Iam-Fei Pun

Current physical models often struggle to accurately capture the spatial structure of sea surface height (SSH) changes induced by tropical cyclones (TCs) because they do not adequately account for TC features. To address this limitation, a new multimodal fusion framework, namely FusionNet, is proposed to forecast the spatial structure of SSH changes induced by TCs. By integrating TC features and pre-storm ocean conditions whilst preserving structural information and spatial patterns, the proposed model effectively captures the amplitudes of TC-induced SSH changes across various TC intensity categories, accurately forecasting both troughs and sea level rise along TC tracks. Moreover, FusionNet consistently outperforms the baseline derived from the widely used ocean reanalysis fields in capturing and modelling the spatial variations, as evidenced by results from an independent testing set and a case study of Super Typhoon Mangkhut (2018). Ablation analysis further disclosed the respective contributions of TC features and pre-storm ocean conditions in modelling the physical processes underlying TC-induced SSH responses. Notably, incorporating TC features significantly enhances the model's ability to represent trough amplitudes, particularly near the storm center and across different TC intensity groups.

Devastating Japan in October 2019, Supertyphoon (STY) Hagibis was an important typhoon in the history of the Pacific. A striking feature of Hagibis was its explosive RI (rapid intensification). In 24 h, Hagibis intensified by 100 kt, making it one of the fastest-intensifying typhoons ever observed. After RI, Hagibis’s intensification stalled. Using the current typhoon intensity record holder, i.e., STY Haiyan (2013), as a benchmark, this work explores the intensity evolution differences of these 2 high-impact STYs.We found that the extremely high pre-storm sea surface temperature reaching 30.5∘C, deep/warm pre-storm ocean heat content reaching 160 kJ cm-2, fast forward storm motion of ~8 m s-1, small during-storm ocean cooling effect of ~ 0.5∘C, significant thunderstorm activity at its center, and rapid eyewall contraction were all important contributors to Hagibis’s impressive intensification. There was 36% more air-sea flux for Hagibis’s RI than for Haiyan’s.After its spectacular RI, Hagibis’s intensification stopped, despite favorable environments. Haiyan, by contrast, continued to intensify, reaching its record-breaking intensity of 170 kt. A key finding here is the multiple pathways that storm size affected the intensity evolution for both typhoons. After RI, Hagibis experienced a major size expansion, becoming the largest typhoon on record in the Pacific. This size enlargement, combined with a reduction in storm translational speed, induced stronger ocean cooling that reduced ocean flux and hindered intensification. The large storm size also contributed to slower eyewall replacement cycles (ERCs), which prolonged the negative impact of the ERC on intensification.

Marine heatwaves (MHWs) have drawn substantial scientific attention due to their profound biological and economic impacts, especially in the context of climate change. This study highlights the crucial role of tropical cyclones (TCs) in disrupting MHWs by altering ocean temperatures. As demonstrated by Tropical Cyclone Bavi, intense sea surface temperature (SST) cooling, reaching up to 7°C, immediately terminated a mature MHW and suppressed subsequent MHW development for up to 8 months. More generally, on the basis of post-TC MHW cases (reoccurring within 1 month) during 2014 to 2023, we found that MHW disruption duration is proportional to the magnitude of TC-induced SST cooling, ~5 to 7 days per degree of cooling. At a broader scale, a 20-year (2004 to 2023) analysis revealed 99% significant inverse relationships between seasonal MHW and TC activity. These findings suggest that TCs can act as natural regulators, effectively mitigating heat stress from MHWs and potentially serving as a lifeline for marine ecosystems.

Tropical Cyclones (TCs) are devastating natural disasters. Analyzing four decades of global TC data, here we find that among all global TC-active basins, the South China Sea (SCS) stands out as particularly difficult ocean for TCs to intensify, despite favorable atmosphere and ocean conditions. Over the SCS, TC intensification rate and its probability for a rapid intensification (intensification by ≥ 15.4 m s⁻¹ day⁻¹) are only 1/2 and 1/3, respectively, of those for the rest of the world ocean. Originating from complex interplays between astronomic tides and the SCS topography, gigantic ocean internal tides interact with TC-generated oceanic near-inertial waves and induce a strong ocean cooling effect, suppressing the TC intensification. Inclusion of this interaction between internal tides and TC in operational weather prediction systems is expected to improve forecast of TC intensity in the SCS and in other regions where strong internal tides are present.

Due to the cold water temperatures, the East China Sea (ECS) is usually unfavorable for typhoon development. Recently, in a rare event, Typhoon Bavi (2020) reached major typhoon status and became the strongest typhoon in the ECS in the past decade. Based on in situ observations and model simulations, we discover that this typhoon is fueled by a marine heatwave, which creates a very warm ocean condition with sea surface temperature (SST) exceeding 30 °C. Also, because of suppressed typhoon-induced SST cooling caused by the shallow water depth (41 m) and strong salinity stratification (river runoff) within the ECS, the SST beneath the typhoon remains relatively high and enhances the total heat flux for the typhoon. More interestingly, due to the fair weather ahead of the typhoon, we find that the rapid development of this marine heatwave is likely, in part, attributed to the typhoon itself. As the risks from typhoons and marine heatwaves are heightening under climate change, this study provides important insights into the interaction between typhoons and marine heatwaves.

The sea surface temperature (SST) beneath a tropical cyclone (TC) is of great importance to its dynamics; therefore, understanding and accurately estimating the magnitude of SST cooling is of vital importance. Existing studies have explored important influences on SST of TC translation speed, maximum surface winds, ocean thermal condition, and ocean stratification. But the influence of the TC wind radii (or collectively called the TC size) on SST has been largely overlooked. In this study, we assess the influence of wind radii uncertainty on SST cooling by a total of 15,983 numerical simulations for the western North Pacific during the 2014–2018 seasons. Results show a 6%–20% SST cooling error induced using wind radii from the Joint Typhoon Warning Center official forecast and a 35%–40% SST cooling error using wind radii from the operational runs of the Hurricane Weather Research and Forecasting (HWRF) model. Our results indicate that SST cooling is most sensitive to the radius of 64 kt winds (R64) due to its effects on the integrated kinetic energy of the TC and subsequent mixing of the ocean surface layer. It is also found that the correlation between SST cooling induced by the TC and its size is 0.49, which is the highest among all the parameters tested. This suggests that it is extremely important to get TC size correct in order to predict the SST cooling response, which then impacts TC evolution in numerical weather prediction models.

The sea surface temperature (SST) beneath a tropical cyclone (TC) is of great importance to its dynamics; therefore, understanding and accurately estimating the magnitude of SST cooling is of vital importance.  Existing studies have explored important influences on SST such as TC translation speed, maximum surface winds, ocean thermal condition and ocean stratification.  But the influence of the TC wind radii (or collectively called the TC size) on SST has been largely overlooked.  In this study we assess the influence of wind radii uncertainty on SST cooling by a total of 15,983 numerical simulations for the western North Pacific during the 2014-2018 seasons.  Results show a 6-20% SST cooling error induced using wind radii from the Joint Typhoon Warning Center official forecast and a 35-40% SST cooling error using wind radii from the operational runs of the Hurricane Weather Research and Forecasting (HWRF) model.  Our results indicate that SST cooling is most sensitive to the radius of 64 kt winds.  The correlation between SST cooling induced by the TC and its size is 0.49, which is highest among all the parameters tested.  This suggests that it is extremely important to get TC size correct in order to predict the SST cooling response, which then impacts TC evolution in numerical weather prediction models.

The 2014 Northeast Pacific hurricane season was highly active, with above-average intensity and frequency events, and a rare landfalling Hawaiian hurricane. We show that the anomalous northern extent of sea surface temperatures and anomalous vertical extent of upper ocean heat content above 26 °C throughout the Northeast and Central Pacific Ocean may have influenced three long-lived tropical cyclones in July and August. Using a variety of satellite-observed and -derived products, we assess genesis conditions, along-track intensity, and basin-wide anomalous upper ocean heat content during Hurricanes Genevieve, Iselle, and Julio. The anomalously northern surface position of the 26 °C isotherm beyond 30° N to the north and east of the Hawaiian Islands in 2014 created very high sea surface temperatures throughout much of the Central Pacific. Analysis of basin-wide mean conditions confirm higher-than-average storm activity during strong positive oceanic thermal anomalies. Positive anomalies of 15–50 kJ cm⁻² in the along-track upper ocean heat content for these three storms were observed during the intensification phase prior to peak intensity, advocating for greater understanding of the ocean thermal profile during tropical cyclone genesis and development.

On 23 August, 2017, Typhoon Hato rapidly intensified by 10 kt within 3 h just prior to landfall in the city of Macau along the South China coast. Hato’s surface winds in excess of 50 m s⁻¹ devastated the city, causing unprecedented damage and social impact. This study reveals that anomalously warm ocean conditions in the nearshore shallow water (depth < 30 m) likely played a key role in Hato’s fast intensification. In particular, cooling of the sea surface temperature (SST) generated by Hato at the critical landfall point was estimated to be only 0.1–0.5 °C. The results from both a simple ocean mixing scheme and full dynamical ocean model indicate that SST cooling was minimized in the shallow coastal waters due to a lack of cool water at depth. Given the nearly invariant SST in the coastal waters, we estimate a large amount of heat flux, i.e., 1.9k W m⁻², during the landfall period. Experiments indicate that in the absence of shallow bathymetry, and thus, if nominal cool water had been available for vertical mixing, the SST cooling would have been enhanced from 0.1 °C to 1.4 °C, and sea to air heat flux reduced by about a quarter. Numerical simulations with an atmospheric model suggest that the intensity of Hato was very sensitive to air-sea heat flux in the coastal region, indicating the critical importance of coastal ocean hydrography.

Supertyphoon Megi (2010) left behind two very contrasting SST cold-wake cooling patterns between the Philippine Sea (1.5°C) and the South China Sea (7°C). Based on various radii of radial winds, the authors found that the size of Megi doubles over the South China Sea when it curves northward. On average, the radius of maximum wind (RMW) increased from 18.8 km over the Philippine Sea to 43.1 km over the South China Sea; the radius of 64-kt (33 m s⁻¹) typhoon-force wind (R64) increased from 52.6 to 119.7 km; the radius of 50-kt (25.7 m s⁻¹) damaging-force wind (R50) increased from 91.8 to 210 km; and the radius of 34-kt (17.5 m s⁻¹) gale-force wind (R34) increased from 162.3 to 358.5 km. To investigate the typhoon size effect, the authors conduct a series of numerical experiments on Megi-induced SST cooling by keeping other factors unchanged, that is, typhoon translation speed and ocean subsurface thermal structure. The results show that if it were not for Megi’s size increase over the South China Sea, the during-Megi SST cooling magnitude would have been 52% less (reduced from 4° to 1.9°C), the right bias in cooling would have been 60% (or 30 km) less, and the width of the cooling would have been 61% (or 52 km) less, suggesting that typhoon size is as important as other well-known factors on SST cooling. Aside from the size effect, the authors also conduct a straight-track experiment and find that the curvature of Megi contributes up to 30% (or 1.2°C) of cooling over the South China Sea.