第4次南極地域観測海氷調査報告

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Introduction Since m/s "SOYA" had reached the ice edge on Dec. 27, 1959 at 65°15'S, 48°35'E, the officers on watch carried out continuous visual ice observations, under the supervision of the navigating officer, until the ship left the ice off Riiser-Larsen Peninsula on Feb. 21, 1960. The items of observation were nearly identical with those of the previous three expeditions. As the ship was not bound with drift ice, several surveying cruises were performed between 33°E and 50°E and eleven oceanographic stations were set up in the region. Miscellaneous observations other than general, for instance, observations on the drift of ice field and icebergs, were also made on occasion. 1. Some comments on ice terminology and recording form Several terms were supplemented to the WMO ice terminology mainly from Russian source. Terms were rearranged and compiled in "A glossary of ice terminology to be used in JARE". An example of supplemented terms is secondary slush, which is a combined conception of melting sludge in Baltic ice code and ledyanaya podushka in Russian terminology. Observed items were nearly identical with those required by the U. S. Hydrographic Office's reporting form. Obtained results were compiled in ice charts. Notations used in charts were also similar to those of the U. S. Hydrographic Office. However, the following changes were made both in observed items and in chart notations in order to describe encountered ice conditions more adequately: a) The size of predominant floes was recorded by nearest meters as the index of mechanical decaying. The item "puddling" seems to be insufficient to describe the dacaying state of Antarctic drift ice which decays mechanically before puddles have developed on it. Another index of mechanical decaying such as whether floes have angular or smooth outlines should have been recorded. b) Inter-ice melting sludge was excluded from the first group in the item "concentration by size" and its existence was indicated by a prefix Sl. Though it is difficult to distinguish between ice cakes and melting sludge from the air, the ship observer must distinguish them because the latter behaves quite differently from the former during the compression or the dispersion of an ice field. Melting sludge, if exists, usually covers inter-ice area completely. The compression (or dispersion) of the ice field, not causing hummocking (or formation of open water), only changes the thickness of sludge. The latter, which has important effects on the navigability, should have been recorded at least qualitatively. 2. General ice observations 2a) Note on observation practice. Separate measurements of snowdepth and ice thickness were difficult because the boundary was not clear to identify whether it is porous ice or firnized snow. For convenience' sake, the upper layer easily separable by turning over floes was regarded as snow. In this sense, most floes were covered by snow 0.3-0.5m thick. Estimation of the age of floes was also difficult. The age of little hummocked floes was estimated by their thickness, as usually done. But, this method seemed inadequate for summer ice, for level ice less than 1m thick often had two planktonrich brown thin layers. 2b) Ice conditions along ice edges and 2c) Those in ice field. Main results are shown in Figs. 1-10. Along edges, ice cakes were predominant and inter-ice area was usually covered by melting sludge. If the edge was under the dispersing effect of wind, there appeared along it a narrow region of a small ice concentration composed only of brashes (no melting sludge). Newly developed ice fields composed of small pancake ice were only seen after the middle of February. On Jan. 21 giant clusters of floebergs were seen at 45°30'E on the edge. Vast and big floes encountered in the region south of 67°20'S during the first penetration of the ship into ice area were probably fragments of the shore ice. The heights of reliefs on them were more than 3m, but the fraction of hummocked area was rather small (2/10-3/10). 2d) The shore ice off the western part of the Prince Olav Coast and the lead off the edge of the shose ice. Although the width of the lead on 40°E meridian had narrowed from several kilometers on Jan. 3 to several hundred meters on Jan. 15 and finally the lead disappeared on Feb. 4, the position of the edge did not change during the observation period (Jan. 3-Feb. 10). The constant position of the edge must have some relations with the bottom relief (the sea suddenly deepens northward from about 300m to 1000m at several kilometers north of the edge). Along the edge there was a hummocked narrow zone (2-3km wide). South of the zone there was another a little wider zone (less than 20km wide) where sasturgi running NEN-WSW were developed. Farther south the surface was flat. Puddles were developed little even in the beginning of February. 3. Miscellaneous observations 3a) Icebergs. The first iceberg was seen at 53°09'S, 32°17'E on Dec. 23, 1959 and the last at 54°00'S, 25°10'E on Feb. 26, 1960. The largest iceberg which was observed off Cape Ann was supposed to be identical with the vast iceberg observed by Russian ice breaker "LENA" at 65°40'S, 88°14'E on Feb.16, 1957. 3b) Drift of icebergs. Only the drifts in the lead were analyzed showing the existence of a current of about 0.3knot there (Table 1). 3c) Drift of ice field. From Jan. 22 to Jan. 27 the ship drifted in an ice field composed of 50% cakes and 50% melting brashes and sludge (Fig. 6). The relative speed of the ship to the cakes (and also to sludge) was surprisingly low (only several meters per hour). This shows that the resistance of the melting sludge against the movement of the ship was very strong. The drift of the ship was analyzed as the drift of the ice field (Table 2). 3d) Dispersion of ice edge by the wind. A return survey of the edge between 42°E and 33°E was made on Dec. 29 and 30. The edge was dispersing northward under the effect of W-SW wind. A rough analysis gave the wind coefficient of about 0.08 for the drift of scattered brashes. 4. Summer ice diminution in observed area On Dec. 7. Japanese whaling fleets observed ice edge at about 62°S. As it was at about 65°S-66°S on Dec. 27-29, the retiring speed along the meridian amounted to about 20km/day. This rapid retiring was undoubtedly due to the intense melting of the ice field dispersing under the effect of prevailing westerly there. South of 66°S, the diminution process in the region A was considered to differ from that in the region B (Fig. 11). Ice conditions in the region A indicated that the ice field there had not experienced any intense movement. There the ice field was probably under the dispersing effect of weak westerly and the diminution of ice was chiefly due to melting. In the region B, on the other hand, the ice field were continuously moving west-south-westward. There the diminution of ice was partly attributed to the decrease in the amount of ice supply from Enderby Land. By the beginning of February, the ice field off the Prince Olav Coast had retired within 70km from the coast. Off Cape Ann no drift ice existed in the middle of February. 5. The meaning of oceanographic observations to the analysis of the ice diminution process Summer ice diminution is determined by two factors: the melting of ice and the movement of ice. The latter is caused primarily by wind. Now, the most characteristic feature of wind regime in the Antarctic coastal region is the existence of easterly near coast and of westerly off the sea. The ice field is dispersed by westerly and then melts rapidly. Therefore, early in summer (at the end of December or at the beginning of January) the ice edge is probably at about the boundary between easterly and westerly. Wind regime reflects on sea regime and the said boundary appears in the sea as the so-called Antarctic Divergence. Thus, to know the position of the Antarctic Divergence becomes important for ice diminution considerations. Our eleven stations were not adequately located for the determination of the position of the Antarctic Divergence. But, from the obtained temperature distribution, the concave of the Antarctic Divergence on 38°E meridian was deduced (The author considers that the Divergence will be nearly pallarel to 1℃ isotherm in Fig. 14.). Concerning the melting, it is difficult to estimate the amount of melted ice from heat balance considerations. But, assuming that no advection occurs in the sea, it may be estimated from considerations of the dilution of the surface water. The obtained oceanographic data were analyzed from this point of view (Table 3, where D, S_1 and S_2 mean the thickness of the surface water, the salinity in winter and the mean salinity in summer, respectively). Reasonable values were obtained for stations 1, 2, 3 and 4, showing that advection corrections for these stations were negligible. This again indicates that in the region A the ice field had not experienced intense moving. Acknowledgements As mentioned in the introduction, the ice observations were carried out by the members of the Navigation Section of the "SOYA". The author expresses his sincere thanks to Mr. TETSUO SHIMOMATSU, the then navigating officer, and other members of the Navigation Section. Mr. KOSEI YOSHIDA, the then fourth officer, compiled the excellent ice charts, on which Figs. 1-10 of the text are based. The contents of §§ 2 and 3 are also mainly due to him. Oceanographic data used in §5 were kindly offered before publication by Mr. SHIGERU FUKASE of the Nagasaki Marine Meteorological Observatory, chief oceanographer of the expedition. Asst. Prof. KOU KUSUNOKI and Mr. NOBUO ONO of the Institute, both the former members of JARE, gave many usufull suggestions. The author expresses his hearty thanks to all of them.
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第4次南極地域観測海氷調査報告

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