Uncertainties in tropical cyclone landfall decay

Tropical cyclones are devastating natural disasters, and their impact on coastal and inland regions is increasing. Recent research by Li and Chakraborty (2020) and Song et al. (2021) indicated a significant slowdown of decay for landfalling hurricanes in the North Atlantic and western North Pacific, respectively, potentially linked to rising sea surface temperatures or dynamic changes. However, Chan et al. (2022) questioned the methodology and robustness of these findings, suggesting that decay depends more on landfalling track modes and the effective area of oceanic moisture supply. Phillipson and Toumi (2021) also found no significant decay slowdown. This study aims to systematically investigate the global climatology of landfall decay to clarify the current conflicting evidence.

Existing literature presents conflicting views on the trend of tropical cyclone decay after landfall. Li and Chakraborty (2020) found a significant slowdown in North Atlantic hurricane decay, attributing it to rising sea surface temperatures. Song et al. (2021) observed a similar slowdown in the western North Pacific, but suggested a dynamic rather than thermodynamic cause. Chan et al. (2022) criticized the methodology of previous studies, highlighting the importance of landfalling track modes and oceanic moisture supply. Phillipson and Toumi (2021) contradicted the findings of the slowdown, adding to the uncertainty.

This study uses best-track data from the US National Hurricane Center (HURDAT2), China Meteorological Administration (CMA), and Joint Typhoon Warning Center (JTWC) from the IBTRACS database. The study incorporates three study periods: the entire period of available data, the period starting in 1967 (as used in previous studies), and the period since the global satellite era (1980 onwards). Two landfall intensity thresholds are considered: ≥34 kt (gale force) and ≥64 kt (hurricane force). A decay timescale (τ) is calculated following existing methods, based on an exponential decay model applied to the first four continuous inland data points. The effective oceanic area of moisture supply (Ao) is calculated as the average oceanic area within a 200-km radius of the cyclone center. Landfalling track modes ("hard-strike," "cross-over," "lingering," "catwalk," "skirting") are categorized based on track characteristics. Analysis includes season-level and event-level decay timescales, using linear regression to assess trends. A 0.1° resolution land-sea mask is used to improve accuracy compared to previous studies.

The study reveals significant inconsistencies in tropical cyclone landfall decay trends across different basins, time periods, data sources, and methodologies. Trends vary among basins, with some showing increasing and others decreasing decay. Different data sources show differing trends even within the same basin, highlighting data inconsistencies. Trends also change significantly with different study periods, indicating potential decadal to multi-decadal variability. The choice of landfall intensity threshold strongly influences the results. The use of different land-sea mask resolutions also results in differing trends, particularly affecting studies that use coarser resolutions. Season-level and event-level analyses yielded different results. The positive correlation between the effective oceanic area of moisture supply (Ao) and decay timescale (τ) is globally significant, confirming the importance of Ao in determining landfall decay. This relationship is more pronounced for stronger landfalling cyclones (≥64 kt). Variations in subtropical high patterns are linked to changes in landfalling track modes and decay, particularly in the North Atlantic and western North Pacific. The western North Pacific shows a significant increase in decay times for weak "hard-strike" events in recent decades.

The study's findings demonstrate that claims of a universal, climate-driven slowdown in tropical cyclone decay are premature. The inconsistencies in trends across various factors highlight the complexities involved and the uncertainties inherent in existing data and methodologies. The significant relationship between Ao and τ supports the hypothesis that the effective area of oceanic moisture supply is a crucial determinant of landfall decay, rather than solely sea surface temperature. The observed variations in subtropical high patterns and their influence on landfalling track modes provide further insight into regional variations in decay trends.

This study comprehensively evaluates global tropical cyclone landfall decay, revealing significant uncertainties in previously reported slowdown trends. The findings underscore the need for more comprehensive analyses, larger datasets with consistent intensity estimates, and consideration of decadal variability. Future research should focus on improving data consistency, refining methodologies, and investigating the complex interplay of factors that influence landfall decay.

The study's conclusions are limited by the availability and consistency of best-track data, particularly for some basins with smaller sample sizes. The methodology used for determining landfall events and calculating decay timescales may also introduce uncertainties. The study focuses primarily on observed trends and does not directly address future projections under climate change.