A Diagnostic Study of the Influence of Early Spring Soil Moisture in Southeastern China on Interannual Variability of the East Asian Subtropical Summer Monsoon Onset

The effect of soil moisture (SM) on the onset of East Asian subtropical summer monsoon (EASSM) is investigated based on multiple sets of reanalysis data in the period of 1981–2010. It is found that the EASSM is characterized by persistent 2-m s⁻¹ southerly winds for about 3 months in spring at 850 hPa over the subtropical region of East Asia. Considering this feature of the meridional winds, we define the EASSM onset date, and obtain that the climatologi-cal onset date is pentad 17.7, around 26 March. On the interannual timescale, the onset date of EASSM exhibits stati-stically significant correlation with the SM in southeastern China in the month preceding the onset, with wetter (drier) conditions being associated with later (earlier) onset. The physical process by which the preceding SM affects the EASSM onset is further explored by examining the surface energy balance as well as its impacts. Positive (negative) SM anomalies in southeastern China in the month before onset may induce negative (positive) surface temperature anomalies. The decreased (increased) surface temperature in southeastern China before the EASSM onset weakens (strengthens) the zonal sea–land thermal contrast in the surface and low-level atmosphere in the subtropical East Asia. The zonal sea–land thermal contrast in wetter (drier) years induces anomalous northerly (southerly) winds over southeastern China, which tends to delay (advance) the zonal thermal seasonal transition in spring and is conducive to a later (earlier) onset of EASSM. These results are helpful for understanding and prediction of the variability of EASSM and the EASSM onset.