Antarctic Sea Ice Region Cloud-radiation Flux Changes And Their Climatic-environmental Effects

In the context of global warming,the climate change in Polar Regions is also receiving increasing scientific attention due to it as an"indicator"and"amplifier"of global climate change.However,in contrast to the dramatic decline of Arctic sea ice in recent decades,Antarctic sea ice has shown an increasing trend,which has attracted extensive attention and research.Although the Antarctic sea ice rise phenomenon has been explained by many scholars from different perspectives,the current study of cloud-radiation fluxes on the surface of Antarctic sea ice has many shortcomings due to the special geographical location of Antarctic,severe weather conditions and natural environment.The main problems with the existing studies are the short time span,the small spatial scale,the single parameter of the analysis and the use of fewer data sources.However,the surface radiation balance is closely linked to the melting and growing of sea ice,and further knowledge and understanding of changes in cloud-radiation fluxes in the Antarctic sea ice region require a comprehensive study based on the inadequacy of existing research.The surface radiation balance of Antarctic sea ice mainly refers to the surface net radiation flux(SNR)and cloud radiative forcing effect(CRF).The SNR mainly determines the balance of radiation energy at the sea ice surface.Generally,long-wave(LW)emitted by the earth's surface has a cooling effect on the surface,while short-wave(SW)radiation from the sun has a heating effect on the surface.The total net radiation flux(Rnet)to the sea ice surface is calculated from the LW net radiation(LWnet)and SW net radiation(SWnet).When Rnet is positive,it means that the sea ice is in a state of heat absorption;when Rnet is negative,it means that the sea ice is in a state of heat release.The thicker clouds over Antarctica have a significant impact on the estimation of surface radiation fluxes,it not only reflecting a large amount of downwelling SW radiation(SW?)back into space,but also blocking a large amount of upwelling LW radiation(LW?)from escaping into space,so the CRF in the Antarctic sea ice region is stronger.The study of the SNR has significant implications for understanding changes in sea ice,while the radiative forcing effects caused by clouds are of great help in studying Antarctic climate change,and their systematic analysis is therefore essential.The polar day phenomenon in Antarctica during the summer causes a large amount of SW?to reach the sea ice surface,leading to various degrees of melting of sea ice in different regions of Antarctica,indicating the complex and variable features of the Antarctic climate change during the summer.Sea ice concentration(SIC),surface albedo(SAL)and sea surface temperature(SST)have significant implications for the estimation of radiation fluxes at the sea ice surface.Based on the changing characteristics of SIC,SAL,and SST,combined with studies on the radiation balance at the Antarctic sea ice surface,it is possible to provide in-depth analysis and understanding of the changes in the Antarctic climate environment during the summer.The main findings of this paper are the following:(1)Three-dimensional histograms and two-dimensional planograms showed that the monthly average of cloud fractional cover(CFC)of both the entire Antarctic sea ice region and five other small areas has a demarcation line around 2000.Time-series changes in CFC between 1982-2015 show a downward trend in both annual and seasonal averages.At the same time,through the seasonal changes in the monthly averages,it can be found that the highest and lowest values of CFC in a year occur in summer and winter,respectively,the highest value in December can reach about 90%,while the lowest value in June and July is less than 65%.The spatial distribution of seasonal averages shows that CFC is above 90%in most areas during the summer,while the Weddell Sea and the Ross Sea exist large areas of low values of CFC in the winter.(2)As LW?escapes into outer space,LWnet is in a negative state all year round,which mainly has a cooling effect on the Antarctic sea ice surface,with the strongest cooling effect in summer,around-50 W m^-2.Because of the polar night phenomenon,the SWnet is almost 0 W m^-2 in winter,while it reaches up to around 100 W m^-2 in summer,causing a stronger heating effect on the Antarctic sea ice surface.Rnet is mainly influenced by LWnet in winter and SWnet in summer,so it has cooling and heating effects on the sea ice surface in winter and summer,respectively.The summer averages of net radiation fluxes in the Antarctic sea ice region show a decreasing trend between 1982-2015,with LWnet,SWnet and Rnet decreasing at-0.722 W m^-2/decade,-3.652 W m^-2/decade and-4.398 W m^-2/decade,respectively.The spatial distribution of Rnet summer averages in the Antarctic sea ice region shows that it causes stronger heating effects at low latitudes than at high latitudes.Rnet shows a decreasing trend in most of the Antarctic Ocean except for partial regions of the Bellingshausen Sea and the Amundsen Sea.(3)The blocking effect of clouds on LW?and the reflection effect of clouds on SW?,which can cause LW CRF and SW CRF to produce about 100 W m^-2 insulating effect and about-150 W m^-2 cooling effect on the Antarctic sea ice surface in summer,respectively.So the All-wave CRF mainly exhibits a cooling effect in the summer,about-30 W m^-2.The summer average of the LW CRF in the Antarctic sea ice region showed a steady and slowly downward trend between 1982-2015,while the time-varying curves of the SW CRF showed a trough phenomenon around 1994/1995.The summer averages of All-wave CRF can cause a cooling effect of less than-50 W m^-2 at the sea-ice margin,which manifests as a heating effect in the other three seasons.Except for some areas near the Antarctic continent,the All-wave CRF shows a declining trend of varying degrees over most of the Antarctic Ocean,especially from the east of the Weddell Sea to the 135°E region,where it declining rate faster than-2 W m-2/decade.(4)Except for the Bellingshausen-Amundsen Sea,the summer averages of SAL between 1982-2015 showed an upward trend in all other regions,therefore,the SAL in the entire Antarctic sea ice region showed an increasing trend,with a rate of 0.851%/decade.The variation curve of SIC is very similar to that of SAL,indicating a strong positive correlation between them.The spatial distribution of summer averages shows that the highest values of SAL and SIC exist in the Weddell Sea,above 60%and 80%,respectively,while SST has the lowest values in that region,below-3°C.The detection experiments of transition points in SAL,SIC and SST suggested that a climate turning point maybe existed in the entire Antarctic sea ice region around 2001.A comparison of SAL trends around2001 reveals that the climate of West Antarctica has transformed from warming before2001 to cooling after 2001 for most areas,especially in the east of the Antarctic Peninsula,while the climate in East Antarctica had shown a cooling trend in recent decades.