Chen Jianhui’s research while affiliated with Lanzhou University and other places

With the continued global warming, the frequency of flood or drought disaster caused by extreme changes of East Asian Summer Monsoon(EASM)will increase, thus it is important to investigate the variability of the EASM on various time-scales and to explore its underlying forcing mechanisms. In order to figure out aforementioned problem, we chose the recently-published quantitative precipitation reconstruction from Gonghai Lake(38°54'N, 112°14'E), a hydrologically-closed alpine lake, which is located on the northeastern margin of the Chinese Loess Plateau, northern China, a typical EASM dominated region. The lake sediment core(GH09B) is from the central part of Gonghai Lake and the age model is based on a combination of 210Pb/137Cs dating of the uppermost 0.35m and 25 accelerator mass spectrometry(AMS) 14C dates of terrestrial plant macrofossils, all converted to calendar years which spans the last 14.7cal.ka B.P., and the resulting record was used to reconstruct annual precipitation change in northern China based on state-of-the-art quantitative paleoecological techniques, a modern pollen reference dataset for China(2689 samples)and fossil pollen assemblages from the lake(771 samples with an average temporal resolution of ca.20a). The Gonghai Lake record can be regarded as a direct and highly reliable record of changes in EASM intensity. In this paper, we utilized aforementioned EASM record and methods of Ensemble Empirical Mode Decomposition(EEMD), wavelet analysis and spectral analysis to explore the various EASM variation periodicity on different time scales and the possible driving mechanism during Holocene. we obtained 7 Intrinsic Mode Function(IMF)component(corresponding to the 7 main periods)and a Residue(Res). Different components of the IMF wavelet analysis shows that the IMF 1 component in the time domain did not show periodic change continuously, and the continuous periodic signal occurred mainly in the Late Holocene, and the periodicity of the IMF 1 from 0 to 3cal.ka B.P. based on spectral analysis showed that the significant period is about 66 years. The periodicity of IMF 2~IMF 7 components are relatively continuous in the Holocene. Judging from the Res, the EASM has a significant decreasing trend in the Holocene. In summery, Holocene EASM varied with 2000-year-period and 4000-year-period on millennium time scale and fluctuated with 200-year, 400-year and 700-year cycles on century time scale. A distinct ca.66-year-cycle of EASM intensity variations happened on decadal time scale during Late Holocene. Furthermore our results suggested that on the millennium time scale, the EASM intensity with about 2000-year-period is mainly controlled by the large oscillations of the El Niño Southern Oscillation(ENSO)by comparing two curves of ENSO and EASM which obtained by Fast Fourier Transform(FFT)band-pass filter(0.0005±0.000025HZ)respectively, which can both are controlled by solar activity; While on the century time scale, the EASM and solar activity has a good corresponding relation in the IMF 2 and IMF 4 components, on the periodicity of about 200 years and 700 years, the EASM and solar activity maintain the same phase fluctuation, may indicate that the solar activity is the main control factor driving EASM; On decadal time scale, we used FFT band-pass filter to abtain EASM and AMO sequence fluctuation curve about 70 years(0.015±0.0025HZ)period, results showed that both phase change is consistent, the warm phase AMO corresponds to the strong EASM, and vice versa. This study gives access to further understanding the dynamic mechanism of EASM changes.