Fortnightly variations in the flow velocity of Rutford Ice Stream, West Antarctica

Most of the ice lost from the Antarctic ice sheet passes through a few fast-flowing and highly dynamic ice streams(1). Quantifying temporal variations in flow in these ice streams, and understanding their causes, is a prerequisite for estimating the potential contribution of the Antarctic ice sheet to global sea-level change(2,3). Here I show that surface velocities on a major West Antarctic Ice Stream, Rutford Ice Stream, vary periodically by about 20 per cent every two weeks as a result of tidal forcing. Tidally induced motion on ice streams has previously been thought to be limited to diurnal or even shorter-term variations(4-9). The existence of strong fortnightly variations in flow demonstrates the potential pitfalls of using repeated velocity measurements over intervals of days to infer long-term change.