Impact of the AMOC Weakening on Upper Troposphere/Lower Stratosphere Warming Over the Extratropical North Pacific

Abstract The Atlantic Meridional Overturning Circulation (AMOC) is a key regulator of the Earth's climate. Here using water hosing experiments with a coupled climate model, we show that a weakened AMOC leads to a significant boreal winter upper troposphere/lower stratosphere (UTLS) warming over the extratropical North Pacific (ENP). This warming is linked to the UTLS vertical motion response through a simple leading‐order thermodynamic balance between the vertical advection of the mean potential temperature by vertical motion anomalies and the zonal advection of temperature anomalies by mean westerlies, which accurately estimates the AMOC‐induced UTLS warming. The UTLS vertical motion response over the ENP is sustained by the advection of the anomalous cyclonic vorticity by mean winds. The cyclonic vorticity anomaly is generated by a stationary wave response (emanating from the tropical Pacific) associated with the AMOC weakening‐induced changes in the tropical North Pacific Walker cell and the eastern tropical Pacific Hadley cell.