Marine Heatwave-Induced Stratification Promoted Subsurface Heat Storage and Supported the Rapid Intensification of Tropical Cyclone Winston

Technical Summary: Marine Heatwave-Induced Stratification and the Rapid Intensification of Tropical Cyclone Winston

Problem Statement
While marine heatwaves (MHWs) are increasingly recognized as modulators of tropical cyclone (TC) intensity, the specific mechanisms linking upper-ocean preconditioning to rapid intensification (RI) remain incompletely understood. Previous research has established that MHWs can enhance TC intensity by creating warm ocean conditions, often citing elevated sea-surface temperatures (SST) and increased ocean-to-atmosphere energy transfer. However, the role of the vertical distribution of heat, specifically how stratification influences the storage and subsequent accessibility of subsurface heat reservoirs during storm passage, requires further investigation. This study addresses the gap in understanding the coupled ocean–atmosphere processes that facilitated the rapid intensification of Tropical Cyclone (TC) Winston (2016), a historic Category 5 event in the Southwest Pacific, which occurred during an extreme MHW.

Methodology
The study employs a multi-source diagnostic approach combining observational data, reanalysis products, and high-resolution ocean modeling to reconstruct the oceanic state before, during, and after TC Winston.

• Data Sources: The analysis utilizes daily Optimum Interpolation Sea Surface Temperature (OISST) for MHW detection; ERA5 atmospheric reanalysis for surface heat fluxes and wind fields (corrected for TC core winds using a parametric–reanalysis method); and the GLORYS2V4 global ocean reanalysis for subsurface temperature and salinity.
• Modeling: To resolve sub-daily upper-ocean variability (diurnal warming, vertical mixing, and entrainment) not captured by daily reanalysis, the Coastal and Regional Ocean Community (CROCO) model was initialized with GLORYS2V4 profiles and forced with hourly ERA5 atmospheric fields.
• Analytical Framework:
  • MHW Detection: Defined using the Hobday et al. (2016) framework, identifying periods where SST exceeded the 90th percentile for at least five consecutive days.
  • Heat Budgets: A Mixed-Layer Heat Budget (MLHB) was calculated to partition temperature tendencies into surface fluxes, horizontal advection, and residual mixing terms.
  • Stratification: Brunt–Väisälä frequency ($N^2$) was computed to quantify vertical stability.
  • Heat Reservoirs: Ocean Heat Content (OHC) anomalies were calculated for the mixed layer ($OHC'_{MLD}$), the upper 100m ($OHC'_{100}$), and down to the 26°C isotherm ($OHC'_{D26}$ or TCHP).
  • Persistence Metrics: An equivalent cooling timescale ($t_{eq}$) was derived to assess the resilience of the heat reservoir against turbulent surface cooling.
  • Track-Based Diagnostics: Along-track analyses of thermal structure, heat-content depletion, and coupled ocean–atmosphere energy exchange (kinetic energy, turbulent fluxes, vertical wind shear) were performed during the RI phases.

Key Results

1. MHW Preconditioning and Stratification: During the 2016 MHW (late January–mid-February), sustained radiative forcing (dominated by enhanced shortwave radiation) and weak surface winds (<5 m/s) generated a shallow mixed layer (~13–17 m) and significantly enhanced upper-ocean stratification. The Brunt–Väisälä frequency in the upper 50 m increased by approximately 68% compared to pre-MHW conditions.
2. Subsurface Heat Storage: Mixed-layer heat budget analysis revealed that while surface heat input was substantial, the observed temperature tendency within the mixed layer was small. Instead, a significant fraction of the heat was redistributed beneath the mixed layer. Consequently, anomalous heat storage extended into the upper thermocline (30–60 m). Heat content integrated to the 26°C isotherm ($OHC'_{D26}$) reached ~170 MJ m⁻², substantially exceeding anomalies confined to the mixed layer. The equivalent cooling timescale increased from <2 days to 8–10 days, indicating a persistent, vertically structured heat reservoir.
3. Storm-Induced Access and Depletion: During TC Winston's rapid intensification, storm-induced mixing accessed this stored heat. Contrary to the expectation that cooling is surface-confined, the strongest heat-content reductions occurred in subsurface reservoirs (300–600 m depth) rather than within the mixed layer itself. The mixed layer actually deepened slightly, but the most significant thermal changes and fractional depletion were observed in the deeper reservoirs ($H_{300}$, $H_{600}$) and TCHP, which approached near-complete depletion by the third RI phase.
4. Coupled Energy Exchange: Rapid intensification coincided with the simultaneous occurrence of elevated subsurface heat availability, weak vertical wind shear (VWS), and enhanced ocean-to-atmosphere turbulent heat fluxes. Despite substantial ocean heat loss, elevated turbulent fluxes were maintained, consistent with the entrainment of anomalously warm subsurface waters that mitigated the typical cooling associated with deep mixing. The intensification was characterized by episodic bursts of kinetic energy growth driven by this coupled response.

Key Contributions

• Mechanistic Insight: The study demonstrates that the contribution of MHWs to TC intensification extends beyond elevated SSTs. It highlights the critical role of MHW-induced stratification in creating a vertically structured heat reservoir that stores energy beneath the mixed layer.
• Subsurface Dynamics: It provides evidence that the energetically important oceanic response to a TC is concentrated beneath the surface layer. The mobilization of subsurface heat, rather than just surface heat, was the primary driver sustaining air–sea energy transfer during RI.
• Diagnostic Framework: The paper establishes a coupled ocean–atmosphere diagnostic framework suitable for data-limited regions like the Southwest Pacific, utilizing reanalysis and high-resolution modeling to infer subsurface processes where in-situ observations are sparse.

Significance and Claims
The paper claims that the rapid intensification of TC Winston was supported by a specific oceanic preconditioning state: a shallow, strongly stratified mixed layer that trapped heat and facilitated its accumulation in the upper thermocline. The study concludes that MHWs influence TC intensity not merely by increasing the magnitude of upper-ocean heat storage, but by modifying the vertical distribution and accessibility of that heat. The findings suggest that reliance on SST alone may underestimate the potential for TC intensification in MHW environments, as the critical energy reservoir often resides below the surface. This work underscores the necessity of monitoring upper-ocean thermal structure and stratification to better understand and forecast compound MHW–TC extremes in a warming climate, particularly in the data-sparse Southwest Pacific.