Asian-Australian monsoon variations at orbital scales – insights from model simulations

<p>The Asian-Australian monsoon system is mainly composed of East Asian monsoon, Australian monsoon and South Asian monsoon. The Australian monsoon and East Asian monsoon can form the East Asian-Australian monsoon system through the cross-equatorial flow over Maritime Continent and the atmospheric lateral coupling. On the other hand, the East Asian monsoon and the Australian monsoon can be relatively independent regional climate systems due to the difference of land-sea configuration and the influence of tropical ocean. Therefore, under different forcings, the evolutionary relationship between the two may be different on different time scales.</p> <p>In this study, we investigated the evolutionary relationship between East Asian winter monsoon (EAWM) and Australian summer monsoon (AUSM) at orbital scales, utilizing a set of transient simulations over past 300,000 years conducted with NCAR-CCSM3. It is found that, the variations of EAWM and AUSM are significantly in-phase at obliquity scale. In the case of high obliquity, when the northern Hemisphere (NH) is winter/the southern Hemisphere (SH) is summer, the NH receives less insolation while the SH receives more, leading to both intensified EAWM and AUSM. Meanwhile, the cross-equatorial flow is also enhanced due to the strengthened hemispheric insolation gradient, which "consolidates" the positive relationship between EAWM and AUSM. Whereas at precession scale, EAWM and AUSM show out-of-phase relationship. Taking low precession for instance, when the summer solstice of the NH is located at perihelion, the insolation over NH and SH both decrease in boreal winter/southern summer, leading to strengthened EAWM. AUSM, however, is weakened due to the reduced insolation. This may be because the AUSM is mainly controlled by local insolation. That is, the reduction of local insolation directly leads to the weakening of the AUSM, and “cut off” the link between EAWM and AUSM.</p> <p>The above results are based on the monsoon index defined by the wind field. We also notice that the East Asian summer monsoon precipitation and the Australian summer monsoon precipitation show significant out-of-phase relationship at precession scale, while in-phase relationship at obliquity scale, which could be well explained by the seasonal variation of insolation caused by the change of precession and obliquity.</p> <p>The different relationship between EAWM and AUSM at precession and obliquity scales is robust in pure orbital run and three forcings run, which is forced by orbital parameters, greenhouse gases and icesheet.</p>