Mechanisms modulating warm Atlantic Water temperatures and inflow toward Helheim Glacier in southeastern Greenland

Abstract:

Melting at the ice-ocean boundary through interactions with relatively warm ocean water, called Atlantic Water (AW), is one mechanism for recent ice loss at many Greenland outlet glaciers. However, as a result of limited field observations, it is unclear how large of a role the ocean has played in past glacier retreat events. Here, we reconstruct AW intrusion variability in a bathymetric trough leading to Sermilik Fjord, abutting Helheim Glacier, in southeastern Greenland using MODIS sea surface temperatures from 2000 to 2019, and investigate mechanisms for the intrusions. We find that AW temperatures along the trough do not always track AW properties offshore. AW intrusions have a strong seasonal and interannual variability, and these correlate with trough AW temperatures. This relationship suggests that AW intrusion inshore, not solely offshore North Atlantic Ocean temperatures, modulate AW dilution as it crosses the shelf and, thus, the deeper AW temperatures that flow toward Helheim.

In examining the mechanisms driving these intrusions, we delineate between two modes of intrusion that interact to control AW transport inshore. In the first, the reversal of alongshore winds drive rapid AW intrusions onto the continental shelf on synoptic timescales. Examining the weather patterns associated with the wind events, we find that only offshore cyclones traveling toward the Denmark Strait consistently lead to intrusions that cross the entire shelf and produce warming at deep moorings at Sermilik Fjord mouth and mid-fjord. In the second mode of intrusion, we confirm previous work observing a spreading of AW onto the continental shelf that coincides with variability in the East Greenland Coastal Current (EGCC), the relatively cold and fresh current flowing south along the coast. Offshore cyclones and EGCC variability effectively serve as an accelerator and brake, respectively, for increasing AW inflow into Sermilik Fjord. We show historical records of cyclone and EGCC variability in the context of Helheim discharge and thinning variability. We suggest that periods with a weakened EGCC makes southeastern Greenland fjords more susceptible to AW intrusion and, when that coincides with offshore cyclone activity, Helheim Glacier, and perhaps others, may experience anomalously high AW inflow toward the glacier terminus.