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Subtropical countercurrent variations in cooling climates induced by freshwater forcing over the subarctic North Atlantic

  • Cunjie Zhang
  • Xiaopei Lin
  • Cong Zhang
  • Yongqing Guo
Article

Abstract

The subtropical countercurrent (STCC) could affect ocean eddy activity and the atmosphere above. How the STCC varied in a cooling scenario, such as the stadials during the last deglaciation (19–11 ka; ka = 1000 years ago), is still unclear. By 1-Sv (1 Sv = 106 m3 s− 1) freshwater hosing over the subpolar North Atlantic to suppress the Atlantic Meridional Overturning Circulation (AMOC), the STCC variations under pre-industrial (PI) and last glacial maximum (LGM, ~ 21 ka) conditions are analyzed. In both hosing experiments, the northern hemisphere cools. The upper ocean becomes less stratified, and the wintertime mixed layer deepens in the Kuroshio–Oyashio extension (KOE) region. As a result, more mode waters (~ 20%) are formed, and they become more vertically uniform. The enhanced mode waters induce a stronger vertical shear and accelerate the STCC based on thermal wind relationship. The strengthened STCC further induces warm ocean temperature and upward air-sea heat flux anomalies along it. While the STCC strengthens in both hosing experiments, the location variations are different. In the PI hosing experiment, the Bering Strait is open. Fresh water enters the North Pacific through the Bering Strait and increases the stratification in the downstream of the KOE region. The mixed layer front shifts westward and induces a robust westward shift of mode waters and the STCC. In the LGM hosing experiment, on the contrast, there is no obvious location shift because the Bering Strait is closed.

Keywords

Subtropical countercurrent Substantially weakened AMOC Bering strait Mode waters 

Notes

Acknowledgements

This work is supported by the China’s national key research and development Projects (2016YFA0601803), the National Natural Science Foundation of China (41490641, 41521091 and U1606402) and the Qingdao National Laboratory for Marine Science and Technology (2017ASKJ01). We thank the National Center for Atmospheric Research (NCAR) and the Earth System Grid Federation (ESGF) for sharing the LGM simulations with the public. We thank Link Ji and Ping Chang from Texas A&M University for providing the simulation outputs under PI conditions. The PI condition experiments data used in this work are available at Ocean and Atmosphere Data Center of Ocean University of China (coadc.ouc.edu.cn). We also thank three anonymous reviewers for their constructive comments and suggestions.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Physical Oceanography Laboratory/CIMSTOcean University of China and Qingdao National Laboratory for Marine Science and TechnologyQingdaoChina
  2. 2.First Institute of OceanographyState Oceanic AdministrationQingdaoChina
  3. 3.Laboratory for Regional Oceanography and Numerical ModelingQingdao National Laboratory for Marine Science and TechnologyQingdaoChina

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