Patterns Of Temporal And Spatial Variability Of Surface Melt Over Greenland Ice Sheet:1958-2020

With global warming,the Arctic cryosphere has become the fastest-warming region in the world.Since the end of last century,the Greenland ice sheet(Gr IS)in the Arctic has presented constant mass loss,which has been proven to be due to increasing surface melt.In this study,the Gr IS was divided into eight regions based on the hydrologic boundary from the Ice Sat data.By regression analysis,correlation analysis,trend analysis and other statistical methods,the Gr IS surface melt changes from 1958 to 2020 were analyzed systematically and quantitatively,using surface melt flux from two high-resolution polar regional climate models,i.e.,RACMO(1 km×1km)and MAR(10 km×10 km).Furthermore,based on near-surface air temperature,surface albedo and energy flux data from RACMO(MAR)and 500 hPa geopotential height and 700 hPa wind data from atmosphere reanalysis ERA5,the potential mechanisms of the fast increase in the surface melt of the Gr IS were discussed.The main conclusions are as follows:(1)During the study period,the Gr IS averagely produced 641.6(541.7)Gt annual surface melt.The contribution of summer surface melt to annual surface melt is up to 85.3(92.2)%.The surface melt major occurred at the edge of the ablation areas and low-elevation accumulation areas.For the whole Gr IS,the multi-year averaged contribution of surface melt of 500-1500 m elevation areas was 59.4(61.0)%.At the edge of the west and southeast ice sheet,the annual surface melt was generally above 2000 mm w.e./a.(2)The Gr IS surface melt showed decreasing trend before the mid-1980s.Since the 1990s,surface melt integrated by the whole Gr IS increased fastly,with a trend of 8.2±5.3(10.0±6.0)Gt/a~2.Compared to before the 1990s,June is the month with the most significant increase in the contribution of monthly surface melt to annual total surface melt.In the terms of elevation,the areas with the most significant increase of surface melt major occurred at 1000-1500 m,and the surface melt gradually extended to high-altitude inland areas.There were obvious regional differences in the increase of surface melt,with trends from 0.5±0.7(0.9±0.9)%/a-3.6±1.8(4.3±2.2)%/a.The south-north difference was significant.The increase in the north Gr IS performed stronger than that in the south Gr IS.The increase of NO has surpassed the southwest region,the major contributor to the whole Gr IS.The relative contribution of NO has even increased by 15.5(20.3)%.(3)In the typical negative anomaly years of Gr IS surface melt,the eastern and western ice sheets often show strong negative melt characteristics.The significant negative ablation in July is the reason for the negative anomaly in the annual total melt.1992 is the year with the lowest surface melt.This year,the entire edge of the ice sheet showed a negative melt anomaly exceeding-220 mm w.e./a.In the typical years of positive anomalous ice sheet surface melt,almost all low-elevation areas showed general positive melt anomalies,and at the same time showed continuous positive anomalies in the main melt months.In 2012,the inland ice sheet also showed a positive melt anomaly of more than 10 mm w.e./a.The continuous extreme melt phenomenon from late June to early August was the reason why 2012 became the most intense year of melt on record.(4)The phase transition of the Greenland blocking index(GBI)and the North Atlantic Oscillation index(NAO)played a critical role in the increase of the Gr IS surface melt from the1990s.Since the 1900s,GBI(NAO)gradually transferred to the positive(negative)phase.Compared to the negative GBI years,a blocking high pressure centred on the southern part of the ice sheet formed in the positive GBI year,and the emergence of a 700 hPa anticyclone system brought more heat to the Greenland ice sheet.The change of GBI can explain 70%of the variance of summer surface melt anomaly.In the context of this atmosphere circulation,the occurrence and increase of surface melt were promoted by the increase in summer near-surface air temperature,decrease in summer surface albedo and the change of radiation energy flux,of which spatial difference further explained the regional difference of the increase of surface melt.