Role Of Synoptic Eddy-Heat-Flux-Induced Feedback In Modulating NAO

The North Atlantic Oscillation(NAO),considered as the dominant mode of low-frequency climate variability over the North Atlantic,strongly influences regional and even Northern Hemispheric weather and climate.The thermodynamic structure and dynamic structure of NAO can be influenced by synoptic-scale eddy feedbacks.The role and mechanism of synoptic eddy-heat-flux-induced feedback in modulating the winter-mean thermodynamic structure and dynamic structure of the NAO are investigated by statistics related to eddy feedback,eddy structure decomposition,the diagnostic framework of the transformed eddy potential vorticity flux,etc.We show that the low-frequency NAO-related flow can barotropically regulate the synoptic eddy heat fluxes,which are directed to the left of the NAO flow and diverge from(converge into)the anticyclonic(cyclonic)NAO flow.That can give rise to an eddy heat transport that is down-gradient(up-gradient)of the NAO-related potential temperature anomalies,thus directly creating negative(positive)feedback on NAO-related thermodynamic structure in the middle-lower(upper)troposphere.Effects of eddy heat flux on NAO-related thermodynamic structure vary with isobar levels;this might be a direct result of NAO-related thermodynamic structure varying with isobar levels.The eddy heat transport will influence the vorticity transport by secondary circulation,and thus creating feedback on NAO-related dynamic structure.The eddy-heat-flux-induced feedback on NAO-related dynamic structure is negative in the upper troposphere and positive in the middle-lower troposphere,and the net contribution is positive which can maintain the NAO-related dynamic structure.In addition,there is a phase asymmetry of eddy-heat-flux-induced feedback on both NAO-related thermodynamic structure and dynamic structure which characterized by a higher feedback efficiency under positive NAO in the middle-lower troposphere.Thus,the positive NAO condition might be more favorable to eddy-heat-flux-induced feedback in the middle-lower troposphere.Using eddy structure decomposition,we calculated the two primary terms of anomalous eddy heat flux: the BA term is the product of basic eddy velocity structure and anomalous eddy thermodynamic structure,and the AB term is the product of anomalous eddy velocity structure and basic eddy thermodynamic structure.In the lower troposphere,compared with the eddy dynamic structure,eddy thermodynamic structure has a phase difference of about one-fourth cycle.Under NAO condition,eddy thermodynamic structure shifts meridionally,thus the anomalous eddy thermodynamic structure shows a meridional in-quadrature dipole relative to basic eddy dynamic structure.The induced meridional BA eddy heat flux is directed to the left of the NAO flow.It is a major contributor to the anomalous eddy heat flux and forcing.Specifically,under negative NAO,eddy dynamic structure slants counterclockwise,thus the anomalous eddy dynamic structure shows a meridional in-phase dipole relative to basic eddy thermodynamic structure.The induced zonal AB eddy heat flux is smaller in magnitude,and creates positive eddy heat forcing upstream of the NAO flow.