Ice Core Records As A Proxy For Deriving The Variation Of Southern Indian Ocean Sea Ice In The Recent300Years

Sea ice plays a crucial role in the global climate system: apart from its direct influence onradiative balance via its high albedo, it also has a large influence on ocean-atmosphereexchange progresses and atmospheric circulation. However, despite its importance, sea iceremains a poorly constrained component in model simulations due to the paucity of informationabout past sea ice conditions. Satellite imagery has allowed sea ice patterns to be monitored atvery high spatial and temporal resolution while this is limited only in recent decades. Forinvestigation of sea ice change beyond the satellite era, proxy-based sea ice reconstructions areneeded. Recently, progress has been made using methanesulfonic acid (MSA) and sea saltaerosol (Na~+) measured in Antarctic ice cores are used as proxies for regional sea ice extent. Anice core from LGB69site, Lambert Glacier basin, eastern Antarctica was used to findcorrelations between sea ice extent of the adjacent South Indian Ocean and the sea ice proxiesin the ice core and then to reconstruct the historical sea ice extent changes.The ice core was dated based on the well-preserved seasonal cycles of Na~+, Mg2+and Cl-,which were counted to establish the depth-age relationship with high accuracy. The102.18m icecore record extends for293years (1708-2001A.D.). The accumulated errors, attributable to afew ambiguous seasonal cycles, are estimated to be only±2years at the end of the record. Thedating results provide a good match with volcanic records used as reference layers. All thoseabove have testified the accuracy of our dating work for LGB69ice core. In comparison to theGlobal Volcanism Program, all explosive volcanic eruptions (VEI≥5) that might havedeposited large amounts of strong acids in Antarctic sheets (eruptions located in the tropics orsouthern hemisphere) have been identified in the LGB69ice core, which proves that our workis of high accuracy.The SIE used in our analysis was calculated as the mean ice edge latitude (or extent) forAugust, September and October of each year (1973-2000), which we define as SIEmax. AnnualSIEmaxwas compared with mean annual MSA concentration (centered on the following summer)and annual Na~+concentration respectively to discuss the correlations between proxies inLGB69ice core and the South Indian Ocean sea ice extent. The MSA and Na~+records werecompared with SIEmaxaveraged from around the entire Antarctic continent and with individual10°sectors, with a view to identifying a source region for MSA and Na~+deposited at LGB69.Mean annual MSA concentrations are found to be remarkable positive correlated with the SIEmaxof the70-100°E sectors (r=0.41,P<0.05,1973-2000a) and mean annual Na~+concentrations are found weakly positive correlated with the SIEmaxof the40-90°E sectors(r=0.26，P<0.2,1973-2000a). Furthermore, both the MSA and Na~+records have significantpositive correlation with the sea-ice duration of the on the edge of the sea ice extent.The meteorological conditions associated with MSA variability at LGB69are examinedusing NCAR/NCEP Re-Analysis data and surface wind data from Davis station, suggesting thatincreased (decreased) ice core MSA is associated with years of northeasterly (southwesterly)wind anomalies that from along the coastline. The meteorological conditions are also wellcorrelated with the ice core Na~+record, meaning the transport of Na~+is a key factor for the Na~+concentrations in LGB69ice core.Before1900a, there are minimum around1760s and1820s, indicating there was little seaice in the South Indian Ocean those times. The LGB69MSA series indicate an apparentdecrease in South Indian Ocean sea ice extent through the20thcentury, which is coincident withthe other reconstructions of Antarctic historical sea ice extent. There is a clear decrease du ringthe early1960s to the early1980s, followed by a slight increase in sea ice extent over the1980-2000, which could be driven by increasing greenhouse gas concentrations and bystratospheric ozone depletion according model simulations.On the whole, sea ice extent of South Indian Ocean has been in a stable state while it hasgone decrease and increase last300years. Even on the background of global warming, changesof South Indian Ocean sea ice extent are within its natural variability. That is to say, SouthIndian Ocean sea ice changes do not in a great extent by the effects of Global Warming.