The strengthened relationship between the Yangtze River Valley summer rainfall and the Southern Hemisphere annular mode in recent decades

This paper reveals a change in the relationship between the Yangtze River valley (YRV) summer rainfall (YRVSR) and the spring Southern Hemisphere annular mode (SAM) during the period of 1958–2012. It is found that the positive correlation between the YRVSR and the previous May SAM is insignificant during the period of 1958–1987 (P1), while it becomes significant during the period of 1988–2012 (P2). Such interdecadal change is relevant to the interdecadal change of the relationship between the May SAM and the Indian Ocean sea surface temperature anomaly (SSTA). During P2, the strong exchange of the air-sea heat flux related to the May SAM can excite a prominent meridional Indian Ocean tri-pole (IOT) SSTA pattern, whereas during P1, the May SAM associated air-sea heat exchange is weak and the IOT SSTA is not evident. Further observational and simulated evidences show that the May SAM associated IOT SSTA can persist through the following boreal summer and modulate the rainfall anomalies over the Maritime Continent (MC) by changing the zonal circulation over the tropical Indian Ocean-MC region. Subsequently, wave energies generated by the latent heat release of the MC rainfall (MCR) anomalies can propagate northward to the YRV along the great circle route. Responding to the positive (or negative) MCR heating forcing, anticyclonic (or cyclonic) circulation anomalies are excited over the western North Pacific, transporting more (or less) moisture to the YRV. Correspondingly, abnormal ascending (or sinking) motions are induced over the YRV, favoring sufficient (or insufficient) YRVSR. Thus, the IOT SSTA and MCR act as the “ocean” and “atmosphere” bridge role in connecting the May SAM and the YRVSR, respectively. Those models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) which well capture the May SAM-YRVSR relation can reproduce the IOT SSTA pattern and MCR anomalies associated with the May SAM. This further affirms that both the IOT SSTA and MCR are responsible for the strengthened relationship between the SAM and the YRVSR.