Potential causes of megapluvials in the Yangtze–Huaihe River valley over the last millennium

Abstract

The potential causes of megapluvials over Yangtze–Huaihe River valley (YHRV) during the last millennium were investigated using the Community Earth System Model-Last Millennium Ensemble (CESM-LME) simulation data and multiple proxy data (i.e., reconstruction and paleoclimate assimilation data). Herein, we first depicted the spatial hydroclimatic pattern over the Northern Hemisphere when megapluvials occurred in the YHRV during the last millennium. The proxy and simulation data both showed that megapluvials in YHRV were accompanied by extensive decadal drought in western Asia and northwestern India, also located in mid to low latitudes. Then, by diagnosing the dynamic processes, we found that the baroclinic structure over western Asia and northwestern India caused anomalous geopotential heights, and hence triggered the negative phase of the Circumglobal Teleconnection (CGT). The eastwards wave train at mid-latitudes disturbed by the CGT mode, triggered lower-level northerlies and upper-level southerlies over the northern part of the YHRV and, as a consequence, led to anomalous strong convection near the YHRV. Together with abundant water vapour from the Pacific Northwest, there was a synergistic effect on megapluvial events over the YHRV. Finally, we analysed the linkage of surface temperature in western Asia and northwestern India and megapluvials in the YHRV. Land surface heating over western Asia and northwestern India may largely stimulate the formation of a negative CGT mode through local feedback of air–land interactions and, as a result, affect precipitation anomalies downstream, such as in the YHRV. It is therefore necessary to consider anomalous high surface temperatures in western Asia and northwestern India as important predictors for interdecadal severe floods in the YHRV.