Changes in flow of Crosson and Dotson ice shelves, West Antarctica, in response to elevated melt

Abstract

Abstract. Crosson and Dotson ice shelves are two of the most rapidly changing outlets in West Antarctica, displaying both significant thinning and grounding-line retreat in recent decades. We used remotely sensed measurements of velocity and ice geometry to investigate the processes controlling their changes in speed and grounding-line position over the past 20 years. We combined these observations with inverse modeling of the viscosity of the ice shelves to understand how weakening of the shelves affected this speedup. These ice shelves have lost mass continuously since the 1990s, and we find that this loss results from increasing melt beneath both shelves and the increasing speed of Crosson. High melt rates persisted over the period covered by our observations (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014), with the highest rates beneath areas that ungrounded during this time. Grounding-line flux exceeded basin-wide accumulation by about a factor of 2 throughout the study period, consistent with earlier studies, resulting in significant loss of grounded as well as floating ice. The near doubling of Crosson's speed in some areas during this time is likely the result of weakening of its margins and retreat of its grounding line. This speedup contrasts with Dotson, which has maintained its speed despite increasingly high melt rates near its grounding line, likely a result of the sustained competency of the shelf. Our results indicate that changes to melt rates began before 1996 and suggest that observed increases in melt in the 2000s compounded an ongoing retreat of this system. Advection of a channel along Dotson, as well as the grounding-line position of Kohler Glacier, suggests that Dotson experienced a change in flow around the 1970s, which may be the initial cause of its continuing retreat.

How Accurately Should We Model Ice Shelf Melt Rates?

“…They have exhibited significant grounding line retreat in recent years, with the Smith grounding line retreating at rates upward of 2 km/a . Ice-sheet modeling suggests that this retreat may have been induced by a decrease in buttressing from the Crosson and Dotson Ice Shelves (Goldberg et al, 2015), consistent with observations of increased velocities close to the grounding line of these ice streams (Lilien et al, 2018;Mouginot et al, 2014).…”

“…For purpose of discussion, we adopt terminology from Khazendar et al (2016) and Gourmelen et al (2017) and refer to them (in east-to-west order) as Pope, Smith,Kohler East,and Kohler West (Figure 3a). Ice-sheet modeling suggests that this retreat may have been induced by a decrease in buttressing from the Crosson and Dotson Ice Shelves (Goldberg et al, 2015), consistent with observations of increased velocities close to the grounding line of these ice streams (Lilien et al, 2018;Mouginot et al, 2014). They have exhibited significant grounding line retreat in recent years, with the Smith grounding line retreating at rates upward of 2 km/a .…”

“…In fact, Lilien et al (2018) infer weakening of the Dotson-Crosson margin from 1996-2011. However, there are other processes that can affect ice-shelf buttressing.…”

“…Ice stiffness (the Glens law parameter) influences ice flow in a similar manner to thickness, and ice-shelf weakening can have a similar effect to melt-induced thinning. In fact, Lilien et al (2018) infer weakening of the Dotson-Crosson margin from 1996-2011. Adjoint sensitivity to Glen's law parameter (not shown) has a pattern similar to that of melting, and it is possible that observed speedup of Smith, Pope, and Kohler East is due to weakening in this shear margin.…”

“…This drop in buttressing may be related to submarine melt-induced thinning, which can decrease buttressing (e.g., Shepherd et al, 2004). High melt rates have been observed for both Dotson and Crosson in recent years (Depoorter et al, 2013;Gourmelen et al, 2017;Lilien et al, 2018;Miles et al, 2016;Randall-Goodwin et al, 2015;Rignot et al, 2013). Between 2003 and 2008, Dotson and Crosson had net average thinning rates of 3.1 and 6.5 m/a, respectively ; and both have had strong thinning trends for the last two decades (Paolo et al, 2015).…”