Abstract
Knowledge of spatial and temporal hydroclimatic differences is critical in understanding climatic mechanisms. Here we show striking hydroclimatic contrasts between northern and southern parts of the eastern margin of the Tibetan Plateau (ETP), and those between East Asian summer monsoon (EASM) and Indian summer monsoon (ISM) areas during the past ~2,000 years. During the Medieval Period, and the last 100 to 200 years, the southern ETP (S-ETP) area was generally dry (on average), while the northern ETP (N-ETP) area was wet. During the Little Ice Age (LIA), hydroclimate over S-ETP areas was wet, while that over N-ETP area was dry (on average). Such hydroclimatic contrasts can be broadly extended to ISM and EASM areas. We contend that changes in sea surface temperatures (SSTs) of the tropical Pacific Ocean could have played important roles in producing these hydroclimatic contrasts, by forcing the north-south movement of the Intertropical Convergence Zone (ITCZ) and intensification/slowdown of Walker circulation. The results of sensitivity experiments also support such a proposition.
Highlights
Climatic changes over N-ETP areas are well-synchronized on centennial/multi-decadal time scales (Fig. 2)
The long term trends in precipitation at Lake Qinghai inferred from sedimentary C/N ratios and grain size[7] are similar to those reconstructed from tree ring widths at Qilian Mt.[8], Dulan[9] (Fig. 2), and Delingha[10] (Fig. S3), and similar to the trend of drought/flood index derived from historical literature[11]
Because precipitation in S-ETP areas is controlled by the ISM7,12,16,17, and that in N-ETP areas is likely dominated by the EASM on centennial/decadal time scales during the past 2,000 years, it is necessary to examine whether the hydroclimatic contrasts between S-ETP and N-ETP areas can be extended to EASM and ISM areas
Summary
Because precipitation in S-ETP areas is controlled by the ISM7,12,16,17, and that in N-ETP areas is likely dominated by the EASM on centennial/decadal time scales during the past 2,000 years (see Supplementary 2), it is necessary to examine whether the hydroclimatic contrasts between S-ETP and N-ETP areas can be extended to EASM and ISM areas. The drought index inferred from historical literature at Seoul, South Korea clearly indicates wetter conditions during the Medieval Period than those during the LIA27. Dry climatic conditions during the Medieval Period can be seen from pollen records in peat from the Dajiuhu area, Hubei province, mid-southern China (Figs 1 and 3)[31]. In the Gulf of Oman, the presence of Impagidinium paradoxum indicates a lack of freshwater discharge into the ocean around this time
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