in Dronning Maud Land, Antarctica

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 104, NO. D16, PAGES 19,191-19,211, AUGUST 27, 1999 Atmospheric signals and characteristics of accumulation in Dronning Maud Land, Antarctica David Noone, John Turner, and Robert Mulvaney British Antarctic Survey, National Environmental Research Council, Cambridge, England Abstract. With the planned European Programme for Ice Coring in Antarctica in Dronning Maud Land it is importanto understand the processes leading to accumulation for successful interpretation of core data. Because it is impractical to obtain precipitation observations with a large spatial coverage and on a daily timescale in Antarctica, model-generated precipitation must be considered for a comprehensive study of the region. However, without observational data it is difficult to check the veracity of the model data. Precipitation data from the European Centre for Medium-Range Weather Forecasts reanalysis project shows that 89% of days have low (under 0.2 mm) precipitation resulting in 31% of the annual total. At the other extreme, less than 1% of days have high (over 1 mm) precipitation, which results in 20% of the annual total. It is reasoned that the changes in the frequency of extreme precipitation events could alter the trace record in ice cores and lead to a bias in reconstructed paleotemperatures. Case studies reveal that high-precipitation days have amplified upper level planetary waves directing warm moist air to the region. Associated with this is the presence of a cyclone in or at the northeast extreme of the Weddell Sea. Commonly, the longwaves provide a blocked anticyclone in the South Atlantic to form a dipolar channeling of the air mass. The accumulation variability is linked to the variability in the intensity of these storms and their tracks. It is seen that this is related to the E1 Nifio-Southern Oscillation and a semiannual cycle. 1. Introduction Ice cores drilled in the polar ice caps capture the signature of atmospheric processes over a time period of many thousands of years [e.g., Dansgaard et al., 1993; GRIP Members, 1993]. Analysis of ice core samples can reveal much of the climatic history of the Earth via consideration of various chemical constituents [Delmas and Legrand, 1989]. Examples include changes in the general strength of the midlatitude winds [Petit et al., 1991], changes in sea surface humidity [douzel et al., 1982], and the global mean temperature [e.g., Aristarain et al., 1986; douzel et al., 1987]. other proxy data (particularly marine sediment cores) are performed so that even a greater understanding of the climate history can be obtained. The European Programme for Ice Coring in Antarctica (EPICA) [douzel et al., 1996] aims to obtain further deep ice cores at two locations in Antarctica. The first of the EPICA cores will be drilled at Dome Concordia in East Antarctica to examine the major climate shifts over several glacial cycles. The second core will be taken in a relatively high accumulation zone in Dronning Maud Land (DML) to obtain a higher resolution time series in the Atlantic sector, allowing examination of the rapid climate oscillations seen throughouthe last glacial cycle. This is an With strong evidence supporting a rise in temperatures attractive location for coring which compliments Greenland through the twentieth century [e.g., Peel et al., 1988; Jones, cores in assessing the importance of the Atlantic circulation in 1990] it is worth the consideration that the Earth may be in the climate system. the process of changing its climate regime. As such, it is To interprethe results of any ice core, the glaciological importanto study paleoenvironments and, in particular, the and atmospheric processes that lead to the formation of the manner in which the environment changes (rapidly or ice sheet mass must be understood. Chemical diffusion and otherwise) between two climatic states. By developing an kinematic effects within the ice can be estimated sufficiently, understanding of the ways in which the Earth system reacts to and allowance can be made in the analysis. However, a climate shifts historically, we can obtain knowledge on which complete understanding of the atmospherichemistry is still to base a climate prediction. needed for many of the analyzed species. The origin of The successful extraction of the long Vostok core in meteoric water over polar regions is one of the most debated Antarctica and the GRIP and GISP-2 cores in Greenland topics [e.g., Kato, 1978; Koster et al., 1986; Petit et al., alone provides important paleoclimatic insight. Comparison 1991], and further, the processes involved are yet without and cross matching between cores from two hemispheres and comprehensive documentation. Before ice core data can be fully utilized, an understanding of the atmospheric conditions under which accumulation occurs must be obtained. INow at School of Earth Sciences, University of Melbourne. The purpose of the present study is to provide an assessment of accumulation and precipitation in DML and to Copyright 1999 by the American Geophysical Union. facilitate effective interpretation of the EPICA ice cores. In Paper number 1999JD the following section we wish to evaluate previously known /99/1999JD aspects of DML accumulatioh and surface mass balance from 19,191

2 ,,,..,, 19,192 NOONE ET AL' DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS Kong H6k0n VII Hov Weddell Sea., Ne.uma,. SANAE undsen"'-s'( '-' "-E- Figure 1. Map of Antarctica showing locations mentioned in the text. The Dronning Maud Land area defined is shaded. Research stations are indicated by solid circles and the position of cores is marked as crosses. The main orographic features are contoured at 0.5 km spacing. both glaciological and atmospheric perspectives. The data In a study based on all in situ glaciological observations used in this study are discussed, and the basic characteristics for the continent, Giovinetto and Bentley [1985] describe the of accumulation and precipitation are presented. An ice sheet flow of DML as moving west into the Weddell Sea. examination of high-precipitation events follows. We then They examine a profile of surface mass balance from the examine the main features of cyclones in the DML region. On coast near SANAE Station inland. A rapid decrease is seen the basis of our findings a discussion of in situ data and between the coastal high accumulation and the plateau above analysis are then given. This interdisciplinary assessment 2000 m where it falls below 50 ram/year. High spatial forms a basis from which an understanding of changes in variability is found to correspond to abrupt changes in the climate circulation can be formed. orography. A similar result was reported by Richardson et al. [1997] from data obtained using a ground-based radar along a 2. Previous Studies transect from the coast near Neumayer to 1000 km inland on the DML plateau. /saksson and Karl n [1994a] reporthat The high-plateau region of DML (Figure 1) with which we higher than 2500 m, there is an annual accumulation rate of are concerned has been the focus of very few studies. In the 100 mm of water equivalent with an interannual variability of coastal areas, atmospheric and glaciological measurements up to 50%. In agreement, recent surveying in DML has have been made since the establishment of research stations revealed accumulation rates from short ice cores of around 80 in the 1950s. These locations are generally under the to 100 mm/year (work in progress). These values are influence of slight to moderate katabatic winds froin the high approximately double that generally accepted for the region plateau, and surface balance estimates reflect the coastal [e.g., Giovinetto and Bentley, 1985]. storms that are active throughouthe year [e.g., Reinwarth et On the coastal incline below 2000 m, Isaksson and Karl n al., 1985]. Only limited amounts of ice core data are available [1994a] find that the accumulation has a decreasing trend on the high altitude inland plateau, notably a series of papers over the years , and they suggest this is associated by Isaksson [1994] that describe the inland mass balance from with general cooling in the area, indicated by a cooling of IøC a study of short firn cores and a longer core. at Halley for that period. Indeed, they show a strong

3 NOONE ET AL.: DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS 19,193 relationship between the annual mean temperature at Halley and the annual accumulation over a 31 year period. They reveal both series to have a large peak around They suggest that this is associated with changes in the cyclone motion along the coast during that period. For the same coastal region, lsaksson and Karl n [1994b] find a local Bromwich [1988] shows the annual cycle of accumulation at coastal stations in East Antarctica to have a maximum in the winter months of April to September, and stations in the Ross and Weddell Seas tend to have a peak during March. The inland stations exhibit a annual cycle with a maximum in winter. Considering poleward moisture fluxes, Bromwich regression between temperature and 8180 values of 1.16%o/øC concludes that the greatest flux is associated with the coastal while a previous study in the same area reported 1.31%o/øC storms, with most of the precipitation falling in the poleward [Orheim et al., 1986]. As both results are high compared to those of, for example, Dansgaard et al. [1973] and Lorius and Merlivat [1977], it is suggested that this discrepancy is related to changes in the moisture origin and the storm tracks. Analyzing a coastal ice core, Isaksson et al. [1996] also moving air mass to the east of the cyclone centers. This is closely related to the offshore position of the storm centers and subjecto the variability of storm frequency in any given year. The moisture from these systems would not be expected to penetrate the continental interior due to the steep implicate the synoptic storms as the basis of the discrepancy orography, but it is likely that the atmospheric moisture between the observed increase in 81sO-derived temperature and the decreased accumulation rate. This argument was applied by Morgan et al. [1991] for East Antarctica. Another possible explanation given by lsaksson et al. [1996] is that the temperature change is only during certain (low accumulation) months. As such, a signature of this in an ice core would be difficult to identify. content of the plateau region would be affected by cyclonic advection. In a study of Greenland, Chen et al. [1997] also identify synopticyclones as the key to understanding polar precipitation. They explain that the position of the cyclones can act to increase (or decrease) the monthly precipitation totals as the cyclone approaches (or is downstream from) the Greenland orography. They suggest the possibility of changes The methane sulfonic acid record in Antarctic ice cores is in precipitation should the climatology of cyclones change known to be influenced by the El Nifio-Southern Oscillation (ENSO) [Legrand and Feniet-Saigne, 1991], and indeed, an ice core at 75øS, 2øE [Isaksson, 1994] shows an ENSO signal. This suggests possible changes in the transport or deposition processes for the different phases of ENSO in the DML area. Cullather et al. [1996] discuss a link between precipitation under different climate regimes, say, glacial versus interglacial, and indeed altered ice sheet heights. Sinclair [1981], however, studied an unusually high temperature event that affected much of Antarctica in December This was found to be associated with a large storm penetrating DML. Rapidly moving warm, moist air was associated with the low in the Amundsen Sea and ENSO. forced by strong northerly winds onto the continental interior. ENSO is seen to affect the Southern Hemispheric pressure at mean sea level (Pmsl) and height fields [van Loon and Shea, Here the air mass slowly cooled with warm temperatures recorded at the pole for the following four days. It is 1987; Karoly, 1989] and sea ice concentration [Simmonds suggested [Sinclair, 1981 ] that this type of event, while rare, and 3'acka, 1995]. Subsequently, it is likely that the accumulation record and the (second order) trace record is affected by ENSO. Bromwich [1988] provides an overview of precipitation for would have a substantial effect on the continental moisture and heat budget due to their extreme nature. With this exception there has been little or no published quantification of the occurrence or importance of these penetrative storms. the Antarctic continent and gives a succinct description of two mechanisms for precipitation. Coastal precipitation 3. Data Considerations associated with storms is complicated by orographically induced low-level blocking. This is common to mountainous precipitation globally and results in a low-level wind maximum parallel to the orography. The cause of the block is thought to be a stabilizing of low-level air. As such, when the air mass meets the orographic incline, the flow diverts rather than ascends. At higher levels, the air mass flows over the block with precipitation forming as it rises. Bromwich suggests that this process explains the relatively constant isotopic ratios below 1000 m. Here the precipitation is formed Studies of DML are particularly limited by data paucity. A glaciological survey of the region is currently under way as part of the EPICA project (including short ice core collection, airborne radio echo sounding, measurement of atmospheric parameters); however, these data are not currently available. The glaciological data considered in this study are the "Epica" core data presented by Isaksson et al. [1996] at 75øS, 2øE in DML and the core of Mulvaney and Wolff [1993] at 77øS, 22.5øW in Coats Land. We also present 1 year of highduring uniform condensation conditions above the block. resolution accumulation measurements from a thermistor Isotopic values above 1000 m show the common altitude dependence when this intruding air comes in contact with the terrain. The other key type of precipitation is clear-sky precipitation (also known as "diamond dust"). This occurs in a cold atmosphere when very tenuous cirrus clouds form just above the snow surface with fine ice crystals precipitating and is generally associated with moisture advection. Diamond dust can also form at night when sublimation from the surface during summer days reforms into crystals as the air cools with the normal diurnal cycle. This would not affect the accumulation amount for a given region (i.e.., sublimation and precipitation of the same water), but various chemical constituents could be altered. stick located at 77øS, 10øW. These locations are marked in Figure 1. A number of research stations have operated along the coastal region of the Atlantic sector for many decades. These stations are also marked in Figure 1. Station synoptic observations are routinely made every three hours and are available via the Global Telecommunications System for use in forecasting and numerical model initialization. They include measurements of basic meteorological parameters (temperature, pressure, humidity, wind) along with a subjective indication of "present weather" and twice daily radiosonde profiles. While precipitation is not recorded for reasons of practicality, the present weather reports indicate

4 19,194 NOONE ET AL.: DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS precipitation conditions when they occur. Observations from Amundsen-Scott Station at the pole are likely to be free of the large temporal variability that the coastal stations are subject to in association with the intense cyclonic storms. However, their relevance to DML is questionable due to their proximity. Nonetheless, station data are also included in this study for verification where possible. Complex numerical weather prediction models are run daily at most large forecasting centers to aid in operational forecasting. The initial conditions for these simulations are generated via four-dimensional assimilation. This process takes raw'observations (from manned and automatic stations, satellites, drifting buoys) and statistically produces an analysis consistent with the dynamics of the model. Part of this process supplies data-sparse regions with a physically based estimate of the atmospheri conditions. As a result, a spatially and temporally comprehensive data set is obtained. While the model analyses are thoughto be of high quality, it,?, must be recognized that the model used is imperfect, and the "best guess" analyses may not accurately reproduce the true d F M A M j d A S 0 N D d atmosphere where no other data exist. Similarly, verification Day of the model performance in these regions is difficult, having few observations with which to compare. Figure 2. Cumulative accumulation for 1997 measured by the thermistor instrument at 77øS 10øW (thick solid, left axis). In recent years there have been a number of projects to Mean daily model precipitation totals for model grid points reanalyze observational data using state-of-the-art models and within a square 10 ø x 3 ø centered on the site. A thin sold assimilation techniques. The key advantage of a reanalysis curve (11 day "moving window" function) for the data is product rather over operational analyses is that the model is shown (right axis). the same for the entire period. This ensures that data set is temporally self-consistent. In comparison, operational models are frequently modified to improve the output for guidance in weather forecasting. As such, it is difficult differentiate actual trends from trends introduced by the changed model. One example of particular concern in this study is the effect of increasing the spatial resolution of the model. It has been seen that model precipitation depends strongly on the ability of a model to represent orography [Chen et al., 1997], which is in turn dependent on resolution. Because we are interested in interannual trends in precipitation, we must choose to use reanalyses rather than operational data. 100 km). Because precipitation (and evaporation) is not among the analyzed fields, 0-24 hour forecast data are used. The data are subject to even greater inaccuracy because they are dependent only on the physics of the model. Further, forecast fields are susceptible to distortion associated with model spin-up. Spin-up is associated with the adjustment of the simulated atmosphere to be fully consistent with the physics at model initialization. In the Antarctic there are only limited observations, so the initial conditions To date, there have been two reanalysis projects dependent on the physics alone, and the spin-up can be completed. These are those of the National Centers for Environmental Prediction (NCEP) and the European Centre for Medium-Range Weather Forecasts (ECMWF). The ECMWF reanalysis (ERA) covers a 15 year period while that of NCEP covers 40 years. Studies of the NCEP analyses have revealed a flaw in the representation of moisture over the Antarctic resulting in poor accumulation estimates [Bromwich et al., 1995]. In contrast, the ERA model is thoughto predicthe Antarctic moisture budget and associated circulation to a remarkably high standard, given the lack of observational data [e.g., Turner et al., 1998a]. To extend the data set temporally, we also use the ECMWF operational analyses (EOP) for the years The EOP precipitation estimates are thought to be of higher quality than the ERA precipitation [Genthon and Krinner, expected to be small. Genthon and Krinner [1998] find that the precipitation data from 6 hour forecasts are of the order of 10% smaller than the 12 hour forecast and suggest that most of the spin-up occurs in the first 6 hours. However, precipitation from later in the forecast period is more susceptible to forecast error. Unique high-resolution in situ accumulation measurements were made in 1997 at 77øS, 10øW using a thermistor stick installed in Antarctica of that year. On the stick, 20 thermistors were equally spaced at 10 mm intervals. The thermistors were interrogated midday and again at midnight to reveal the diurnal temperature cycle. While in the air, the temperature fluctuations were evident. However, when the accumulated snow had buried a thermistor, the fluctuations were much diminished. By examining the year long time 1998]; however, we require the data set to be self-consistent, series of thermistor data and the temperature difference and therefore we will primarily consider the ERA data in this study. We must note that an intercomparison of the ERA and EOP data sets goes beyond the scope of this study. The version of the ECMWF global model used in the between thermistors, one can obtain the dates when the accumulated snow had buried each 10-mm-spaced thermistor. This can be translated into accumulation. Figure 2 shows the cumulative accumulation indicated by the instrument. Twenty reanalysis project had 31 vertical levels and the horizontal increments were covered in just under a year, indicating over resolution defined by triangular truncation of the spectral 80 mm of water equivalent accumulation in this particular series at wavenumber 106 (approximately equivalent to year. Note that the relatively flat sections observed in the will be strongly

5 NOONE ET AL.: DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS 19,195 accumulation curve must indicate a slow input of snowfall to counter the steady slight fall in the surface level due to melting and compaction of the snow grains. In Figure 2 we also show the average daily precipitation for the model grid points in a square 10 ø x 3 ø (longitude, latitude) centered on the site. The model precipitation for this box indicates about twice the precipitation recorded by the thermistor. This clearly shows the problem in relating model results to individual glaciological observations. The difference is probably associated with both a failing of the model (in particular the parameterization of sea ice in the Weddell Sea and the topography error in the DML area) and the sampling problem with using a single site for glaciological measurements. Therefore the model precipitation series is best viewed here as an indicator of the occurrence of precipitation related to synoptic activity rather than site accumulation amount per se. The thermistor data show a number of rapid increases. With the exception of one, they are all associated with periods of precipitation in the model. However, there are more high-precipitation events in the model data than recorded in the thermistor data. While this may be a sampling problem with the still relatively low temporal resolution of the thermistor data, it may also indicate a limitation of the model and, in particular, an over estimation of the frequency of high-precipitation events. It must be noted that calibration and interpretation of the thermistor data are not yet well understood. The data are presented here to show that the model precipitation relates encouragingly to in situ results. While we do not wish to suggesthat the model and thermistor data are exceptionally similar, the importance of the coincidence is twofold. First, we may conclude that the model at least partially reproduces realistic atmospheric Zwally and Giovinetto [1995] with in situ data. This distribution shows more variability around the coast similar to that in Figure 3c. This is particularly so in the region of coastal basins that would experience local orographically forced precipitation. For various regional sectors, using both radiosonde data [e.g., Connolley and King, 1993; Cullather et al., 1998] and atmospheric model data [Bromwich et al., 1995; Cullather et al., 1998], accumulation estimates from vertically integrated moisture flux also agree well with the in situ estimates. ERA, however, shows generally lower accumulation, particularly in central East Antarctica. The DML area lies inland of the 50 mm contour of annual water equivalent accumulation of Figure 3c. Estimates from ice cores in DML (pre-epica surveying) are around 80 mm. There are many factors that may contribute to the inaccuracies. In particular, the model assumes a climatological sea ice distribution and has a coarse representation of the ocean/land boundary, which may have a significant effect on the hydrology at the coast. Also, the ERA model assumes an orography up to 1300 m higher than reality in western DML [e.g., Genthon and Braun, 1995]. Nonetheless, the mean spatial representation of accumulation is quite reasonable on a regional scale [Turner et al., 1999]. However, even if the model is sufficiently accurate spatially, there remains a question as to the temporal reliability. This is of particular concern in examining extreme events. Clearly, model results are less than ideal and must be viewed with caution. On the other hand, model precipitation estimates are the only data available for much of the Antarctic and on short (daily) timescales. The model can describe the physical processes leading to precipitation and is thought to signals. In particular, there is evidence in glaciological represent the resulting amount with some accuracy. In this observations for some of the high-precipitation events indicated by the model. Second, the in situ measurements at regard we view the ERA data as valuable in understanding certain aspects of Antarctic mass balance. this site contain some of the accumulation signal for the region. This comparison shows the validity and benefit of a 4. Accumulation Characteristics combined modeling/glaciological approach to understanding Antarctic mass balance and climate in general. We define the DML area, shaded in Figure 1, as grid Figure 3 shows the spatial distribution of the annual mean points above 2600 m (of model orography) and bound by precipitation, evaporation and accumulation from ERA. It is 70øS and 80øS and 15øW and 25øE. This represents 51 points seen clearly that the modeled evaporation in the interior of the in the model data, which is thought sufficient to generate continent is small compared to the precipitation. As such, the inland accumulation (estimated as the difference between precipitation and evaporation) is almost identical to the useful statistics for the region. A time series representation of modeled accumulation for the DML area is presented in Figure4 for the ERA/EOP period. The mean annual precipitation. Antarctic evaporation is not generally thought accumulation is 32 mm water equivalent per year with to be so small [van den Broke, 1997], and therefore the ERA individual years ranging from 21 mm (1984) to 45 mm evaporation is probably an underestimate. Descending the (1981). These are likely to be underestimates yet give an steep terrain to the coastal sea ice and open ocean areas, the important indication of the variability. From the figure we can ERA evaporation rises quickly as surface moisture is more see that summer month totals are consistently lower than the abundant. Indeed, at the coastal land/sea boundary the mean. However, within any given year, the monthly evaporation is around 150 mm a year, which has an important variability can be quite large. In particular, the January effect on the accumulation total. It is of interest to note that maximum in 1981 influences the annual signal, while other the aggregation of the precipitation and evaporation leads to months have less representation in the annual record. Years local accumulation maxima near SANAE and with lower annual accumulation tend to exhibit lower Novolazarevskaya bases. These tend to be associated with the complex coastal orography. monthly variability; or to state the converse, highprecipitation years tend to be associated with a number of There is general agreement between derived Antarctic contributing months with particularly high precipitation. accumulation based on different methods [Cullather et al., 1998]. Giovinetto and Bentley [1985] used in situ At higher temporal resolution the accumulation can be categorized by the contribution from various daily observations to give the first definitiv estimate of the spatial precipitation amounts. Figure 5 presents a frequency distribution of accumulation. Recently, Faughan et al. [1999] statistically merged the passive microwave observations of distribution of the amount of annual precipitation attributed to particular daily totals. From this figure the importance of the

6 . 19,196 NOONE ET AL.' DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS I\ Figure 3. (a) European Centre for Medium-Ranged Weather Forecasts Reanalysis (ERA) spatial distribution of precipitation, (b) evaporation, and (c) accumulation. Contours are at 2, 5, 10, 15, 20 in units of 10 mm water equivalent, and thereafter the interval is 10. small precipitation totals is clear. Some 96% of days have under 0.5 mm precipitation, which produce over 61% of the annual accumulation. Further, 89% of days have under 0.2mm precipitation, which leads to 31% of the annual accumulation. Consequently, clear-sky precipitation is clearly important in the moisture budget and in the composition of the ice sheet. Radok and Lile [1977] examine data from the inland Plateau Station in the International Geophysical Year and suggesthat 87% of precipitation falls as fine ice crystals, which occurs mostly on days with clear skies. There is, however, no indication of the precipitation type from the model, so we can only estimate the amount of diamond dust crudely on the basis of the daily totals. As Plateau is of higher altitude than DML, temperatures will be colder and the relative contribution of diamond dust greater. However, the systematic error in the precipitation data induced by model spin-up may influence the distribution of amounts, and there may be a bias toward small background precipitation totals. This would lead to an overestimate of diamond dust and an underestimate of days with no precipitation in DML. Also from Figure5, 20% of the mean accumulation can be attributed to precipitation events greater than l mm a day. This represents only 56 days in the 15 year ERA data (about 1% of days). The higher amounts fall on days when the region is under the influence of strong cyclonic activity on the coast. We now examine the precipitation for a point 75øS on the

7 NOONE ET AL.' DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS 19, Yeor Figure 4. Time series of monthly modeled accumulation " (right axis) in the Dronning Maud Land (DML) region for the 20 ERA and European Centre for Medium-Ranged Weather Forecasts operational analysis (EOP) period in millimeters of water equivalent. The annual totals are shown as horizontal bars (left axis). lo Greenwich meridian rather than the DML region used earlier. This point is chosen as representative of the area with respect to individual precipitation events rather than an area average. It is also in close proximity to the Isaksson et al. [1996] core and likely to be close to the EPICA drilling. We subdivide the annual data into categories based on daily totals, bound by the values 0.1, 0.2, 0.5, 1, and 2 mm and above 2 mm. These totals are shown in Figure 6, stacked to indicate the annual total. Indeed, the annual totals are qualitatively similar to those for the region (Figure 4). This allows some reassurance that the daily totals at the 75øS point may be reflected more widely in the DML region. We are now able to examine the nature of the interannual variability in detail. In particular, we notice that the years with highest precipitation have significant contributions from events over lmm. While only 10 of the 19 years show the occurrence of events over 2mm, these are again important contributions for the annual total (about 16% in those years). The total for events below 0.5mm shows less interannual variability. The maxima at , 1992, and 1997 are reflected at each category level. However, Precipitation amount (mm / day) Figure 5. Spectrum of modeled daily precipitation totals in Dronning Maud Land. Totals are grouped into bins of 0.1mm and are shown as a percentage of the mean annual total (left axis). The curve shows the cumulative total with increasing daily totals (right axis). o Year All Figure 6. Time series of model annual precipitation at 75øS, 0øE. Precipitation is grouped by daily totals bound by 0.1, 0.2, 0.5, 1, 2, and greater than 2 mm/day. These are indicated from dark to light shading and are stacked into columns from the lowest category. the peak is not seen throughout the spectrum, rather a maximum in can be seen for the lower categories. This suggests that 1981 was unique with respecto the larger precipitation events and, indeed, dories and Simmonds [1993] indicate the summer of 1981 as a period of particularly deep depressions around the hemisphere. Our time series shows a discontinuity at the boundary, where we have used EOP rather than ERA data. It is uncertain whether this trend is reasonable or if it results from different data, which is likely to be very different in character. Baroclinic cyclones are a major feature of Southern Hemisphere circulation. These storms result, fundamentally, from an unstable collapse of the equator/pole temperature gradient and are important in the redistribution of energy on a global scale. Therefore the cyclones would be expected to reflect global signals. dories and Simmonds [1.993] showed that the mean central pressure of Southern Hemisphere depressions exhibit the largest interannual variability south of 55øS. Winter cyclones are more intense during years of the warm ENSO phase of! and (their Figure 13). Their data do not cover the E1 Nifio period from 1991 onward. Marshall and King [1.998] observe a relationship between ENSO and upper level (500 hpa) circulation anomalies with respecto a dipole "seesaw" across the Antarctic Peninsula, with a maximum in the Weddell Sea during the warm phase. With these findings one could expect

8 19,198 NOONE ET AL' DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS,,_,, 3 E E.9._o_,- 2 o J F M A M J J A $ 0 N D Month All Figure 7. Annual cycle of model precipitation 75øS, 0øE. Precipitation is grouped by daily totals as in Figure 5. Categories are stacked from lowesto highest to form the monthly means and are shaded from dark to light. Indeed, the latter show this signal is maintained in the latitude band 63øS to 71øS in that sector. Simmonds and Jones [1998] show a strong SAO in Southern Hemisphere Pmsl from a 20 year data set through the 1970s and 1980s. They find the SAO to exist south of 60øS with a maximum in the second harmo ic at around 70øS. This persists to the pole although it is weaker. Since the SAO is a cycle on a hemispheric scale, one could expect a secondary migration of storm tracks in phase with changes in the meridional temperature gradient [e.g., Hurrell and van Loon, 1994]. As such, this secondary southern migration of the storms would lead to the increased precipitation observed in Figure 6. This is in agreement with Cullather et al. [1998] who show the importance of southern storm track variability in determining Antarctic precipitation. They illustrate the annual cycle of moisture transport broken into mean and eddy components. A SAO is seen in the eddy and in the total data for western DML (their Figure 16f), while this is reduced farther east. We show the mean seasonal cycle of the sea level pressure at the five manned' stations for the duration of their inhabitance (of the order of 40 years) in Figure 8. A clear SAO is seen in the data, although it is strongest for the coastal stations. The data from Amundsen-Scott display one primary mode of variability with a minimum in autumn. The SAO is seen only as a leveling of the pressure trend in spring. This is not surprising because the SAO is associated with the baroclinic flow, which has a reduced influence on sites farther inland. However, this does suggesthat there are two different precipitation signals (coastal and inland) in agreement with Bromwich [1988]. Between these extremes the precipitation signal in the DML area would contain a superposition of the two effects. Also in this figure is 1 year (1997) barometric pressure data from DML (77øS, 10øW). The DML curve reveals a signal more likened to that of the Amundsen-Scott data, although it is dominated by the synoptic conditions that greater than normal advection of heat and moisture in DML from the increased eddy activity. It would follow that the.....'1 I-k strong El Nifio years display higher precipitation. Isaksson et al [1996] observed a correlation between DML accumulation /. and Halley temperatures. This can be explained by greater 990I 985 ', '... - advection and cyclone activity associated with ENSO, as 980 indicated by Marshall and King [1998] for Faraday Station. 975 " While a moderate ENSO relation is observed qualitatively in Amundsen-Scott Figure 6, the length of the time series limits the degree of 6 o," o,ley,- -'- certainty with which such a causal effect can be identified. Figure7 shows the mean annual cycle of the ERA 685 precipitation at the 75øS point categorized by daily totals in the same manner as Figure6. The data show a clear 680 maximum in the extended winter period from March to June ,,,,, This indeed corresponds to the intensification and southward DML (77S 10W) 750 migration of the baroclinic zone. Again, the stronger storm systems would be likely to advect more moisture and heat 745 from the lower latitudes into the Antarctic region. The highest (>2 ram) precipitation events do not appear biased toward any 740 _ particular month or season. 755 Evidence for a smaller maximum may be seen in spring, d F M A M d d A S O N D particularly in the daily totals below 0.5 mm/d. This can be Month explained by examining the dynamics of the hemispheric Figure 8. Mean annual cycle of station pressure at mean sea flow; namely, a semiannual oscillation (SAO) [e.g., van Loon, level (Pmsl) at coastal (SANAE, Novolazarevskaya, 1967] has been observed in precipitation on the Antarctic Neumayer, and Halley) and Amundsen-Scott Stations. Also, Peninsula by Turner et al. [1997] and Turner et al. [1999]. barometric pressure at 77øS, 10øW in DML from [ ovol zarev k a - eum ye SAN

9 NOONE ET AL.' DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS 19, OO 80O - //, ' \\\ I\\ / \ ii / // \ ; ii I / / / \ / / / \ I I1 ( - / \ "', i\\\ X', '-.. ".',/ 7///// \, "... / t// vj 'J,9. ///, / // /// /// / / ;/ 111 ] J F M A M J J A S 0 N D Month Figure 9. Annual cycle of model poleward moisture flux anomalies with height. Positive values (solid curves) indicate increased poleward moisture flux. Negative anomalies (dashed curves) indicate reduced po_leward moisture flux. Contours are in units of l x103 kg m-2 s-1. The zero contour is solid. circulation patterns are sufficient to alter the source region, perhaps substantially, even within the duration of 1 year. To address the question of source regions in a general sense, we now examine mean poleward moisture flux in the sector 10øW to 15øE as cross sections for the May and February extremes (Figure 10). In May there is substantial poleward flux throughouthe troposphere with a maximum mass transport at around hPa. It appears that the intrusion reaches the pole, indicating that moisture from coastal storms reaches the interior. The February case reveals a reduction in the coastal intrusion, while the inland flux prevails, now below the wintertime inversion height. For mass continuity, this inland moisture flux requires a source, which must instead result from the zonal flow. Indeed, examination of the westerly component (not shown) reveals a flux to the continent south of 77øS. This latitude corresponds to the southern extent of the Weddell Sea. Here cyclonic activity directs air eastward up the orographic rise and is consistent year-round, as indicated by Figure 10. Furthermore, this area is at the precise location of the greatest error in the model orography, which results in an occurred in 1997 rather than the climatological trend. This conclusion requires that the ERA data are indeed representing the seasonal cycle accurately. We cannot be sure that differences between the cycle in Figure 7 and those shown by Bromwich [1988] do not indicate the flaw in the ERA precipitation. If we consider the poleward moisture flux into the DML region, a clear SAO is seen. Figure 9 shows the annual cycle of anomalies in moisture flux. Positive values represent greater moisture transporthan the mean, and while there is still moisture transporto DML, negative values indicate that it is at a reduced rate. The May maximum (and indeed the mean) inflow is found at approximately 600 hpa. Phillpot and Zillman [1970] indicate a typical height of the wintertime inversion of 650hPa. During summer the inversion breaks down and allows moisture to arrive near the ice sheet surface. 200 o This can be seen in the November maximum and in the February minimum. The height difference could suggestwo different synoptic origins for the summer and winter o 400 \ - precipitation. Different chemical traces would be expected in._m 600 core samples simply from different source regions rather than, say, purely temperature-driven changes in the chemical trace, as is the case for stable water isotopes. Observational studies suggest a winter source near the sea ice edge at 55øS [Karo, 1978], while model studies [Petit et al., 1991] suggest a midlatitude origin 30ø-40øS. Bromwich [1988] suggests that this discrepancy is associated with the nature of the analysis methodology; namely, the moisture budget approach considers net flux for an atmospheric Figure 10. Monthly mean poleward moisture flux from ERA, volume, while an isotopic model approach indicates the zonally history of a conglomerate of individual parcels. Fisher and February. averaged Contour from intervals 10øW are to of 15øS l x103 for kg m -2 - The zero contour is solid. The typical continental height is shaded. Alt [1985] indicate that the source for precipitation at 75øS is Negative values show northward moisture flux and are shown equally partitioned in the region from 30øS to 75øS. While we as dashed contours. Values below -10x103 kg m-2 at the have not attempted to quantify the synoptic origins of coast are shown as solid-dashed contours with an interval of moisture here, it is apparent that the seasonal changes in 10x 103 kg m -2. o 400._m O ø f 'F' '''...'...'... eb b 800, 1000, Lotitude (a) Ma_Ysa d (b)

10 19,200 NOONE ET AL.: DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS Figure 11. Pressure at mean sea level at 1200 UTC on (a) January 13, (b) 15, (c) 17, and (d) Contour interval is 4hPa. overestimated zonal surface gradient and may act to underestimate the Weddell Sea source. North of 77øS, however, the zonal component is easterly. As such, we may a consequence, there is lower accumulation than one would expect from simple altitude considerations. This can be observed in Figure 3. conclude that the mean moisture flux to DML approaches from the northeast, while moisture farther inland passes over Coats Land and is of Weddell Sea origin. This is indicated by the mean moisture flux field [e.g., Cullather et al., 1996, Figure 9]. This may explain the observation that an ice core in Coats 5. Case Studies of Extreme Precipitation Events As we have seen, a large percentage of the accumulation can be attributed to intense baroclinic storms. To understand the general mechanisms underlying the high daily Land has lower accumulation than cores from similar precipitation caused by these storms we now presentwo case altitudes (R. Mulvaney, personal communication, 1998). Air studies in some detail. As we are using model data, it is from the southern Weddell Sea will have lower moisture plausible that such storms are merely an artifact of the climate content compared to that of the coastal storms in Kong H tkon VII Hav. This follows from considering, for example, the reduced amount of evaporation over dense sea ice regions. As of the model. We should like to diagnose whether precipitation statistics generated from model data are representative of the actual precipitation in these high-

11 NOONE ET AL.: DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS 19,201 Figure 12. The 500 to 1000 hpa thickness (indicating mean tropospheric temperature) and midtropospheric (500hPa) wind vectors at 1200 UTC on (a) January 13, (b) 15, (c) 17, and (d) 19 January Thickness contour interval is 50m, and wind fields scaling vector is 25 knots. precipitation events. To achieve this we, incorporate available station observations and satellite imagery for verification. High-precipitation events were selected on the criterion that more than 2 mm of precipitation fall at the 75øS, 0øE point. In the 15 years of ERA, 13 cases were identified, while 18 were counted in the subsequent 4 years of EOP data. As some of these represent synoptic events that lasted more than 1 day, a total of 25 result. There is a difference in the frequency of these events between the ERA and the EOP data and it is likely that this is associated with the differences in the models themselves. One important difference may be the overestimated topography in the ERA model. This would prevent moisture ascending to the DML plateau and reducing the observed number of high-precipitation events. If these events play a crucial role in the Antarctic heat and moisture balance [Sinclair, 198 l], this could lead to the underestimate of accumulation in DML in the ERA data The case of January 17 and 19, 1981 This event was responsible for the spike seen in Figure 4. For that month, 14.5ram of precipitation fell at the 75øS point, while nearly 1 lmm were from the 6 days of this event. This represents 30% of the mean annual total and 25% for that year. It could be expected that this would leave a significant, uncharacteristic signature in ice cores. Modeled

12 19,202 NOONE ET AL.' DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS precipitation falls in greatest amounts (4.8 and 2.9 mm at the (Figures l lc and 12c) the 5400 m hpa thickness 75øS point) on January 17 and 19. contour crossed the coast. The coastal low had moved rapidly A sequence of ERA Pmsl is shown in Figure I 1 at 2-day to the east and dissipated, while the Weddell Sea low (now in intervals from January 1.3 at UTC. Figure 12 shows the the Kong Hfikon VII Hav) remained blocked. Greatly 1000 to 500hPa thickness as an indicator of mean amplified longwaves ensure a cross pole northwesterly of tropospheric temperature, along with the wind field at around 20 knots at Amundsen-Scott, allowing strong moisture 500hPa. A depression from near the tip of the Antarctic aad heat advection to the continent. Heavy precipitation (4.8 Peninsula had moved into the western Weddell Sea ram) occurred in DML on that day. (Figure 1 l a). There was another depression in the Atlantic The prevailing westerly flow moved the ridge of warm air sector at around 20øE. A large anticyclone was also seen in only 15 ø eastward by January 19 (Figure 12d) as precipitation the Atlantic sector. Seven cyclonic centers were analyzed in continued. Onshore winds persisted but were reduced. At the hemispheric flow, giving a wavenumber 6 or 7 pattern. SANAE Station, 11 knots were reported. The high persisted This unstable regime leads to the rather disorganized structure in the eastern Atlantic with another depression moving of Figure 1 l a. With the flow blocked in the Atlantic by the through the Drake Passage (Figure 1 ld). The stalled situation high-pressure system the thickness (Figure 12a) shows a broke down rapidly, with a retreat of thickness contours and poleward ridge around 30øW. Halley Station recorded dissipation of the coastal cyclone. Subsequently, the 500 hpa easterly winds and began reporting intermittent snowfall after winds shifted 90 ø to become mostly zonal. no change in the previous 24 hours. SANAE and Figure 13 shows the spatial distribution of precipitation for Novolazarevkaya recorded northeasterly wind under the the event period January Clearly, this indicates the influence of the coastal system. importance of the orography with the greatest spatial The data for January 15 indicate intensification of the concentration of precipitation on the plateau above Weddell Sea cyclone. Winds at Halley doubled to around 30 Neumayer. The region of precipitation is, in fact, quite large, knots still from the east, while the 500hPa winds indicate a suggesting that the effects of the event would be captured in westerly on the eastern side of the Weddell Sea. the ice sheet over much of DML and Coats Land. The model Novolazarevskaya had 100% humidity. The Atlantic high even indicates over 1 mm of precipitation the pole. became elongated, forming a dipole with the Weddell Sea cyclone (Figure l lb). A warm air intrusion is seen in 5.2. The case of November 5, 1997 Figure 12b with strong upper level advection. While Pmsl over the continent interior is less than ideal as an indicator of The EOP 700 hpa height fields are shown in Figure 14 for the flow, little change is observed. this case to offer another view of a similar event. Unlike The Atlantic high continued to block the flow, allowing Pmsl, the 700 hpa level is above the ice sheet and is therefore the Weddell Sea cyclone to deepen farther. The surface wind not subjecto nonphysical extrapolation below the continental is westerly at Novolazarevskayand SANAE. The warm surface. These charts are shown for each day November 2, 3, intrusion continued to move southward. By January 17 4, and 5 with 0.2, 0.9,!.9, and 2.4 mm of precipitation falling on those days. While satisfying our "extreme" selection criterion, this event is more typical than the January case given above. Although we chose a DML point for the selection of this event, its effects are visible in the thermistor data from Coats Land (Figure 2). Figure 14a reveals a cyclone in the western Kong Hfikon VII Hav on November 2. This feature is also visible at higher levels in the atmosphere the same location, typical of Antarctic weather systems. Around the hemisphere there were three major systems indic. ating a relatively stable flow regime. A high-pressure ridge extended from the Atlantic along 10øE associated with a blocked anticyclone. Consequently, there was a warm air mass over the continent with the northerly advection. Neumayer Station reported winds up to 42 knots from the east with the cyclonic flow and heavy blowing snow. There was also blowing snow at Halley, while Novolazarevskaya, under the pressure ridge, reports only light winds (2-4 knots) from the north. The data from November 3 show that the cyclone had moved slightly southward into the northern Weddell Sea; otherwise, the pattern around the hemisphere had changed o, o little (Figure 14b). The intruding ridge had pushed farther into the continent. Amundsen-Scott had clear-sky precipitation. Neumayer continued to report blowing snow, now interspersed with reports of heavy snowfall. Composite Figure 13. Total precipitation for the days January infrared satellite imagery from the NOAA polar orbiting Contours are 1, 2, 5, and 10mm, and thereafter the spacecraft (obtained from the University of Wisconsin Space interval is 10mm. Light, medium and dark shading is shown Sciences and Engineering Department archives) is shown for above 5, 10, and 20mm. November 3 in Figure 1.5. The Weddell Sea cyclone shows

13 NOONE ET AL.: DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS 19,203 Figure 14. The 700hPa height and 500hPa wind field at 1200 UTC on (a) November 2, (b) 3, (c) 4, and (d) Height contour interval is 50m, and wind scaling vector is 25 knots. the characteristic spiral band cloud associated with a cold front. This cloud extended from the southern Weddell Sea to eastern Antarctic indicated by the model. The blocking high and the Weddell cyclone remained static. Subsequently, the east across DML. Precipitation from the associated temperatures at Amundsen-Scott increased a further 10øC moisture at the 75øS point on November 4 gave a total of 1.9 mm. Again, the warm moist air mass has continued to with continued precipitation. Other station temperatures begin to fall as the cyclone weakened slightly. This allows relative penetrate southward and affects much of the continent humidities to remain high and precipitation to continue. This (Figure 14c). Conditions on November 5 (Figure 14d) gave 2.4 mm of event was also captured by the thermistor measurements made at 77øS, 10øW (Figure 2) and suggests the region precipitation in DML with all four stations reporting experienced precipitation over an extended (2 weeks) period. precipitation (Amundsen-Scott still had falling ice prisms). Temperatures at the stations had been steadily increasing 5.3 General Comments on High-Precipitation Events (12øC at Neumayer, 16øC at Halley, 19øC at Of the 25 individual events there are three main synoptic Novolazarevskay and 15øC at Amundsen-Scott) over the 4- day period representative of temperatures over most of types that can be identified. The two examples given show greatly amplified longwaves with a low in the Weddell Sea

14 19,204 NOONE ET AL.: DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS Figure 15. Composite polar infrared satellite imagery at 1200 UTC on November , showing spiral cloud band associated with a cyclone in the Weddell Sea. and an Atlantic block forcing enhanced northerly advection. time of the air mass in uncertain. Trajectories were generated This situation is common to 19 of the 25 events identified. The second type follows from intense cyclogenesis in the lee of the Antarctic Peninsula. A third type has a cyclone crossing the base of the Antarctic Peninsul and deepening in the Weddell Sea. Common to the latter two, westerly flow is for every six hours from 0000 UTC and revealed a surprising insensitivity to the arrival time, indicating the persistence of the circulation. We presentrajectories arriving at 1200 UTC in Figure 16. The main feature is, of course, rapid convergence from the north. Some of the events exhibit an induced perpendicular to the coast at Halley, and a pressure easterly component near the coast associated with cyclonic ridge in the Kong Hfikon VII Hay allows the familiar dipolar structure to develop, in a manner not dissimilar to the examples of the first type given. For all 25 extreme cases this, again, is associated with amplified longwaves with pronounced northerly flow at upper levels bringing midlatitude air uncharacteristically south. Another common feature of these systems is their rapid dispersion shortly after the high precipitation is observed. In some instances the activity in the Kong Hfikon VII Hav in agreement with the general moisture flux for the region. A subset of these paths approach over Neumayer, occurring when a cyclone is in the Weddell Sea. These have shorter trajectories, indicating that the moisture has a more local origin than the cases that have a blocked anticyclone in the Atlantic. These two origins would have different signatures in the chemical trace of ice core data. Weddell Sea cyclone remains and deepens again to give more It should be noted that a selection criterion based on precipitation. Using a three-dimensional advection scheme, trajectory paths for air parcels arriving at the 75øS point at 500 hpa can be calculated for the 25 high-precipitation days identified. The 500 hpa was selected to be representative of the precipitation amounts at a different point or indeed for the DML area, yields a different set of "extreme" cases. Points farther to the east are likely to show a reduced influence of the Weddell Sea lows. The strongest precipitation events, such as the January event above, generally affect the whole tropospheric flow and close to the height of moisture region (as seen by its presence in the accumulation for the transport (around 600 hpa), as shown previously. It is at this level that precipitation is thought to form [e.g., Robin, 1977]. region shown in Figure 4). Some of the 25 events fail to yield substantial precipitation at model points even 5 ø (zonal) Trajectories started from different levels show qualitatively away. It is believed this is not an artifact of the model but an similar paths. This suggests that the systems are uniform throughout the depth of the atmosphere (i.e., barotropic). Because we have only daily precipitation totals, the arrival example of the complicated nature of precipitation as a locally discrete phenomenon and the difficulties of examining regional signals by assessing individual cases.

15 NOONE ET AL.' DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS 19,205 Figure 16. Reverse 5 day parcel trajectories for parcel arrival at 500hPa on the 25 days with precipitation totals over 2mm at 75øS, 0øW. 6. Behavior of Cyclones Weddell Sea. This was also found by dones and Simmonds [1993] and is strongest during winter. They also show the Changes in the storm tracks are often referred to as the Weddell Sea as a location for both cyclogenesis and cause of distinct shifts in glaciological data. However, there is cyclolysis throughout the year, which is confirmed by our rarely an attempto describe this change. It is important data. The concentration of storms increases eastward to a therefore to document the predominant behavior of cyclones maximum at around 30øE. Accumulation in eastern DML in the region. We consider the location of cyclonic depression would be strongly influenced by these systems. By centers associated with precipitation events. Depressions are considering equilibrium, it could be argued that this accounts identified in the six hourly ERA Pmsl data using the objective for the orographic incline across DML to the east. The greater finding and tracking scheme devised by Murray and moisture advection is offset by and in balance with the high Simmonds [1991]. As we are interested synoptic scale lows orography. rather than intermittent or "inflection" features also identified The subset of depressions existing on higher precipitation by the scheme, we require that cyclones exist for at least 36 days (more than 0.5 mm falling over the DML area) was hours. This selection method is preferred over a movement found and is shown as a density anomaly in Figure 17b. criterion such as that used by Sinclair [1994]. We have also Recall that this represents only 5% of days yet almost 61% of chosen to discount all centers identified over the continent the annual accumulation results. A maximum in the anomaly above 1000m because they are a nonphysical artifact of the extends from the western Kong Hfikon VII Hav to the scheme. This results in a total of different depression Weddell Sea. A greater number of cyclones in these locations tracks over the 15 ERA years. would allow advection along the trajectory paths indicated Depression locations can be examined by generating a earlier. The negative depression density anomaly at 25øE density field using a Cressman weighting scheme, here with a indicates that there are relatively few cyclones in this area 600 km radius of influence. The density field can be during precipitation events. A similar result was found for interpreted as the climatologically preferred location of Greenland by Chen et al. [1997]. The negative anomaly storms. Figure 17a shows that the Weddell Sea and the further indicates the possibility of a dipole structure, as was western Kong Hfikon VII Hav region have fewer cyclones the case for the extreme precipitation events under the than the remainder of the hemisphere. This is due to the influence of blocking. Antarctic Peninsula acting as a barrier that limits the In comparison, the days with moderate (0.1 to 0.5 mm) predominant westerly motion of the storms [e.g., Turner et precipitation in DML show a much weaker signal al., 1998b]. There is a moderate local maxilnum seen in the (Figure 17c). The positive anomaly is at 15øW and close to

16 19,206 NOONE ET AL.' DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS...-"):'5.?-...' :..,., o o Figure 17. (a) Density of depression centers for ERA cyclones lasting at least 36 hours. (b) Depression density anomalies for days with more than 0.5mm and (c) between 0.1 and 0.5mm of precipitation falling at 75øS, 0øE. Contours are 1 x 103 centers within a 600km radius. Negative anomalies have dashed contours, the zero contour is solid. Areas above 2 and 3 are shaded light and dark. The continent has been masked above 1000m. the coast. However, the positive region is broader, suggesting that cyclones anywhere in the Kong Hfikon VII Hav give moisture advection to the continent. The maximum in the Weddell Sea is much diminished, which suggests that it is a feature of higher-precipitation conditions. The negative anomaly is also weaker and farther to the east, suggesting the dipolar flow configuration is less important. Under these cyclonic conditions, the moisture flux would be from the northeast in accordance with the climatological mean. As we have just seen, cyclones in the Weddell Sea and the western Kong Hfikon VII Hav are very important in developing precipitation conditions. To examine the mobility of the cyclones, we categorize them based on the life cycle of individual tracks using three criteria. First, tracks existing in the Weddell Sea (60øW to 30øW and south of 50øS) (WST) are identified. Second, tracks existing in the Atlantic sector (30øW to 0øE, south of 50øS) (AST) but not of the WST type are found. Third, we choose systems that undergo cyclogenesis in the Atlantic sector (30øW to 0øE, south of 50øS) (ACG). These groupings account for all the cyclones in the sector. In the mean, there are 120, 18, and 38 cyclone tracks per year of the WST, AST, and ACG types. These are presented in Figure 18. Cyclones of the WST type can be seen to move east and

17 NOONE ET AL.: DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS 19,207 Figure 18. Depression track density for cyclones lasting more than 36 hours and grouped as (a) WST, (b) AST, and (c) ACG. Contours are in units of tracks within a 600 kin/year. Sectors outlined in solid indicate the region used to select tracks of each type. The continent has been masked above 1000m. along the preferred latitude near 65øS (Figure 18a). A smaller number move first northward before joining the predominant path or dissipating. Cyclones in the southern Weddell Sea move northward along the coast near Coats Land and generally have a lower intensity. The Weddell Sea is known as a region of storm development in the lee of the Antarctic Peninsula. The large localized maximum seen for the WST cyclones suggests stationary development and decay. This was also observed by Jones and Simmonds [1993] and Turner et al. [1998b]. Some cyclones appear in the Weddell Sea after crossing the Peninsula. This occurs at about 72øS at the base of the Peninsula or, more commonly, at 65øS across the tip of the peninsula. In our data it is rare for cyclones to cross the peninsula (eight tracks per year), yet it is considered an overestimate because the peninsula in the ERA orography is 1000 m too low. Nonetheless, cyclones crossing the peninsula tend to be more mobile after deepening in the Weddell Sea. Another origin of the WST cyclones is downstremn of South America and the Drake Passage. Cyclones of the AST category tend to develop under the downstream influence of the Andes. Figure 18b shows that their movement is to the southeast, approaching the 65øS hemispheric band. By definition, these cyclones develop at quite a distance from the Antarctic coast. Northerly advection will have prevailed in the southern Atlantic as the cyclones move to the DML coastal sector, and there will be meteoric

18 19,208 NOONE ET AL.' DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS water available for precipitation. They converge to a maximum occurs near 65øS 0øE. This corresponds to the general increase in storm density eastward seen in Figure 17a. Cyclones at this location would be important for DML mean temperature for that year. For soluble species in the atmosphere, the effect of an unusually large snowfall depends, to some extent, on the reservoir of the individual chemical. In the case of a limited reservoir the trace chemical precipitation with moisture flux from the northeast. Similarly, would be depleted quickly, and the ice core profile would ACG cyclones also have a maximum at this location (Figure 18c), and when formed, they move eastward. The ACG density shows a broadening northward rather than being show low concentrations during the high-snowfall event. This is possibly visible for nitrate in 1981 in both the published profiles from the Coats Land [Mulvaney and Wolff, 1993, confined to the coast. This suggests cyclone development Fig. 2] and the Epica [Isaksson et al., 1996, Figure 3] cores. farther north and subsequent southerly migration into the dominant 65øS cyclone track in some instances. In the case of a reservoir that is quickly replenished, a high accumulation would bring with it a constant flux of the trace chemical, making the integrated annual signal abnormally 7. Discussion of in Situ Data Analysis For the reconstruction of past atmospheric trends from the analysis of ice cores, it is generally assumed that the snowfall samples the atmosphere at a steady rate through the year. Further, a well-defined annual cycle of trace constituents is high. If it were found that there were a few evaporation periods that contributed a large fraction of the annual mean (associated with, say, particularly high winds and unusual temperatures), again there would be implications for the ice core analyses and a possible bias in derived temperatures. The Epica ice core shows an annual accumulation of required for accurate dating by layer-counting techniques. 77ram compared to 32mm in our ERA data. The difference The evidence shown here is that in DML, steady year-round accumulation of small amounts of precipitation is an important contributor to the annual accumulation budget, implying a steady sampling through the year of the atmospheric trace chemistry composition, and of the water vapor isotope composition. These conditions are ideal for the archiving of the seasonal characteristics of the atmosphere. However, it is also clear from the model results that intermittent and highly variable heavier precipitation events occur infrequently, and in some years, this could be up to a quarter of the annual accumulation. This has the potential to severely disrupt the seasonal pattern and affect the annual may be a result of the difficulties the model has with estimating precipitation over Antarctica (in particular, the interior) and may be connected with the poor orographic representation in western DML (overestimated by 1300 m). It is not possible to say whether discrepancies indicate a failure to model the influence of intense baroclinic storms or the general flux of moisture (clear-sky precipitation). Further, differences may be associated with small-scale affects not resolved by the model. The ERA accumulation does not include calculations of drifting snow. Gall e. [1997] suggests that blowing-snow attributes for removal of around 32 mm/year averaged over total of trace constituents in the ice. We can consider a the Antarctic ice sheet. Snowdrift tends to be smaller in number of cases. The /5180 values generally reflect the condensation history of the moisture transport and are used orographically smooth regions and areas away from strong katabatic winds. As such, snowdrift on the DML plateau is by the palcoclimate com nunity as a proxy for temperature. thought to be at least an order of magnitude lower than the Generally, in deep cores these proxies are averaged over accumulation (T. Lachlan-Cope, personal communication, many years. If, over the long term, the seasonal accumulation 1998) but remains a source of uncertainty in our findings. It is profile is biased toward particular months, then the derived temperature will be biased toward the temperature of the higher snowfall months. If, in other climatic regimes (e.g., the suggested therefore that the model estimate is quite reliable in assessing the behavior of area-averaged accumulation for DML. Samples from a single core are subject to the local glaciated period), the storm tracks prevailing in the current topographically influenced snowdrifts and anomalies climate changed to alter the seasonal pattern of snowfall, then associated with sastrugi in any given year, which cause this would result in an apparent change in temperature. "deposition noise" in the ice-core-derived accumulation Krinner et al. [1997] estimated the temperature bias from changes of local meteorology, including the seasonality of precipitation, by considering a model simulation of the Last Glacial Maximum. Using a proxy for isotopically derived surface temperatures, they find that isotopic data from much series. Also, the ERA model does not explicitly include the effects of timing as a surface process (formation of ice on the surface directly from the vapor phase) which may be significant at certain sites. However, moisture and heat are conserved quantities, and the precipitation scheme is of the East Antarctic would contain only a minor temperature sufficien to resolve condensation low temperatures. bias. In DML, however, the estimated bias ranges from near 10øC between the coast and 75øS to, again, minimal bias on the plateau closer to the pole. They reason that the changes in Both the ERA and the Isaksson et al. [1996] accumulation series show variability of up to around 50%. Isaksson [1994] relates the core data to ENSO over a 127 year period. It was the meteorology may explain the large difference between seen earlier that the combined ERA/EOP series shows higher isotopically derived temperature and borehole thermometry in the case of central Greenland. Their assessment shows that totals in warm ENSO years. Also, the "stacked core" given by Isaksson and Karldn [1994a] is remarkably similar to our such biases may exist and will need to be resolved ERA time series (see their Figure 9). In particular, there are quantitatively for the EPICA coring site in DML. In high-resolution ice core studies, where the individual annual layers are dated and studied in detail, any year with an unusually biased accumulation pattern (for example, the very strong peaks seen in and again in This is an exciting result given that neither a single glaciological series nor the series from an at nospheric model may have been expected to be accurate in this regard. Our qualitative high January snowfall in 1981 contributing 30% of the annual comparison suggests that both in situ and modeled accumulation reported in this paper) is likely also to show a high mean annual b180, which would be interpreted as a high accumulation capture some signature of atmospheric signals on a global scale. It follows then that this is encouraging for

19 NOONE ET AL.: DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS 19,209 the selection of DML as a deep coring site because global teleconnections are a matter of investigation [douzel et al., 1996]. Paleoclimate modeling suggests differences in storm track locations and intensity compared to the present climate [e.g., Krinner and Genthon, 1998]. We have shown that the precipitation is a result of these storms. As such, we would Sinclair [1981] remarks that large systems would play an important role in the heat budget of the continent. The present study has not addressed the extent to which transported moisture is removed from the atmosphere during the event. It could be that the moisture from these storms resides in the continental air until conditions allow further precipitation, or that the moisture of lighter precipitation is more local in expect altered accumulation during the glacial periods. origin and falls as soon as it reaches a critical temperature in Estimates of the precise nature of past atmospheric circulation the polar atmosphere. are difficult, as there are many factors that must be quantified. One of the important findings of this study is that there are For example, it is generally assumed that the topographic a number of matters regarding Antarctic precipitation that are features of the ice sheet remain similar. Also, influences on the atmosphere, such as the sea ice concentration and therefore the heat fluxes near the continent, need to be addressed. Further, variations in the source of various yet to be resolved, even under a present day climate regime. Among these are a successful estimate of the moisture origins, a full understanding of inland precipitation, and the role of extreme events on timescales from synoptic to chemicals must be estimated. In each case we find that an millennial. In the absence of observational data, we have understanding lies behind the conundrum: To be able to use a proxy to imply climate history, one needs to know the processes affecting the proxy, while to infer a history of the proxy, one needs to know the climatological influences. extensively used the daily precipitation estimates from an atmospheric model. While it is thought that the model is realistic in some respects, our findings must be viewed with caution given the limitations of the model and the fact that we have only considered results from a single model. The 8. Conclusions ECMWF plans to again reanalyze atmospheric observations. A full summary of precipitation in DML has been given. It has been shown that both small amounts of precipitation falling on many days and large amounts falling on a few days have equal contributions to the annual accumulation. While the general trend in the interannual variability is across both, the higher precipitation amounts are associated with the greatest variability. This has been linked to the behavior of the cyclonic variability of the hemisphere. Storms centered in and to the north of the Weddell Sea are seen to play an important role in the accumulation. For the highestprecipitation amounts, there are links to the occurrence of blocking and related amplification of long planetary waves. This feature was also noted by Bromwich [1988] to play an important role for Antarctic precipitation in general. The associated strong upper level advection brings moisture and higher temperatures during the events. As the warmer air is generally of midlatitude origin, the snow accumulated would represent nonlocal conditions. Noone and Simmonds [1998] note that an ice core would thus represent atmospheric conditions that are warmer than the local mean. These data will be for a 40 year period and will utilize the most recent model improvements, including the west DML orography. The data will be crucial in verifying the results reported here and understanding trends that were difficult to identify in our 15 year data. Also, the EPICA pre-drilling glaciological survey has already revealed interesting results in this much unexplored area. This will be furthered by full interdisciplinary analysis of the deep EPICA core. Acknowledgments. Special thanks are due to Steven Leonard for obtaining and manipulating the atmospheric data sets. Gareth Marshall is also thanked for providing the cyclone track data. M. Whittaker of the Antarctic Meteorology Research Center in the Space Science and Engineering Center at the University of Wisconsin-Madison is applauded for his work, maintaining the satellite data archive from which we obtained the NOAA polar imagery. We also wish to thank the British Atmospheric Data Centre for disseminating the ERA data. References Aristarain, A. J., J. Jouzel, and M. Pourchet, Past Antarctic Peninsula climate ( ) deduced from an ice core isotope While precipitation (and accumulation) is considered to be record, Clim. Change, 8, 69-89, biased by strong local effects [Isaksson et al., 1996], our Bromwich, D. H., Snowfall at high southern latitudes, Rev. results show that there is a definite signal attributed to larger Geopohys., 26, , Bromwich, D. H., F. M. Robasky, R. I. Cullather, and M. L. Van (synoptic) scale phenomena. Indeed, the storms are related to Woert, The atmospheric hydrologi cycle over the Southern the global ENSO and SAO. 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21 NOONE ET AL.: DRONNING MAUD LAND ACCUMULATION CHARACTERISTICS 19,211 Vaughan, D. G., J. L. Bamber, M. Giovinetto, J. Russell, and A. P. R. Cooper, Reassessment of net surface mass balance in Antarctica, J. Clint., 12, Zwally, H. J., and M. B. Giovinetto, Accumulation in Antarctica and Greenland derived from passive-microwave data: A comparison with contoured compilations, Ann. Glaciol., 21, , R. Mulvaley, and J. Turner, British Antarctic Survey, National Environmental Research Council, High Cross, Madingley Road, Cambridge, CB3 0ET, England. (rmu bas.ac.uk; jtu bas.ac.uk.) D. Noone, School of Earth Sciences, University of Melbourne, Parkville, VIC, 3052, Australia. (dcn met.unimelb.edu.au.) (Received September 16, 1998; revised May 4, 1999; accepted May 19, 1999.)