LONG-TERM FAST-ICE VARIABILITY OFF DAVIS AND MAWSON STATIONS, ANTARCTICA

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1 Ice in the Environment: Proceedings of the 16th IAHR International Symposium on Ice Dunedin, New Zealand, 2nd 6th December 2002 International Association of Hydraulic Engineering and Research LONG-TERM FAST-ICE VARIABILITY OFF DAVIS AND MAWSON STATIONS, ANTARCTICA Petra Heil 1,2 and Ian Allison 3,4 ABSTRACT Fast-ice and atmospheric measurements from two locations off the East Antarctic coast (Davis and Mawson) are available for individual years in the 1950s to 1970s and more frequently during the 1980s and 1990s. We study changes in the annual maximum ice thickness, snow depth, and the date of maximum ice thickness. Annual maximum ice thickness does not show distinct longterm trends at either location, however the interannual variability increased significantly during the 1980s at Mawson and during the 1990s at Davis. Interannual variability in snow depth does not affect maximum ice thickness. At Davis and Mawson there is a trend in the date of annual maximum ice thickness, with the maximum occuring later in the 1990s than in the 1950s. At Davis the rate is +4.3 days per decade; at Mawson this delay is +4.8 days per decade. Our analysis suggests that these changes in the fast-ice properties are likely to be related to changes in the large-scale atmospheric forcing. INTRODUCTION Observations of the Antarctic pack ice (e.g., Parkinson, 1994; Worby et al., 1998) have shown interannual variability in the physical parameters of the drifting ice. Changes in equatorward ice extent and ice concentration, as well as in the duration of the seasonal seaice coverage have been identified at several locations in the Antarctic pack. Information on long-term changes in the ice pack is mostly restricted to recent decades (mid 1970s onwards) when satellite-based monitoring became available. To supplement the relatively short pack-ice record, we investigate data from two measurement sites on the landfast sea-ice off East Antarctica. Davis (77 58 E, S) and Mawson (62 52 E, S) provide intermittent time-series of fast-ice thickness measurements. While fast-ice data were consistently collected during the 1980s and 1990s, individual measurements were taken during the 1950s, 1960s and 1970s. Hence the total fast-ice record provides useful insight into long-term changes in the fast-ice characteristics. At Davis the growth of fast ice is largely determined by atmospheric forcing. In the 1 IARC/Frontier, University of Alaska, Fairbanks, USA 2 Now at: TPAC, University of Tasmania, Hobart, Australia 3 Antarctic CRC, University of Tasmania, Hobart, Australia 4 Australian Antarctic Division, Kingston, Australia

2 vicinity of the measurement site the water depth is 5 to 33 m, and there is little oceanic heat available at the underside of the fast ice during winter after the oceanic mixed layer has been cooled to the freezing point temperature. Near Mawson a glacial canyon (in excess of 300 m) penetrates the continental shelf towards the coast. This allows the onshelf movement of water that has a higher heat content due to intrusions of warm offshore water, such as Circumpolar Deep Water. Hence, at Mawson oceanic forcing also influences ice growth, as shown in a previous study (Heil et al., 1996), which used a thermodynamic model of sea-ice growth combined with meteoreological and fast-ice data to estimate the oceanic heat flux. The sites chosen here thus represent two different ocean-atmosphere forcing regimes for East Antarctic fast ice. FAST-ICE OBSERVATIONS Fast ice forms and decays in a seasonal cycle along the Antarctic coast. Fast ice is immobile, hence, in the absense of platelet ice accretion, its growth is determined by thermodynamic processes only. Observations of fast-ice parameters are useful in studying the direct link between external forcing, by the ocean and atmosphere, and ice growth. Annual measurements of ice and snow thickness from the single Davis site (M1) are available for 1957 and 1958, for 1979 to 1986 and from 1992 onwards. In this study we use data from two different locations near Mawson to maximize the number of years with available data. Observations at West Bay (site A) near Mawson were made in 1958, 1965, 1969, and intermittently from 1976 onwards. Measurements from the harbour (site B) off Mawson Station are available for 1955, 1957, 1958, 1962, 1967 and , 1981, 1982, 1984, 1987 to 1989 and from 1995 onwards. The two sites are within 1 km of each other, separated by a small peninsula. Data obtained at Mawson Harbour in 1955, 1957, 1962, 1967, 1970 and 1971 are combined with the observations from West Bay to form one time-series with 27 years of data. The Mawson harbour data are corrected for slight differences in the annual maximum ice thickness ( m) and dates of fastice formation ( 3.0 days) between the two sites, derived from years when measurements were made at both. The blended data set is identified as site A. The accuracy of the thickness measurements is estimated to be better than 0.02 m (Heil et al. 1996). Time-series from the two data sets are used to analyze the long-term changes in East Antarctic fast ice in annual maximum ice thickness and the date of this maximum. Annual maximum ice thickness For locations, where the summer ice break-out occurs well after the date of annual maximum ice thickness, the maximum ice thickness is a good measure of the integrated (ocean and atmosphere) climatological condition at that site. Based on 17 years of data the overall annual maximum ice thickness measured at Davis Station is m. The standard error, which is an estimate of the unexplained variation in the ice thickness and which can be derived by dividing the standard deviation of all ice thicknesses by the square root of the number of data points available (Kreyszig, 1988), is ± m. The interannual variability is high (Fig. 1). In 1993 the fast-ice thickness peaked at m, while in 1998 the annual maximum thickness was only m. This low ice thickness is caused by an atypical severe midwinter breakout during July (Day of Year [DoY] 207) 1998 at site M1, a measurement site that is normally not affected by midwinter loss of fast ice. The Davis

3 time-series shows decreasing annual maximum ice thickness later in the 1990s (excluding data from 1998) but relatively constant annual maximum ice thicknesses during the 1980s and the early 1990s. Maximum ice thickness (m) Mawson, Site A * Year Figure 1: Time-series of the annual maximum fast-ice thickness for Davis (top) and Mawson (bottom). Also shown are the decadal trends of annual maximum thickness (solid lines) and the long-term average (dotted line). See text for an explanation on the data point for For Mawson, 27 years of available fast-ice data from West Bay (site A ) have a mean annual maximum ice of m (standard error of ± m). The thinnest annual maximum ice occurred in 1970 (1.293 m) and the thickest in 1986 (1.910 m). Analyses of the meteorological conditions at both locations suggest that the thinner ice at Mawson is most likely caused by the supply of oceanic heat underneath the ice rather than by differences in atmospheric conditions. There is a much larger long-term variability in annual maximum ice thickness at Mawson than at Davis. Values in the 1950s and 1960s are about 0.25 to 0.35 m less than the mean annual maximum ice thickness during the 1980s. There is a 0.22 m decrease in mean annual maximum ice thickness from the 1980s to the 1990s. Heil et al. (1996) identified the increase of ice thickness at Mawson from the 1950s and 1960s to the 1980s as a consequence of changed ocean conditions. They hypothesized that in the 1980s reduced ice concentration in Prydz Bay, upstream from Mawson, cooled the ocean so that less heat flux was available to the ice downstream. Over the last two

4 decades (1980s and 1990s) a trend of 0.42 m decrease per decade (statistically not significant) is seen in the Davis maximum ice thickness (excluding data for 1998). At Mawson the thinning trend over the 1980s and the 1990s is 0.19 m per decade (statistically significant at the 90 % level). It has not been established if the thinning of the Mawson fast ice during the late 1990s has been triggered by upstream events that influence the ocean properties, or whether atmospheric changes alone are likely to have contributed to the recent thinning. At both sites interannual variability dominates the time series of annual maximum ice thickness. At Mawson the interval from the late 1970s to the late 1980s exhibits larger interannual variability than during the 1990s, and vice versa at Davis. Decadal means have been calculated albeit for variable numbers of years of data availability. The decadal means at Mawson (1950s (3 years): 1.43 m; 1960s (4 years): 1.37 m; 1970s (5 years): 1.50 m; 1980s (9 years): 1.72 m; 1990s (6 years): 1.49 m) are not as constant at Davis (1950s (3 years): 1.68 m; 1980s (8 years): 1.69 m; 1990s (6 years): 1.67 m). The contemporaneous increase in annual maximum ice thickness at both East Antarctic locations suggests that large-scale changes in the atmosphere causes the fast-ice variability. Snow thickness at annual maximum ice thickness The growth of sea ice is not only a function of the atmospheric and oceanic forcings, but it also depends on the thermal insulation provided by the sea ice and the overlying snow (e.g., Maykut, 1982). To further evaluate the changes in annual maximum ice thickness we study the annual evolution of the snow cover and the snow thickness during time of maximum ice thickness. The measurement sites at Mawson are adjacent to the Antarctic plateau, and strong katabatic winds prevail in the area. This offshore wind generally keeps the fast ice clear of snow (Fig. 2 bottom). Note that there are no observations of snow thickness at West Bay for 1955, 1957, 1967, 1980, 1996 or The measurement site near Davis is on the edge of the Vestfold Hills, a low lying icefree area adjacent to eastern Prydz Bay. At the observational site there are no katabatic winds and strong wind events are rare. Falling snow is often redistributed within the fast-ice area while bare ice is common close to the coast, snow deposited in the lee of islands can grow in excess of 1 m. Site M1 at Davis is about 800 m from the nearest coast in an area usually covered with sastrugi. The dominant time of snowfall at Davis is during autumn and early winter. Accordingly, the snow depth grows rapidly in late autumn, and levels out towards an equilibrium depth within about 3 months of fast-ice formation (Fig. 3). New snow fall frequently occurs during storm events and is immediately redistributed by high wind. The snow cover on Mawson fast ice is thin in all years and there is no correlation between the interannual variability in snow thickness and fast-ice thickness at that location. The changes in annual maximum ice thickness that have been identified at Mawson cannot be attributed to variability in the snow cover. At Davis also, the interannual variability in annual maximum ice thickness does not correlate with interannual changes in snow depth (R d = 0.14). This is different than in the Arctic, where interannual changes in the snow cover may be reflected in the interannual variability of ice thickness (Brown and Cote,

5 Snow during Zmax(ice) (m) Mawson, Site A * Year Figure 2: Time-series of snow thickness at time of annual maximum fast-ice thickness for Davis (top) and Mawson (bottom). Also shown are the decadal trends (solid lines) and the long-term average (dotted line) Snow depth (m) Time (DoY) Figure 3: Seasonal evolution of measured snow depth at site M1 Davis station (dots) based on data from The solid line is obtained by applying a five-term binomial filter.

6 1992). Date of annual maximum ice thickness Interannual variability in the date of maximum ice thickness is another indicator of change in the forcing parameters. At Davis the fast ice usually reaches its maximum thickness between mid October and late November (Fig. 4 top), while the Mawson ice thickness reaches its maximum during October (Fig. 4 bottom), about 3 weeks earlier than at Davis. The date of maximum ice thickness does not correlate highly with the date of ice formation (e.g., at Davis the correlation is 0.02). At both sites there is considerable interannual variability in the date of maximum ice thickness. At both sites there is a trend towards later occurrence of the date of maximum ice thickness (Davis: +4.3 days per decade, statistically not significant; Mawson: +4.8 days per decade statistically significant). An exception to this trend occurred at Davis during 1998 (star in Fig. 4 top) due to an atypical midwinter breakout, which also removed the fast ice from site M1. DoY of max. ice thickness Mawson Site A * Year Figure 4: Time-series of the date of annual maximum ice thickness at Davis (top) and Mawson (bottom). The dashed lines represent the interannual trend. See text for an explanation on the data point for The long-term trend towards later occurrence of the annual maximum ice thickness is similar at both locations. This suggest that large-scale atmospheric (or oceanic) changes are responsible for the trend. Delayed occurrence of maximum ice thickness is likely

7 to be a consequence of warmer surface temperatures during winter, as have been identified from meteorological data from Davis Station by Heil (in preparation). That analysis further shows that at Davis the winter warming is accompanied by summer and autumn cooling. The annual mean temperature at Davis appears to have warmed slightly since the mid 1970s. A similar analysis of data from Mawson reveals trends towards winter and spring warming, which are counterbalanced by a strong summer and autumn cooling. Annual mean Mawson temperatures show a slight cooling trend for the interval from 1975 to 2000, which is in agreement with annual temperature trends reported for 1959 to 1996 by Jacka and Budd (1998) and for 1979 to 1998 by Comiso (2000). The seasonal pattern of winter warming and summer and autumn cooling is compatible with the mechanism of a Southern Hemisphere annual mode (SAM; Thompson and Solomon, 2002). From 1985 onwards the lower polar stratosphere has cooled significantly ( 10 K by the late 1990s) over the months of October and November (Trenberth and Olson, 1989; Randel and Wu, 1999). Over the same time interval the springtime breakdown of the polar vortex in both the troposphere and stratosphere has been delayed and takes now place in summer rather than in spring (e.g., Hurrell and van Loon, 1994; or Waugh et al., 1999). Thompson and Solomon (2002) argue that the vertical coupling between troposphere and lower stratosphere is strongest at times of perturbation in the polar vortex. This together with the spring time cooling in the lower polar stratosphere would cause tropospheric cooling from summer onwards over East Antarctica. DISCUSSION AND CONCLUSIONS Observations from two fast-ice locations off the East Antarctic coast show a recent decrease in ice thicknesses and a delayed occurrence of the annual maximum ice thickness. These trends are however embedded in large interannual variability in the physical parameters. The annual maximum ice thickness at Davis is slightly higher than that at Mawson. This is due to the availability of oceanic heat at the latter, from the onshore transport of warmer deep waters. The larger interannual variability in maximum ice thickness at Mawson is explained by variability in the oceanic heat flux at that location. As shown by Heil et al. (1996) the variability in oceanic heat flux at Mawson may be influenced by upstream pack ice conditions. The recent (decadal) decline in the annual maximum ice thickness at both sites is most likely associated with changes in the atmospheric forcing. We have shown that interannual changes in the depth of the snow cover do not influence the variability in the annual maximum ice thickness. The delayed occurrence of the annual maximum ice thickness in recent decades agrees well with observations of trends, such as milder winter temperatures during the 1990s, which may be interpreted as a consequence of the occurrence of more severe SAM events (Thompson and Solomon, 2002). ACKNOWLEDGMENT Volunteers of numerous Australian National Antarctic Research Expeditions (ANARE) are thanked for their assistance in collecting fast ice and snow measurements off Davis and Mawson stations. The atmospheric data used here were obtained by the Bureau of Meteorology (BoM), Tasmania and Antarctica Regional Office, Hobart, Australia and

8 supplied via the BAS/ICD meteorological information database. This research was in part funded by the Frontier Research System for Global Change through the International Arctic Research Center, University of Alaska, Fairbanks (USA). REFERENCES Brown, R.D. and Cote, P. Interannual variablity of landfast ice thickness in the Canadian High Arctic, Arctic, 45: (1992). Comiso, J.C. Variability and trends in Antarctic surface temperature from in situ and satellite infrared measurements. J. Clim. 13: (2000). Heil, P. Interactions between fast ice and local atmospheric conditions at Davis Station, East Antarctic: A case study. 15pp (in preparation). Heil, P., Allison, I. and Lytle, V.I. Seasonal and interannual variations of the oceanic heat flux under a landfast Antarctic sea ice cover. J. Geophys. Res. 101: (1996). Hurrell, J.W. and van Loon, H. A modulation of the atmospheric annual cycle in the Southern Hemisphere. Tellus 46: (1994). Jacka, T.H. and Budd, W.F. Detection of temperature and sea-ice extent changes in the Antarctic and Southern Ocean, Ann. Glaciol. 27: (1998). Kreyszig, E. Advanced engineering mathematics. John Wiley & Sons, New York, 6th edition (1988) 1294p. Maykut, G.A. Large-scale heat exchange and ice production in the central Arctic. J. Geophys. Res. 87: (1982). Parkinson, C.L. Spatial patterns in the length of the sea ice season in the Southern Ocean. J. Geophys. Res. 99: (1994). Randel, W.J. and Wu, F. Cooling of the Arctic and Antarctic polar stratosphere due to ozone depletion. J. Clim. 12: (1999). Thompson, D.W.J. and Solomon, S. Interpretation of recent Southern Hemisphere climate change. Science 296: (2002). Trenberth, K.E. and Olson, J.G. Temperature trends at the South Pole and McMurdo Sound. J. Climate 2: (1989). Waugh, D.W., Randel, W.J., Pawson, S., Newman, P.A. and Nash, E.R. Persistence of the lower stratospheric polar vortices. J. Geophys. Res. 104: (1999). Worby, A.P., Massom, R.A., Allison, I., Lytle, V.I. and Heil, P. East Antarctic sea ice: a review of its structure, properties and drift. In Antarctic Sea Ice Physical Processes, Interactions and Variability, Antarct. Res. Ser., 74, M. O. Jeffries, ed., AGU, Washington, D.C. (1998)