Modulations of NAO on the Interplay Between Monsoon and Westerlies in Asia on Interdecadal Timescales

Among the global monsoon systems, the Asian summer monsoon (ASM) reaches the furthest north into eastern Asia and separates it into the monsoon- and westerlies-dominated regions. Paleoclimate studies revealed different hydroclimate patterns between the monsoon- and westerlies-dominated regions at glacial-interglacial and centennial timescales. The monsoon-westerlies boundaries are difficult to be quantified with the instrumental data. Herein, a monsoon-westerlies dipole pattern was identified by empirical orthogonal function (EOF) analysis of the Palmer Drought Severity Index (PDSI) for the 1900–2018 period on interdecadal timescales. This dipole pattern showed significant correlations with variations of the monsoon-related western Pacific sea surface temperature (SST) and the westerlies-related North Atlantic Oscillation (NAO). We then reconstructed the dipole pattern back to 1511 by tree rings, which was consistent with the reconstructed NAO at interdecadal timescale. Our reconstruction revealed long-term changes of monsoon-westerlies boundaries and provided evidence of their linkages with the NAO. Apart from anthropogenic forcing, climate internal variability also play a critical role in modulations of the dipole mode between ASM and westerlies at interdecadal timescale.