Effects of Long-term Climate Change on Typhoon Rainfall Associated with Southwesterly Monsoon Flow near Taiwan: Mindulle (2004) and Morakot (2009)

Typhoons Morakot (2009) and Mindulle (2004) were two of the rainiest and most damaging typhoons to hit Taiwan on record, where both cases are associated with a strong low-level southwesterly monsoon flow. The moisture-rich southwesterly monsoon flow and the typhoon-induced northwesterly current usually converge on Taiwan’s Central Mountain Range to produce catastrophic rainfall. The two storms are simulated with a cloud-resolving model (CRM) using the pseudo-global-warming (PGW) methodology to assess the fraction of precipitation attributable to long-term climate change. For each storm, two scenarios are simulated and compared—the control run in present-day climate and the sensitivity test in a past environment four decades ago, where the climate-change signal (“deltas”) is computed using global reanalysis data as the difference between 1990–2009 and 1950–1969. Being realistically reproduced by the CRM at a 3-km grid size in the control run, both typhoons progress in the sensitivity test with highly similar evolution to their present-day counterpart, even though the background in the sensitivity run is slightly cooler and drier than the present. Under the current climate, Morakot and Mindulle produce more rainfall by about 5 mm per day within 300–400 km from the center during their lifespan (equal to an increase of ~4–8%) compared to their counterparts in past climates. Such results are in close agreement with previous studies, and the shift in mean daily rainfall is tested as statistically significant at a confidence level of 99.5%. The water budget analysis shows that the increased rainfall from past to present climate is accounted for mainly by the low-level convergence of moisture associated with a more vigorous secondary circulation and a higher precipitable water amount.