J. Clim. http://doi.org/nsg (2013)

The interglacial period that occurred 500,000 years ago — known as Marine Isotope Stage 13 — is thought to have been cooler than the present interglacial, with persistent continental ice in the Northern Hemisphere. A numerical simulation suggests that the East Asian summer monsoon was stronger during this interval.

Helene Muri of Université catholique de Louvain, Belgium, and colleagues used a coupled ocean–atmosphere general circulation model to assess the climate conditions that resulted from the combination of relatively high incoming solar radiation and low levels of atmospheric greenhouse gases during Marine Isotope Stage 13. In the simulations, global mean temperatures during the Northern Hemisphere summer were similar to those in pre-industrial times, whereas winter temperatures were one to two degrees cooler. Over east Asia, however, summer temperatures were several degrees warmer than in the pre-industrial period, with a weak temperature gradient between the eastern and western Pacific Ocean and a different pattern of zonal atmospheric circulation. In addition, an intense high pressure system persisted over the North Pacific. The presence of ice sheets in North America and western Eurasia further altered atmospheric circulation and precipitation patterns.

In sum, the simulated climate conditions increased summer monsoon precipitation over East Asia, and pushed precipitation farther north than present, in agreement with palaeoclimate records.