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

Abstract:

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 of the NAO, in terms of the pattern of the winter-mean potential temperature anomaly in the troposphere, can be influenced by synoptic-scale eddy heat fluxes. The role and mechanism of eddy heat feedback in modulating the winter-mean thermodynamic structure of the NAO are investigated by statistics related to eddy feedback, eddy structure decomposition, 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. In addition, the positive NAO phase has a negatively stronger eddy heat feedback than the negative phase; this is related to the higher persistence of the anomalous potential temperature pattern under negative NAO.

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.