Marine heatwave events strengthen the intensity of tropical cyclones

The study investigates how and by which processes marine heatwaves (MHWs)—extreme ocean warming events lasting days to weeks—affect the short-term evolution and intensification of tropical cyclones (TCs). While prior work in a warming climate suggests fewer TCs but a higher proportion of intense storms and increased TC-related precipitation, most assessments emphasize annual statistics rather than day-to-day evolution. Case studies have linked exceptionally high sea surface temperatures and warm upper-ocean structures to TC intensification, but comprehensive, multi-storm analyses have been lacking. Motivated by documented MHW–TC interactions (e.g., Hurricane Michael 2018; TC Amphan 2020), the authors conduct a basin-scale statistical analysis of 312 TCs in the western North Pacific (WNP) and Atlantic (ATL) to quantify MHW impacts on TC intensification from five days before to two days after the lifetime maximum intensity (LMI), focusing on convection and air–sea interaction processes.

Prior studies using high-resolution climate models indicate that while overall TC frequency may decrease with warming, the incidence of intense (Category 3+) TCs increases, and precipitation rates rise due to higher atmospheric moisture. Case studies show that exceptionally warm SSTs and enhanced upper-ocean heat content can fuel rapid TC intensification, particularly when strong stratification reduces mixing and preserves warm subsurface layers. Observations have documented MHWs forming from atmospheric heatwaves and preceding TCs, with resulting warm stratified oceans supplying energy that intensified subsequent storms (e.g., Hurricane Michael 2018; TC Amphan 2020). Research on convective processes highlights the importance of convective bursts, vortical hot towers, and diabatic heating near the storm center for intensification. Air–sea interaction studies point to latent heat fluxes governed by surface wind speed and air–sea moisture disequilibrium as key drivers of intensification, with rapidly intensifying TCs typically exhibiting elevated latent heat fluxes.

Study domain and period: TCs over the western North Pacific (WNP) and Atlantic (ATL) during May–October (MJJASO), 1982–2019. TC position and intensity: JTWC Best Track (WNP) and HURDAT2 (ATL). Sea surface temperature (SST): NOAA OISST v2 (0.25°). Atmospheric variables (winds, specific humidity, latent heat flux): ECMWF ERA5 (0.25°). Precipitation: TRMM TMPA L3 V7 (0.25°, 3-hourly) for 1998–2019; composited to daily to remove diurnal cycle. Daily vertical ocean temperature: GLORYS (data referenced in availability statement). MHW detection: Following Hobday et al., MHWs are periods when daily SST exceeds the seasonally varying 90th percentile for at least 5 consecutive days. The 90th percentile climatology is based on 1982–2011, smoothed with an 11-day running window. MHW characteristics include duration and intensity (SST anomaly above climatology). TC sample definition and filters: From 312 TCs that intensified, the analysis focuses on the intensification window from Day −5 to Day +2 relative to each storm’s LMI. MHW TC: a TC whose center remained within 1° of an MHW area for ≥2 days before LMI. Non-MHW TC: a TC that never encountered an MHW during its lifetime. TCs encountering an MHW within 24 h before LMI or only after LMI were excluded. To minimize formation-location biases, only TCs forming in main development regions were considered: WNP (128°–180°E, 5°–20°N) and ATL (80°–15°W, 10°–20°N). LMI latitude was constrained to <30°N. Final sample sizes: 128 MHW TCs and 184 non-MHW TCs (WNP: 100 MHW, 137 non-MHW; ATL: 28 MHW, 47 non-MHW). Analysis framework: TC intensity evolution was composited from Day −5 to LMI and beyond, and Saffir–Simpson category proportions were compared between groups. Precipitation was composited within a 5° radius of the TC center. Surface latent heat flux (LHF) from ERA5 was analyzed spatially around the TC center and averaged temporally (6-hourly) to examine evolution, along with its key drivers: air–sea moisture disequilibrium qsat(Tsk) − q and 10 m wind speed U10. Statistical significance was assessed, with 95% confidence intervals shown where applicable.

- MHWs substantially strengthen TC intensity: The average LMI for MHW TCs is 106.72 kt, 27.92–28 kt higher than non-MHW TCs (78.80 kt), corresponding to about 35.4% stronger maximum intensity. - Accelerated intensification prior to LMI: From Day −3 onward, MHW TCs intensify faster, with rates about threefold higher than non-MHW TCs on Day −2 (12.5 vs. 4.5 kt day−1) and 17.32 kt day−1 on Day −1 (MHW). - Higher likelihood of major storms: The proportion of Category 4–5 storms is 53.9% for MHW TCs versus 22.8% for non-MHW TCs, more than doubling the chance of reaching super typhoon/hurricane status. - Basin-specific results: Similar strengthening occurs in both basins. At LMI, intensity differences MHW–non-MHW are ~27.3 kt in both WNP and ATL. MHW TCs’ Day −1 intensification rates are ~20.0 kt day−1 in WNP and ~7.9 kt day−1 in ATL. Super-typhoon/hurricane ratios for MHW vs non-MHW: WNP 58% vs 25.6%; ATL 39.3% vs 14.9%. - Precipitation near the TC center is enhanced: Within 5° of the center, MHW TCs exhibit ~1.5–2× greater mean precipitation than non-MHW TCs, with rapid increases from Day −4 to Day −2 (≈75% increase, 5.3 to 9.3 mm h−1). Heavy precipitation (>10 mm h−1) concentrates within 1–2° of the center from Day −2 to LMI only for MHW TCs. - Latent heat flux is elevated and reorganizes toward the eyewall: LHF for MHW TCs is ~1.8–3× higher throughout intensification. From Day −4 to Day −2, the spatially averaged LHF increases by ~82.97% (from −98.11 to −179.51 W m−2), later concentrating around the eyewall as storms intensify. - Mechanisms: Early (Day −5 to −3) LHF enhancement is dominated by larger air–sea moisture disequilibrium over extremely warm oceans with elevated upper-ocean heat content; later, stronger surface winds in intensifying MHW TCs further increase LHF, creating a positive feedback that accelerates intensification. - Atmospheric conditions: Large-scale vertical wind shear is not more favorable for MHW TCs (shear magnitudes are actually 1.4–4.7 m s−1 larger than in non-MHW cases), whereas thermodynamic conditions (moisture convergence, mid-level relative humidity) are more favorable over MHW regions, consistent with elevated SST and enhanced LHF.

The analyses demonstrate that MHWs provide a physically favorable environment for rapid TC intensification by boosting ocean-to-atmosphere latent heat flux and enhancing convection and precipitation near the storm center. Early-stage intensification is initiated primarily by strong moisture disequilibrium due to anomalously warm and stratified upper-ocean conditions during MHWs, which increase upper-ocean heat content and sustain elevated LHF. As storms strengthen, TC-induced winds amplify LHF further, reinforcing a positive feedback that accelerates intensification up to LMI. Despite somewhat higher vertical wind shear in MHW cases, enhanced thermodynamic conditions (e.g., moisture convergence, mid-level humidity) associated with warm SST anomalies enable more efficient convective organization (including potential aggregation of vortical hot towers), leading to stronger, precipitation-rich TCs. These findings address the core question by establishing the day-to-day physical pathway from MHW ocean states to TC intensification and highlight that as MHWs become more frequent and persistent in a warming climate, the risk of higher-intensity, precipitation-richer TCs increases in both the WNP and ATL basins.

The study provides multi-storm, basin-wide evidence that marine heatwaves intensify tropical cyclones by increasing latent heat flux from the ocean and promoting heavy, center-concentrated precipitation, thereby accelerating intensification prior to LMI. MHW TCs reach substantially higher maximum intensities and are more than twice as likely to become Category 4–5 storms compared to non-MHW TCs. Mechanistically, early-stage intensification is driven by enhanced air–sea moisture disequilibrium over anomalously warm, stratified upper-ocean layers, followed by wind-driven LHF amplification as storms strengthen. As MHWs are projected to become more frequent and longer-lasting under global warming, the presented MHW vs. non-MHW comparison framework offers a means to understand how short-term TC evolution mechanisms may shift. Future work could extend this framework to other ocean–atmosphere phenomena (e.g., tropical oceanic MCSs and offshore rainfall systems), incorporate additional basins and seasons, and integrate process-resolving ocean observations to refine causal attribution.

- Spatial and temporal scope: Analyses are limited to the WNP and ATL during May–October (1982–2019) and to storms with LMI latitudes <30°N, which may affect generalizability to other basins, seasons, or higher latitudes. - Sampling and filtering: Restricting genesis to main development regions and excluding TCs encountering MHWs close to or after LMI reduces potential biases but also limits sample diversity. - Data constraints: Precipitation analyses rely on TRMM TMPA (available 1998–2019). Key variables (LHF, winds, humidity) are from reanalysis (ERA5) and satellite products, which carry uncertainties. Daily precipitation compositing removes diurnal variability that may influence convective evolution. - Classification thresholds: The definition of MHW TCs (within 1° of MHW area for ≥2 days before LMI) and MHW detection (≥5 consecutive days above the 90th percentile) may influence case attribution and sensitivity of results. - Causality: The study is based on statistical composites and process-consistent interpretations; while mechanisms are physically plausible, definitive causal attribution at event scale may require targeted coupled observations and modeling.