Fig. 1: Cenozoic evolution of boreal SPR.

Spring rainfall (shading, unit: mm d−1) and its fraction of annual rainfall (contours, interval: 5%) over (a, c, e, g) North America and (b, d, f, h) East Asia in the Eocene, Miocene, Pliocene and present day derived from CESM coupled simulations. Filled circles, filled circles with white dots are climate proxies based on lithological and paleobotanical data, respectively, with blue (red) color representing humid (arid) climates. Dashed boxes denote the area of SPR over North America and East Asia.