Contrasting Near‐Inertial Internal Wave Responses to Single Versus Consecutive Typhoons Forcing in the Xisha Islands Waters, South China Sea

Abstract

Mooring observations capture the responses of near‐inertial internal waves (NIWs) to three sequential typhoons (Trami, Yinxing, and Man‐yi) over the Xisha Islands from October to November 2024, using high‐resolution ADCP measurements. Typhoon forcing excites near‐inertial kinetic energy (NIKE) mainly in the 0–100 m layer. Although >90% of NIKE is dissipated within the main thermocline, the remaining energy penetrates below 1,200 m. Nonlinear interactions between DITs and NIWs generate fD1 waves, observed at both the surface and deep layer, indicating their generation is not restricted to the upper ocean. The fD1 waves closely correlate with NIWs and DITs and exhibit pronounced vertical shear, which enhances mixing throughout the water column. Numerical simulations further demonstrate that typhoon‐induced NIWs substantially enhance both near‐inertial potential energy and energy flux. Compared to a single typhoon, consecutive typhoons excite broader near‐inertial spectra and are more favorable for fD1 wave generation. NIKE from a preceding typhoon can superimpose on and resonate with NIKE from subsequent typhoons. NIKE induced by Trami couples with and facilitates downward transmission of NIKE induced by Yinxing and Man‐yi, accompanied by elevated modal structures. Vertically, NIKE shows a layered response: alternating enhancements and weakenings occur in the surface layer (0–200 m), weakening dominates the subsurface (200–600 m), and enhancement prevails below 600 m. As a secondary fluctuation, fD1 wave also undergoes modulation, with enhancement throughout the water column, further promoting mixing. These findings reveal the collective role of consecutive typhoons in modulating NIW dynamics and promoting deep‐ocean mixing.