2 Polar Research Institute of China,Shanghai 200136,China
1 Introduction Antarctica is the cold source of the global atmosphere. As a key factor of Antarctica,sea ice plays an importantrole in global climate processes[1, 2, 3]. It prevents the direct exchange of heat flow between the atmosphere and ocean by both increasing surface albedo and insulating the relatively warm ocean from the atmosphere in winter. Heat flux in the pack ice zone is complex because of the wide difference in albedos between open water,snow,and sea ice and because of the thermodynamic interactions between the pack ice and leads. What’s more,wave-ice interaction has become the focus of research on sea ice dynamics because it is the most recurrent environmental influence of the marginal ice zone[6, 7]. To obtain a better understanding of all the processes mentioned above,it is necessaryto establish basic information regarding sea ice,e.g.,sea ice extent and its variation,and snow and ice thicknesses and their distributions.
Several methods have been applied to survey the thickness and extent of sea ice in Antarctica. Satellite remote sensing is an effective tool for assessing large-scale ice cover[8, 9],rapidly obtaininglarge amounts of sea ice data. However,some limitations should be taken into consideration. For example,satellite sensors have low spatial resolution whendistinguishing water from sea ice. Furthermore,the reliability of ice thicknesses determined by satellite altimeters depends on the accuracy of the snow depth data set and the densities of the snow and ice[10, 11]. In addition,satellite-derived ice concentrations could cause errors when dealing with high or low concentrations of sea ice. In situ measurement is another method for acquiring basic parameters of sea ice and snow in Antarctica. Although it can provide an accurate data set,the amount of information recorded from ground-based observations is generally poor in the region of interest because of the adverse climaticenvironment. Owing to their mobility,ship-based observations[14, 15] provide a larger survey platformthan ground-based observations. Compared with satellite remote sensing,they have higher spatial resolution and provide relatively reliable results. Thus,it is preferential that satellite remote sensing results should be combined with ship-based observations to evaluate sea ice and snow in Antarctica.
As part of the 29th ChineseNational Antarctic Research Expedition,ship-based observations were conducted to survey the sea ice and snow in Prydz Bay and the surrounding waters. In this paper,we presentdetails of the sea ice extent and its variation,and the ice and snow thickness distributions and their variations with time in the observed zone.2 Observations 2.1 Study area and cruise tracks Both the thicknessand the extent of sea ice in Prydz Bay and the surrounding waters change seasonally. In winter,Prydz Bayand the surrounding waters are covered completely by sea ice,which can be up to 2-m thick. In summer,most sea ice in this region melts,although some,particularly near the coast,survivesthroughout the summer. We conducted three ship-based observational campaigns from 28 November 2012to 3 February2013 in the study area (64.40°S-69.40°S,76.11°E-81.29°E). Figure 1 shows the study area and the three tracks of the R/ V XUE LONG icebreaker. The start and end dates of each track are indicated in Table 1. The three observational campaigns lasted five,three and four days,respectively.
|Track No.||Date (Year/Month/Day)||Pack ice extent||Open water extent||Fast ice extent|
Because ship-basedobservations can only observe ice along the cruise track,we presentremotely sensed images of sea ice extent and concentration in Prydz Bay in Figure 2. The first anspan>d second observations were conducted in the Antarcticmidsummer with intense solar radiation,and the sea ice extent changed drastically during this period. According to the data recorded by the meteorological station onboard the R/V XUE LONG icebreaker,the daily mean air temperatures and wind speeds from 27 November to 29 Novemberwhen the R/V XUE LONG icebreakercruised south through the pack ice zone were −0.6,−1.7,and −2.8℃ and 27.3,15.6,and7.5 m∙s-1,respectively. When the R/V XUE LONG icebreaker moved north through the pack ice zone from 17 December to 18 December,the daily mean air temperatures and wind speeds were 1.2 and −1.1℃ and 3.2 and 6.0 m∙s-1,respectively. During the first observation,the northern edge ofthe pack ice in the study area was between 63.50°S and 64.40°S and the southernedge was between 66.50°S and 67.20°S.Compared with the first observations,the southern edge of the pack ice during the second observational period changedlittle,whereas the northern edge exhibited significant retreat.Wind-generated waves can break pack ice into smaller pieces and a rise of air temperature can contribute to the ice melting. As a supplement,sea ice drift may also play a role in the retreatof the northern edge of the pack ice. The Antarctic Divergence near Prydz Bay and the surrounding waters is generally between 65°S-66°S. The northernpart of the pack ice was locatednorth of the Antarctic Divergence,where the mean current was to the east. Mean ocean circulation can cause sea ice drift over long time scales,but a drasticdecline in ice extent within a short period may also be influencedby synoptic factors,for example,sea ice drift driven by wind. Despite the enlargement of the open-water area in the south of the pack ice over a long time scale,the southern edge of the pack ice may move southwards in a short time,causing adecrease in the open-water area. This phenomenon can be found by comparingthe southern edge of the pack ice in Figure s 2c and 2d. Ice melt in the inner part of the pack ice zonewas slower,becauseof the relatively calm environment. Ice concentration was lower at the edge of the pack ice zone than in the inner part,as demonstrated in Figure 2. The northernedge of the fast ice retreated by only 6.7 km,which indicates a decay rate of 0.37 km∙d-1. Compared with thepack ice in the same sector,the fast ice in the higher-latitude zone was thicker and suffered less environmental impact,which lead to a significantly slower variation in extent and much longer melting period.3.2 Ice thickness distribution We obtained481,588,and 29 ice thickness values via the ship-basedvideo camera method during the three observational campaigns,respectively. We also obtained 23 ice thickness values in Huaxia Fjord by drilling holes,covering the latitude range of 64.40°S-69.40°S. In addition,77 and 56 values of ice concentration from the pack ice zone were recordedduring the first and second observational campaigns,respectively. Most thicknessvalues of the two earlier observational campaigns were obtained from the fast ice zone,while the third observational campaign contained only values of pack ice thickness. Thicknesses were divided into groups accordingto ice condition and the number of thicknesssamples at different latitudes. The means and standarddeviations of the ice thicknesses in each groupwere determined. Figure 3 presents the ice thicknessvariation with latitude,in which a,b,and c representthe three tracksshown in Table 1. Figure 4 presents the pack ice concentration with latitude during the first and second observational campaigns.
Generally,ice showed an incrementin thickness with increasinglatitude from the end of November to the middle of December. In the pack ice zone,ice thickness did not increasedirectly with latitude,but it fluctuated. In Figure 3a,the mean values of ice thicknesson the edge of the pack ice zone (64.40°S-64.60°S,67.67°S) were 38 and 18 cm,respectively. These are much smaller than in the inner part,where the values ranged from 42 to 77 cm. This was attributed to the open water. On the one hand,water without ice cover tends to form waves more easily under the action of wind,promoting the breaking up and melting of the pack ice.On the other hand,water absorbs more heat from solar radiationthan ice and snow do; thus,the heat flow in the horizontal direction is enhanced,which results in significant lateral melting of the pack ice. Therefore,ice concentration at the edge of the pack ice zone was lower than that in the inner part (Figure 4). In Figure 3b,the extent of the pack ice can be seen to be narrower than that shown in Figure 3a. Pack ice thicknesstends to be uniform; the mean value between 65.50°S and 66.50°S was around 65 cm,which is greater than the thicknesses of 42 and 58 cm at the same latitudes in Figure 3a. The thicknessdistribution of Antarctic pack ice is determinedby both thermodynamic and dynamic processes. Sea ice drift physically redistributes the pack ice and changes its extent and concentration. Through deformations,such as rafting,piling up,and ridge building,the distribution of ice thicknessis changed. Furthermore,the cruise tracks of the observational campaignscannot coincide exactly,which is whygreater ice thicknesses were found in the pack ice zone at some latitudes during the second observational campaign than in the first. In Figure 3c,pack ice thickness is shown to have changed little with latitude;the mean values are around 30 cm.
In contrastto the pack ice,ice thickness in the fast ice zone increasedobviously with decreasing distance to the coast,which can be established from the mean values. The mean value of fast ice thicknesson the outer part of the fast ice zone (69.10°S) in Figure 3a was 88 cm. However,the value obtained by drilling holes in Huaxia Fjord (69.40°S) near the coast,increased rapidly to 140 cm. The same regularity can be found in Figure 3b. Ice in the inner part ofthe fast ice zone,taking Huaxia Fjord as an example,was thicker than in the outer part,which was caused by the geomorphology. Many small islands are scattered in the coastal area off ZhongshanStation,and a large number of icebergs become grounded in this area,making it a place that suffers little from dynamic action. Thus,the growth anddegradation of sea ice near the coast is almost entirely determined by thermodynamic process,and the sea ice there is thicker than further offshore. We discovered that the mean values decreasedby about 10 cm,when comparing the fast ice thicknesses in Figure 3b with the ice thicknesses at similar latitudes (69.15°S-69.20°S) in Figure 3a; the thicknessvariation over time in the fast ice zone was slow.
Standard deviationsof pack ice thickness in Figure 3 show no obvious regularity with latitude. The standard deviations in Figure 3a range from 5.4 to 33.8 cm,which is attributedto the small number of ice thickness samples captured by the video camera in the pack ice zone,randomness of ice monitoring by video camera,and the complex ocean dynamic and thermodynamic environment of the area. In contrast to the pack ice,the dispersion degree of fast ice thicknesses increases with increasing latitude (Figure s 3a and 3b),leadingto larger standard deviations.
Figure 5 demonstrates the frequency distributions of ice thicknesses in the first two observational campaigns,μ representsthe expected value of thickness,i.e.,abscissa values of the peak on curve; σ representsmean square deviation. The probabilities of ice thickness distributed within the range of μ±σ in Figure s 5a and 5b are 0.690 and 0.641,respectively,while that of the standard normal distribution is similar at 0.683. Ice thickness followed an approximate normal distribution in the period between late November (Figure 5a) and mid-December (Figure 5b). Two peaks of 65 and 90 cm are found in Figure 5a. However,in Figure 5b,it changes to a unimodal distribution with a maximum of 75 cm. Theexpected value (μ) changes little between the two earlier observations,but the mean square deviation (σ) decreases by about 10 cm. In Figure 5b,the proportion of ice thinner than 10 cm and thicker than 120 cm decreasescompared with Figure 5a,whereas it increasesfor ice thickness between 60 and 120 cm. Young ice,which is generallythin and located in the pack ice zone,melts quickly. The decrease of thick ice occurs in the fast ice zone and thus,ice thickness distribution tends to be more concentrated and homogeneous in mid-December (79.1±19.1 cm) than it is in late November (79.7±28.9 cm). During the third observational campaign,the sea ice was in the final phase of the melting period and only 29 pack ice thicknesses were obtained between 68.50°S and 68.90°S.This sample size is too small to represent the real situation accurately and therefore,the statistical parameters of μ and σ were ignored. As a comparison,the mean value of the 29 thicknesses from the third cruise was 29.5 cm,which is much smaller than that determined from the two earlier observational campaigns.3.3 Snow thickness distribution We obtained 226 and 409 snow thickness values by ship- based methods during the first two observational campaigns,respectively. Snow thickness values were not acquired during the third cruise because sea ice concentration during this timewas low and the snow thicknesses were so thin that
it could not be recognizedby the video camera. Figure 6 presents the snow thickness variation with latitude,which is quite different to that of the ice. Ice thickness shows a huge difference between the pack ice and fast ice,whereas snow thickness shows no obviousdifference betweenthem. The mean value of the variationof snow thickness with latitude is insignificant; the ranges of variation in Figure s 6a and 6b are 5.5-15.2cm and 10.5-22.2 cm,respectively,with a maximum extent of 11.7 cm. In the pack ice zone,snow at the same latitude(65.0°S-67.0°S) during the second cruise wasthicker than that in the first,similar to the ice thickness mentioned in section 3.2. Because snow exists on top of the sea ice,movement of sea ice causes the redistribution of snow thicknessto some degree. Furthermore,persistently strong winds over the Antarctic sea ice,typically observed following precipitation events,is another key element of snow thicknessdistribution[23, 24]. The redistribution of dry unconsolidated snow is initiatedat wind speeds in excess of 6-8 m∙s-1. As an addition,slush generated by rain and melted snow refreezesinto ice,reducing the snow thickness. Despitethe unimpressive distribution of mean snow thicknesswith latitude,the mean snow thickness reaches a peak in the highest latitude of the fast ice zone in Figure 6; the latitude at which the ice was relatively thick. Mean snow thicknessdecreases by about 4.7 cm in Figure 6b comparedwith Figure 6a at similar latitude (69.10°S- 69.20°S) in the fast ice zone. The standard deviation of snow thickness has a similar variationtrend as the mean value (Figure 6),i.e.,the area with thicker snow generally has a greater dispersion degree of snow thickness,and vice versa.
Figure 7 demonstrates the frequency distributions of snow thicknesses in the two earlier cruises. Snow thicknesses follow an approximate normal distribution,but tailing slightly to the right; thus,the fitted normal distribution curves are unsymmetrical in Figure 7,which is different to the curve ofice thickness distribution in Figure 5. Therefore,there is a much greater probability of snow that is thinner than the mid-valueof thickness than thicker. Multiplepeaks are found around μ and μ±σ in Figure 7a becausethe snow thickness distribution was scatteredduring the first cruise. Similar to the distribution of ice thicknessin Figure 5b,there is only onepeak within the range of μ±σ in Figure 7b. The variation of snow thicknessdistribution in Figure 7 can be explained by the decreasing proportion of snow that is thinner than 6 cm and between 14 and 20 cm. Similar to the variation of ice thickness distribution in Figure 5,the snow thickness values in Figure 7b are more concentrated than in Figure 7a,mostly distributed over the range of 9.6±3.4 cm. An interesting and unusual fact is that the proportion of snow between 25 and 35 cm increases,rather than decreases. This could be interpreted as follows. We observed snow in a higher-latitude zone (69.25°S)during the second observational campaign (Figure 6b) than during the first (Figure 6a); the mean value of snow thicknessin the higher-latitude zone was 22.2 cm,which is larger than any other observedzone. By comparing the expected value (μ) of snow and ice thickness during the two earlier cruises,it can be seen that the former decreases by 2 cm,whereas the latter almost retains the same value. As mentioned above,the mean thickness of ice in latitudes 69.15°S-69.20°S,decreased by about 10 cm,whereas that of snow in similarlatitudes of 69.10°S-69.20°S decreased by 4.7 cm. Considering that the mean thickness of ice is about 5-7 times larger than that of snow in those latitudes,we conclude that snow is more sensitiveto environmental change than ice is.4 Conclusions We conducted three ship-based observational campaigns tosurvey the sea ice and snow in Prydz Bay and the surroundingwaters (64.40°S-69.40°S,76.11°E-81.29°E) during the period 18 November 2012 to 3 February2013. A wide open-water zone was found between the pack ice zone and fast ice zone in Prydz Bay during the end of November and middle of December 2012,the position of which changed with time. Distribution ranges of pack ice varied under the impact of dynamic and thermodynamic factors; the southern edge of the pack ice changed little,whereas the northern edge retreated significantly. Fast ice showed a significantly slower variation in extent with a mean decay rate of 0.37 km∙d-1. The third cruise was conductedat the end of January and in early February 2013,at which time the pack ice was distributed only within 68.50°S-68.90°S,whereas the fast ice had melted completely.
Generally,sea ice showed an incrementin thickness with increasing latitude,especially in the fast ice zone,from the end of Novemberto mid-December. Both the northern andsouthern parts of the pack ice zone adjoined open water,which made the ice thinner at those points than in the middle part. Ice concentration at the edge of the pack ice zone was lower than that in the inner part. Ice thicknessfollowed an approximate normal distribution during the two earlier cruises. The expected value of ice changed little between the first two observational periods,but the mean square deviation decreased by 10 cm. Ice thickness distribution tended to bemore concentrated and homogeneous during the middle of December (79.1±19.1 cm) than at the end of November (79.7±28.9 cm).
The variationof the mean value of snow thickness with latitude is unimpressive; however,the thickestsnow was found in the highest latitude of the fast ice zone. Snow thickness followed an approximate normal distribution. The expected value of snow thicknessduring mid-December is 2 cm less than that between 28 Novemberand 2 December 2012. The snow thicknessfrequency distribution exhibited multiple peaks at the end of November,which turned into a single-peak distribution and tended to be more concentrated and homogeneous by the middle of December,similarto that found for the ice distribution.Acknowledgments This work is supported by the Foundation for InnovativeResearch Groups of the National Natural Science Foundation of China (Grant no. 51221961),the National Basic Research Program of China (Grant no. 2010 CB950301),and the International Science and Technology Cooperation Project (Grant no. 2011DFA22260). The fieldwork was performedduring the 29th Chinese National Antarctic Research Expedition organized by the Chinese Arctic and Antarctic Administration. Data were issued by the Data-sharing Platform of Polar Science (http://www.chinare.org.cn) maintained by Polar Research Institute of China (PRIC) and Chinese National Arctic & Antarctic Data Center (CN-NADC). We thank the PRIC for providing excellent logistic support.
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