Topographic Organization of Abyssal Intraseasonal Variability in the Northern Philippine Basin

Abstract

Deep intraseasonal variability (ISV) in weakly stratified marginal basins remains poorly understood, especially where complex bathymetry may modulate abyssal motions. Here we examine its structure and driving mechanisms in the northern Philippine Basin (NPB) using multi‐year, nearly full‐depth mooring observations at 128°E, 20.45°N. Velocities spanning the surface to 4,900 m reveal two dominant ISV bands near ∼83 and ∼113 days in the deep zonal flow, both intensifying toward the bottom with vertically coherent structures consistent with topographic Rossby wave (TRW)‐like motions. Local dispersion‐consistency diagnostics confirm that both bands fall within the admissible topographic Rossby wave period range and yield characteristic horizontal wavelengths of order 10 ²  km. Modal decomposition further shows that the ∼113‐day band projects more strongly onto the barotropic mode, whereas the ∼83‐day band retains a larger low‐order baroclinic contribution. Estimated barotropic Rossby normal‐mode periods for the connected deep basin are consistent with both observed bands, suggesting time‐scale compatibility with basin‐scale intrinsic adjustment, while the bands' TRW‐like character is supported independently by bottom intensification and dispersion‐relation consistency. The two bands also differ in their dominant forcing: the >100‐day band is more closely linked to upstream sea level anomaly variability organized along potential vorticity pathways, whereas the <100‐day band is more strongly associated with mesoscale eddy activity east of the mooring. Together, these results demonstrate that abyssal ISV in the NPB is shaped by temporally varying external forcing acting within a deep‐basin system constrained by basin geometry, with different forcing regimes exciting distinct intraseasonal bands.