Selective Interaction between Monsoon and ENSO: Effects of Annual Cycle and Spring Predictability Barrier

This paper reviews research on the role of annual cycles of monsoon and ENSO in monsoon-ENSO interaction, spring predictability barrier (SPB), Webster-Yang monsoon index (WYI), and precursory signals preceding the Asian summer monsoon mainly based on the study of Webster and Yang (1992). As the major sources of predictability of global weather and climate, the Asian monsoon and ENSO show prominent annual variations and seasonally "phase-locked" features. During the boreal autumn and winter, the weakest Asian monsoon convection occurs, accompanied by the peak intensity of ENSO. During the boreal spring, however, the Asian monsoon strengthens explosively, accompanied by a quick decay of ENSO signals. The out-of-phase variation in the monsoon and ENSO makes the tropical ocean-atmosphere system most unstable during boreal spring. Thus, small random errors could grow unlimitedly and eventually result in a rapid drop of predictive skill of ENSO. Essentially, the Asian summer monsoon circulation is a low-frequency Rossby wave response of the atmosphere to latent heating over the subtropics with strong vertical wind shear, the physical basis for defining WYI. A larger WYI indicates stronger easterly wind shear over South Asia i.e., stronger Asian monsoon circulation, and vice versa. Regressing antecedent physical fields onto the summer WYI reveals several precursory signals preceding a stronger Asian summer monsoon. Significant easterly anomaly occurs over the Indian Ocean and subtropical Asia, while prominent westerly anomaly appears over the higher latitudes. Moreover, significant increase in soil moisture content is found in the subtropics such as the Indian subcontinent, the Indo-China Peninsula, and extratropical East Asia. In addition, decreased snow density occurs in mid-latitude regions especially the middle and western portions of the Tibetan Plateau. These precursors preceding the Asian summer monsoon can be applied to construct dynamical statistical models for improving seasonal prediction of the Asian summer monsoon.