Multi-decadal to centennial-scale variability in Australian-Indonesian monsoon intensity over the past two millennia

Abstract:

The Australian-Indonesian monsoon (AIM) system is an important determinant of the climate system in the tropical Indo-Pacific region. The factors controlling past variability of the AIM are, however, less well understood. We present 2000 years high-resolution records of Australian-Indonesian winter monsoon (AIWM) winds and summer monsoon (AISM) rainfall from the same sedimentary archive retrieved offshore southern Indonesia. Records of upper water column temperature and planktonic foraminifera faunal composition are used as proxies for upwelling variability and thus changes in austral winter monsoonal winds. Bulk sedimentary element analysis records represent a measure for riverine detrital input and thus changes in austral summer monsoonal rainfall. Upwelling and hence AIWM winds were strong during the Little Ice Age (LIA) and weak during the Medieval Warm Period (MWP) and the Roman Warm Period (RWP). We find that rainfall was strong during the LIA and weak during the MWP and RWP. Based on modern instrumental records, we suggest that changes in the background state of the tropical Pacific so-called El Niño-like or La Niña-like conditions may have substantially contributed to upwelling variations and hence changes in austral winter monsoon winds over the past 2000 years. In analogy to the modern instrumental records, stronger upwelling off southern Indonesia and thus intensified Australian/Indonesian cross-equatorial winter winds during the LIA might suggest an El Niño-like mean state whereas weaker upwelling and weaker cross-equatorial winds during the MWP and RWP indicate more La Niña-like dominated conditions. However, the prevalence of El Niño-like conditions during the LIA and La Niña-like conditions during the MWP and RWP conflicts with our AISM rainfall reconstructions. It is expected from modern observations that periods of more frequent and/or intense El Niño events, such as during the LIA have resulted in reduced rainfall and subsequent drought in the AISM region. This disagreement might suggest that either the centennial-scale variations of the AISM and AIWM during the past 2000 years are unrelated to ENSO dynamics, or that El Niño events covary with La Niña events on that time scales, which typically would result in increased rainfall over central and southern Indonesia and northern Australia.