Multidecadal Oscillation of the Atlantic Meridional Overturning Circulation and its Association with Mean Subpolar Gyre in CMIP5 Models

The multidecadal variability of the Atlantic meridional overturning circulation (AMOC) is a major component of the natural climate variability in the North Atlantic Ocean. However, its fundamental mechanism has yet to be fully understood. Current state-of-the-art climate simulations show a wide range in frequency and amplitude of multidecadal AMOC oscillation. It is important to identify the causes of this strong model dependency. In this study, we examine the AMOC oscillation in preindustrial simulations of 500 to 1000 years long, produced by five Coupled Model Intercomparison Project Phase 5 (CMIP5) climate systems and the Community Climate System Model, version 3.0. A multichannel singular spectrum analysis is used to objectively identify the dominant oscillation in each of these climate models. A phase composite based on the dominant mode is used to construct a composite lifecycle of the oscillation to examine its temporal-spatial structure for each model.

Our results show that, although the AMOC oscillates with widely different periods and strengths in these models, they follow a common sequence of phases within a composite lifecycle: A positive anomalous overturning centered at 40^o-50^oN is first reversed from north with warm subsurface ocean temperature anomalies in the central basin of the northern North Atlantic. The reversed anomalous AMOC then extends southward as the warm signals propagate further westward. On the other hand, these warm temperature anomalies are previously generated during the strong AMOC phase near the North Atlantic Current and advected northward along the track of the subpolar gyre (SPG). An air-sea feedback may influence the process because the subsurface induced warm sea surface temperature anomalies in northern North Atlantic weaken the North Atlantic Oscillation. The process is reversed during the cold phases. The mean SPG structure plays an important roe in determining the frequency and strength of the dominant AMOC oscillation. In general, an intensive SPG within northwestern Atlantic generates strong AMOC oscillation with shorter period while a weak SPG elongated eastward generates a weak oscillation with a longer period.