We investigate the link between land surface processes and variability in the East Asian Summer Monsoon. Such a linkage is suggested by observation of a dominant intraseasonal mode of variability, indicating an inverse relationship in precipitation between East Asian land, and oceanic regions to the south and east. This mode is believed to be 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 (SSTA) forcing. Here, we hypothesize that the inverse precipitation relationship between East Asian land and sea (East Asian precipitation dipole or EAPD) results from interaction of the large scale general circulation with the land surface energy and hydrologic cycles, and the internal positive and negative feedback found between them.
Ensembles of computer simulations of the summer monsoon from May 1 to September 30 are performed to test the hypothetical relationship. The following results are found:
The common element which weakens or delays the East Asian summer monsoon is reduction of planetary boundary layer (PBL) moist static energy (MSE) available for moist convection. When diurnal solar forcing is removed, the convective threshold is reached later because the MSE diurnal variation is removed, even though the mean MSE in the PBL is larger. This delays the initial convection required for monsoon onset.
On the other hand with initial Eurasian drought, dry ground results in reduced evaporation, less mean MSE in the PBL, and generally suppressed convection. This reduces the vertical transport of latent heat to drive large scale convection and the summer monsoon circulation. Additionally, with initial Eurasian drought and the resulting high land surface temperatures, the extratropical circulation over Asia moves to its July state more quickly. As a result, the June phase of East Asian monsoon advance is largely skipped, resulting in perpetuation of the initial East Asian drought. The cause here is lower tropospheric moisture convergence, which also reduces MSE in the PBL.
These results suggest that sufficient MSE is a critical element for Asian monsoon onset. Examination of processes affecting the monsoon should take into account effects on lower tropospheric MSE.