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Mid-Holocene monsoons in South and Southeast Asia: dynamically downscaled simulations and the influence of the Green Sahara

Abstract. Proxy records suggest that the Northern Hemisphere during the
mid-Holocene (MH), to be assumed herein to correspond to 6000 years ago,
was generally warmer than today during summer and colder in the winter due
to the enhanced seasonal contrast in the amount of solar radiation reaching
the top of the atmosphere. The complex orography of both South and Southeast
Asia (SA and SEA), which includes the Himalayas and the Tibetan Plateau (TP)
in the north and the Western Ghats mountains along the west coast of India
in the south, renders the regional climate complex and the simulation of the
intensity and spatial variability of the MH summer monsoon technically
challenging. In order to more accurately capture important regional features
of the monsoon system in these regions, we have completed a series of
regional climate simulations using a coupled modeling system to dynamically
downscale MH global simulations. This regional coupled modeling system
consists of the University of Toronto version of the Community Climate
System Model version 4 (UofT-CCSM4), the Weather Research and Forecasting
(WRF) regional climate model, and the 3D Coastal and Regional Ocean Community
model (CROCO). In the global model, we have taken care to incorporate Green
Sahara (GS) boundary conditions in order to compare with standard MH
simulations and to capture interactions between the GS and the monsoon
circulations in India and SEA. Comparison of simulated and reconstructed
climates suggest that the dynamically downscaled simulations produce
significantly more realistic anomalies in the Asian monsoon than the global
climate model, although they both continue to underestimate the inferred
changes in precipitation based upon reconstructions using climate proxy
information. Monsoon precipitation over SA and SEA is also greatly
influenced by the inclusion of a GS, with a large increase particularly
being predicted over northern SA and SEA, and a lengthening of the monsoon
season. Data–model comparisons with downscaled simulations outperform those
with the coarser global model, highlighting the crucial role of downscaling
in paleo data–model comparison.
