Analysis of the record-breaking August 2021 rainfall over the Greenland Ice Sheet

Rainfall was witnessed for the first time at the highest area of the Greenland Ice Sheet on 14 August,2021.The thermodynamic mechanisms supporting the rainfall are revealed by ERA5 reanalysis,in-situ and satellite data.We find that a strong southward intrusion of the polar vortex favored the maintenance of a deep cyclone over Baffin Island and an amplification of anticyclonic circulation over the southeastern ice sheet,which pumped warm and moist air toward Greenland from anomalously warm waters south of Greenland.Across a wide swath of the ice sheet,atmospheric uplift maintained above-melting and rainfall conditions via condensation and enhanced downward infrared irradiance.Without the low-level liquid clouds,the spatial extent and duration of the rainfall would have been smaller.Over the ice sheet topographic summit,the air temperature from the ground to 250 hPa level was~2℃higher than the previous record set on 12 July,2012.Such events may occur more frequently with the decreased temperature contrast between the Arctic and the mid-latitude regions that drives highly amplified jet streams.Thus,this extreme event serves as a harbinger of a more likely wet surface condition across all elevations of the ice sheet.

Co-influence of the start of thermal growing season and precipitation on vegetation spring green-up on the Tibetan Plateau

The climate in the Tibetan Plateau(TP)has undergone significant change in recent decades,mainly in thermal and water conditions,which plays a crucial role in phenological changes in vegetation spring phenology.However,how the start of the thermal growing season(SOS-T)and the start of the rainy season(SORS)as key climatic factors affect vegetation green-up remains unclear.Given that these factors characterize thermal and water conditions required for vegetation green-up,this study investigated changes in the SOS-T and SORS from 1961 to 2022,using observation-based datasets with long time series.We found that the SOS-T and SORS have advanced across the TP in 1961-2022 and have shown a spatial pattern of advancement in the east and delay in the west in 2000-2022.Further,the co-effect of temperature and precipitation change on the start of vegetation growing season(SOS-V)in 2000-2022 was observed.Averaged across TP,the SOS-V had an early onset of 1.3 d per decade during 2000-2022,corresponding to advanced SOS-T and SORS.Regionally,the SOS-V generally occurred nearly at the same time as the SOS-T in the high-altitude meadow region.A substantial delay in the SOS-V relative to the SOS-T was observed in the desert,shrub,grassland and forest regions and generally kept pace with the SORS.Furthermore,for 50%of the vegetated regions on the TP,inter-annual variation in the delay in the SOS-V relative to the SOS-T was dominated by precipitation change,which was profound in warm-climate regions.This study highlights the co-regulation of precipitation and temperature change in the SOS-V in different vegetation cover regions in the TP,offering a scientific foundation for comprehending the impact of climate change and prospects for vegetation phenology on the TP.

Impacts of Extreme Droughts on Ecosystem Water Use Efficiency Diverge between Forest and Grassland

Ecosystem-scale water use efficiency (WUE),as a quantification of the coupling between carbon and water cycle,determines whether vegetation could survive under severe drought stress.Nevertheless,how extreme droughts affect ecosystem-scale WUE and its difference among regions and biomes are still poorly understood.In this study,using data-oriented gridded products of gross primary productivity (GPP) and evapotranspiration (ET),we assessed the ecosystem WUE change during extreme drought years over China,and further compared drought impacts on WUE between forest and grassland ecosystems.We found a spatial heterogeneity of WUE change in response to extreme droughts across regions.Particularly,grassland WUE was substantially reduced concurrent with suppressed photosynthesis,while most of forest exhibited slightly decreased or even increased WUE under extreme droughts.In addition,we demonstrated that drought characteristics and environmental conditions modulated drought impacts on WUE.Temperature anomalies during droughts and leaf area were found to be the predominant factors driving WUE change for both forest and grassland.With increasing occurrence of compound dry and hot extremes,our results,therefore,would be an insightful supplement to the current understanding of the influence of extreme events on terrestrial ecosystems.

Desertification dynamics in China's drylands under climate changeAuthor links open overlay panel

Much of drylands has been hit by land degradation during the recent decades.But whether and how desertification is related to climate change is poorly understood.Here,using the Normalized Difference Vegetation Index(NDVI)time series and climatic variables,we assessed the desertification dynamics over China's drylands from 1982 to 2016 and explored the climate change impacts,with a particular focus on the influence of the 2015/2016 El Niño event.We found a fluctuant downward trend of barren land area from 1982 to the early 2010s,but followed by a sharp increase afterwards.Decrease in precipitation combined with high temperature are major contributors to the barren land expansion.During the 2015/2016 El Niño,the concurrence of severe drought and heat stress gave rise to the most serious land desertification in the record.Hyper-arid and arid drylands are the predominant contributors to the abrupt barren land area increase during 2015/2016.Our findings,therefore,highlight the climate change impacts on dryland desertification processes.Future dryland expansion and accompanying drought stress may exacerbate the risk of land degradation in these regions.