‐sea coupling, thereby influencing climate variability in the North Atlantic.