Multi-Decadal Modulations Observed in the Variability of the East Asian Summer Monsoon

Tuesday, June 16, 2015: 9:15 AM
Hisashi Nakamura, Tasuku Machimura, Shinichi Ogawa, Kazuaki Nishii, Takafumi Miyasaka and Yu Kosaka, The University of Tokyo, RCAST, Tokyo, Japan
Abstract:
The East Asian summer monsoon fluctuates from its climatological activity on monthly and interannual time scales, and the most dominant pattern of the variability is known as the Pacific-Japan (PJ) pattern. Characterized by a meridional teleconnection in anomalous activity of the Meiyu/Baiu rainband, tropical storms and a surface subtropical anticyclone (i.e., the Bonin High) in between, the PJ pattern exerts substantial influence on summertime climatic conditions over East Asia and the western North Pacific. Despite the recent warming trend observed in its background state, no assessment thus far has been performed on how substantially the PJ has undergone, if any, multi-decadal modulations in its structure or dominance. Through an EOF analysis applied to a new dataset of global atmospheric reanalysis (JRA-55), the predominance of the pattern is confirmed as being extracted in the leading EOF of lower-tropospheric monthly vorticity anomalies over 55 recent years. Both efficient conversion of barotropic and baroclinic energy from the climatological-mean state and generation of available potential energy from anomalous convective activity over the tropical western Pacific are found essential for the maintenance of the monthly atmospheric anomalies of the PJ pattern over the entire 55-year period. At the same time, however, the same EOF analysis as above but applied separately to each of the sub-periods reveals a distinct signature of long-term modulations in amplitude and thus the dominance of the PJ pattern. While being extracted in the first EOF up to the 1980s, the PJ pattern is extracted in the second EOF in the period since the 1990s with marked reductions in both the variance fraction explained and the efficiency of energy conversion/generation. Slight modulations are also found in its structure. The associated modulations of the summertime meridional teleconnection are also discussed.