THE ASIAN SUMMER PRECIPITATION OVER THE PAST FIVE CENTURIES

Abstract

Sparse long-term Asian monsoon (AM) records have limited our ability to understand and accurately model low-frequency AM variability. This dissertation presents a gridded 544-year (from AD 1470 to 2013) Reconstructed Asian summer Precipitation (RAP) dataset by the weighted merging of two complementary proxies including 453 tree ring width chronologies and 71 historical documentary records. Verification against observations and evaluation with various proxies (speleothem, ice core and upwelling) supports the RAP as a valuable dataset for study of large-scale low-frequency Asian summer precipitation variability. Four major modes of variability of the Asian summer precipitation are identified with the long record of RAP, including a biennial El Niño-Southern Oscillation (ENSO) mode, a low-frequency ENSO mode, a central Pacific El Niño-like decadal mode, and an interdecadal mode. It is shown that the relationship between the RAP and ENSO is ENSO phase-dependent since 1470. Two major modes of interannual variability are found to be associated with the ENSO developing and decaying phases, respectively. The mechanisms behind the modern monsoon-ENSO relationship can reasonably well explain the past monsoon behavior. In response to a developing ENSO, precipitation anomalies from the Maritime Continent (MC) via India to northern China are in phase, and this “chain reaction” tends to be largely steady since around 1620 AD; during the decaying phase, however, the summer rainfall-ENSO relationship over the Yangtze River Valley-southern East China (YRV-SEC), the MC and central Asia, has gone through large multidecadal to centennial changes over the past five centuries. The Pacific Decadal Oscillation and ENSO intensity are speculated to be associated with these multidecadal to centennial changes of rainfall-ENSO relations. An area-averaged All Asian Rainfall Index (AARI) was constructed with the RAP, and significant low-frequency periodicities of the AARI are found on decadal (8-10 years), quasi-bidecadal (22 years), and semi-centennial or multidecadal (50-54 years), as well as centennial time scales. A remarkable abrupt frequency shift from semi-centennial to decadal is detected around AD 1700 across the entire Asian land area, which nearly concurs with a dramatic upswing of the Indian summer monsoon and AAR. The leading EOF modes on the decadal, multidecadal, and centennial timescales all exhibit a similar spatially uniform structure, suggesting a tendency of in-phase variations among the rainfall over South Asia, East Asia, and MC across the three time scales. The leading mode of the decadal variation of AAR is associated with a mega-El Niño-Southern Oscillation (ENSO) in the Pacific, a cool western Indian Ocean, and a warm North Atlantic and cool tropical South Atlantic. The leading mode of semi-centennial (or multi-decadal) variation exhibits a more spatially uniform pattern and significantly correlated with the reconstructed Atlantic Multidecadal Oscillation (AMO) and the proxy mega-ENSO. Centennial variation of the AAR follows more closely the volcanic forcing variation, while is uncorrelated with solar forcing variability. Both the AARI-mega-ENSO and AARI-AMO relationships are nonstationary and experience significant centennial changes, especially around AD 1700; so is the AARI-volcanic forcing relation.