1. Introduction More than a hundred years ago, Fridtjof Nansen led a four-year drifting expedition to the Arctic between 1893 and 1986 in his wooden vessel Fram (Nansen 1897). This was the first observational cruise for the Arctic Ocean. Since then, the data in the multi-year ice zone of Arctic Ocean have been obtained from ice camps, icebreaker cruises, air-borne observation, submarine cruises, and ice-drifting buoys. With regard to ice drifting buoy observation in the Arctic Ocean, however, most buoys observe mainly weather and sea ice, while only a few are equipped with sensors for measuring the sea under the multi-year ice. The first buoy to measure water temperature and salinity beneath the ice was the SALinity Argos (SALARGOS) buoy developed in 1986 by J.H. Morison of Polar Science Center, Applied Physics Laboratory, University of Washington (PSC/APL/UW) and S. Burke of Polar Research Laboratory Inc. (PRL) (Burke and Morison, 1987). In the late 1980s, the multifunctional and cheaper Polar Ocean Profiler (POP) buoy was developed for extensive and long-term observation of water temperature and salinity under the Arctic ice (Morison et al. 1991). The POP buoys of 20 or more were installed in the multi-year ice of the Arctic Ocean from 1980s to 1990s and had measured the oceanographical conditions. On the other hand, in 1991-93, about the same time when the POP buoy was deployed in the Arctic Ocean, Japan Marine Science and Technology Center (JAMSTEC) instituted research into the Arctic Ocean with the joint development of the ice ocean environmental buoy (IOEB) with the Woods Hole Oceanographic Institution (WHOI) (Krishfield et al., 1993, Hatakeyama, 1994). The first IOEB was deployed in the Beaufort Sea in April 1992, and the second was deployed into the Arctic Transpolar Drift in April 1994. The IOEBs are equipped not only with weather, sea-ice, and physical oceanographic sensors but also with various other sensors as well: including optical sensors to determine the activities of marine organisms and time series data collection devices. However, the IOEB lacked mobility and there was little consistency in the sea areas observed and the data obtained, because of the large number of different kinds of sensors used to collect data that made the system complex. In addition, each buoy was expensive and large-scale camps with extensive equipment and material had to be set up on the ice to install these buoys, and then they had to be recovered to analyze collected sediment samples. Over the last few decades, it became well known that the Arctic is a significant component of the global climate system and several studies showed the Arctic environment is in the midst of drastic change seen in the atmosphere, sea-ice, and ocean (e.g. Serreze et al., 2000, Morison et al., 2000). For example, sea-ice extent of the Arctic Ocean decreased about 2.9 percent per decade (Cavalieri et al., 1997) during the last two decade as well as sea ice draft in the Arctic Ocean, decreasing by 1.3 meters over the past 30 to 40 years (Rothrock et al., 1999). A noticeable salinization of the surface water, i.e. a retreat of the cold halocline layer, was found in the Eurasian Basin during the 1990s (Steele and Boyd, 1998), and the influence of the Atlantic Water has increased in the Arctic Ocean (Morison et al., 1998). Such key variables of the Arctic climate are changing in location and over the time scales that are monitored with the present observation system. However, the lack of dense environmental data coverage in the Arctic Ocean due to inaccessibility has been a barrier to the progress of the Arctic climate research. To establish new sustainable time series observations in the Arctic Ocean, the development of automated drifting buoy is practical. For the Arctic Ocean observations, the use of the drift buoy is more cost effective than by ship, ice camp, and other methods. Moreover, the buoy is capable of providing us with observational data in basin wide areas through the year. In 1999, JAMSTEC and MetOcean Data System Ltd. developed a new drifting buoy, named J-CAD (JAMSTEC Compact Arctic Drifter), to conduct long-term observations in the Arctic multi-year ice zones (Hatakeyama and Monk, 2001). The J-CAD is a state-of-the-art compact and inexpensive drifting buoy, designed and developed using the latest technologies, and incorporated the vast amount of engineering knowledge and experience gained from the IOEB program. Since 2000, we have been conducting operations of the J-CADs in order to measure the structure of upper-ocean currents and water properties under the multi-year ice of the Arctic Ocean for a better understanding the role of the Arctic Ocean for the global climate. As of now, JAMSTEC has installed five J-CADs into the sea-ice of the Arctic Ocean and has been collecting recent oceanographical and meteorological data. In this data report, we briefly describe the J-CAD technology and its operations. Also, we provide the archived data of the J-CAD operations and some geographical results.