Role of Eddy Heat Feedback in Modulating Winter‐Mean NAO‐Related Thermodynamic Structure

The thermodynamic structure of the North Atlantic Oscillation (NAO), in terms of the pattern of the winter‐mean potential temperature (PT) anomaly in the troposphere, is a crucial indicator of regional climate variations and can be influenced by synoptic‐scale eddy heat (EH) fluxes. The role and mechanism of EH feedback in modulating the winter‐mean thermodynamic structure of the NAO are investigated. We show that the low‐frequency NAO‐related flow can barotropically regulate EH flux and give rise to an EH transport that is down‐gradient (up‐gradient) of the NAO‐related PT anomalies, thus creating negative (positive) EH feedback on PT anomalies in the middle‐lower (upper) troposphere. Using eddy structure decomposition, we calculated the two primary terms of anomalous EH flux: the BA term is the product of basic eddy velocity and anomalous eddy PT, and the AB term is the product of anomalous eddy velocity and basic eddy PT. The meridional BA EH flux is caused by the meridional shift of eddy PT structure under NAO. It is a major contributor of the anomalous EH flux and forcing, which weaken NAO‐related PT anomalies. The zonal AB EH flux is smaller in magnitude and is induced by the zonal slant of the eddy streamfunction structure in the negative NAO phase. It creates EH feedback upstream of the NAO flow. In addition, the positive NAO phase has a negatively stronger EH feedback than the negative phase; this is related to the higher persistence of the anomalous PT pattern under negative NAO.