Cause of the Recent Tendency of Tropical Cyclones Approaching Coasts as Revealed by HighResMIP-PRIMAVERA Simulations

Abstract Tropical cyclones developing over the ocean threaten society when they approach land. A recent study has shown that the global mean distance of tropical cyclones at their lifetime maximum intensity (LMI) to the land has decreased over the last 40 years. However, whether this trend is due to global warming or natural variability still needs to be determined. This study aims to identify the primary driver of the observed trend. We analyzed both atmosphere-only and coupled simulations following the CMIP6 High Resolution Model Intercomparison Project (HighResMIP) protocol with global models from the PRIMAVERA project. The former can depict the responses to anthropogenic forcing and natural climate variability constrained by sea surface temperature (SST), while the latter can reproduce only the forced response. The observed LMI distance shows statistically significant trends in the Pacific basin. For this basin, model results indicate that the atmosphere-only simulations reproduced the observed trend of the LMI distance, but the atmosphere–ocean coupled simulations did not, suggesting that the observed trend is not due to global warming but to natural variability. The results of atmospheric-only simulations supported the previously proposed hypothesis based on the observational evidence that the low-frequency variability of SST and the LMI distance to the coasts are linked by the atmospheric steering wind anomaly associated with the Pacific decadal oscillation (PDO) or, equivalently, the interdecadal Pacific oscillation (IPO), suggesting that the recent decadal trend in the LMI distance is driven by natural low-frequency SST variability. These findings bear societal significance, as the recently observed trend may not persist with progressing global warming but depends on future changes in PDO/IPO. Significance Statement In recent years, a tendency has been observed for tropical cyclones to become closer to the land at their strongest stage. We want to understand whether this trend will continue in the future due to global warming or whether it may change due to climate variability, but it is difficult to elucidate from only observational data. Therefore, we analyzed multiple global simulation data. Through this analysis, we checked the reproducibility of the observed trend in the models and used statistical methods to explore the factors behind the trend. The results suggest that the trend is not primarily driven by global warming but rather by decadal natural variability in the Pacific sea surface temperatures. Therefore, the observed recent trend may not continue in the future.