Ocean impact on the 79 North Glacier, Northeast Greenland

Abstract

The retreat and acceleration of marine-terminating glaciers around the coast of Greenland observed over the last two decades have partly been attributed to a warming of Atlantic Water (AW) circulating around the subpolar North Atlantic. This thesis investigates the impact of the ocean circulation on the 79 North Glacier (79NG), which has Greenland's largest floating ice tongue. One overall hypothesis tested in this thesis is whether a long-term warming of AW in Fram Strait has spread across the continental shelf off Northeast Greenland (NEG) toward the 79NG, which may explain the recent thinning observed at the floating ice tongue. A detailed bathymetry is crucial for studying pathways of AW across the NEG continental shelf. Thus, the first part of this thesis is devoted to an accurate representation of the continental shelf bathymetry and 79NG cavity geometry. Based on a collection of hydrographic data obtained between 1979 and 2016 it is shown that the Norske Trough, the southern branch of the characteristic C-shaped trough system on the continental shelf, is filled up by warm Atlantic Intermediate Water (AIW) exceeding 1 degree Celsius at depths below 200-250 m. Current velocities from moored and lowered ADCPs inside Norske Trough indicate that a boundary current transports warm AIW toward the 79NG. The results show that Norske Trough provides the main pathway for warm AIW from the continental shelf break toward the 79NG. Anomalies in AW temperatures in Fram Strait could reach the 79NG within 1.5 years. A unique data set comprising bathymetric, hydrographic, and current velocity observations obtained during the R/V Polarstern cruise PS100 in front of the 79NG calving front emphasizes the importance of the complex bathymetry for the heat transport into the cavity below the floating ice tongue. A density-driven gravity plume, steered by the bathymetry outside the cavity, transports warm AIW into the subglacial cavity. These findings imply that the AIW layer thickness on the continental shelf plays an important role in determining the strength of the overturning in the cavity and thereby melting at the base of the floating ice tongue. For the first time, the major exchange flow across the 79NG calving front (i.e., between the continental shelf and the 79NG cavity) was captured by synoptic hydrographic and velocity observations. A bottom-intensified flow transports 140 plus/minus 20 GW of heat into the cavity via a 500 m deep and 2 km wide depression. A heat supply of this magnitude causes an average melt rate of 8.3 plus/minus 2.1 m/yr at the ice base of the 79NG. By mixing of AIW and glacial meltwater (i.e., both basal meltwater and subglacial runoff), a shallow outflow of glacially modified AIW is generated. The observed ocean exchange flow across the 79NG calving front suggests that the overturning in the cavity has a strength of 36 plus/minus 17 mSv. Combining historic and recent hydrographic data from the 79NG cavity and the NEG continental shelf reveals a warming of AIW by 0.4 plus/minus 0.1 degree Celsius between the late 1990s and recent years. Numerical experiments performed with a one-dimensional ice-shelf plume model suggest that the observed changes go along with a 44 plus/minus 13% increase in average basal melt rates. The results verify the hypothesis that a long-term warming of AW in Fram Strait has spread across the continental shelf into the 79NG cavity and emphasize that the ocean is the main driver of the ice thickness loss at the 79NG. Warm AIW is also present at the Zachariae Isstrom (ZI) located 50 km south of the 79NG, which was shown for the first time from temperature measurements. The warming in AIW presumably has driven the disintegration of the floating ice tongue of ZI in 2012/14. A further warming of waters below the 79NG may cause a similar rapid loss of the entire floating ice tongue as observed at ZI.