Expansion of the Asian Monsoon into the Mongolian Steppe at Insolation Maxima

Variations in Northern Hemisphere summer insolation are thought to drive northward expansions of the East Asian monsoon (EAM); however, the magnitude of these shifts and the monsoons' sensitivity to external forcings are poorly constrained. To evaluate the magnitude and timing of extreme northward EAM expansions, we reconstructed the lake-level history of Lake Khukh, a closed-basin lake located at 50°N in the East Mongolian Steppe. The results show that over the past 125 ka, Lake Khukh experienced pronounced highstands during northern hemisphere summer insolation maxima, indicating extreme northward incursions of the EAM into the Mongolian Steppe. The highstands are synchronous with depleted δ¹⁸O periods in the Chinese Caves record, indicating that orbital-scale northward expansions accompany EAM intensifications. We compiled lake-level data from closed-basin lakes across East Asia and compared them with the CCSM3 transient climate simulation. The results demonstrate that the model reproduces both the spatial distribution of moisture and the northern expansion of the EAM. Our results indicate that extreme shifts of the EAM into the high latitudes during insolation maxima are reliably reproduced by the CCSM3 climate simulation, thereby providing evidence of the model's skill and strengthening the fidelity of its future projections of EAM variability.