The decay of near-inertial oscillations (NIOs), strongly modulated by mesoscale eddies, reflects the persistence of surface near-inertial energy and its transfer to the ocean interior. Its variability is important for understanding regional energy pathways and mixing processes. Using hourly surface drifter observations and daily satellite altimetry from 1995 to 2021 in the East Sea (Japan Sea), we estimated key parameters of NIOs: relative frequency shift (RFS), near-inertial variance (NIV), and inverse excess bandwidth (IEB, representing the decay timescale). We then examined their relationship with altimeter-derived mesoscale vorticity fields. Additionally, moored ADCP measurements from the EC1 mooring in the southwestern East Sea, collected since 2009, were used to examine non-seasonal variability in NIO decay timescales that cannot be resolved from drifter observations alone due to their inherent spatiotemporal sampling limitations. IEB exhibits a positive relationship with NIV, while it shows a negative relationship with the magnitude of RFS. This suggests that near-inertial oscillations with longer persistence tend to have greater energy, whereas large frequency shifts induced by mesoscale vorticity correspond to shorter decay timescales. Importantly, IEB also decreases with increasing magnitude of the vorticity gradient. Spatially, IEB was generally lower in the south of the subpolar front than north (11 days and 12.2 days, corresponding to decay timescales of 3.5 and 4 days, respectively), consistent with the stronger mesoscale vorticity gradients in the southern East Sea. In the southwestern East Sea, autumn-mean IEB showed a negative relationship with vorticity gradient magnitude on interannual timescales, with the lowest IEB (IEB » 7 days) occurring in 2012 when the vorticity gradient was strongest (|∇ζ / f| ~ 2.7×10⁻⁶ m^-1). The in-situ current observation from EC1 subsurface mooring further confirms rapid decay (e.g., an IEB of 5 days) under strong vorticity gradients (|∇ζ / f| > 1.3×10⁻⁶ m⁻¹), where the decay timescale of NIOs may vary within a month. Our results show that NIOs decay faster south of the subpolar front and, during autumn, tend to decay faster in years with stronger vorticity gradients. This demonstrates that mesoscale vorticity structure plays a key role in regulating the spatial and temporal variability of NIO decay in the East Sea.

Near-inertial oscillations,Decay timescale,East Sea (Japan/East Sea),Mesoscale eddies