Abstract
The spatial pattern of the first mode of interannual variability associated with the East Asian summer monsoon (EASM), obtained from a multivariate Empirical Orthogonal Functions (MV-EOF) analysis, corresponds to the Pacific–Japan (PJ) pattern and is referred to as the PJ-mode. The present study investigates the interannual variation of the PJ-mode from the perspective of the intraseasonal timescale. In particular, the impact of the Madden–Julian oscillation (MJO) on the interannual variation of the PJ-mode is investigated. The results show that the MJO has a significant influence on the interannual variation of the PJ-mode mainly in the lower troposphere (850 hPa) and that the former accounts for approximately 11% of the amplitude of the latter. The major part of the contribution comes from a change in frequency of the different phases of the MJO, especially that of MJO phase 6. This suggests that intraseasonal variation of the convection anomalies over the tropical eastern Indian and western Pacific Oceans plays an important role in the interannual variation of the PJ-mode. In addition, MJO phase 7 also contributes to the interannual variability of the PJ-mode, in this case induced by both the change in frequency and the change in circulation anomalies associated with MJO phase 7.
Highlights
The East Asian summer monsoon (EASM) is an energetic component of the global climate system, bringing rainfall to East Asia, one of the most densely populated regions in the world
The first mode of the East Asian summer monsoon (EASM) corresponds to the Pacific–Japan pattern, the interannual variability of which is closely associated with floods and droughts along the East Asian (Meiyu/ Changma/Baiu) rain band
The results show that the Madden–Julian oscillation (MJO) has an influence on the interannual variation of the PJ-mode and the former contributes about 11% to the latter
Summary
The East Asian summer monsoon (EASM) is an energetic component of the global climate system, bringing rainfall to East Asia, one of the most densely populated regions in the world. Tropical diabatic heating anomalies are mostly provided by the Madden–Julian Oscillation (MJO) (Madden and Julian 1971, 1972) or the closely related Boreal Summer Intraseasonal Oscillation (BSISO), the major modes of intraseasonal variability in the atmosphere over the tropical Indian and western Pacific Oceans. Li et al (2018) show that the positive phase of the PJ-mode cannot totally offset the negative phase of the PJ-mode during a specific summer (see Fig. 3a in Li et al 2018 and note that the average over all MJO phases is not zero), suggesting that the MJO may have an influence on the interannual variation of the PJ-mode This hypothesis is tested in the present study.
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