Simulated Influence of Solar Spectral Irradiance on the East Asian Monsoon Rainband on the Decadal Scale and the Mechanism

The possible amplification of the small solar variation signal in the earth climate, in particular at certain regions, is of great scientific interest. This study simulated the atmospheric and oceanic characteristics of three sunspot cycles from 1974 to 2005 with the Whole Atmosphere Community Climate Model (WACCM) by using CMIP6 and CMIP5 recommended solar forcing respectively. The CMIP6 dateset has obvious difference with CMIP5 recommended data at the ultraviolet bands, which shows a larger solar-cycle variability contribution to TSI than in CMIP5 (50% compared to 35 %). The results show that the simulated East Asian rainband based on CMIP6 data is closer to the observation than that based on the CMIP5. It can reproduce the relationship between the quasi-10-year meridional oscillation of the rainband of the East Asian summer monsoon and the sunspot cycle. And after band-pass filtering the quasi-10-year oscillation of the simulated rainband becomes significant, and its phase lags slightly behind the sunspot cycle phase by 1-2 years. These characteristics are consistent with the observations. The comparison of atmospheric circulation fields shows that the characteristics of CMIP6 circulation fields are much closer to the actual observation than those of CMIP5. The main area of the monsoon in the high years of the sun is relatively large, and the abnormal subsidence in the south of the Yangtze River is not conducive to precipitation, which makes the rainband prone to northward, causing the meridional oscillation of the rainband to synchronize with (or lag slightly behind) the sunspot cycle. This indicates that the numerical simulation experiments based on more realistic SSI forcing data can better simulate the results close to observations. Due to the difference in ultraviolet radiation between the two SSI data, the comparison of the two simulations implies that the influence mechanism of solar ultraviolet radiation is likely important for the possible modulation of solar activity on decadal variations of the East Asian monsoon rainband. Further study shows that the low level wind, in particular the meridional wind at the low solar year is essential in reproducing the observed rainband. And the difference of low level circulation based on CMIP6 and CMIP5 recommended solar forcing can be traced to the difference of simulated ozone distribution and the thermal dynamic difference in the stratosphere.