Multiscale oceanic response to Typhoon Noru (2022) in the South China Sea: modulation by submesoscale processes

A comprehensive understanding of the upper-ocean response to typhoons is critical for improving typhoon intensity prediction. Although background mesoscale environments modulate the upper ocean, how submesoscale dynamics specifically drive the three-dimensional reshaping of the post-typhoon wake remains unclear. Our study employs a one-way nested model configuration (9 km parent grid and 3 km child grid) to examine the multiscale oceanic response to Typhoon Noru (2022) within the frontogenetic Vietnam Offshore Current region of the South China Sea. Mixed-layer heat budget analysis revealed that wind-driven vertical mixing primarily governed the forced-stage cooling magnitude, regardless of model resolution. However, the thermal evolution within the post-typhoon cold wake exhibited a pronounced contrast between the two resolutions. Spatial scale decomposition confirmed that the enhanced fine-scale thermal variability was primarily driven by submesoscale flows advecting background temperature gradients. Furthermore, the high-resolution simulation successfully resolved coherent ageostrophic secondary circulations along these submesoscale fronts. These localized overturning motions produced a persistent positive vertical buoyancy flux that slumped horizontal density fronts, thereby driving rapid restratification. These fine-scale vertical exchanges effectively transported warm surface water downward, governing the thermal restructuring within the post-typhoon cold wake. The findings highlight the importance of resolving submesoscale dynamics and provide a critical dynamical basis for improving sub-grid parameterizations of vertical transport in coupled prediction models.