Multi-timescale Variation And Attribution Of The Relationship Between Asian Sub-monsoon Systems Over The Past Millennium

Using the climate simulation outputs of CESM-LME(Community Earth System Model for Last Millennium Ensemble),the variations and relationships between three sub-monsoon systems of the Asian Summer Monsoon during the past millennium have been explored on three distinct time-scales,i.e.interannual,decadal and centennial timescales.The three sub-monsoon systems include East Asian Summer Monsoon(EASM),Indian Summer Monsoon(ISM)and West Northern Pacific Summer Monsoon(WNPSM)On the basis of the validation of simulation results,analysis of the spatial-temporal variations of Asian Summer Monsoon before and after the Industrial Revolution(1850 CE)has been underwent by using the method of Multivariate Empirical Orthogonal Function(MV-EOF),and further attribution has been carried out by comparing the correlation coefficients of the PC1 between different single forcing experiments to show the main forcings of each sub-monsoon system's variations and the relationships between them.First of all,the climatological average fields of the summer precipitation,sea level pressure(SLP)as well as winds at 850 and 200 h Pa from 1958 to2000 have been calculated both by using the CESM-LME and ERA-40,the reanalysis data,to validate the reliability of the simulation results,which turns out to show that the spatial coefficients of three fields are 0.73,0.76 and 0.61 respectively,and all of them are significant at the 95% confidence level.It means that the model's capability of simulating the climate change in Asia Monsoon Area is acceptable.The first modes of MV-EOF of three sub-monsoon systems have showed little differences between the two periods before and after 1850 CE.with the spatial pattern being dipole pattern in East Asia as well as in West-northern Pacific Ocean,while the one being consistent in India.However,the time series tend to show distinguishable differences.Before the Industrial Revolution,all of the three sub-monsoons have a significant interannual cycle of 4 years or so,while EASM and WNPSM also have a quasi-150-year centennial cycle.The EASM and WNPSM tend to show a significant trend after the Industrial Revolution,while ISM has the opposite tendency,with no significant cycle existing in all of the sub-systems.Before the period,there is a significant positive relationship between EASM and ISM,with the coefficient being 0.48,a negative relationship between EASM and WNPSM,with the coefficient being-0.31,while the one between ISM and WNPSM is relatively weak.After the Industrial Revolution,the positive relationship between EASM and ISM remains,while others are disappeared.As for the attribution,it has been found that volcanic activities are the main external forcing affecting the variations of ISM and WNPSM before the period with relatively high correlation coefficients,while the forcing to ISM changes into solar radiation variations after the period.Among the three pairs of relationships,the one between EASM and ISM turns to be the highest,which diminishes after the period due to the volcanic variation.And the one between ISM and WNPSM is the lowest.Besides,there is a reversal in the coefficient of EASM and WNPSM,which is positive before 1850,and negative after.