Figure 1: Winter atmospheric circulation in Asia during Quaternary and Eocene times.

Graph shows the location of pressure highs (H, in purple) and associated winter winds (black arrows), the average annual position of the subtropical jet stream (dashed line) as well as the areas of significant summer rainfall (blue shaded areas) for interglacials (a), glacials (b) and hypothesized circulations for the Eocene (c,d). Xining area (square) lays at the interface between the Chinese Loess Plateau (in yellow) and the Tibetan Plateau (TP, in orange). Displayed on a middle Eocene paleogeographic reconstruction¹⁵, Eocene hypothesis 1 (c) proposes a semi-permanent subtropical high in eastern China^13,14; Eocene hypothesis 2 (d) proposes a 20° northward shift of the synoptic atmospheric system and the anchoring of the ITCZ to the Tibetan Plateau¹⁸.