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

Abstract Typhoons Morakot (2009) and Mindulle (2004) were two of the most-rainy and damaging typhoons to hit Taiwan on record, where both cases are associated with strong low-level southwesterly monsoon flows. To quantitatively assess the fraction of rainfall that is attributable to long-term climate change, the two storms are simulated with a cloud-resolving model (CRM) using the pseudo-global-warming (PGW) methodology. For each storm, two scenarios are simulated and compared—the control run in present-day climate and the sensitivity test in past climate four decades ago, where the climate-change signal (“deltas”) is computed using global reanalysis data as the difference between 1950–1969 and 1990–2009. Being realistically reproduced by the CRM at 3-km grid size in the control run, both typhoons progress in the sensitivity test with highly similar evolution with their present-day counterpart, where background of the sensitivity run is slightly cooler and drier. Under modern climate, both 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 climate. 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 in the control run is accounted for mainly by the low-level convergence of moisture associated with both a stronger secondary circulation and a higher precipitable water amount.