Marine heatwave as a supercharger for the strongest typhoon in the East China Sea

The study investigates why Typhoon Bavi (August 2020) unusually intensified to Category 3 over the East China Sea (ECS), a shallow marginal sea typically hostile to typhoon development due to strong vertical wind shear and sharp ocean temperature gradients. Historically, most typhoons weaken over the ECS, and only about 1.3% of Western North Pacific typhoons achieve lifetime maximum intensity within the ECS. The research examines the hypothesis that an exceptionally warm ocean due to a marine heatwave, combined with shallow bathymetry and strong salinity stratification, suppressed typhoon-induced SST cooling and enhanced ocean-to-atmosphere heat flux, enabling Bavi to intensify and maintain major-typhoon strength in this region. The study’s purpose is to quantify the roles of marine heatwave warming, shallow water depth, and salinity stratification in modulating SST cooling and enthalpy fluxes under Typhoon Bavi, using in situ air–sea measurements and one-dimensional mixed-layer modeling. This has broader importance for understanding typhoon–ocean interactions and risks under climate change.

Background literature establishes that typhoon-induced SST cooling typically limits intensification by reducing air–sea enthalpy flux; cooling can reach up to 10 °C in the ECS due to strong stratification and cold bottom water. The vertical mixing depth under typhoons depends on stratification and storm properties; in shallow waters, bathymetry caps mixing depth, potentially suppressing cooling and occasionally facilitating intensification of landfalling storms. However, strong pre-existing stratification and cold bottom water on continental shelves can still lead to profound cooling despite depth limitations. Recent work highlights marine heatwaves as extreme ocean warming events that can increase coastal ocean heat content and potentially enhance subsequent tropical cyclone intensity. Prior studies also note barrier-layer effects from freshwater input (e.g., Yangtze River) that inhibit vertical mixing, and show the importance of absolute SST (not just cooling magnitude) for determining heat flux direction and intensity. This study builds on and integrates these strands, examining a rapid marine heatwave in the ECS preceding Bavi and its compound interaction with shallow bathymetry and salinity stratification.

Data and observations: The study uses in situ air–sea observations from the Ieodo Ocean Research Station (IORS) located at the ECS–Yellow Sea boundary in ~41 m water depth. Measurements include 10-minute wind speed, sea-level pressure, near-surface air temperature and relative humidity, and ocean temperature and salinity at 3 m, 20.5 m, and 38 m. Satellite datasets include daily 0.25° SST (OISST), sea surface salinity (SMAP), and wind vectors (CCMPv2), outgoing longwave radiation (NOAA CDR), and bathymetry (ETOPO2). JTWC best track data document storm intensity and track. Marine heatwave definition: Following Hobday et al. (2016), a marine heatwave is defined where local daily SST exceeds the 90th percentile of daily SST at that location over the baseline (1998–2021) for at least 5 consecutive days. Microwave SST was used; results are consistent with NOAA OISST. Modeling approach: A suite of one-dimensional Price–Weller–Pinkel (PWP) ocean mixing model experiments was conducted to isolate effects of (1) marine heatwave warming (MHW, control), (2) absence of marine heatwave (No_MHW), (3) no salinity stratification (No_SALT), and (4) no bottom depth constraint (DEEP). Simulations span 1600 UTC 24 Aug to 0400 UTC 27 Aug 2020 with 1 m vertical resolution and 15 min timestep, driven by IORS wind forcing. PWP resolves static, bulk, and shear instabilities using standard Richardson number criteria; wind stress uses a high-wind drag coefficient (Powell et al. 2003). Initial profiles: MHW uses averaged IORS T/S (1600 UTC 21–23 Aug). No_MHW replaces the temperature profile to match long-term August mean SST (27.4 °C) while leaving subsurface stratification as configured; No_SALT sets salinity homogeneous at 27.8 psu; DEEP imposes infinite depth with temperature extrapolated below 41 m from observed gradients and salinity set to the 38 m value. A systematic 2-hour time calibration (forward shift) is applied to PWP outputs to account for a small timing bias relative to observations. Air–sea heat fluxes: Sensible and latent heat fluxes are computed via bulk aerodynamic formulas using observed wind speed, near-surface air temperature, relative humidity, and SST from observations or PWP outputs. Exchange coefficients for sensible and latent heat are 1.3×10⁻³; air density is 1.2 kg m⁻³; standard constants for air heat capacity and latent heat of vaporization are used. Total heat flux is the sum of sensible and latent fluxes. Heat flux evolutions are analyzed relative to storm approach, impact at IORS (time 0), and recession. Event characterization: Satellite SST analyses quantify the rapid ECS warming from early to late August 2020, the spatial extent of the marine heatwave (Aug 15–24), and anomalies up to ~+3 °C in Bavi’s lifetime maximum intensity region. OLR and surface wind fields before Bavi document suppressed convection, clear-sky conditions, and weak winds (<5 m s⁻¹), consistent with enhanced solar heating and reduced turbulent heat loss ahead of the storm.

- A rapid, basin-wide marine heatwave developed in the ECS from Aug 15–24, 2020, with SST anomalies up to ~+3 °C relative to the August climatology and absolute SSTs reaching 31 °C by Aug 23. SST rose ~4 °C between Aug 9–23 in the interior ECS. - IORS observations during Bavi’s closest approach (peak intensity 100 kt) recorded 45 m s⁻¹ winds, minimum SLP 966 hPa, and maximum SST cooling of ~9 °C at 3 m (from 30.5 to 21.5 °C). The water column at 3, 20.5, and 38 m became well mixed shortly after impact, with an immediately mixed temperature near 23.7 °C (observed), stabilizing around ~23.5 °C. Inertial oscillations (~22.6 h period) likely modulated post-passage temperature fluctuations. - PWP simulations matched observed SST evolution with a ~2 h timing bias; final well-mixed temperature was 23.2 °C (MHW experiment). Calibrated simulations reproduced observed cooling rates and flux transitions. - Marine heatwave effect (MHW vs No_MHW): Despite larger cooling in MHW, absolute SST under the storm was warmer with MHW by about 1.5 °C at impact and 0.9 °C after mixing. Crucially, total heat flux at impact remained positive with MHW (+76 W m⁻²), indicating continued ocean-to-storm energy transfer, whereas without MHW it would be strongly negative (−259 W m⁻²) and near zero even before strong mixing, precluding maintenance of major intensity. No_MHW produced smaller total cooling (5.1 °C) but much colder during-typhoon SST due to lower initial SST. - Shallow water depth effect (DEEP): Removing the 41 m depth constraint increased final cooling to 8.2 °C (+1.2 °C; +17% relative to actual depth) and drove SST to 22.1 °C after passage. Negative total heat flux in the rear half of the storm became more negative (−169 W m⁻² vs −107 W m⁻²; ~58% stronger energy loss), implying faster decay post-IORS and reduced landfall intensity without depth limitation. Depth effects became evident about 3 hours after impact. - Salinity stratification effect (No_SALT): Removing salinity-induced barrier layer increased the cooling rate by ~1.5× (0.57 to 0.86 °C h⁻¹) and enhanced during-typhoon cooling by ~1.0 °C (+19%). During-typhoon total heat flux became negative (−173 W m⁻²) without salinity stratification, indicating energy loss to the ocean; effects vanished after full mixing (~3 h post-impact), with unchanged terminal mixed temperature. - Spatial flux asymmetry: Observed total heat flux at IORS was positive and increasing on the front half (+246 to +471 W m⁻²) and turned negative immediately after center passage (−89 to −335 W m⁻²), consistent with Bavi’s subsequent weakening. Simulations show that without MHW, fluxes would be negative throughout the passage; shallow depth limits rear-half negative flux magnitude; salinity stratification boosts positive flux near the core. - Self-preconditioning: Clear-sky, subsidence-dominated conditions and weak winds ahead of Bavi likely facilitated rapid ECS warming, suggesting the marine heatwave development was partly attributable to the approaching typhoon’s environmental influence.

The findings demonstrate that an intense coastal marine heatwave can overturn the typically unfavorable ECS environment for typhoon intensification by keeping absolute SST high enough to sustain positive ocean-to-atmosphere enthalpy flux at the time of maximum impact. While SST cooling magnitude is a key negative feedback, it is the absolute SST under the storm that governs flux direction and intensity. In Bavi, marine heatwave warming, combined with shallow bathymetry and salinity-induced stratification that limited vertical mixing and cooling, enabled maintenance of Category 3 intensity over the ECS and delayed decay. The positive fluxes observed on the storm’s front half and the rapid switch to negative flux after passage align with the documented intensity evolution. The study also highlights a feedback whereby fair-weather subsidence and reduced winds in the storm’s outer environment can rapidly precondition coastal shelves, contributing to marine heatwave onset and providing a heat reservoir for the incoming cyclone. These results underscore the importance of monitoring and modeling compound ocean processes (heatwave warming, stratification, bathymetric constraints) in predicting typhoon intensity changes over continental shelves under a warming climate.

This work shows that Typhoon Bavi’s unusual major intensity over the East China Sea was primarily enabled by a rapid marine heatwave that elevated absolute SST and sustained positive enthalpy fluxes at impact, with additional contributions from shallow water depth and salinity stratification that suppressed mixing-driven cooling. In the absence of the marine heatwave, fluxes would have been negative throughout, making such intensification unlikely. The analysis provides a mechanistic framework for typhoon–marine heatwave interactions on continental shelves and indicates that typhoons may partly precondition their ocean environment through subsidence-induced clear-sky warming ahead of landfall. Future research should: deploy higher-resolution subsurface observations to resolve vertical thermal structures during marine heatwaves; investigate full water column heatwave signatures; incorporate tides, waves, and lateral advection; and use fully coupled atmosphere–ocean models to quantify the coupled feedbacks and their impact on landfall intensity and decay rates.

- Modeling simplification: The one-dimensional PWP framework focuses on wind-driven vertical mixing and does not resolve lateral advection, upwelling, tides, wave-current interactions, or mesoscale processes that can influence SST and post-mixing cooling. A small timing bias (~2 h) indicates missing background flows or horizontal advection. - Observational coverage: IORS provided three-depth temperature/salinity in a 41 m water column; subsurface structure below and spatial variability across the ECS were not fully resolved. It remains uncertain how deep marine heatwave anomalies extended; surface-confined warming would be rapidly diluted by mixing. - Flux estimation: Bulk formulas with fixed exchange coefficients under high winds introduce uncertainty; air–sea flux parameterizations at extreme winds remain a source of error. - Event specificity: Findings are based on a single event in a specific shelf setting; generalizability to other basins and storm characteristics requires additional cases and coupled modeling studies. - Coastal complexity: The interplay of river runoff, barrier layers, and bathymetry is site-specific; detailed representation of freshwater inputs and stratification evolution over time is limited.