Delayed impacts of the IOD: cross-seasonal relationships between the IOD, Tibetan Plateau snow, and summer precipitation over the Yangtze–Huaihe River region

Abstract

Significant anomalous signals are found during the ensuing summer after the Indian Ocean Dipole (IOD) forcing mode dissipates over the tropical Indian Ocean. These signals manifest as increased summer precipitation over the Yangtze–Huaihe River (YHR) region associated with abnormal water vapor convergence contributed mainly by southerly water vapor input along the western flank of an anomalous anticyclone over the subtropical western North Pacific (WNP). The mechanism of these delayed impacts of the IOD is investigated and summarized as a relationship between the IOD, Tibetan Plateau (TP) snow, and summer precipitation. The evolution of the moisture circulation response to the IOD reveals that the IOD can modulate the precipitation over the TP during winter and deepen the snow distribution over the southern TP. Anomalous snow conditions over the TP are a likely capacitor of the delayed effect of the IOD in that they can exert further influence on the ensuing summer monsoon. The influence of deeper snow over the southern edge of the TP on summer precipitation is confirmed: (1) the melting of excessive snow over the TP can lead to a weakened thermal land-sea contrast and weakened Asian summer monsoon westerlies; (2) easterly anomalies can further induce divergence over the WNP, meanwhile stimulating a Pacific-Japan teleconnection pattern; and (3) the anomalous anticyclone over the WNP strengthens the subtropical high and benefits southerly water vapor input to the YHR, while the anomalous cyclone dominates Japan. In between, the subtropical upper-level westerly jet accelerates, and the divergence at its right entrance can enhance the ascending vertical motion over the YHR, which is also favourable for precipitation.