Dynamics of the Extratropical Response to a Tropical Atlantic SST Anomaly

Abstract

Abstract Previous atmospheric general circulation model (AGCM) experiments revealed that atmospheric responses to a tropical Atlantic sea surface temperature anomaly (SSTA) were asymmetric with respect to the sign of the SSTA. A positive SSTA produced a south–north dipole in geopotential heights, much like the North Atlantic Oscillation (NAO), while a negative SSTA yielded an eastward-propagating wave train, with the northern lobe of the NAO absent. Here these height responses are decomposed into components that are symmetric or antisymmetric with respect to the sign of the SSTA. The symmetric, or notionally linear, component is a nearly south–north dipole projecting on the NAO, while the antisymmetric, or notionally nonlinear, component is a different dipole. Experiments with a diagnostic linear baroclinic model (LBM) suggest that both components are maintained primarily by transient-eddy forcing. Dynamical mechanisms for the formation of the two components are explored using the LBM and a nonlinear barotropic vorticity equation model (BVM). Transient-eddy feedback is sufficient to explain the linear response. The NAO-like linear response occurs when the initial heating induces transient-eddy forcing in the exit of the Atlantic jet. The structure of the background absolute vorticity in this region is such that this transient-eddy forcing induces a nearly north–south dipole in anomalous geopotential heights. When the nonlinear self-interaction of this transient-induced low-frequency perturbation is included in the BVM, the dipole axis tilts to the east or west, resulting in a response that is nonlinear about the sign of the forcing.