The Northern Annular Mode: Atlantic vs. Pacific

The first EOF of the Northern Hemisphere winter-averaged zonal mean zonal wind, i.e. the Northern Annular Mode (NAM), is a dipole pattern which represents variability in the meridional position of the eddy-driven jet. The second EOF characterizes the strength of the eddy-driven jet. Previous work has shown that the NAM is the dominant mode of variability due to a positive feedback between the zonal mean anomalies and the eddy momentum fluxes.

If an analysis is performed on the zonal mean zonal wind over the Atlantic and Pacific ocean basins separately, the resulting EOF's are spatially well correlated in each sector. The dynamical interpretation, though, is quite different due to differences of the time averaged zonal flow over the Atlantic and Pacific sectors. In the Atlantic sector the first EOF is still associated with the meridional position of the eddy-driven jet, but the first EOF in the Pacific sector is associated with the strength of the jet. This result suggests that the strength of the positive feedback mechanism, which causes the first EOF of the global zonal mean zonal wind to be the NAM, is weaker over the Pacific sector.

Numerical simulations in a dry primitive equation model are used to explore separately the dynamics of the Atlantic and Pacific sectors. It is found that the strength of the feedback mechanism is dependent on the relationship between the eddy-driven and subtropical (Hadley-driven) jets. Specifically, the feedback is very weak when the eddy-driven and subtropical jets are nearly collocated, which occurs over the Pacific basin.