Basal shear stress of the Ross ice streams from control method inversions Ian Joughin 1 Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA Douglas R. MacAyeal Department of Geophysical Sciences, University of Chicago, Chicago, Illinois, USA Slawek Tulaczyk Earth Sciences Department, University of California, Santa Cruz, California, USA Received 30 December 2003; revised 23 June 2004; accepted 16 July 2004; published 24 September 2004. [1] We used control method inversions to determine the basal shear stress beneath the Ross ice streams where new high-resolution velocity data sets have recently become available. The inversion algorithm was adapted from an earlier viscous bed algorithm to allow solution for the basal shear stress corresponding to a weak plastic bed. We performed several experiments using synthetic data to determine the quality of the inversions. These experiments indicate that with high-quality surface elevation data (e.g., errors <5 m), the inversions are relatively robust with respect to errors in ice flow velocity and bed topography. The inversions are consistent with seismic and borehole observations and indicate that the Ross ice streams lie atop a bed that is nearly everywhere weak. In contrast, the tributaries feeding these ice streams overlie alternating patches of strong and weak bed. INDEX TERMS: 1827 Hydrology: Glaciology (1863); 9310 Information Related to Geographic Region: Antarctica; 0933 Exploration Geophysics: Remote sensing; 1640 Global Change: Remote sensing; 3260 Mathematical Geophysics: Inverse theory; KEYWORDS: glaciology, Antarctica, ice streams, remote sensing Citation: Joughin, I., D. R. MacAyeal, and S. Tulaczyk (2004), Basal shear stress of the Ross ice streams from control method inversions, J. Geophys. Res., 109, B09405, doi:10.1029/2003JB002960. 1. Introduction [2] Large areas beneath the West Antarctic Ice Sheet (WAIS) rest on weak unconsolidated sediments (till) [Blankenship et al., 1986; Kamb, 2001] believed to enable the relatively fast motion of the Ross ice streams (Ice Streams A, B, C, D, E, and F, also called Mercer, Whillans, Kamb, Bindschadler, MacAyeal, and Echelmeyer). These ice streams are remarkable in that their driving stresses are roughly an order of magnitude less than typical outlet glaciers values, yet their flow speeds fall within the range of typical outlet glacier speeds [Alley et al., 1986]. Under- standing the frictional properties of subglacial till is thus important to predicting the future contribution of WAIS to sea level. In addition, many of the northern hemisphere ice sheets during the last glacial are believed to have flowed over soft (e.g., unconsolidated sediments) beds, which may have reduced their response time to rapid climate change [Clark et al., 1999]. As the last remaining marine-based ice sheet, WAIS provides a ‘‘natural laboratory’’ with which to understand the processes in former ice sheets that led to instabilities and rapid response to climatic conditions. 1.1. Background [3] The portion of WAIS that drains into the Ross Sea Embayment varies its discharge on a number of time- scales. Since the Last Glacial Maximum, the grounding line has retreated by distances of up to 1300 km [Conway et al., 1999]. Over the last millennium, flow stripes preserved in the surface of the Ross Ice Shelf record a history of significant variability in ice stream discharge [Fahnestock et al., 2000] with the best known example being the shutdown of the lower region of Ice Stream C about 150 years ago [Retzlaff and Bentley , 1993]. Over the last two decades, Ice Stream B has decelerated by 23% (1974–1997) [Joughin et al., 2002]. Over much shorter timescales, Ice Streams B and D have exhibited diurnal variation with significant changes in velocity occurring over periods as short as several minutes [Anandakrishnan et al., 2003; Bindschadler et al., 2003]. The longer-term variations have had a direct impact on sea level. For example, the stagnation of Ice Stream C and slowdown JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, B09405, doi:10.1029/2003JB002960, 2004 1 Now at Polar Science Center, Applied Physics Laboratory, University of Washington, Seattle, Washington, USA. Copyright 2004 by the American Geophysical Union. 0148-0227/04/2003JB002960$09.00 B09405 1 of 20