Abstract
Abstract Optimal perturbations of sea surface salinity are obtained for an idealized North Atlantic basin using a 3D planetary geostrophic model—optimality is defined with respect to the intensity of the meridional overturning circulation. Both optimal initial and stochastic perturbations are computed in two experiments corresponding to two different formulations of the surface boundary conditions: the first experiment uses mixed boundary conditions (i.e., restoring surface temperature and prescribed freshwater flux), whereas the second experiment uses flux boundary conditions for both temperature and salinity. The latter reveals greater responses to both initial and stochastic perturbations that are related to the existence of a weakly damped oscillatory eigenmode of the Jacobian matrix, the optimal perturbations being closely related to its biorthogonal. The optimal initial perturbation induces a transient modification of the circulation after 24 yr. The spectral response to the optimal stochastic perturbation reveals a strong peak at 35 yr, corresponding to the period of this oscillatory eigenmode. This study provides an upper bound of the meridional overturning response at multidecadal time scales to freshwater flux perturbation: for typical amplitudes of Great Salinity Anomalies, initial perturbations can alter the circulation by +2.25 Sv (1 Sv ≡ 106 m3 s−1; i.e., 12.5% of the mean circulation) at most; stochastic perturbations with amplitudes typical of the interannual variability of the freshwater flux in midlatitudes induce a circulation variability with a standard deviation of 1 Sv (i.e., 5.5% of the mean circulation) at most.
Highlights
A strong modification of surface air temperature in the North Atlantic during the past century has been established (Mann et al 1999)—in the context of global warming
We have investigated the influence of surface salinity modifications on the ocean circulation, paying special attention to the thermohaline circulation, the slow component of the ocean and of the climate system, transporting heat poleward
We have addressed two types of optimal sea surface salinity perturbations: initial perturbations, inducing the largest change in the MOC intensity, and stochastic perturbations, inducing the largest variability of the MOC
Summary
A strong modification of surface air temperature in the North Atlantic during the past century has been established (Mann et al 1999)—in the context of global warming. Sun et al (2005) extended this work in a coupled ocean–atmosphere box model and suggested that for weak amplitude perturbations, the linear approach remains valid In this linear context, Zanna and Tziperman (2005) studied the transient growth at 40 yr of an optimal initial perturbation in a simple ocean–atmosphere coupled model (latitude–depth ocean model with only two levels on the vertical and a one-layer atmosphere): the growth mechanism is due to the advection by the circulation anomaly, which increases both temperature and salinity anomalies. Stochastic optimal perturbations are relevant to understanding how high-frequency atmospheric forcing related to midlatitude weather regimes [the North Atlantic Oscillation (NAO), for instance] can induce longterm variability of the thermohaline circulation These analyses provide the upper bounds on the modification of the MOC. We discuss our results and propose some outlook (section 6)
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