GRL - Reduced Ice Thickness In Arctic Transpolar Drift Favors Rapid Ice Retreat

[hatrack]

Christian Haas

Alfred Wegener Institute for Polar and Marine Research,
Bremerhaven, Germany

Now at Department of Earth and Atmospheric Sciences, University of Alberta,
Edmonton, Alberta, Canada

Andreas Pfaffling

Alfred Wegener Institute for Polar and Marine Research,
Bremerhaven, Germany

Now at Norwegian Geotechnical Institute,
Oslo, Norway

Stefan Hendricks and Lasse Rabenstein

Alfred Wegener Institute for Polar and Marine Research,
Bremerhaven, Germany

Jean-Louis Etienne

Septieme Continent,
Paris, France

Ignatius Rigor

Polar Science Center, Applied Physics Laboratory, University of Washington,
Seattle, Washington, USA

Abstract

<1> Helicopter-borne electromagnetic sea ice thickness measurements were performed over the Transpolar Drift in late summers of 2001, 2004, and 2007, continuing ground-based measurements since 1991. These show an ongoing reduction of modal and mean ice thicknesses in the region of the North Pole of up to 53 and 44%, respectively, since 2001. A buoy derived ice age model showed that the thinning was mainly due to a regime shift from predominantly multi- and second-year ice in earlier years to first-year ice in 2007, which had modal and mean summer thicknesses of 0.9 and 1.27 m. Measurements of second-year ice which still persisted at the North Pole in April 2007 indicate a reduction of late-summer second-year modal and mean ice thicknesses since 2001 of 20 and 25% to 1.65 and 1.81 m, respectively. The regime shift to younger and thinner ice could soon result in an ice free North Pole during summer.

Received 24 April 2008; revised 27 June 2008; accepted 17 July 2008; published 3 September 2008.

Keywords: Arctic sea ice thickness, EM thickness profiling, buoy ice age model.

Index Terms: 0750 Cryosphere: Sea ice (4540); 0762 Cryosphere: Mass balance (1218, 1223).
1. Introduction

<2> The summer of 2007 saw another record low sea-ice coverage of the Arctic Ocean, with a minimum monthly ice extent of 4.28 × 106 km2, 23% less than during the previous minimum in 2005 . Questions arise whether this drastic reduction of ice extent is just the result of natural variability superimposed on a generally declining trend, or if the Arctic sea ice cover has transitioned into a different climatic state where completely ice-free summers would soon become normal . The rapidity of the Arctic summer sea ice decline is also surprising as it is much faster than predicted by any of the Intergovernmental Panel of Climate Change model scenarios . A better representation of sea ice in these models is complicated by the variety of different processes contributing to the presence of sea ice. For example, anomalous wind patterns, air temperatures, and radiation regimes have all been considered as causes for the minimum ice coverage in 2007 . It is unclear how much the warming of the Atlantic layer contributes to increases in ocean heat flux and therefore ice reduction . These factors all overlay a general, continued reduction of the fraction of older ice in the Arctic Ocean, as shown by drifting buoys and satellite radar maps . The latter also implies an overall shrinkage of ice volume, as the thickness of sea ice generally increases with age .

<3> However, due to methodological and logistical constraints, little is known about recent changes of ice thickness. The ice thickness distribution includes important information about both, thermodynamic and dynamic boundary conditions of ice formation and development . The mode of the thickness distribution, or modal thickness, represents level ice thickness, which is a result of winter accretion and summer ablation. Mean ice thickness is dominated by the tail of the thickness distribution which represents the thickness and amount of deformed ice as a result of ice convergence and shear.

EDIT

19] Our ice thickness data set is very heterogeneous as it has been obtained during sporadic expeditions to varying but overlapping regions of the Transpolar Drift. However, the uniformity of the derived thickness distributions and the inclusions of ice age information in our interpretations justify the conclusion of a rapidly thinning ice cover. Both, modal and mean SYI and FYI thicknesses have decreased in the region of the North Pole. The reduction of modal thicknesses is a result of increased atmospheric and ocean heat fluxes to the ice. Unfortunately, we are not able to distinguish between the individual processes. However, the good agreement with the FDD model suggests an important role of increased air temperature, while increases in ocean heat flux might have played a minor role in the present thinning. Ice concentration was remarkably high in our study region in 2004 and 2007, limiting the deposition of heat in the mixed layer . The presented results provide valuable information for the validation and improvement of numerical sea ice models.

EDIT

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