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Optimal excitation of Atlantic ocean variability and implications for predictability

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The Atlantic meridional overturning circulation (MOC) plays an important role in regulating high-latitude climate. Evidence from numerical models indicates that the strength of the MOC varies on a wide range of timescales with a relatively high potential predictability on decadal timescales. In order to explore the variability and predictability of the Atlantic MOC , we study the fastest growing perturbations arising from the ocean dynamics in a general circulation model.

The analysis reveals that the MOC is most sensitive to deep density perturbations located in the northern part of the basin. The excitation of such perturbations can generate a large amplification of MOC anomalies after approximately 7 years. This information may help in recognizing for example where additional observations could better constrain interannual and multi-decadal variability associated with the growth of perturbations. The mechanism associated with the growth of MOC anomalies can be understood by examining the time evolution of deep zonal density gradients (related to the MOC via the thermal wind relation). The propagation of density anomalies, which depends on the mean flow and the mean density gradient, determines the growth time scale of the MOC anomalies.

Moreover, the growth timescale of the perturbations gives an estimate of error growth in initial conditions, and therefore an estimate on how such growth can potentially limit the predictability of the MOC . In the present model, MOC anomalies are found to grow faster when deep ocean perturbations are allowed rather than when only the surface of the ocean is perturbed. Therefore, uncertainties in the deep ocean may limit the predictability of the ocean circulation and predictability experiments in which only the atmospheric state (equivalent to perturbing only the ocean surface) is perturbed may overestimate of the ocean predictability time.

References: Zanna L, P. Heimbach, A.M. Moore and E. Tziperman, 2009: On the predictability and variability of the North Atlantic ocean. Submitted to J. of Climate Zanna L, P. Heimbach, A.M. Moore and E. Tziperman, 2009: Excitation of interannual variability of the meridional overturning circulation. Submitted to J. of Phys. Oceanogr.

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