Abstract
The Inner Tibetan Plateau (ITP), the central and western part of the Tibetan Plateau (TP), covers about one-fourth of the entire TP and contains more than 800 endorheic lakes larger than 1 km2. These lakes are important water reservoirs and sensitive to TP climate changes. They regulate regional water circulations, and further influence local ecosystems. Many lakes in ITP are surrounded by conspicuous paleoshorelines indicating much higher past lake levels. Previous studies found that lakes in the western ITP (west of ∼86°E) apparently expanded to higher levels than those to the east during the Holocene high lake level stage, however, there is no in-depth study on the reasons for the spatial differences of high lake levels within the ITP. In this study, we first identify Holocene lake level (or lake extent) changes over the ITP by combining published lake level variation data with our reconstruction of Dagze Co lake level variations. We then investigate spatial differences in the magnitude of lake expansions and explore the underlying forces driving these differences using the transient climate evolution of the last 21 ka (TraCE-21ka) and Kiel Climate Model (KCM) simulation results. We find that lakes in the ITP expanded to their highest levels during the early Holocene when the Indian summer monsoon (ISM) greatly intensified. After the mid-Holocene, lake levels fell as a result of the weakening of the ISM. The early Holocene northward shift of the westerly jet and a positive phase of the Atlantic multidecadal oscillation (AMO) resulted in the intensification of southwesterly winds on the southwest TP flank. Concurrently, westerly winds over the TP weakened, causing a differential increase in water vapor transport to the ITP with higher precipitation levels in the southwestern ITP and lower levels to the northeast. These wind-driven differential precipitation levels caused lakes in the southwestern ITP to expand to higher levels than those in the central, northern and northeastern ITP. During the early Holocene, expansion of lakes in the northwestern ITP was enhanced by an increase in glacier melt water besides the increased summer rainfall associated with the intensified ISM.
Highlights
The Tibetan Plateau (TP) is the largest elevated landform on Earth, occupying an area of ∼300 × 104 km2 (Zhang et al, 2020)
Several researches reported that the climate in southern and western TP were wettest during the early Holocene, but became dry since ∼7.5 ka (Gasse et al, 1991; Bird et al, 2014; Chen et al, 2020; Zhang et al, 2021). These researches are consistent with paleoshoreline dating results that the highest lake level for most lakes in central, southern, and western TP occurred during the early Holocene
It has been suggested that glacier melt water from an ice cap in the western TP was at maximum during the early Holocene (Li et al, 2021), and as a result we propose that lakes in western Inner Tibetan Plateau (ITP) may have expanded due to increased glacier melt water supply as well as to increased rainfall related to the intensified Indian summer monsoon (ISM)
Summary
The Tibetan Plateau (TP) is the largest elevated landform on Earth, occupying an area of ∼300 × 104 km (Zhang et al, 2020). At the seven other ITP lakes the highest lake levels all occurred between 10 and 8 ka (Figure 5), consistent with lacustrine and stalagmite records indicating summer season rainfall substantially increased during the early Holocene in the region influenced by the ISM (Fleitmann et al, 2007; Bird et al, 2014). These researches are consistent with paleoshoreline dating results that the highest lake level for most lakes in central, southern, and western TP occurred during the early Holocene These proxies cannot provide valuable information about changes in precipitation amount and its spatial variations during the Holocene over the TP. As with modern precipitation shifts, this “up-and-over” moisture transport pattern resulted in a substantial increase in precipitation in the southwestern ITP since water vapor crossing the high mountains initially arrives in that part of the ITP, producing more rainfall than further to the north and northeast (Liu et al, 2019). The amount of enhanced precipitation decreased from the southwestern to northeastern ITP, causing lakes in the southwestern ITP to expand to higher levels than those in the central, northern, and northeastern ITP
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