The mid-Pliocene warm period was the most recent geological period in Earth’s history that featured long-term warming. Both geological evidence and model results indicate that East Asian summer winds (EASWs) strengthened in monsoonal China, and that East Asian winter winds (EAWWs) weakened in northern monsoonal China during this period, as compared to the pre-industrial period. However, the corresponding mechanisms are still unclear. In this paper, the results of a set of numerical simulations are reported to analyze the effects of changed boundary conditions on the mid-Pliocene East Asian monsoon climate, based on PRISM3 (Pliocene Research Interpretation and Synoptic Mapping) palaeoenvironmental reconstruction. The model results showed that the combined changes of sea surface temperatures, atmospheric CO₂ concentration, and ice sheet extent were necessary to generate an overall warm climate on a large scale, and that these factors exerted the greatest effects on the strengthening of EASWs. The orographic change produced significant local warming and had the greatest effect on the weakening of EAWWs in northern monsoonal China in the mid-Pliocene. Thus, these two factors both had important but different effects on the monsoon change. In comparison, the effects of vegetational change on the strengthened EASWs and weakened EAWWs were relatively weak. The changed monsoon winds can be explained by a reorganization of the meridional temperature gradient and zonal thermal contrast. Moreover, the effect of orbital parameters cannot be ignored. Results showed that changes in orbital parameters could have markedly affected the EASWs and EAWWs, and caused significant short-term oscillations in the mid-Pliocene monsoon climate in East Asia.

The differences of boreal summer 850 hPa winds (units: m s^？1) between experiments and the contribution of the combined changes of SCIS, orographic change and vegetational change in the MP experiment compared to the PI experiment. In panels (a, e, f), only changes that are significant at the 95% confidence level for 850 hPa meridional winds (using the Student’s t-test) are shaded. Regions with an elevation above 1500 m are left blank. 

Citation: Zhang, R., D. Jiang, and Z. Zhang, 2015: Causes of mid-Pliocene strengthened summer and weakened winter monsoons over East Asia. Adv. Atmos. Sci., 32(7), doi: 10.1007/s00376-014-4183-3.

Download：http://159.226.119.58/aas/EN/abstract/abstract2647.shtml