Optimal heat source for the interannual variability of the western North Pacific summer monsoon

ABSTRACT Using 132-member experiments based on a linear baroclinic atmospheric model (LBM), this study investigates the optimal heat source forcing the interannual variability of the western North Pacific summer monsoon (WNPSM). The 132 members are forced by localized atmospheric heat sources distributed homogeneously over regions from 55°S to 55°N, each 10° latitude × 30° longitude in size. The atmospheric responses to all the heating constitute an ensemble to examine the relative contribution of each local heat source to the strength of the WNPSM. The result indicates that the combination of an atmospheric heating (cooling) source over the subtropical Northwest Pacific and a cooling (heating) source over the tropical Indian Ocean and the midlatitudes from China to the southern part of Japan is the pattern most effective at enhancing (weakening) the WNPSM. Besides, the optimal heat source pattern identified by the LBM simulations is similar to the observed atmospheric heating anomalies associated with WNPSM interannual variability. The results suggest that any external forcing that leads to a similar heating structure as the optimal thermal forcing pattern could lead to an anomalous WNPSM. Graphical Abstrac