The Lake Qinghai SMI record generally resembles the changing trends of ASM records derived from speleothems21,22
in China over the last 20 ka (). However, the SMI is relatively low and has lower-amplitude fluctuations during the 32 to 11.5 ka interval compared to the speleothem records. Furthermore, the SMI record has an extremely abrupt transition into the Holocene, whereas the cave δ18
O transition is more gradual.
From 32 to 20 ka, our SMI record suggests that the ASM was very weak, exhibiting no long-term change in the mean. We infer that the monsoon front rarely penetrated sufficiently northwest to reach Lake Qinghai, at the fringes of the area affected by the modern ASM. Part of the reason may be that the distance from Lake Qinghai to the oceanic moisture source increased during the LGM, when the coastline moved southeastward by as much as 1000 km (Supplementary Fig. S8
). This implies that long distance translation of the coastline in East and Southeast Asia associated with the sea-level change may have played an important role in regulating the summer monsoon over inland Asia on glacial-interglacial timescales. After 20 ka, the SMI indicates slight strengthening of the ASM and then an abrupt increase at 11.5 ka, consistent with the abrupt rises in sea level and associated tropical ocean warming23
, and the timing of rapid rise of sea level resulting from the Melt Water Pulse 1B24
, respectively. These factors likely contributed to the strengthening of the ASM through increased moisture supply at these times. During the Holocene, both the amplitude and variability of the ASM increased markedly, especially in the early Holocene, suggesting the front of intensified summer monsoon penetrated to Lake Qinghai and even further inland. This is consistent with a relatively high lake level in the early Holocene6
. The weakened ASM during the last glaciation and the strengthened ASM in the Holocene broadly support insolation as a major control on the ASM25
The WI indicates that the mid-latitude Westerlies climate dominated the Lake Qinghai area in glacial times and that the strength and variability of the Westerlies climate were notably larger than during the Holocene. Our data and previous studies6
suggest that the basin was occupied by a shallow, brackish water body under cold, arid and windy conditions. This pattern of WI variation is generally similar to that of NGRIP δ18
during the last 32 ka, although the WI exhibits higher amplitude fluctuations in the pre-Holocene interval (). Global climate modeling has demonstrated that ice sheet growth and lower North Atlantic sea-surface temperature during the LGM led to an increase in the meridional (latitudinal) temperature gradient and southward migration of the polar front and the Westerlies9,25
. Therefore, the Westerlies are most likely the link between the climatic variations in the high north latitudes and the northeastern TP.
Over the past 32 kyr, the Westerlies systematically weakened while the ASM climate strengthened beginning at 20 ka, especially after 11.5 ka. Thus, there is an anti-phase relationship between the Westerlies and the ASM climates on glacial-interglacial timescales (). This kind of interplay can be ascribed to forcing by north hemisphere insolation, ice volume, and associated high north-latitude climate and sea level changes.
Comparison of Lake Qinghai records with other records since 11.5 ka.
Abrupt, large-amplitude increases in the WI, most of which correlate well with changes in the NGRIP δ18
O record, a phenomena similar to the loess deposits in the Ili Basin12
, indicate markedly intensified Westerlies climate during the Younger Dryas (YD), LGM, and (within the relative accuracies of the lake and ice core chronologies) Heinrich events 1, 2 and 3 (). These cold-dry and dust-rich events are marked by visible loess-like sediments and by increases in the coarse fraction, suggesting strong instabilities in the Westerlies-dominated climate during glacial times. Strengthening (or weakening) of the Westerlies on the TP could be produced by shifts in the mean position of the Westerlies. During these cold events, the core of the Westerlies shifted southward to a position over Lake Qinghai during times when the North Atlantic Ocean was cold and relatively ice-covered. This would be consistent with studies that have documented southward shifts of other climate systems such as the Intertropical Convergence Zone27
or the Southern Hemisphere Westerlies28
. Contemporaneously, muted summer monsoon variation at Lake Qinghai and weakened ASM indicated by the speleothem δ18
O records demonstrate an anti-phase relationship between the ASM and Westerlies-dominated climates on millennial timescales. Within these events, increased frequency of dust storms, represented by high WI values, suggests large variability in the Westerlies climate on centennial timescales. Even with the uncertainty of our chronology, the WI suggests damped Westerlies influences during DO oscillations 1 to 4 (). It is plausible that cold air excursions of high-latitude, North Atlantic origin influenced the climate on the northeastern TP, and also caused cold-dry events in the East Asian monsoon areas, having been transmitted through the Westerlies and the Siberia High1
The WI sequence in the Holocene also correlates with the NGRIP δ18
in terms of long-term trends, but the WI record differs in having greater amplitude of abrupt climate change at the centennial and millennial scales (). Most of the dry-cold events inferred from increased WI appear to correspond to the Bond events in the North Atlantic29
. These suggest that the Westerlies may be a cold-air conveyer between the North Atlantic and the northeastern TP in the Holocene as well as in the glacial. In addition, the WI shows oscillations of Westerlies climate from 11.6 to 8.0 ka not recognized in previous work in western China30
In the Holocene, the ostracod δ18
O record from Lake Qinghai reflects the precipitation/evaporation budget associated with the ASM system6,31,32
. In the early Holocene, the depleted δ18
O of E
indicates increases in effective precipitation, implying strengthened monsoon intensity. In contrast, the notably enriched δ18
O values of L
, which are consistent with those of E. mareotica6,31,33
, reflect decreased monsoon intensity in the late Holocene. Our high-resolution ostracod δ18
O data show detailed ASM variations with a similar overall trend to that of previous studies (Supplementary Fig. S9
, see Materials and methods). Both the SMI and the ostracod δ18
O appear to follow northern hemisphere summer insolation, and document significant ASM variability at millennial time scales in the Holocene, as does the δ18
O record from Dongge Cave (). This SMI variability may be linked to North Atlantic abrupt events associated with the Atlantic's overturning circulation and sea-ice variations34
. The resemblance of two events at 8.2 and 9.3 ka BP implies a common cause and suggests that freshwater forcing played a prominent role in millennial scale climate change35,36,37
. Furthermore, weak monsoon events in the SMI sequence can be correlated with many of the intervals of low solar activity (high Δ14
C and low Total Solar Irradiance)38,39
, especially the events around 5.3 and 10 ka. These correlations further support the idea that decreased solar activity may weaken ASM circulation21,40
through amplifying processes41
Spectral analysis of the atmosphere Δ14
indicates concentrations of variance at 207 (de Vries), 149, 130, 104, and 88 yrs (Gleissberg) periods. Analysis of the SMI and WI records from Lake Qinghai indicate similar concentrations of variance with a particularly well-resolved peak centered on the Gleissberg period (Supplementary Fig. S10
). These findings support the robustness of our Holocene age model and the idea that solar changes are at least partly responsible for variations in ASM21,43
and Westerlies climate in the Holocene.
Climatic changes over the semi-arid areas of the TP, as represented by the Lake Qinghai records, are controlled by variations in both the ASM and the mid-latitude Westerlies. In turn, these two circulation systems respond to external forcing (orbital, perhaps solar output) and internal boundary conditions (ice volume, sea-level, and ocean circulation). The Lake Qinghai record exhibits a high degree of sensitivity to changes in these climatic controls. At Lake Qinghai, warm-humid ASM climates are anti-correlated with cold-dry Westerlies climates on glacial-interglacial and glacial millennial scales, and this variation is a manifestation of the interplay between the Westerlies and ASM. The importance of both the ASM and the Westerlies is similar to the case for Lake Hovsgol in the Lake Baikal catchment, where the ASM contributed much moisture during the early Holocene and the Westerlies were dominant during glacial Heinrich events5
. Similarly, interplay between high- and low-latitude climate variability occurred in the Aegean Sea during the early and middle Holocene44
. Thus, the alternating influence of the Westerlies and the ASM on glacial-interglacial timescales likely has been a major pattern of climate change in these regions for most of the Quaternary. Enhanced ASM at the boundary between the humid monsoon areas and arid inland regions is most likely associated with increased temperature in Northern Hemisphere. The climatic characteristics reported here provide key observational constraints for understanding climate changes in the humid/arid transition zone in Asia and improving models and their prediction of ASM and Westerlies changes linked to global warming.