Variability in the western North Pacific summer monsoon in 140-year-long AGCM hindcast experiments: SST impact on the cyclonic anomaly around 1890s–1930s

In this study, we examine the long-term variability in the western North Pacific (Philippine Sea) summer monsoon (WNPSM) from the 1880s to the present. To determine the sea surface temperature (SST)-induced atmospheric variability, atmospheric general circulation model (AGCM) ensemble simulations are conducted with the observed SST. For the AGCM experiments, we use two convection schemes and SST datasets to examine the uncertainty in the convective parameterization and robustness of the simulated atmospheric response. In the AGCM simulation (T42; approximately 300-km horizontal resolution, 3-member ensemble in each), interdecadal variability can be seen as a cyclonic anomaly in the 1890s–1930s. This cyclonic anomaly is robust to different convection schemes and SST datasets. The observed rainfall over the land demonstrates consistent features. In our AGCM ensemble simulations, sensitivity experiments with several SST patterns show that the interbasin effect (i.e., contribution of tropical Indian Ocean and Atlantic) played a key role in the WNPSM cyclonic anomaly during the 1890s–1930s. Further, idealized SST experiments show that reduction in SST over the tropical Indian Ocean and Atlantic causes the WNPSM cyclonic anomaly. Additionally, using high-resolution AGCM experiments (T119; approximately 100-km horizontal resolution, 4-member ensemble in each), we investigate the increase in tropical cyclone (TC) activity over the western North Pacific during the 1890s–1930s. However, variability of the TC activity is sensitive to the convective parameterization.