Wetting and greening Tibetan Plateau in early summer in recent decades

Abstract

The Tibetan Plateau (TP) plays an essential role in the global hydrological cycle. Unlike the well‐recognized surface warming, changes in precipitation over the TP and the underlying mechanisms remain ambiguous. A significant increase in the amount of precipitation over the southeastern TP in May over 1979–2014 (13.46% decade−1 of the climatology) is identified in this study, based on homogenized daily rain gauge data. Both the increased precipitation frequency and intensity have contributions. The coherent increases in soil moisture content and vegetation activities further confirm the precipitation trend, indicating a wetting and greening TP in the early summer in recent decades. The moisture budget analysis shows that this wetting trend in the past four decades is dominated by the increased water vapor convergence due to circulation changes, while increases in specific humidity play a minor role. The wetting trend over the TP in May results directly from the earlier onset of the South Asian summer monsoon (ASM) since the late 1970s associated with the phase transition of Interdecadal Pacific Oscillation around the late 1990s. The earlier onset of the ASM triggers low‐level southwesterly anomalies over the northern Indian Ocean, promoting moisture convergence and increased precipitation over the TP in May. Specifically, the increased amount of precipitation after the onset of the ASM explains 95% of the increase in the total amount of precipitation in May.

Climate Change Trends and Impacts on Vegetation Greening Over the Tibetan Plateau

“…For the southwestern TP, the greening trend is dominant in winter, spring, and summer (Figures a, b, d–f, and h). The general wetting and greening trend of the TP revealed by our study is consistent with several former studies (e.g., Ding et al, ; Zhang et al, ). Due to the differences in the study periods and NDVI data sources, some discrepancies are observed among these studies.…”

“…For example, Ding et al () identified an increase in the NDVI in the southern and eastern TP. Zhang et al () found that the wetting and greening of the TP occurred in early summer. Our study revealed that the wetting and greening trends have heterogeneous characteristics.…”

“…Areas dominated by human activities are much smaller than those dominated by climate. Using daily measurements from a rain gauge, Zhang et al () found a significant increase in the amount of precipitation over the southeastern TP in May from 1979–2014, which demonstrated the wetting and greening of the TP in early summer in recent decades. Zhu et al (2016) identified the global greening trends by using ecosystem models and satellite data.…”

Observationally constrained projection of the reduced intensification of extreme climate events in Central Asia from 0.5 °C less global warming

“…This cold bias can strongly affect future projections of the whole region. In addition, the TP is generally regarded in many studies as a specific high-altitude region that should be separately discussed (Su et al 2013;Zhang et al 2017;You et al 2018). Thus, in this study, we focus on the region of Central Asia, with the TP masked out.…”

Indian summer monsoon: Extreme events, historical changes, and role of anthropogenic forcings

“…Therefore, changes in the timing and magnitude of monsoonal rains can have particularly severe impacts on the region's agriculture, surface water and ground water resources (Asoka, Gleeson, Wada, & Mishra, ; Barnett, Adam, & Lettenmaier, ; Gadgil & Gadgil, ; Russo & Lall, ). Changes in the timing of the monsoon can also have non‐local impacts such as over the Tibetan Plateau, where a trend toward early Indian monsoon onset has been associated with enhanced precipitation and greening since the 1970s (W. Zhang, Zhou, & Zhang, ).…”