Precursory influence of the Antarctic Oscillation on the onset of Asian summer monsoon

The Asian summer monsoon (ASM) begins firstly over the Indo-China Peninsula in early May and over the South China Sea (SCS) in mid-May. The different monsoon onset dates can exert distinct effects on the summer rainfall in Asia. Statistical results indicate that the Antarctic Oscillation (AAO) in the boreal winter has a significant precursory influence on the ASM onset dates. In stronger AAO years, both the Mascarene high and the Australia high in March are stronger owing to the "see-saw" structure of atmospheric circulation over the subtropics and higher latitudes in the Southern Hemisphere, and the tropical intertropical convergence zone (ITCZ) is deeper. Thus, the pressure gradient between the subtropical and tropical regions increases in spring. As a result, the Somalia cross-equatorial flow (SCEF) occurs earlier, strengthens, and enhances the westerlies over the tropical Indian Ocean. The enhanced westerlies impel an eastward withdrawal of the western Pacific subtropical high and intensify the convergence and rising motion at the lower troposphere, accelerating the burst of ASM. Differently, weaker AAO weakens the pressure gradient between the tropical and subtropical regions and delays the establishment of SCEF, resulting in a delayed onset of ASM. This study extends the leading time of seasonal forecast of ASM onset from the previous spring to winter and provides useful information about precursory signals in climate prediction operation.