Interannual and decadal covariabilities in East Asian and Western North Pacific summer rainfall for 1979\u20132016

Abstract

Understanding the variabilities of East Asian summer rainfall (EASR) and Western North Pacific summer rainfall (WNPSR) is essential because they play a key role to control the energy and water supply. Monsoon index is generally defined by seasonal mean, but it is not appropriate for considering various timescale characteristics. Because it includes the influence of not only the interannual but also longer timescales. Thus, the several timescale variances in the indices and background fields should be isolated as each variability to focus on their own features. Here, the present study defines the EASR and WNPSR indices to explore potential factors that may trigger the enhanced rainfall for the period 1979–2016. Interannual variation is shown to be dominant in both indices, while decadal variation is more significant in WNPSR than EASR. Each index is decomposed to obtain a signal on interannual and decadal timescales with 1–7 and 8–15 years. The interannual components in both indices perform opposite each other in atmospheric and oceanic fields and have a negative relationship with high covariance. Both location of subtropical jet and thermal condition over the Indian Ocean contribute to interannual EASR and WNPSR as factors. In contrast, related fields upon both decadal indices represent distinguished patterns. The EASR is modulated by Pacific sea surface temperature anomalies (SSTA) and interdecadal oscillation patterns in the decadal timescale. However, the WNPSR is controlled by the tri-polar SSTA over the North Atlantic. Consequently, the decadal and interannual variabilities show differing mechanisms to adjust rainfall during monsoon events.