Ship-based Sea Ice Observations in Lützow-holm Bay, East Antarctica

The authors conducted the extensive ship-based sea ice observations from December 2000 to February 2001 in Lützow-Holm Bay, Antarctica. The authors focused on the observations of sea ice thickness in the land fast ice, which provides fundamental information on the unique growth and decay process of the sea ice in this area. The thickness of ice plus snow was measured using a portable Electro-Magnetic induction (EMI) sensor as well as a video camera onboard the Japanese Antarctic research vessel “Shirase”. The authors also conducted visual observations of sea ice conditions based on the SCAR ASPeCt program. The authors presented the preliminary analyses of the thickness profile of the land fast ice in Lützow-Holm Bay. It was observed that there were apparent discontinuities in the ice thickness profile, which might be attributed to the past breakups of the land fast ice. INTRODUCTION Lützow-Holm Bay, where Japanese Syowa station is located, is covered with the thick land fast ice. Satellite observations indicate that the extent of the land fast ice shows significant temporal variation due to its breakup. Ushio et al. (1998) reported that the last large breakup occurred from early autumn to winter season in 1997. The possible change of the ice thickness as a consequence of breakups has been reflected by the ship transit time to Syowa station. However the mechanism of breakups is not fully understood. In this regard, Lützow-Holm Bay is one of the remarkable fields for investigating sea ice growth and decay process. It is needless to say that monitoring sea ice thickness is indispensable for understanding the growth and decay process of the land fast ice in Lützow-Holm Bay. In this regards, the Japanese Antarctic Research vessel “Shirase” is an excellent observation platform, because “Shirase” navigates along a regular track annually in the land fast ice in Lützow-Holm Bay. Indeed, Japanese Antarctic Research Expedition (JARE) has conducted the ship-based sea ice thickness observations so far. In particular, JARE30, 31 and 32 conducted the extensive sea ice research project from 1988 to 1991. In this project, sea ice thickness was observed on board using a downward-looking video camera (Shimoda et al., 1997). Recently, JARE39 and 41 conducted sea ice thickness 1 National Maritime Research Institute observations by the same method. Although this method gives reliable data for level and moderately deformed ice, it takes very long time to analyze sea ice thickness from video images. Recently, Haas (1998) conducted the ship-based sea ice thickness observations in the Bellingshausen and Amundsen Seas using a portable Electro-Magnetic induction (EMI) sensor. It was reported that the existence of the seawater-filled gap in the summer melting ice decreased measurement accuracy. Meanwhile, Worby et al. (1999) concluded that a portable EMI had great potential to measure the thickness of undeformed component of the winter and spring ice. If the influence of the internal melting layer is negligible, this method is applicable for measuring the thickness of summer ice in Lützow-Holm Bay, because the land fast ice here mainly consists of level and moderately deformed ice. One of the authors joined the JARE42 and conducted extensive ship-based sea ice thickness observations using 1) EMI, 2) Video and 3) Visual method by the SCAR ASPeCt program (Worby et al. 1999). In this paper, we present the preliminary analyses of the thickness profile observed in the land fast ice in Lützow-Holm Bay. MEASUREMENTS Ship-based sea ice observations were conducted from mid December 2000 to mid February 2001. Fig. 1 shows the schematic of the ship-based sea ice monitoring system on board “Shirase”. EMI Measurement The EMI sensor has two coils at both ends. A transmitter coil emits a primary electro-magnetic field and a receiver coil detects a secondary field, which is mainly induced at the bottom of sea ice. This is because of the large contrast in electrical conductivity between sea ice and seawater. The secondary-primary field ratio can be expressed in terms of an apparent conductivity (σa). If the electrical conductivity of sea ice and seawater is constant, σa is only dependent on the distance from the EMI sensor to the bottom of sea ice (ZE). The distance to the sea ice surface (ZL) can be measured using a laser distance sensor. Subtraction of ZL from ZE gives total thickness (ZI). Here the total thickness denotes ice plus snow thickness. Details of the ship-based EMI observation are shown in Haas (1998). Photos 1 and 2 show the installation of the EMI and the laser sensor. A wooden frame, which contains the EMI sensor (EM-31, GEONICS) and the laser distance sensor (LD-3100HS, RIEGL), was suspended at the height of 4 to 5 m over ice surface and about 7m outside from the starboard side of the vessel. Conductivity-Thickness Transformation The calibration of the EMI sensor was conducted for transforming from σa to ZE. Firstly, we changed the height of the EMI and the laser sensor incrementally and measured σa and ZL. Secondly, we made drill-hole measurements of sea ice and snow thickness just below the sensors to determine ZI. Then we obtained the relation between σa and ZE. Forward looking Video (Concentration, Ice type, Floe size) Visual Observation EMI and Laser (Total Thickness, Roughness) Downward looking Video (Ice and Snow Thickness) Fig. 1: Sea Ice Monitoring on board “Shirase” System. Photo 1: Experimental Setup of EMI Photo 2: Experimental Setup of EMI and Laser Sensors –1 and Laser Sensors -2 Fig. 2 shows the calibration data as well as the data by Haas (1998) and the theoretical curve. Present results agree fairly well with theory. Meanwhile, there exists large discrepancy between present results and that by Haas (1998). It is reported that the existence of seawater-filled gap in summer ice caused such large difference. Even in the present results, there exists small difference between two datasets over the land fast ice in Lützow-Holm Bay. It is probably that this difference is attributed to the existence of internal melting layer. Indeed it was observed that the sea ice at Ongul Strait (denoted as ▼ in Fig. 2) had thicker internal melting layer than that off Benten-jima (△). Unfortunately, we did not measure the thickness and salinity of these layers. However, it should be pointed out that the internal melting layer has much less influence on the EMI measurements than that by Haas (1998), although further accumulation of calibration data is needed for improving its accuracy. We determined the following σa-ZE equation by the least-square fitting of all data including those over open sea in Amundsen Bay. ZE = a0 + a1 exp(-a2σa) + a3 exp(-a4σa) a0 = 2.924, a1 = 5.044, a2 = 0.019, a3 = 2.955, a4 = 0.114 Video Measurement Kawamura et al. (1993) reported that snow cover significantly affects the sea ice formation in Lützow-Holm Bay. However the EMI method cannot measure snow thickness separately. Thus a downward-looking video camera was installed for observing snow thickness as well as ice thickness. A video measurement also serves as a complementary method for an EMI measurement. Details of video measurements are shown in Uto et al. (1999). 80