The effects of teleconnections on carbon fluxes of global terrestrial ecosystems

Lee

Water Resources Research

et al. 2019

As drought involves diverse natural and human‐made processes in hydrologic cycles, it is necessary to track a process‐specific variable for efficient drought risk management. In this study, we employed a state‐of‐the‐art formulation of the generalized complementary relationship (GCR) to estimate terrestrial evapotranspiration (ET). The GCR ET estimates were used to assess historical droughts over the contiguous United States (CONUS) in relation to long‐term natural fluctuations. Results showed that the GCR provided strong performance in reproducing water balance ET estimates and correlations to the El Niño–Southern Oscillation, when compared to observation‐driven estimates and land surface models. For 1982‐2011, the GCR yielded better performance than land surface models even with estimated radiation inputs and no wind speed variations. The Standardized Evapotranspiration Deficit Index (SEDI) based on the GCR provided a consistent drought assessment with precipitation‐based drought identification and remotely sensed vegetation conditions. The SEDI seems to capture the heat wave‐driven flash drought together with water deficit‐driven droughts. In the CONUS, the La Niña‐like conditions were likely to trigger extensive vegetation droughts with extreme severity. This study found the three long‐term environmental factors that affect the general trend of ET deficits over the CONUS, which are the Atlantic Multidecadal Oscillation, the mean air temperature, and the Pacific Decadal Oscillation. Especially, the Atlantic Oscillation seems to be the most significant predictor that could explain the long‐term variation of vegetation droughts over the CONUS. This study highlights that the GCR‐based SEDI could be useful for monitoring and assessment of drought risks at a continental scale.

Section: Discussionmentioning

confidence: 76%

Historical Drought Assessment Over the Contiguous United States Using the Generalized Complementary Principle of Evapotranspiration

Lee

Water Resources Research

et al. 2019

“…Regarding the possible causes of RN reductions, previous studies have suggested that cloud coverage and air pollution and aerosol loading from rapid urbanization are potential reasons (Qian, Kaiser, Leung, & Xu, 2006;Xia, Wang, Chen, & Liang, 2006;Yang et al, 2009). Zhu et al (2017) found that large-scale atmospheric or coupled atmosphere-ocean modes are strongly associated with cloud cover across the world, which leads to RN variations. Moreover, with a weaken WND in China, air pollution and aerosol loading cannot be rapidly dispersed, which results in the further decrease in RN (Yang et al, 2009).…”

Section: Causes For Trends In Meteorological Factorsmentioning

confidence: 99%

“…Furthermore, by considering the importance of ET 0 for the estimation of crop water demand and drought forecasting systems (McEvoy et al, 2016), obtaining detailed information about ET 0 is critical before developing countermeasures for policy makers. Large-scale atmospheric or coupled atmosphere-ocean modes of climate variability (usually referred to as teleconnections due to their long-distance range of influence) can provide a useful framework for linking ET 0 to climate fluctuations via influencing climate variables (e.g., wind speed, solar radiation, temperature, and relative humidity; Chen, Li, & Pryor, 2013;Li & Chen, 2014;Yu, Zhong, Bian, & Heilman, 2015;Zhu et al, 2017). Given more knowledge of atmospheric system processes and the improved capability of future teleconnection predictions (Alexander, Matrosova, Penland, Scott, & Chang, 2008;Derome, Lin, & Brunet, 2005;Yang et al, 2015;Zheng, Fang, Zhu, Yu, & Li, 2016), estimating future ET 0 has become more possible using historical and predicted teleconnection patterns.…”

mentioning

confidence: 99%

Changes in reference evapotranspiration over China during 1960–2012: Attributions and relationships with atmospheric circulation

Chai

Sun

Chen

et al. 2018

Hydrological Processes

This study investigates reference evapotranspiration (ET0) trends in China from 1960 to 2012 based on the Penman–Monteith equation and gridded meteorological measurements. Under the combined impacts of factors influencing ET0 (i.e., net radiation [RN], mean temperature [TAVE], vapour pressure deficit [VPD], and wind speed [WND]), both seasonal and annual ET0 for the whole China and more than half of the grids decreased over the past 53 years. The attribution analyses suggest that for the whole China, the WND is responsible for annual and seasonal ET0 decreases (excluding summer, where RN is responsible). Across China, the annual cause of WND with the largest spatial extent (43.1% of grids) mainly derives from north of the Changjiang River Basin (CJRB), whereas VPD (RN) as a cause is dispersedly distributed (within and to the south of the CJRB). In summer, RN is dominant in more than half of the grids, but the dominance of VPD and WND accounts for approximately 90% of grids during the remaining seasons. Finally, the correlation coefficients between ET0 and the Atlantic Oscillation (AO), North AO, Indian Ocean Dipole (IOD), Pacific Decadal Oscillation (PDO), and El Niño Southern Oscillation (ENSO) indices with different lead times are calculated. For the whole China, annual and seasonal ET0 always significantly correlate with these indices (excluding the IOD) but with varied lead times. Additionally, near half of the grids show significant and maximum (i.e., the largest one between ET0 and a certain index with a lead time of 0–3 seasons) correlation coefficients of ET0 with PDO in spring and summer, ENSO in autumn, and AO in winter. This study is not only significant for understanding ET0 changes, but it also provides preliminary and fundamental reference information for ET0 prediction.

“…In any case, those concerned with temperature recognise 1) that around half the solar energy that falls on land is converted into the evaporation of water thus cooling the land surface (Pokorny et al 2010;Wang and Dickinson 2012), 2) that water vapour is the dominant greenhouse gas on our planet (Ravishankara 2012;Sherwood et al 2010) and 3) that the distribution of clouds and snow cover exert a major influence on planetary albedo (the proportion of incident light reflected back into space) and energy balance (Donohoe and Battisti 2011;. Those concerned with carbon recognise that water is the most limiting factor for terrestrial ecosystem carbon uptake, and that uncertainties over water imply uncertainties over biomass and carbon fixation (Polis 1999;Good et al 2013;Bernacchi and VanLoocke 2015;Thorley et al 2015;Viglizzo et al 2016;Taylor et al 2017;Zhu et al 2017). Furthermore, those concerned with environmental conservation, stability and the maintenance of species diversity recognise both the significance of freshwater biodiversity (supporting over 126,000 species of plants and animals, many of them vulnerable, on 0.8% of the world's surface, Garcia-Moreno et al 2014) and the links between terrestrial diversity and moisture (Kreft and Jetz 2007;Sheil et al 2016;Viglizzo et al 2015).…”

Section: Introductionmentioning

confidence: 99%

Forests, atmospheric water and an uncertain future: the new biology of the global water cycle

Sheil

2018

For. Ecosyst.

144

Theory and evidence indicate that trees and other vegetation influence the atmospheric water-cycle in various ways. These influences are more important, more complex, and more poorly characterised than is widely realised. While there is little doubt that changes in tree cover will impact the water-cycle, the wider consequences remain difficult to predict as the underlying relationships and processes remain poorly characterised. Nonetheless, as forests are vulnerable to human activities, these linked aspects of the water-cycle are also at risk and the potential consequences of large scale forest loss are severe. Here, for non-specialist readers, I review our knowledge of the links between vegetation-cover and climate with a focus on forests and rain (precipitation). I highlight advances, uncertainties and research opportunities. There are significant shortcomings in our understanding of the atmospheric hydrological cycle and of its representation in climate models. A better understanding of the role of vegetation and tree-cover will reduce some of these shortcomings. I outline and illustrate various research themes where these advances may be found. These themes include the biology of evaporation, aerosols and atmospheric motion, as well as the processes that determine monsoons and diurnal precipitation cycles. A novel theory-the 'biotic pump'-suggests that evaporation and condensation can exert a major influence over atmospheric dynamics. This theory explains how high rainfall can be maintained within those continental land-masses that are sufficiently forested. Feedbacks within many of these processes can result in non-linear behaviours and the potential for dramatic changes as a result of forest loss (or gain): for example, switching from a wet to a dry local climate (or visa-versa). Much remains unknown and multiple research disciplines are needed to address this: forest scientists and other biologists have a major role to play. New ideas, methods and data offer opportunities to improve understanding. Expect surprises.