Evidence for an early Holocene climatic optimum in the Antarctic deep ice-core record

In the interpretation of the Antarctic deep ice-core data, little attention has been given to the Holocene part of the records. As far as translation of the stable isotope content in terms of temperature is concerned, this can be understood because expected temperature changes may be obscured by isotopic noise of various origins and because no ¹⁴C dating has yet been available for this type of sequence. In this article, we focus on the Dome C and Vostok cores and on a new 850-m long ice core drilled out at Komsomolskaïa by the Soviet Antarctic Expeditions. These three sites are located in East Antarctica, on the Antarctic plateau, in a region essentially undisturbed by ice-flow conditions, so that their detailed intercomparison may allow us to identify the climatically significant isotopic signal. Our results compare well with the proximal records of Southern Hemisphere high latitudes and support the existence of a warmer climatic optimum between 10 and 6 ka y BP. Maximum temperatures are reached just at the end of the last deglaciation, which confirms previous observations at high latitudes, in contrast with later dates for the Atlantic and hypsithermal optima in Europe and North America.Contribution to Clima Locarno — Past and Present Climate Dynamics; Conference September 1990, Swiss Academy of Sciences — National Climate Program     

High resolution simulation of recent Arctic and Antarctic stratospheric chemical ozone loss compared to observations

Simulations of polar ozone losses were performed using the three-dimensional high-resolution (1^∘ × 1^∘) chemical transport model MIMOSA-CHIM. Three Arctic winters 1999–2000, 2001–2002, 2002–2003 and three Antarctic winters 2001, 2002, and 2003 were considered for the study. The cumulative ozone loss in the Arctic winter 2002–2003 reached around 35% at 475 K inside the vortex, as compared to more than 60% in 1999–2000. During 1999–2000, denitrification induces a maximum of about 23% extra ozone loss at 475 K as compared to 17% in 2002–2003. Unlike these two colder Arctic winters, the 2001–2002 Arctic was warmer and did not experience much ozone loss. Sensitivity tests showed that the chosen resolution of 1^∘ × 1^∘ provides a better evaluation of ozone loss at the edge of the polar vortex in high solar zenith angle conditions. The simulation results for ozone, ClO, HNO₃, N₂O, and NO _y for winters 1999–2000 and 2002–2003 were compared with measurements on board ER-2 and Geophysica aircraft respectively. Sensitivity tests showed that increasing heating rates calculated by the model by 50% and doubling the PSC (Polar Stratospheric Clouds) particle density (from 5 × 10⁻³ to 10⁻² cm⁻³) refines the agreement with in situ ozone, N₂O and NO _y levels. In this configuration, simulated ClO levels are increased and are in better agreement with observations in January but are overestimated by about 20% in March. The use of the Burkholder et al. (1990) Cl₂O₂ absorption cross-sections slightly increases further ClO levels especially in high solar zenith angle conditions. Comparisons of the modelled ozone values with ozonesonde measurement in the Antarctic winter 2003 and with Polar Ozone and Aerosol Measurement III (POAM III) measurements in the Antarctic winters 2001 and 2002, shows that the simulations underestimate the ozone loss rate at the end of the ozone destruction period. A slightly better agreement is obtained with the use of Burkholder et al. (1990) Cl₂O₂ absorption cross-sections.     

Evidence of a Convective Boundary Layer Developingon the Antarctic Plateau during the Summer

Summary The development of a convective boundary layer over the Antarctic Plateau is documented by a Doppler minisodar data-set recorded during a 10 day campaign in January 1997. The vertical velocities associated with thermals do not exceed 1 m/s, while the depth of the convective layer, usually less than 200 m, never surpasses 300 m. Measurements of momentum flux, sensible heat flux, wind speed and radiation budget show characteristics that are typical of a convective boundary layer evolution. The diurnal behaviour of absolute humidity, however, exhibits features that are not expected, e.g. anticorrelation with incoming net radiation and air temperature. Received October 30, 1998 Revised May 26, 1999     

Model Simulations on the Variability of Particulate MSA to Non-Sea-Salt Sulfate Ratio in the Marine Environment

A box model was constructed to investigate connections between the particulate MSA to non-sea-salt sulfate ratio, R, and DMS chemistry in a clean marine boundary layer. The simulations demonstrated that R varies widely with particle size, which must be taken into account when interpreting field measurements or comparing them with each other. In addition to DMS gas-phase chemistry, R in the submicron size range was shown to be sensitive to the factors dictating sulfate production via cloud processing, to the removal of SO₂ from the boundary layer by dry deposition and sea-salt oxidation, to the entrainment of SO₂ from the free troposphere, to the relative concentration of sub- and supermicron particles, and to meteorology. Three potential explanations for the increase of R toward high-latitudes during the summer were found: larger MSA yields from DMS oxidation at high latitudes, larger DMSO yields from DMS oxidation followed by the conversion of DMSO to MSA at high latitudes, or lower ambient H₂O₂ concentrations at high latitudes leading to less efficient sulfate production in clouds. Possible reasons for the large seasonal amplitude of R at mid and high latitudes include seasonal changes in the partitioning of DMS oxidation to the OH and NO₃ initiated pathways, seasonal changes in the concentration of species participating the DMS-OH reaction pathway, or the existence of a SO₂ source other than DMS oxidation in the marine boundary layer. Even small anthropogenic perturbations were shown to have a potential to alter the MSA to non-sea-salt sulfate ratio.     

Joining Australia to Antarctica GCM implications for the Cenozoic record of Antarctic glaciation

A previous GCM study concerning the formation and maintenance of Antarctic glaciation is expanded to include the joining of Australia to Antarctica; the two continents were physically connected prior to about 40 million years ago. It has been proposed that the increased continentality resulting from the enlarged landmass inhibited glaciation by increasing the degree of summer heating. However, simulations with the NCAR CCM1 suggest little change in the net Antarctic snow accumulation when Australia is joined to Antarctica, even under extreme variations in SST and topography. If anything, there is a slight increase in the net accumulation with the larger landmass. The climate of Australia does change markedly, consistent with the roughly 30° poleward shift in latitude. These results may not be inconsistent with paleoclimatic data from the early Cenozoic and the Cretaceous, with temperate flora and fauna along the coast, and large ice sheets inland.     

Response of the Antarctic ice sheet to future greenhouse warming

Possible future changes in land ice volume are mentioned frequently as an important aspect of the greenhouse problem. This paper deals with the response of the Antarctic ice sheet and presents a tentative projection of changes in global sea level for the next few hundred years, due to changes in its surface mass balance. We imposed a temperature scenario, in which surface air temperature rises to 4.2° C in the year 2100 AD and is kept constant afterwards. As GCM studies seem to indicate a higher temperature increase in polar latitudes, the response to a more extreme scenario (warming doubled) has also been investigated. The mass balance model, driven by these temperature perturbations, consists of two parts: the accumulation rate is derived from present observed values and is consequently perturbed in proportion to the saturated vapour pressure at the temperature above the inversion layer. The ablation model is based on the degree-day method. It accounts for the daily temperature cycle, uses a different degree-day factor for snow and ice melting and treats refreezing of melt water in a simple way. According to this mass balance model, the amount of accumulation over the entire ice sheet is presently 24.06 × 10¹¹ m³ of ice, and no runoff takes place. A 1°C uniform warming is then calculated to increase the overall mass balance by an amount of 1.43 × 10¹¹ m³ of ice, corresponding to a lowering of global sea level with 0.36 mm/yr. A temperature increase of 5.3°C is needed for the increase in ablation to become more important than the increase in accumulation and the temperature would have to rise by as much as 11.4°C to produce a zero surface mass balance. Imposing the Bellagio-scenario and accumulating changes in mass balance forward in time (static response) would then lower global sea level by 9 cm by 2100 AD. In a subsequent run with a high-resolution 3-D thermomechanic model of the ice sheet, it turns out that the dynamic response of the ice sheet (as compared to the direct effect of the changes in surface mass balance) becomes significant after 100 years or so. Ice-discharge across the grounding-line increases, and eventually leads to grounding-line retreat. This is particularly evident in the extreme case scenario and is important along the Antarctic Peninsula and the overdeepened outlet glaciers along the East Antarctic coast. Grounding-line retreat in the Ross and Ronne-Filchner ice shelves, on the other hand, is small or absent.     

The sudden stratospheric warming and chemical ozone loss in the Antarctic winter 2019: comparison with the winters of 1988 and 2002

Theoretical and Applied Climatology - Sudden stratospheric warmings (SSWs) are associated with rapid rise in temperature in a short period of time in the polar vortex and reversal of the zonal...     

Study of background aerosols in the Antarctic troposphere

Systematic year-round observations of submicron aerosols were carried out at Syowa Station (69°00'S, 39°35'E) in 1978. On the basis of the results of these observations, it is concluded that two types of aerosols originating from different sources are present in the Antarctic croposphere. With the intrusion of maritime air, mostly in the polar night months, sea salt particles and ammonium sulfate particles contained originally in the clean maritime air are dominant. The size distribution of such aerosols is monomodal, having a single mode at around 0.03 m in radii. On the other hand, in the sunlit months, sulfuric acid droplets are predominant and the size distribution is bimodal, having an additional mode at around 0.005 m in radii. Those sulfuric acid particles seem to be formed photochemically within a specific layer in the mid to lower troposphere over Antarctica. Most Antarctic submicron particles are of tropospheric origin, not of stratospheric nor anthropogenic origin.     

Signature of the Antarctic oscillation in the northern hemisphere

Using the ECWMF daily reanalysis data, this paper investigates signatures of the Antarctic Oscillation (AAO) in the upper troposphere of the northern hemisphere. It is found that during boreal winter, a positive (negative) phase of the AAO is associated with anomalous easterlies (westerlies) in middle-low latitudes (~30–40°N) and anomalous westerlies (easterlies) in middle-high latitudes (~45–65°N) of the upper troposphere about 25–40 days later. While there is also a response in zonal wind in the tropics, namely over the central-eastern Pacific, to some extent, these tropical zonal wind anomalies can trigger a Pacific/North American teleconnection patterns (PNA)-like quasi-stationary Rossby waves that propagate into the Northern Hemisphere and gradually evolve into patterns which resemble North Atlantic teleconnection patterns. Furthermore, these quasi-stationary Rossby waves might give rise to anomalous eddy momentum flux convergence and divergence to accelerate anomalous zonal winds in the Northern Hemisphere.     

Verification and Improvement of the Ability of CFSv2 to Predict the Antarctic Oscillation in Boreal Spring

The boreal spring Antarctic Oscillation(AAO) has a significant impact on the spring and summer climate in China. This study evaluates the capability of the NCEP's Climate Forecast System, version 2(CFSv2), in predicting the boreal spring AAO for the period 1983–2015. The results indicate that CFSv2 has poor skill in predicting the spring AAO, failing to predict the zonally symmetric spatial pattern of the AAO, with an insignificant correlation of 0.02 between the predicted and observed AAO Index(AAOI). Considering the interannual increment approach can amplify the prediction signals, we firstly establish a dynamical–statistical model to improve the interannual increment of the AAOI(DY AAOI), with two predictors of CFSv2-forecasted concurrent spring sea surface temperatures and observed preceding autumn sea ice. This dynamical–statistical model demonstrates good capability in predicting DY AAOI, with a significant correlation coefficient of 0.58 between the observation and prediction during 1983–2015 in the two-year-out cross-validation. Then, we obtain an improved AAOI by adding the improved DY AAOI to the preceding observed AAOI. The improved AAOI shows a significant correlation coefficient of 0.45 with the observed AAOI during 1983–2015. Moreover, the unrealistic atmospheric response to March–April–May sea ice in CFSv2 may be the possible cause for the failure of CFSv2 to predict the AAO. This study gives new clues regarding AAO prediction and short-term climate prediction.     

Hydrological Studies of Lake Outbursts in the Antarctic Oases

The results of a detailed hydrological study of the Larsemann Hills Oases (East Antarctica) during field seasons of 2017 to 2019 are presented. The study investigates a variety of lakes’ hydrological regime and the characteristics of outburst floods resulting from the lakes’ water flowing through tunnels in the snow-ice dams. The hydrographs calculated by the mathematical modeling do not generally contradict the physical essence of the process of outburst flood formation.

     

Observational Diagnosis of Cloud Phase in the Winter Antarctic Atmosphere for Parameterizations in Climate Models

The cloud phase composition of cold clouds in the Antarctic atmosphere is explored using data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) instruments for the period 2000--2006. We used the averaged fraction of liquid-phase clouds out of the total cloud amount at the cloud tops since the value is comparable in the two measurements. MODIS data for the winter months (June, July, and August) reveal liquid cloud fraction out of the total cloud amount significantly decreases with decreasing cloud-top temperature below 0oC. In addition, the CALIOP vertical profiles show that below the ice clouds, low-lying liquid clouds are distributed over ～20% of the area. With increasing latitude, the liquid cloud fraction decreases as a function of the local temperature. The MODIS-observed relation between the cloud-top liquid fraction and cloud-top temperature is then applied to evaluate the cloud phase parameterization in climate models, in which condensed cloud water is repartitioned between liquid water and ice on the basis of the grid point temperature. It is found that models assuming overly high cut-offs (》-40oC) for the separation of ice clouds from mixed-phase clouds may significantly underestimate the liquid cloud fraction in the winter Antarctic atmosphere. Correction of the bias in the liquid cloud fraction would serve to reduce the large uncertainty in cloud radiative effects.     

On the sensitivity of Antarctic sea ice model biases to atmospheric forcing uncertainties

Climate Dynamics - Although atmospheric reanalyses are an extremely valuable tool to study the climate of polar regions, they suffer from large uncertainties in these data-poor areas. In this work,...     

Causes of the record-low Antarctic sea-ice in austral summer 2022

2022年夏季(2021年12月至2022年2月)南极海冰面积达到历史新低,西南极减少最显著.2021年8～10月南半球环状模接近历史最强和7～9月海洋性大陆附近海温显著增暖是两个关键因素.由于平流层臭氧破纪录减少使得前者维持历史最强或接近最强,导致阿蒙森低压(ASL)加深并向西南移动,有利于海冰减少.后者持续到夏季,有利于拉尼娜的发展,通过激发罗斯贝波列加深ASL.在热力上,臭氧减少导致向下净短波辐射增加,引起西南极增暖.此外,净短波辐射-海温-云形成正反馈,与埃克曼输送一起,放大表面增暖,从而促进海冰融化.     

南极地区对全球变化的响应与反馈作用研究

南极地区对全球变化的响应与反馈作用研究是"九五”国家重点科技攻关项目(98-927).我院承担了该项目"南极大气和空间物理过程对全球变化的响应研究”专题中的两个子专题,即"南极臭氧和紫外辐射的监测和变化机制研究”和"南极冰雪与大气相互作用过程和辐射持征的观测研究”.2000年的主要研究进展有: