Ice mass-balance buoys: a tool for measuring and attributing changes in the thickness of the Arctic sea-ice cover Jacqueline A. RICHTER-MENGE, 1 Donald K. PEROVICH, 1 Bruce C. ELDER, 1 Keran CLAFFEY, 1 Ignatius RIGOR, 2 Mark ORTMEYER 2 1 US Army Cold Regions Research and Engineering Laboratory, 72 Lyme Road, Hanover, NH 03755-1290, USA E-mail: Jacqueline.A.Richter-Menge@erdc.usace.army.mil 2 Polar Science Center, Applied Physics Laboratory, University of Washington, 1013 NE 40th Street, Seattle, WA 98105-6698, USA ABSTRACT. Recent observational and modeling studies indicate that the Arctic sea-ice cover is undergoing significant climate-induced changes, affecting both its extent and thickness. The thickness or, more precisely, the mass balance of the ice cover is a key climate-change indicator since it is an integrator of both the surface heat budget and the ocean heat flux. Accordingly, efforts are underway to develop and deploy in situ observing systems which, when combined with satellite remote-sensing information and numerical models, can effectively monitor and attribute changes in the mass balance of the Arctic sea-ice cover. As part of this effort, we have developed an autonomous ice mass-balance buoy (IMB), which is equipped with sensors to measure snow accumulation and ablation, ice growth and melt, and internal ice temperature, plus a satellite transmitter. The IMB is unique in its ability to determine whether changes in the thickness of the ice cover occur at the top or bottom of the ice cover, and hence provide insight into the driving forces behind the change. Since 2000, IMBs have been deployed each spring from the North Pole Environmental Observatory and in several other areas, including a few in the Beaufort Sea and Central Basin. At this point, the collective time series is too short to draw significant and specific conclusions regarding interannual and regional variability in ice mass balance. Comparisons of available data indicate that ice surface ablation is greater in the Beaufort region (67–80 cm), relative to the North Pole (0–30 cm), consistent with a longer period of melt in the more southerly location. Ablation at the bottom of the ice (22 cm), maximum ice thickness (235 cm) and maximum snow depth (28 cm) were comparable in the two regions. INTRODUCTION Recent studies indicate that the Arctic sea-ice cover is undergoing significant climate-induced changes, affecting both its extent and thickness. For instance, satellite-derived estimates of maximum ice extent suggest a net reduction between 1978 and 1999, at an average rate of 3% per decade (e.g. Parkinson and others, 1999; Parkinson and Cavalieri, 2002). A report by Comiso (2002) indicates an even more rapid reduction in the perennial sea-ice cover, of 9% per decade. During the summers of 2002–05, there has been an unprecedented series of extreme ice-extent minima (Stroeve and others, 2005; personal communication from J.C. Stroeve, 2006). Ice-thickness data, derived from submarine-based upward-looking sonar, also suggest a net thinning of the perennial sea-ice cover since 1958 (Rothrock and others, 1999; Wadhams and Davis, 2000; Tucker and others, 2001). Model results, used to extend these obser- vations in both space and time, are consistent with this conclusion and further suggest that the decline in perennial ice thickness was most rapid in the 1990s (Rothrock and others, 2003). It is important that we continue and expand efforts to monitor these changes to (a) improve the fundamental understanding of the role of the sea-ice cover in the global climate system and its influence on the Arctic ecosystem and (b) take advantage of the sensitivity of the sea-ice cover as an early indicator of the magnitude and impact of climate change. The thickness or, more precisely, the mass balance of the ice cover is a key climate-change indicator, since it is an integrator of both the surface heat budget and the ocean heat flux. If there is net warming over time, then there will be thinning of the ice. Conversely, a net cooling leads to thicker ice. The mass balance of the sea-ice cover is a function of its extent and thickness, which combine to give its volume. The extent of the sea-ice cover is effectively monitored from satellite platforms using passive microwave imagery. Moni- toring changes in the ice thickness is more problematic. As with ice extent, the ideal platform for monitoring ice thickness is a satellite because it provides a full-basin perspective. Until recently, no technique had been ad- equately developed to obtain reliable satellite-based meas- urements of ice thickness. Results reported by Laxon and others (2003) and Kwok and others (2004) suggest possible breakthroughs in the use of satellite altimeter measurements of ice freeboard to determine the mean ice-thickness field and its variability. As this and other satellite-based technol- ogies develop, we must also find ways to make more effective use of ice-thickness measurements collected from other platforms, including submarines, aircraft, sea-floor moorings and drifting buoys. While these measurement platforms have spatial limitations, they can play a central role in the validation and calibration of satellite-based instruments. Further, their capacity to collect data at higher temporal and spatial resolutions can provide information necessary to understand and attribute observed changes in the ice thickness. This paper gives a detailed description of an ice mass- balance buoy (IMB), designed to make in situ observations of changes in the mass balance of the ice cover. We Annals of Glaciology 44 2006 205