Asynchronous evolution of the Indian and East Asian Summer Monsoon indicated by Holocene moisture patterns in monsoonal central Asia

Abstract

a b s t r a c t a r t i c l e i n f oThe numerical meta-analysis of 92 proxy records (72 sites) of moisture and/or temperature change confirms earlier findings that the dominant trends of climatic evolution in monsoonal central Asia since the Last Glacial roughly parallel changes in Northern Hemisphere summer insolation, i.e. the period following the Last Glacial Maximum was characterized by dry and cold conditions until 15 cal. kyr BP, followed by a warm, wet period coincident with the Bølling/Allerød warm period and terminated by a cold, dry reversal during the Younger Dryas period. After an abrupt increase at the start of the Holocene, warm and wet conditions prevailed until ca. 4 cal. kyr BP when moisture levels and temperatures started to decrease. Ordination of moisture records reveals strong spatial heterogeneity in moisture evolution during the last 10 cal. kyr. The Indian Summer Monsoon (ISM) areas (northern India, Tibetan Plateau and southwest China) exhibit maximum wet conditions during the early Holocene, while many records from the area of the East Asian Summer Monsoon indicate relatively dry conditions, especially in north-central China where the maximum moisture levels occurred during the mid-Holocene. We assign such phenomena to strengthened Hadley Circulation centered over the Tibetan Plateau during the early Holocene which resulted in subsidence in the East Asian monsoonal regions leading to relatively dry conditions. Our observations of the asynchronous nature of the two Asian monsoon subsystems on millennial time scales have also been observed on annual time-scales as well as implied through the spatial analysis of vertical air motion patterns after strong ascending airflows over the Tibetan Plateau area that were calculated from NCEP/NCAR reanalysis data for the last 30 years. Analogous with the early Holocene, the enhancement of the ISM in a 'future warming world' will result in an increase in the asynchronous nature of the monsoon subsystems; this trend is already observed in the meteorological data from the last 15 years.

Spatial variability of Holocene changes in the annual precipitation pattern: a model-data synthesis for the Asian monsoon region

“…Seasonal variations in moisture: Considering the rough information deduced from the proxy data, it is impossible to acquire the seasonal variations, limiting the comparability of the index and the modelling results (Wang et al 2010b). Even more problematic, the single proxies may represent moisture signals of different seasons of the year and most proxies integrate over a certain or even varying period of the year e.g.…”

“…Precipitation is decreased in southern Indochina (up to 0.6 mm/day), on the south-eastern Tibetan Plateau (up to 1 mm/day) and in the Yangtze-Huanghe-Plain (up to 0.4 mm/day). Figure 3 shows the reconstructed moisture index (Wang et al 2010b) in comparison with the simulated relative precipitation difference between mid-Holocene and present-day. Overall, the model and reconstructions show qualitatively a similar pattern of moisture change with wetter conditions in North India and large parts of the Tibetan Plateau as well as in south-western China.…”

“…Generally, the moisture indices are relative, in a semi-quantitative sense for each individual case and therefore can only be compared qualitatively to the model results. For detailed information concerning the methods and original results for most sites we refer to Wang et al (2010b). Meanwhile, during producing the moisture index, we are aware of the following potential uncertainties:…”

“…So far, the reconstructions have lacked a consistent age control and revealed large disparities in the sample resolution that might have influenced the interpretation of these records and their applicability in comparative studies of climate modelling results and reconstructions. Recently, Wang et al (2010b) have provided a synthetic review of the spatial and temporal pattern of moisture evolution in monsoonal central Asia that is based on more than 90 synchronised proxy records.…”

Vegetation, Climate, Man—Holocene Variability in Monsoonal Central Asia

“…Paleoclimate records covering the past 18 cal. ka BP were collected from monsoonal Central Asia (Wang et al 2010). After recalibrating radiocarbon ages, semi-quantitative climate indices for moisture and temperature were calculated in 100-year intervals for the Holocene and 200-year intervals for the late glacial period.…”