| We investigate the linkage between land surface processes and variability in the East Asian Summer Monsoon (EASM), suggested by observation of a dominant intraseasonal mode of variability. This mode indicates an inverse precipitation relationship between East Asian land and ocean to the south and east, and is believed related to major drought and flood conditions over East Asia. In previous numerical simulations, the temporal behavior of this mode appeared to depend on anomalous sea surface temperature forcing. Here, we hypothesize that the inverse precipitation relationship between East Asian land and sea (East Asian precipitation dipole [EAPD]) results from interaction between the general circulation and land surface energy and hydrologic cycles, including positive and negative feedbacks.; Computer simulations from May 1 to September 30 are performed to test the hypothesis. The following results are found: (1) The EAPD is an intrinsic intraseasonal mode of variability of the EASM system. (2) Removing the diurnal cycle of incoming solar radiation delays Indian monsoon rainfall onset and reduces the June (Mei-Yu) rains over China. (3) Initial Eurasian scale drought weakens the simulated EASM, despite increased land-sea thermal contrast.; The common element which weakens or delays the EASM is reduction of planetary boundary layer (PBL) moist static energy (MSE) available for moist convection. Without diurnal solar forcing, convective threshold is reached later because the MSE diurnal variation is also removed, although the mean MSE in the PBL is larger, delaying the initial convection required for monsoon onset.; With initial Eurasian drought, there is reduced evaporation, less mean MSE in the PBL, and suppressed convection. This reduces vertical latent heat transport, which further reduces large-scale convection and the summer monsoon circulation. Additionally, with a dry extratropical Eurasia, the extratropical Asian circulation shifts early to its July state. Consequently, the June phase of East Asian monsoon advance is skipped, resulting in perpetuation of initial East Asian drought, because of less lower tropospheric moisture convergence, which reduces MSE in the PBL.; These results suggest that sufficient MSE is critical for Asian monsoon onset. Examination of processes affecting the monsoon should take into account effects on lower tropospheric MSE. |
