The Relay Effect of Soil and Ocean on the Response of East Asian Summer Monsoon to Spring Surface Forcing over Tibetan Plateau

Abstract The influence of spring surface forcing over the Tibetan Plateau (TP) on the East Asian summer monsoon (EASM) has been extensively studied, but the physical mechanism remains unclear: How could the signal of surface forcing over the TP be maintained to boreal summer? We found that both soil temperature (ST) over the TP and sea surface temperature (SST) over the western Pacific (WP) play key roles for the maintenance of spring TP forcing on the EASM. The key time scales are about 30 days for ST and 40 days for SST. The results show that the triple pattern of precipitation over eastern Asia in summer (especially in July) is highly correlated with spring TP surface forcing, characterized by surface potential vorticity (SPV) forcing as a new metric, which is different from the dipole pattern of other TP metric like the sensible heat (SH) in previous study. Further analysis suggested that TP surface forcing leads to local near-surface heating of air and warm top 0–7-cm surface soil maintaining from early June to early July. Furthermore, the TP surface forcing triggers updrafts and causes increased geopotential height at 200 hPa, stimulating a stationary wave that propagates downstream to the WP and leads to a warm-cored barotropic anticyclone over the WP. Under the high pressure, the top net shortwave radiation (TNSR) increases, heating the near-surface air temperature and leading to warm SST and seawater from 0 to 150 m below the WP, which sustain from mid-June to late July. Our study provides new evidence for the understanding of spring TP forcing on the subseasonal variations of the EASM. Significance Statement The objectives of our study were to analyze the impact of spring Tibetan Plateau (TP) surface forcing on the Asian summer monsoon (ASM) based on the new metric of surface potential vorticity (SPV) and clarify the different time scales on which land and sea play roles for storing the signal of TP forcing. We found that both soil temperature (ST) over the TP and the sea surface temperature (SST) over the western Pacific (WP) play key roles in the maintenance of spring TP forcing on the East Asian summer monsoon (EASM). This result importantly reveals that spring TP surface forcing could modulate the EASM by air–land–sea interaction.