Physics

Chinese Journal of Oceanology and Limnology

, Volume 31, Issue 6, pp 1368-1374

First online:

Optimal nonlinear excitation of decadal variability of the North Atlantic thermohaline circulation

  • Ziqing Zu祖子清Affiliated withLASG, Institute of Atmospheric Physics, Chinese Academy of SciencesUniversity of Chinese Academy of Sciences
  • , Mu Mu穆穆Affiliated withLASG, Institute of Atmospheric Physics, Chinese Academy of SciencesKey Laboratory of Ocean Circulation and Wave, Institute of Oceanology, Chinese Academy of Sciences Email author 
  • , Henk A. DijkstraAffiliated withInstitute for Marine and Atmospheric Research Utrecht, Department of Physics and Astronomy, Utrecht University

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Abstract

Nonlinear development of salinity perturbations in the Atlantic thermohaline circulation (THC) is investigated with a three-dimensional ocean circulation model, using the conditional nonlinear optimal perturbation method. The results show two types of optimal initial perturbations of sea surface salinity, one associated with freshwater and the other with salinity. Both types of perturbations excite decadal variability of the THC. Under the same amplitude of initial perturbation, the decadal variation induced by the freshwater perturbation is much stronger than that by the salinity perturbation, suggesting that the THC is more sensitive to freshwater than salinity perturbation. As the amplitude of initial perturbation increases, the decadal variations become stronger for both perturbations. For salinity perturbations, recovery time of the THC to return to steady state gradually saturates with increasing amplitude, whereas this recovery time increases remarkably for freshwater perturbations. A nonlinear (advective) feedback between density and velocity anomalies is proposed to explain these characteristics of decadal variability excitation. The results are consistent with previous ones from simple box models, and highlight the importance of nonlinear feedback in decadal THC variability.

Keyword

thermohaline circulation decadal variability conditional nonlinear optimal perturbation nonlinear advective feedback