Annular Mode Variability of the Meridional Circulation

The annular modes are the leading modes of intraseasonal variability in the extratropics and describe a poleward shift of the midlatitude zonal jet during the high-index phase in both hemispheres. As the midlatitude jet is maintained by eddies, the storm track must shift polewards as well. The goal of this work is to understand the changes this shift has on the meridional overturning circulation and global transports of mass and energy.

As the atmosphere is turbulent in nature, one must take temporal and zonal averages to obtain a coherent picture of the meridional circulation. However, the choice of vertical coordinate on which the averages are taken can result in circulations with significant qualitative and quantitative differences. Averaging on pressure surfaces gives rise to the classical three-celled Eulerian circulation with a thermally indirect Ferrel cell in the midlatitudes. The equatorward energy transport implied by the Ferrel cell is a misleading representation of the total energy transport because it neglects the eddy contribution which is polewards and dominant in the midlatitudes. The circulation averaged on isentropic coordinates better accounts for the eddy transport and consists of a single overturning circulation.

Here, we investigate the annular mode variability of the Eulerian and dry and moist isentropic circulations. The isentropic circulations are analyzed using the Statistical Transformed Eulerian Mean (STEM) framework, developed by Pauluis et al. (2011), which provides a generalization of the Transformed Eulerian Mean (TEM) framework to arbitrary and possibly unstratified vertical coordinates; in this case the vertical coordinates are taken to be the dry and moist static energies. Like the TEM formulation, the STEM streamfunction is decomposed into an Eulerian mean component and an eddy component, which allows for studying the annular mode responses separately using a linear regression analysis.

It is found that the in the high-index phase of the annular mode, the eddy momentum flux convergence and Ferrel cell show polewards shifts that support the shift in the eddy-driven jet. However, the isentropic circulation responses show apparent equatorward shifting in the midlatitudes. An analysis using the STEM decomposition shows that, in the Northern hemisphere, the circulation driven by eddy heat fluxes responds with only a weak polewards shift that is dominated by the response of the mean circulation. This is in contrary to the annual mean isentropic circulation, in which the midlatitude eddy component dominates that of the mean.