| There are significant spatial and temporal differences in climate over the Asian continent, which comprises two distinct climatic regions: monsoon circulation–dominated humid eastern and southern part of Asia and mid-latitude westerlies–controlled Asian inland. In the past decades, under the context of greenhouse gas–induced global warming, the intensity of the Asian summer monsoon(ASM), especially the East Asian summer monsoon(EASM) in northern China, experienced a persistent weakening trend, whereas climate in Asian inland has shifted from dry to wet. It is found that the out-of-phase relationship between moisture changes in Asian inland and monsoon-dominated Asia also existed on decadal to centennial time scales during the past millennium. However, their relationship on millennial and orbital time scales remains controversial. Lake records from northern Xinjiang in China, a core region influenced by the westerlies, suggest a wetting trend in Asian inland and an inverse relationship with the ASM history during the Holocene. Lake records from a wider area, which is divided apart from the EASM region based on modern ASM limit, however suggest a dry early Holocene, a wetter early to middle Holocene, and a moderately wet late Holocene, which was out-of-phase relationship with the ASM evolution throughout the Holocene. Speleothem oxygen isotope(δ18O) records from Kesang Cave in Asian inland however indicate a drying trend there during the Holocene, which was in-phase relationship with the ASM history.Climate changes over the past millennium, especially over the last century, have been remarkably affected by human activities apart from the natural variability of the climate system. Better understanding of the natural climate variability is scientifically important to estimate the relative contribution of human activities to climate changes that is of the essence of predicting future climate. Paleoclimate during the time with little human disturbance(e.g., the Holocene) hides the information of the natural climate variability that help our predictions of future climate changes. Therefore, this paper investigates the moisture histories in Asian inland and monsoon-dominated Asia, as well as the physical mechanisms throughout the Holocene using results from a transient simulation with a global climate model forced by changes in Earth’s orbital parameters and their comparison with proxy records.During the Holocene, proxy records reveal a gradually weakening trend in the strength of the Indian summer monsoon(ISM), which is a response to orbital forcing with decreasing summer insolation. For the evolutionary history of the EASM, there are differences among different types of proxy records. Speleothem oxygen isotope(δ18O) records indicate a gradually decreasing trend of the EASM intensity, whereas lake and loess records suggest a relatively strong early Holocene, a stronger middle Holocene, and a weak late Holocene. In spite of the differences, it is generally believed that the strength of the EASM is controlled by summer insolation during the Holocene. Hence, the strength of the ASM has generally shifted from strong to weak throughout the Holocene.For the great controversy of the moisture history in Asian inland among different proxy records, it is essential to understand their climate significance. In this study, the climate significance for different proxy records from Asian inland are reanalyzed via comparing proxy records with meteorological observation data, the National Centers for Environmental Prediction-National Center for Atmospheric Research(NCEPNCAR) daily reanalysis data, and simulation results. Some new results are derived and provided as follow:1. The ranges of monsoon circulation–dominated humid part of Asia and midlatitude westerlies–controlled Asian inland are not fixed but changing over time. The intertropical convergence zone(ITCZ) shifted much more northward during the early Holocene compared with the present, resulting in an extension of the area influenced by the ASM and the corresponding reduction in the area affected by the mid-latitude westerlies over Asian inland. Along with the weakening of the ASM throughout the Holocene, the areas influenced by the ASM has decreased whereas the area affected by the mid-latitude westerlies over Asian inland has increased. However, the boundary between the ASM influenced region and mid-latitude westerlies influenced region in previous studies were mainly based on modern ASM limit, which raises a question that some proxy records influenced by the EASM during the early to middle Holocene were categorized into mid-latitude westerlies–controlled Asian inland, causing erroneous estimation of the moisture evolution there with wettest climate during the middle Holocene due to the disturbance from the EASM.2. The climatic significance of speleothem δ18O records from Asian inland(Kesang cave) are remarkably different from that from the ASM influenced region. Meteorological observation data suggests that precipitation δ18O values in Asian inland(Wulumuqi weather station near Kesang cave site) are relative high in humid summer, which is significant different from that in the ASM influenced region. Moisture from the Indian Ocean can arrive over Asian inland through monsoonal winds over the east and north of the Qinghai–Tibet Plateau, which contributes enormously to heavy precipitation at Wulumuqi weather station. However, the moisture source for light precipitation comes mainly from the North Atlantic Ocean via westerly winds. Heavy precipitation at Wulumuqi weather station possesses relatively high δ18O values compared to light precipitation. This is different from precipitation δ18O in the ASM influenced region, in which heavy precipitation possesses relatively low δ18O values(precipitation amount effect). Therefore, the positive shift of speleothem δ18O values at Kesang Cave throughout the Holocene suggests a wetting trend in Asian inland, which is completely opposite to the moisture history in the ASM influenced region.3. Model results reveal a persistent wetting trend in mid-latitude westerlies–controlled Asian inland both in boreal summer and winter throughout the Holocene, coinciding with the proxy records. Model–data comparisons suggest an inverse relationship in the moisture evolution between the ASM influenced region and midlatitude westerlies–controlled Asian inland.This paper further provides the physical mechanisms for the inverse relationship in the moisture evolution between the ASM influenced region and mid-latitude westerlies–controlled Asian inland. Summer precipitation in Asian land is closely related the circumglobal teleconnection(CGT), which is triggered by the diabatic heating of ISM precipitation. During the negative phase of the CGT(weak ASM), monsoonal moisture from the Indian Ocean can be transported to Asian inland to produce heavy precipitation there due to weakened southwesterly winds over northern China, and vice versa. The CGT is positively associated with ASM precipitation. The CGT has shifted from positive to negative throughout the Holocene. Therefore, the CGT provides a viable explanation for the out-of-phase relationship in the moisture evolution but an in-phase relationship in the speleothem δ18O between the ASM influenced region and mid-latitude westerlies–controlled Asian inland during the Holocene. The CGT is affected by the combined effects of summer insolation, ISM, North Atlantic and Indian Ocean–western Pacific Ocean sea surface temperature(SST). Hence, the CGT can relate regional climate change in the ASM influenced region and mid-latitude westerlies–controlled Asian inland to global climate change.Boreal winter precipitation has been revealed to contribute a great deal to moisture evolution in Asian inland, which is controlled by the westerly winds, moisture transportation upstream(North Atlantic, Mediterranean, Black and Caspian Seas, and East Asian winter monsoon(EAWM). The westerly wind intensity is influenced by the insolation gradient at mid-latitudes that controls the meridional thermal difference. The water vapour content is closely related to insolation at mid-latitudes that affects surface temperature and thus the evaporation. The EAWM is closely related to the land–sea thermal contrast, which is also controlled by winter insolation. Therefore, changes in the seasonal cycle of incoming solar radiation driven by Earth’s orbital changes have probably also played an important role in the out-of-phase relationship in the moisture evolution between the ASM influenced region and mid-latitude westerlies–controlled Asian inland during the Holocene. |
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