Tas D. van Ommen

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 AD1700 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 multi-decadal variability and anthropogenic effects and, as demonstrated by the paleoclimate record, on the other hand to multi-decadal 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° ± 1oC, 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 Earths climate and oceans.

A dynamic early East Antarctic Ice Sheet suggested by ice-covered fjord landscapes

The first Cenozoic ice sheets initiated in Antarctica from the Gamburtsev Subglacial Mountains and other highlands as a result of rapid global cooling ∼34 million years ago. In the subsequent 20 million years, at a time of declining atmospheric carbon dioxide concentrations and an evolving Antarctic circumpolar current, sedimentary sequence interpretation and numerical modelling suggest that cyclical periods of ice-sheet expansion to the continental margin, followed by retreat to the subglacial highlands, occurred up to thirty times. These fluctuations were paced by orbital changes and were a major influence on global sea levels. Ice-sheet models show that the nature of such oscillations is critically dependent on the pattern and extent of Antarctic topographic lowlands. Here we show that the basal topography of the Aurora Subglacial Basin of East Antarctica, at present overlain by 2–4.5 km of ice, is characterized by a series of well-defined topographic channels within a mountain block landscape. The identification of this fjord landscape, based on new data from ice-penetrating radar, provides an improved understanding of the topography of the Aurora Subglacial Basin and its surroundings, and reveals a complex surface sculpted by a succession of ice-sheet configurations substantially different from today’s. At different stages during its fluctuations, the edge of the East Antarctic Ice Sheet lay pinned along the margins of the Aurora Subglacial Basin, the upland boundaries of which are currently above sea level and the deepest parts of which are more than 1 km below sea level. Although the timing of the channel incision remains uncertain, our results suggest that the fjord landscape was carved by at least two iceflow regimes of different scales and directions, each of which would have over-deepened existing topographic depressions, reversing valley floor slopes.

Calibrating the ice core paleothermometer using seasonality

High-resolution oxygen isotope measurements on the Dome Summit South (DSS) ice core from Law Dome, Antarctica, provide a seasonal profile that is sufficiently stable and undistorted by biases in accumulation to permit comparison with measured temperature seasonality. This comparison yields an isotope-temperature relation with a gradient (dδ/dT) of 0.44±0.02‰/°C and provides a new method for exploring the isotope-temperature relationship at high-accumulation sites. If applied to the observed isotope record from the DSS core, which extends through the last glacial and beyond, this calibration suggests that at its coldest the last glaciation was ∼13°C colder than present at this site (after correcting for elevation change). This finding compares with a temperature difference of ∼8°C computed by using the local spatially derived calibration.

Paleoclimate Data-Model Comparison and the Role of Climate Forcings over the Past 1500 Years*

The past 1500 years provide a valuable opportunity to study the response of the climate system to external forcings. However, the integration of paleoclimate proxies with climate modeling is critical to improving the understanding of climate dynamics. In this paper, a climate system model and proxy records are therefore used to study the role of natural and anthropogenic forcings in driving the global climate. The inverse and forward approaches to paleoclimate data–model comparison are applied, and sources of uncertainty are identified and discussed. In the first of two case studies, the climate model simulations are compared with multiproxy temperature reconstructions. Robust solar and volcanic signals are detected in Southern Hemisphere temperatures, with a possible volcanic signal detected in the Northern Hemisphere. The anthropogenic signal dominates during the industrial period. It is also found that seasonal and geographical biases may cause multiproxy reconstructions to overestimate the magnitude of the long-term preindustrial cooling trend. Inthesecondcasestudy,themodelsimulationsarecomparedwithacorald 18 OrecordfromthecentralPacific Ocean. It is found that greenhouse gases, solar irradiance, and volcanic eruptions all influence the mean state of the central Pacific, but there is no evidence that natural or anthropogenic forcings have any systematic impact on El Ni~ Oscillation. The proxy climate relationship is found to change over time, challenging the assumption of stationarity that underlies the interpretation of paleoclimate proxies. These case studies demonstrate the value of paleoclimate data–model comparison but also highlight the limitations of current techniques and demonstrate the need to develop alternative approaches.

High-precision dating of volcanic events (A.D. 1301–1995) using ice cores from Law Dome, Antarctica

A record of volcanic activity over the period A.D. 1301–1995 has been extracted from three Law Dome ice cores (East Antarctica). The record dating is unambiguous at the annual level from A.D. 1807 to 1995 and has an uncertainty of ±1 year at A.D. 1301. Signals from 20 eruptions are preserved in the record, including those of two unknown eruptions with acid deposition beginning in A.D. 1810.8 and A.D. 1685.8. The beginning of the ice core signal from the A.D. 1815 Tambora eruption is observed in the austral summer of A.D. 1816/1817. The mean observed stratospheric transport and deposition time to Law Dome from the eruption site is 1.5 years (σ = 0.6 years) from 11 well-dated eruptions. The largest eruption observed in the Law Dome record has its maximum in A.D. 1460 with volcanic sulfate deposition beginning in the austral winter of A.D. 1459. This event is also observed in other ice core records and is attributed to the volcano Kuwae, with an eruption date in the range A.D. 1455.9–1459.9 if all sources of error are considered. This is at least three years later than the date previously ascribed by dendrochronological and historical studies.

Deglacial and Holocene changes in accumulation at Law Dome, East Antarctica

Abstract Dating constraints have been combined with an ice-flow model to estimate surface accumulation rates at Law Dome, East Antarctica, to approximately 80 kyr BP. Results indicate that the present high-accumulation regime (~0.7ma–1 ice equivalent) was established some time after ~7 kyr BP, following an increase of approximately 80% from early to mid-Holocene. The accumulation rate at the Last Glacial Maximum is estimated at less than ~10% of the modern value. The record reveals an approximately linear dependence between temperature (inferred from isotope ratio) and accumulation rate through the glacial period. This dependence breaks down in the early Holocene, and this is interpreted as a change to a mode in which moisture-transport changes have a stronger influence on accumulation than temperature (via absolute humidity). The changes in accumulation, including the large change in the early to mid-Holocene, are accompanied by changes in sea-salt concentrations which support the hypothesis that Law Dome climate has shifted from a glacial climate, more like that of the present-day Antarctic Plateau, to its current Antarctic maritime climate. The change between these two modes occurred progressively through the early Holocene, possibly reflecting insolation-driven changes in atmospheric moisture content and circulation.

A Millennial Proxy Record of ENSO and Eastern Australian Rainfall from the Law Dome Ice Core, East Antarctica

ENSO causes climate extremes across and beyond the Pacific basin; however, evidence of ENSO at high southern latitudes is generally restricted to the South Pacific and West Antarctica. Here, the authors report astatisticallysignificantlinkbetweenENSOandseasaltdepositionduringsummerfromtheLawDome(LD) ice core in East Antarctica. ENSO-related atmospheric anomaliesfrom the central-western equatorialPacific (CWEP) propagate to the South Pacific and the circumpolar high latitudes. These anomalies modulate high- latitude zonal winds, with El Nino (La Nina) conditions causing reduced (enhanced) zonal wind speeds and subsequent reduced (enhanced) summer sea salt deposition at LD. Over the last 1010 yr, the LD summer sea salt(LDSSS)recordhasexhibitedtwobelow-average(ElNino-like)epochs,1000-1260 ADand1920-2009 AD, and a longer above-average (La Nina-like) epoch from 1260 to 1860 AD. Spectral analysis shows the below- average epochs are associated with enhanced ENSO-like variability around 2-5 yr, while the above-average epoch is associated more with variability around 6-7 yr. The LDSSS record is also significantly correlated with annual rainfall in eastern mainland Australia. While the correlation displays decadal-scale variability similar to changes in the interdecadal Pacific oscillation (IPO), the LDSSS record suggests rainfall in the modern instrumental era (1910-2009 AD) is below the long-term average. In addition, recent rainfall declines in some regions of eastern and southeastern Australia appear to be mirrored by a downward trend in the LDSSS record, suggesting current rainfall regimes are unusual though not unknown over the last millennium.

The International Trans-Antarctic Scientific Expedition (ITASE): an overview

Abstract From its original formulation in 1990 the International Trans-Antarctic Scientific Expedition (ITASE) has had as its primary aim the collection and interpretation of a continent-wide array of environmental parameters assembled through the coordinated efforts of scientists from several nations. ITASE offers the ground-based opportunities of traditional-style traverse travel coupled with the modern technology of GPS, crevasse detecting radar, satellite communications and multidisciplinary research. By operating predominantly in the mode of an oversnow traverse, ITASE offers scientists the opportunity to experience the dynamic range of the Antarctic environment. ITASE also offers an important interactive venue for research similar to that afforded by oceanographic research vessels and large polar field camps, without the cost of the former or the lack of mobility of the latter. More importantly, the combination of disciplines represented by ITASE provides a unique, multidimensional (space and time) view of the ice sheet and its history. ITASE has now collected >20 000km of snow radar, recovered more than 240 firn/ice cores (total length 7000 m), remotely penetrated to ~4000m into the ice sheet, and sampled the atmosphere to heights of >20 km.

A 700 year record of Southern Hemisphere extratropical climate variability

Abstract Annually dated ice cores from West and East Antarctica provide proxies for past changes in atmospheric circulation over Antarctica and portions of the Southern Ocean, temperature in coastal West and East Antarctica, and the frequency of South Polar penetration of El Niño events. During the period AD 1700–1850, atmospheric circulation over the Antarctic and at least portions of the Southern Hemisphere underwent a mode switch departing from the out-of-phase alternation of multi-decadal long phases of EOF1 and EOF2 modes of the 850 hPa field over the Southern Hemisphere (as defined in the recent record by Thompson and Wallace, 2000; Thompson and Solomon, 2002) that characterizes the remainder of the 700 year long record. From AD 1700 to 1850, lower-tropospheric circulation was replaced by in-phase behavior of the Amundsen Sea Low component of EOF2 and the East Antarctic High component of EOF1. During the first phase of the mode switch, both West and East Antarctic temperatures declined, potentially in response to the increased extent of sea ice surrounding both regions. At the end of the mode switch, West Antarctic coastal temperatures rose and East Antarctic coastal temperatures fell, respectively, to their second highest and lowest of the record. Polar penetration of El Niño events increased during the mode switch. The onset of the AD 1700–1850 mode switch coincides with the extreme state of the Maunder Minimum in solar variability. Late 20th-century West Antarctic coastal temperatures are the highest in the record period, and East Antarctic coastal temperatures close to the lowest. Since AD 1700, extratropical regions of the Southern Hemisphere have experienced significant climate variability coincident with changes in both solar variability and greenhouse gases.

Ice‐core evidence for a small solar‐source of atmospheric nitrate

A precision-dated ice-core nitrate record from Law Dome, coastal East Antarctica is compared to the timing of known solar proton events and geomagnetic storms over the period 1888 to 1995. We find statistical evidence for a significant (P < 0.03) but small elevation in mean nitrate concentrations following the solar events (∼11% over the 12 months beginning 3 months post-event). While some solar events are identifiable in the nitrate record, most are not distinguishable from the background noisy signal (which has numerous large peaks), and some solar events show no nitrate elevation above even mean levels.