Force-perturbation analysis of recent thinning and acceleration of Jakobshavn Isbræ, Greenland
Force-perturbation analysis of recent thinning and acceleration of Jakobshavn Isbræ, Greenland
复制标题
格陵兰岛雅各布港最近变薄和加速的力扰动分析
作者：
R. Thomas
摘要 在1997年至2001年期间对格陵兰岛雅各布港冰川的观测发现，在其15公里长的漂浮冰舌约4公里发生崩解之后，下游部分的流速立即增加了30%，厚度减薄达60米，这表明加速可能是由崩解引发的。假设与这种弱化相关的力扰动能迅速向上游冰川远处传播，我推导出了一些方程来估算这种扰动。最初，观测到的变化与崩解相关的相对较小的扰动是相符的。此后，这种加速可能是由剩余冰舌以约80米/年的速度减薄所维持的。否则，随着冰川变薄，纵向蠕变的静水驱动力减小，力扰动很快就会被平衡，流速会回落到原来的值。计算得出的力扰动在接地线向内陆约10公里处达到最大值，这与冰川在向海倾斜的基岩上变薄时重力减小是一致的。再向内陆，力扰动逐渐减小，可能是因为随着冰川加速，边缘阻力增加了。在这里以及在漂浮冰舌上，边缘的冰似乎由于局部强烈的剪切对冰温及/或冰晶组构的影响而变软了。更近的观测显示出持续的加速和减薄，并且大部分剩余的冰舌在2003年4月崩解掉了，所以减薄可能会继续。
Abstract Observations between 1997 and 2001, of a 30% velocity increase and up to 60 m thinning of downstream parts of Jakobshavn Isbræ, Greenland, immediately following calving of about 4 km of its 15 km floating ice tongue, suggest that acceleration may have been initiated by the calving. Assuming that the force perturbation associated with such weakening is swiftly transmitted far up-glacier, I develop equations to estimate the perturbation. Initially, the observed changes are consistent with the comparatively small perturbation associated with the calving. Thereafter, it was probably sustained by thinning of the remaining ice tongue at rates of about 80 ma–1. Otherwise, the force perturbation would soon have been balanced by reduction in the hydrostatic driving force for longitudinal creep as the glacier thinned, with velocities dropping to their former values. The calculated force perturbation increases to a maximum about 10 km inland of the grounding line, consistent with decreasing weight forces as the glacier thins over bedrock that slopes uphill seawards. Further inland, it progressively decreases, probably because marginal drag increased as the glacier accelerated. Both here and on the floating tongue, marginal ice appears to have been softened by the influence of locally intense shear on ice temperature and/or fabric. More recent observations show continued acceleration and thinning, and most of the remaining ice tongue calved away in April 2003, so thinning is likely to continue.