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

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.

Predictability and Decadal Variability of the North Atlantic Ocean State Evaluated from a Realistic Ocean Model

“…Also, the results of this study could be extended to a fully nonlinear framework. Specifically, conditional nonlinear optimal perturbations (Mu and Zhang 2006;Zu et al 2013;Li et al 2014) provide a natural extension of the LOPs. Together with considering the effects of ocean-atmosphere coupling, these ideas will offer directions for future work.…”

Optimal Initial Excitations of Decadal Modification of the Atlantic Meridional Overturning Circulation under the Prescribed Heat and Freshwater Flux Boundary Conditions

“…Moreover, the absolute maximum of SST anomalies is around 38C, of the same amplitude as strong seasonal SST anomalies in the HadSST dataset (Kennedy et al 2011). Zu et al (2013) calculated the NOIP of SSS under mixed boundary conditions. Because of the atmospheric damping of SST anomalies, the NOIP of SSS has different characteristics: the negative anomalies in the northwest are weaker, whereas the positive anomalies become stronger and move to the western subbasin near 408N (with respect to the results under flux boundary conditions).…”

“…In the past few years, the CNOP method has been used in the study of the predictability of ENSO events Mu et al 2007), the Kuroshio large meander variability (Wang et al 2012(Wang et al , 2013, the sensitivity of ecosystems (Sun and Mu 2011), the sensitivity and multidecadal variability of AMOC Zu et al 2013), and to determine adaptive observation strategies of typhoons (Qin and Mu 2012).…”

“…Therefore, the condition nonlinear optimal perturbation (CNOP) methodology was used to determine the nonlinear optimal initial perturbation (NOIP) patterns that optimally grow in the fully nonlinear flow Sun et al 2005;Zu et al 2013). Using the CNOP method and a box model, Mu et al (2004) found that the difference between the NOIP and LOIP increases with increasing amplitude of the perturbations.…”

“…Within a three-dimensional ocean circulation model of the North Atlantic, Zu et al (2013) obtained two kinds NOIPs, corresponding to the global and local maxima of the objective functions, respectively. They found that these two kinds of the perturbations led to significantly different multidecadal variations of the AMOC, which was explained through a nonlinear advective feedback between density and velocity anomalies.…”