The Importance of the Transition Zone to Decadal AMOC Variability

On short timescales variability of the AMOC primarily reflects that of the wind field, and meridional coherence is limited. However, on decadal timescales the AMOC exhibits meridionally coherent modes of variability, which are driven by a complex mixture of wind and thermohaline processes. Models differ substantially in the spatial patterns and dominant timescales of low-frequency AMOC variability, and isolating robust mechanisms of AMOC variability remains challenging. Despite this, we argue that the region near the Grand Banks where the Gulf Stream/North Atlantic Current (NAC) and the deep western boundary current cross over, henceforth called the Transition Zone, is a key region influencing large-scale decadal AMOC variability. It is here that we observe the Mann eddy, an intense anticyclone swirling to the southeast of the NAC. Variability in this key region is implicitly reflected in the AMOC indices commonly utilized by the modeling community. Processes that are important in creating buoyancy anomalies in the Transition Zone are expected to play an important role in AMOC variability. Such processes include local atmospheric forcing, advection of anomalies by mean currents, west- ward propagating (wind or buoyancy forced) baroclinic Rossby waves, anomalies resulting from large-scale ocean circulation changes (such as shifts of the Gulf Stream path), and anomalies advected/propagated from high latitudes. The complex ocean dynamics in transition zone likely explain why AMOC variability is so sensitive to model formulation, both between models and in the same model when changes are made to its resolution, overflow parameterizations, etc.