Variation of the summer Asian westerly jet over the last millennium based on the PMIP3 simulations/

Investigation of the evolution of summer Asian westerly jet (AWJ) in Earth history may deepen our understanding of the behavior of AWJ in the present and future. Here, the variation of summer westerlies and the associated mechanisms over West and East Asia are examined using the last millennium simulations from the Paleoclimate Modeling Intercomparison Project Phase III (PMIP3). The multimodel ensemble mean of 11 PMIP3 models reproduces the spatial distribution of modern zonal wind at 200 hPa and captures the location and intensity of the summer AWJ well. The results illustrate that the location of summer westerlies over West and East Asia shows no clear long-term trend over the last millennium, whereas there is an enhancing trend for the intensity, but with significant interannual and multidecadal variability. Summer westerlies over West Asia mainly oscillate within ~25-year band in terms of the location and intensity, while the principal periodicity over East Asia oscillate within ~25- and ~15-year band, respectively. Moreover, summer AWJ broadly moves poleward and weakens during the ‘Medieval Climate Anomaly’ (MCA, AD 1000–1200) relative to the ‘Little Ice Age’ (LIA, AD 1400–1600), which is attributed to the reduced meridional temperature gradient during the MCA. This poleward shift is consistent with the reconstructed moisture conditions over Central Asia, but the lack of direct proxy for westerlies restricts detailed model–data comparisons. Nevertheless, a possible scenario is presented on the spatiotemporal variation of summer AWJ over the last millennium, which may shed light on the response of the AWJ to natural forcings.