Abstract
A comparative analysis of East Asian summer monsoon (EASM) precipitation is performed to reveal the drivers and mechanisms controlling the similarities of the mid-Pliocene EASM precipitation changes compared to the corresponding pre-industrial (PI) experiments derived from atmosphere-only (i.e. AGCM) and fully coupled (i.e. CGCM) simulations, as well as the large simulated differences in the mid-Pliocene EASM precipitation between the two simulations. The area-averaged precipitation over the EASM domain is enhanced in the mid-Pliocene compared to the corresponding PI experiments performed by both the AGCM (LMDZ5A) and the CGCM (IPSL-CM5A). Moisture budget analysis reveals that it is the surface warming over East Asia that drives the area-averaged EASM precipitation increase in the mid-Pliocene in both simulations. The surface warming increases the atmospheric moisture content, as revealed by an increase in the thermodynamic component of vertical moisture advection, resulting in enhanced mid-Pliocene EASM precipitation compared to PI in both simulations. Moist static energy diagnosis identifies the combined effect of enhanced zonal thermal contrast and column-integrated meridional stationary eddy velocity $$\overline{{v^{*} }}$$ and its convergence $$\frac{{\overline{{\partial v^{*} }} }}{\partial y}$$ as the physical mechanisms that sustain the enhancement of mid-Pliocene EASM precipitation in both simulations compared to the PI experiments. This takes place through a strengthening of the EASM circulation and moisture transport into the EASM domain associated with an increase in local moisture convergence in the mid-Pliocene in both simulations. Moisture budget analysis also reveals that the larger area-averaged mid-Pliocene EASM precipitation increase in the CGCM compared to its AGCM component is mainly caused by the dynamical component contributing more to the vertical moisture advection in the CGCM (i.e. IPSL-CM5A) compared to its AGCM (LMDZ5). The large simulated differences in the spatial pattern of the mid-Pliocene EASM precipitation between the two simulations result from the combined effect of enhanced meridional thermal contrast over the EASM domain and increased $$\overline{{v^{*} }}$$ convergence over South China in the CGCM simulation compared to the AGCM simulation.
Highlights
The mid-Pliocene warm period (~3.3–3.0 Ma) is widely considered as the most recent warm period in Earth’s history when the global average surface air temperature (SAT)was higher than the present day (Haywood et al 2000; Haywood and Valdes 2004)
The drivers and mechanisms of enhanced summer monsoon precipitation over East Asia during the mid-Pliocene in atmosphere-only and coupled simulations with IPSLCM5A, as well as the simulated differences between the two simulations, are revealed in the present paper by performing a comparative analysis. This analysis demonstrates that the increase in area-averaged East Asian summer monsoon (EASM) summer precipitation in the mid-Pliocene simulations compared to the PI simulations is mostly driven by the increase in SAT over East Asia in both the mid-Pliocene simulations
Moist static energy (MSE) diagnosis provides an effective way to identify the combined effect of enhanced thermal contrast and increased column-integrated meridional stationary eddy velocity (〈v∗〉) as the principal mechanism controlling the large similarities and differences of the spatial patterns of EASM precipitation changes in the midPliocene between the two simulations
Summary
The mid-Pliocene warm period (~3.3–3.0 Ma) is widely considered as the most recent warm period in Earth’s history when the global average surface air temperature (SAT). East Asian climate during the mid-Pliocene warm period has been investigated (Yan et al 2012a, b), revealing a stronger than present East Asian summer monsoon (EASM) using the Community Atmosphere Model version 3.1 (CAM3.1). This result was subsequently confirmed by multi-model comparison analysis, in both atmosphere-only and coupled model experiments from the PlioMIP (Zhang et al 2013). The differences between the mid-Pliocene EASM precipitation simulated by the CGCM and AGCM are discussed in Sect.
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