State of the Antarctic and Southern Ocean Climate System

This paper reviews developments in our understanding of the state of the Antarctic and Southern Ocean climate and its relation to the global climate system over the last few millennia. Climate over this and earlier periods has not been stable, as evidenced by the occurrence of abrupt changes in atmospheric circulation and temperature recorded in Antarctic ice core proxies for past climate. Two of the most prominent abrupt climate change events are characterized by intensification of the circumpolar westerlies (also known as the Southern Annular Mode) between ∼6000 and 5000 years ago and since 1200–1000 years ago. Following the last of these is a period of major trans-Antarctic reorganization of atmospheric circulation and temperature between A.D. 1700 and 1850. The two earlier Antarctic abrupt climate change events appear linked to but predate by several centuries even more abrupt climate change in the North Atlantic, and the end of the more recent event is coincident with reorganization of atmospheric circulation in the North Pacific. Improved understanding of such events and of the associations between abrupt climate change events recorded in both hemispheres is critical to predicting the impact and timing of future abrupt climate change events potentially forced by anthropogenic changes in greenhouse gases and aerosols. Special attention is given to the climate of the past 200 years, which was recorded by a network of recently available shallow firn cores, and to that of the past 50 years, which was monitored by the continuous instrumental record. Significant regional climate changes have taken place in the Antarctic during the past 50 years. Atmospheric temperatures have increased markedly over the Antarctic Peninsula, linked to nearby ocean warming and intensification of the circumpolar westerlies. Glaciers are retreating on the peninsula, in Patagonia, on the sub-Antarctic islands, and in West Antarctica adjacent to the peninsula. The penetration of marine air masses has become more pronounced over parts of West Antarctica. Above the surface, the Antarctic troposphere has warmed during winter while the stratosphere has cooled year-round. The upper kilometer of the circumpolar Southern Ocean has warmed, Antarctic Bottom Water across a wide sector off East Antarctica has freshened, and the densest bottom water in the Weddell Sea has warmed. In contrast to these regional climate changes, over most of Antarctica, near-surface temperature and snowfall have not increased significantly during at least the past 50 years, and proxy data suggest that the atmospheric circulation over the interior has remained in a similar state for at least the past 200 years. Furthermore, the total sea ice cover around Antarctica has exhibited no significant overall change since reliable satellite monitoring began in the late 1970s, despite large but compensating regional changes. The inhomogeneity of Antarctic climate in space and time implies that recent Antarctic climate changes are due on the one hand to a combination of strong multidecadal variability and anthropogenic effects and, as demonstrated by the paleoclimate record, on the other hand to multidecadal to millennial scale and longer natural variability forced through changes in orbital insolation, greenhouse gases, solar variability, ice dynamics, and aerosols. Model projections suggest that over the 21st century the Antarctic interior will warm by 3.4° ± 1°C, and sea ice extent will decrease by ∼30%. Ice sheet models are not yet adequate enough to answer pressing questions about the effect of projected warming on mass balance and sea level. Considering the potentially major impacts of a warming climate on Antarctica, vigorous efforts are needed to better understand all aspects of the highly coupled Antarctic climate system as well as its influence on the Earth\u27s climate and oceans

Snowpack properties in Dronning Maud Land, Antarctica, compared to Envisat ASAR and scatterometer measurements

The knowledge of snow pack properties and its temporal and spatialvariability is of importance for the interpretation of backscatteredsignals in the microwave region. For example, forthcoming radaraltimetric satellite missions, like ESA's CRYOSAT, aim at improvedmeasurements of both ice surface height and surface height fluctuation,which is especially needed at the margins of the continental ice sheetswhere the topography is more complex.This investigation focuses on the area between the German Neumayer base(70°39'S, 08°15'W) at the ice shelf Ekströmisen and the drilling sitefor the EPICA (European Project on Ice Coring in Antarctica) ice core atKohnen station (75°S, 0°W, 2850m a.s.l.) in Dronning Maud Land. ERSwindscatterometer data are used to retrieve the incidence angledependence and azimuthal anisotropy of backscattering of various snowpack types. These parameters are used to normalize Envisat ASAR(Advanced Synthetic Aperture Radar) wide swath data, which were acquiredin 2004 using the same polarization. Envisat data are compared to snowaccumulation, derived by stake readings and firn core drilling, and snowpack properties, derived from snow pit measurements. Finally,backscattering data in C-VV are compared to C-HH (Envisat, Radarsat) andKu-VV and Ku-HH (NSCAT).The analysis demonstrates that coarse resolution scatterometer data andhigh resolution SAR data can be usefully merged over the flat terrain ofthe Antarctic inland in order to improve the classification of varioussnowpack types

Multipolarizational signatures of snow compared to snowpack properties in Dronning Maud Land, Antarctica

The knowledge of snow pack properties and its temporal and spatial variability are of importance for the interpretation of backscattered signals in the microwave region. For example, forthcoming radar altimetric satellite missions, like ESA's CRYOSAT, aim at improved measurements of both ice surface height and surface height fluctuation, which is especially needed at the margins of the continental ice sheets where the topography is more complex. However, a change in surface height does not imply a mass imbalance, as snow and ice density may change. Spaceborne scatterometers provide valuable information on backscattering characteristics at low spatial but high temporal resolution. In addition, the surface is illuminated at different viewing angles during a single overflight. Previous studies demonstrated that anisotropies in the scatterometer signal are related to surface features like sastrugis, which also cause anomalies in the altimeter signal.This investigation focuses on the area between the German Neumayer base (70°39'S, 08°15'W) at the ice shelf Ekströmisen and the drilling site for the EPICA (European Project on Ice Coring in Antarctica) ice core at Kohnen station (75°S, 0°W, 2850m a.s.l.) in Dronning Maud Land. The backscattering properties in Ku band at vertical and horizontal polarization from two satellite borne scatterometers are compared at several sites to snow pack properties such as accumulation rate and grain size. In addition, SAR (Synthetic Aperture Radar) images with high spatial resolution are compared to long term accumulation rates around Kohnen station, which are derived by means of ice penetrating radar and firn cores.Differences in the backscattering signatures reflect the variable pattern in the snow morphology, although the relation is not straightforward. The normalized backscattering cross section is classified to localize those areas, where additional field measurements should be conducted to obtain an improved understanding of backscattering and snowpack properties. The signature study provides additional information for a better understanding of signals like radar altimeter and high resolution SAR

Spatial distribution of surface mass balance on Amundsenisen plateau, Antarctica, derived from ice-penetrating radar studies

The distribution of surface mass balance on Amundsenisen, Dronning Maud Land, Antarctica, is investigated along a continous profile line. Ice-penetrating radar is used to map variations in ice-layer thickness within the upper 100 m of the ice sheet. The route passes several firn- and ice- core drilling sites over a distance of 320 km. Dielectric-profiling data of ice cores is used to calculate the depths of selected reflection horizons and the cumulative mass of the ice column. The local surface mass balance is determined as a temporal average, covering a time span of almost two centuries. The findings indicate a complex accumulation pattern superimposed on a general low surface mass balance, which is related to small-scale surface undulations. The results of the radar soundings are in general in good agreement with surface mass balance data derived from firn-core studies. Discrepancies between these two datasets can be explained by spatial mismatch or by minor quality of either ice core profiles or radar data. For regional comparison of radar-based accumulation data we use an accumulation distribution interpolated from point measurements. The surface mass balance varies up to 50 % over short distances, with correlation lengths of <10 km. We conclude that the current utilization schemes of point sampling are only capable of reproducing local values and regional trends but provide no information on the small-scale variability of surface mass balance

Räumliche Verteilung von Schneeakkumulation und Schneedeckeneigenschaften in Dronning Maud Land, Antarktis: Observationstechniken und Methoden der Netto-Massenbilanzbestimmung polarer Eisschilde

Primary objective of this thesis is the investigation of the spatial distribution of snow accumulation and its influencing factors in Dronning Maud Land (DML), Antarctica, by using a combination of data from different observation techniques. Field measurements of surface mass balance are only sparsely available for most parts of the Antarctic ice sheet. The main focus of this study is on exploring the potential of radar satellite imagery for a spatially continuous assessment of accumulation patterns in high resolution. For this, radar backscattering properties of the Antarctic snowpack are analysed by means of low-resolution C- and Ku-band scatterometer data to establish links between returned radar signals and snow morphology. As a result, a continent-wide snowpack classification is presented, which shows similar patterns for both frequencies. Furthermore, the narrow percolation zone at the margin of the ice sheet could be distinguished from the interior dry-snow zone. This study contributes to a better understanding of the complexity of snow accumulation in the Antarctic

Abschätzung der physikalisch-morphologischen Eigenschaften sowie Akkumulation der polaren Schneedecke mit aktiven Mikrowellensensoren in Dronning Maud Land, Antarktis.

Durch die Kombination verschiedener satellitengestützter aktiver Mikrowellensensoren, in erster Linie niedrig auflösendes Scatterometer und hochauflösendes SAR, soll ein Beitrag zur präziseren Abschätzung von Schneeeigenschaften sowie der räumlichen Variation der Akkumulation polarer Eisschilde geleistet werden. Scatterometer erlauben die Erfassung der Rückstreueigenschaften einer Oberfläche mit umfangreicher Blick- und Einfallswinkelgeometrie. Durch Ableitung bestimmter Parameter können Regionen gleicher Rückstreueigenschaften abgegrenzt werden. Innerhalb gewisser Schneedeckenklassen ist es möglich, Akkumulationsraten durch einfache lineare Regression aus SAR-Daten zu parametrisieren und dadurch zu einer flächenhaften, hochaufgelösten Abschätzung der Massenbilanz einer Region zu gelangen. Voraussetzung hierfür sind Bodenreferenzdaten, welche in Form von Schneepegelmessungen, Schneeschachtstudien, Firnkernbohrungen und Bodenradarmessungen (GPR) zur Verfügung stehen. Die Ergebnisse werden verglichen mit einer aktuellen Akkumulationskarte für Dronning Maud Land, Antarktis, interpoliert aus ca. 120 Firnkerndaten, woraus sich die Repräsentativität derartiger Punktmessungen für den vorliegenden Raum bewerten lässt