The Physical Processes Dominating the Impact of the Summer North Atlantic Oscillation on the Eastern Tibetan Plateau Summer Rainfall

Abstract The summer North Atlantic Oscillation (SNAO), an important climate signal in regulating the interannual variability of Tibetan Plateau (TP) summer rainfall, is closely related to a meridional precipitation dipole pattern between the southeastern and northeastern TP. In this study, based on diagnoses of observations and multiple realizations of the CESM2 historical simulation, we find that there are fundamental differences between the formation processes dominating the SNAO-related summer rainfall anomaly in the southeastern and northeastern TP. An atmospheric moisture budget analysis reveals that the anomalous vertical (horizontal) moisture advection makes the largest contribution to the southeastern (northeastern) TP summer rainfall anomaly. During the negative phase of SNAO, the increased precipitation in the southeastern TP is related to the anomalous ascending flows, which are driven by two processes according to the moist static energy budget. The first is the southward shift of the subtropical westerly jet stream, which produces positive anomalous zonal advection of the climatological moist enthalpy in the upper-middle troposphere over the southeastern TP. The second is related to the enhanced transport of anomalous warm moist air to the south of TP, which produces positive anomalous meridional advection of anomalous moist enthalpy into the lower troposphere over the southeastern TP under the control of climatological monsoonal meridional circulations. This positive moist enthalpy advection enhances the atmospheric moist static energy and facilitate enhanced local convection. For the northeastern TP, the decreased precipitation is dominated by negative anomalous horizontal moisture advections due to the SNAO-induced equivalent-barotropic anomalous cyclone near the eastern edge of the TP.