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- Author or Editor: Qing Wu x
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Abstract
The Haiyang-2A (HY-2A; HaiYang means ocean in Chinese) satellite was successfully launched in China on 16 August 2011, carrying the nation’s first operational radar altimeter along with three other microwave sensors. In this study, HY-2A altimeter significant wave height (SWH) data have been validated against National Data Buoy Center (NDBC) buoy and Jason-2 altimeter SWH data over a period of 27 months (from 1 October 2011 to 31 December 2013). During the collocation, the effects of different thresholds of several flags are carefully studied. These flags prove to be useful for the SWH selection and different thresholds are observed to change the results remarkably. The final results show that HY-2A SWHs, with a 0.339-m root-mean-square (RMS) difference and a negative bias of 0.231 m in buoy comparison, have reached the mission target (0.5-m RMS). Nonetheless, the Jason-2 altimeter performs better with a lower RMS difference of 0.292 m and a positive bias of only 0.016 m. In addition, by analyzing the residuals (altimeter minus buoy), the bias for the HY-2A altimeter is found to decline monotonically over the whole range with an overestimation at low sea state (SWH < 1 m), a minor underestimation at middle sea state (1 m < SWH < 5 m), and a severe underestimation at high sea state (SWH > 5 m). However, only an underestimation at high sea state is found for the Jason-2 altimeter. A linear regression is also proposed. The 20 days of the newly processed HY-2A SWHs are investigated and discussed as well, and a slight quality improvement has been observed using these data.
Abstract
The Haiyang-2A (HY-2A; HaiYang means ocean in Chinese) satellite was successfully launched in China on 16 August 2011, carrying the nation’s first operational radar altimeter along with three other microwave sensors. In this study, HY-2A altimeter significant wave height (SWH) data have been validated against National Data Buoy Center (NDBC) buoy and Jason-2 altimeter SWH data over a period of 27 months (from 1 October 2011 to 31 December 2013). During the collocation, the effects of different thresholds of several flags are carefully studied. These flags prove to be useful for the SWH selection and different thresholds are observed to change the results remarkably. The final results show that HY-2A SWHs, with a 0.339-m root-mean-square (RMS) difference and a negative bias of 0.231 m in buoy comparison, have reached the mission target (0.5-m RMS). Nonetheless, the Jason-2 altimeter performs better with a lower RMS difference of 0.292 m and a positive bias of only 0.016 m. In addition, by analyzing the residuals (altimeter minus buoy), the bias for the HY-2A altimeter is found to decline monotonically over the whole range with an overestimation at low sea state (SWH < 1 m), a minor underestimation at middle sea state (1 m < SWH < 5 m), and a severe underestimation at high sea state (SWH > 5 m). However, only an underestimation at high sea state is found for the Jason-2 altimeter. A linear regression is also proposed. The 20 days of the newly processed HY-2A SWHs are investigated and discussed as well, and a slight quality improvement has been observed using these data.
Abstract
Anomalous warming occurred over the Tibetan Plateau (TP) before and during the disastrous freezing rain and heavy snow hitting central and southern China in January 2008. The relationship between the TP warming and this extreme event is investigated with an atmospheric general circulation model. Two perpetual runs were performed. One is forced by the climatological mean sea surface temperatures in January as a control run; and the other has the same model setting as the control run except with an anomalous warming over the TP that mimics the observed temperature anomaly. The numerical results demonstrate that the TP warming induces favorable circulation conditions for the occurrence of this extreme event, which include the deepened lower-level South Asian trough, the enhanced lower-level southwesterly moisture transport in central-southern China, the lower-level cyclonic shear in the southerly flow over southeastern China, and the intensified Middle East jet stream in the middle and upper troposphere. Moreover, the anomalous TP warming results in a remarkable cold anomaly near the surface and a warm anomaly aloft over central China, forming a stable stratified inversion layer that favors the formation of the persistent freezing rain. The possible physical linkages between the TP warming and the relevant resultant circulation anomalies are proposed. The potential reason of the anomalous TP warming during the 2007–08 winter is also discussed.
Abstract
Anomalous warming occurred over the Tibetan Plateau (TP) before and during the disastrous freezing rain and heavy snow hitting central and southern China in January 2008. The relationship between the TP warming and this extreme event is investigated with an atmospheric general circulation model. Two perpetual runs were performed. One is forced by the climatological mean sea surface temperatures in January as a control run; and the other has the same model setting as the control run except with an anomalous warming over the TP that mimics the observed temperature anomaly. The numerical results demonstrate that the TP warming induces favorable circulation conditions for the occurrence of this extreme event, which include the deepened lower-level South Asian trough, the enhanced lower-level southwesterly moisture transport in central-southern China, the lower-level cyclonic shear in the southerly flow over southeastern China, and the intensified Middle East jet stream in the middle and upper troposphere. Moreover, the anomalous TP warming results in a remarkable cold anomaly near the surface and a warm anomaly aloft over central China, forming a stable stratified inversion layer that favors the formation of the persistent freezing rain. The possible physical linkages between the TP warming and the relevant resultant circulation anomalies are proposed. The potential reason of the anomalous TP warming during the 2007–08 winter is also discussed.
Abstract
Floods and droughts hit southwest China (SWC) frequently, especially over the last decade. In this study, the dominant modes of summer rainfall anomalies over SWC on the interannual time scale and the possible causes are investigated. Interannual variability of the summer rainfall over SWC has two dominant modes. The first mode features rainfall increases over most of SWC except central Sichuan, and the second mode exhibits wet conditions in the north but dry conditions in the south. The suppressed convection over the Philippine Sea affects the first mode by inducing anomalous anticyclones over the western North Pacific and to the south of the Tibetan Plateau, which transport more water vapor to eastern Tibet and eastern SWC and hence favor above-normal rainfall there. The enhanced convection over the western Maritime Continent could generate similar atmospheric circulation anomalies associated with the suppressed convection over the Philippine Sea but with a northward shift, resulting in significant increases in rainfall over northeastern SWC but weak decreases in rainfall over southeastern SWC. As a result, the rainfall anomalies over SWC tend to be different between El Niño–Southern Oscillation decaying and developing phases because their different impacts on the convection over the Philippine Sea and the western Maritime Continent. Meanwhile, the sea surface temperature in the tropical southeastern Indian Ocean also plays an important role in variability of the rainfall over SWC because of its significant impact on the convection over the western Maritime Continent.
Abstract
Floods and droughts hit southwest China (SWC) frequently, especially over the last decade. In this study, the dominant modes of summer rainfall anomalies over SWC on the interannual time scale and the possible causes are investigated. Interannual variability of the summer rainfall over SWC has two dominant modes. The first mode features rainfall increases over most of SWC except central Sichuan, and the second mode exhibits wet conditions in the north but dry conditions in the south. The suppressed convection over the Philippine Sea affects the first mode by inducing anomalous anticyclones over the western North Pacific and to the south of the Tibetan Plateau, which transport more water vapor to eastern Tibet and eastern SWC and hence favor above-normal rainfall there. The enhanced convection over the western Maritime Continent could generate similar atmospheric circulation anomalies associated with the suppressed convection over the Philippine Sea but with a northward shift, resulting in significant increases in rainfall over northeastern SWC but weak decreases in rainfall over southeastern SWC. As a result, the rainfall anomalies over SWC tend to be different between El Niño–Southern Oscillation decaying and developing phases because their different impacts on the convection over the Philippine Sea and the western Maritime Continent. Meanwhile, the sea surface temperature in the tropical southeastern Indian Ocean also plays an important role in variability of the rainfall over SWC because of its significant impact on the convection over the western Maritime Continent.
Abstract
Observations from TRITON buoys in the warm/fresh pool and a global ocean general circulation model are used to study the interannual variability of the equatorial western Pacific and the relationship between the zonal warm water transport, meridional convergence, and the warm water volume (WWV). The simulated temperature, salinity, and zonal warm water transport are validated with the mooring observations for the period 2000–14. The model results are then used to examine the WWV balance in ENSO cycles in an extended period from 1980 to 2014. It is shown that the zonal transport is highly correlated with meridional convergence and leads by about 4–5 months, and their phase offset determines the WWV changes. This result differs from the recharge paradigm in which the meridional convergence is supposed to be mainly responsible for the WWV changes. There is also no apparent change in relationship between zonal and meridional transports since 2000, unlike that between WWV and SST. The study suggests that the zonal warm water transport from the western boundary could have major implications for ENSO dynamics.
Abstract
Observations from TRITON buoys in the warm/fresh pool and a global ocean general circulation model are used to study the interannual variability of the equatorial western Pacific and the relationship between the zonal warm water transport, meridional convergence, and the warm water volume (WWV). The simulated temperature, salinity, and zonal warm water transport are validated with the mooring observations for the period 2000–14. The model results are then used to examine the WWV balance in ENSO cycles in an extended period from 1980 to 2014. It is shown that the zonal transport is highly correlated with meridional convergence and leads by about 4–5 months, and their phase offset determines the WWV changes. This result differs from the recharge paradigm in which the meridional convergence is supposed to be mainly responsible for the WWV changes. There is also no apparent change in relationship between zonal and meridional transports since 2000, unlike that between WWV and SST. The study suggests that the zonal warm water transport from the western boundary could have major implications for ENSO dynamics.
Abstract
Variability in soil moisture has implications for regional terrestrial environments under a warming climate. This paper focuses on the spatiotemporal variability in the intra-annual persistence of soil moisture in China using the fifth-generation reanalysis dataset by the European Centre for Medium-Range Weather Forecasts for the period 1979–2018. The results show that in China, the mean intra-annual persistence in the humid to arid zones increased from 60 to 115 days in the lower layer but decreased from 19 to 13 days and from 25 to 14 days in the upper and root layers, respectively. However, these changes were strongly attenuated in extremely dry and wet regions due to the scarcity of soil moisture anomalies. Large changes in persistence occurred in the lower soil layer in dryland areas, with a mean difference of up to 40 days between the 2010s and the 1980s. Overall increasing trends dominated the large-scale spatial features, despite regional decreases in the eastern arid zone and the North and Northeast China plains. In the root layer, the two plains experienced an expanded decrease while on the Tibetan Plateau it was dominated by decadal variability. These contrasting changes between the lower and root layers along the periphery of the transition zone was a reflection of the enhanced soil hydrological cycle in the root layer. The enhanced persistence in drylands lower layer is an indication of the intensified impacts of soil moisture anomalies (e.g., droughts) on terrestrial water cycle. These findings may help the understanding of climate change impacts on terrestrial environments.
Abstract
Variability in soil moisture has implications for regional terrestrial environments under a warming climate. This paper focuses on the spatiotemporal variability in the intra-annual persistence of soil moisture in China using the fifth-generation reanalysis dataset by the European Centre for Medium-Range Weather Forecasts for the period 1979–2018. The results show that in China, the mean intra-annual persistence in the humid to arid zones increased from 60 to 115 days in the lower layer but decreased from 19 to 13 days and from 25 to 14 days in the upper and root layers, respectively. However, these changes were strongly attenuated in extremely dry and wet regions due to the scarcity of soil moisture anomalies. Large changes in persistence occurred in the lower soil layer in dryland areas, with a mean difference of up to 40 days between the 2010s and the 1980s. Overall increasing trends dominated the large-scale spatial features, despite regional decreases in the eastern arid zone and the North and Northeast China plains. In the root layer, the two plains experienced an expanded decrease while on the Tibetan Plateau it was dominated by decadal variability. These contrasting changes between the lower and root layers along the periphery of the transition zone was a reflection of the enhanced soil hydrological cycle in the root layer. The enhanced persistence in drylands lower layer is an indication of the intensified impacts of soil moisture anomalies (e.g., droughts) on terrestrial water cycle. These findings may help the understanding of climate change impacts on terrestrial environments.
Abstract
This paper reports a consistent seesaw relationship between interdecadal precipitation variability over North China and the Southwest United States, which can be found in observations and simulations with several models. Idealized model simulations suggest the seesaw could be mainly driven by the interdecadal Pacific oscillation (IPO), through a large-scale circulation anomaly occupying the entire northern North Pacific, while the Atlantic multidecadal oscillation (AMO) contributes oppositely and less. Modulation of precipitation by the IPO tends to be intensified when the AMO is in the opposite phase, but weakened when the AMO is in the same phase. The warm IPO phase is associated with an anomalous cyclone over the northern North Pacific; consequently, anomalous southwesterly winds bring more moisture and rainfall to the Southwest United States, while northwesterly wind anomalies prevail over North China with negative rainfall anomalies. The east–west seesaw of rainfall anomalies reverses sign when the circulation anomaly becomes anticyclonic during the cold IPO phase. The IPO-related tropical SST anomalies affect the meridional temperature gradient over the North Pacific and adjacent regions and the mean meridional circulation. In the northern North Pacific, the atmospheric response to IPO forcing imposes an equivalent barotropic structure throughout the troposphere. An important implication from this study is the potential predictability of drought-related water stresses over these arid and semiarid regions, with the progress of our understanding and prediction of the IPO and AMO.
Abstract
This paper reports a consistent seesaw relationship between interdecadal precipitation variability over North China and the Southwest United States, which can be found in observations and simulations with several models. Idealized model simulations suggest the seesaw could be mainly driven by the interdecadal Pacific oscillation (IPO), through a large-scale circulation anomaly occupying the entire northern North Pacific, while the Atlantic multidecadal oscillation (AMO) contributes oppositely and less. Modulation of precipitation by the IPO tends to be intensified when the AMO is in the opposite phase, but weakened when the AMO is in the same phase. The warm IPO phase is associated with an anomalous cyclone over the northern North Pacific; consequently, anomalous southwesterly winds bring more moisture and rainfall to the Southwest United States, while northwesterly wind anomalies prevail over North China with negative rainfall anomalies. The east–west seesaw of rainfall anomalies reverses sign when the circulation anomaly becomes anticyclonic during the cold IPO phase. The IPO-related tropical SST anomalies affect the meridional temperature gradient over the North Pacific and adjacent regions and the mean meridional circulation. In the northern North Pacific, the atmospheric response to IPO forcing imposes an equivalent barotropic structure throughout the troposphere. An important implication from this study is the potential predictability of drought-related water stresses over these arid and semiarid regions, with the progress of our understanding and prediction of the IPO and AMO.
Deriving AMVs from Geostationary Satellite Images Using Optical Flow Algorithm Based on Polynomial ExpansionAbstract
An optical flow algorithm based on polynomial expansion (OFAPE) was used to derive atmospheric motion vectors (AMVs) from geostationary satellite images. In OFAPE, there are two parameters that can affect the AMV results: the sizes of the expansion window and optimization window. They should be determined according to the temporal interval and spatial resolution of satellite images. A helpful experiment was conducted for selecting those sizes. The limitations of window sizes can cause loss of strong wind speed, and an image-pyramid scheme was used to overcome this problem. Determining the heights of AMVs for semitransparent cloud pixels (STCPs) is challenging work in AMV derivation. In this study, two-dimensional histograms (H2Ds) between infrared brightness temperatures (6.7- and 10.8-μm channels) formed from a long time series of cloud images were used to identify the STCPs and to estimate their actual temperatures/heights. The results obtained from H2Ds were contrasted with CloudSat radar reflectivity and CALIPSO cloud-feature mask data. Finally, in order to verify the algorithm adaptability, three-month AMVs (JJA 2013) were calculated and compared with the wind fields of ERA data and the NOAA/ESRL radiosonde observations in three aspects: speed, direction, and vector difference.
Abstract
An optical flow algorithm based on polynomial expansion (OFAPE) was used to derive atmospheric motion vectors (AMVs) from geostationary satellite images. In OFAPE, there are two parameters that can affect the AMV results: the sizes of the expansion window and optimization window. They should be determined according to the temporal interval and spatial resolution of satellite images. A helpful experiment was conducted for selecting those sizes. The limitations of window sizes can cause loss of strong wind speed, and an image-pyramid scheme was used to overcome this problem. Determining the heights of AMVs for semitransparent cloud pixels (STCPs) is challenging work in AMV derivation. In this study, two-dimensional histograms (H2Ds) between infrared brightness temperatures (6.7- and 10.8-μm channels) formed from a long time series of cloud images were used to identify the STCPs and to estimate their actual temperatures/heights. The results obtained from H2Ds were contrasted with CloudSat radar reflectivity and CALIPSO cloud-feature mask data. Finally, in order to verify the algorithm adaptability, three-month AMVs (JJA 2013) were calculated and compared with the wind fields of ERA data and the NOAA/ESRL radiosonde observations in three aspects: speed, direction, and vector difference.
Abstract
Annual precipitation anomalies over eastern China are characterized by a north–south dipole pattern, referred to as the “southern flooding and northern drought” pattern (SF/ND), fluctuating on decadal time scales. Previous research has suggested possible links with oceanic forcing, but the underlying physical mechanisms by which sea surface temperature (SST) variability impacts the dipole pattern remains unclear. Idealized atmospheric general circulation model experiments conducted by the U.S. CLIVAR Drought Working Group are used to investigate the role of historical SST anomalies associated with Pacific El Niño–Southern Oscillation (ENSO)-like and the Atlantic multidecadal oscillation (AMO) patterns in this dipole pattern. The results show that the Pacific SST pattern plays a dominant role in driving the decadal variability of this dipole pattern and the associated atmospheric circulation anomalies, whereas the Atlantic SST pattern contributes to a much lesser degree. The direct atmospheric response to the Pacific SST pattern is a large-scale cyclonic or anticyclonic circulation anomaly in the lower troposphere occupying the entire northern North Pacific. During the warm phase of the Pacific SST pattern, it is cyclonic with northwesterly wind anomalies over northern China pushing the monsoon front to the south and consequently SF/ND. During the cold phase of the Pacific SST pattern, the circulation anomaly reverses with southeasterly winds over northern China allowing the monsoon front and the associated rainband to migrate northward, resulting in southern drought and northern flooding. The Atlantic SST pattern plays a supplementary role, enhancing the dipole pattern when the Pacific SST and Atlantic SST patterns are in opposite phases and weakening it when the phases are the same.
Abstract
Annual precipitation anomalies over eastern China are characterized by a north–south dipole pattern, referred to as the “southern flooding and northern drought” pattern (SF/ND), fluctuating on decadal time scales. Previous research has suggested possible links with oceanic forcing, but the underlying physical mechanisms by which sea surface temperature (SST) variability impacts the dipole pattern remains unclear. Idealized atmospheric general circulation model experiments conducted by the U.S. CLIVAR Drought Working Group are used to investigate the role of historical SST anomalies associated with Pacific El Niño–Southern Oscillation (ENSO)-like and the Atlantic multidecadal oscillation (AMO) patterns in this dipole pattern. The results show that the Pacific SST pattern plays a dominant role in driving the decadal variability of this dipole pattern and the associated atmospheric circulation anomalies, whereas the Atlantic SST pattern contributes to a much lesser degree. The direct atmospheric response to the Pacific SST pattern is a large-scale cyclonic or anticyclonic circulation anomaly in the lower troposphere occupying the entire northern North Pacific. During the warm phase of the Pacific SST pattern, it is cyclonic with northwesterly wind anomalies over northern China pushing the monsoon front to the south and consequently SF/ND. During the cold phase of the Pacific SST pattern, the circulation anomaly reverses with southeasterly winds over northern China allowing the monsoon front and the associated rainband to migrate northward, resulting in southern drought and northern flooding. The Atlantic SST pattern plays a supplementary role, enhancing the dipole pattern when the Pacific SST and Atlantic SST patterns are in opposite phases and weakening it when the phases are the same.
Abstract
The spatial distribution of summer rainfall anomalies over eastern China often shows a tripole pattern with rainfall anomalies over the Yangtze River basin varies in opposite phase with North China and South China. It is not clear whether this tripole pattern is an intrinsic atmospheric mode or it is remotely forced. Using two sets of model outputs from 20 models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5), this paper investigates the driving mechanisms of this leading rainfall mode and its major influencing factors. One set (piControl) is fully coupled atmosphere–ocean simulations under constant preindustrial forcing and the other (sstClim) is atmosphere-alone models forced by prescribed climatological sea surface temperatures (SSTs). By comparing results from these two different sets of simulations, it is found that the tripole pattern is the leading mode of summer precipitation variability over eastern China with or without oceanic forcing. It can be regarded as an intrinsic atmospheric mode although air–sea interaction can modify its temporal variability. The cyclonic–anticyclonic atmospheric circulation anomaly over the northern North Pacific is identified as a key factor in both experiments. As atmospheric internal variability, it is related to a circumglobal zonal wave train propagating along the westerly jet stream. When air–sea interactions involved, modulation from SST anomalies is exerted through the meridional Pacific–Japan/East Asia–Pacific wave train propagating along the East Asian coast. Our results suggest that the North Pacific could be another key region providing potential predictability to the East Asian monsoon in addition to the Indo-Pacific.
Abstract
The spatial distribution of summer rainfall anomalies over eastern China often shows a tripole pattern with rainfall anomalies over the Yangtze River basin varies in opposite phase with North China and South China. It is not clear whether this tripole pattern is an intrinsic atmospheric mode or it is remotely forced. Using two sets of model outputs from 20 models participating in phase 5 of the Coupled Model Intercomparison Project (CMIP5), this paper investigates the driving mechanisms of this leading rainfall mode and its major influencing factors. One set (piControl) is fully coupled atmosphere–ocean simulations under constant preindustrial forcing and the other (sstClim) is atmosphere-alone models forced by prescribed climatological sea surface temperatures (SSTs). By comparing results from these two different sets of simulations, it is found that the tripole pattern is the leading mode of summer precipitation variability over eastern China with or without oceanic forcing. It can be regarded as an intrinsic atmospheric mode although air–sea interaction can modify its temporal variability. The cyclonic–anticyclonic atmospheric circulation anomaly over the northern North Pacific is identified as a key factor in both experiments. As atmospheric internal variability, it is related to a circumglobal zonal wave train propagating along the westerly jet stream. When air–sea interactions involved, modulation from SST anomalies is exerted through the meridional Pacific–Japan/East Asia–Pacific wave train propagating along the East Asian coast. Our results suggest that the North Pacific could be another key region providing potential predictability to the East Asian monsoon in addition to the Indo-Pacific.
Abstract
Changing pathways of soil moisture loss, either directly from soil (evaporation) or indirectly through vegetation (transpiration), are an indicator of ecosystem and land hydrological cycle responses to the changing climate. Based on the ratio of transpiration to evaporation, this paper investigates soil moisture loss pathway changes across China using five reanalysis-type datasets for the past and Coupled Model Intercomparison Project Phase 6 (CMIP6) climate projections for the future. The results show that across China, the ratio of vegetation transpiration to soil evaporation has generally increased across vegetated land areas, except in grasslands and croplands in north China. During 1981–2014, there was an increase by 51.4 percentage points (pps, p < 0.01) on average according to the reanalyses and by 42.7 pps according to 13 CMIP6 models. The CMIP6 projections suggest that the holistic increasing trend will continue into the twenty-first century at a rate of 40.8 pps for SSP585, 30.6 pps for SSP245, and −1.0 pps for SSP126 shared socioeconomic pathway scenarios for the period 2015–2100 relative to 1981–2014. Major contributions come from the increases in vegetation transpiration over the semiarid and subhumid grasslands, croplands, and forestlands under the influence of increasing temperatures and prolonged growing seasons (with twin peaks in May and October). The future increasing vegetation transpiration ratio in soil moisture loss implies the potential of regional greening across China under global warming and the risks of intensifying land surface dryness and altering the coupling between soil moisture and climate in regions with water-limited ecosystems.
Abstract
Changing pathways of soil moisture loss, either directly from soil (evaporation) or indirectly through vegetation (transpiration), are an indicator of ecosystem and land hydrological cycle responses to the changing climate. Based on the ratio of transpiration to evaporation, this paper investigates soil moisture loss pathway changes across China using five reanalysis-type datasets for the past and Coupled Model Intercomparison Project Phase 6 (CMIP6) climate projections for the future. The results show that across China, the ratio of vegetation transpiration to soil evaporation has generally increased across vegetated land areas, except in grasslands and croplands in north China. During 1981–2014, there was an increase by 51.4 percentage points (pps, p < 0.01) on average according to the reanalyses and by 42.7 pps according to 13 CMIP6 models. The CMIP6 projections suggest that the holistic increasing trend will continue into the twenty-first century at a rate of 40.8 pps for SSP585, 30.6 pps for SSP245, and −1.0 pps for SSP126 shared socioeconomic pathway scenarios for the period 2015–2100 relative to 1981–2014. Major contributions come from the increases in vegetation transpiration over the semiarid and subhumid grasslands, croplands, and forestlands under the influence of increasing temperatures and prolonged growing seasons (with twin peaks in May and October). The future increasing vegetation transpiration ratio in soil moisture loss implies the potential of regional greening across China under global warming and the risks of intensifying land surface dryness and altering the coupling between soil moisture and climate in regions with water-limited ecosystems.
