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Original Research Papers

Transient response of the Atlantic Meridional Overturning Circulation to enhanced freshwater input to the Nordic Seas–Arctic Ocean in the Bergen Climate Model

Authors:

Odd Helge Otterå ,

Nansen Environmental and Remote Sensing Center, Edv. Griegsvei 3A, 5059 Bergen; Bjerknes Centre for Climate Research, Allégt. 55, 5007 Bergen, NO
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Helge Drange,

Nansen Environmental and Remote Sensing Center, Edv. Griegsvei 3A, 5059 Bergen; Bjerknes Centre for Climate Research, Allégt. 55, 5007 Bergen; Geophysical Institute, University of Bergen, Allégt. 70, 5007 Bergen, NO; Nansen-Zhu International Research Centre, Beijing 100029, CN
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Mats Bentsen,

Nansen Environmental and Remote Sensing Center, Edv. Griegsvei 3A, 5059 Bergen; Bjerknes Centre for Climate Research, Allégt. 55, 5007 Bergen, NO
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Nils Gunnar Kvamstø,

Bjerknes Centre for Climate Research, Allégt. 55, 5007 Bergen; Geophysical Institute, University of Bergen, Allégt. 70, 5007 Bergen, NO
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Dabang Jiang

Nansen-Zhu International Research Centre, Beijing 100029, CN
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Abstract

The transient response of the climate system to anomalously large freshwater input to the high latitude seas is examined using the newly developed Bergen Climate Model. A 150-yr twin-experiment has been carried out, consisting of a control and a freshwater integration. In the freshwater integration, the freshwater input to the Arctic Ocean and the Nordic Seas is artificially increased by a factor of 3, or to levels comparable to those found during the last deglaciation. The obtained response shows a reduced maximum strength of the Atlantic Meridional Overturning Circulation (AMOC) over the first 50 yr of about 6 Sv (1 Sv =106 m3 s—1), followed by a gradual recovery to a level comparable to the control integration at the end of the period.

The weakened AMOC in the freshwater integration is caused by reduced deep-water formation rates in the North Atlantic subpolar gyre and in the Nordic Seas, and by a reduced southward flow of intermediate water masses through the Fram Strait. The recovery of the AMOC is caused by an increased basin-scale upwelling in the Atlantic Ocean of about 1 Sv, northward transport of saline waters originating from the western tropical North Atlantic, and a surface wind field maintaining the inflow of Atlantic Water to the Nordic Seas between the Faroes and Scotland.

Associated with the build-up of more saline waters in the western tropical North Atlantic, a warming of ~0.6 °C over the uppermost 1000 m of the water column is obtained in this region. This finding is consistent with paleo records during the last deglaciation showing that the tropics warmed when the high latitudes cooled in periods with reduced AMOC.

Furthermore, the results support the presence of a coupled North-Atlantic-Oscillation-like atmosphere–sea-ice–ocean response mode triggered by the anomalous freshwater input. Throughout most of the freshwater integration, the atmospheric circulation is characterized by anomalously low sea level pressure in the Nordic Seas and anomalously high sea level pressure over Spain. This forces the North Atlantic Drift to follow a more easterly path in the freshwater integration than in the control integration, giving an asymmetric sea surface temperature response in the northern North Atlantic, and thereby maintaining the properties of the AtlanticWater entering the Nordic Seas between the Faroes and Scotland throughout the freshwater integration.

How to Cite: Otterå, O.H., Drange, H., Bentsen, M., Kvamstø, N.G. and Jiang, D., 2004. Transient response of the Atlantic Meridional Overturning Circulation to enhanced freshwater input to the Nordic Seas–Arctic Ocean in the Bergen Climate Model. Tellus A: Dynamic Meteorology and Oceanography, 56(4), pp.342–361. DOI: http://doi.org/10.3402/tellusa.v56i4.14421
  Published on 01 Jan 2004
 Accepted on 29 Jan 2004            Submitted on 6 May 2003

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