MOS-1/1b MESSR Observations of the Antarctic Sea Ice: Ice Bands and Ice Streamers

Transcription

1 Journal of Oceanography, Vol. 55, pp. 417 to MOS-1/1b MESSR Observations of the Antarctic Sea Ice: Ice Bands and Ice Streamers KUNIMITSU ISHIDA 1, KAY I. OHSHIMA 2, TAKASHI YAMANOUCHI 3 and HIROSHI KANZAWA 4 1 Toba National College of Maritime Technology, 1-1, Ikegami-cho, Toba-shi, Mie , Japan 2 The Institute of Low Temperature Science, Hokkaido University, Kita-19, Nishi-8, Kita-ku, Sapporo , Japan 3 National Institute of Polar Research, Kaga, Itabashi-ku, Tokyo , Japan 4 National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki , Japan (Received 1 April 1998; in revised form 4 December 1998; accepted 9 January 1999) Meso- or submeso-scale features of the Antarctic sea ice are investigated using the MOS- 1/1b MESSR images (spatial resolution of approximately 50 m) received at Syowa Station. Particular attention is paid to the ice bands and ice streamers in coastal polynyas. In the Antarctic Ocean, ice bands can be often seen not only at the ice edge but also in the ice interior zone throughout the year and they extend for hundreds of kilometers in the latitudinal direction. It is found that the width and spacing of ice bands tend to decrease from winter to summer. The width of ice band is about 2 6 km in August and September, and km in December. The spacing of ice bands is about 3 10 km in August and September, and km in December. In coastal polynyas, ice streamers, which are composed of new ice, are sometimes observed. In general, the row of the streamers is spaced at km with a width of km. Keywords: Ice band, ice streamer, coastal polynya, sea ice, Antarctic Ocean, MESSR MOS-1/1b. 1. Introduction Satellite remote sensing offers us the capability of synoptically mapping sea ice. Since 1973, satellite passivemicrowave data have provided global distributions of sea ice on a near-daily basis, which has contributed a great deal to sea ice studies (e.g. Shuchman and Onstott, 1990). The information can currently be obtained from the Defense Meteorological Satellite Program (DMSP)-Special Scanning Microwave/Imager (SSM/I). However, since passive microwave observations provide data with a resolution of approximately 30 km, features with a scale of less than tens of kilometers cannot be detected. The National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) is useful in supplying ice information with a good resolution (order of 1 km). The drawback is that sea ice can be seen only on cloud-free days. At Syowa Station (69 00 S, E), Antarctica, NOAA AVHRR images were received daily from 1980 to From these NOAA AVHRR images, off East Queen Maud Land, Yamanouchi and Seko (1992) reported several features of sea ice; Ishikawa et al. (1996) reported the behavior of coastal polynyas; and Fukamachi et al. (1998) reported the mesoscale ice features in the summer marginal ice zone. Synthetic Aperture Radars (SARs) on European Remote Sensing Satellite (ERS)-1/2, Japanese Earth Resources Satellite (JERS)-1, and Radar Satellite (Radarsat) have great potential for providing information on sea ice with a m resolution, without impact from atmospheric conditions (e.g. Onstott, 1992). Another merit of SARs is that they can provide the internal properties of sea ice as well as surface information. The Multispectral Electronic Self Scanning Radiometer (MESSR) installed on Marine Observation Satellite (MOS)- 1/1b is a visible and infrared radiometer of high horizontal resolution, about 50 m field of view. The MESSR cannot be used for sea ice study under cloudy conditions, at night, and during polar nights. The resolution of MESSR images is 1 2 orders higher than that of NOAA images, so that the MESSR data can afford a more precise determination of ice conditions using the high resolution. Yamanouchi et al. (1991) showed the effectiveness of the MESSR images for sea ice observations of smaller scale features. The advantage of MESSR data used in this study is that considerable data were obtained with high spatial resolution. For example, during we obtained approximately 2,300 images of 100 km 100 km length on which sea ice can be seen. Thus these images have a potential for yielding common features from many images. In this paper, we describe sea ice features which have a scale less than tens of kilometers using the MESSR images, special attention being paid to the following two phenomena. One is ice bands, which are a regular series of linear bands of ice (e.g. Bauer and Martin, 1980). They have Copyright The Oceanographic Society of Japan. 417

2 Fig. 1. Study region. The heavy lines indicate the area of MESSR images shown in Fig. 3. Rectangular and shaded areas denote locations of MESSR images shown in Figs. 5 and 8. Path numbers indicate paths (World Reference System) of MOS-1 descending orbits. The inset map of Antarctica at top left indicates the study area. been observed along the ice edge in many seas; the Bering Sea (e.g. Muench and Charnell, 1977), the East Greenland Sea (Wadhams, 1981) and the Labrador Sea (LeBlond, 1982). The other is ice streamers, which organize frazil ice and/or new ice into several rows. These are commonly observed in coastal polynyas and along ice edges (Muench et al., 1983). Martin and Kauffman (1981) and Ushio et al. (1993) showed that in polynya the Langmuir circulation organizes the ice into streaks with a scale of tens of meters in width. However, the scale of the ice streamers ( km width) described in this paper is 1 2 orders larger than that of the Langmuir streaks. Section 2 describes the state of reception of MESSR images at Syowa Station and the data used in this study. An example of a mosaic from MESSR images is also shown. In Section 3, we examine the characteristics of ice bands. In Section 4, we describe ice streamers observed in coastal polynyas. A summary is given in Section Data 2.1 Reception of MESSR images at Syowa Station MOS-1/1b data were received at Syowa Station from February 1989 to April During that period, a considerable number of images of MESSR were obtained from 1,447 paths. The recurrent period of MOS-1/1b satellites is 17 days. Figure 1 shows the study region observed by Fig. 2. Histogram of the number of MESSR images in which sea ice can be observed, from March 1989 to December K. Ishida et al.

3 Fig. 3. A mosaic of 51 MESSR images obtained from December 5 to 11, 1989 (paths 56E 62E). The sea ice field is divided into five regions: A, coastal polynyas; B, region of giant floes; C, close pack ice region with numerous fractures; D, transition areas between C and E; E, ice band region. MESSR Observations of the Antarctic Sea Ice 419

4 MESSR images. At Syowa Station, data are received between 22 to 55 E (paths 54 to 70). Since the interval of adjacent paths is about 57 km at 70 S and the swath width is about 100 km, an MESSR image overlaps with that of the adjacent path (Yamanouchi et al., 1991). In principle, data were received every day during the summer (half year) and every 3 days during the winter (half year). One path is composed of about 60 images of 100 km 100 km length (Yamanouchi et al., 1991). 2.2 Images used in the study Negative films of Quick look (QL) photos (visible channel) of all MESSR images are archived at National Institute of Polar Research (NIPR), Japan. In this study, we Fig. 4. A visible NOAA AVHRR image on December 10, 1989 received at Syowa Station. The solid lines indicate the region of the mosaic shown in Fig. 3. have used these QL photos from February 1989 to January During that period, MESSR images were obtained from 669 paths. Among all these images, we selected the images on which sea ice can be seen. The selected images are approximately 2,300 in number, with 380 paths, representing about 6% of the total number of received images. Figure 2 shows the histogram of the number of usable images for each month during the period. No data could be obtained from mid-april to mid-august, since the solar elevation is low. 2.3 An example of mosaic from MESSR images Figure 3 shows a mosaic composed of 51 MESSR images from December 5 to 11, This is a rare example in which sea ice cover can be seen over a wide area. Dark areas are open waters or thin-ice covered regions. Using these mosaic images, the field is roughly divided into five regions according to the morphological feature of sea ice distribution: A, coastal polynyas; B, region of giant floes; C, close pack ice region with numerous fractures; D, transition areas between C and E; E, ice band region. From most views obtained in our study, the sea ice field seems to consist of all (or a part) of these regions in general. Figure 4 shows an NOAA AVHRR image (channel 2, resolution; 2.2 km) obtained on December 10, 1989, in the same period as Fig. 3. It is found that the ice band features cannot be resolved in the AVHRR images. Ice band features can be often seen in other images from the ice edge to the ice interior zone throughout the year. MESSR images with high spatial resolution are effective for examining such features. In Section 3, we describe the ice band features and its seasonal variations in detail. 3. Ice Bands In this study, ice band is defined as a parallel bandlike structure of sea ice, as shown in region E of Fig. 3, and band spacing is defined as the distance between the centers of neighboring bands. Ice band features from past Table 1. Summary of ice bands features from past studies. Reference Period Region Observations Band Band Ice bands width spacing zone (km) (km) (km) Muench and Charnell (1977) Nov Bering Sea NOAA 3/ Mar Bauer and Martin (1980) Mar Bering Sea Aerial photograph 1 ~15 Martin et al. (1983) Mar Bering Sea NOAA ship Surveyor Johannessen et al. (1992) Feb East of Bjørnøya, Aircraft SAR X and C-bands Barents Sea Liu et al. (1994) Jan Bering Sea ERS-1 SAR Wadhams et al. (1996) Apr Odden Region, Greenland Sea ERS-1 SAR K. Ishida et al.

5 Fig. 5. MESSR images in which typical ice bands can be observed from August to December, MESSR Observations of the Antarctic Sea Ice 421

6 Fig. 6. Seasonal variations in the width and spacing of ice bands from August to December, Solid circles show the mean values with maximum and minimum values indicated by the vertical bar. Thin solid lines are regression lines. See text for further description. 422 K. Ishida et al.

7 Fig. 7. Map showing the ice band distributions detected from MESSR images from August, 1990 to April, One symbol indicates approximate position of an MESSR image in which ice bands appear. The ice edge positions from SSM/I (15% ice concentration) are also shown, where the concentration is calculated by using the NASA Team algorithm and data provided by the National Snow and Ice Data Center, University of Colorado. studies are summarized in Table 1. The formation area of ice band (ice band zone) ranges from tens of kilometers to about 150 km from the satellite images in these papers. Muench and Charnell (1977) showed, from analysis of NOAA images, that ice edge bands have lengths on the order of 10 km and widths on the order of 1 km. In a field study, Bauer and Martin (1980) showed that the ice that makes up the bands comes from the outer 5 10 km of the pack ice. The SAR images acquired during the Seasonal Ice Zone Experiment (SIZEX 89) showed that ice edge bands, 5 30 km in length and km in width, extended out from an ice edge and were oriented normal to the wind direction, but along the ice edge (Johannessen et al., 1992). From ERS-1 SAR images, Liu et al. (1994) reported that the ice bands are observed near the ice edge, perpendicular to the wind direction. From ERS-1 SAR images and field observations, Wadhams et al. (1996) reported a substantial ice band of about 5 km width, oriented at right angles to the wind, with streamers oriented parallel to the wind. Figure 5 shows typical ice bands from the MESSR images from August to December, Ice bands with an eddy- or wave-like pattern can be seen in Figs. 5(b) and 5(d). The direction of the band is not necessarily the same in each image. After December 2, the size of the ice band reduces rapidly. On December 26, the ice band zone is composed of fine elements that cannot clearly be discriminated in the image. One remarkable point in Figs. 5(a) 5(h) is that the width and spacing of the band seems to decrease from winter to summer. Figure 6 shows the seasonal variation of band width and spacing during Photographic prints of QL images are ~125 mm in width, corresponding to 100 km of the MESSR swath. The scale is measured with a magnifying glass with accuracy of 0.1 mm, which correspond to ~80 m of actual state. The bar length in Fig. 6 represents the difference between minimum and maximum values, indicating the amount of scatter in measured values. Figure 6 clearly demonstrates that the band width and band spacing decreases from winter to summer every year except in In general, the width of an ice band is about 2 6 km in August and September, and km in December. The spacing of ice bands is about 3 10 km in August and September, and km in December. Figure 7 shows the spatial distributions of ice bands superimposed on the positions of the ice edge from August, 1990 to April, A symbol corresponds to an MESSR image in which ice bands appear. Figure 7 shows that the ice bands extend from an ice edge to an ice interior zone and that the ice band zone extends to hundreds of kilometers in the latitudinal direction. From past studies, the ice band zone is considered to be a feature that occurs mainly at the ice edge with an extension of at most 150 km. In the study area (off Syowa Station), however, the ice band zone is found to be a quite common feature, even in the ice interior region away from the ice edge, and this region spans a distance of hundreds of kilometers. In the Antarctic Ocean, sea ice field, current system and wind field are circumpolar, so that the study area is a representative one. Thus it is expected that the MESSR Observations of the Antarctic Sea Ice 423

8 observed ice bands features can be seen at the whole of the Antarctic Ocean. Several mechanisms have been proposed for the formation of ice bands. Wadhams (1983) proposed that the wave radiation pressure of the fetch-limited sea produced by the off-ice wind plays a major role in the generation of multiple bands (wave radiation theory). Muench et al. (1983) proposed that ice bands can be formed by the interactions between internal waves and sea ice (internal wave theory). Since we do not have reliable in-situ meteorological data, we cannot discuss the formation mechanism in precise terms. However, the fact that the scale of ice bands decreases from winter to summer may give a hint about the mechanism. According to the wave radiation theory, the band scale decreases as the ice floe size decreases. Generally, the floe size decreases with melting toward summer, so that the wave radiation theory is consistent with the observed seasonal variation of the ice band scales. On the internal wave theory, the band scale increases as the internal wave phase velocities increase. The phase velocity usually increases toward summer, since the stratification becomes strong due to the melting and heating of the upper layer. Thus the band scale is expected to increase toward summer in this theory. This is not consistent with the observed results. According to the seasonal variation of the band scale, the wave radiation theory is a more likely mechanism for the observed ice bands. A more precise understanding awaits future study. Therefore it is suggested that the wave radiation theory is proper to explain the seasonal variation of the ice bands. Moreover, the divergent ice field of the Antarctic Ocean seems to be a basic factor underlying the large extension of the ice band zone. 4. Ice Streamers in Coastal Polynyas MESSR images have an advantage in examining finescale structure in coastal polynyas. Figure 8(a) shows an MESSR image of coastal polynya in Breid Bay on March 30, The polynya is often observed in Breid Bay from NOAA AVHRR images (Ishikawa et al., 1996). The most remarkable feature of this image is that in the polynya rows of sea ice flow offshore with a width of km and a spacing of km. In the present, we call this feature ice streamers hereafter. Since the observed time of Fig. 8(a) corresponds to the time when ice production begins, the ice streamers seem to be composed of new ice. The ice Fig. 8. MESSR images of the coastal polynyas. (a) Breid Bay on March 30, (b) The area off Cape Boothby on October 30, The locations are shown in Fig. 1. Fig. 9. Schematic diagram showing a formation process of the shape of landfast ice edge. 424 K. Ishida et al.

9 Table 2. Ice streamer features from March 1989 to October Date Path Width of ice streamer (km) Spacing of ice streamer (km) March 26, W November 16, W March 14, E October 6, E November 15, W October 6, E October 13, E March 1, E March 19, W March 29, E October 13, E October 19, E streamers seem basically to drift under the influence of the wind. Near the coast, the direction of the streamers is northward, while it gradually shifts to the westward with distance from the coast. This suggests that the streamers are affected by the westward mean current off Breid Bay (Ohshima et al., 1996; Miyakawa and Ohshima, 1997). Figure 8(b) shows an image off Cape Boothby (around 66 S, 57 E) on October 30, The coastal polynya appears with 6 10 km in width (the distance from the pack ice to the landfast ice). Ice streamers can also be seen in this image. The row of the streamers is spaced at km with a width of km. It is also seen that the fast-ice edge is shaped like a sawtooth. This feature is also seen in Fig. 8(a). We speculate a formation mechanism as follows: new ice tend to be accumulated in the east side of each fast-ice cap due to the current and/or the wind, then the cap gradually grows by projecting new ice toward the west as schematically shown in Fig. 9. In our survey of all the images where coastal polynya can be seen, we have found that such ice streamers sometimes occur in coastal polynyas. Table 2 shows the list of ice streamers detected from all the images. As the table shows, ice streamers are not observed in summer. In general, the row of the streamers is spaced at km with a width of km on average. Ushio et al. (1993) used aircraft to observe frazil ice streaks with a width of tens of meters in the polynya off Syowa Station. They suggest that this is caused by a Langmuir circulation, as shown by Martin (1981). The streamers in Figs. 8(a) and 8(b) are morphologically similar to grease-ice plumes which are organized by the Langmuir circulation. However, the scale of the streamers in Fig. 8 is 1 2 orders larger than that of Langmuir circulation. Two possible scenarios for the formation processes are considered. One is the systematic gathering of Langmuir circulation. The other is the plumes associated with the deep-reaching convection (Kämpf and Backhaus, 1998). Actually in coastal polynyas the deep-reaching convection possibly occurs owing to heavy brine rejection by active ice production. If the convective plumes are affected by the wind, the streaks as a result of collection of new ice can be induced along zones of convergent surface currents (Kämpf, pers. comm.). The simulated streaks have similar size to the observed ice streamers. For a precise understanding of the streamers, insitu observations of ice and ocean are needed. 5. Summary We have described meso- or submeso-scale features of the Antarctic sea ice using a number of MESSR images received at Syowa Station, Antarctica. Ice bands can be seen not only near the ice edge but also in the ice interior zone throughout the year. It seems to be usual in the Antarctic Ocean that the ice bands extend to hundreds of kilometers in a latitudinal direction. The band width and spacing decrease from winter to summer (Figs. 5 and 6). The width of an ice band is about 2 6 km in August and September, and km in December. The spacing of ice bands is about 3 10 km in August and September, and km in December. The ice streamers, which are composed of new ice, are sometimes observed in the coastal polynyas. In general, the row of streamers is spaced at km with a width of km, which is 1 2 orders larger than that of Langmuir circulation. Finally, it is noted from the MESSR data that sea ice can be observed only on cloud-free conditions, which may introduce some bias in drawing general features. Furthermore, MOS-1/1b satellites have low temporal resolution, with a recurrent period of 17 days. To minimize such defects, the combined use of other satellite data such as SSM/I, NOAA AVHRR and SAR data may be helpful for future investigation. Acknowledgements We are grateful to the members of the 30 33rd Japanese Antarctic Research Expedition for their kind assistance in receiving the MESSR data. We are also grateful to Prof. M. Ejiri, National Institute of Polar Research, for his effort in starting the reception of MOS-1 satellite data at Syowa Station. We wish to thank Dr. J. Ukita of International Arctic Research Center for his instructive discussion. We would like to thank Dr. Y. Fukamachi of Hokkaido University for his helpful discussions during the course of this study, and providing Fig. 4. Thanks are extended to Prof. M. Wakatsuchi of Hokkaido University, for his encouragement and support. Comments and suggestions from reviewers were helpful in the improvement of this paper. This work was partly supported by Grant-in-Aid (No ) for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan. MESSR Observations of the Antarctic Sea Ice 425

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