Modulation of the M2 Internal Tides Near the Luzon Strait by Westward-Propagating Mesoscale Eddies in the Philippine Sea

Mesoscale eddies are prevalent in the Philippine Sea and frequently impinge on the Kuroshio Current, affecting internal tides (ITs) originating from the Luzon Strait (LS). This process is investigated in this study using idealized experiments, which provide a controlled environment to quantify variations of the M2 ITs as a single cyclonic eddy (CE) or anticyclonic eddy (AE) approaches the LS from the east. Our results reveal an asymmetric influence: AEs induce more pronounced changes in the M2 tidal conversion than CEs. Variations in conversion rates are primarily controlled by changes in bottom pressure perturbations, which are linked to the product of subtidal buoyancy gradients and IT vertical velocity. Another noteworthy finding is that these mesoscale eddies influence the M2 ITs differently in two stages: during propagation across the Philippine Sea and upon impingement with the Kuroshio Current. In the first stage, mesoscale eddies modulate the propagation of the M2 ITs through direct interaction, inducing incoherent M2 ITs in the Philippine Sea. The percentage of incoherent ITs exceeds 50% along the eddy paths. Advection from eddy-induced currents contribute significantly more than background stratification and relative vorticity. During the second stage, as the eddies impinge on the Kuroshio Current and move northward, variations in the M2 IT energetics within the LS are evident. The IT incoherence increases not only in the Philippine Sea but also in the LS and South China Sea. These findings are important for understanding the IT variability and incoherence in the Philippine Sea and South China Sea.