The Leading, Interdecadal Eigenmode of the Atlantic Meridional Overturning Circulation in a Realistic Ocean Model
Journal of Climate
Variations in the strength of the Atlantic meridional overturning circulation (AMOC) are a major potential source of decadal and longer climate variability in the Atlantic. In this study, we analyze continuous integrations of tangent linear and adjoint versions of an ocean General Circulation Model (OPA) and rigorously show the existence of a weakly-damped oscillatory eigenmode centered in the North Atlantic and controlled solely by linearized ocean dynamics. In this particular GCM the mode
... ar GCM the mode period is roughly 24 years, its e-folding decay timescale is 40 years, and the mode is the least damped in the system. Its mechanism is related to the westward propagation of temperature anomalies in the northern Atlantic in the latitudinal band between 30 and 60 • N: these temperature anomalies modify density in the upper 1000 m of the ocean and hence, by geostrophic balance, ocean currents which then affect the temperature field. Salinity variations partially compensate the effect of temperature on density but, in general, have a smaller role in the oscillation (except during the excitation of the mode by initial perturbations). The westward propagation of temperature anomalies results from of a competition between mean eastward zonal advection, equivalent anomalous westward advection due to the mean meridional temperature gradient, and westward propagation typical of long baroclinic Rossby waves. When a temperature anomaly arrives at the basin western boundary, the ensuing geostrophic adjusment modifies the AMOC in about 2 years. Further, we show that the system is nonnormal, which implies that the structure of the eigenmode is different from the least-damped mode of the adjoint model. The latter mode describes the sensitivity of the system, i.e. which patterns, chosen as initial conditions, are most efficient for exciting the eigenmode. An idealized model is presented to highlight the role of the background meridional temperature gradient in the North Atlantic in the mechanism of the interdecadal mode and the nonnormality of the system.