antarctic sea ice climatology variability and late twentieth century change in ccsm4 abstracta preindustrial control run and an ensemble of twentieth century integrations of the community climate system model version 4 ccsm4 are evaluated for antarctic sea ice climatology modes of variability trends and covariance with related physical variables such as surface temperature and sea level pressure compared to observations the mean ice cover is too extensive in all months this is in part related to excessively strong westerly winds over 50 60 s which drive a large equatorward meridional ice transport and enhanced ice growth near the continent and also connected with a cold bias in the southern ocean in spite of these biases in the climatology the model s sea ice variability compares well to observations the leading mode of austral winter sea ice concentration exhibits a dipole structure with anomalies of opposite sign in the atlantic and pacific sectors both the el nino southern oscillation and the southern annular mode sam project onto this mode in twentieth century inte

freshening of the ross sea during the late 20th century ocean measurements in the ross sea over the past four decades one of the longest records near antarctica reveal marked decreases in shelf water salinity and the surface salinity within the ross gyre these changes have been accompanied by atmospheric warming on ross island ocean warming at depths of 300 meters north of the continental shelf a more negative southern oscillation index and thinning of southeast pacific ice shelves the freshening appears to have resulted from a combination of factors including increased precipitation reduced sea ice production and increased melting of the west antarctic ice sheet

modeling the influence of greenland ice sheet melting on the atlantic meridional overturning circulation during the next millennia a three dimensional earth system model of intermediate complexity including a dynamic ice sheet component has been used to investigate the long term evolution of the greenland ice sheet and its effects on the atlantic meridional overturning circulation amoc in response to a range of stabilized anthropogenic forcings our results suggest that the greenland ice sheet volume should experience a significant decrease in the future for a radiative forcing exceeding 7 5 w m 2 the modeled ice sheet melts away within 3000 years a number of feedbacks operate during this deglaciation implying a strong nonlinear relationship between the radiative forcing and the melting rate only in the most extreme scenarios considered the freshwater flux from greenland into the surrounding oceans of ca 0 1 sv during a few centuries induces a noticeable weakening of the amoc in the model

the effect of increased fresh water from antarctic ice shelves on future trends in antarctic sea ice observations show that in contrast to the arctic the area of antarctic sea ice has increased since 1979 a potential driver of this significant increase relates to the mass loss of the antarctic ice sheet subsurface ocean warming causes basal ice shelf melt freshening the surface waters around antarctica which leads to increases in sea ice cover with climate warming ongoing future mass loss rates are projected to accelerate which has the potential to affect future antarctic sea ice trends here we investigate to what extent future sea ice trends are influenced by projected increases in antarctic freshwater flux due to subsurface melt using a state of the art global climate model ec earth in standardized climate model intercomparison project phase 5 cmip5 climate change simulations virtually all cmip5 models disregard ocean ice sheet interactions and project strongly retreating antarctic sea ice applying various freshwater flux scenarios we find that the additional fresh water significantly offsets the decline in sea ice area and is even able to reverse the trend in the strongest freshwater forcing scenario that can reasonably be expected especially in austral winter the model also simulates decreasing sea surface temperatures ssts with the sst trends exhibiting strong regional variations that largely correspond to regional sea ice trends

modulation of the seasonal cycle of antarctic sea ice extent related to the southern annular mode through analysis of remotely sensed sea surface temperature sst and sea ice concentration data we investigate the impact of winds related to the southern annular mode sam on sea ice extent around antarctica we show that positive sam anomalies in the austral summer are associated with anomalously cold ssts that persist and lead to anomalous ice growth in the following autumn while negative sam anomalies precede warm ssts and a reduction in sea ice extent during autumn the largest effect occurs in april when a unit change in the detrended summertime sam is followed by a 1 8 0 6 105 km2 change in detrended sea ice extent we find no evidence that sea ice extent anomalies related to the summertime sam affect the wintertime sea ice extent maximum our analysis shows that the wind anomalies related to the negative sam during the 2016 2017 austral summer contributed to the record minimum antarctic sea ice extent observed in march 2017

observations of large scale ocean atmosphere interaction in the southern hemisphere abstract the authors provide a detailed examination of observed ocean atmosphere interaction in the southern hemisphere sh focus is placed on the observed relationships between variability in sh extratropical sea surface temperature sst anomalies the southern annular mode sam and the el nino southern oscillation enso results are examined separately for the warm november april and cold may october seasons and for monthly and weekly time scales it is shown that the signatures of the sam and enso in the sh sst field vary as a function of season both in terms of their amplitudes and structures the role of surface turbulent and ekman heat fluxes in driving seasonal variations in the sam and enso related sst anomalies is investigated analyses of weekly data reveal that variability in the sam tends to precede anomalies in the sst field by 1 week and that the e folding time scale of the sam related sst field anomalies is at least 4 months the persistence of the sam related sst anomalies is

30 year satellite record reveals contrasting arctic and antarctic decadal sea ice variability a 30 year satellite record of sea ice extents derived mostly from satellite microwave radiometer observations reveals that the arctic sea ice extent decreased by 0 30 0 03 x 10 exp 6 square kilometers per 10 yr from 1972 through 2002 but by 0 36 plus or minus 0 05 x 10 exp 6 square kilometers per 10yr from 1979 through 2002 indicating an acceleration of 20 in the rate of decrease in contrast the antarctic sea ice extent decreased dramatically over the period 1973 1977 then gradually increased over the full 30 year period the antarctic ice extent decreased by 0 15 plus or minus 0 08 x 10 exp 6 square kilometers per 10 yr the trend reversal is attributed to a large positive anomaly in antarctic sea ice extent in the early 1970 s an anomaly that apparently began in the late 1960 s as observed in early visible and infrared satellite images

two modes of bottom water in the australian antarctic basin 1 recent data from the deep australian antarctic basin show that waters colder than about 0 4 c are fresher by as much as 0 02 than data collected prior to 1994 it is possible that the freshening represents a fundamental change in the sources of bottom water for the basin but it is more likely that the fresh mode recurs periodically and is underrepresented in the historical database the freshening may be caused by a change in fresh bottom water input from the antarctic coast between 120 e 160 e or by a reduction in the outflow of saline ross sea bottom water

a possible link between the weddell polynya and the southern annular mode abstract shortly after the advent of the first imaging passive microwave sensor on board a research satellite an anomalous climate feature was observed within the weddell sea during the years 1974 1976 a 250 103 km2 area within the seasonal sea ice cover was virtually free of winter sea ice this feature the weddell polynya was created as sea ice formation was inhibited by ocean convection that injected relatively warm deep water into the surface layer though smaller less persistent polynyas associated with topographically induced upwelling at maud rise frequently form in the area there has not been a reoccurrence of the weddell polynya since 1976 archived observations of the surface layer salinity within the weddell gyre suggest that the weddell polynya may have been induced by a prolonged period of negative southern annular mode sam during negative sam the weddell sea experiences colder and drier atmospheric conditions making for a saltier surface layer with reduced pycnocline stability t

a recount of ross sea waters abstract oceanographic observations within the ross sea have grown dramatically in recent years both in number and quality this has prompted a parallel recount of the circulation and structure of all water masses in the southwestern continental margins of the pacific ocean a high resolution set of horizontal property distributions was combined into a new climatology which in turn is the basis of a fine volumetric s census of all ross sea water masses inshore of the shelf break 700 m isobath the ross sea volume 25 10 4 km 3 partitioning into layers of neutral density n is 25 in the top layer n 3 of antarctic surface water aasw 22 in the middle layer 28 00 kg m 3 n 3 of oceanic thermocline water 12 and modified circumpolar deep water mcdw 10 and 53 in the bottom layer n 28 27 kg m 3 of shelf water sw 1 85 c 22 precursor of the antarctic bottom water aabw offshore aasw flows westward along the outer shelf and southward along the eastern coast past cape colbeck oceanic thermocline waters 28 00 kg m 3 n 3 cross the shelf break west of 170 w and follow southward paths along banks shoaling progressively to near the sea surface of the central and western inner shelves winter buoyancy loss converts aasw and oceanic thermocline water into denser types of mcdw and sw which move cyclonically toward the sills of major troughs the census shows a continuous mode of relatively dense mcdw 28 10 kg m 3 n 3 directly linked to the circumpolar deep water cdw offshore effectively ventilating and freshening the deep ocean at levels below the salinity maximum and above aabw mcdw outflows from the ross sea shelf are traced to the antarctic circumpolar current near 155 e into the australian antarctic basin along the northern flank of the southwest pacific ridge and to near 135 w past the eastern end of the ross gyre two sources of salty sw are inferred near the coastal polynyas in the terra nova bay and western ross ice shelf areas the latter contributes to a major outflow of ice sw that reaches the shelf break along 180 vertical mixing of mcdw and sw produce msw over the shelf continuing over the sills as new aabw export of low salinity aabw s

interdecadal water mass changes in the southern ocean between 30 e and 160 e interdecadal water mass changes in the indian western pacific sectors of the southern ocean were investigated using the japanese antarctic research expeditions and historical hydrographic observations from the 1950s to 1990s freshening and cooling occurred on the neutral density surfaces of 27 0 kgm3 equatorward of sub antarctic front results for the area south of the polar front show warm and saline anomalies and oxygen decreases on the surfaces around 27 9 kgm3 which correspond to the upper circumpolar deep water these latter anomalies are most simply explained by the mixing of these shallow waters with warmer and fresher surface waters steric sea level has also increased with an average change of 1mmyr1 from the 1970s to 1990s the changes are larger north of the sub antarctic front implying a strengthening of the antarctic circumpolar current it appears that the observed changes are consistent with the results from coupled climate model results for a similar period

rapid freshening of antarctic bottom water formed in the indian and pacific oceans 1 repeat hydrographic sections occupied in 1995 and 2005 reveal a rapid decline in the salinity and density of antarctic bottom water throughout the australian antarctic basin the basin wide shift of the deep potential temperature salinity s relationship reflects freshening of both the indian and pacific sources of antarctic bottom water the s curves diverge for waters cooler than 0 1 c corresponding to a layer up to 1000 m thick over the antarctic continental slope and rise changes over the last decade are in the same direction but more rapid than those observed between the late 1960s and the 1990s when combined with recent observations of similar freshening of north atlantic deep water these results demonstrate that dense water formed in both hemispheres is freshening in response to changes in the high latitude freshwater balance and rapidly transmitting the signature of changes in surface climate into the deep ocean

modification of the quaternary stratigraphic framework of the inner continental shelf by holocene marine transgression an example offshore of fire island new york abstract the inner continental shelf off fire island new york was mapped in 2011 using interferometric sonar and high resolution chirp seismic reflection systems the area mapped is approximately 50 km long by 8 km wide extending from moriches inlet to fire island inlet in water depths ranging from 8 to 32 m the morphology of this inner continental shelf region and modern sediment distribution patterns are determined by erosion of pleistocene glaciofluvial sediments during the ongoing holocene marine transgression much of the shelf is thus an actively forming ravinement surface remnants of a pleistocene outwash lobe define a submerged headland offshore of central fire island east of the submerged headland relatively older pleistocene outwash is exposed over much of the inner continental shelf and covered by asymmetric sorted bedforms interpreted to indicate erosion and westward transport of reworked sediment erosion of the eastern flank of the submerged pleistocene headland over the last 8000 years yielded an abundance of modern sand that was transported westward and reworked into a field of shoreface attached ridges offshore of western fire island west of the submerged headland erosion of pleistocene outwash continues in troughs between the sand ridges resulting in modification of the lower shoreface comparison of the modern sand ridge morphology with the morphology of the underlying ravinement surface suggests that the sand ridges have moved a minimum of 1000 m westward since formation comparison of modern sediment thickness mapped in 1996 1997 and 2011 allows speculation that the nearshore shoreface sedimentary deposit has gained sediment at the expense of deflation of the sand ridges

the relationship between the southern hemisphere annular mode and antarctic peninsula summer temperatures analysis of a high resolution model climatology abstract the large regional summer warming on the east coast of the northern antarctic peninsula ap which has taken place since the mid 1960s has previously been proposed to be caused by a trend in the southern hemisphere annular mode sam the authors utilize a high resolution regional atmospheric model climatology 14 km grid spacing to study the mechanisms that determine the response of the near surface temperature to an increase in the sam t sam month to month variations in near surface temperature and surface pressure are well represented by the model it is found that north of 68 s t sam is much larger on the eastern lee side than on the western windward side of the barrier this is because of the enhanced westerly flow of relatively warm air over the barrier which warms and dries further as it descends down the lee slope the downward motion on the eastern side of the barrier causes a decrease in surface mass balance and cloud cover south of 68 s vertical deflection across

interpretation of recent antarctic sea ice variability 1 trends in the satellite derived antarctic sea ice concentrations 1979 2002 show pronounced increase decrease in the central pacific sector bellingshausen western weddell sector by 4 10 per decade confidence levels for these regional trends exceed 95 positive polarities of the antarctic oscillation aao lead to more less ice in the eastern ross amundsen sector bellingshausen northern weddell sector which are qualitatively opposite to the impacts of positive polarities of the el nino southern oscillation enso the mechanisms responsible for the covariability between the ice and the a aao and b enso are demonstrated over the last 24 years a positive aao trend and a slightly negative enso trend produce a spatial pattern of ice changes similar to the regional ice trends however the magnitude of the ice changes associated with the aao and enso is much smaller than the regional ice trends more local or less understood large scale processes should be investigated for the explanations

twentieth century sea ice trends in the ross sea from a high resolution coastal ice core record we present the first proxy record of sea ice area sia in the ross sea antarctica from a 130 year coastal ice core record high resolution deuterium excess data show prevailing stable sia from the 1880s until the 1950s a 2 5 reduction from the mid 1950s to the early 1990s and a 5 increase after 1993 additional support for this reconstruction is derived from ice core methanesulphonic acid concentrations and whaling records while sia has continued to decline around much of the west antarctic coastline since the 1950s concurrent with increasing air and ocean temperatures the underlying trend is masked in the ross sea by a switch to positive sia anomalies since the early 1990s this increase is associated with a strengthening of southerly winds and the enhanced northward advection of sea ice

analysis of the projected regional sea ice changes in the southern ocean during the twenty first century using the set of simulations performed with atmosphere ocean general circulation models aogcms for the fourth assessment report of the intergovernmental panel on climate change ipcc ar4 the projected regional distribution of sea ice for the twenty first century has been investigated averaged over all those model simulations the current climate is reasonably well reproduced however this averaging procedure hides the errors from individual models over the twentieth century the multimodel average simulates a larger sea ice concentration decrease around the antarctic peninsula compared to other regions which is in qualitative agreement with observations this is likely related to the positive trend in the southern annular mode sam index over the twentieth century in both observations and in the multimodel average despite the simulated positive future trend in sam such a regional feature around the antarctic peninsula is absent in the projected sea ice change for the end of the twenty first century the maximum decrease is indeed located over the central weddell sea and the amundsen bellingshausen seas in most models changes in the oceanic currents could play a role in the regional distribution of the sea ice especially in the ross sea where stronger southward currents could be responsible for a smaller sea ice decrease during the twenty first century finally changes in the mixed layer depth can be found in some models inducing locally strong changes in the sea ice concentration

the impact of a changing southern hemisphere annular mode on antarctic peninsula summer temperatures since the mid 1960s rapid regional summer warming has occurred on the east coast of the northern antarctic peninsula with near surface temperatures increasing by more than 2 degrees c this warming has contributed significantly to the collapse of the northern sections of the larsen ice shelf coincident with this warming the summer southern hemisphere annular mode sam has exhibited a marked trend suggested by modeling studies to be predominantly a response to anthropogenic forcing resulting in increased westerlies across the northern peninsula observations and reanalysis data are utilized to demonstrate that the changing sam has played a key role in driving this local summer warming it is proposed that the stronger summer westerly winds reduce the blocking effect of the antarctic peninsula and lead to a higher frequency of air masses being advected eastward over the orographic barrier of the northern antarctic peninsula when this occurs a combination of a climatological temperature gradient across the barrier and the formation of a fohn wind on the lee side typically results in a summer near surface temperature sensitivity to the sam that is 3 times greater on the eastern side of the peninsula than on the west sam variability is also shown to play a less important role in determining summer temperatures at stations west of the barrier in the northern peninsula similar to 62 degrees s both at the surface and throughout the troposphere this is in contrast to a station farther south similar to 65 degrees s where the sam exerts little influence

antarctic ocean and sea ice response to ozone depletion a two time scale problem theresponseofthesouthernoceantoarepeatingseasonalcycleofozonelossisstudiedintwocoupledclimate models and is found to comprise both fast and slow processes the fast response is similar to the interannual signature of the southern annular mode sam on sea surface temperature sst onto which the ozone hole forcing projects in the summer it comprises enhanced northward ekman drift inducing negative summertime sst anomalies around antarctica earlier sea ice freeze up the following winter and northward expansion of the sea ice edge year round the enhanced northward ekman drift however results in upwelling of warm waters from below the mixed layer in the region of seasonal sea ice with sustained bursts of westerly winds induced by ozone hole depletion this warming from below eventually dominates over the cooling from anomalous ekman drift the resulting slow time scale response years to decades leads to warming of ssts around antarctica and ultimately a reduction in sea ice cover year round this two time scale behavior rapid cooling followed by slow but persistent warming is found in the two coupled models analyzed one with an idealized geometry and the other with a complex global climate model with realistic geometry processes that control the time scale of the transition from cooling to warming and their uncertainties are described finally the implications of these results are discussed for rationalizing previous studies of the effect of the ozone hole on sst and sea ice extent

coupled ocean atmosphere ice response to variations in the southern annular mode abstract the coupled ocean atmosphere ice response to variations in the southern annular mode sam is examined in the national center for atmospheric research ncar community coupled climate model version 2 the model shows considerable skill in capturing the predominantly zonally symmetric sam while regional deviations between model and observation sam winds go a long way in explaining the generally small differences between simulated and observed sam responses in the ocean and sea ice systems vacillations in the position and strength of the circumpolar winds and the ensuing variations in advection of heat and moisture result in a dynamic and thermodynamic forcing of the ocean and sea ice both meridional and zonal components of ocean circulation are modified through ekman transport which in turn leads to anomalous surface convergences and divergences that strongly affect the meridional overturning circulation and potentially the pathways of intermediate water ventilation a heat budget analysis de

ice core evidence for antarctic sea ice decline since the 1950s the instrumental record of antarctic sea ice in recent decades does not reveal a clear signature of warming despite observational evidence from coastal antarctica here we report a significant correlation p

an analysis of the atmospheric processes driving the large scale winter sea ice variability in the southern ocean the response of the sea ice in the southern ocean to the variability of the atmosphere on the period 1979 2004 is investigated using both model and observational data on the one hand our results show that in line with previous investigations the classical modes of atmospheric variability do not explain a large part of the winter sea ice extent variability integrated over the entire southern ocean on the other hand the regression between the ice extent and the atmospheric pressure displays a pattern with low pressure areas over the south atlantic over the indian ocean and on the southwestern part of the pacific ocean and a high pressure area over the antarctic region extending toward both the southeastern pacific and the region south of australia this pattern which does not correlate significantly with any of the known modes of atmospheric variability induces changes in the sea ice extent by influencing the air temperature the ice production and both the meridional and the zonal ice velocity however there is no clear link between the different centers of action of this pattern furthermore the correlation of the sea ice extent between the different sectors is generally low as a conclusion we must consider that the sea ice extent in the southern ocean over the last 30 years does not behave as a single entity its variability is the result of the combination of regional sea ice changes each sector should thus be examined separately

simulated changes in the extratropical southern hemisphere winds and currents 1 the results from 12 global climate models show a remarkably consistent strengthening and poleward shifting of the zonal wind stress through the 20th and 21st centuries at extratropical southern hemisphere latitudes changes in the zonal circulation of the ocean in the region are broadly consistent with the changes in zonal wind stress in particular the climate models simulate a strengthening and a poleward shift of the antarctic circumpolar current the strengthening of the zonal wind stress also results in intensifying northward ekman transport across the antarctic circumpolar current which in the unblocked latitudes of drake passage implies increasing southward geostrophic transport in the ocean below about 2000 m zonal wind stress changes such as these may be expected to enhance the mesoscale eddy activity in the southern ocean

abyssal ocean warming and salinification after weddell polynyas in the gfdl cm2g coupled climate model abstractthe role of weddell sea polynyas in establishing deep ocean properties is explored in the noaa geophysical fluid dynamics laboratory s gfdl coupled climate model cm2g using statistical composite analysis of over 30 polynya events that occur in a 2000 yr long preindustrial control run the temperature salinity and water mass changes associated with the composite event are quantified for the time period following the composite polynya cessation termed the recovery warming between 0 002 and 0 019 c decade 1 occurs below 4200 m in the southern ocean basins temperature and salinity changes are strongest in the southern ocean and the south atlantic near the polynya formation region comparison of the model results with abyssal temperature observations reveals that the 1970s weddell polynya recovery could account for 10 8 of the recent warming in the abyssal southern ocean for individual southern ocean basins this percentage is as little as 6 11 or as much as 34 13

interpretation of recent southern hemisphere climate change climate variability in the high latitude southern hemisphere sh is dominated by the sh annular mode a large scale pattern of variability characterized by fluctuations in the strength of the circumpolar vortex we present evidence that recent trends in the sh tropospheric circulation can be interpreted as a bias toward the high index polarity of this pattern with stronger westerly flow encircling the polar cap it is argued that the largest and most significant tropospheric trends can be traced to recent trends in the lower stratospheric polar vortex which are due largely to photochemical ozone losses during the summer fall season the trend toward stronger circumpolar flow has contributed substantially to the observed warming over the antarctic peninsula and patagonia and to the cooling over eastern antarctica and the antarctic plateau

ice core records as sea ice proxies an evaluation from the weddell sea region of antarctica 1 ice core records of methanesulfonic acid msa from three sites around the weddell sea are investigated for their potential as sea ice proxies it is found that the amount of msa reaching the ice core sites decreases following years of increased winter sea ice in the weddell sea opposite to the expected relationship if msa is to be used as a sea ice proxy it is also shown that this negative msa sea ice relationship cannot be explained by the influence that the extensive summer ice pack in the weddell sea has on msa production area and transport distance a historical record of sea ice from the northern weddell sea shows that the negative relationship between msa and winter sea ice exists over interannual 7 year period and multidecadal 20 year period timescales national centers for environmental prediction national center for atmospheric research ncep ncar reanalysis data suggest that this negative relationship is most likely due to variations in the strength of cold offshore wind anomalies traveling across the weddell sea which act to synergistically increase sea ice extent sie while decreasing msa delivery to the ice core sites hence our findings show that in some locations atmospheric transport strength rather than sea ice conditions is the dominant factor that determines the msa signal preserved in near coastal ice cores a cautious approach is thus required in using ice core msa for reconstructing past sea ice conditions including the need for networks of ice core records and multiproxy studies to assess the significance of past msa changes at different locations around antarctica

evaluation of the simulation of the annual cycle of arctic and antarctic sea ice coverages by 11 major global climate models 1 comparison of polar sea ice results from 11 major global climate models gcms and satellite derived observations for 1979 2004 reveals that each of the models is simulating annual cycles that are phased at least approximately correctly in both hemispheres each is also simulating various key aspects of the observed ice cover distributions such as winter ice not only throughout the central arctic basin but also throughout hudson bay despite its relatively low latitudes however some of the models simulate too much ice others simulate too little ice in some cases depending on hemisphere and or season and some match the observations better in one season versus another several models do noticeably better in the northern hemisphere than in the southern hemisphere and one does noticeably better in the southern hemisphere in the northern hemisphere all simulate monthly average ice extents to within 5 1 106 km2 of the observed ice extent throughout the year in the southern hemisphere all except one simulate the monthly averages to within 6 3 106 km2 of the observed values all the models properly simulate a lack of winter ice to the west of norway however most obtain more ice immediately north of norway than the observations show suggesting an under simulation of the north atlantic current the spread in monthly averaged ice extents among the 11 model simulations is greater in the southern hemisphere than in the northern hemisphere and greatest in the southern hemisphere winter and spring

definition of antarctic oscillation index following walker s work about his thmous three oscillations published during the 1920 30s many papers were written about atmospheric oscillations a iburth atmospheric oscillation in the middle and high southern latitudes was lbund and nanted the grotarctic oscillation ao a refiers to a large scale alternation of atmospheric mass between the mid latitudes and high latitudes surthce pressure in order to understand the spatial structure of sea level pressure variation in detail empirical orthogonal fitnction analysis is applied an objective index of the antarctic oscillation index aoi is defined as the diffierence of zonal mean sea level pressure between 40os and 65os the aoi has the potential tbr clarifying climate regimes in the southern hemisphere similar to how the na and the npo has been used in the northern hemisphere were there other oscillation s besides nao npo and s especially in the southern hemisphere it had been supposed that there might be other atmospheric oscillation s in the high southern latitudes although as early as in the first quarter of this century walker 1928 had stated that just as in the north atlantic there is a pressure opposition between the azores and iceland there is an opposition between the high pressure belt across chile and the argentine on the one hand and the low pressure area of weddell sea and the bellingshausen sea on the other the scarcity of data in the southern hemisphere hindered the search tbr new oscillation s during the last two decades more comprehensive data over the southern hemisphere became available and more analyses were carried out see kidson 1975 rogers and van loon 1982 mo and l 7 ite 1985 2 4o 1986 connolley 1997 gong and wang 1998 a tburth atmospheric oscillation in the middle and high southern latitudes was tbund and nanted as the antarctic oscillation a wang 1992 gong and wang 1998 the term ao relrs to a large scale alternation of atmospheric mass between the mid latitude sur hce pressure

the future of ice sheets and sea ice between reversible retreat and unstoppable loss we discuss the existence of cryospheric tipping points in the earth s climate system such critical thresholds have been suggested to exist for the disappearance of arctic sea ice and the retreat of ice sheets once these ice masses have shrunk below an anticipated critical extent the ice albedo feedback might lead to the irreversible and unstoppable loss of the remaining ice we here give an overview of our current understanding of such threshold behavior by using conceptual arguments we review the recent findings that such a tipping point probably does not exist for the loss of arctic summer sea ice hence in a cooler climate sea ice could recover rapidly from the loss it has experienced in recent years in addition we discuss why this recent rapid retreat of arctic summer sea ice might largely be a consequence of a slow shift in ice thickness distribution which will lead to strongly increased year to year variability of the arctic summer sea ice extent this variability will render seasonal forecasts of the arctic summer sea ice extent increasingly difficult we also discuss why in contrast to arctic summer sea ice a tipping point is more likely to exist for the loss of the greenland ice sheet and the west antarctic ice sheet

antarctic ice sheet melting provides negative feedbacks on future climate warming we show by using a three dimensional climate model which includes a comprehensive representation of polar ice sheets that on centennial to millennial time scales antarctic ice sheet ais can melt and moderate warming in the southern hemisphere by up to 10 degrees c regionally in a 4 x co2 scenario this behaviour stems from the formation of a cold halocline in the southern ocean which limits sea ice cover retreat under global warming and increases surface albedo reducing local surface warming furthermore we show that ais melting by decreasing antarctic bottom water formation restrains the weakening of the atlantic meridional overturning circulation which is a new illustration of the effect of the bi polar oceanic seesaw consequently it appears that ais melting strongly interacts with climate and ocean circulation globally it is therefore necessary to account for this coupling in future climate and sea level rise scenarios

instrument and tree ring based estimates of the antarctic oscillation abstract an estimate of the strength of the austral summer antarctic oscillation using station sea level pressure records for the period 1878 2000 is presented the first to the authors knowledge the reconstruction was obtained by relating the antarctic oscillation aao intensity derived from ncep ncar reanalysis data to the leading principal components of station records using multiple regression analysis particular effort has been made to fit the model in a way that is robust to the questionable trends in the ncep ncar data in the southern hemisphere the trends in the reconstruction are derived from the station data not from the ncep data cross validation with the ncep data and comparison with other analyses of the aao over the late instrumental period give confidence that this station based reconstruction can be regarded as trustworthy with regard to the whole reconstruction period some unquantifiable uncertainty stems from potential instability of the statistical relationships to extend this

influence of the southern annular mode on the sea ice ocean system 1 the global sea ice ocean model orca2 lim driven by the ncep ncar national centers for environmental prediction national center for atmospheric research reanalysis daily 2 m air temperatures and 10 m winds and by monthly climatologies for precipitation cloud cover and relative humidity is used to investigate the impact of the southern annular mode sam on the antarctic sea ice ocean system our results suggest that the response of the circumpolar southern ocean consists of an annular and a nonannular component for the sea ice cover the non annular component seems to be the most important the annular component strongly affects the overall patterns of the upper ocean circulation when the sam is in its positive phase a northward surface ekman drift a downwelling at about 45 degreess and an upwelling in the vicinity of the antarctic continent are simulated the non annular component has a significant impact at the regional scale especially in the weddell ross amundsen and bellingshausen seas in those regions the pressure pattern associated with the sam induces meridional winds which advect warmer air in the weddell sea and around the antarctic peninsula and colder air in the amundsen and ross seas this implies a dipole response of sea ice to the sam with on average a decrease in ice area in the weddell sea and around the antarctic peninsula and an increase in the ross and amundsen seas during years with a high sam index the long term trend in the observed sea ice area does not appear to be related to the trend in the sam index

consistent past half century trends in the atmosphere the sea ice and the ocean at high southern latitudes simulations performed with the climate model loveclim aided with a simple data assimilation technique that forces a close matching of simulated and observed surface temperature variations are able to reasonably reproduce the observed changes in the lower atmosphere sea ice and ocean during the second half of the twentieth century although the simulated ice area slightly increases over the period 1980 2000 in agreement with observations it decreases by 0 5 106 km2 between early 1960s and early 1980s no direct and reliable sea ice observations are available to firmly confirm this simulated decrease but it is consistent with the data used to constrain model evolution as well as with additional independent data in both the atmosphere and the ocean the simulated reduction of the ice area between the early 1960s and early 1980s is similar to the one simulated over that period as a response to the increase in greenhouse gas concentrations in the atmosphere while the increase in ice area over the last decades of the twentieth century is likely due to changes in atmospheric circulation however the exact contribution of external forcing and internal variability in the recent changes cannot be precisely estimated from our results our simulations also reproduce the observed oceanic subsurface warming north of the continental shelf of the ross sea and the salinity decrease on the ross sea continental shelf parts of those changes are likely related to the response of the system to the external forcing modifications in the wind pattern influencing the ice production melting rates also play a role in the simulated surface salinity decrease

important role for ocean warming and increased ice shelf melt in antarctic sea ice expansion changes in sea ice significantly modulate climate change because of its high reflective and strong insulating nature in contrast to arctic sea ice sea ice surrounding antarctica has expanded1 with record extent2 in 2010 this ice expansion has previously been attributed to dynamical atmospheric changes that induce atmospheric cooling3 here we show that accelerated basal melting of antarctic ice shelves is likely to have contributed significantly to sea ice expansion specifically we present observations indicating that melt water from antarctica s ice shelves accumulates in a cool and fresh surface layer that shields the surface ocean from the warmer deeper waters that are melting the ice shelves simulating these processes in a coupled climate model we find that cool and fresh surface water from ice shelf melt indeed leads to expanding sea ice in austral autumn and winter this powerful negative feedback counteracts southern hemispheric atmospheric warming although changes in atmospheric dynamics most likely govern regional sea ice trends4 our analyses indicate that the overall sea ice trend is dominated by increased ice shelf melt we suggest that cool sea surface temperatures around antarctica could offset projected snowfall increases in antarctica with implications for estimates of future sea level rise

ediacaran biota in the aftermath of the kotlinian crisis asha group of the south urals abstract the asha group of the south urals is dominated by laminated shales and thin siltstone sandstone alternations interpreted as a prograding low energy inner shelf depositional system the succession includes several thick sharp based sandstone bodies comprising interstratified fine to medium grained fine laminated planar laminated hummocky convolute and wave bedded planar and trough cross bedded sandstones occasionally pebble to boulder conglomerates regarded as tidal deltaic and shoreface depositional systems the sandstone shoreface bodies are interpreted as forced regressive deposits stranded in offshore positions during subsequent transgressions although palaeoecological and taphonomic context of the asha group is favourable for the ediacara type biofacies the associated fossil assemblages nevertheless are depauperate and consist of frondomorph holdfast structures palaeopascichnids microbial colonies arumberiamorph structures as well as lithified microbial substrates shagreen texture biolaminites in terms of fidelity and fossil completeness preservation of ediacaran fossils in the asha group is by no means inferior to that seen in other ediacaran macrofossil localities the low biodiversity of ediacaran macrofossils can be attributed to a relatively young ediacaran age of the asha group which is constrained by a u pb zircon date of 547 6 3 8 ma from an ash bed in the lower part of the sedimentary succession the uppermost part of the asha group has yielded bilobed burrows with a backfill structure suggesting an affinity with the ichnogenus didymaulichnus from the ediacaran cambrian boundary strata the asha group therefore offers an important glimpse into the history of the edacaran biota in the aftermath of the kotlinian crisis that caused extinction of dickinsoniomorphs tribrachiomorphs and bilateralomorphs in wave and current agitated shoreface depositional systems 550 million years ago but did not significantly affect frondomorphs and palaeopascichnids

the influence of sea ice on ocean heat uptake in response to increasing co2 two significant changes in ocean heat uptake that occur in the vicinity of sea ice cover in response to increasing co2 are investigated with community climate system model version 3 ccsm3 a deep warming below 500 m and extending down several kilometers in the southern ocean and warming in a 200 m layer just below the surface in the arctic ocean ocean heat uptake caused by sea ice retreat is isolated by running the model with the sea ice albedo reduced artificially alone this integration has a climate response with strong ocean heat uptake in the southern ocean and modest ocean heat uptake in the subsurface arctic ocean the arctic ocean warming results from enhanced ocean heat transport from the northern north atlantic at the time of co2 doubling about 1 3 of the heat transport anomaly results from advection of anomalously warm water and 2 3 results from strengthened inflow at the same time the overturning circulation is strengthened in the northern north atlantic and arctic oceans wind stress changes cannot explain the circulation changes which instead appear related to strengthened convection along the siberian shelves deep ocean warming in the southern ocean is initiated by weakened convection which is mainly a result of surface freshening through altered sea ice and ocean freshwater transport below about 500 m changes in convection reduce the vertical and meridional temperature gradients in the southern ocean which significantly reduce isopycnal diffusion of heat upward around antarctica the geometry of the sea ice cover and its influence on convection have a strong influence on ocean temperature gradients making sea ice an important player in deep ocean heat uptake in the southern ocean