The tale of two ice shelves: Zachariae Isstrøm and Nioghalvfjerdsfjorden, Northeast Greenland

Oceanography and gravity data of Northeastern Greenland reveal ocean temperature and bathymetry in front of Zachariae Isstrøm (ZI) and Nioghalvfjerdsfjorden (79N), which hold a 1.1-m sea-level rise ice volume equivalent but underwent different evolutions. Sub-surface, warm, salty water of Atlantic origin has easier access to ZI than to 79N because of bathymetric barriers. We reconstruct ice removal by the ocean at the grounding line and floatation retreat from thinning to explain the observed grounding line retreat since 1979. The agreement between observed and calculated retreat demonstrates that ocean thermal forcing is a major control on the glacier evolution and undercutting of grounded ice must be included in numerical ice sheet models to reproduce the high rates of retreat.

格陵兰东北部的海洋学与重力数据，揭示了扎卡里亚伊斯特勒姆冰川（Zachariae Isstrøm，ZI）与尼欧哈尔夫峡湾（Nioghalvfjerdsfjorden，79N）前缘海域的海水温度与海底地形（bathymetry）。二者蕴含的冰体量相当于可引发1.1米海平面上升的规模，但演化路径截然不同。受海底地形屏障（bathymetric barriers）制约，源自大西洋的暖咸次表层水，相较于抵达79N冰川海域，更易抵达ZI前缘。为阐释1979年以来观测到的接地线（grounding line）后退现象，我们重建了海洋在接地线处对冰体的掏蚀过程，以及由冰体变薄驱动的浮化后退过程。观测所得冰后退速率与模拟计算结果高度吻合，表明海洋热强迫（ocean thermal forcing）是调控冰川演化的核心控制因子；若要复现观测到的高速冰后退现象，数值冰盖模型（numerical ice sheet models）必须纳入基岩冰体的掏蚀过程。