Ocean Dynamics

, Volume 57, Issue 3, pp 223–235 | Cite as

Recent changes of the thermohaline circulation in the subpolar North Atlantic

  • Manfred Bersch
  • Igor Yashayaev
  • Klaus Peter Koltermann
Article

Abstract

Time series of hydrographic sections in the northern North Atlantic from the period 1990 to 2004 are analyzed for changes in the characteristics and distribution of water masses that are involved in the thermohaline circulation (THC). During the 1990s, the North Atlantic Oscillation (NAO) alternates from a positive phase (strong westerlies) to a negative phase (weak westerlies). The reduced ocean heat loss confined the convection in the Labrador Sea to the upper 1,200 m, generating a new salinity minimum layer characterizing the Upper Labrador Sea Water (ULSW), and led to a warming and salinization of the older LSW below due to lateral mixing. The Lower LSW, formed in the first half of the 1990s, spread in the subpolar gyre and reached the Newfoundland and Irminger basins after about 1 to 2 years, where the associated isopycnal doming contributed to eastward frontal shifts in the upper layer. After 5 and 6 years, it arrived in the Iceland and West European basins, respectively. The collapse of the isopycnal dome in the Labrador Sea, associated with the drainage of the Lower LSW, resulted in a slowing of the cyclonic circulation of the subpolar gyre. This was accompanied in the upper layer by a westward shift of the southeastern extension of the gyre and a northward advection of warm and saline subtropical water in its eastern part, which finally reached the Labrador Sea after about 7 years. In the upper layer of the Labrador Sea, the advection of warm and saline water dominated over the heat loss to the atmosphere and the freshwater gain from melting ice and precipitation in the NAO-low period, so that no accumulation of freshwater but an increase of the heat and salt contents were observed, as in the whole eastern part of the subpolar gyre. Within 1 to 2 years after the drop of the NAO in the winter of 1995/1996, the Subarctic (Subpolar) Front shifted northward and westward north of about 50°N, favored by the retreat of the low-salinity tongue extending eastward from the southern Labrador Sea, and it shifted southward and eastward in the Newfoundland Basin. Therefore, the enhanced northward advection of subtropical waters in the northeastern North Atlantic is balanced by the enhanced southward advection of subarctic waters, including Lower LSW in the Newfoundland Basin, indicating a strong response of the gyre component of the THC.

Keywords

Thermohaline circulation Subpolar North Atlantic Labrador Sea Water Subpolar gyre Subarctic Front North Atlantic Oscillation 

Notes

Acknowledgment

We thank the sailors, technicians, and scientists who were involved in the hydrographic measurements. Helpful discussions with Hendrik van Aken, Helmuth Haak, and Johann Jungclaus are appreciated. The study was supported by the Bundesministerium für Bildung und Forschung under grant no. 03F0377E and 03F0378A (CLIVAR-marin).

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Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  • Manfred Bersch
    • 1
  • Igor Yashayaev
    • 2
  • Klaus Peter Koltermann
    • 3
  1. 1.Institut für MeereskundeUniversität HamburgHamburgGermany
  2. 2.Bedford Institute of OceanographyDartmouthCanada
  3. 3.Bundesamt für Seeschifffahrt und HydrographieHamburgGermany

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