Dynamic Changes Of Ice Shelves/glaciers In The East Indian Ocean Sector In Antarctica

The East Antarctic Ice Sheet has the world's largest freshwater reserves.The accurate estimation of its mass balance has become one of the most uncertain factors for the prediction of future sea level rise.Nevertheless,because of the conventional view that the East Antarctic Ice Sheet is relatively stable and is not highly sensitive to global warming,it receives less attention than the Greenland Ice Sheet and West Antarctic Ice Sheet.However,recent observations suggest that the East Antarctica Ice Sheet may be more vulnerable than previously thought.Large sections of the East Antarctic Ice Sheet are grounded below sea level and are deemed vulnerable to marine ice sheet instability.The marine-based ice sheets in East Antarctica are five times those in West Antarctica and are thus enough to raise the global sea level by 19 m.At present,the monitoring of the dynamic changes of the East Antarctic Ice Sheet is gradually increasing,but we still lack a deep understanding of its response mechanism to global warming.For example,how will the rising temperatures,warming oceans,and changes in sea ice concentration affect glacier movement? What are the temporal and spatial characteristics of the ice shelf basal melt? What are the main factors that control the ice–ocean interaction at the bottom? The lack of a thorough understanding of these issues also indirectly leads to great uncertainties in estimating the contribution of the East Antarctic Ice Sheet to sea level rise.Therefore,on the basis of multi-source remote sensing data,this study focuses on the dynamic changes of ice shelves/glaciers,including glacier flow rate,melt rate at the bottom of the ice shelf,and ice sheet mass balance.The causes of these dynamic changes and the feedback mechanism of ice shelves/glaciers to global warming are also investigated.The physical mechanism of the interannual and seasonal variations of ice velocity is analyzed on the basis of an accurate estimation of ice velocity and other environmental factors.The spatial and temporal distributions of ice shelf basal melting are obtained.The mass balance of the ice sheet is precisely assessed as well.The main research contents and results are as follows.(1)Study on glacier surface flowMulti-source remote sensing images are used to study the dynamic changes of the ice velocity of the Polar Record Glacier,which is located near the Zhongshan station in East Antarctica.The spatial and temporal characteristics of interannual and seasonal ice velocity changes are analyzed on the basis of the measurements between 2005 and 2015.The results show a long-term increasing trend for ice velocity over the ice shelf,but no obvious acceleration trend is found at the ice sheet section.The general trend of seasonal variation shows that the ice flow starts to accelerate from September/October,reaches the highest velocity in December,and then maintains the maximum speed until January.From February to April,the velocity declines rapidly and reaches the lowest point of the year.Then,the influence of a series of environmental factors on the change of flow velocity is analyzed on the basis of air temperature,sea water temperature,glacier surface melt area,and sea ice concentration.The interannual change of glacier velocity is found to be mainly controlled by the invasion of the ice shelf cavity by the warm m CDW.The intrusion of m CDW into the cavity leads to strong melting at the bottom of the ice shelf,and the thinning of the ice shelf leads to the reduction of back stress,thus accelerating the glacier.A significant correlation exists between annual speed and mean ocean temperatures.The seasonal velocity change is determined by different factors in different stages.Sea ice begins to lose its rigidity and gradually breaks up in spring.This condition initiates ice shelf acceleration in September/October.Thereafter,ice flow acceleration may be mainly related to glacier surface meltwater.Flow speed and air temperatures reach a maximum in early January,particularly in the area of supraglacial melt lakes;this condition indicates peak meltwater production.We therefore hypothesize that summer acceleration is due to the weakening of the ice shelf by the englacial penetration of meltwater and/or filling of crevasses.Such phenomenon increases ice temperature.(2)Study on ice shelf basal meltThe elevation change rate and basal melting rate of the Shackleton ice shelf are studied and analyzed using satellite altimetry data.A model is used to calculate the elevation and elevation change rate of the ice shelf surface on the basis of Cryo Sat-2 data.The average elevation change rate of the ice shelf surface is-0.084±0.39 m/yr in the Eulerian framework,with the highest value reaching approximately 1 m/yr.By contrast,the average elevation change rate in the Lagrangian framework is-0.26±0.84 m/yr.Comparing the spatial distributions of the two results indicates that the noise of the resulting Lagrangian elevation change is obviously reduced and presents obvious spatial characteristics.The mean value of the RMSE decreases from 0.92 m/yr in the Eulerian framework to 0.78 m/yr in the Lagrangian framework,indicating that the Lagrangian approach is best suited in areas with rough surfaces and significant advection.Then,the spatial distribution characteristics of the total basal melt and melt rate of the Shackleton shelf are estimated by using the Lagrangian elevation change rate.The basal mass balance of the Shackleton ice shelf from 2010 to 2018 was 53.3±9.4 Gt/yr,with the average melt rate being 2.1±0.4 m/yr,which is slightly less than that recorded in 2003–2008.The basal melt of the ice shelf near the ground line is greater than 50 m/yr in some areas and is much higher than previously estimated.The value is even greater than that for the Totten ice shelf,which is considered to have the strongest basal melt in East Antarctica.Finally,our analysis shows that the bedrock elevation data of the Shackleton ice shelf area are prone to errors.This drawback prevents researchers from carrying out a detailed study of the ice–ocean interaction at the bottom of the ice shelf and indicates the need for ocean/ice shelf observations in the Shackleton ice shelf area.(3)Study on mass balance of ice sheet basinThe ice shelf surface velocity of Wilkes Land in 2018 is calculated,and the changes in velocity between 2008 and 2018 are studied and analyzed.Then,the ice fluxes of the three basins are calculated on the basis of the results of ice velocity,and the changes of the ice fluxes between 2008 and 2018 are analyzed.For the C-Cp basin,the ice flux in 2018 was 118.1±35.9 Gt/yr while the ice flux in 2008 was 112.8±34.5 Gt/yr,which is about 5.4 Gt/yr lower than that in 2018.We speculate that the increase of the ice flux in 2018 is mainly caused by the acceleration of the Denman glacier.For the Cp-D basin,the ice flux in 2018 was 219.8±29.8 Gt/yr,and the ice flux in 2008 was 219.0±25.1 Gt/yr.The ice flux did not change significantly between 2008 and 2018,with the difference being 0.8 Gt/yr only.This behavior is consistent with the steady state of the Totten glacier located in this basin after 2007.For the D-Dp basin,the ice flux in 2018 was 105.9±13.1 Gt/yr,and the ice flux in 2008 was 104.9±13.5 Gt/yr.Similar to that in the Cp-D basin,the ice flux in the D-Dp basin did not change significantly from 2008 to 2018,with the difference being 0.9 Gt/yr only.The total ice flux in Wilkes Land increased by about 7.1 Gt/yr from 2008 to 2018.Considering the uncertainty of the ice flux calculation,we believe that the total ice flux in the Wilkes Land remained stable from 2008 to 2018.The mass balance of Wilkes Land,combined with the SMB data,is calculated.The results show that all the three basins were in weak positive mass balance between 2008 and 2018,with the values being 11.8±43.4,8.6±53.2,and 13.9±27.4 Gt/yr.In general,Wilkes Land is in a slightly positive mass balance.As a result of the continuous positive mass balance in Dronning Maud Land,we speculate that East Antarctica was still in a weak positive mass balance state between 2008 and 2018.