Mid-Pliocene global land monsoon from PlioMIP1 simulations

The mid-Piacenzian (Pliocene) warm period (approximately 3.3-3.0 Ma) monsoon not only provides us with a useful insight into the future monsoon under a warming scenario with high atmospheric CO2 concentration but is also hypothesized to play a key role in climate feedback through impact on the carbon cycle on a geological timescale. The monsoon evolution over time is one key to test this hypothesis. In this sense, we examine the mid-Piacenzian global land monsoon area (GLMA), precipitation (GLMP), and GLMP intensity (GLMPI) using all available model data from the Pliocene Model Intercomparison Project phase 1 (PlioMIP1). Relative to the pre-industrial period, both the GLMA and GLMP increase in the mid-Piacenzian and by an average of 7.3% and 5.8%, respectively. On a large scale, the increased GLMP comes mainly from northern Africa, Asia, and northern Australia. Quantitatively, it is the variation of GLMA, not of GLMPI, that makes a major contribution to the GLMP change. Further analysis reveals that anomalous inland water vapor transportation due to the reduced large-scale meridional thermal contrast in the mid-to-upper troposphere, together with the variation of vertical moisture advection and evaporation, explains most of the global land monsoon changes. A model-data comparison shows that the simulated monsoon precipitation variations are roughly consistent with the reconstructions in northern Africa, Asia, northern Australia, Central America, and eastern South America. Moreover, the intensified mid-Piacenzian Asian monsoon, which is supported by both simulations and reconstructions, or an overall intensified global land monsoon is hypothesized to contribute to the lowering of atmospheric CO2 concentration and the subsequent onset of the sustained major Northern Hemisphere glaciation.