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Aeolian dunes in many arid environments on Earth are shaped by seasonally varying bimodal wind regimes. However, the dynamics of dune evolution under such wind regimes are difficult to investigate at the time and length scales of laboratory experiments(1). These bedforms, in their natural environments(2-4), may also be influenced by unknown initial conditions and a variety of factors such as sediment availability(5), vegetation(6) and cohesion(7). Here we report results from a landscape-scale experiment in which we examine the evolution of bedforms under asymmetric bimodal winds. After flattening an experimental dunefield across 16 hectares of the Tengger Desert in Inner Mongolia, we measured winds and topography from March 2008 to October 2011 to reveal the development of regular dune patterns with a constant wavelength and increasing amplitude. On a seasonal timescale, we show that individual dunes propagate in different directions according to the prevailing wind. We find that the orientation of dune crests is controlled by the combination of the normal contributions of the two dominant winds, with respect to their relative strengths and directions, such that crests form an oblique angle of 50 degrees with the resultant sand flux. Our landscape-scale experiment suggests that the alignment of aeolian dunes can be used to determine wind forcing patterns on the Earth and other planetary bodies.