The East China Sea (ECS), typically unfavorable for typhoon development due to cold water temperatures, experienced an unusual event in late August 2020 when Typhoon Bavi intensified to a Category 3 typhoon, becoming one of the strongest typhoons ever recorded in this shallow-water region. This intensification, a 25-knot increase in approximately 1.5 days, is exceptionally rare. Over 30 years (1991–2020), only 6.7% of western North Pacific typhoons reached the ECS, with most experiencing decay. The ECS's environment hinders typhoon development due to factors such as strong vertical wind shears, sharp ocean temperature gradients, and cold bottom water temperatures (as low as 12°C in winter). During summer, strong stratification leads to warm surface water and cold bottom water, facilitating typhoon-induced SST cooling—a significant negative feedback that reduces energy transfer from ocean to typhoon. Previous research suggests shallow bathymetry near coasts may facilitate landfalling typhoon intensification by suppressing vertical mixing and SST cooling. However, strong stratification and cold bottom water can still lead to SST cooling, even in shallow waters. This study investigates the role of an extreme ocean warming event, a marine heatwave, in altering this unfavorable condition and facilitating Typhoon Bavi's intensification.

Existing literature highlights the detrimental effects of the ECS environment on typhoon intensification. Strong vertical wind shear, sharp ocean temperature gradients, and cold bottom waters contribute to significant typhoon-induced sea surface temperature (SST) cooling, a crucial negative feedback mechanism. Studies have shown that shallow bathymetry near continental coasts can influence typhoon intensification, often by limiting the depth of typhoon-induced mixing and thereby reducing SST cooling. However, the interplay between shallow water depth, ocean stratification, and SST cooling remains complex and depends on the pre-existing temperature structure of the water column. While prior research acknowledges these factors, the influence of marine heatwaves in such contexts, particularly their interaction with typhoon intensification, remains less explored. The research presented here seeks to address this gap by examining Typhoon Bavi's atypical behavior.

The study employed a combination of in situ observations from the leodo Ocean Research Station (IORS) and numerical simulations using the Price-Weller-Pinkel (PWP) ocean mixing model. The IORS, situated at the ECS-Yellow Sea boundary, provided crucial air-sea interaction data before, during, and after Typhoon Bavi's passage. The PWP model, a one-dimensional model driven by surface forcing and widely used to study ocean response to typhoons, was used to simulate SST cooling under various scenarios. The researchers defined marine heatwaves following the procedure outlined in Hobday et al. (2016), using SST data exceeding the 90th percentile of daily SST values over the past 30 years for at least 5 consecutive days. Four PWP experiments were conducted: a control experiment (MHW) using pre-typhoon observed conditions; a No_MHW experiment excluding the marine heatwave; a No_SALT experiment removing the salinity stratification; and a DEEP experiment eliminating the bottom depth constraint. Air-sea heat fluxes (sensible and latent) were calculated using bulk aerodynamic formulas. The study used SST data from the optimally interpolated SST product (OISST), wind vector data from the Cross-Calibrated Multi-Platform (CCMP) Version 2.1 product, and other relevant datasets from NOAA and other sources.

Satellite observations revealed that the ECS experienced a marine heatwave reaching a maximum intensity of about 1.5 °C (above the 90th percentile level) just before Typhoon Bavi's arrival. The SST in the ECS was unusually high, reaching 31°C, significantly exceeding the long-term August mean of 28-29°C. The IORS measurements showed a maximum SST cooling of 9°C during Typhoon Bavi's passage, but the water column quickly mixed to a temperature around 23.7°C. PWP model simulations demonstrated that the marine heatwave significantly reduced SST cooling (by 1.9°C) compared to the No_MHW experiment. Without the marine heatwave, the during-typhoon SST would be much colder (by 1.5°C), leading to a negative total heat flux (-259 W m⁻²) implying energy transfer from the atmosphere to the ocean. The MHW experiment showed a positive total heat flux (+76 W m⁻²) during the typhoon's passage, indicating continued energy transfer from the ocean. The shallow water depth and salinity stratification played supplementary roles in influencing SST cooling and heat fluxes. Without the shallow water depth limitation, SST cooling was stronger (8.2°C vs 7°C), and negative heat fluxes were more pronounced during the typhoon's rear passage, potentially affecting its decay rate. The removal of salinity stratification also led to faster and more pronounced SST cooling, causing a negative total heat flux (-173 W m⁻²) under the typhoon's core area. Analyses of total heat flux at the IORS showed a continuous positive flux before the typhoon's center passed, followed by a rapid shift to negative flux once the center passed. The marine heatwave's contribution was crucial; without it, the total heat fluxes would have remained negative throughout the typhoon's passage. The study also suggests a possible feedback mechanism where the typhoon itself, through fair weather ahead of its convective zones, might contribute to the marine heatwave's development.

The findings directly address the research question regarding the factors contributing to Typhoon Bavi's exceptional intensity. The marine heatwave emerges as the primary driver, providing a substantial energy source that allowed the typhoon to maintain its Category-3 strength in a region typically unfavorable for such intensification. The significant role of marine heatwaves in modulating typhoon behavior challenges conventional understandings of typhoon intensification and highlights the complex interactions between atmospheric and oceanic processes. The shallow water depth and salinity stratification played complementary roles, primarily by affecting SST cooling and subsequently influencing the heat flux, particularly during the typhoon's later stages. The observed relationship between the typhoon and preceding marine heatwave suggests a potential positive feedback mechanism. The results emphasize the growing need for detailed, high-resolution monitoring of both surface and subsurface ocean conditions to improve typhoon forecasting and prepare for the impacts of these events under a changing climate. The interplay between these factors necessitates further research using fully coupled ocean-atmosphere models to better understand the nuances of this intricate relationship.

This study demonstrated the crucial role of marine heatwaves in the exceptional intensification of Typhoon Bavi in the East China Sea. The marine heatwave acted as the primary energy source, while shallow water depth and salinity stratification played secondary but notable roles in modulating SST cooling and heat fluxes. A positive feedback loop between the typhoon and the development of the marine heatwave was suggested. The results underscore the increased risks associated with landfalling typhoons under climate change and highlight the critical need for enhanced monitoring and modeling capabilities to better understand and predict these events.

The study primarily focused on the upper ocean's thermal structure, limiting a full understanding of the marine heatwave's impact on the entire water column. While the IORS provided valuable in situ data, a more detailed vertical temperature profile is needed for a comprehensive analysis. The PWP model, a one-dimensional model, simplified the complex coastal ocean dynamics. Processes like cold water advection, not fully represented in the model, may influence SST cooling. Further studies using fully coupled typhoon-ocean models are recommended to improve the understanding of these interactions.