Marine heatwaves (MHWs), periods of extreme ocean warming lasting days to weeks, are increasingly prevalent due to global warming. Their devastating impacts on marine ecosystems and fisheries are well-documented. Beyond these ecological and economic effects, MHWs, characterized by warm ocean thermal structures, interact with atmospheric phenomena such as the El Niño Southern Oscillation, atmospheric heatwaves, and tropical cyclones (TCs). The growing frequency and intensity of MHWs necessitate a comprehensive understanding of their influence on marine weather systems. Tropical cyclones (TCs), intense weather systems lasting days to weeks, cause significant societal and economic damage. While high-resolution climate models suggest that global warming may decrease the overall frequency of TCs, they also indicate an increase in the frequency of intense TCs (Category 3 or higher) and higher TC-related precipitation rates due to increased atmospheric moisture. However, previous studies primarily focused on annual averages rather than the short-term evolution of TCs. Although some studies have shown the impact of extremely high Sea Surface Temperature (SST) on TC intensity, these were largely limited to specific cases and a small number of TCs. The warm ocean thermal structure created by MHWs may provide favorable conditions for TCs. Increased shortwave radiation due to shallower mixed layer depth during MHWs enhances warming and increases upper ocean stratification, reducing vertical mixing and heat transport. This elevates upper ocean heat content and brings warm anomalies to the surface. Previous case studies have shown how MHWs, characterized by high SST and strong stratification, supplied thermal energy to intensify hurricanes. For example, the MHW in the northern Gulf of Mexico in 2018, fueled by an atmospheric heatwave and a preceding TC, contributed to the intensification of Hurricane Michael to Category 5. Similarly, the MHW in the Bay of Bengal in 2020 was linked to the intensification of TC Amphan. These case studies highlight the need for a comprehensive analysis examining the effect of MHWs on multiple TCs.

Existing literature demonstrates a clear link between increasing global temperatures and the frequency and intensity of marine heatwaves. Studies such as Frölicher & Laufkötter (2018) highlight the emerging risks associated with these events, emphasizing their devastating ecological and economic consequences. Research by Oliver et al. (2018) and Lee et al. (2023) provide evidence for the increasing duration and frequency of marine heatwaves, particularly in specific regions. The impact of MHWs on various atmospheric phenomena has also been investigated, including their interaction with the El Niño Southern Oscillation (Feng et al., 2013; Pearce & Feng, 2013; Oliver et al., 2017; Lee et al., 2020) and their role in intensifying tropical cyclones (Dzwonkowski et al., 2020; Rathore et al., 2022). However, a comprehensive, statistically robust analysis examining the effect of MHWs on the intensification of a large number of TCs across multiple ocean basins has been lacking until now.

This study analyzed 312 TCs that intensified in the western North Pacific (WNP) and Atlantic (ATL) Oceans from 1982 to 2019. The focus was on the intensity changes of TCs when an MHW occurred before the TC reached its maximum intensity. The day-to-day TC intensification was examined from 5 days before the lifetime maximum intensity (LMI) to 2 days after the LMI. To understand the intensification process, the study analyzed convection and air-sea interaction, focusing on precipitation and surface latent heat flux near the TC center. Data sources included the Joint Typhoon Warning Center (JTWC) Best Track data and HURDAT2 for TC information, NOAA Optimum Interpolation Sea Surface Temperature (OISST) version 2 for SST, and the European Centre for Medium-Range Weather Forecasts ERA5 reanalysis for atmospheric variables. TRMM (TMPA) Level 3 Version 7 data provided daily precipitation information. MHW events were identified based on Hobday et al. (2016), defining them as periods when daily SST exceeded a seasonally varying 90th percentile for at least five consecutive days. An MHW TC was defined as a TC whose center remained within 1 degree of an MHW area for at least two days before reaching its LMI. TCs never encountering MHWs were classified as non-MHW TCs. To avoid bias from TC formation location, geographical limitations were applied to the analysis focusing on main development regions. The latitude of the LMI was also limited to below 30°N. This resulted in a dataset of 128 MHW TCs and 184 non-MHW TCs. The analysis involved comparing the daily evolution of TC intensity and precipitation patterns, as well as examining the surface latent heat flux and its contributing factors (wind speed and moisture disequilibrium) for MHW and non-MHW TCs.

The study found significant differences in the lifetime maximum intensity (LMI) of TCs influenced by MHWs compared to those not influenced. The average LMI for MHW TCs (106.72 knots) was 28 knots higher than for non-MHW TCs (78.80 knots). The intensity difference was apparent three days before LMI, with MHW TCs intensifying more rapidly from this point onwards. The TC intensification rates for MHW TCs were approximately three times higher than those for non-MHW TCs. MHW TCs were over twice as likely to reach Category 4 or 5 compared to non-MHW TCs. Analysis of precipitation showed that MHW TCs exhibited significantly higher precipitation levels than non-MHW TCs, particularly within 5 degrees of the TC center. This increased precipitation was observed even before a significant difference in TC intensity emerged, indicating that the MHW influence precedes the increase in TC intensity. The increased precipitation was concentrated near the TC center, suggestive of vigorous convection fueled by the enhanced latent heat flux. The study also examined surface latent heat flux, finding that MHW TCs displayed a much larger latent heat flux than non-MHW TCs throughout the intensification period (1.8 to 3 times greater). The increased latent heat flux was linked to two factors: greater moisture disequilibrium between the ocean surface and the atmosphere, particularly at the early stages of intensification, and stronger TC-induced surface winds, playing a more significant role as the TC intensified. This positive feedback loop, where increased latent heat flux leads to more vigorous convection, heavier precipitation, and further intensification, is key to the observed enhancement in TC intensity during MHW events. The analysis also showed that while large-scale vertical wind shear was higher during MHW events, favorable atmospheric thermodynamic conditions (moisture convergence and mid-level relative humidity) supported the intensification process.

This research provides strong evidence that MHWs play a crucial role in strengthening TCs. The observed increased intensity and faster intensification rates in MHW TCs are directly linked to the significantly higher latent heat flux and precipitation near the TC center. The enhanced latent heat flux, stemming from both increased moisture disequilibrium and stronger surface winds, fuels vigorous convection and leads to heavy precipitation, creating a positive feedback loop that amplifies TC intensification. The findings highlight the importance of considering short-term, event-based interactions between MHWs and TCs, rather than relying solely on annual average TC activity. This is particularly relevant in light of climate change projections indicating increased frequency and duration of MHWs. The results suggest that future intensification of TCs may be more significantly influenced by MHWs than previously understood.

This study demonstrates a clear and robust link between MHW events and the intensification of TCs. The enhanced latent heat flux and increased precipitation associated with MHWs create a positive feedback mechanism, leading to more intense and rapidly strengthening TCs. The findings highlight the crucial need to integrate MHWs into models and projections of future TC activity. Future research should investigate the broader implications of this interaction for other marine weather systems and explore region-specific responses to MHW events. Further investigation is needed into the role of MHWs on TCs across diverse geographical locations and the complex interplay of oceanographic and atmospheric conditions.

The study's reliance on satellite data introduces potential limitations related to the accuracy and resolution of these measurements, especially for precipitation data where diurnal variations were removed. The spatial and temporal resolution of the datasets used might influence the detection of MHW-TC interactions. Furthermore, while the study controlled for some confounding factors, such as TC formation location, other unmeasured variables could potentially impact the relationship between MHWs and TC intensity. Finally, the study focuses on TC intensification; future research should expand to consider the full life cycle of TCs under MHW conditions.