Warming of waters in an East Greenland fjord prior to glacier retreat: mechanisms and connection to large-scale atmospheric conditions

P Christoffersen,A Luckman,T J Benham,J P M Syvitski,R I Mugford,K J Heywood,J A Dowdeswell,I Joughin

doi:10.5194/tc-5-701-2011

Abstract

Abstract. Hydrographic data acquired in Kangerdlugssuaq Fjord and adjacent seas in 1993 and 2004 are used together with reanalysis from the NEMO ocean modelling framework to elucidate water-mass change and ice-ocean-atmosphere interactions in East Greenland. The hydrographic data show that the fjord contains warm subtropical waters and that fjord waters in 2004 were considerably warmer than in 1993. The ocean reanalysis shows that the warm properties of fjord waters in 2004 are related to a major peak in oceanic shoreward heat flux into a cross-shelf trough on the outer continental shelf. The heat flux into this trough varies according to seasonal exchanges with the atmosphere as well as from deep seasonal intrusions of subtropical waters. Both mechanisms contribute to high (low) shoreward heat flux when winds from the northeast are weak (strong). The combined effect of surface heating and inflow of subtropical waters is seen in the hydrographic data, which were collected after periods when along-shore coastal winds from the north were strong (1993) and weak (2004). The latter data were furthermore acquired during the early phase of a prolonged retreat of Kangerdlugssuaq Glacier. We show that coastal winds vary according to the pressure gradient defined by a semi-permanent atmospheric high-pressure system over Greenland and a persistent atmospheric low situated near Iceland. The magnitude of this pressure gradient is controlled by longitudinal variability in the position of the Icelandic Low.

Highlights

The mass balance of the Greenland Ice Sheet has changed from a state close to balance in the 1980s and 1990s (Rignot et al, 2008; Hanna et al, 2005) to a state where net annual losses exceed 200 Gt yr−1 (Rignot and Kanagaratnam, 2006; Velicogna and Wahr, 2006; Chen et al, 2006; Rignot et al, 2008; van den Broeke et al, 2009)
In 1993, Atlantic Water (AW) with potential temperature θ > 2 ◦C and salinity S ∼35 penetrated beneath cold Polar Surface Water (PSW) with θ < 0 ◦C and S < 32 (Fig. 1b)
The concurrent warming of surface waters and the deeper inflows of AW is consistent with the hydrographic data acquired in Kangerdlugssuaq Fjord (KFj) in 2004, and both aspects of warming were a result of weak northeasterly air flow associated with low P and a persistent eastern position of the Icelandic Low (IL) over the Labrador Sea (Fig. 10b)

Summary

Introduction

The mass balance of the Greenland Ice Sheet has changed from a state close to balance in the 1980s and 1990s (Rignot et al, 2008; Hanna et al, 2005) to a state where net annual losses exceed 200 Gt yr−1 (Rignot and Kanagaratnam, 2006; Velicogna and Wahr, 2006; Chen et al, 2006; Rignot et al, 2008; van den Broeke et al, 2009). Recent studies have suggested a sensitive interaction of the Greenland Ice Sheet with its surrounding seas (Holland et al, 2008; Rignot et al, 2010; Murray et al, 2010), ice-ocean interactions in Greenland remain uncertain and poorly documented It is not yet clear if recent changes in the flow of tidewater glaciers were caused by increased transport of warm subtropical waters to coastal environments and fjords or by changes in the properties of these waters. The combined effect of surface heating and the deeper inflow of subtropical waters is consistent with the hydrographic data, which was collected after periods when northerly winds were strong (1993) and weak (2004)

Hydrographic surveys

Ocean reanalysis

Hydrographic observations from KFj and KTr

Shelf water exchange in ocean reanalysis

Summary and conclusions