The Response of ENSO to a Weakening of the Atlantic Meridional Overturning Circulation

Abstract:

The El Nino-Southern Oscillation (ENSO) drives the largest interannual climate variability globally. However, how and why ENSO has changed in the past and how it may change in the future in association with changes in the mean state of the climate system are poorly understood. We propose that the response of ENSO to a freshwater perturbation in the North Atlantic that weakens the Atlantic Meridional Overturning Circulation is highly sensitive to the tropical Pacific mean state biases present in Atmosphere-Ocean General Circulation Models (GCMs) and that the presence or absence of tropical Pacific mean state biases can lead to a range of ENSO responses. We demonstrate these principles using a linearized coupled atmosphere-ocean model of the tropical Pacific in addition to fully coupled GCM simulations with the Community Earth System Model (CESM) in which freshwater perturbations were imposed to the North Atlantic. CESM runs were performed with and without the addition of heat flux corrections to reduce mean state biases in the tropics before the freshwater forcing was imposed. The processes responsible for the changes in ENSO variability due to the freshwater perturbation (with and without biases in the starting mean state) are quantified using an offline intermediate model of the tropical Pacific. Understanding the sensitivity of ENSO to such mean state changes and to biases in the mean state of the tropical Pacific in GCMs is critical for constraining past and future ENSO variability.